Prof. Dr.-Ing. A. Erman Tekkaya

Institut für Umformtechnik und Leichtbau
TU Dortmund University

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  • Augmented Reality Application for the Mobile Measurement of Strain Distributions
    Mogylenko, O. and Selvaggio, A. and Upadhya, S. and Grodotzki, J. and Tekkaya, A.E.
    Advances in Intelligent Systems and Computing 1231 AISC (2021)
    In mechanical engineering studies hands-on laboratories are an integral part of the students’ education. In manufacturing related fields, material characterization labs are often used to enable students to foster their understanding of different materials. To enhance the laboratory experience and to educate about specific aspects of the uniaxial tensile test, an Augmented Reality (AR) application has been developed. With this applications, it is possible to visualize the inhomogeneous strain field that arises during the experiment on the surface of the specimen. The technical components and structure of the implementation are described in this paper. The usability of several algorithms, technical and software implementation is discussed and evaluated. © 2021, Springer Nature Switzerland AG.
    view abstract10.1007/978-3-030-52575-0_19
  • Coin minting by additive manufacturing and forming
    Pragana, J.P.M. and Rosenthal, S. and Alexandrino, P. and Araújo, A. and Bragança, I.M.F. and Silva, C.M.A. and Leitão, P.J. and Tekkaya, A.E. and Martins, P.A.F.
    Proceedings of the Institution of Mechanical Engineers, Part B: Journal of Engineering Manufacture 235 (2021)
    Additive manufacturing is proposed as a novel alternative to coin blank’s production routes based on rolling, blanking and edge rimming. The presentation draws from laser powder bed fusion of cylinders, slicing into individual coin blanks by electro discharge machining and surface preparation by polishing, to coin minting in a laboratory press-tool system. Special emphasis is given to material deposition and coin minting due to the originality of producing coin blanks with complex intricate contoured holes and to the necessity of subjecting the additive manufactured coin blanks to extreme compressive stresses that are typical of coin minting. Numerical and experimental results confirm the excellent performance of the additive manufactured coin blanks. The new design layouts included in the additive manufactured coin blanks open the way to produce high value-added singular collector coins, which are disruptively different from those available in the market nowadays. © IMechE 2020.
    view abstract10.1177/0954405420971128
  • Methods for measuring large shear strains in in-plane torsion tests
    Traphöner, H. and Clausmeyer, T. and Tekkaya, A.E.
    Journal of Materials Processing Technology 287 (2021)
    The in-plane torsion test achieves true strains far beyond 1.0 for sheet metals, especially using specimens with circular grooves. The accurate measurement of these high strains is a challenge for the conventional digital image correlation (DIC). Thus, the determination of flow curves is limited and fracture strains for very ductile materials cannot be measured. A new grooved specimen is introduced to avoid strain localization. Shear stress and shear strain along a defined area in the groove are constant so that strains can be measured independently of the DIC system setting without error due to strain localization. Furthermore, three methods for the measurement of very high shear strains in the in-plane torsion test are presented: Firstly, the limit of the optical strain measurement is extended by multiple renewal of the digital image correlation (DIC) pattern on the samples. Secondly, the shear strain for the planar specimen is calculated exactly from the rotation angle curve. Lastly, a new incremental method is presented. This method enables to determine shear strains for plane and grooved specimen exactly by only measuring the torque and the angle of rotation. All methods were applied for three steel sheet materials namely DP1000, CP1000 and DC04. The equivalent strain in a grooved in-plane torsion test of sheet steel DC04 was determined as 3.3 with the new incremental method. Such high strains far exceed conventional methods for determining the flow curve. © 2019 The Author(s)
    view abstract10.1016/j.jmatprotec.2019.116516
  • Remote Lab to Illustrate the Influence of Process Parameters on Product Properties in Additive Manufacturing
    Upadhya, S. and Grodotzki, J. and Selvaggio, A. and Mogylenko, O. and Tekkaya, A.E.
    Advances in Intelligent Systems and Computing 1231 AISC (2021)
    Additive manufacturing, which enables the production of highly complex components that were previously next to impossible to manufacture, has evolved from a tool for rapid prototyping to an integral part of many production lines in the metal manufacturing industry. Therefore, it is critical that today’s students, the manufacturing engineers of tomorrow, get a fundamental understanding of the process and the influence of the various process parameters on the process and the final product properties. The developed remote lab offers the students an opportunity to vary different process parameters and characterize the performance of specimens manufactured under different conditions with help of the uniaxial tensile test and to quantitatively analyze the interplay of different process parameters on the final product. From the educator’s point of view, the remote lab will allow for a higher throughput of students in the field of additive manufacturing without compromising on the machine and user safety, as well as the effectiveness of the lab. © 2021, Springer Nature Switzerland AG.
    view abstract10.1007/978-3-030-52575-0_37
  • A quick model for demonstrating high speed forming capabilities
    Hahn, M. and Tekkaya, A.E.
    Mechanics Research Communications 108 (2020)
    High speed metal forming processes apply dynamic pressures on a sheet metal blank. To predict the forming capabilities inherent to these processes, suitable modeling approaches are of interest. Often, complex and large finite element (FE) simulations are necessary. A simplified novel mechanical approach, named chain model, is introduced and compared with experiments employing the innovative vaporizing foil actuator technology. It allows for a fast basic assessment of different material laws and dynamic loading conditions without the need of advanced soft- or hardware resources. The model consists of allocated masses connected by plastic segments. Despite its simplicity, efficiency, and some remaining challenges, the model predicts the experimental observation acceptably. For a next step, model results indicate that applying vaporizing actuators in a varied two-sided fashion might enable a toolless manufacture of some part geometries under free forming conditions. © 2020 Elsevier Ltd
    view abstract10.1016/j.mechrescom.2020.103579
  • Adiabatic blanking of advanced high-strength steels
    Schmitz, F. and Winter, S. and Clausmeyer, T. and Wagner, M.F.-X. and Tekkaya, A.E., (1)
    CIRP Annals 69 (2020)
    Adiabatic blanking of advanced high-strength steels with initial flow stresses above 1300 MPa is investigated. The blanked edge exhibits a unique S-shape. Localisation and properties of shear bands are analysed in shear-compression tests. A shear-compression stress state before separation leads to blanked edges without fracture zone, burr, or roll-over. Numerical modelling predicts the characteristic shape of the blanked edge satisfactorily. Physics-based models reveal that the strain rate sensitivity of the workpiece material is the key parameter affecting the width and the surface hardness of the shear band. Stress triaxiality and strain rate sensitivity determine the minimum size of radii in complex parts blanked without failure. © 2020 CIRP
    view abstract10.1016/j.cirp.2020.03.007
  • Analysis of incremental die bending of wires and tubes
    Holstein, V. and Hermes, M. and Tekkaya, A.E.
    Production Engineering 14 (2020)
    The use of bent wires, tubes, and profiles has a broad application in the industry as it is based on a great number of possible applications. Both structures with a high light-weight construction potential as well as functional and aesthetic geometry can be realised expediently. Known processes are often limited in case of tight radii and complex bending contours that cut each other or that are similar to a loop. The so called incremental die bending provides the remedy. It is an innovative process for the production of complexly shaped workpieces in the form of bent wires, tubes, or profiles. The bending process in case of incremental die bending is different to conventional processes, since a die is reshaped. With this process the semi-product is put into a hard bending shape by a moving feeding unit. The process is very accurate and at the same time cost-effective, since almost any bending contours can be produced form-based in one working stage. Furthermore, the die geometry enables the manufacture of contours that were previously very work-intensive or not producible at all. © 2020, The Author(s).
    view abstract10.1007/s11740-020-00952-1
  • Analytical process design for interference-fit joining of rectangular profiles
    Weber, F. and Müller, M. and Haupt, P. and Gies, S. and Hahn, M. and Tekkaya, A.E.
    Journal of Materials Processing Technology 276 (2020)
    The classical joining of round tubes by die-less hydroforming is extended to joining of rectangular profiles. A new analytical approach for the calculation of required process parameters is developed. It allows the calculation of the interference pressure p and the required fluid pressure range given by a lower and upper limit (pi,min and pi,max). These limits are the result of the deformation state of the inner and outer joining partner. The minimum pressure, where plastic deformation of the inner joining partner occurs first, characterizes the value at which the joint formation starts. The maximum pressure is the value, at which plastic deformation of the outer joining partner occurs. The analytical model is validated by experimental studies where two rectangular aluminum profiles (EN AW-6060) with different heat treatment conditions were joined. Above pi,max, the increase of the achievable joint strength with respect to the internal pressure is not significant anymore. The maximum deviation between the analytically and experimentally determined maximum fluid pressure pi,max is about 12% in case of a wall thickness of 3 mm and 11% in case of a wall thickness of 5 mm. © 2019 Elsevier B.V.
    view abstract10.1016/j.jmatprotec.2019.116391
  • Characterization of damage in forward rod extruded parts
    Hering, O. and Dunlap, A. and Tekkaya, A.E. and Aretz, A. and Schwedt, A.
    International Journal of Material Forming 13 (2020)
    In addition to strain hardening and residual stress, damage influences the product performance of forward rod extruded parts. Damage is usually neglected and difficult to quantify. The evolution of ductile damage in metal forming is closely correlated to the load path. An experimental approach using automated energy dispersive X-ray spectroscopy (EDX) particle analysis in scanning electron microscopy (SEM) is used to successfully quantify the void area fraction and obtain information on ductile damage. The method is performed on forward rod extruded 16MnCrS5 workpieces with varying extrusion strains and shoulder opening angles (and thus varying underlying load paths). The quantified damage is directly correlated to the load path, which can be described by the stress triaxiality evolution during forming. Density measurements were performed to further validate the results. By observing the change of strain-weighted stress triaxiality and maximum stress triaxiality, it is shown, that the maximum stress triaxiality is the decisive parameter enabling void nucleation. © 2019, Springer-Verlag France SAS, part of Springer Nature.
    view abstract10.1007/s12289-019-01525-z
  • Damage in metal forming
    Tekkaya, A.E., (1) and Bouchard, P.-O. and Bruschi, S., (1) and Tasan, C.C.
    CIRP Annals 69 (2020)
    Physical mechanisms of ductile damage in metal forming, experimental characterization methods for damage, and models predicting the damage level in formed components are reviewed. Applications of damage analysis in metal forming processes reveal that damage in metal formed parts is not failure, but a product property that accumulates between processes. Various metal forming process designs demonstrate that damage in formed products can be reduced and their performance can be increased. Static and fatigue strength, impact toughness, stiffness, and formability are typical examples of performance indicators that can be improved by damage-based process design. Potential scientific and technological challenges are addressed to realize damage-controlled metal forming processes. © 2020 CIRP
    view abstract10.1016/j.cirp.2020.05.005
  • Damage-induced performance variations of cold forged parts
    Hering, O. and Tekkaya, A.E.
    Journal of Materials Processing Technology 279 (2020)
    Forming processes play a key role in the manufacturing of metal components. They allow for the economical production of geometrical shapes with reproducibly high quality. Strain hardening and residual stresses affect the performance of the produced parts. These factors are controllable and can even be utilized to increase the performance of the component. This, however, does not apply to damage. Damage in metals describes the decrease of the load-bearing capacity due to the appearance and evolution of voids. The aim is to analyse, predict, and control the evolution of damage in cold forging, to allow for a production of cold forged components with a defined, load-adapted performance. It was investigated numerically to what extent the load path, which is responsible for the damage evolution, is affected in cold forging. Subsequently, the effect of load path changes on the product performance was determined experimentally in the region of the central axis where the load path is affected most by the extrusion parameters. Hereby, a correlation between the occurring triaxiality during forming and the product performance by means of number of cycles to failure in multi-step fatigue tests, impact energy and Young's modulus was observed. © 2019 Elsevier B.V.
    view abstract10.1016/j.jmatprotec.2019.116556
  • Development of a Remote Compression Test Lab for Engineering Education
    Selvaggio, A. and Upadhya, S. and Grodotzki, J. and Erman Tekkaya, A.
    Lecture Notes in Networks and Systems 80 (2020)
    A fully automated compression test for material characterization in forming technology is presented. This formability test is performed in a tele-operative testing cell consisting of a universal testing machine Zwick Roell Z250, an industrial robot KUKA KR 30-3, and other necessary components for the automation and execution of experiments. First, a methodology is introduced explaining how the remote compression test is realized. Afterwards, the integration of the compression test into the existing tele-operative testing cell is presented. The practical application of theoretical concepts is realized through the analysis of the final results of the experimental data. © Springer Nature Switzerland AG 2020.
    view abstract10.1007/978-3-030-23162-0_45
  • Development of a Remote Tube Bending Lab to Illustrate Springback and Determine Process Limits
    Upadhya, S. and Selvaggio, A. and Ortelt, T.R. and Grodotzki, J. and Tekkaya, E.A.
    Lecture Notes in Networks and Systems 80 (2020)
    The benefits of remote labs in overcoming the time and accessibility constraints of conventional labs is well known. To complement the already existing labs at the IUL, a remote tube bending lab incorporating an industry-relevant process was developed to aid in the understanding of important process limits and features of profile bending. The remote lab was developed in a bottom-to-top approach where the underlying tube bending lab was conceptualized and developed keeping in mind the best way to educate the students. Based on a problem based learning approach, the lab gives the students the freedom to explore and discover the field of tube bending on their own. The state of the art bending machine, with open platform communication (OPC) capabilities, coupled with a dedicated specimen handling robot, allows for easy automation of the lab while the use of augmented reality and web cameras facilitate easy visualization of the process and the product. © Springer Nature Switzerland AG 2020.
    view abstract10.1007/978-3-030-23162-0_10
  • Distortion and Dilution Behavior for Laser Metal Deposition onto Thin Sheet Metals
    Tebaay, L.M. and Hahn, M. and Tekkaya, A.E.
    International Journal of Precision Engineering and Manufacturing - Green Technology 7 (2020)
    The combination of additive manufacturing and incremental sheet forming offers great flexibility in the manufacture of function-integrated parts. In this study, both processes were carried out by the same CNC machine. This offers the possibility to manufacture large-scale lightweight parts with smaller additive parts on it in one machine and clamping device. Additionally, the process combination can lead to a reduced energy and material consumption for small batch sizes. DC01 sheets are used as a substrate with two different initial conditions. The first condition is as delivered steel sheet and the second is an incrementally formed with a thickness of 0.5 mm. The additive manufacturing was conducted by laser metal deposition (LMD). The powder material is a stainless steel 316 L. A segmentation of the cladding surface was applied and the path strategy of the laser movement was varied simultaneously to analyse the warpage of the thin substrate. It is shown that there is a dependency between the build-up strategies and the melt pool temperature, the thermal distortion, the dilution and the size of the cladding area. A segmentation of the working surface causes a lower melt pool temperature and thermal distortion. The lower melt pool temperature also generates a reduced dilution zone. © 2020, Korean Society for Precision Engineering.
    view abstract10.1007/s40684-020-00203-9
  • Influence of anisotropic damage evolution on cold forging
    Langenfeld, K. and Schowtjak, A. and Schulte, R. and Hering, O. and Möhring, K. and Clausmeyer, T. and Ostwald, R. and Walther, F. and Tekkaya, A.E. and Mosler, J.
    Production Engineering 14 (2020)
    This contribution deals with the influence of anisotropic material degradation (damage) within numerical simulations of cold forging. For that purpose, two constitutive frameworks for modeling ductile damage are presented: an isotropic and an anisotropic model. In a first step, both models are calibrated based on a uniaxial tensile test. Then, the forward rod extrusion process is simulated with the isotropic model. The deformation of a characteristic element is transferred to the anisotropic model and the local response is investigated. Both models are compared to one another in terms of the process induced ductile damage. It will be shown, that the magnitude of the induced damage agrees reasonably well, but that the orientation of ductile damage is of major importance. © 2020, German Academic Society for Production Engineering (WGP).
    view abstract10.1007/s11740-019-00942-y
  • Interaction of process parameters, forming mechanisms, and residual stresses in single point incremental forming
    Maaß, F. and Hahn, M. and Tekkaya, A.E.
    Metals 10 (2020)
    The residual stress state of a sheet metal component manufactured by metal forming has a significant influence on the mechanical properties, and thus determines the time until the component fails, especially for dynamic loads. The origin of the resulting residual stress state of incrementally formed parts with regard to the forming mechanisms of shearing, bending, and the normal stress component is still under investigation. The relationship between the process parameters, the forming mechanisms, and the resulting residual stress state for a complex part geometry manufactured by single point incremental forming (SPIF) is presented in this publication. For this purpose, a validated numerical process model is used to analyze the influence of the step-down increment ∆z for truncated cones on the characteristics of the forming mechanisms and the resulting residual stress state. For the first time the forming mechanisms are evaluated numerically on both sides of the formed component. A relationship between the process parameters, forming mechanisms, residual stresses, and the mechanical properties of an incrementally formed component is shown. Shearing-induced hardening is identified as a relevant influence on the residual stress state of cones. © 2020 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstract10.3390/met10050656
  • Interface formation during collisionwelding of aluminum
    Niessen, B. and Schumacher, E. and Lueg-Althoff, J. and Bellmann, J. and Böhme, M. and Böhm, S. and Tekkaya, A.E. and Beyer, E. and Leyens, C. and Wagner, M.F.X. and Groche, P.
    Metals 10 (2020)
    Collision welding is a high-speed joining technology based on the plastic deformation of at least one of the joining partners. During the process, several phenomena like the formation of a so-called jet and a cloud of particles occur and enable bond formation. However, the interaction of these phenomena and how they are influenced by the amount of kinetic energy is still unclear. In this paper, the results of three series of experiments with two different setups to determine the influence of the process parameters on the fundamental phenomena and relevant mechanisms of bond formation are presented. The welding processes are monitored by different methods, like high-speed imaging, photonic Doppler velocimetry and light emission measurements. The weld interfaces are analyzed by ultrasonic investigations, metallographic analyses by optical and scanning electron microscopy, and characterized by tensile shear tests. The results provide detailed information on the influence of the different process parameters on the classical welding window and allow a prediction of the different bond mechanisms. They show that during a single magnetic pulse welding process aluminum both fusion-like and solid-state welding can occur. Furthermore, the findings allow predicting the formation of the weld interface with respect to location and shape as well as its mechanical strength. © 2020 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstract10.3390/met10091202
  • Joining dissimilar thin-walled tubes by Magnetic Pulse Welding
    Lueg-Althoff, J. and Bellmann, J. and Hahn, M. and Schulze, S. and Gies, S. and Tekkaya, A.E. and Beyer, E.
    Journal of Materials Processing Technology 279 (2020)
    Welding dissimilar metal tubes attracts interest for a wide range of automotive, aeronautical, and plant engineering applications as well as other consumables. Hybrid driveshafts or structural elements can meet mechanical requirements at a reduced weight. However, joining materials with strongly different thermo-physical properties is a challenge for conventional fusion welding processes. In Magnetic Pulse Welding (MPW), the weld formation is based on the high-velocity collision between the joining partners, without additional heat input. This allows for the fabrication of sound “cold” welds. MPW of tubular parts is usually realized by the radial electromagnetic compression of the outer “flyer” part and the subsequent impact on the inner “parent” part. This impact represents a harsh loading for the parent, which therefore is usually designed as a thick-walled or solid part to avoid damage or unwanted deformations. To further increase the lightweight potential, the objective of the present manuscript is the comprehensive analysis of MPW with thin-walled parent parts. Experimental and analytical investigations are presented, which enable to reduce the parent thickness without affecting the joint strength. The approaches comprise the observation of the impact and deformation behavior by inline laser-based measurement technology as well as the development of adequate, re-usable mandrels to support the parent parts. The focus is on aluminum flyer parts, which are welded to steel and copper parent parts. Critical values for the parent wall thickness are deduced and recommendations for the process design of MPW with thin-walled tubes are given. © 2019 Elsevier B.V.
    view abstract10.1016/j.jmatprotec.2019.116562
  • Lightweight in Automotive Components by Forming Technology
    Rosenthal, S. and Maaß, F. and Kamaliev, M. and Hahn, M. and Gies, S. and Tekkaya, A.E.
    Automotive Innovation 3 (2020)
    Lightweight design is one of the current key drivers to reduce the energy consumption of vehicles. Design methodologies for lightweight components, strategies utilizing materials with favorable specific properties and hybrid materials are used to increase the performance of parts for automotive applications. In this paper, various forming processes to produce light parts are described. Material lightweight design is discussed, covering the manufacturing processes to produce hybrid components like fiber–metal, polymer–metal and metal–metal composites, which can be used in subsequent deep drawing or combined forming processes. Approaches to increasing the specific strength and stiffness with thermomechanical forming processes as well as the in situ control of the microstructure of such components are presented. Structure lightweight design discusses possibilities to plastically form high-strength or high-performance materials like magnesium or titanium in sheet, profile and tube forming operations. To join those materials and/or dissimilar materials, new joining by forming technologies are shown. To economically produce lightweight parts with gears or functional elements, incremental sheet-bulk metal forming is presented. As an important part property, the damage evolution during the forming operations will be discussed to enable even lighter parts through a more reliable design. New methods for predicting and tailoring the mechanical properties like strength and residual stresses will be shown. The possibilities of system lightweight design with forming technologies are presented. A combination of additive manufacturing and forming to produce highly complex parts with integrated functions will be shown. The integration of functions by a hot extrusion process for the manufacturing of shape memory alloys is presented. An in-depth understanding of the newly developed processes, methodologies and effects allows for a more accurate dimensioning of components. This facilitates a reduction in the total mass and an increasing performance of vehicle components. © 2020, The Author(s).
    view abstract10.1007/s42154-020-00103-3
  • Micro-magnetic damage characterization of bent and cold forged parts
    Samfaß, L. and Baak, N. and Meya, R. and Hering, O. and Tekkaya, A.E. and Walther, F.
    Production Engineering 14 (2020)
    Damage can have a strong impact on the fatigue performance of bulk formed parts for example produced by cold forging and sheet metal formed parts for example produced by bending. One suitable method to detect damage non-destructively in a time-efficient way is the micro-magnetic material characterization. In this paper, the suitability of harmonic analysis of the tangential magnetic field strength for the detection of damage in bent DP800-parts and cold forged 16MnCrS5-parts is discussed. For differently formed parts a correlation between the magnitude of damage and the behavior of the upper harmonics parameters is shown. © 2019, German Academic Society for Production Engineering (WGP).
    view abstract10.1007/s11740-019-00934-y
  • On mesh dependencies in finite-element-based damage prediction: application to sheet metal bending
    Sprave, L. and Schowtjak, A. and Meya, R. and Clausmeyer, T. and Tekkaya, A.E. and Menzel, A.
    Production Engineering 14 (2020)
    The properties of a local and a regularised gradient-enhanced continuum damage model are highlighted and both types of models are applied to the simulation of an air bending process. Constitutive relations are summarised for both Lemaitre-type models and a brief description of their implementation into Abaqus user material subroutines is given. With (several) material parameters obtained from a basic parameter identification process, an air bending experiment is simulated with different mesh densities. By means of the damage evolution as well as the distribution of representative damage and hardening variables, the mesh dependence of the local model in contrast to the mesh independence of the gradient-enhanced model is analysed for two air bending processes with different die width. © 2019, German Academic Society for Production Engineering (WGP).
    view abstract10.1007/s11740-019-00937-9
  • Particle ejection by jetting and related effects in impact welding processes
    Bellmann, J. and Lueg-Althoff, J. and Niessen, B. and Böhme, M. and Schumacher, E. and Beyer, E. and Leyens, C. and Tekkaya, A.E. and Groche, P. and Wagner, M.F.-X. and Böhm, S.
    Metals 10 (2020)
    Collision welding processes are accompanied by the ejection of a metal jet, a cloud of particles (CoP), or both phenomena, respectively. The purpose of this study is to investigate the formation, the characteristics as well as the influence of the CoP on weld formation. Impact welding experiments on three different setups in normal ambient atmosphere and under vacuum-like conditions are performed and monitored using a high-speed camera, accompanied by long-term exposures, recordings of the emission spectrum, and an evaluation of the CoP interaction with witness pins made of different materials. It was found that the CoP formed during the collision of the joining partners is compressed by the closing joining gap and particularly at small collision angles it can reach temperatures sufficient to melt the surfaces to be joined. This effect was proved using a tracer material that is detectable on the witness pins after welding. The formation of the CoP is reduced with increasing yield strength of the material and the escape of the CoP is hindered with increasing surface roughness. Both effects make welding with low-impact velocities difficult, whereas weld formation is facilitated using smooth surfaces and a reduced ambient pressure under vacuum-like conditions. Furthermore, the absence of surrounding air eases the process observation since exothermic oxidation reactions and shock compression of the gas are avoided. This also enables an estimation of the temperature in the joining gap, which was found to be more than 5600 K under normal ambient pressure. © 2020 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstract10.3390/met10081108
  • Potential and status of damage controlled forming processes
    Hirt, G. and Tekkaya, A.E. and Clausmeyer, T. and Lohmar, J.
    Production Engineering 14 (2020)
    In modern process design of metallic components, the influence of the metal forming process on the component properties can be taken into account. However, damage occurring concurrent to forming cannot be accounted for yet. Due to the complex multi-scale, multi-mechanism nature of damage, it is very challenging to predict its evolution through any metal forming process chain. In order to enable such damage controlled forming processes in the future three research questions need to be addressed in detail: How are the mechanisms governing the damage initiation and evolution in metals best characterized? How can the damage mechanisms be described and the damage evolution be predicted using models? How do forming processes influence the damage evolution? Answering these questions and considering damage during process design will in the long term lead to improved lightweight components that do not require conventional safety factors, as their performance is already fully known. © 2020, The Author(s).
    view abstract10.1007/s11740-019-00948-6
  • Prediction and analysis of damage evolution during caliber rolling and subsequent cold forward extrusion
    Schowtjak, A. and Wang, S. and Hering, O. and Clausmeyer, T. and Lohmar, J. and Schulte, R. and Ostwald, R. and Hirt, G. and Tekkaya, A.E.
    Production Engineering 14 (2020)
    Damage in the sense of voids influences material as well as product properties and thus is important for the performance of formed components. In this work, the influence of caliber rolling prior to a cold forward extrusion process in terms of damage evolution is investigated. To this end, an uncoupled Lemaitre-type damage model considering the effects of strain-rate and temperature dependent plasticity is employed. The damage-related parameters are identified in an inverse manner based on notched tensile tests. Numerical investigations of the forming processes show that damage increases throughout the process chain. The simulations are validated in terms of density measurements and microscopic investigations. The experimental measurements and numerical simulations are in good qualitative agreement. It is shown that the influence of rolling on subsequent cold forward extrusion is negligible for this experimental setup. For other process parameters, however, the influence could be significant. © 2019, German Academic Society for Production Engineering (WGP).
    view abstract10.1007/s11740-019-00935-x
  • Special Issue on Automotive Lightweight
    Tekkaya, A.E. and Min, J.
    Automotive Innovation 3 (2020)
    view abstract10.1007/s42154-020-00117-x
  • Strain hardening under large deformation for AA5182
    Zhang, C. and Wang, Y. and Shang, H. and Wu, P. and Fu, L. and Lou, Y. and Clausmeyer, T. and Tekkaya, A.E. and Zhang, Q.
    IOP Conference Series: Materials Science and Engineering 967 (2020)
    In this study, an aluminium alloy of AA5182 is taken as the research object to study strain hardening under large plastic deformation. Tensile tests are done for four specimens, including dog-bone specimens, notched specimens, specimens with a central hole and in-plane shear specimens. Bulging tests are also conducted to measure strain hardening under balanced biaxial tension. In addition, an experimental method called in-plane torsion test is also used for shear loading. At least three experiments are completed for each type of specimens along the rolling direction (RD), diagonal direction (TD), and transverse direction (DD). The stroke of each tests is measured by a digital image correlation (DIC) system, and the load-stoke curves were obtained for the tests. Combined with an inverse engineering method, the strain hardening properties are calibrated for the alloy under different loading conditions of shear, uniaxial tension, plane strain tension, and balanced biaxial tension. The strain hardening under various loading conditions is compared and modelled by various yield functions to evaluate their performance. It is concluded that inverse engineering approach is a simple but powerful method to obtain the stress-strain curve up to large plastic deformation. It is also observed that it needs to develop yield functions to model yielding behaviour under complex loading conditions. © 2020 Published under licence by IOP Publishing Ltd.
    view abstract10.1088/1757-899X/967/1/012030
  • Strain path dependency in incremental sheet-bulk metal forming
    Wernicke, S. and Hahn, M. and Gerstein, G. and Nürnberger, F. and Tekkaya, A.E.
    International Journal of Material Forming (2020)
    Incremental sheet-bulk metal forming (iSBMF) enables the manufacture of functional lightweight components featuring a load-adapted shape with a high material efficiency. The flexibility of the incremental forming process allows for the modification of the strain path through the adjustment of the tool motion while maintaining the final product geometry. These modifications generate both a different strain hardening and damage evolution. In this paper, a numerical and experimental investigation of the different strain paths is carried out to identify their impact on the resulting load capacity of gears. In experiments on the quasistatic load capacity of the gears it is validated that forming of gears with a strain path showing a reduced damage potential leads to a 50% higher load capacity compared to the most unfavorable strain path. Moreover, all investigated load paths present load changes that have to be taken into account in numerical modeling of iSBMF processes. Therefore, a new approach for a material characterization under multiple load changes and high effective plastic strain is tested. Compared to numerical modeling with a characterized monotonically flow curve, this approach decreases the deviation force prediction by around 80% without increasing the calculation time. © 2020, Springer-Verlag France SAS, part of Springer Nature.
    view abstract10.1007/s12289-020-01537-0
  • Analysis of the influence of fibers on the formability of metal blanks in manufacturing processes for fiber metal laminates
    Mennecart, T. and Gies, S. and Khalifa, N.B. and Tekkaya, A.E.
    Journal of Manufacturing and Materials Processing 3 (2019)
    In the one-step manufacturing process for fiber metal laminate parts, the so-called in situ hybridization process, the fabrics are interacting with metal blanks. During deep drawing, the liquid matrix is injected between the metal sheets through the woven fiber layers. The metal blanks can be in contact with dry or with infiltrated fibers. The formability of the blanks is influenced by the variation of the starting time of injection. The reason for that is that, due to high contact forces, the fibers are able to deform the metal surface locally, so that movement and the strain of the blanks is inhibited. To investigate the influence of different fibers on the formability of metals, Nakazima tests are performed. In these tests, two metal blanks are formed with an interlayer of fibers. The results are compared with the formability of two blanks without any interlayer. It is shown that in with fibers between sheets, the formability decreases compared to the formability of two metal blanks without interlayers. Based on a simplified numerical model for different types of fibers, the interactions of the fibers with the metal blank are analyzed. It could be shown that the friction due to contact has more influence than the friction due to the form fit caused by the imprints. © 2019 by the authors.
    view abstract10.3390/jmmp3010002
  • Analytical and experimental bond strength investigation of cold forged composite shafts
    Ossenkemper, S. and Dahnke, C. and Tekkaya, A.E.
    Journal of Materials Processing Technology 264 (2019)
    Composite cold forging denotes the simultaneous processing of hybrid raw parts by cold forging operations. The objective is the manufacturing of composite components by means of joining by plastic deformation. For lightweight purposes, composite shafts were produced by forward rod extrusion of backward extruded steel cups, into which an aluminum core has been inserted. The final composite shafts possess a wear-resistant outer steel sleeve and a light aluminum core. An analytical model has been developed to predict the strength of the force fit of composite shafts produced by cold forging. Push-out tests were conducted in order to experimentally determine the bond strength for the validation of the model. The analytically estimated bond strengths are in good accordance with the experimental values determined by push-out tests. The bond strength can be increased significantly by structuring the inner surface of the cups for the hybrid raw part to cause a form fit between steel sleeve and aluminum core after forward rod extrusion. Metallurgical bonds are not established between steel and aluminum in the investigated composite cold forged shafts. © 2018 Elsevier B.V.
    view abstract10.1016/j.jmatprotec.2018.09.008
  • Analytical prediction of wall thickness reduction and forming forces during the radial indentation process in Incremental Profile Forming
    Grzancic, G. and Löbbe, C. and Ben Khalifa, N. and Tekkaya, A.E.
    Journal of Materials Processing Technology 267 (2019)
    Incremental Profile Forming (IPF) is a recently introduced flexible tube forming technology, which allows the manufacture of tubular structures with varying cross-sectional geometries along the longitudinal axis of the part. The process is characterized mainly by the operation of several tools, laterally moving, indenting and deforming the initial tubular workpiece. In kinematic IPF the use of universal tools with hemispheric tool shapes allow the flexible manufacture of highly complex parts since its geometry is mainly defined by the tool motions. Thinning of the tube material in the tool contact region is typical for kinematic IPF forming processes. In order to predict the forming behavior, an analytical model is developed taking the tube dimensions, the tool geometry as well as the tube material into account. Based on the predicted forming behavior, the process force during the indentation process is also determined analytically. The validation of the analytical model is performed by experimental and numerical investigations. After the geometrical analysis of the tool contact region and the tube deformations, the plastic strain distribution in the forming zone is described, in order to predict the reduction of the wall thickness. Furthermore, the analytical model allows the prediction of the forming force course over the indenting depth for various process parameters. © 2018 Elsevier B.V.
    view abstract10.1016/j.jmatprotec.2018.12.003
  • Bending moment in incremental tube forming
    Nazari, E. and Staupendahl, D. and Löbbe, C. and Tekkaya, A.E.
    International Journal of Material Forming 12 (2019)
    Incremental tube forming is a combination of free-form bending and spinning, which enables the direct production of curved tailored tubes under greatly reduced forming forces. To predict the effect of radial and circumferential superposed stresses, generated by the spinning rolls, on the bending moment, an analytical model is proposed. The model takes into account the isotropic hardening behaviour of the material as well as the amount of diameter and thickness reduction, simultaneously. The analytical model is validated by experimental studies with various spinning and bending parameters. The bending moment from the analytical model is used to calculate the springback. The calculated springback ratio is in agreement with experiments and shows a deviation of only 5% for the studied material. © 2018, Springer-Verlag France SAS, part of Springer Nature.
    view abstract10.1007/s12289-018-1411-x
  • Developments in composite extrusion of complex profiles for automotive applications
    Schulze, A. and Dahnke, C. and Tekkaya, A.E.
    Materials Today: Proceedings 10 (2019)
    In recent years, the composite extrusion process has been developed to manufacture aluminum profiles with embedded high-strength steel reinforcements. Based on the fundamental understanding, the process is extended to manufacture reinforced profiles for automotive applications. The manufacturing of reinforced composite profiles under industrial conditions causes new challenges regarding the die design. In comparison to existing die designs for the academic environment, the new concept has to consider lower wall thicknesses and higher extrusion speeds. To overcome the mentioned challenges, FEM simulations are carried out to optimize the tool design and ensure a reliable economical production of the profiles. © 2018 Elsevier Ltd.
    view abstract10.1016/j.matpr.2018.10.399
  • Effect of plastic strain and ductile damage on elastic modulus of multiphase steel and its impact on springback prediction
    Münstermann, S. and Sparrer, Y. and Yao, Y. and Lian, J. and Meya, R. and Clausmeyer, T. and Tekkaya, A.E.
    AIP Conference Proceedings 2113 (2019)
    The springback behavior of cold formable steel of grade DP 1000 is assessed experimentally and numerically. Bending tests according to the VDA test specification are interrupted at three characteristic roller displacements, and the unloading characteristics are investigated. The tests are simulated with four different material models: i.) elastic plastic simulation with constant elastic modulus, ii.) elastic plastic simulation with plastic strain-dependent elastic modulus, iii.) damage mechanics simulation with constant elastic modulus, iv.) damage mechanics simulation with plastic strain-dependent elastic modulus. To provide the required input data for these simulations, the effect of plastic strain on the elastic modulus is studied based on uniaxial tensile tests, whereas the possible effect of ductile damage evolution on springback properties is numerically captured by the modified Bai-Wierzbicki model. The studies reveal that consideration of plastic strain effects on the elastic modulus adds a significant amount of accuracy to the numerical simulations, whereas the consideration of ductile damage does not improve the simulation results as much. This observation has to be related to the fact that steel DP 1000 is characterized by late damage initiation with rapid subsequent damage accumulation. © 2019 Author(s).
    view abstract10.1063/1.5112739
  • Effect of the unbending process on mechanical properties before and after flattening of extruded open tubes of magnesium alloy ME20
    Joghan, H.D. and Izadyar, S.A. and Hahn, M. and Tekkaya, A.E.
    AIP Conference Proceedings 2113 (2019)
    Magnesium alloys are promising candidates for the replacement of steel and aluminium in the transportation industry. Extrusion of open tube profiles and flattening the tubes afterward is one possible method for manufacturing wider magnesium alloy sheets. In this paper, warm-extruded magnesium alloy ME20 sheets are analyzed78 experimentally. In this context, extruded profiles are expanded and finally flattened in a press machine. In order to investigate the influence of the unbending and flattening on the properties of the magnesium sheets, tensile tests at room temperature as well as at elevated temperatures are performed for both the open tube profile and flattened sheet. The microstructure of both states is also investigated. According to the results of the tensile tests, yield stress and ultimate strength are higher in the flattened sheet. The strain-rate sensitivity study for both states shows that the tubes indicate a higher strain-rate sensitivity with rising temperature. The fracture elongation increases differently for both states with increasing temperature. The intersection point of the hardening rate and true stress-strain curve, which hint the necking point, indicates that the sheet shows a higher formability before flattening. The microstructure investigation shows that the flattening process reduces the grain size heterogeneously. In case of the sheet, a higher strain hardening rate dominates the grain size refinement effect on formability, while the reduced grain size leads to higher work hardening. © 2019 Author(s).
    view abstract10.1063/1.5112707
  • Effect of the wall thickness on the forming behavior and welding result during magnetic pulse welding [Einfluss der Wandstärke auf das Umformverhalten und das Schweißergebnis beim Magnetpulsschweißen]
    Bellmann, J. and Lueg-Althoff, J. and Schulze, S. and Hahn, M. and Gies, S. and Beyer, E. and Tekkaya, A.E.
    Materialwissenschaft und Werkstofftechnik 50 (2019)
    Magnetic pulse welding is a promising technology for the joining of dissimilar metals. Since the input of thermal energy is significantly reduced compared to conventional fusion welding technologies, critical intermetallic phases can largely be avoided. Therefore, proper collision conditions are necessary. Those require a careful adjustment of the energetic and geometric parameters at the impact welding setup. The thickness of the accelerated joining partner (flyer) determines the necessary energy input for a successful weld. However, at the same time, it has an effect on the weld formation. This study utilizes a novel optical measurement system to explain these findings and to gain insights into the forming behavior of the flyer parts. It is shown that the collision angle depends on the flyer tube thickness and, thus, directly has an effect on the welding result. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/mawe.201900024
  • Experimental and numerical investigations of joining by electromagnetic forming for aeronautical applications
    Lueg-Althoff, J. and Beu, M.-A. and Güzel, A. and Rohr, T. and Hahn, M. and Tekkaya, A.E.
    AIP Conference Proceedings 2113 (2019)
    Due to the demanding requirements on cost, weight, and safety for helicopters, a continuous improvement of the manufacturing processes is necessary. Increasing interest in unmanned aircrafts as well as developments in the direction of hybrid-electric propulsion systems are some of the other drivers, which encourage the aircraft industry to permanently look for alternative manufacturing processes for lightweight components to improve cost and weight. The present study investigates the electromagnetic crimping process as an alternative to thermally welded joints with different form-fit elements like grooves, pockets, and knurlings. The electromagnetic joining technology is based on pulsed magnetic fields to shape components made of electrically conductive materials and it is able to manufacture form-fit or welded joints. First, the analytical methods are presented to design a lightweight helicopter's cyclic stick by the electromagnetic joining process based on maximum applied pilot control forces. Furthermore, approaches to calculate the maximum axial and torsional load transfer between the joining partners are given. The results are used in a two dimensional finite-element simulation to determine the process parameters and to optimize the groove design with respect to shear stresses. A good agreement between the numerical results and the experimental investigations is shown. The pull-out force is set as the failure criterion of the connection and the specific joint strength of different groove shapes is compared with the analytical model. Due to the slight increase of the total weight at the presented weight analysis, proposals for design optimization with focus on the joining zone are made. Despite this fact, the cost analysis shows a reduction of production costs. The achieved main goal of the presented study is the proof of feasibility of substituting thermally welded connections with electromagnetically crimped joints made of lightweight components and the proof of the remarkable potential of reducing production costs and time of aeronautic components. © 2019 Author(s).
    view abstract10.1063/1.5112576
  • Experimental study on the magnetic pulse welding process of large aluminum tubes on steel rods
    Bellmann, J. and Schettler, S. and Dittrich, S. and Lueg-Althoff, J. and Schulze, S. and Hahn, M. and Beyer, E. and Tekkaya, A.E.
    IOP Conference Series: Materials Science and Engineering 480 (2019)
    Solid state welding technologies enable dissimilar metal welding without critical intermetallic phase formation. Magnetic Pulse Welding (MPW) is a promising joining method for hybrid sheet connections in car body production or for manufacturing of dissimilar tube connections. Given a suitable MPW process design, the shear testing of MPW joints usually leads to failure in the weaker base material. This finding emphasizes the high strength level of the joining zone itself. Consequently, the transmission of higher forces or torques, respectively, requires stronger materials or adapted geometries. In the present experimental study, the diameter of an exemplary driveshaft was doubled to 80 mm at constant tube wall thickness to increase the load bearing capability. The characteristic impact flash was recorded at different positions around the tube's circumference and it was used to adjust the most relevant process parameters, i.e. working length and acceleration gap, at the lower process boundary. In metallographic analysis, the final shapes of both joining partners were compared with the original driveshaft dummies on macroscopic and microscopic scale. The typical wavy interface between aluminum and steel was analyzed in detail. Doubling the tube diameter lead to four times higher torque levels of failure during quasistatic and cyclic torsion tests. © Published under licence by IOP Publishing Ltd.
    view abstract10.1088/1757-899X/480/1/012033
  • Flow curves up to high strains considering load reversal and damage
    Hering, O. and Kolpak, F. and Tekkaya, A.E.
    International Journal of Material Forming 12 (2019)
    The new idea is to produce specimens by forward rod extrusion, where in the core of the extrudate a deviatoric tension-loading is present, which is superposed by an adjustable hydrostatic pressure. Various damage levels are hence possible in the extrudate. Conducting tensile and upsetting tests with the pre-strained specimens both the influence of a load reversal as well as the material weakening through ductile damage on the resulting flow curve is explored. Not only can the results be utilized to identify flow curves of materials up to high strains (ε > 1.7), but also to get new insights into the plastic material behaviour, which can be used for generating or adapting new damage models as well as kinematic hardening models under cold forging conditions. The proposed method was first assessed by means of analytical and numerical methods and then validated experimentally, by the example of the typical cold forging steel 16MnCrS5. © 2018, The Author(s).
    view abstract10.1007/s12289-018-01466-z
  • Forming mechanisms-related residual stress development in single point incremental forming
    Maaß, F. and Hahn, M. and Tekkaya, A.E. and Dobecki, M. and Poeche, A. and Brömmelhoff, K. and Reimers, W.
    Production Engineering 13 (2019)
    The mechanical properties of a component are significantly influenced by the prevailing residual stress state. A deliberate induction of compressive residual stresses or a reduction of tensile residual stresses can improve the component properties, such as fatigue strength. Single point incremental forming is a flexible manufacturing process to produce complex shaped parts by the computerized numerically controlled movement of a hemispherical forming tool. Because the process parameters can be locally adjusted it is possible to influence the residual stress state of the component. The influence of the forming mechanisms bending, shearing and membrane stretching, as well as the role of the hydrostatic compression on the residual stress state is widely unknown. This work aims to fill this gap. Therefore, linear grooves are formed into AA5083 sheets in a single-stage incremental forming process. The residual stress state of the unclamped sheet is measured on both sides of the groove center by means of X-ray diffraction. The relative intensity of the dominant forming mechanism is adjusted by adapting the relevant process parameters step-down increment Δz and tool radius R Tool . The forming mechanisms are analyzed numerically by splitting the total plastic energy into the three forming mechanisms bending, shearing and membrane stretching. The numerical results for bending and membrane stretching could be validated by crystallographic analysis. A shift in the energy ratio of the forming mechanism from bending to shearing with increasing relative step-down increment Δz/R Tool could be observed numerically. The maximum residual stress amplitudes are found for Δz/R Tool  < 1. The results indicate that a deliberate residual stress state can be induced by adjusting the dominant forming mechanism of the process. © 2018, German Academic Society for Production Engineering (WGP).
    view abstract10.1007/s11740-018-0867-3
  • Forming properties of additively manufactured monolithic Hastelloy X sheets
    Rosenthal, S. and Platt, S. and Hölker-Jäger, R. and Gies, S. and Kleszczynski, S. and Tekkaya, A.E. and Witt, G.
    Materials Science and Engineering A 753 (2019)
    Additive manufacturing (AM) of semi-finished sheets for a subsequent forming operation has not been investigated yet. The potentials in resource efficiency and effective use of build-chamber-volumes, by combining laser powder bed fusion of metals and forming technology are demonstrated. The overarching aim of this process chain are time savings of up to 50% and the benefit of material strengthening by work hardening. The scope of this paper is the understanding and characterization of the flow behavior of additively manufactured semi-finished parts for the use in a subsequent forming application of the nickel-based superalloy, Hastelloy X. Characterization methods used in sheet metal forming are applied to monolithic additively manufactured tensile, compression and in-plane torsion specimens. The resulting characterization and yield criteria can be used to predict the forming behavior of additively manufactured semi finished parts with integrated functions like cooling channels that are formed in its final geometry. A correction function is introduced to consider the surface roughness in the stress-strain diagrams. The material shows a high anisotropic yield stress with a nearly isotropic hardening behavior in the as-build condition. The heat treatment reveals a homogenization of the material accompanied with an isotropic initial yield stress but anisotropic yield behavior. To numerically model those effects, different yield surfaces based on the preceding material characterization are discussed. It turns out, that the additively manufactured Hastelloy X shows high potential in terms of formability combined with high tensile strengths. © 2019 Elsevier B.V.
    view abstract10.1016/j.msea.2019.03.035
  • Global and high-resolution damage quantification in dual-phase steel bending samples with varying stress states
    Meya, R. and Kusche, C.F. and Löbbe, C. and Samman, T.A. and Kerzel, S.K. and Tekkaya, A.E.
    Metals 9 (2019)
    In a variety of modern, multi-phase steels, damage evolves during plastic deformation in the form of the nucleation, growth and coalescence of voids in the microstructure. These microscopic sites play a vital role in the evolution of the materials’ mechanical properties, and therefore the later performance of bent products, even without having yet led to macroscopic cracking. However, the characterization and quantification of these diminutive sites is complex and time-consuming, especially when areas large enough to be statistically relevant for a complete bent product are considered. Here, we propose two possible solutions to this problem: an advanced, SEM-based method for high-resolution, large-area imaging, and an integral approach for calculating the overall void volume fraction by means of density measurement. These are applied for two bending processes, conventional air bending and radial stress superposed bending (RSS bending), to investigate and compare the strain- and stress-state dependent void evolution. RSS bending reduces the stress triaxiality during forming, which is found to diminish the overall formation of damage sites and their growth by the complimentary characterization approaches of high-resolution SEM and global density measurements. © 2019 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstract10.3390/met9030319
  • Influence of SMA-induced stress on shape memory alloy metal matrix composites manufactured by continuous composite extrusion
    Dahnke, C. and Pottmeyer, F. and Pinter, P. and Weidenmann, K.A. and Tekkaya, A.E.
    Smart Materials and Structures 28 (2019)
    In shape memory alloy metal matrix composites manufactured by continuous composite extrusion the strategies of active property tuning and active strain energy tuning are used for the improvement of the mechanical properties of the components. Due to the thermal activation of the embedded NiTi wires (SM495), compressive stresses are transferred to the surrounding aluminum matrix (AA6060). At elevated temperatures tensile tests, three-point-bending tests and notch impact test show the influence of temperature, prestrain and reinforcing volume on the component performance. In tensile testing, a simultaneous increase of strength and ductility can be found, leading to an increase of the energy abortion capacity. Results of the three-point-bending test and notch impact test also show an increase of the required work depending on the thermomechanical treatment of the specimens. © 2019 IOP Publishing Ltd.
    view abstract10.1088/1361-665X/ab2d6a
  • Influence of the preheating strategy on the deep drawing of extruded magnesium alloy ME20 sheets
    Joghan, H.D. and Hahn, M. and Traphöner, H. and Tekkaya, A.E.
    IOP Conference Series: Materials Science and Engineering 651 (2019)
    Due to the hexagonal crystal structure of magnesium alloys, a high forming limit can only be achieved at elevated temperatures. For the material characterization of extruded magnesium alloy ME20 sheets, at elevated temperatures, the in-plane torsion test and a multi-layer upsetting test were conducted. Also, FLCs were determined at elevated temperatures. For the deep drawing, two different heating strategies are investigated. In the first method, specimens are placed in an oven at 400 °C for around 10 minutes and then rapidly transferred to the tool. In the second method the specimens are directly heated in the deep drawing tool. In both methods the specimens are painted with Bornitrid lubricant. The effect of the preheating on the coefficient of friction is investigated by using strip tensile tests. FEM simulations for the deep drawing comparing two different material models (Barlat 2000, CPB06) are executed. The results show that specimens heated in the tool show a better formability than oven-heated specimens. The numerical results present that there is no significant difference between Barlat 2000 and CPB06 in an isothermal deep drawing condition. The numerical results are in good agreement with the deep drawing experiments which also indicates that the warm FLCs allow for a good failure prediction. © Published under licence by IOP Publishing Ltd.
    view abstract10.1088/1757-899X/651/1/012067
  • Influence of tool path strategies on the residual stress development in single point incremental forming
    Maaß, F. and Hahn, M. and Dobecki, M. and Thannhäuser, E. and Erman Tekkaya, A. and Reimers, W.
    Procedia Manufacturing 29 (2019)
    The prevailing residual stress state in a formed metal component highly affects its mechanical properties. The single point incremental forming (SPIF) process allows a flexible adjustment of the process parameters during the forming process to influence the residual stress state. A selective induction of compressive residual stresses in the component can improve product properties, such as the fatigue strength. The effect of the dominant forming mechanisms on the residual stress state of the manufactured component is analyzed. Linear grooves are formed into aluminum alloy 5083 sheets and their resulting residual stress state is measured by means of X-ray diffraction (XRD). The grooves are manufactured in a multiple-stage process to ensure the transferability of the results to the incremental forming process of complex-shaped components. Different unidirectional and bidirectional tool path strategies were used to analyze the residual stress development. To set the dominant forming mechanism, the process parameter step-down increment Δz was adjusted, the other process parameters were kept constant. The dominant forming mechanisms were evaluated numerically. The results suggest that the influence of the tool path strategy on the resulting residual stress amplitude is unincisive. Distinctive residual stresses can be observed in bending-dominated regions. In case of shear domination, the resulting residual stress state is less pronounced. © 2019 The Authors. Published by Elsevier B.V.
    view abstract10.1016/j.promfg.2019.02.105
  • Investigation of evolving yield surfaces of dual-phase steels
    Hou, Y. and Min, J. and Guo, N. and Lin, J. and Carsley, J.E. and Stoughton, T.B. and Traphöner, H. and Clausmeyer, T. and Tekkaya, A.E.
    Journal of Materials Processing Technology (2019)
    The aim of this paper is to describe the evolving yield behavior of dual-phase steels during plastic deformation characterized for ten loading paths using a series of mechanical tests including uniaxial tension, uniaxial compression, in-plane torsion and cruciform biaxial tension with the aid of digital image correlation techniques for strain measurement. Large plastic strains in the gauge area of cruciform specimens tested were enabled by a laser deposition process to strengthen the arms in order to measure deformation behavior of the sheet without arbitrarily thinning the gauge section. Experimental yield loci were determined for three dual phase steels with different strength levels up to equivalent plastic strains of ˜0.11 for DP590, ˜0.07 for DP780, ˜0.05 for DP980, respectively. Several existing anisotropic yield criteria under both associated flow rule (AFR) and non-associated flow rule (non-AFR) were applied to describe the anisotropic yield behavior of these DP steels. A comparative study was preformed to validate prediction accuracy of yield criteria with experimental measurements including yield loci, yield stresses and rφ -values under uniaxial tension in seven orientations as well as yield stresses and rb -value under equi-biaxial tension. The results show that non-AFR significantly improved prediction accuracy of both stresses and r-values simultaneously. Under non-AFR, an order of two in the yield stress function is sufficient to accurately predict flow stresses. The evolution of both yield stress and plastic potential surfaces of DP steels were illustrated by changing parameters in the yield criterion as functions of equivalent compliance λ¯. © 2019 Elsevier B.V.
    view abstract10.1016/j.jmatprotec.2019.116314
  • Investigation of martensite-transformation and forming properties of additively reinforced 22MnB5 sheet metals
    Ünsal, I. and Hirtler, M. and Sviridov, A. and Bambach, M. and Hama-Saleh, R. and Weisheit, A. and Schleifenbaum, J.H. and Kamaliev, M. and Tekkaya, A.E.
    AIP Conference Proceedings 2113 (2019)
    Especially in crash relevant car body structures, local reinforcements of sheet metal parts can combine high crash performance and lightweight design. Patchwork blanks (PB) can be used to manufacture locally reinforced hot stampings. However, PB may entail disadvantages like corrosion in the gap between patch and base material or low formability of the joining zone and low shear strength at the spot welds. Additively manufactured local patches with metallurgical bonding to the substrate material are a promising alternative. The patches are generated by Laser Metal Deposition (LMD), where metal powder is melted together with the surface of the substrate to build a metallurgical bond. This new approach has already been tested with steel and aluminum alloys. For hot stamping of 22MnB5 steel, the hardenability of the additively manufactured material is of key importance. With quenching experiments, a similar hardening behavior of additively manufactured 22MnB5 compared to conventional sheet material can be shown. Based on this, a component with additive reinforcements was manufactured by hot stamping. For this purpose, rolled 22MnB5 sheets were locally reinforced by LMD and subsequently formed in a hot stamping process. The formability of locally reinforced sheets and the hardenability of the additively manufactured 22MnB5 patches are analyzed and compared with quenching experiments. Additionally, the hardenability and martensite/bainite transformation of additively manufactured 22MnB5 is investigated using dilatometer experiments and compared to conventional 22MnB5. © 2019 Author(s).
    view abstract10.1063/1.5112735
  • Light enough or go lighter?
    Hahn, M. and Gies, S. and Tekkaya, A.E.
    Materials and Design 163 (2019)
    A novel concept for evaluating the lightweight design of structural components, named the true lightweight degree, is developed. It is found that traditional lightweight parameters just constitute a lower bound for stiffness-oriented designs. This can be attributed to the low underlying design freedom. However, this is often not suitable anymore as today's manufacturing processes and materials typically allow for an increased design freedom. With a simplified analytical model, it is shown that a combination of a requirement-based equivalent strain, the specific Young's modulus, and the yield strength gives an upper bound. Numerical topology optimizations prove that this theoretical upper bound can serve as a good qualitative criterion for the mass-minimizing material choice. The investigations reveal that the right material choice can be quite sensitive to the degree of design freedom. For example, under lower-bound design constraints, an aluminum alloy having a yield strength of 280 MPa enables a slightly lighter component mass than a steel alloy with a yield strength of 800 MPa. In contrast, the steel alloy yields a considerably lower mass if full geometric design freedom is assumed. Yet, the concept derived within this work is only valid for elastically loaded, stiffness-oriented components made of isotropic material. © 2018 The Authors
    view abstract10.1016/j.matdes.2018.107545
  • Magnetic pulse welding of tubular parts
    Bellmann, J. and Lueg-Althoff, J. and Schulze, S. and Hahn, M. and Gies, S. and Beyer, E. and Tekkaya, A.E.
    AIP Conference Proceedings 2113 (2019)
    Magnetic Pulse Welding (MPW) enables to join metals with very low heat input, compared to conventional fusion welding technologies. Thus, dissimilar material combinations are producible without the formation of critical intermetallic phases. The process consists of two stages, an electromagnetically driven forming process and the controlled high-speed collision with the joining partner. Adjusting the time-depended magnetic field distribution along the part's surface is key to achieve proper collision conditions and to generate a sound solid state weld. MPW is preferably applied to join axisymmetric parts since the usual shape of a coil results in strong compressive forces on the electrically conductive tube when it is placed in the center of the working coil. Field shapers are often applied to intensify the magnetic field of the coil and to ensure a sufficient acceleration of the work piece. The position of the field shaper's slot is of special interest during process adjustment, since the magnetic field shows inhomogeneities that can lead to collision conditions outside of the "welding window". Furthermore, geometrical disturbances of the parts, such as variations in wall thickness, working length and joining standoff affect the forming behavior of the tube and correspondingly the collision behavior. In this paper, the influence of the mentioned disturbance variables on the welding result is studied both experimentally and numerically. Additionally, the impact flash spewing out of the joining gap is recorded and evaluated. The targeted manipulation of the MPW process highlights the ranges of certain disturbance values that must not be exceeded during production of a generic aluminum-steel assembly. © 2019 Author(s).
    view abstract10.1063/1.5112579
  • Manufacturing of reinforced profiles by means of combined continuous and discontinuous composite extrusion
    Dahnke, C. and Kolpak, F. and Kloppenborg, T. and Tekkaya, A.E.
    Materials Today: Proceedings 10 (2019)
    Discontinuous composite extrusion allows the partial embedding of cylindrical, spherical or rectangular reinforcing elements into an aluminum profile. However, continuous composite extrusion allows the embedding of wires or flat ribbons over the complete profile length. In recent investigations, a new die concept has been developed to combine the advantages of both processes and manufacture a semi-finished product for a subsequent hot forging process. The combination generates new challenges regarding the die design. For this reason, the material flow of the concept was analyzed by FEM. Subsequently to the manufacturing, the die was experimentally tested and the embedding quality was analyzed. © 2018 Elsevier Ltd.
    view abstract10.1016/j.matpr.2018.10.397
  • Novel roll stand for flexible profile bending
    Martschin, J. and Meya, R. and Löbbe, C. and Tekkaya, A.E.
    IOP Conference Series: Materials Science and Engineering 651 (2019)
    A new method and roll stand for the manufacturing of structural profiles with tailored dimensions and properties based on the combination of rolling and profile bending is developed. Through this merged process the outer contour of a semi-finished product is shaped by a set of driven flexible and rigid rollers. The profile inner side is supported by an adjustable mandrel, so that an arbitrary variation of the cross-section along the profile can be achieved. Additionally, a moveable guiding roll facilitates the bending of the product. Conventionally, the manufacturing of load-optimized bent profiles requires a sequence of operations, whereas the novel method enables the manufacturing of those components in one process step. This leads to a reduction in tooling cost and setup time. Hence, the production of lightweight structures in small batch sizes is facilitated. In this contribution, the new forming method and the design of the developed roll stand are presented alongside with an investigation of the effects of stress superposition on bending stresses during combined bending and rolling as well as the possible processes applications. © Published under licence by IOP Publishing Ltd.
    view abstract10.1088/1757-899X/651/1/012076
  • Optimization of the Linear Coil Winding Process by Combining New Actuator Principles on the Basis of Wire Forming Analysis
    Hofmann, J. and Komodromos, A. and Fleischer, J. and Tekkaya, A.E.
    2018 8th International Electric Drives Production Conference, EDPC 2018 - Proceedings (2019)
    Due to the electrification of the automotive drive train new challenges in production technology must be faced. One of the big challenges are the copper losses within the electric drive that can be reduced by an optimized layer structure of the winding on the coil. This paper is targeting an optimized linear coil winding process with a special focus on the first layer which is decisive for the quality of the following layers. Here, the forming influence of the wire during winding on the bobbin is examined in particular. Crucial parameters in this context are the change in diameter through bending and the development of the clearance between wire and coil bobbin including their main influencing parameters. Especially the wire guide represents a machine element which influences the clearance negatively. This paper focuses on deriving critical process points from a process model and deriving forming strategies for controlling the winding process. For the first time, two actuator principles are combined to compensate the fluctuations in wire tensile force during winding and also to minimize the influence of the wire guide by moving it according to a FE simulation. Therefore, firstly the state of the art is analyzed and characterized in order to derive systematically the selection of the actuators and the control strategy. This is done in the context of achieving a higher efficiency of the electric motor through a deeper understanding of the forming process. On the one hand the integration of a fluidic muscle is serving as a compensation of the free wire length between the wire guide and the coil bobbin for a normalization of the wire tensile force. On the other hand, a piezo actuator is preventing the pre-deformation of the wire by the wire guide for keeping the clearance at a low level. © 2018 IEEE.
    view abstract10.1109/EDPC.2018.8658264
  • Predicting weld-quality in direct hot extrusion of aluminium chips
    Kolpak, F. and Schulze, A. and Dahnke, C. and Tekkaya, A.E.
    Journal of Materials Processing Technology 274 (2019)
    Based on a physically motivated weld strength model by Cooper and Allwood a weld quality model for the prediction of the process success in direct hot extrusion of aluminium chips is presented. The results are validated against a variety of literature results as well as against the results of additional hot extrusion experiments utilizing flat-face dies for the production of round bars and porthole dies for the production of hollow profiles. A procedure is presented to determine the minimum required weld quality value necessary to provide a process success by the example of chips made of the aluminium alloy AA6060. The weld model can be used to save time- and cost-consuming experimental try-outs, which may make the process an alternative to conventional hot extrusion due to its clear environmental benefits in the production of complex profiles. © 2019 Elsevier B.V.
    view abstract10.1016/j.jmatprotec.2019.116294
  • Properties of components with incrementally formed gears
    Wernicke, S. and Sieczkarek, P. and Gies, S. and Tekkaya, A.E.
    Metals 9 (2019)
    The process class of sheet-bulk metal forming (SBMF) involves several advantages for the manufacture of functional components. Its incremental variant (iSBMF) enables a very flexible dimensioning of components. To treat the unfavorable manufacturing time of the incremental approach, this investigation is focused on an alternative process route using rotating forming tools, which decrease the process time significantly. After an analysis of the mechanical properties as well as the micro- and macroscopic surface quality, a quasi-static benchmark test was performed. Normalized by the weight of the component, gears manufactured by iSBMF and BS600 steel presented the same load capacity as gears manufactured by blanking with subsequent hardening. Here, using innovative high strength steels with a significant strain hardening behavior like high manganese steels enables for weight-reduced gears. © 2019 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstract10.3390/met9050515
  • Setting component properties in incremental forming
    Maaβ, F. and Dobecki, M. and Hahn, M. and Reimers, W. and Tekkaya, A.E.
    MS and T 2019 - Materials Science and Technology 2019 (2019)
    The mechanical properties of formed components are significantly influenced by the prevailing residual stress state. Single point incremental sheet metal forming is a flexible manufacturing process for complex shaped parts that enables to adjust the residual stress state by a variation of the process parameters. In this paper, truncated cone geometries with increasing relative tool step-down increments are cyclically loaded to analyze the influence of the process parameters on the fatigue strength of the formed components. The fatigue strength decreases due to resulting tensile residual stresses on the tool-side of the cone with decreasing relative tool step-down increments. The maximum reduction of the fatigue strength caused by residual stresses is 42% in the investigated range of the relative tool step-down increments. The tensile residual stress amplitudes on the tool-side increase with an increasing number of step-down increments for the same final geometry. Copyright © 2019 MS&T19®
    view abstract10.7449/2019/MST_2019_1176_1182
  • Simulation approach for three-point plastic bending of additively manufactured Hastelloy X sheets
    Rosenthal, S. and Hahn, M. and Tekkaya, A.E.
    Procedia Manufacturing 34 (2019)
    Additive manufacturing as a process for producing semi-finished parts for a subsequent forming operation has not been investigated yet. This paper examines the forming behavior of an additively manufactured monolithic sheet metal in three point bending. An approach for a numerical simulation is proposed, with the results of the material characterization of additively manufactured Hastelloy X. The anisotropic behavior is implemented with the Hill (1948) yield criterion. The results reveal that it is possible to characterize an additively manufactured material employing testing procedures commonly used in forming technologies and to use the material properties and flow curves to numerically simulate a bending operation with a difference of not more than 4 % compared to the experiment. © 2019 The Authors. Published by Elsevier B.V.
    view abstract10.1016/j.promfg.2019.06.201
  • Simultaneous deep drawing and cold forging of multi-material components: Draw-forging
    Napierala, O. and Dahnke, C. and Tekkaya, A.E.
    CIRP Annals 68 (2019)
    A new process combining cold forging and deep drawing is introduced for forming multi-material components. The components consist of a cold extruded core with a deep drawn and redrawn cup acting as shell. Process mechanics, failures, and process window are investigated for a steel-aluminium pairing. The joining mechanisms between the steel shell and the aluminium core is of force- and form-fit type. The joining strength is larger than 40% of the shear yield stress of the weakest material. Alternative material pairings, chip-cores, double stepped shafts, and deep drawing with sequential backward cup-extrusion, are explored demonstrating the technological potential of the process. © 2019 CIRP
    view abstract10.1016/j.cirp.2019.03.001
  • Stress State Analysis of Radial Stress Superposed Bending
    Meya, R. and Löbbe, C. and Tekkaya, A.E.
    International Journal of Precision Engineering and Manufacturing 20 (2019)
    Radial stress superposed bending is a sheet metal bending process, which superposes predetermined radial stresses. Stress superposition is mandatory to enable the reduction of the triaxiality in bending, resulting in delayed damage evolution and an improved product performance. The knowledge of the stress state is essential for damage-controlled bending as the triaxiality is the driving force for the void evolution. To control the stress state in radial stress superposed bending, an additional counter force responsible for the pressure in the outer fiber is applied. To predict the effect of the counter force on the radial stress and the triaxiality an analytical model is proposed. The prediction of the reaction forces in the system is required for the process design and for the calculation of the stress superposition. The stress state for plane strain bending with stress superposition is derived, and pressure calculations are made using the theory of Hertz. The model and the assumptions are verified in numerical and experimental studies for various counter pressures and bending ratios. Finally, a discussion of the load path depending on the transient counter pressure is carried out and experimental evidence for a inhibited damage evolution due to stress superposition is given. © 2019, Korean Society for Precision Engineering.
    view abstract10.1007/s12541-019-00040-0
  • Thermomechanical behavior of shape memory alloy metal matrix composite actuator manufactured by composite extrusion
    Dahnke, C. and Reeb, A. and Pottmeyer, F. and Weidenmann, K.A. and Tekkaya, A.E.
    Smart Materials and Structures 28 (2019)
    Continuous composite extrusion represents a new possibility for the manufacturing of shape memory alloy metal matrix composites (SMA-MMC). During the process SMA wires are embedded into aluminum or magnesium profiles by means of modified porthole dies. Due to the high flexibility regarding the profile geometry, the materials as well as number, thickness and position of the SMA elements, the process can be used for the generation of a profile integrated bending function. The bending function of the actuator profile depends on the temperature and is thermally activated. The parameters influencing the behavior of the manufactured composited actuators are experimentally investigated. It is found that the radius of curvature mainly depends on the recovery stress and the eccentric position of the SMA wire as well as on the bending stiffness of the actuator profile. The bending mechanism and the experimental results are described by the use of an analytical model as well as a finite element analysis. Based on the results the analytical model is used for the targeted design of a profile with multiple embedded NiTi wires, which is able to perform a repeatable, pure elastic deflection within a defined temperature range between room temperature and 75 °C. © 2019 IOP Publishing Ltd.
    view abstract10.1088/1361-665X/ab0ef5
  • Analysis of residual stress state in sheet metal parts processed by single point incremental forming
    Maaß, F. and Gies, S. and Dobecki, M. and Brömmelhoff, K. and Tekkaya, A.E. and Reimers, W.
    AIP Conference Proceedings 1960 (2018)
    The mechanical properties of formed metal components are highly affected by the prevailing residual stress state. A selective induction of residual compressive stresses in the component, can improve the product properties such as the fatigue strength. By means of single point incremental forming (SPIF), the residual stress state can be influenced by adjusting the process parameters during the manufacturing process. To achieve a fundamental understanding of the residual stress formation caused by the SPIF process, a valid numerical process model is essential. Within the scope of this paper the significance of kinematic hardening effects on the determined residual stress state is presented based on numerical simulations. The effect of the unclamping step after the manufacturing process is also analyzed. An average deviation of the residual stress amplitudes in the clamped and unclamped condition of 18 % reveals, that the unclamping step needs to be considered to reach a high numerical prediction quality. © 2018 Author(s).
    view abstract10.1063/1.5035043
  • Combination technology of deep drawing and back-moulding for plastic/metal hybrid components
    Hopmann, C. and Schild, J. and Wurzbacher, S. and Tekkaya, A.E. and Hess, S.
    Journal of Polymer Engineering 38 (2018)
    The combined deep drawing and back-moulding process shows great potential to reduce manufacturing costs of plastic/metal hybrids for structural components. To achieve this, a new mould technology with the components of both forming technologies is developed. By closing the mould, the inserted metal sheet is first deep drawn, and when the mould is fully closed, it is further formed by melt pressure. It can be shown that the forming quality of the second forming step mainly depends on the packing pressure. For a controlled flow of the metal, a downholder is necessary. By adjusting the downholder, force on the thinning of the metal through forming can be controlled. © 2018 Walter de Gruyter GmbH, Berlin/Boston.
    view abstract10.1515/polyeng-2017-0354
  • Damage mechanisms and mechanical properties of high-strength multiphase steels
    Heibel, S. and Dettinger, T. and Nester, W. and Clausmeyer, T. and Tekkaya, A.E.
    Materials 11 (2018)
    The usage of high-strength steels for structural components and reinforcement parts is inevitable for modern car-body manufacture in reaching lightweight design as well as increasing passive safety. Depending on their microstructure these steels show differing damage mechanisms and various mechanical properties which cannot be classified comprehensively via classical uniaxial tensile testing. In this research, damage initiation, evolution and final material failure are characterized for commercially produced complex-phase (CP) and dual-phase (DP) steels in a strength range between 600 and 1000 MPa. Based on these investigations CP steels with their homogeneous microstructure are characterized as damage tolerant and hence less edge-crack sensitive than DP steels. As final fracture occurs after a combination of ductile damage evolution and local shear band localization in ferrite grains at a characteristic thickness strain, this strain measure is introduced as a new parameter for local formability. In terms of global formability DP steels display advantages because of their microstructural composition of soft ferrite matrix including hard martensite particles. Combining true uniform elongation as a measure for global formability with the true thickness strain at fracture for local formability the mechanical material response can be assessed on basis of uniaxial tensile testing incorporating all microstructural characteristics on a macroscopic scale. Based on these findings a new classification scheme for the recently developed high-strength multiphase steels with significantly better formability resulting of complex underlying microstructures is introduced. The scheme overcomes the steel designations using microstructural concepts, which provide no information about design and production properties. © 2018 by the authors.
    view abstract10.3390/ma11050761
  • Design of process parameters for the incremental tube forming (ITF) by FEM to control product properties
    Nazari, E. and Löbbe, C. and Gallus, S. and Izadyar, S.A. and Tekkaya, A.E.
    AIP Conference Proceedings 1960 (2018)
    The incremental tube forming (ITF) is a process combination of the kinematic tube bending and spinning to shape high strength and tailored tubes with variable diameters and thicknesses. In contrast to conventional bending methods, the compressive stress superposition by the spinning process facilitates low bending stresses, so that geometrical errors are avoided and the shape accuracy is improved. The study reveals the interaction of plastic strains of the rolling and bending process through an explicit FEM investigation. For this purpose, the three-dimensional machine set-up is discretized and modeled in terms of the fully disclosed spinning process during the gradual deflection of the tube end for bending. The analysis shows that, depending on the forming tool shape, the stress superposition is accompanied by high plastic strains. Furthermore, this phenomenon is explained by the three dimensional normal and shear strains during the incremental spinning. Analyzing the strains history also shows a nonlinearity between the strains by bending and spinning. It is also shown that process parameters like rotational velocity of the spinning rolls have a huge influence on the deformation pattern. Finally, the method is used for the manufacturing of an example product, which reveals the high process flexibility. In one clamp a component with a graded wall thickness and outside diameter along the longitudinal axis is produced. © 2018 Author(s).
    view abstract10.1063/1.5035046
  • Development of a procedure for forming assisted thermal joining of tubes
    Chen, H. and Löbbe, C. and Staupendahl, D. and Tekkaya, A.E.
    AIP Conference Proceedings 1960 (2018)
    With the demand of lightweight design in the automotive industry, not only the wall-thicknesses of tubular components of the chassis or spaceframe are continuously decreased. Also the thicknesses of exhaust system parts are reduced to save material and mass. However, thinner tubular parts bring about additional challenges in joining. Welding or brazing methods, which are utilized in joining tubes with specific requirements concerning leak tightness, are sensitive to the gap between the joining partners. Furthermore, a large joining area is required to ensure the durability of the joint. The introduction of a forming step in the assembled state prior to thermal joining can define and control the gap for subsequent brazing or welding. The mechanical pre-joint resulting from the previously described calibration step also results in easier handling of the tubes prior to thermal joining. In the presented investigation, a spinning process is utilized to produce force-fit joints of varying lengths and diameter reduction and form-fit joints with varying geometrical attributes. The spinning process facilitates a high formability and geometrical flexibility, while at the achievable precision is high and the process forces are low. The strength of the joints is used to evaluate the joint quality. Finally, a comparison between joints produced by forming with subsequent brazing and original tube is conducted, which presents the high performance of the developed procedure for forming assisted thermal joining. © 2018 Author(s).
    view abstract10.1063/1.5034879
  • Development of forming and product properties of copper wire in a linear coil winding process
    Komodromos, A. and Lobbe, C. and Tekkaya, A.E.
    2017 7th International Electric Drives Production Conference, EDPC 2017 - Proceedings 2017-December (2018)
    Since electric motors are becoming more important in many application fields, e. g. hybrid electric vehicles, the optimization of the linear coil winding process is an important contribution to a higher productivity and flexibility. For the investigation of the forming behavior of the winding wire the material behavior is characterized in different experimental setups using wire diameters of 0.63 mm-3.35 mm. By comparing the results of tensile tests and compression tests, a tension-compression anisotropy of the wire behavior can be noticed. The Young's Modulus, measured in cyclic tensile tests from 99-116 GPa (dependent on the amount of strain), is used for the characterization of the elastic behavior of the copper wire. Subsequently, numerical investigations of the linear winding process in a case study for a rectangular bobbin are carried out in order to analyze the influence forming parameters have on the resulting properties of the wound coil. The wire tensile force represents a key parameter concerning the geometrical properties of the wound wire and the clearance between wire and bobbin. Numerical results show that the wire cross-section decreases through bending already at standard wire tensile forces. Additionally, the clearance between wire and bobbin increases with larger wire diameters. The aforementioned results serve as fundamentals for a comprehensive modeling of the linear winding process of noncircular coil bobbins with copper wire. © 2017 IEEE.
    view abstract10.1109/EDPC.2017.8328143
  • Effect of multiple forming tools on geometrical and mechanical properties in incremental sheet forming
    Wernicke, S. and Dang, T. and Gies, S. and Tekkaya, A.E.
    AIP Conference Proceedings 1960 (2018)
    The tendency to a higher variety of products requires economical manufacturing processes suitable for the production of prototypes and small batches. In the case of complex hollow-shaped parts, single point incremental forming (SPIF) represents a highly flexible process. The flexibility of this process comes along with a very long process time. To decrease the process time, a new incremental forming approach with multiple forming tools is investigated. The influence of two incremental forming tools on the resulting mechanical and geometrical component properties compared to SPIF is presented. Sheets made of EN AW-1050A were formed to frustums of a pyramid using different tool-path strategies. Furthermore, several variations of the tool-path strategy are analyzed. A time saving between 40% and 60% was observed depending on the tool-path and the radii of the forming tools while the mechanical properties remained unchanged. This knowledge can increase the cost efficiency of incremental forming processes. © 2018 Author(s).
    view abstract10.1063/1.5035057
  • Energy saving by manufacturing technology
    Tekkaya, A.E.
    Procedia Manufacturing 21 (2018)
    There are different ways of saving energy in production engineering and thereby reducing the environmental stress. Here, different possibilities of saving energy by manufacturing technology are presented. On the one hand, this is possible through lightweight components. The production of components for the automotive industry with low weight while maintaining the same mechanical properties helps to reduce the fuel consumption of the vehicles. This can be achieved by means of load-adjusted components. Furthermore, suitable materials can be selected for the respective function of the part. This is in interaction with the selected manufacturing process of the component. Press hardening can be mentioned as an example. Here, the production of complex components with high strength gets only possible by the forming process. On the other hand, energy can be saved directly during the production of the components. This is possible through energy efficient production and begins with the selection of the production process. A huge part of the energy used for the production of a component results from the manufacturing of the raw materials. Consequently, a production process should be selected which produces as little as possible excess material. For example, the production of a screw by forming results in a high utilization of the material. If this part would be produced by machining, there would be relatively much waste of material in form of chips. This in turn would have a negative impact on the environmental stress. However, the production of the forming tools must also be considered. Therefore, this example is mainly valid for large-scale production. This consideration is also important for the sector of additive manufacturing where the entire material has to be melted and therefore a large amount of energy is required. Nevertheless, if advantages with regard to lightweight components are obtained, this increased energy expenditure can be justified. © 2018 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.promfg.2018.02.136
  • Experimental and numerical investigations of wire bending by linear winding of rectangular tooth coils
    Komodromos, A. and Tekkaya, A.E. and Hofmann, J. and Fleischer, J.
    AIP Conference Proceedings 1960 (2018)
    Since electric motors are gaining in importance in many fields of application, e.g. hybrid electric vehicles, optimization of the linear coil winding process greatly contributes to an increase in productivity and flexibility. For the investigation of the forming behavior of the winding wire the material behavior is characterized in different experimental setups. Numerical examinatons of the linear winding process are carried out in a case study for a rectangular bobbin in order to analyze the influence of forming parameters on the resulting properties of the wound coil. Besides the numerical investigation of the linear winding method by using the finite element method (FEM), a multi-body dynamics (MBD) simulation is carried out. The multi-body dynamics simulation is necessary to represent the movement of the bodies as well as the connection of the components during winding. The finite element method is used to represent the material behavior of the copper wire and the plastic strain distribution within the wire. It becomes clear that the MBD simulation is not sufficient for analyzing the process and the wire behavior in its entirety. Important parameters that define the final coil properties cannot be analyzed in the manner of a precise manifestation, e.g. the clearance between coil bobbin and wire as well as the wire deformation behavior in form of a diameter reduction which negatively affects the ohmic resistance. Finally, the numerical investigations are validated experimentally by linear winding tests. © 2018 Author(s).
    view abstract10.1063/1.5035014
  • Improved failure prediction in forming simulations through pre-strain mapping
    Upadhya, S. and Staupendahl, D. and Heuse, M. and Tekkaya, A.E.
    AIP Conference Proceedings 1960 (2018)
    The sensitivity of sheared edges of advanced high strength steel (AHSS) sheets to cracking during subsequent forming operations and the difficulty to predict this failure with any degree of accuracy using conventionally used FLC based failure criteria is a major problem plaguing the manufacturing industry. A possible method that allows for an accurate prediction of edge cracks is the simulation of the shearing operation and carryover of this model into a subsequent forming simulation. But even with an efficient combination of a solid element shearing operation and a shell element forming simulation, the need for a fine mesh, and the resulting high computation time makes this approach not viable from an industry point of view. The crack sensitivity of sheared edges is due to work hardening in the shear-affected zone (SAZ). A method to predict plastic strains induced by the shearing process is to measure the hardness after shearing and calculate the ultimate tensile strength as well as the flow stress. In combination with the flow curve, the relevant strain data can be obtained. To eliminate the time-intensive shearing simulation necessary to obtain the strain data in the SAZ, a new pre-strain mapping approach is proposed. The pre-strains to be mapped are, hereby, determined from hardness values obtained in the proximity of the sheared edge. To investigate the performance of this approach the ISO/TS 16630 hole expansion test was simulated with shell elements for different materials, whereby the pre-strains were mapped onto the edge of the hole. The hole expansion ratios obtained from such pre-strain mapped simulations are in close agreement with the experimental results. Furthermore, the simulations can be carried out with no increase in computation time, making this an interesting and viable solution for predicting edge failure due to shearing. © 2018 Author(s).
    view abstract10.1063/1.5034939
  • Influence of cutting tool stiffness on edge formability
    Levin, E. and Larour, P. and Heuse, M. and Staupendahl, D. and Clausmeyer, T. and Tekkaya, A.E.
    IOP Conference Series: Materials Science and Engineering 418 (2018)
    For the evaluation of the forming behaviour of cut edges of AHSS the Hole Expansion Test (HET) standardized in the ISO 16630 is generally used. However, the observed Hole Expansion Ratio (HER) is prone to significant scatter. One reason for this scatter is the subjective observation of the through thickness crack by the machine operator. Additionally, the ISO is not very specific in its description of the test setup and material preparation, actually allowing great variations in the cutting process. Although the nominal cutting clearance is specified, a low stiffness of the punching machine or tool can cause a non-uniform clearance along the circumference. Additionally, an oblique position of the punch can cause an angled or eccentric sheared hole, resulting in additional crack initiation sites on the shear cut surface. FE simulations are used to investigate the stiffness effects of a C-frame press design. A comparative round robin analysis based on ISO16630 was performed by cross-testing both cutting and hole expansion setups for a high strength hot rolled steel grade. Significant differences were registered in the HER values, whereby the cutting process was identified to have the largest influence on HER variation. The homogeneity of the burnished zone along the circumference was observed to give valuable information about the cut edge quality and subsequent level of HER values. © Published under licence by IOP Publishing Ltd.
    view abstract10.1088/1757-899X/418/1/012061
  • Influence of manufacturing processes on material characterization with the grooved in-plane torsion test
    Traphöner, H. and Heibel, S. and Clausmeyer, T. and Tekkaya, A.E.
    International Journal of Mechanical Sciences (2018)
    In-plane torsion tests offer advantages such as a proportional loading path and a homogeneous stress and strain distribution when characterizing the material behavior in the state of in-plane shear. The use of a grooved specimen is mandatory for the characterization of the damage behavior. The manufacturing of the groove by turning, milling, and electrical discharge machining for a DP600, DP1000, and CP1000 showed a strong influence on the experimentally measurable strain values at failure. Fine machining by milling displayed good results for all tested materials. The notch-effect in turned grooves led to an early fracture initiation. A straightforward parameter identification scheme for the Hosford-Coulomb fracture criterion as proposed by [16] was used to show the sensitivity of fracture curves on the experimental database. © 2018 Elsevier Ltd.
    view abstract10.1016/j.ijmecsci.2017.12.052
  • Influence of the flyer kinetics on magnetic pulse welding of tubes
    Lueg-Althoff, J. and Bellmann, J. and Gies, S. and Schulze, S. and Tekkaya, A.E. and Beyer, E.
    Journal of Materials Processing Technology 262 (2018)
    In this paper, the influence of different pulse generators with their characteristic discharge frequencies on the process parameters of magnetic pulse welding (MPW) of aluminum EN AW-6060 tubes on steel C45 cylinders is analyzed. Experimental, numerical, and analytical investigations focus on the radial impact velocity vi,r, the time-dependent collision angle βt, and the impact pressure pi. The influence of the temporal course of the magnetic pressure pmt is discussed. It is shown that the minimum radial impact velocity required for welding with the same geometrical setup can be reduced significantly at low discharge frequencies compared to high ones. This is attributed to a different deformation behavior of the tubular flyer part and, consequently, more favorable collision angles. Geometric changes to the joining setup enable a targeted modification of βt and allow for a reduction of vi,r even at high-frequency systems. During the design of an MPW process it is essential to consider the pulse characteristics. The advanced analysis methods presented in this paper contribute to the targeted establishment of favorable collision conditions for MPW, taking the distinctive features of the applied equipment into account. © 2018 Elsevier B.V.
    view abstract10.1016/j.jmatprotec.2018.06.005
  • Investigation of the effects of process and geometrical parameters on formability in tube hydroforming using a modular hydroforming tool
    Joghan, H.D. and Staupendahl, D. and Hassan, H.U. and Henke, A. and Keesser, T. and Legat, F. and Tekkaya, A.E.
    AIP Conference Proceedings 1960 (2018)
    Tube hydroforming is one of the most important manufacturing processes for the production of exhaust systems. Tube hydroforming allows generating parts with highly complex geometries with the forming accuracies needed in the automotive sector. This is possible due to the form-closed nature of the production process. One of the main cost drivers is tool manufacturing, which is expensive and time consuming, especially when forming large parts. To cope with the design trend of individuality, which is gaining more and more importance and leads to a high number of product variants, a new flexible tool design was developed. The designed tool offers a high flexibility in manufacturing different shapes and geometries of tubes with just local alterations and relocation of tool segments. The tolerancing problems that segmented tools from the state of the art have are overcome by an innovative and flexible die holder design. The break-even point of this initially more expensive tool design is already overcome when forming more than 4 different tube shapes. Together with an additionally designed rotary hydraulic tube feeding system, a highly adaptable forming setup is generated. To investigate the performance of the developed tool setup, a study on geometrical and process parameters during forming of a spherical dome was done. Austenitic stainless steel (grade 1.4301) tube with a diameter of 40mm and a thickness of 1.5mm was used for the investigations. The experimental analyses were supported by finite element simulations and statistical analyses. The results show that the flexible tool setup can efficiently be used to analyze the interaction of the inner pressure, friction, and the location of the spherical dome and demonstrate the high influence of the feeding rate on the formed part. © 2018 Author(s).
    view abstract10.1063/1.5035009
  • Joining by die-less hydroforming of profiles with oval cross section
    Müller, M. and Gies, S. and Tekkaya, A.E.
    Key Engineering Materials 767 KEM (2018)
    Joining by die-less hydroforming is used to produce overlap joints by means of hydraulic expansion. Due to a difference in the elastic recovery of the two joining partners an interference pressure p remains at their contact area. Due to the possibility to produce multi-material joints without relying on heat, the process has great potential for joining parts in lightweight applications. Therefore, the process limits were extended so that profiles with non-rotationally symmetric cross-sections can be joined. For this purpose a new tool for profiles with oval cross-section was developed. The inner and the outer joining partner ware made of aluminum 6060 and aluminum 6082 respectively. The influence of the overlap length and different wall thicknesses of the outer joining partner were investigated by numerical simulations and validated by experiments. An upper limit in interference pressure was observed which was also found previously for profiles with circular cross sections. The fluid pressure limit is compared with the analytically calculated value for a configuration with circular tubes under equivalent conditions. The analytical model underestimated the pressure limit. In contrast to circular tubes, the strain distribution of profiles with oval cross sectional shapes is not uniform, which results in superposed bending stresses. Also a difference in stiffness of the inner and outer joining partner leads to a pressure depended contact area which is assumed constant in the analytical model. © 2018 Trans Tech Publications, Switzerland.
    view abstract10.4028/www.scientific.net/KEM.767.405
  • Material characterization for plane and curved sheets using the in-plane torsion test – An overview
    Traphöner, H. and Clausmeyer, T. and Tekkaya, A.E.
    Journal of Materials Processing Technology 257 (2018)
    The in-plane torsion test offers a broad range of applications for the characterization of mechanical properties of sheet metal materials and components. True plastic strains beyond 1.0 can be achieved. Such data can be used for the numerical analysis without extrapolation of the flow curve. The stress and strain state correspond to truely ideal simple shear during the entire process. The full-field strain and stress analysis of the test area makes it possible to determine an almost arbitrary number of cyclic flow curves with only one single specimen. By using a grooved specimen, the ideal simple shear state can be obtained until fracture of the material without loosing the shear state. New investigations show that the in-plane torsion test is also suitable for the determination of flow curves of sheets with curved surfaces. Investigations on curved rotationally symmetrical as well as a tubular shape sheets were performed. Finally, first results for the determination of the local strength at arbitrary positions of a sheet component are presented. © 2018 Elsevier B.V.
    view abstract10.1016/j.jmatprotec.2018.02.030
  • Mechanics of the reciprocal effects of bending and torsion during 3D bending of profiles
    Staupendahl, D. and Tekkaya, A.E.
    Journal of Materials Processing Technology 262 (2018)
    Profiles with circular cross-sections can be geometrically described by the shape of the bending line. To achieve 3D bending lines with kinematic bending processes, a continuous change of the bending plane is needed, resulting in bending force vectors that change direction accordingly. These force vectors generate a bending moment in the forming zone and, hence, longitudinal tensile and compressive stresses. For profiles with non-circular cross-sections the orientation of the cross-section along the bending line needs to be controlled additionally. This can be achieved by applying a specific torque to the bending process and, thus, introducing desired shear stresses into the forming zone. Until now, this fundamental aspect of 3D profile bending has not been studied in a coherent fashion. To take into account the reciprocal effects of the various stresses applied to the forming zone and their effect on the bending moment and, thus, on springback, a comprehensive analytical process model was set up. The model is validated by experimental investigations performed using the Torque Superposed Spatial (TSS) profile bending machine and by comprehensive numerical investigations. Analyses were performed during plane bending as well as during bending superposed with torsion. The investigations show that the applied bending force and torque not only result in stress superposition, but actually also affect the development of shear strains over the cross-section of the profile. Similar to the longitudinal strains, the shear strains decrease linearly from the intrados and extrados of the profile to the neutral axis. Considering this newly observed behavior in the analytical process model, the absolute bending force deviation was reduced by a factor of four down to 7.6%. © 2018 Elsevier B.V.
    view abstract10.1016/j.jmatprotec.2018.07.025
  • Mechanisms for controlling springback and strength in heat-assisted sheet forming
    Löbbe, C. and Tekkaya, A.E.
    CIRP Annals 67 (2018)
    The recently developed multi-step sheet forming technology with inductive in-situ heating serves to increase the productivity and to reduce costs compared to hot stamping with furnace heating. The paper reveals the mechanisms for the flexible setting of geometric and mechanical properties such as the bending angle and strength in terms of a closed loop control. The air and die bending as representative forming processes are analyzed with respect to the mechanisms overbending, springback and thermal distortion by experimental, numerical, and analytical investigations. The mechanisms for controlling strength in carbon steels by grain refinement, grain growth and cooling rate are described. © 2018
    view abstract10.1016/j.cirp.2018.04.013
  • Parameter identification for magnetic pulse welding applications
    Bellmann, J. and Joern, L.-A. and Schulze, S. and Gies, S. and Beyer, E. and Tekkaya, A.E.
    Key Engineering Materials 767 KEM (2018)
    Magnetic pulse welding (MPW) is a promising technology to join dissimilar metals and to produce multi-material structures, e.g. to fulfill lightweight requirements. During this impact welding process, proper collision conditions between both joining partners are essential for a sound weld formation. Controlling these conditions is difficult due to a huge number of influencing and interacting factors. Many of them are related to the pulse welding setup and the material properties of the moving part, the so-called flyer. In this paper, a new measurement system is applied that takes advantage of the high velocity impact flash. The flash is a side effect of the MPW process and its intensity depends on the impact velocity of the flyer. Thus, the intensity level can be used as a welding criterion. A procedure is described that enables the user to realize a fast parameter development with only a few experiments. The minimum energy level and the optimum distance between the parts to be joined can be identified. This is of importance since a low energy input decreases the thermal and mechanical shock loading on the tool coil and thus increases its lifetime. In a second step, the axial position of the flyer in the tool coil is adjusted to ensure a proper collision angle and a circumferential weld seam. © 2018 Trans Tech Publications, Switzerland.
    view abstract10.4028/www.scientific.net/KEM.767.431
  • The Journal of Materials Processing Technology 2007-17
    Allwood, J.M. and Tekkaya, A.E.
    Journal of Materials Processing Technology 251 (2018)
    view abstract10.1016/j.jmatprotec.2017.09.009
  • Adaptive wear model for shear-cutting simulation with open cutting line
    Cha, W.-G. and Hammer, T. and Gutknecht, F. and Golle, R. and Tekkaya, A.E. and Volk, W.
    Wear 386-387 (2017)
    Shear-cutting is one of the most important manufacturing processes involving sheet metal. The local pressure on the tools as well as the sliding speed between tools and work piece are much larger in shear cutting than in typical forming processes such as deep-drawing. This makes the prediction of tool life more critical in a shear-cutting simulation, in contrast to a forming simulation. A FE-program for wear prediction was used so far in the forming simulation. This tool was adjusted to the shear-cutting process through modifications of the shear-cutting simulation and of the interpolation algorithm to stabilize the wear calculation for the process. Endurance tests for the validation of the simulation results were performed with a hardened cold working steel (1.2379, 58 HRC) as tool material and a dual phase steel (DP600) with a thickness of 1 mm as sheet material. The FE-tool permits the qualitative prediction of the wear of the active elements in shear-cutting processes. © 2017 Elsevier B.V.
    view abstract10.1016/j.wear.2017.05.019
  • Analysis of dislocation structures in ferritic and dual phase steels regarding continuous and discontinuous loading paths
    Gerstein, G. and Clausmeyer, T. and Gutknecht, F. and Tekkaya, A.E. and Nürnberger, F.
    Minerals, Metals and Materials Series Part F6 (2017)
    In sheet-bulk metal forming processes the hardening behavior of the material depends on the sequence of deformation steps and the type of deformation. Loading path changes induce transient hardening phenomena. These phenomena are linked to the formation and interaction of oriented dislocation structures. The aim of this study is to investigate the effect of continuous and discontinuous loading path changes on the dislocation microstructure in ferritic and ferritic-martensitic dual-phase steel, respectively. For the experiments a biaxial test stand was used, which permits to continuously change the load from tension to shear. In the ferrite single-phase steel transmission-electron microscopy reveals a reduced evolution of oriented dislocation structures for continuous loading path changes compared to discontinuous loading path changes. This evolution is further decreased in dual-phase steel compared to the ferritic steel. Microstructural results for the ferritic steel are accompanied by simulation results with a transient hardening model. © The Minerals, Metals & Materials Society 2017.
    view abstract10.1007/978-3-319-51493-2_20
  • Analytical prediction of Joule heat losses in electromagnetic forming coils
    Gies, S. and Tekkaya, A.E.
    Journal of Materials Processing Technology 246 (2017)
    Coils in electromagnetic forming operations are exposed to mechanical as well as thermal loads. Especially in case of high volume production the thermal loading due to Joule heat losses needs to be considered in the coil and process design to prevent thermal overstressing. For this purpose an analytical approach to calculate the Joule heat losses in a one-turn coil with rectangular cross section is presented. It takes the geometrical and physical properties of coil and workpiece as well as the amplitude, frequency, and damping behavior of the discharge current into account. While a simplified approach assumes a constant electrical conductivity of the coil, the enhanced approach considers the temperature-dependent course of the electrical conductivity. Verification of the analytical model is realized using a combination of experimental and numerical investigations. Fiber-optical measurements of the coil temperature are used to verify the simulation. The Joule heat loss per unit length determined with the verified numerical model is then used to evaluate the prediction quality of the analytical approach. Different conductor materials (CuCr1Zr, Cu-ETP, and EN AW-1050A), conductor geometries, and discharge energies are analyzed in this verification procedure. Compared to the numerical Joule heat prediction an average deviation of 13.1% and 9.0% is determined for the simplified and the enhanced analytical model, respectively. Especially in case of higher discharge energies the enhanced model shows an improved accuracy and should be preferred over the simplified approach. Considering complexity and accuracy, the analytical approach is a proper instrument for the thermal coil and process design in electromagnetic forming operations. © 2017 Elsevier B.V.
    view abstract10.1016/j.jmatprotec.2017.03.008
  • Ball burnishing under high velocities using a new rolling tool concept
    Hiegemann, L. and Erman Tekkaya, A.
    ASME 2017 12th International Manufacturing Science and Engineering Conference, MSEC 2017 collocated with the JSME/ASME 2017 6th International Conference on Materials and Processing 1 (2017)
    Ball burnishing is a process used to smooth rough surfaces. For not rotational symmetric parts, the process is typically conducted on milling machines. Since it is an incremental process, it is relatively time consuming. Therefore, a rolling tool is developed, which superposes the rotation of the milling spindle with the feed of the machine to increase the rolling velocity. In order to achieve constant rolling forces, hydrostatic ball burnishing tools are used. Within this work, the influence of this tool concept on the processing time as well as on the leveling of surface irregularities is investigated. This is achieved by a comparison with a conventional ball burnishing process. Finally, the rotating tool is used to investigate the influence of high rolling speeds on the leveling of the surface. All experiments were carried out with thermally coated specimens. A model for calculating the strain rates at the roughness peaks during ball burnishing is derived. For the experiments carried out with the rotating rolling tool, rolling velocities of 50,000 mm/min were realized. Calculations with the developed model showed that this results in local strain rates at the roughness peaks of up to 1,384 s-1. In addition, the flow stresses at the roughness peaks were calculated. Compared with quasi static experiments, the flow stress drops to less than the half under high velocities. This results in a better leveling of the surface for rolling velocities between 10,000 mm/min and 25,000 mm/min. A further rise of the rolling speed increases the flow stress again and thereby reduces the possible leveling. ©2017 ASME.
    view abstract10.1115/MSEC20172644
  • Combination technology of deep drawing and back-moulding for plastic/metal hybrid components
    Hopmann, C. and Schild, J. and Wurzbacher, S. and Tekkaya, A.E. and Hess, S.
    Journal of Polymer Engineering (2017)
    The combined deep drawing and back-moulding process shows great potential to reduce manufacturing costs of plastic/metal hybrids for structural components. To achieve this, a new mould technology with the components of both forming technologies is developed. By closing the mould, the inserted metal sheet is first deep drawn, and when the mould is fully closed, it is further formed by melt pressure. It can be shown that the forming quality of the second forming step mainly depends on the packing pressure. For a controlled flow of the metal, a downholder is necessary. By adjusting the downholder, force on the thinning of the metal through forming can be controlled. © 2017 Walter de Gruyter GmbH, Berlin/Boston.
    view abstract10.1515/polyeng-2017-0354
  • Deformation characteristics of thermoplastics in single point incremental forming
    Maaß, F. and Gies, S. and Tekkaya, A.E.
    AIP Conference Proceedings 1896 (2017)
    Single point incremental forming (SPTF) is a manufacturing process to produce complex shaped parts by the CNC controlled movement of a hemispherical forming tool. The poor geometrical accuracy is one of the dominant process limits in SPTF. Several references deal with approaches to improve the geometrical accuracy in SPTF, especially for sheet metal forming. However, forming thermoplastic materials with high geometrical accuracy is even more difficult due to the large elastic recovery upon unloading. As already shown for SPTF of metal sheets, overbending the workpiece is one opportunity to improve the geometrical accuracy. The aim of this research is to investigate how SPTF process parameters effect the resulting geometry and bulging of thermoplastic sheets. The research is done experimentally using Polyvinylchloride (PVC) and high density Polyethylene (PE-HD). Tt includes a strategy to reduce bulging effects by overbending the material. The experiments were performed on a CNC milling machine equipped with a single point forming tool and a simple support to fix the workpiece. To investigate the influence of the process parameters on the geometrical accuracy, linear grooves were manufactured into thermoplastic sheets. The geometric quality of the formed thermoplastic parts was evaluated by means of three-dimensional optical measurement. Tool radius, initial sheet thickness and workpiece material showed a strong correlation with the bulge height of the final part. Depending on the considered process parameter, the bulge height can be reduced up to 98%. © 2017 Author(s).
    view abstract10.1063/1.5008050
  • Demonstration of deep drawing experiments in a remote lab environment
    Grodotzki, J. and Selvaggio, A. and Ortelt, T.R. and Tekkaya, A.E.
    Proceedings of 2017 4th Experiment at International Conference: Online Experimentation, exp.at 2017 (2017)
    Understanding fundamental process limits is a crucial skill for all types of engineers. In mechanical engineering, this especially applies to the field of metal forming. To have the students understand the different limits of the commonly used deep drawing process, e.g. the influence of the clamping force, a tele-operative testing cell was developed at the Institute of Forming Technology and Lightweight Components of TU Dortmund University. The live experiments that can be conducted using this testing cell are included in different lectures as well as in remote labs which are accessible online for students around the world. In either case, the experiments are used to have the students realize on their own what different types of limits exist and when they occur. © 2017 IEEE.
    view abstract10.1109/EXPAT.2017.7984374
  • Development of a FEM-lab for the virtual experimentation in forming processes
    Grodotzki, J. and Ortelt, T.R. and Tekkaya, A.E.
    Proceedings of 2017 4th Experiment at International Conference: Online Experimentation, exp.at 2017 (2017)
    In nowadays engineering, the ability to use and understand Finite-Element-Method (FEM) simulation software has become a crucial skill. If applied correctly, it can provide insight into various processes, such as forming operations, without the need to actually perform the real experiment. A novel, fully virtual FEM-Lab is currently under development at the Institute of Forming Technology and Lightweight Components of TU Dortmund University, giving access to undergraduate students so they can learn about forming processes in detail by actively changing the process parameters. Fundamental knowledge about Finite-Element-Analysis (FEA) is provided during numerical experimentation such that previous knowledge is not necessary to use this kind of virtual laboratory. © 2017 IEEE.
    view abstract10.1109/EXPAT.2017.7984417
  • Effect of workpiece deformation on Joule heat losses in electromagnetic forming coils
    Gies, S. and Tekkaya, A.E.
    Procedia Engineering 207 (2017)
    Coils in electromagnetic forming operations are exposed to mechanical as well as thermal loads. Especially in case of high volume production the thermal loading due to Joule heating needs to be considered in the coil and process design to prevent thermal overstressing. An analytical approach for the calculation of Joule heat losses in electromagnetic forming coils considering the changing gap width between coil and workpiece is presented. An electromagnetic sheet metal forming application using a straight one-turn coil is used as reference case to prove the accuracy of the model. A comparison of the analytical calculation and the numerical results based on a simulation with a coupling of structural, thermal, and electromagnetic effects is provided. It is proved that the coil heating in case of a deforming workpiece ranges between the heating without workpiece and the heating with rigid workpiece. It is shown that with increasing workpiece velocities the heat losses in the coil tend more and more towards the lower bound represented by a coil without workpiece.
    view abstract10.1016/j.proeng.2017.10.785
  • Experimental analysis of anisotropic damage in dual-phase steel by resonance measurement
    Gerstein, G. and Clausmeyer, T. and Isik, K. and Nürnberger, F. and Tekkaya, A.E. and Bruchanov, A.A. and Maier, H.J.
    International Journal of Damage Mechanics 26 (2017)
    The ductile damage in deformed dual-phase steel sheets (DP600) was investigated based on measurements of the degradation of the direction-dependent Young's modulus. The study focuses on the material-induced damage anisotropy in such advanced high-strength steel. The elastic properties in the direction of applied loading of the deformed sheets were determined by measuring the resonance frequency of rectangular samples. The material was investigated in the as-delivered condition and after annealing at 220 for 48 h. Tensile strains of up to 10% were applied after annealing. Tensile tests were performed in different directions with respect to the rolling direction to determine the evolution of damage in different directions. The comparison of the obtained results with the electron micrographs shows that the damage in the steel sheets occurs in the form of nano and micro damages near the grain boundary and interfaces of phases. The maximum decrease of the Young's modulus in the transverse direction was observed for the largest applied deformation of 10% tensile strain in the transverse direction. An efficient calculation method to obtain information on the distribution of anisotropy in the plane of the sheet was applied. This calculation method relies on an efficient representation of the material's texture. In order to assess the influence of texture, the texture was determined experimentally. © 2017 SAGE Publications.
    view abstract10.1177/1056789516650245
  • Experimental and analytical investigation of the force requirements in shear cutting of metal-polymer-metal composites
    Groche, P. and Übelacker, D. and Stein, P. and Steinbach, F. and Erman Tekkaya, A.
    International Journal of Material Forming (2017)
    The increasing demand for lightweight design requires the use of multi materials such as metal-polymer-metal composites. These so-called sandwich panels offer a good stiffness-to-weight ratio. Production technologies like shear cutting have to be adapted for these materials. For a targeted adaption, a comprehensive knowledge about the cutting phases of the shear cutting process of sandwich panels is essential. Therefore, within this paper, the shear cutting process of sandwich panels is studied in detail. The conducted experimental studies indicate that the shear cutting process can be divided into five stages. Based on these findings, a new analytic model is introduced to predict the force displacement curves of sandwich panels. The quality of the new model is proven by comparison with existing analytic models for monolithic materials as well as with the experimental data. © 2017 Springer-Verlag France
    view abstract10.1007/s12289-017-1343-x
  • Failure assessment in sheet metal forming using a phenomenological damage model and fracture criterion: Experiments, parameter identification and validation
    Heibel, S. and Nester, W. and Clausmeyer, T. and Tekkaya, A.E.
    Procedia Engineering 207 (2017)
    In this contribution microstructural and macroscopic experimental findings are used to calibrate the phenomenological damage model GISSMO (Generalized Incremental Stress State dependent Model) and the fracture criterion FFL/SFFL (Fracture Forming Limit line/ Shear Fracture Forming Limit line) to assess failure in sheet metal forming simulation. It is shown that macroscopic failure in a commercial dual-phase steel is initiated through a combination of ductile damage mechanisms and local shear banding at a characteristic, stress-state dependent optical or tactile measureable fracture strain. The parameter identification for both models is based on ductile fracture experiments representing characteristic stress states. GISSMO is calibrated inversely using experimental stress-strain curves, optical measured fracture strains and simulation data. The FFL and SFFL are constructed in a direct manner with the optically and tactilely measured fracture strains. Both models are validated comparatively on a cross-die cup showing ductile fracture with slight necking. The use of the fracture criterion in combination with a direct method of determining the fracture lines based on tactile strain measurements leads to an overestimation of the instant of fracture initiation. With the inversely identified parameters of the phenomenological damage model the onset of fracture initiation can be accurately predicted. © 2017 The Authors. Published by Elsevier Ltd.
    view abstract10.1016/j.proeng.2017.10.1065
  • Finite element analysis of combined forming processes by means of rate dependent ductile damage modelling
    Kiliclar, Y. and Vladimirov, I.N. and Wulfinghoff, S. and Reese, S. and Demir, O.K. and Weddeling, C. and Tekkaya, A.E. and Engelhardt, M. and Klose, C. and Maier, H.J. and Rozgic̀, M. and Stiemer, M.
    International Journal of Material Forming 10 (2017)
    Sheet metal forming is an inherent part of todays production industry. A major goal is to increase the forming limits of classical deep-drawing processes. One possibility to achieve that is to combine the conventional quasi-static (QS) forming process with electromagnetic high-speed (HS) post-forming. This work focuses on the finite element analysis of such combined forming processes to demonstrate the improvement which can be achieved. For this purpose, a cooperation of different institutions representing different work fields has been established. The material characterization is based on flow curves and forming limit curves for low and high strain rates obtained by novel testing devices. Further experimental investigations have been performed on the process chain of a cross shaped cup, referring to both purely quasi-static and quasi-static combined with electromagnetic forming. While efficient mathematical optimization algorithms support the new viscoplastic ductile damage modelling to find the optimum parameters based on the results of experimental material characterization, the full process chain is studied by means of an electro-magneto-mechanical finite element analysis. The constitutive equations of the material model are integrated in an explicit manner and implemented as a user material subroutine into the commercial finite element package LS-DYNA. © 2015 Springer-Verlag France
    view abstract10.1007/s12289-015-1278-z
  • Formability analysis of thin press hardening steel sheets under isothermal and non-isothermal conditions
    Georgiadis, G. and Tekkaya, A.E. and Weigert, P. and Horneber, S. and Aliaga Kuhnle, P.
    International Journal of Material Forming 10 (2017)
    Aiming at the enhancement of the lightweight potential of press hardening steels, investigations on the formability of thin, boron alloyed, hot dip aluminized steel sheets are carried out. The material formability is described through Forming Limit Diagram (FLD), determined by means of Nakajima formability test of thin 22MnB5 sheets (0.50 mm, 0.80 mm, 1.25 mm) at elevated temperatures. The influence of sheet thickness on forming limits is evaluated under both isothermal and non-isothermal conditions. The effect of different deformation start temperatures is examined. The non-isothermal behavior is further investigated via microstructural analysis and a study on temperature profile during Nakajima test. The results show a significant difference regarding the influence of sheet thickness under isothermal and non-isothermal conditions. Increasing the sheet thickness results, as expected, in higher forming limits for isothermal conditions, whereas for non-isothermal conditions the opposite effect on formability is observed. The obtained Forming Limit Curves (FLCs) are validated through hot stamping simulation and subsequent analysis of different thin components, concluding that in case of thin sheets the isothermal FLC constitutes a more conservative approach, while the non-isothermal one reaches the formability limits with higher accuracy. © 2016 Springer-Verlag France
    view abstract10.1007/s12289-016-1289-4
  • Forming-induced damage and its effects on product properties
    Tekkaya, A.E. and Ben Khalifa, N. and Hering, O. and Meya, R. and Myslicki, S. and Walther, F.
    CIRP Annals - Manufacturing Technology 66 (2017)
    Damage is caused in the microstructure of metals during forming. Damage is not a failure, but affects the mechanical properties of the component under service loads. This paper explores experimentally the effect of metal forming process parameters on the evolution of damage and the resulting product properties. As a representative for bulk forming processes, cold forging is investigated. It is shown that an increase of the extrusion ratio leads to lower damage and increased fatigue strength. Air bending, as a sheet forming process, is analysed, exhibiting that damage can be influenced by process design such as the superposition of stresses. © 2017
    view abstract10.1016/j.cirp.2017.04.113
  • High temperature and dynamic testing of AHSS for an analytical description of the adiabatic cutting process
    Winter, S. and Schmitz, F. and Clausmeyer, T. and Tekkaya, A.E. and Wagner, M.F.-X.
    IOP Conference Series: Materials Science and Engineering 181 (2017)
    In the automotive industry, advanced high strength steels (AHSS) are widely used as sheet part components to reduce weight, even though this leads to several challenges. The demand for high-quality shear cutting surfaces that do not require reworking can be fulfilled by adiabatic shear cutting: High strain rates and local temperatures lead to the formation of adiabatic shear bands (ASB). While this process is well suited to produce AHSS parts with excellent cutting surface quality, a fundamental understanding of the process is still missing today. In this study, compression tests in a Split-Hopkinson Pressure Bar with an initial strain rate of 1000 s-1 were performed in a temperature range between 200 °C and 1000 °C. The experimental results show that high strength steels with nearly the same mechanical properties at RT may possess a considerably different behavior at higher temperatures. The resulting microstructures after testing at different temperatures were analyzed by optical microscopy. The thermo-mechanical material behavior was then considered in an analytical model. To predict the local temperature increase that occurs during the adiabatic blanking process, experimentally determined flow curves were used. Furthermore, the influence of temperature evolution with respect to phase transformation is discussed. This study contributes to a more complete understanding of the relevant microstructural and thermo-mechanical mechanisms leading to the evolution of ASB during cutting of AHSS. © Published under licence by IOP Publishing Ltd.
    view abstract10.1088/1757-899X/181/1/012026
  • Improvement strategies for the formfilling in incremental gear forming processes
    Sieczkarek, P. and Wernicke, S. and Gies, S. and Tekkaya, A.E. and Krebs, E. and Wiederkehr, P. and Biermann, D. and Tillmann, W. and Stangier, D.
    Production Engineering 11 (2017)
    Incremental Sheet-Bulk Metal Forming offers an innovative and flexible approach for the manufacturing of gears. An insufficient formfilling of the generated gearing, especially of the first tooth formed, is observed. Aiming for a formfilling improvement of the first tooth element, three influencing factors were investigated. First, the prevailing friction is analyzed and a possibility for its adjustment is offered by a tailored adaption of the tool surface topographies. These were manufactured by micromilling, EDM and polishing processes and partially covered by CrAlN PVD-coatings. Based on ring-compression tests, which were performed to determine the resulting friction conditions, the analyzed topographies were transferred onto real tool surfaces and used in the incremental gear forming process. Second, the influence on the formfilling of the blank cutting process and the resulting sheet edge properties were investigated. The third aspect to enhance the formfilling of the gear elements was the modification of the process strategy of the incremental forming process. Due to different conditions for the initial and the following indentations, a preforming operation was investigated in order to realize a similar material flow for all indentations. With the combination of the best parameters regarding the tool surface, the blank cutting process and the forming strategy, an improvement of the formfilling of the first formed gear element by up to 33% and for the following gears by up to 13% was achieved. © 2017 German Academic Society for Production Engineering (WGP)
    view abstract10.1007/s11740-017-0764-1
  • Increasing the formability of ferritic stainless steel tube by granular medium-based hot forming
    Chen, H. and Staupendahl, D. and Hiegemann, L. and Tekkaya, A.E.
    Journal of Physics: Conference Series 896 (2017)
    Ferritic stainless steel without the alloy constituent nickel is an economical substitution for austenitic stainless steel in the automotive industry. Its lower formability, however, oftentimes prevents the direct material substitution in forming processes such as hydroforming, necessitating new forming strategies. To extend the forming capacity of ferritic stainless steel tube, the approach of forming at elevated temperatures is proposed. Utilizing granular material as forming medium, high forming temperatures up to 900°C are realized. The forming process works by moving punches axially into the granular medium, thereby, compressing it and causing axial as well as radial pressure. In experimental and numerical investigations it is shown that interfacial friction between the granular medium and the tube inherently causes tube feed, resulting in stain states in the tension-compression region of the FLD. Formability data for this region are gained by notched tensile tests, which are performed at room temperature as well as at elevated temperatures. The measured data show that the formability is improved at forming temperatures higher than 700°C. This observed formability increase is experimentally validated using a demonstrator geometry, which reaches expansion ratios that show fracture in specimens formed at room temperature. © Published under licence by IOP Publishing Ltd.
    view abstract10.1088/1742-6596/896/1/012009
  • Influence of Different Yield Loci on Failure Prediction with Damage Models
    Heibel, S. and Nester, W. and Clausmeyer, T. and Tekkaya, A.E.
    Journal of Physics: Conference Series 896 (2017)
    Advanced high strength steels are widely used in the automotive industry to simultaneously improve crash performance and reduce the car body weight. A drawback of these multiphase steels is their sensitivity to damage effects and thus the reduction of ductility. For that reason the Forming Limit Curve is only partially suitable for this class of steels. An improvement in failure prediction can be obtained by using damage mechanics. The objective of this paper is to comparatively review the phenomenological damage model GISSMO and the Enhanced Lemaitre Damage Model. GISSMO is combined with three different yield loci, namely von Mises, Hill48 and Barlat2000 to investigate the influence of the choice of the plasticity description on damage modelling. The Enhanced Lemaitre Model is used with Hill48. An inverse parameter identification strategy for a DP1000 based on stress-strain curves and optical strain measurements of shear, uniaxial, notch and (equi-)biaxial tension tests is applied to calibrate the models. A strong dependency of fracture strains on the choice of yield locus can be observed. The identified models are validated on a cross-die cup showing ductile fracture with slight necking. © Published under licence by IOP Publishing Ltd.
    view abstract10.1088/1742-6596/896/1/012081
  • Influence of Different Yield Loci on Failure Prediction with Damage Models
    Heibel, S. and Nester, W. and Clausmeyer, T. and Tekkaya, A.E.
    Journal of Physics: Conference Series 896 (2017)
    Advanced high strength steels are widely used in the automotive industry to simultaneously improve crash performance and reduce the car body weight. A drawback of these multiphase steels is their sensitivity to damage effects and thus the reduction of ductility. For that reason the Forming Limit Curve is only partially suitable for this class of steels. An improvement in failure prediction can be obtained by using damage mechanics. The objective of this paper is to comparatively review the phenomenological damage model GISSMO and the Enhanced Lemaitre Damage Model. GISSMO is combined with three different yield loci, namely von Mises, Hill48 and Barlat2000 to investigate the influence of the choice of the plasticity description on damage modelling. The Enhanced Lemaitre Model is used with Hill48. An inverse parameter identification strategy for a DP1000 based on stress-strain curves and optical strain measurements of shear, uniaxial, notch and (equi-)biaxial tension tests is applied to calibrate the models. A strong dependency of fracture strains on the choice of yield locus can be observed. The identified models are validated on a cross-die cup showing ductile fracture with slight necking. © Published under licence by IOP Publishing Ltd.
    view abstract10.1088/1742-6596/896/1/012081
  • Material characterization for plane and curved sheets using the in-plane torsion test-an overview
    Traphöner, H. and Clausmeyer, T. and Tekkaya, A.E.
    Procedia Engineering 207 (2017)
    The in-plane torsion test offers a broad range of applications for the characterization of mechanical properties of sheet metal materials and components. True plastic strains up to 1.0 can be achieved. Such data can be used for the numerical analysis without extrapolation of the flow curve. The stress and strain state correspond to ideal simple shear during the entire process. The application range of the plane torsion test has been constantly expanded by different approaches and a more sophisticated evaluation has been developed. The full-field analysis of the test area makes it possible to determine an almost arbitrary number of cyclic flow curves with only one single specimen. By using a grooved specimen, the ideal simple shear state can be obtained until fracture of the material without inhomogeneities. New investigations show that the in-plane torsion test is also suitable for the determination of flow curves for curved surfaces. Investigations on curved rotationally symmetrical as well as a tubular shape were performed. Finally, first results for the local determination of the strength' arbitrary locations of a component are presented. © 2017 The Authors. Published by Elsevier Ltd.
    view abstract10.1016/j.proeng.2017.10.964
  • Material flow analysis for the incremental sheet-bulk gearing by rotating tools
    Wernicke, S. and Sieczkarek, P. and Grodotzki, J. and Gies, S. and Khalifa, N.B. and Tekkaya, A.E.
    ASME 2017 12th International Manufacturing Science and Engineering Conference, MSEC 2017 collocated with the JSME/ASME 2017 6th International Conference on Materials and Processing 1 (2017)
    The manufacturing of gear elements by forming offers advantages regarding the resulting mechanical properties of the functional components. One possible approach is offered by the incremental sheet-bulk metal forming of gears using a linear motion punch. This method is highly flexible in terms of shape and position of the functional elements to be produced, but inefficient from an economical point of view due to the high process time. This paper presents a new sheet-bulk gear forming process using rotating tools in order to speed up the manufacturing process of load-Adapted gears. Here, different concepts with rotating tools being synchronized and nonsynchronized to the workpiece are investigated to form highstrength, load-Adapted gears made of bainitic steel BS600. The focus is on the analysis of the occurring material flow which is examined by means of finite element analysis and microstructural investigations to ensure the manufacture of fully functional geared components by this sheet-bulk metal forming process. ©2017 ASME.
    view abstract10.1115/MSEC20173029
  • Microstructural characterization and simulation of damage for geared sheet components
    Gerstein, G. and Isik, K. and Gutknecht, F. and Sieczkarek, P. and Ewert, J. and Tekkaya, A.E. and Clausmeyer, T. and Nürnberger, F.
    Journal of Physics: Conference Series 896 (2017)
    The evolution of damage in geared components manufactured from steel sheets was investigated, to analyse the influence of damage caused by the sheet-bulk-metal forming. Due to the inhomogeneous and multi-axial deformation in the investigated parts, different aspects such as the location-dependent shape and size of voids are analysed by means of various microscopic methods. In particular, a method to characterize the state of damage evolution, i. e. void nucleation, growth and coalescence using scanning electron microscopy (SEM) is applied. The investigations reveal a strong dependence of the void area fraction, shape of voids and thus damage evolution on the loading mode. The microstructural analysis is complemented with FEM simulations using material models which consider the characteristics of the void evolution. © Published under licence by IOP Publishing Ltd.
    view abstract10.1088/1742-6596/896/1/012076
  • Microstructural characterization and simulation of damage for geared sheet components
    Gerstein, G. and Isik, K. and Gutknecht, F. and Sieczkarek, P. and Ewert, J. and Tekkaya, A.E. and Clausmeyer, T. and Nürnberger, F.
    Journal of Physics: Conference Series 896 (2017)
    The evolution of damage in geared components manufactured from steel sheets was investigated, to analyse the influence of damage caused by the sheet-bulk-metal forming. Due to the inhomogeneous and multi-axial deformation in the investigated parts, different aspects such as the location-dependent shape and size of voids are analysed by means of various microscopic methods. In particular, a method to characterize the state of damage evolution, i. e. void nucleation, growth and coalescence using scanning electron microscopy (SEM) is applied. The investigations reveal a strong dependence of the void area fraction, shape of voids and thus damage evolution on the loading mode. The microstructural analysis is complemented with FEM simulations using material models which consider the characteristics of the void evolution. © Published under licence by IOP Publishing Ltd.
    view abstract10.1088/1742-6596/896/1/012076
  • Modeling of ductile fracture from shear to balanced biaxial tension for sheet metals
    Lou, Y. and Chen, L. and Clausmeyer, T. and Tekkaya, A.E. and Yoon, J.W.
    International Journal of Solids and Structures 112 (2017)
    A ductile fracture model is proposed to describe shear fracture of sheet metals from shear to balanced biaxial tension via uniaxial and plane strain tension. The fracture criterion models plastic damage as strain-induced void nucleation, triaxiality-governed void enlargement, Lode-controlled void torsion, and shear-restrained coalescence of voids. Its flexibility is investigated by a parameter study of the ductile fracture model proposed. The fracture model is employed to describe ductile fracture behavior of an aluminum alloy AA6082 T6 (thickness: 1.0. mm). Dogbone specimens are strained to characterize the strain hardening properties, while another four different specimens are tested to characterize fracture behavior in shear, uniaxial tension, plane strain tension and balanced biaxial tension. The loading processes are analyzed numerically with the stress invariant-based Drucker yield function which is for the first time specified for body-centered cubic and face-centered cubic metals. Fracture strains in various loading conditions are measured with a hybrid experimental-numerical approach. The measured fracture strains are then used to calibrate the ductile fracture model proposed. The ductile fracture model calibrated above is employed to predict the onset of ductile fracture for these four specimens. For the purpose of comparison, the predicted fracture strokes of these four loading conditions are compared with those predicted by the modified Mohr-Coulomb model (), and two micromechanism-inspired criteria proposed recently (). The comparison reveals that the proposed model predicts the fracture behavior in much better agreement compared with experimental results from shear to the balanced biaxial tension. Accordingly, the proposed ductile fracture criterion is recommended for the prediction of ductile fracture in sheet metal forming processes, optimization of forming parameters and design of tools for both solid elements and shell elements. Besides, the ductile fracture model proposed can also be applied in various bulk metal forming processes in case that the model is calibrated by proper sets of experiments. © 2017.
    view abstract10.1016/j.ijsolstr.2016.11.034
  • Stress state dependency of unloading behavior in high strength steels
    Sumikawa, S. and Ishiwatari, A. and Hiramoto, J. and Yoshida, F. and Clausmeyer, T. and Tekkaya, A.E.
    Procedia Engineering 207 (2017)
    Accuracy of springback prediction strongly depends on whether the unloading behavior of the material is properly considered in the material model or not. It is known that the stress-strain relationship for steel sheet during unloading is nonlinear. For accurate springback prediction, the nonlinear unloading behaviors should be observed not only under uniaxial, but also under multi-axial stress state, and properly considered in FEsimulations. In this study, unloading stress-strain curves of high strength steels under four stress states: uniaxial tension, plane strain tension, biaxial tension and shear, were experimentally obtained in several material tests. From the obtained curves with different prestrains, the average elastic moduli were calculated and converted into the equivalent modulus using the isotropic Hooke's law. Moreover, the nonlinearity of the unloading stress-strain curve was evaluated by the instantaneous stress-strain slope. The average elastic modulus and the nonlinearity obviously differ by stress states, prestrains and types of steel. The investigated steels show the stress state dependency of the unloading behavior. © 2017 The Authors. Published by Elsevier Ltd.
    view abstract10.1016/j.proeng.2017.10.758
  • The reciprocal effects of bending and torsion on springback during 3D bending of profiles
    Staupendahl, D. and Tekkaya, A.E.
    Procedia Engineering 207 (2017)
    Profiles with circular cross-sections can be geometrically described by the shape of the bending line. To achieve 3D bending lines with kinematic bending processes, a continuous change of the bending plane is needed, resulting in bending force vectors that change in direction accordingly. These force vectors generate a bending moment in the forming zone and, thus, longitudinal tensile and compressive stresses. For profiles with non-circular cross-sections, the orientation of the cross-section along the bending line needs to be additionally controlled. This can be achieved by applying a specific torque to the bending process and, thus, introducing desired shear stresses into the forming zone. Up until now, this fundamental aspect of 3D profile bending has not been regarded in a coherent fashion. To take into account the reciprocal effects of the stresses applied to the forming zone and their effect on the bending moment and, thus, on springback, a comprehensive analytical process model was set up. The model is validated by experimental investigations performed using the TSS profile bending machine and comprehensive numerical investigations. Analyses were performed during plane bending as well as bending superposed with torsion. The investigations show that the applied bending force and torque not only result in stress superposition but actually also affect the development of shear strains over the cross-section of the profile. Similar to the longitudinal strains, the shear strains decrease linearly from the intrados and extrados of the profile to the neutral axis. Considering this newly observed behavior in the analytical process model, the bending moment prediction is in accordance with the experimental and numerical results. © 2017 The Authors. Published by Elsevier Ltd.
    view abstract10.1016/j.proeng.2017.10.1002
  • Thermally activated lightweight actuator based on hot extruded shape memory metal matrix composites (SMA-MMC)
    Dahnke, C. and Shapovalov, A. and Tekkaya, A.E.
    Procedia Engineering 207 (2017)
    Based on modified porthole dies, continuous composite extrusion allows the manufacturing of shape memory alloy metal matrix composites (SMA-MMC). In order to extend the functionality of SMA-MMC, the present work shows a new actuator concept based on integrated NiTi wires within an aluminum matrix. Due to an eccentric positioning of the wires as well as a prestraining, a bending moment is generated within the profile when the temperature is increased to a critical value. Depending on the process parameters, the occurring bending moment is able to cause an elastic or elastic-plastic deformation of the profile or specimen. © 2017 The Authors. Published by Elsevier Ltd.
    view abstract10.1016/j.proeng.2017.10.1083
  • Accurate springback prediction in deep drawing using pre-strain based multiple cyclic stress-strain curves in finite element simulation
    Ul Hassan, H. and Traphöner, H. and Güner, A. and Tekkaya, A.E.
    International Journal of Mechanical Sciences 110 (2016)
    The focus of this work is the accurate prediction of springback in deep drawing for DP600 material. A novel method is used for the characterization of material which leads to simultaneous generation of multiple cyclic stress-strain curves with different magnitude of plastic strains in a single experiment. An enhanced finite element simulation model is also presented which is capable of application of these multiple pre-strain based cyclic stress-strain curves in a single simulation. After each increment, the elements are grouped based on their pre-strain levels independent of the element location and assigned the relevant stress-strain curve. Simulations are performed with the Yoshida Uemori (YU) model, Chaboche-Roussilier (CR) model and an isotropic hardening model for the prediction of springback for hat geometry and tunnel geometry. The maximum deviation between the geometries of experiment and the springback simulation for hat and tunnel geometry for model with multiple cyclic stress-strain curves is 0.8 mm and 1.6 mm respectively in contrast to the deviation of 1.8 mm and 4.2 mm for the simulation model with single cyclic stress-strain curve respectively. It is shown that the simulation model with multiple cyclic stress-strain curves predicts the springback more accurately than the other models with single stress-strain curve. © 2016 Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.ijmecsci.2016.03.014
  • Additive manufacture of tools and dies for metal forming
    Hölker-Jäger, R. and Tekkaya, A.E.
    Laser Additive Manufacturing: Materials, Design, Technologies, and Applications (2016)
    In metal forming processes, additive manufacturing technologies are mostly used to manufacture dies for traditional forming processes, and not for the direct mass production of parts, since additive manufacturing processes to date are often not as fast-and thereby as time- and cost-efficient-compared with conventional manufacturing processes (machining, casting, forming). Furthermore, additive manufacturing processes are not yet as accurate as desired regarding surface quality, which means that producing a net-shape is often not possible. For that reason, in metal forming processes, additive manufacturing technologies are mostly used to manufacture dies with high geometric complexity with, for example, inner channels for a functional integration, which leads to an additional benefit for the whole process, or dies that cannot be manufactured by conventional subtractive methods because of production-related restrictions. Nevertheless, minor surface finishing often has to be applied locally using subtractive methods. According to the condition or shape of the base material and the manner of its deposition, additively manufactured dies for metal forming applications are presented in this chapter, grouped into layer-laminated tools (. Section 17.1), the powder bed (. Section 17.2), and the powder nozzle-based tools (. Section 17.3). © 2017 Elsevier Ltd. All rights reserved.
    view abstract10.1016/B978-0-08-100433-3.00017-8
  • Advancements in the manufacturing of dies for hot aluminum extrusion with conformal cooling channels
    Hölker, R. and Tekkaya, A.E.
    International Journal of Advanced Manufacturing Technology 83 (2016)
    To prevent an overheating of the workpiece material and to increase the productivity in hot aluminum extrusion, the application of extrusion dies with conformal cooling channels manufactured additively by selective laser melting is known. Since, to date, the additive manufacturing processes are often accompanied with higher manufacturing time and costs in comparison to conventional subtractive methods, a new concept for a hybrid extrusion die is presented. Here, the large volume but geometrically simple die part, the die bridge, is manufactured conventionally by subtractive methods, and the smaller part with geometrical complexity, the tip of the mandrel, is built-up on it additively by laser melting. A further novelty of the developed die is the isolated feeding of the coolant up to the target area, close to die bearings, where the cooling shall be localized. Numerical and experimental investigations revealed that the profile’s exit temperature can be reduced locally and controlled which leads only to a moderate increase of the extrusion force. The experimental results show that the hybrid tools withstand the high mechanical and thermal loads which occur during hot aluminum extrusion. © 2015, Springer-Verlag London.
    view abstract10.1007/s00170-015-7647-4
  • An analytical model to predict the shock pressure amplitude in vaporizing foils
    Cai, S. and Weddeling, C. and Tekkaya, A.E.
    Journal of Materials Processing Technology 231 (2016)
    Metal wires or foils can be vaporized when high currents are applied. The generated metal gas or plasma (at higher temperature) will expand very rapidly with high pressure. A shock wave is induced thereafter and then transmits through a polyurethane plate and finally provides the pressure pulse to the sheet metal, which results in a deformation of the sheet. In this work, an analytical model is introduced which describes the shock pressure on the sheet metal. In order to verify the analytical model, measurements of shock pressure pulses with different charging energies and polyurethane plate thicknesses are conducted. It turns out that the results predicted by the analytical model are in good agreement with the experimental data. Hence, the analytical model is successfully established for analyzing the shock pressures induced by vaporizing foils. © 2016 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.jmatprotec.2016.01.013
  • Analysis of shear cutting of dual phase steel by application of an advanced damage model
    Gutknecht, F. and Steinbach, F. and Hammer, T. and Clausmeyer, T. and Volk, W. and Tekkaya, A.E.
    Procedia Structural Integrity 2 (2016)
    Shear cutting is still the most preferred process in industry for separation of sheets. An enhanced fully-coupled Lemaitre model is applied for the description of the material behaviour. The local damage model considers the influence of shear and compression-dominated stress states on the propagation of damage. A time-efficient approach for parameter identification is used to obtain proper material parameters from different tensile and torsion tests. Shear cutting experiments for dual phase steel are performed to validate the simulation model. An accurate prediction of the cutting force is obtained with the process model. Furthermore, it is shown that the triaxiality at fracture has to be considered in combination with the predicted geometry to determine the characteristics of the cutting surface, i.e. the burnish and the fracture zone. Copyright © 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license.
    view abstract10.1016/j.prostr.2016.06.215
  • Analysis of shear cutting of dual phase steel by application of an advanced damage model
    Gutknecht, F. and Steinbach, F. and Hammer, T. and Clausmeyer, T. and Volk, W. and Tekkaya, A. E.
    21st European Conference on Fracture, (ecf21) 2 (2016)
    Shear cutting is still the most preferred process in industry for separation of sheets. An enhanced fully-coupled Lemaitre model is applied for the description of the material behaviour. The local damage model considers the influence of shear and compression-dominated stress states on the propagation of damage. A time-efficient approach for parameter identification is used to obtain proper material parameters from different tensile and torsion tests. Shear cutting experiments for dual phase steel are performed to validate the simulation model. An accurate prediction of the cutting force is obtained with the process model. Furthermore, it is shown that the triaxiality at fracture has to be considered in combination with the predicted geometry to determine the characteristics of the cutting surface, i.e. the burnish and the fracture zone. Copyright (C) 2016 The Authors. Published by Elsevier B.V.
    view abstract10.1016/j.prostr.2016.06.215
  • Analytical approach for magnetic pulse welding of sheet connections
    Hahn, M. and Weddeling, C. and Lueg-Althoff, J. and Tekkaya, A.E.
    Journal of Materials Processing Technology 230 (2016)
    An analytical model to calculate the acting forming pressure in magnetic pulse welding by determining the magnetic field strength between the flyer sheet and a one-turn coil was presented. By neglecting plastic deformation of the flyer, the model allows to calculate the transient velocity and displacement behavior, too. The electromagnetic acceleration of 5000-series aluminum alloy sheets was investigated under various experimental parameters. Utilizing Photon Doppler Velocimetry revealed that the analytical model appropriately describes the influence of current amplitude, coil geometry, and, especially, discharge frequency on the velocity-displacement curve of the flyer and hence on the impact velocity. The model introduced was applied to compute the impact velocity for the welding of long lap joints of 5000-series aluminum alloy sheets and 6000-series aluminum alloy hollow profiles. Through peel tests it was shown that the weld strength at least complied with the strength of the weaker base material as failure always happened in the flyer sheet. The wavy interface pattern typical for impact welding was identified with the help of metallography. © 2015 Elsevier B.V.
    view abstract10.1016/j.jmatprotec.2015.11.021
  • Analytical contact pressure model for predicting roughness of ball burnished surfaces
    Hiegemann, L. and Weddeling, C. and Tekkaya, A.E.
    Journal of Materials Processing Technology 232 (2016)
    Within this work, an analytical model to predict the roughness after a ball burnishing process for thermally coated surfaces is developed. For this purpose, first a model is derived which determines the contact pressure between rolling ball and workpiece. By extending a previously developed model and by using the calculated contact pressure the prediction of the roughness after a rolling process of total areas without conducting experimental preliminary tests becomes possible. The model and the assumptions are verified in experimental studies for various rolling parameters and materials. The agreement between the calculated values and the experiments is suitable. Furthermore, a discussion of the influence of the individual parameters is carried out based on the developed model. © 2016 Elsevier B.V.
    view abstract10.1016/j.jmatprotec.2016.01.024
  • Closed-loop control concept for kinematic 3D-profile bending
    Staupendahl, D. and Chatti, S. and Tekkaya, A.E.
    AIP Conference Proceedings 1769 (2016)
    Kinematic tube and profile bending processes produce bending contours by the relative movement of single process axes. Tools only need to be adapted to fit the cross-section of the tubular material. While offering a great flexibility in production, kinematic bending processes cause a high part springback and as a result, compensatory methods are needed to achieve target contours. These compensatory methods are generally embedded in bending tables or analytical calculations that in turn are embedded into the process control software. This procedure can cope with known material behavior, as for instance gained through a tensile test of the material batch prior to the bending process. Material variations inside a batch cannot be detected however and cause contour deviations. To counter this error, a closed-loop control system can be used, which can quickly adapt axes' movements to produce target shapes and thus reduce scrap. In this paper, two methods to apply closed-loop control to 3D profile bending will be presented. An indirect approach, using the bending force and torque, and a direct approach, by measuring the profile contour after bending. © 2016 Author(s).
    view abstract10.1063/1.4963542
  • Closed-loop control of product properties in metal forming
    Allwood, J.M. and Duncan, S.R. and Cao, J. and Groche, P. and Hirt, G. and Kinsey, B. and Kuboki, T. and Liewald, M. and Sterzing, A. and Tekkaya, A.E.
    CIRP Annals - Manufacturing Technology 65 (2016)
    Metal forming processes operate in conditions of uncertainty due to parameter variation and imperfect understanding. This uncertainty leads to a degradation of product properties from customer specifications, which can be reduced by the use of closed-loop control. A framework of analysis is presented for understanding closed-loop control in metal forming, allowing an assessment of current and future developments in actuators, sensors and models. This leads to a survey of current and emerging applications across a broad spectrum of metal forming processes, and a discussion of likely developments. © 2016 The Author(s)
    view abstract10.1016/j.cirp.2016.06.002
  • Concepts of the international manufacturing remote lab (MINTReLab): Combination of a MOOC and a remote lab for a manufacturing technology online course
    Ortelt, T.R. and Pekasch, S. and Lensing, K. and Gueno, P.-J. and May, D. and Tekkaya, A.E.
    IEEE Global Engineering Education Conference, EDUCON 10-13-April-2016 (2016)
    A concept for a Manufacturing Technology Online Course, which combines the MOOC approach and the use of a remote laboratory is presented. Hence, this online course is a combination of a MOOC-concept and a physically existing but tele-operatively usable experimentation laboratory for real engineering experimentation. Developing this online course can be either used as an advertising for mechanical engineering education in Germany and for the "Master of Science in Manufacturing Technology (MMT)" or as one part of the preparation process for international students coming to Dortmund in advance of their stay in Germany. For doing so the course offers not only core engineering content around the topic of manufacturing technology but also explains the German educational system at higher education institutions and presents cultural information about Germany as a place to live. © 2016 IEEE.
    view abstract10.1109/EDUCON.2016.7474612
  • Development of a cupping test in remote laboratories for engineering education
    Selvaggio, A. and Sadiki, A. and Ortelt, T.R. and Meya, R. and Becker, C. and Chatti, S. and Tekkaya, A.E.
    Proceedings of 2016 13th International Conference on Remote Engineering and Virtual Instrumentation, REV 2016 (2016)
    A remote controlled cupping test for sheet metal as material characterization for forming technology is presented. This formability test is included in a tele-operative testing cell consisting of an additional testing machine, an industrial robot, and other necessary components for the automation and execution of experiments. First, a methodology is introduced explaining how the remote cupping test is realized. Afterwards, the integration of the cupping test in a remote laboratory is presented. © 2016 IEEE.
    view abstract10.1109/REV.2016.7444451
  • Development of a tele-operative control for the incremental tube forming process and its integration into a learning environment
    Meya, R. and Ortelt, T.R. and Selvaggio, A. and Chatti, S. and Becker, C. and Tekkaya, A.E.
    IEEE Global Engineering Education Conference, EDUCON 10-13-April-2016 (2016)
    A deficient access to experimental equipment leads to the usage of remote labs to improve engineering education and open experiments for every student location - and time - independent. The usage of a tele-operative controlled industrial bending process in lecture combines theoretical learning contents with practical experiences. Lecturers can make experiments in interaction with the students, who are able to assist in choosing the process values. The chosen and presented bending process is the incremental tube forming process that uses in contrast to many ordinary bending processes targeted the superposition of stresses. By superposing of stresses, in this process for example a tube bending and a tube spinning process, several fundamental process characteristics can be observed and integrated into lectures to visualize the theoretical fundamentals behind. Incremental tube forming combines the tube spinning process, which affects the diameter of the tube all along the tube and creates a compressive stress, and a bending process. The understanding of superposition of stresses and the process phenomena are ambitious, so that experimental experience is very useful. By using a tele-operative control, the experiment is location- and time-independent available for lecturers and students all over the world. They can interact with the process like stopping it, influencing it during the process or laying it up. The possibilities for a usage in learning environments are described and pointed out. © 2016 IEEE.
    view abstract10.1109/EDUCON.2016.7474535
  • Enhanced granular medium-based tube and hollow profile press hardening
    Chen, H. and Hess, S. and Haeberle, J. and Pitikaris, S. and Born, P. and Güner, A. and Sperl, M. and Tekkaya, A.E.
    CIRP Annals - Manufacturing Technology 65 (2016)
    Active and passive control strategies of internal pressure for hot forming of tubes and hollow profiles with granular media are described. Force transmission and plastic deformation of granular medium is experimentally investigated. Friction between tube, granular medium and die, and the external stress field are shown to be essential for the process understanding. Wrinkling, thinning and insufficient forming of the tube establishes the process window for the active pressure process. By improving the punch geometry and controlling tribological conditions, the process limits are extended. Examples for the passive pressure process reveal new opportunities for hot forming of tubes and hollow profiles. © 2016 CIRP
    view abstract10.1016/j.cirp.2016.04.010
  • Evaluation of Void Nucleation and Development during Plastic Deformation of Dual-Phase Steel DP600
    Isik, K. and Gerstein, G. and Clausmeyer, T. and Nürnberger, F. and Tekkaya, A.E. and Maier, H.J.
    Steel Research International 87 (2016)
    This paper presents investigations on the characterization of ductile damage and identification of the porosity-related material model parameters in a dual-phase steel DP600. As a modeling reference for the damage evolution, a variant from the Gurson model family is taken. The micromechanical investigations related to nucleation and growth of voids have been carried out. In order to show the void-volume evolution during the deformation, post-mortem scanning electron microscope (SEM) analysis of a notched tensile test is used. Using the ion beam slope cutting methodology to prepare the specimens for SEM analysis, the microstructure can be observed in 2D including the voids. In this way, for the dual-phase steel, characteristic damage behavior upon deformation due to interaction of martensite and ferrite can be investigated. The minimum void size (areal) that can be measured is 0.05 µm2. This resolution of the measurements provides the detection of the newly nucleated voids. For the related material parameters, void-size relevant criterion is applied to determine the newly nucleated voids at a certain plastic strain. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/srin.201500483
  • Experimental and numerical analysis of tribological effective surfaces for forming tools in Sheet-Bulk Metal Forming
    Kersting, P. and Gröbel, D. and Merklein, M. and Sieczkarek, P. and Wernicke, S. and Tekkaya, A.E. and Krebs, E. and Freiburg, D. and Biermann, D. and Weikert, T. and Tremmel, S. and Stangier, D. and Tillmann, W. and Matthias, S. and Reithmeier, E. and Löffler, M. and Beyer, F. and Willner, K.
    Production Engineering 10 (2016)
    Sheet-Bulk Metal Forming (SBMF) allows the manufacture of complex parts with integrated functional form elements, such as teeth and thickened areas. Therefore, bulk forming operations are applied to sheets with initial thicknesses of 2 or 3 mm. The design and functionality of the tools are as important as the process itself. Therefore, the working group "Tools" of the Transregional Collaborative Research Centre on Sheet-Bulk Metal Forming (CRC/TR73) focuses on the optimization of the technical tool design. By varying topographies or applying tailored coatings, the friction behavior is changed to achieve a better form filling and to reduce process forces during the forming operations. In this paper, the potential of different tailored surfaces is validated by simulations and experimental studies. The tribological behavior of 14 surface microstructures is evaluated using a half-space model in order to select structures suitable for application. Those were characterized experimentally by ring-compression and pin-extrusion tests. The determined friction factors were used in a forming simulation to predict the form filling of small cavities in a flow forming operation. Furthermore, special attention is paid to the utilization of the anisotropic behavior of specific structures. The results were validated by an incremental gear forming process. © 2016, German Academic Society for Production Engineering (WGP).
    view abstract10.1007/s11740-015-0651-6
  • Experimental and numerical investigation of increased formability in combined quasi-static and high-speed forming processes
    Kiliclar, Y. and Demir, O.K. and Engelhardt, M. and Rozgić, M. and Vladimirov, I.N. and Wulfinghoff, S. and Weddeling, C. and Gies, S. and Klose, C. and Reese, S. and Tekkaya, A.E. and Maier, H.J. and Stiemer, M.
    Journal of Materials Processing Technology 237 (2016)
    The formability of deep drawing can be extended by combining it with a subsequent high-speed forming method such as electromagnetic forming. However, up to now, no sufficient systematic understanding of the underlying principles or of a successful design of such coupled processes has been gained. Hence, in this work, a methodology for the analysis and design of such process chains is presented. This approach comprises a new method for the experimentally based determination of quasi-static and high-speed forming limits along close to proportional strain paths, a constitutive visco-plastic, anisotropic material model with a rate dependent ductile damage formulation, which allows for the accurate numerical prediction of forming limits for complicated forming operations under a largely varying strain rate, and finally the actual application of both to a combined quasi-static and high-speed forming operation. In doing so, material areas are identified that are deep drawn up to a degree immediately before necking occurs and then electromagnetically be formed beyond the quasi-static forming limit without damage. This proves that an extension of formability is here achieved due to a change in strain rate rather than in the strain path. © 2016 Elsevier B.V.
    view abstract10.1016/j.jmatprotec.2016.06.007
  • Failure by fracture in sheet-bulk metal forming
    Isik, K. and Wernicke, S. and Silva, M.B. and Martins, P.A.F. and Tekkaya, A.E.
    Journal of Strain Analysis for Engineering Design 51 (2016)
    This article investigates the possibility of failure by crack-opening mode III (out-of-plane shearing) in sheet-bulk metal forming processes. The investigation makes use of experimentally and theoretically determined fracture-forming limits of aluminium AA1050-H111 sheets with 1 mm thickness, experimental tests in incremental ploughing with a roll-tipped tool and numerical simulation using a commercial finite element programme. Results show that incremental ploughing of thin sheets with a roll-tipped tool under large indentation depths gives rise to transverse cracks that are triggered at the upper groove surface and propagate downward across thickness along an inclined direction to the sheet surface. In contrast to sheet-metal forming processes that only fail by fracture in crack-opening modes I and II, sheet-bulk metal forming processes present the unique ability of failing in all three possible crack-opening modes, namely, in mode III that is typical of bulk metal-forming processes. © Institution of Mechanical Engineers.
    view abstract10.1177/0309324716639773
  • Fracture toughness and failure limits in sheet metal forming
    Silva, M.B. and Isik, K. and Tekkaya, A.E. and Atkins, A.G. and Martins, P.A.F.
    Journal of Materials Processing Technology 234 (2016)
    This paper proposes a link between plastic flow, void coalescence and growth, ductile damage, crack opening modes and fracture toughness in sheet metal forming. This new integrated view is based on an analytical framework that allows estimating the location of the fracture loci in the principal strain space directly from material stress-strain response and from fracture toughness and thickness at fracture obtained from double-notched test specimens loaded in tension and plane torsion (in-plane shear). Experiments in AA1050-H111 aluminium sheets with 1 mm thickness give support to the proposed analytical framework. © 2016 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.jmatprotec.2016.03.029
  • Granular media-based tube press hardening
    Chen, H. and Güner, A. and Ben Khalifa, N. and Tekkaya, A.E.
    Journal of Materials Processing Technology 228 (2016)
    Press hardening process can benefit the formability of 22MnB5 in high temperature and high strength as a final product. It is widely used for weight reduction of car body without sacrifice of its crashworthiness. Nevertheless, not only strength but also stiffness is important for some vehicle components. Press hardening of tube using granular media is the possible technology to realize the press hardening process for tubular components, which have much higher stiffness as compared to sheet metal parts. To choose appropriate granular media, instrumented die compaction test and high pressure direct shear test were established to characterize the material property of granular material. A hot tensile test was used to determine the formability of 22MnB5 tube material. Interaction between granular media and tube material including friction coefficient and heat transfer coefficient was measured by shear test and heat transfer test. Based on these works, a thermal-mechanical coupled finite element model was used to analyses the process. In validation experiment, a T-shape specimen was formed and quenched. Process parameters such as loading force, interfacial friction, and tube geometry were also investigated via numerical and experimental research for a better understanding of the process. The interfacial friction between granular media and tube showed significant effects to the forming result. These effects were represented by process parameters such as friction coefficient, tube length, types of granular media. A multi-type granular media brought out higher pressure transfer effect and also reduced interfacial friction force, which showed better formability. © 2015 Elsevier B.V.
    view abstract10.1016/j.jmatprotec.2015.03.028
  • Incipient and repeatable plastic flow in incremental sheet-bulk forming of gears
    Sieczkarek, P. and Wernicke, S. and Gies, S. and Martins, P.A.F. and Tekkaya, A.E.
    International Journal of Advanced Manufacturing Technology 86 (2016)
    This paper analyses the differences between incipient and repeatable material flow in the incremental sheet-bulk metal forming (SBMF) of gears produced by indentation along the direction perpendicular to the sheet thickness. The underfilling of the punch cavity during the first indentation, which prevents the production of sound disk gears, is explained on the basis of constrained material flow under material strain hardening. A solution based on the utilization of a tailored disk blank is proposed to overcome this defect. The geometry of the tailored disk blank is determined by means of finite element analysis, and the overall methodology involved material characterization and experimentation with DC04 mild steel. The discussion on the extent of the plastic deformation region under constrained and free-material flow during indentation is complemented by experimental results obtained with a flat punch in rectangular sheet blanks of aluminium EN AW-1050A. © 2016, Springer-Verlag London.
    view abstract10.1007/s00170-016-8442-6
  • Integration of new concepts and features into forming technology lectures
    Ortelt, T.R. and Gies, S. and Traphoner, H. and Chatti, S. and Tekkaya, A.E.
    IEEE Global Engineering Education Conference, EDUCON 10-13-April-2016 (2016)
    New concepts and features are shown for forming technology lectures. On the one hand, technological and electronic developments as audience and response system and the integration of a remote lab is presented. On the other hand, didactical approaches focus on the aim to combine theory with practical relevance. The implemented developments were evaluated by the students and the influence to their grade points in exams are presented. Additionally, new concepts in the current winter semester 2015/2016 are shown. © 2016 IEEE.
    view abstract10.1109/EDUCON.2016.7474606
  • Internationalization as a topic in higher engineering education: A quantitative content analysis examining the engineering curricula from ten German technical universities
    May, D. and Lensing, K. and Tekkaya, A.E.
    IEEE Global Engineering Education Conference, EDUCON 10-13-April-2016 (2016)
    Within the last years the demand for intercultural awareness and intercultural competences among the working force in engineering grows significantly. Even if engineers more than other professions have always been working in international settings, the situations of international collaboration in engineering work increased. Moreover, strategy papers on European and national levels endorse the rising demand by industry for such graduates and claim development of internationalization within the German landscape of higher education institutions (HEIs) for several years now. However, if these topics are important for successful engineering careers, the question is "If at all, in how far are these topics addressed in current engineering curricula?". In order to answer this question mechanical engineering curricula of 10 German technical universities have been examined by performing a quantitative content analysis and searching for respective keywords. The underlying assumption is, that if these topics are addressed in engineering study programs this must be visible in the official documents, too. The results of the conducted content analysis presented in this paper are in a way disappointing. Based on the found keywords, the amount of modules that tackle international awareness and international topics in engineering is by far smaller than anticipated. It is in particular even smaller than the significance of these topics in business may suggest. Hence, from our perspective this area still offers substantial untapped potential. © 2016 IEEE.
    view abstract10.1109/EDUCON.2016.7474544
  • Investigations of ductile damage during the process chains of toothed functional components manufactured by sheet-bulk metal forming
    Isik, K. and Gerstein, G. and Schneider, T. and Schulte, R. and Rosenbusch, D. and Clausmeyer, T. and Nürnberger, F. and Vucetic, M. and Koch, S. and Hübner, S. and Behrens, B.-A. and Tekkaya, A.E. and Merklein, M.
    Production Engineering 10 (2016)
    Sheet-bulk metal forming processes combine conventional sheet forming processes with bulk forming of sheet semi-finished parts. In these processes the sheets undergo complex forming histories. Due to in- and out-of-plane material flow and large accumulated plastic strains, the conventional failure prediction methods for sheet metal forming such as forming limit curve fall short. As a remedy, damage models can be applied to model damage evolution during those processes. In this study, damage evolution during the production of two different toothed components from DC04 steel is investigated. In both setups, a deep drawn cup is upset to form a circumferential gearing. However, the two final products have different dimensions and forming histories. Due to combined deep drawing and upsetting processes, the material flow on the cup walls is three-dimensional and non-proportional. In this study, the numerical and experimental investigations for those parts are presented and compared. Damage evolution in the process chains is simulated with a Lemaitre damage criterion. Microstructural analysis by scanning electron microscopy is performed in the regions with high mechanical loading. It is observed that the evolution of voids in terms of void volume fraction is strongly dependent on the deformation path. The comparison of simulation results with microstructural data shows that the void volume fraction decreases in the upsetting stage after an initial increase in the drawing stage. Moreover, the concurrent numerical and microstructural analysis provides evidence that the void volume fraction decreases during compression in sheet-bulk metal forming. © 2016, German Academic Society for Production Engineering (WGP).
    view abstract10.1007/s11740-016-0656-9
  • Investigations of ductile damage in DP600 and DC04 deep drawing steel sheets during punching
    Isik, K. and Gerstein, G. and Gutknecht, F. and Clausmeyer, T. and Nürnberger, F. and Maier, H.J. and Tekkaya, A.E.
    Procedia Structural Integrity 2 (2016)
    The paper presents numerical and microstructural investigations on a punching process of 2 mm thick steel sheets. The dual phase steel DP600 and the mild steel DC04 exhibit different damage and fracture characteristics. To distinguish the void development and crack initiation for both materials, interrupted tests at varied punch displacements are analyzed. The void volume fractions in the shearing zone are identified by scanning electron microscopy (SEM). The Gurson model family, which is recently extended for shear fracture, is utilized to model the elastoplastic behavior with ductile damage. The effect of the shear governing void growth parameter, introduced by Nahshon and Hutchinson (2008), is discussed. © 2016 The Authors. Published by Elsevier B.V.
    view abstract10.1016/j.prostr.2016.06.087
  • Investigations of ductile damage in DP600 and DC04 deep drawing steel sheets during punching
    Isik, K. and Gerstein, G. and Gutknecht, F. and Clausmeyer, T. and Nurnberger, F. and Maier, H. J. and Tekkaya, A. E.
    21st European Conference on Fracture, (ecf21) 2 (2016)
    The paper presents numerical and microstructural investigations on a punching process of 2 mm thick steel sheets. The dual phase steel DP600 and the mild steel DC04 exhibit different damage and fracture characteristics. To distinguish the void development and crack initiation for both materials, interrupted tests at varied punch displacements are analyzed. The void volume fractions in the shearing zone are identified by scanning electron microscopy (SEM). The Gurson model family, which is recently extended for shear fracture, is utilized to model the elastoplastic behavior with ductile damage. The effect of the shear governing void growth parameter, introduced by Nahshon and Hutchinson (2008), is discussed. Copyright (C) 2016 The Authors. Published by Elsevier B.V.
    view abstract10.1016/j.prostr.2016.06.087
  • Live demo of two experiments using a remote lab for forming technology
    Ortelt, T.R. and Meya, R. and Selvaggio, A. and Chatti, S. and Tekkaya, A.E.
    Proceedings of 2016 13th International Conference on Remote Engineering and Virtual Instrumentation, REV 2016 (2016)
    In engineering courses, it is more and more important to combine theory with practice. Therefore, at the Institute of Forming Technology and Lightweight Construction of TU Dortmund University a tele-operative testing cell for material characterization was developed. This remote lab was successfully integrated to different scenarios. On the one hand, the remote lab was integrated to forming technology lectures. Thereby the lecturer configures the experiment in interaction with the students in the lecture hall. After the experiment is finished the results can be discussed face to face. On the other hand, the tele-operative testing cell becomes a part of different online learning scenarios like online courses. With this remote lab lectures and students are able to do experiments of forming technology via internet. In the following the design of the remote lab and two experiments for a live demo are described. © 2016 IEEE.
    view abstract10.1109/REV.2016.7444494
  • Local sheet thickening by in-plane swaging
    Wernicke, S. and Sieczkarek, P. and Martins, P.A.F. and Tekkaya, A.E.
    International Journal of Mechanical Sciences 119 (2016)
    This paper presents a new sheet-bulk forming process to locally pile-up material in thin sheets for subsequent forming or joining operations. An analytical solution is proposed for explaining the influence of the major process parameters, estimating the thickening of the pile-up material and determining the normal and thrust forces applied by the tools. The approach is built upon the slip-line field theory under plane strain assumptions and results are compared against finite element predictions and experimental results using Aluminium EN AW-1050A (EN 573-3) sheets with 3 mm thickness. The last part of the paper introduces a modified tool geometry that is able to control the pile-up material for subsequent mechanical fastening operations. © 2016 Elsevier Ltd
    view abstract10.1016/j.ijmecsci.2016.10.003
  • Manufacturing of functional elements by sheet-bulk metal forming processes
    Gröbel, D. and Schulte, R. and Hildenbrand, P. and Lechner, M. and Engel, U. and Sieczkarek, P. and Wernicke, S. and Gies, S. and Tekkaya, A.E. and Behrens, B.A. and Hübner, S. and Vucetic, M. and Koch, S. and Merklein, M.
    Production Engineering 10 (2016)
    Due to increasing economic and ecological restrictions, conventional sheet and bulk forming operations often reach their limits with regard to part weight and functional integration. One solution to meet those challenges is provided by sheet-bulk metal forming (SBMF) processes. SBMF is defined as the application of bulk forming operations on sheet metal. SBMF can be combined with conventional sheet forming operations and offers the opportunity to form highly functional integrated parts out of sheet metal. It contains the benefit of an optimization of the part weight and a shortening of the process chain. Recent research has found different solutions regarding the actual implementation of SBMF in several process variants. In this paper, a categorisation for functional elements on sheet metal parts is proposed. A selection of possible approaches for their manufacturing is presented. The process variants are compared by means of the main process characteristics. By these means, the choice of a suitable option shall be facilitated for practical manufacturing design and for a particular relevant product. © 2016, German Academic Society for Production Engineering (WGP).
    view abstract10.1007/s11740-016-0662-y
  • Measurement and analysis technologies for magnetic pulse welding: established methods and new strategies
    Bellmann, J. and Lueg-Althoff, J. and Schulze, S. and Gies, S. and Beyer, E. and Tekkaya, A.E.
    Advances in Manufacturing 4 (2016)
    Magnetic pulse welding (MPW) is a fast and clean joining technique that offers the possibility to weld dissimilar metals, e.g., aluminum and steel. The high-speed collision of the joining partners is used to generate strong atomic bonded areas. Critical brittle intermetallic phases can be avoided due to the absence of external heat. These features attract the notice of industries performing large scale productions of dissimilar metal joints, like automotive and plant engineering. The most important issue is to guarantee a proper weld quality. Numerical simulations are often used to predict the welding result a priori. Nevertheless, experiments and the measurement of process parameters are needed for the validation of these data. Sensors nearby the joining zone are exposed to high pressures and intense magnetic fields which hinder the evaluation of the electrical output signals. In this paper, existing analysis tools for process development and quality assurance in MPW are reviewed. New methods for the process monitoring and weld characterization during and after MPW are introduced, which help to overcome the mentioned drawbacks of established technologies. These methods are based on optical and mechanical measuring technologies taking advantage of the hypervelocity impact flash, the impact pressure and the deformation necessary for the weld formation. © 2016, Shanghai University and Springer-Verlag Berlin Heidelberg.
    view abstract10.1007/s40436-016-0162-5
  • Multiple forming tools in incremental forming - Influence of the forming strategies on sheet contour
    Dang, T. and Tebaay, L.M. and Gies, S. and Tekkaya, A.E.
    AIP Conference Proceedings 1769 (2016)
    Single point incremental forming (SPIF) is a well known process which is used for rapid prototyping or for small-quantity production. The feature of this process is the flexible manufacturing of complex hollow shapes with the use of basic equipments. However, this forming process takes very long time. To speed up the process time, multiple forming tools can be used simultaneously. This paper presents the influence of the multiple tools in SPIF on the formed shape. The conventional SPIF with a single tool is taken into account for a comparative analysis. The results in this study showed that the tool arrangements and its distance have a significant effect on the geometrical accuracy. Moreover, it is shown the influence between the vertical step size of the tool and the strain distributions. This knowledge can be used for generation of new forming strategies. © 2016 Author(s).
    view abstract10.1063/1.4963462
  • New process for flexible manufacturing of bent parts with variable arbitrary cross section
    Becker, C. and Grzancic, G. and Chatti, S. and Tekkaya, A.E.
    AIP Conference Proceedings 1769 (2016)
    In accordance to the increasing demands in profile manufacturing the general profile geometries become more and more complex. The complexity is basically characterized by varying cross section geometry along the profile axis as well as a longitudinal curvature. A new process for the flexible manufacturing of curved profiles with variable cross section geometries along the longitudinal axis is introduced. The process is a combination of the incremental profile forming process and a free form bending operation and has a high potential for lightweight construction and energy saving. Beside the description of the process principle and possible machine setups, the potentials of the process are discussed in this article. © 2016 Author(s).
    view abstract10.1063/1.4963541
  • Reducing the stair step effect of layer manufactured surfaces by ball burnishing
    Hiegemann, L. and Agarwal, C. and Weddeling, C. and Tekkaya, A.E.
    AIP Conference Proceedings 1769 (2016)
    The layer technology enables fast and flexible additive manufacturing of forming tools. The disadvantages of this system is the formation of stair steps in the range of tool radii. Within this work a new method to smooth this stair steps by ball burnishing is introduced. This includes studies on the general feasibility of the process and the determination of the influence of the rolling parameters. The investigations are carried out experimentally and numerically. Ultimately, the gained knowledge is applied to finish a deep drawing tool which is manufactured by layer technology. © 2016 Author(s).
    view abstract10.1063/1.4963612
  • Remote labs in ELLI: Lab experience for every student with two different approaches
    Kruse, D. and Frerich, S. and Petermann, M. and Ortelt, T.R. and Tekkaya, A.E.
    IEEE Global Engineering Education Conference, EDUCON 10-13-April-2016 (2016)
    With the goal of extending the possibilities for practical experience in engineering tasks, the project ELLI (Excellent Teaching and Learning in Engineering Sciences) follows the idea of providing virtual and remote labs for students, teachers and even interested people from different disciplines, countries or skill levels. To integrate these labs into the existing curriculum and to get students and teachers used to this tool are challenging processes. Moreover, the technical development of the remote labs is as much important as the integration to an e-learning environment. Two different approaches for the development and the integration are described. © 2016 IEEE.
    view abstract10.1109/EDUCON.2016.7474595
  • Setting mechanical properties of high strength steels for rapid hot forming processes
    Löbbe, C. and Hering, O. and Hiegemann, L. and Tekkaya, A.E.
    Materials 9 (2016)
    Hot stamping of sheet metal is an established method for the manufacturing of light weight products with tailored properties. However, the generally-applied continuous roller furnace manifests two crucial disadvantages: The overall process time is long and a local setting of mechanical properties is only feasible through special cooling techniques. Hot forming with rapid heating directly before shaping is a new approach, which not only reduces the thermal intervention in the zones of critical formability and requested properties, but also allows the processing of an advantageous microstructure characterized by less grain growth, additional fractions (e.g., retained austenite), and undissolved carbides. Since the austenitization and homogenization process is strongly dependent on the microstructure constitution, the general applicability for the process relevant parameters is unknown. Thus, different austenitization parameters are analyzed for the conventional high strength steels 22MnB5, Docol 1400M, and DP1000 in respect of the mechanical properties. In order to characterize the resulting microstructure, the light optical and scanning electron microscopy, micro and macro hardness measurements, and the X-ray diffraction are conducted subsequent to tensile tests. The investigation proves not only the feasibility to adjust the strength and ductility flexibly, unique microstructures are also observed and the governing mechanisms are clarified. © 2016 by the authors.
    view abstract10.3390/ma9040229
  • Simulation of sheet-bulk metal forming processes with simufact.forming using user-subroutines
    Beese, S. and Beyer, F. and Blum, H. and Isik, K. and Kumor, D. and Löhnert, S. and Rademacher, A. and Tekkaya, A.E. and Willner, K. and Wriggers, P. and Zeller, S.
    AIP Conference Proceedings 1769 (2016)
    In this article, we consider the simulation of sheet-bulk metal forming processes, which makes high demands on the underlying models and on the simulation software. We present our approach to incorporate new modelling approaches from various fields in a commercial simulation software, in our case Simufact.forming. Here, we discuss material, damage, and friction models as well as model adaptive techniques. © 2016 Author(s).
    view abstract10.1063/1.4963457
  • Springback prediction and reduction in deep drawing under influence of unloading modulus degradation
    ul Hassan, H. and Maqbool, F. and Güner, A. and Hartmaier, A. and Ben Khalifa, N. and Tekkaya, A.E.
    International Journal of Material Forming 9 (2016)
    Springback is considered as one of the major problems in deep drawing of high-strength steels (HSS) and advanced high-strength steels (AHSS) which occurs during the unloading of part from the tools. With an ever increasing demand on the automotive manufactures for the production of lightweight automobile structures and increased crash performance, the use of HSS and AHSS is becoming extensive. For the accurate prediction of springback, unloading behavior of dual phase steels DP600, DP1000 and cold rolled steel DC04 for the deep drawing process is investigated and a strategy for the reduction of springback based on variable blankholder force is also presented. Cyclic tension compression tests and LS-Opt software are used for the identification of material parameters for Yoshida-Uemori (YU) model. Degradation of the Young’s modulus is found to be 28 and 26 and 14 % from the initial Young’s modulus for DP600, DP1000 and for the DC04 respectively for the saturated value. A finite element model is generated in LS-DYNA based on the kinematic hardening material model, namely Yoshida-Uemori (YU) model. The validation of numerical simulations is also carried out by the real deep drawing experiments. The springback could be predicted with the maximum deviation of 1.1 mm for these materials. For DP1000, the maximum springback is reduced by 24.5 %, for DP600 33.3 and 48.7 % for DC04 by the application of monotonic blankholder force instead of a constant blankholder force of 80 kN. It is concluded that despite the reduction of Young’s modulus, the springback can be reduced for these materials by increasing the blankholder force only in last 13 % of the punch travel. © 2015, Springer-Verlag France.
    view abstract10.1007/s12289-015-1248-5
  • The evaluation of remote laboratories: Development and application of a holistic model for the evaluation of online remote laboratories in manufacturing technology education
    May, D. and Terkowsky, C. and Ortelt, T.R. and Tekkaya, A.E.
    Proceedings of 2016 13th International Conference on Remote Engineering and Virtual Instrumentation, REV 2016 (2016)
    The integration of online remote laboratories is still an emerging field in engineering education, especially in the area of manufacturing technology. Over the last years and in different project contexts the Institute of Forming Technology and Lightweight Construction (IUL) and the Center for Higher Education (zhb) at TU Dortmund University developed a laboratory environment, which gives the opportunity to the students to do experiments like the tensile tests-a core experiment for defining material properties-from the computer at home using online technology. This system already has been used in different teaching contexts and its usage is now expanded step by step to other courses. Hence, its practice-based evaluation is coming more and more into focus in order to improve the technical equipment as well as its imbedding into the educational settings. This means that not only the technology and its functionality are evaluated but also a special focus has to be put on the student-computer interaction. Therefore a holistic model for evaluating the system and its usage has been developed. This model divides into three different perspectives for evaluation: (1) The individual perspective focusing the user's learning process in the laboratory environment, (2) the system-perspective focusing the technical equipment, and finally (3) the course perspective focusing the lab's integration into the course context. This evaluation model was inspired by several other evaluation approaches existing in literature. The aim was to work out both, a model that serves as a fitting evaluation process for the explicit context existing at TU Dortmund University and at the same time as an adequate approach for other remote laboratory contexts. This paper presents the evaluation model with its perspectives as well as the used questionnaires and its first application in context of an international online course making use of the IUL's remote lab. © 2016 IEEE.
    view abstract10.1109/REV.2016.7444453
  • Transnational Connected Learning and Experimentation Using Live Online Classes and Remote Labs for Preparing International Engineering Students for an International Working World
    May, D. and Tekkaya, A. E.
    International Journal of Engineering Pedagogy 6 (2016)
    Students, who are leaving their home country for taking part in an international study program, face several challenges. Not only the new course of studies can be very challenging but also their whole living conditions may change significantly. This can be a severe clash especially for students who are moving to a country with a totally different cultural background in comparison to their home countries. Moreover, it can profoundly complicate the first weeks at the new university. Knowing about the difficulties the Institute of Forming Technology and Lightweight Construction (IUL) at TU Dortmund University in Germany developed a preparational online course for those international students, who are coming to the IUL for their Master of Science program in Manufacturing Technology (MMT; a special international master program). In context of this course the use of the IUL's remote laboratory equipment was a key aspect. The course itself was implemented and delivered for the first time in 2014. By now a second updated edition was delivered in 2015. It was designed to prepare the students as best as possible for their new studies at a German university and at the same time prepare them for transnational collaboration. Hence, this course is a good example for a meaningful integration of remote laboratories into an innovative online course concept. On the one hand making use of remote laboratories and its practical integration in online courses helps to connect the international students and on the other hand it brings them into the situation to interact in context of a typical engineering situation, the experiment. The paper presents the course itself and experiences from its first and second implementation.
    view abstract10.3991/ijep.v6i1.5287
  • Vaporizing foil actuator welding as a competing technology to magnetic pulse welding
    Hahn, M. and Weddeling, C. and Taber, G. and Vivek, A. and Daehn, G.S. and Tekkaya, A.E.
    Journal of Materials Processing Technology 230 (2016)
    Photonic Doppler velocimetry was applied to compare magnetic pulse welding and vaporizing foil actuator welding against each other in the form of lap joints made of 5000 series aluminum alloy sheets under identical experimental conditions which are: charging energies of the pulse generator, specimen geometry, initial distances between flyer and target plate. Impact velocities resulting from rapidly vaporizing aluminum foils were up to three times higher than those of purely electromagnetically accelerated flyer plates. No magnetic pulse welds were achieved, while every vaporizing foil experiment yielded a strong weld in that failure always occurred in the joining partners instead of in the weld seam during tensile tests. An analytical model to calculate the transient flyer velocity is presented and compared to the measurements. The average deviation between model and experiment is about 11% with regard to the impact velocity. Hence, the model may be used for the process design of collision welds generated by vaporizing foil actuators. © 2015 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.jmatprotec.2015.11.010
  • Wear behavior of tribologically optimized tool surfaces for incremental forming processes
    Sieczkarek, P. and Wernicke, S. and Gies, S. and Tekkaya, A.E. and Krebs, E. and Wiederkehr, P. and Biermann, D. and Tillmann, W. and Stangier, D.
    Tribology International 104 (2016)
    The mechanical wear behavior of forming tools is the limiting factor during an incremental gear-forming process. These forming tools with a simply shaped geometry are exposed to high forming forces. Additionally, the necessary workpiece chambering, which is characteristic for this incremental process restricts the dimensioning of the tools. Thereby, the geometrical design of the forming tools is limited, which leads to a decreased lifetime. Functional structures on the tool surfaces can influence the occurring loading and wear behavior by reducing the contact area, the supply of lubricant pockets, and by a controlled influence and adjustment of the occurring material flow. For the extension of the tool's lifetime, different surface concepts and combinations with CrAlN PVD-coatings are investigated. To offer conditions with a high tool load, the investigations are focused on an incremental gear forming process with a simple one-wedge forming tool. The results show abrasive and adhesive wear characteristics, as well as outbreaks, and crack formations. The crack propagation on the flank leads to a chipping of the tool tip, hence limiting the tool life. Compared to the reference tool, a surface structure combined with a PVD-coating provides a significant increase of the tool life of 84%. © 2016 Elsevier Ltd
    view abstract10.1016/j.triboint.2016.08.028
  • Writing for the benefit of the reader
    Allwood, J.M. and Tekkaya, A.E.
    Journal of Materials Processing Technology 227 (2016)
    view abstract10.1016/j.jmatprotec.2015.09.022
  • 60 Excellent inventions in metal forming
    Tekkaya, A.E. and Homberg, W. and Brosius, A.
    60 Excellent Inventions in Metal Forming (2015)
    60 novel approaches in metal forming are presented and explained in detail. Contributions from acknowledged international scientists representing the state-of-art in metal forming open a general view on recent results and a clear view on demands for new research initiatives. © Springer-Verlag Berlin Heidelberg 2015.
    view abstract10.1007/978-3-662-46312-3
  • A grooved in-plane torsion test for the investigation of shear fracture in sheet materials
    Yin, Q. and Soyarslan, C. and Isik, K. and Tekkaya, A.E.
    International Journal of Solids and Structures 66 (2015)
    Abstract The grooved in-plane torsion test is proposed as a shear fracture test for sheet materials. Unlike conventional simple shear tests, which are prone to incipient cracking at the free edges, this test uses radially continuous specimens, as firstly introduced by Marciniak and Kołodziejski (1972). In order to control the fracture position, a radial groove is cut out which allows to keep the fracture away from the clamping area. Thus, this test is able to create material fracture under ideal shear conditions i.e., the condition of vanishing triaxiality at the observable region of the test. Accordingly, the recent shear extended damage and fracture models for the selected material classes can be validated and/or quantified. With the help of finite element analysis (FEA), the corresponding fracture strains for the steel DP1000 were investigated using the proposed shear test and, additionally, three tensile tests conducted on notched specimens which cause fracture at moderate to high triaxialities. These are used to fit the fracture loci of some shear enhanced fracture criteria which have recently been proposed in the literature. The FEA shows that the proposed test provides fracture development under constantly zero triaxiality and zero Lode parameter conditions. Moreover, among the selected criteria, the model proposed by Lou et al. (2012) delivers the best results for selected experimental set. The developed test is ideally suitable for fracture parameter identification of sheet materials which do not show pronounced in-plane anisotropy, e.g. dual phase steels. Furthermore, this test is not limited to metallic materials. © 2015 Elsevier Ltd.
    view abstract10.1016/j.ijsolstr.2015.03.032
  • A new test for determining fracture toughness in plane stress in mode II
    Kerim, I. and Silva, M.B. and Atkins, A.G. and Tekkaya, A.E. and Martins, P.A.F.
    Journal of Strain Analysis for Engineering Design 50 (2015)
    This article presents a new experimental test for determining fracture toughness, in plane stress, in crack opening mode II based on the utilization of double-notched circular test specimens loaded in plane torsion. The proposed methodology for determining fracture toughness involves characterization of the evolution of torque with the degree of rotation for a number of test cases performed with specimens having different lengths of the ligaments between the notches. The work is supported by measurement of the in-plane and gauge length strains in aluminium AA1050-H111, and the overall experimental results show that the new proposed test provides an easy and effective way of evaluating the ability of a sheet metal to resist cracking under in-plane shear loading conditions. © IMechE 2015.
    view abstract10.1177/0309324715576429
  • Analytical methodology for the process design of electromagnetic crimping
    Weddeling, C. and Demir, O. K. and Haupt, P. and Tekkaya, A. E.
    Journal of Materials Processing Technology 222 (2015)
    Modern lightweight design concepts, like the space frame design and multi-material structures, have complex demands on joining technologies, and conventional processes are often pushed to their technological limits. An interesting alternative for connecting extruded aluminum profiles without heating or penetration is joining by electromagnetic crimping. Compared to adhesive bonding and welding, the process also requires a less extensive joining zone preparation. However, existing process design methodologies require either extensive experimental studies or sophisticated numerical modeling. Therefore, an analytical approach for the determination of process parameters, like the applied charging energy, is presented in this article. Besides groove geometry and workpiece properties, the model also considers the electrical characteristics of the electromagnetic forming equipment. Experimental studies of the joining process are performed to verify the developed model. Additionally, the electromagnetic crimping operation is numerically modeled for a more detailed analysis of this process. (C) 2015 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.jmatprotec.2015.02.042
  • Analytical model to determine the strength of form-fit connection joined by die-less hydroforming
    Weddeling, C. and Gies, S. and Khalifa, N.B. and Tekkaya, A.E.
    Journal of Manufacturing Science and Engineering, Transactions of the ASME 137 (2015)
    Modern lightweight concept structures are increasingly composed of several dissimilar materials. Due to the different material properties of the joining partners, conventional and widely used joining techniques often reach their technological limits when applied in the manufacturing of such multimaterial structures. This leads to an increasing demand for appropriate joining technologies, like joining by die-less hydroforming (DHF) for connecting tubular workpieces. The present work introduces an analytical model to determine the achievable strength of form-fit connections. This approach, taking into account the material parameters as well as the groove and tube geometry, is based on a membrane analysis assuming constant wall thicknesses. Besides a fundamental understanding of the load transfer mechanism, this analytic approach allows a reliable joining zone design. To validate the model, experimental investigations using aluminum specimens are performed. A mean deviation between the calculated and the measured joint strength of about 19% was found. This denotes a good suitability of the analytical approach for the design process of the joining zone. © 2015 by ASME.
    view abstract10.1115/1.4030878
  • Approaches for the Simulation of Composite Extrusion - Possibilities and Limits
    Schwane, M. and Kloppenborg, T. and Haase, M. and Khalifa, N.B. and Tekkaya, A.E.
    Materials Today: Proceedings 2 (2015)
    During continuous composite extrusion, endless elements, such as steel wires, are embedded in the aluminum base material. Due to the complex material flow conditions, the design of composite extrusion dies is challenging. Therefore, numerical simulations are applied in order to determine appropriate die geometries and process parameters. In this paper, an overview of different modeling approaches for the simulation of continuous composite extrusion, which have been developed at the Institute of Forming Technology and Lightweight Construction (IUL) in recent years, is presented. In particular, fundamental differences of the methods are related to the utilized finite element approaches, i.e. the Eulerian formulation and the Lagrangian formulation. Furthermore, it is discussed whether it is necessary to consider the embedded elements physically in the simulation model. The advantages, drawbacks, and limits of the different approaches are presented, in particular with regard to material flow, reinforcing element position in the profile, and load on reinforcing elements. Furthermore, the simulation results are compared with experiments. © 2015 Elsevier Ltd.
    view abstract10.1016/j.matpr.2015.10.011
  • Basic investigations in incremental profile forming
    Grzancic, G. and Becker, C. and Khalifa, N.B. and Tekkaya, A.E.
    ASME 2015 International Manufacturing Science and Engineering Conference, MSEC 2015 1 (2015)
    Increasing demands in profile manufacturing lead to a constant increase of the geometric complexity of the profile. Especially in times where lightweight design and load adaption are of huge importance, a need for new profile manufacturing technologies exists. In order to cope with this development a new profile forming method was invented, the Incremental Profile Forming (IPF). IPF allows the flexible manufacturing of profiles with varying cross section geometries along the longitudinal profile axis and offers therefore high potential for the manufacturing of lightweight design parts. Due to the high degree of innovation research work regarding the process fundamentals as well as the process limits is necessary. For this reason, the results of the first basic experimental and numerical investigations are presented. While in the experiments the potential of the process is shown in feasibility studies, first analysis of the forming process as well as the influence of diverse process parameters on the process was carried out in numerical investigations. Finally first analytical approaches for predicting the forming behavior are presented. Copyright © 2015 by ASME.
    view abstract10.1115/MSEC20159406
  • Closed loop springback control in progressive die bending by induction heating
    Löbbe, C. and Hoppe, C. and Becker, C. and Tekkaya, A.E.
    International Journal of Precision Engineering and Manufacturing 16 (2015)
    Large springback of high strength steels in progressive dies hinders accurate manufacturing because a proper prediction of elastic unloading is not possible due to material variations, tool wear, and varying ambient conditions. In order to compensate springback and enhance the forming limit when brittle materials are bent, a warm bending technology for progressive dies through inline induction heating is developed. Within a certain temperature range, there is a linear relation to the springback angle, which allows a direct influence on the final bending angle. Based on this principle, a closed loop control with a feedback of the angle after unloading is implemented, which adjusts the sheet temperature before bending. With the developed discrete controller, finally a stable and fast control mode is achieved so that an initial deviation from the target value is reduced to less than Δθ = ±0.30°. © 2015, Korean Society for Precision Engineering and Springer-Verlag Berlin Heidelberg.
    view abstract10.1007/s12541-015-0314-8
  • Cold extrusion of hot extruded aluminum chips
    Haase, M. and Tekkaya, A.E.
    Journal of Materials Processing Technology 217 (2015)
    The direct conversion of aluminum alloy machining chips into finished parts by hot extrusion with subsequent cold extrusion was investigated. While the process of hot extrusion was utilized to break the oxides covering the individual chips and to lead to bonding of the pure metal, the processes of cold forward rod extrusion as well as cold backward can extrusion were used for the production of chip-based finished parts. For the hot extrusion process, a flat face die and a die with integrated equal channel angular pressing (iECAP die) were used in order to investigate the influence of the deformation route on the quality of the chip-based finished parts. The flat-face die is a conventional tool for the fabrication of solid sections, while the iECAP die is an experimental tool that integrates the severe plastic deformation process of equal channel angular pressing into a conventional hot extrusion die. Tensile tests revealed superior mechanical properties of chips extruded through the iECAP die compared to those of chips extruded through the flat-face die. The hot extruded chips were further processed at room temperature by either backward can extrusion to cans with different wall thicknesses or by forward rod extrusion to shafts with different values of extrusion ratio and cone angle. For all fabricated chip-based finished parts, the mechanical properties and the microstructure were analyzed. Backward can extrusion of chip-based extrudates fabricated with the iECAP die resulted in defect-free cans for all investigated wall thicknesses, while the cans obtained from flat-face die processed chips showed cracks within the walls. Shafts without visible internal defects could be produced by forward rod extrusion of previously hot extruded chips, independent of the hot extrusion die design. However, subsequent compression tests revealed a dependency of the mechanical properties of chip-based shafts on the hot extrusion die design. © 2014 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.jmatprotec.2014.11.028
  • Composite Extrusion of Aluminum with Discontinuous Eccentric Reinforcements
    Levin, E. and Haase, M. and Foydl, A. and Tekkaya, A.E.
    Materials Today: Proceedings 2 (2015)
    The demand for lightweight components can be seen as a long-term objective of the automotive industry, motivated by ecological, economic and political reasons. The combination of multiple materials can be used to fabricate load-adapted products with the potential for lightweight applications. A promising approach for the production of such components is composite hot extrusion. In this process, high strength material is embedded into an aluminum matrix, in order to gain a local improvement of the mechanical properties of the produced component, while retaining the weight advantage of the light alloy. In this paper, the influence of the initial position of the reinforcements within the billet on their final position in the profile is investigated. © 2015 Elsevier Ltd. All rights.
    view abstract10.1016/j.matpr.2015.10.009
  • Determining cyclic flow curves using the in-plane torsion test
    Yin, Q. and Tekkaya, A.E. and Traphöner, H.
    CIRP Annals - Manufacturing Technology 64 (2015)
    A new method of determining cyclic stress-strain curves for characterizing kinematic hardening of sheet materials is introduced utilizing the in-plane torsion test and optical strain measurement. The test enables the simultaneous recording of multiple cyclic stress-strain curves with different amplitudes of plastic strain at load reversal in one single test and with one single specimen. Cyclic flow curves are obtained for DP600, DP800, DX54D, and AA5182, and compared with the Miyauchi shear test. Material parameters for a typical kinematic hardening model are determined. Multiple cyclic stress-strain curves result significantly different model parameters as obtained by a single cyclic curve. © 2015 CIRP.
    view abstract10.1016/j.cirp.2015.04.087
  • Development of a tele-operative testing cell as a remote lab for material characterization
    Ortelt, T.R. and Sadiki, A. and Pleul, C. and Becker, C. and Chatti, S. and Tekkaya, A.E.
    Proceedings of 2014 International Conference on Interactive Collaborative Learning, ICL 2014 (2015)
    Laboratory experiments play a significant role in engineering education. The experience gathered during the labs is one of the most important experiences during studying engineering because there is a strong connection between theory and practical relevance. A tele-operative testing cell for material characterization for forming processes is presented. This testing cell is used as a remote lab so that students can gain their experiences location and time-independent via the internet. In addition, the tele-operative testing cell is also used within the scope of lectures to combine the theory with live experiments in interaction with the students. The main aspects are, on the one hand, the developments in the field of engineering and the implementation of the IT components like iLab and, on the other hand, the integration of the tele-operative testing cell into engineering education. © 2014 IEEE.
    view abstract10.1109/ICL.2014.7017910
  • Enhancement of Lemaitre model to predict cracks at low and negative triaxialities in sheet metal forming
    Isik, K. and Doig, M. and Richter, H. and Clausmeyer, T. and Erman Tekkaya, A.
    Key Engineering Materials 639 (2015)
    Advanced high strength steels are still one of the best alternatives for light weight design in the automotive industry. Due to their good performances like high strength and high energy absorption, those steel grades are excellent for body in white components. Because of their restricted ductility, which sometimes leads to the formation of cracks without or low necking during forming operations, conventional forming limit diagrams may fall short. As a remedy, an enhanced variant of the Lemaitre continuum mechanical damage model (CDM) is presented in this work. Previous model extensions of the Lemaitre model improved the damage prediction for the shear and compression dominated stress states by introducing an additional weighting factor for the influence of compression on damage evolution, the so called crack closure parameter h. However, the possible range of the fracture behavior predicted by such models for low and negative stress triaxialities is limited. In this work, the Lemaitre CDM has been enhanced by considering the maximal shear stress to predict the fracture occurrence under shear. Previous models for the effect of void closure on damage evolution are reviewed and a novel model enhancement taking into account the maximal shear stresses is described. The determination of the damage model parameters is presented for a dual phase steel. For this particular material, the response of model enhancement on the failure prediction is discussed for a test part. © (2015) Trans Tech Publications, Switzerland.
    view abstract10.4028/www.scientific.net/KEM.639.427
  • Enhancement of Lemaitre model to predict cracks at low and negative triaxialities in sheet metal forming
    Isik, K. and Doig, M. and Richter, H. and Clausmeyer, T. and Tekkaya, A. E.
    Key Engineering Materials 639 (2015)
    Advanced high strength steels are still one of the best alternatives for light weight design in the automotive industry. Due to their good performances like high strength and high energy absorption, those steel grades are excellent for body in white components. Because of their restricted ductility, which sometimes leads to the formation of cracks without or low necking during forming operations, conventional forming limit diagrams may fall short. As a remedy, an enhanced variant of the Lemaitre continuum mechanical damage model (CDM) is presented in this work. Previous model extensions of the Lemaitre model improved the damage prediction for the shear and compression dominated stress states by introducing an additional weighting factor for the influence of compression on damage evolution, the so called crack closure parameter h. However, the possible range of the fracture behavior predicted by such models for low and negative stress triaxialities is limited. In this work, the Lemaitre CDM has been enhanced by considering the maximal shear stress to predict the fracture occurrence under shear. Previous models for the effect of void closure on damage evolution are reviewed and a novel model enhancement taking into account the maximal shear stresses is described. The determination of the damage model parameters is presented for a dual phase steel. For this particular material, the response of model enhancement on the failure prediction is discussed for a test part. © (2015) Trans Tech Publications, Switzerland.
    view abstract10.4028/www.scientific.net/KEM.639.427
  • Environmental assessment of solid state recycling routes for aluminium alloys: Can solid state processes significantly reduce the environmental impact of aluminium recycling?
    Duflou, J.R. and Tekkaya, A.E. and Haase, M. and Welo, T. and Vanmeensel, K. and Kellens, K. and Dewulf, W. and Paraskevas, D.
    CIRP Annals - Manufacturing Technology 64 (2015)
    Solid state recycling techniques allow the manufacture of high density aluminium alloy parts directly from production scrap. In this paper the environmental impacts associated with 'meltless' scrap processing routes based on three different techniques, namely hot extrusion, screw extrusion and spark plasma sintering (SPS), are compared with the corresponding remelting route as reference. Analysis of the obtained results allows clear conclusions on the perspectives offered by solid state recycling for systematic environmental impact reduction of aluminium recycling with material and energy savings as most important influencing factors. An overall impact reduction with a factor 2 for the SPS route and 3-4 for the extrusion routes is found to be realistic. © 2015 CIRP.
    view abstract10.1016/j.cirp.2015.04.051
  • Formability prediction of AL7020 with experimental and numerical failure criteria
    Yue, Z.M. and Badreddine, H. and Dang, T. and Saanouni, K. and Tekkaya, A.E.
    Journal of Materials Processing Technology 218 (2015)
    The formability of aluminum alloy sheet metal is a key issue in its design, analysis and operation of manufacturing processes. The conventional forming limit diagram (FLD) which evaluates the principal strains at failure is often used to quantify the formability limits. Due to the sensitivity of the FLD to the strain paths, the forming limit stress diagram (FLSD) based on principal stresses is shown to be more efficient. In contrast, a fully coupled behavior-damage models are recently proposed, which allow to predict the strain localization and the failure occurrence based on appropriates fully coupled constitutive equations describing the main physical phenomena involved. In this work a fully coupled constitutive equations taking into account the mixed nonlinear isotropic and kinematic hardenings fully coupled with the isotropic ductile damage is used. The microcracks closure is added to affect the equivalent plastic strain at fracture in a large range of stress states. Various tests are conducted to test the formability of aluminum alloy AL7020. The three different methods (FLD, FLSD and the fully coupled model) are identified and validated separately. With the help of Nakazima tests and cross-section deep drawing tests, the quality of the three failure criteria for AL7020 are compared and demonstrated with the investigations of the simulation results. © 2014 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.jmatprotec.2014.11.034
  • Fracture Loci in Sheet Metal Forming: A Review
    Silva, M. B. and Isik, K. and Tekkaya, A. E. and Martins, P. A. F.
    Acta Metallurgica Sinica-english Letters 28 (2015)
    Fracture in sheet metal forming usually occurs as ductile fracture, rarely as brittle fracture, at the operating temperatures and rates of loading that are typical of real processes in two different modes: (1) tensile and (2) in-plane shear (respectively, the same as modes I and II of fracture mechanics). The circumstances under which each mode will occur are identified in terms of plastic flow and ductile damage by means of an analytical approach to characterize fracture loci under plane stress conditions that takes anisotropy into consideration. Fracture loci was characterized by means of the fracture forming limit line and by the shear fracture forming limit line in the fracture forming limit diagram. Experiments were performed with single point incremental forming and double-notched test specimens loaded in tension, torsion and in-plane shear give support to the presentation and allow determining the fracture loci of AA1050-H111 aluminium sheets with 1 mm thickness. The relation between fracture toughness and the fracture forming limits was also investigated by comparing experimental values of the strains at fracture obtained from a truncated conical part produced by single point incremental forming and from double-notched test specimens loaded in tension.
    view abstract10.1007/s40195-015-0341-6
  • Hot Extrusion Dies with Conformal Cooling Channels Produced by Additive Manufacturing
    Hölker, R. and Haase, M. and Khalifa, N.B. and Tekkaya, A.E.
    Materials Today: Proceedings 2 (2015)
    The influence of local inner cooling in hot aluminum extrusion dies was investigated. For the manufacturing of the dies with conformal cooling channels, a layer-laminated manufacturing method and a laser melting process were applied. Extrusion trials with and without applying die cooling were performed. Numerical and experimental investigations revealed that, while maintaining the exit temperature of the extrudate, a distinct increase of the production speed up to 300% can be realized, while the extrusion force increases only slightly. Visioplastic analyses revealed that the rough surfaces, originating from the laser melting, do not disturb the material flow in the welding chamber. © 2015 Elsevier Ltd.
    view abstract10.1016/j.matpr.2015.10.028
  • Identification of fully coupled anisotropic plasticity and damage constitutive equations using a hybrid experimental-numerical methodology with various triaxialities
    Yue, Z.M. and Soyarslan, C. and Badreddine, H. and Saanouni, K. and Tekkaya, A.E.
    International Journal of Damage Mechanics 24 (2015)
    A hybrid experimental-numerical methodology is presented for the parameter identification of a mixed nonlinear hardening anisotropic plasticity model fully coupled with isotropic ductile damage accounting for microcracks closure effects. In this study, three test materials are chosen: DP1000, CP1200, and AL7020. The experiments involve the tensile tests with smooth and notched specimens and two types of shear tests. The tensile tests with smooth specimens are conducted in different directions with respect to the rolling direction. This helps to determine the plastic anisotropy parameters of the material when the ductile damage is still negligible. Also, in-plane torsion tests with a single loading cycle are used to determine separately the isotropic and kinematic hardening parameters. Finally, tensile tests with notched specimens and Shouler and Allwood shear tests are used for the damage parameters identification. These are conducted until the final fracture with the triaxiality ratio• lying between 0 and 1 / 3 (i.e. 0• 1/3). The classical force-displacement curves are chosen as the experimental responses. However, for the tensile test with notched specimens, the distribution of displacement components is measured using a full field measurement technique (ARAMIS system). These experimental results are directly used by the identification methodology in order to determine the values of material parameters involved in the constitutive equations. The inverse identification methodology combines an optimization algorithm which is coded within MATLAB together with the finite element (FE) code ABAQUS/Explicit. After optimization, good agreement between experimental and numerically predicted results in terms of force-displacement curves is obtained for the three studied materials. Finally, the applicability and validity of the determined material parameters are proved with additional validation tests. © 2014 The Author(s) Reprints and permissions.
    view abstract10.1177/1056789514546578
  • In-Situ measurement of loading stresses by means of X-ray diffraction with multi-state sheet specimen
    Güner, A. and Tekkaya, A.E.
    60 Excellent Inventions in Metal Forming (2015)
    view abstract10.1007/978-3-662-46312-3_61
  • Incremental tube forming
    Becker, C. and Hermes, M. and Tekkaya, A.E.
    60 Excellent Inventions in Metal Forming (2015)
    The incremental tube forming process is an innovative approach to manufacture three-dimensionally bent tubes with varying cross sections. The process combination of a tube bending and a tube spinning process leads to a significantly reduced bending moment as well as to a reduced springback compared to a conventional bending process. Therefore, this innovative process combination is especially suitable for the manufacture of high-strength materials as well as thin-walled tubes. Besides the mentioned process phenomena the process allows the manufacture of tubes with varying cross sections. The diameter as well as the wall thickness of the tube can be varied and, therefore, the cross section can be adapted to the load case applied during the subsequent usage of the bent tubular part. © Springer-Verlag Berlin Heidelberg 2015.
    view abstract10.1007/978-3-662-46312-3_54
  • Industrial Benchmark 2015: Process Monitoring and Analysis of Hollow EN AW-6063 Extruded Profile
    Gamberoni, A. and Donati, L. and Reggiani, B. and Haase, M. and Tomesani, L. and Tekkaya, A.E.
    Materials Today: Proceedings 2 (2015)
    The paper reports information in term of simulation settings and output results related to the Industrial benchmark 2015: extrusion benchmark is an event where participants from software houses, industries and academia are requested to simulate an extrusion process case which main experimental data are initially unknown and disclosed only after the submission of simulation results. The industrial benchmark 2015 is focused on the extrusion of a multi-cavities hollow profile with EN AW-6063 aluminum alloy. Thermal field was monitored by means of contactless pyrometers installed on the press and five thermosensors were inserted in key positions in the die. Several extrusion data were continuously acquired including the profile speed, the puller force and the extrusion load. After extrusion the profiles are analyzed in order to determine the position of the seam weld and the microstructure inside the profile cross section after air or water quenching. © 2015 Elsevier Ltd.
    view abstract10.1016/j.matpr.2015.10.004
  • Influencing the forming limits in air bending using incremental stress superposition
    Chatti, S. and Weinrich, A. and El Budamusi, M. and Becker, C. and Maevus, F. and Tekkaya, A.E.
    Key Engineering Materials 651-653 (2015)
    Restricted forming limits of modern high strength steels are a big challenge in manufacturing engineering and crucial in sheet metal bending processes. Looking for solutions for this problem, different modifications of the air bending process have already been developed. The innovative process of incremental stress superposition on air bending is one of these modifications. Studies of this process alternative show a positive effect on the considerable reduction of the sheet metal springback and the efficient extension of forming limits of materials. Using the principle of incremental stress superposition leads to several advantages compared to conventional bending processes. The bending force and, therefore, the consumed energy during air bending with incremental stress superposition are much lower. At the same time the process limits are extended. This paper presents the new process alternative and shows the latest investigation results. © (2015) Trans Tech Publications, Switzerland.
    view abstract10.4028/www.scientific.net/KEM.651-653.1602
  • Investigations for the Embedding of Functional Elements in the Composite Extrusion Process
    Dahnke, C. and Hilbring, J. and Kloppenborg, T. and Haase, M. and Tekkaya, A.E.
    Materials Today: Proceedings 2 (2015)
    In addition to high strength steel elements, composite extrusion offers the possibility to manufacture functional aluminum profiles by the embedding of isolated electric conductors. However, due to the lower strength and lower thermal stability, the embedding of such elements into the metal matrix by the usage of modified porthole dies is challenging. The length of the welding chamber and the pressure inside the welding chamber are the most important criteria for the successful production of a functional profile. In order to overcome the occurring challenges, the investigation of an adapted die design is shown in this paper. © 2015 Elsevier Ltd.
    view abstract10.1016/j.matpr.2015.10.010
  • Joining zone design for electromagnetically crimped connections
    Weddeling, C. and Walter, V. and Haupt, P. and Tekkaya, A.E. and Schulze, V. and Weidenmann, K.A.
    Journal of Materials Processing Technology 225 (2015)
    Space frames and multi-material structures are innovative designs to reduce the weight of a vehicle. But both lightweight design concepts have complex demands on joining technologies with the result that conventional processes are often pushed to their technological limits. Joining by electromagnetic crimping provides an interesting alternative to connect such structures without penetration or external heating. During electromagnetic crimping, pulsed magnetic fields are used to form a profile made out of an electrically conductive material into form-fit elements, like grooves, of the other joining partner. Thereby, an interlock is generated, which enables a load transfer. However, existing joint design methodologies require either extensive experimental studies or numerical modeling. To facilitate the connection design, an analytical approach for the prediction of the joining zone parameters with respect to the loads to be transferred is presented in this article. For the validation of the developed approach, experimental studies regarding the load transfer under quasi-static tension are performed. The major parameters considered in these investigations are the groove dimensions and its shape. In order to reduce the mass of a structure, hollow mandrels can be applied. To analyze how the reduced compressive strength of such inner connection elements influences the joining behavior and the load transfer of electromagnetically crimped connections, experimental studies are performed subsequent to the studies on the general groove parameters. Based on the obtained results, design strategies and a process window for the manufacturing of such joints are developed. To show the potential of electromagnetic form-fit joining, example connections joined in accordance with the established design guidelines are tested under quasi-static and cyclic loading. © 2015 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.jmatprotec.2015.06.009
  • Measurement of working coil temperature in electromagnetic forming processes by means of optical frequency domain reflectometry
    Gies, S. and Langolf, A. and Weddeling, C. and Tekkaya, A.E.
    Case Studies in Nondestructive Testing and Evaluation 3 (2015)
    Joule heating losses in the working coil of up to 50% make thermal loads a crucial influencing factor on the coil lifetime in electromagnetic forming processes. Thus, especially in case of discharge sequences with short cycle times knowledge about the temperature distribution is essential to avoid thermal overstressing. This paper presents an approach for temperature measurement inside the working coil during the electromagnetic forming process. Based on fiber-based optical frequency domain reflectometry (OFDR), the spatio-temporal temperature distribution inside the coil is determined. Temperature profiles for long-term discharge sequences are provided. The results prove the qualification of this measurement technique for electromagnetic forming processes. Due to the comparatively high acquisition rate the temperature increase and drop between two discharges can be analyzed in detail. This renders new possibilities for process analysis and monitoring. Especially the effectiveness of approaches aiming at a decrease of thermal loading can be rated easily using the presented measurement technique. © 2015 The Authors.
    view abstract10.1016/j.csndt.2015.02.001
  • Metal forming beyond shaping: Predicting and setting product properties
    Tekkaya, A.E. and Allwood, J.M. and Bariani, P.F. and Bruschi, S. and Cao, J. and Gramlich, S. and Groche, P. and Hirt, G. and Ishikawa, T. and Löbbe, C. and Lueg-Althoff, J. and Merklein, M. and Misiolek, W.Z. and Pietrzyk, M. and Shivpuri, R. and Yanagimoto, J.
    CIRP Annals - Manufacturing Technology 64 (2015)
    Metal forming is not only shaping the form of a product, it is also influencing its mechanical and physical properties over its entire volume. Advanced analysis methods recently enable accurate prediction of these properties and allow for setting these properties deterministically during the forming process. Effective measurement methods ensure the setting of these predicted properties. Several real examples demonstrate the impressive achievements and indicate the necessity of a paradigm change in designing products by including manufacturing-induced effects in the initial dimensioning. This paradigm change will lead to lightweight components and serve environmentally benign designs. © 2015 CIRP.
    view abstract10.1016/j.cirp.2015.05.001
  • Optimization of Porthole Die for Non-symmetric Composite Profiles
    Citrea, T. and Dahnke, C. and Gagliardi, F. and Haase, M. and Ambrogio, G. and Tekkaya, A.E.
    Materials Today: Proceedings 2 (2015)
    Composite extrusion aims at the improvement of mechanical properties of extrudates by embedding continuous reinforcing elements into a profile using a modified porthole die. The extrusion of non-symmetric reinforced profiles is a challenge, as the material flow has to be homogeneous. Die shape optimization for the production of non-symmetric reinforced profiles was investigated. The optimization phase took into account the process conditions aiming at a localized homogenization of the material flow to avoid profile distortion. The reinforcement effect on the material flow was analyzed by numerical investigations. The numerical results were verified by experiments. © 2015 Elsevier Ltd.
    view abstract10.1016/j.matpr.2015.10.012
  • Prediction of roughness after ball burnishing of thermally coated surfaces
    Hiegemann, L. and Weddeling, C. and Khalifa, N.B. and Tekkaya, A.E.
    Journal of Materials Processing Technology 217 (2015)
    An analytical model for a ball burnishing process is developed, which allows a prediction of the roughness of a thermally sprayed coating after a post treatment. Based on the observation of a roughness peak, a correlation between the leveling during rolling and the surface pressure under the rolling ball as well as a material parameter is derived and made mathematically describable. The model is verified by experimental studies with various rolling parameters and materials. The agreement between the experimental data and the calculated values is satisfactory. © 2014 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.jmatprotec.2014.11.008
  • Scientific Benchmark 2015: Effect of Choking and Bearing Length on Metal Flow Balancing in Extrusion Dies
    Selvaggio, A. and Donati, L. and Reggiani, B. and Haase, M. and Dahnke, C. and Schwane, M. and Tomesani, L. and Tekkaya, A.E.
    Materials Today: Proceedings 2 (2015)
    The Extrusion Benchmark is a biannual event where experts in the field of aluminum extrusion processing come together, not only in order to verify the increase of performance and accuracy of finite element (FEM) simulations for process optimization, but also to share advanced knowledge on the matter. The scientific benchmark 2015 was focused on bearing design and on the effect of choking and bearing length during extrusion of EN AW 6060 round bars. The trials were conducted on a 10 MN extrusion press at the Institute of Forming Technology and Lightweight Construction (IUL), which is equipped with advanced instrumentation for monitoring of process data. The tests were monitored in term of final profiles lengths, extrusion load, thermal evolution of the die in proximity of the six bearings and die deflection. © 2015 Elsevier Ltd.
    view abstract10.1016/j.matpr.2015.10.003
  • Sheet material characterization with the in-plane torsion test: Cyclic loading, grooved specimen and twin bridge specimen
    Traphöner, H. and Yin, Q. and Tekkaya, A.E.
    60 Excellent Inventions in Metal Forming (2015)
    The numerical analysis of springback in sheet metal forming is used to increase the accuracy of shape and the design of tools and processes. The more precise the material used for modeling is described, the more accurate the prediction ability of simulations is. For that reason, mixed isotropic-kinematic hardening models are used whose parameters are described by cyclic flow curves. The in-plane torsion test is used for the determination of cyclic flow curves. The optical strain measurement allows the simultaneous determination of cyclic flow curves with different pre-strain on a single sample. A modified twin bridge specimen is used to test anisotropic material behavior and a specimen with circular groove is used to characterize failure prediction under ideal shear load. © Springer-Verlag Berlin Heidelberg 2015.
    view abstract10.1007/978-3-662-46312-3_3
  • Teaching and learning globally connected using live online classes for preparing international engineering students for transnational collaboration and for studying in Germany
    May, D. and Sadiki, A. and Pleul, C. and Tekkaya, A.E.
    Proceedings of 2015 12th International Conference on Remote Engineering and Virtual Instrumentation, REV 2015 (2015)
    Students who are leaving their home country for taking part in an international study program face several challenges. Not only the new course of studies can be very challenging but also their whole living conditions may change significantly. This can be a severe clash especially for students who are moving to a country with a totally different cultural background in comparison to their home countries. Moreover it can profoundly complicate the first weeks at the new university. Knowing about the difficulties the Institute of Forming Technology and Lightweight Construction (IUL) at TU Dortmund University in Germany developed a three-weeks preparational online course for those international students coming to the IUL for their Master of Science program in Manufacturing Technology (MMT), a special international master program. In context of this course the use of the IUL's remote laboratory equipment was a key aspect. The course itself was implemented and delivered for the first time in 2014. It was designed to prepare the students as best as possible for their new studies at a German university and at the same time prepare them for transnational collaboration. Hence this course is a good example for a meaningful integration of remote laboratories into an innovative curriculum. Without this technology such course concepts with students distributed all over the world would not be possible. Making use if this technology brings the international course participants together and puts them into the situation to interact in context of a typical engineering situation, the experiment. The paper presents the course and experiences from its first implementation. © 2015 IEEE.
    view abstract10.1109/REV.2015.7087275
  • The challenge of specimen handling in remote laboratories for Engineering Education
    Sadiki, A. and Ortelt, T.R. and Pleul, C. and Becker, C. and Chatti, S. and Tekkaya, A.E.
    Proceedings of 2015 12th International Conference on Remote Engineering and Virtual Instrumentation, REV 2015 (2015)
    The robot controlled specimen handling for experiments in the field of material characterization for forming technology is presented. The testing cell consists of testing machines, an industrial robot, and other necessary components for the automation and conduction of experiments. First, a methodology is introduced how the key sequence of the robot tasks are identified, planned, and simulated. Afterwards, the design process of the testing cell is described. Finally, the implementation of the methodology and the integration of the robot tasks in a remote laboratory are presented. © 2015 IEEE.
    view abstract10.1109/REV.2015.7087289
  • The globally competent engineer: What different stakeholders say about educating engineers for a globalized world
    May, D. and Tekkaya, A.E.
    Proceedings of 2014 International Conference on Interactive Collaborative Learning, ICL 2014 (2015)
    The world is becoming more and more globalized. That has an important impact on the working environment of engineers. Designing, producing and distributing products either in international companies, in international working-teams or at least for international markets is part of today's economic reality. Even if goods have been sold or purchased all over the globe for centuries now, today's markets have been grown closer together than ever. This is the potential working environment engineering students are facing. In contrast to that engineering education still is very local oriented. The number of international courses or even programs in which the students can train to act in intercultural settings is small in comparison to the amount of engineering courses at universities. Changing this imbalance means developing possibilities to train globally competent engineers in local educational systems. An essential step in this work is to define the globally competent engineer itself. What are competences or attributes graduates need in order to act successfully in international contexts? Answering this question the paper takes three sequential steps. First of all the term 'intercultural competence' will be discussed. In a second step a literature research on attributes of globally acting engineers will be shown. Finally official documents of four accreditation agencies for engineering programs will be analyzed with focus on international aspects. By doing so four central areas of intercultural competence that are related to engineering can be identified: (International) communication, understanding of the engineering profession in a global context, (international) teamwork and ethical reasoning. In the conclusion all three steps will be connected and a catalogue of four central competences will be discussed. © 2014 IEEE.
    view abstract10.1109/ICL.2014.7017898
  • The impact of torsion on the bending curve during 3D bending of thin-walled tubes - A Case study on forming helices
    Staupendahl, D. and Becker, C. and Tekkaya, A.E.
    Key Engineering Materials 651-653 (2015)
    Three-dimensional tube bending is used in different industrial sectors. This work focuses on the three-roll push bending (TRPB) process to manufacture helices. Firstly, the tube rotation needed to produce helices was measured and compared to resulting helix radii and helix heights. The results were subsequently used to set up an analytical model, which, first of all, describes the tube rotation needed to produce the torsion of the investigated helices and, more importantly, can be generalized to describe the tube rotation needed for the torsion of arbitrary bending curves. © (2015) Trans Tech Publications, Switzerland.
    view abstract10.4028/www.scientific.net/KEM.651-653.1595
  • Thermally sprayed finestructured WC-12Co coatings finished by ball burnishing and grinding as an innovative approach to protect forming tools against wear
    Tillmann, W. and Hollingsworth, P. and Baumann, I. and Hiegemann, L. and Weddeling, C. and Tekkaya, A.E. and Rausch, S. and Biermann, D.
    Surface and Coatings Technology 268 (2015)
    The forming of high-strength steel sheets offers novel possibilities to produce lightweight structural parts with a high stiffness for the aircraft and automotive industries. However, the employment of such sheet materials leads to intense wear and thus reduces the lifetime of forming tools. At the same time, the requirements concerning their performance, their geometrical complexity, and their shape accuracy have been significantly increased. To counteract this problem, the tools are either treated by different thermo-chemical processes (e.g. hardening, nitriding) or are coated using thin film techniques such as PVD or CVD. In this study, thermal spraying is presented as a cost efficient and more flexible approach to protect the surface of forming tools against wear. For this purpose, planar samples as well as cylindrical deep drawing dies were coated by means of the high velocity oxy-fuel (HVOF) flame spraying technique, utilizing a fine WC-12Co powder (agglomerate size 2-10. μm) with a carbide size of 400. nm. Prior to the coating operation, a comprehensive parameter optimization was performed based on the statistical design of experiments (DOE) to achieve coatings with improved mechanical and tribological properties. The planar samples were used to ascertain the sliding and rolling wear behavior within two standardized test methods (Ball-on-Disc and Taber Abraser tests). The coated dies were smoothened by ball burnishing as well as grinding and afterwards evaluated within the deep drawing of high strength (HC380LA) steel sheets. In addition, the results were compared to those achieved with an uncoated conventional cold work steel die, which is commonly employed for this operation. In contrast to the cold work steel, both coated dies, the ball burnished as well as the ground die, showed a significantly better wear performance after the forming of 10,000 parts. © 2014 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2014.06.039
  • TwinTool
    Kwiatkowski, L. and Tekkaya, A.E.
    60 Excellent Inventions in Metal Forming (2015)
    The TwinTool is a tooling concept to speed up the incremental sheet forming process. In this concept two additional linear axes of motion are added to the conventional tooling setup, each of them featured with an own forming tool. While working in parallel, this tooling concept aims at decreasing the process time. However, the TwinTool is a result of a comprehensive study where concepts different in kinematics and flexibility are compared to each other with the objective to incoperate multiple forming zones acting simultaneously during the incremental sheet forming process. Those variants as well as a realized TwinTool prototype are described in this paper. © Springer-Verlag Berlin Heidelberg 2015.
    view abstract10.1007/978-3-662-46312-3_25
  • Wall thickness distribution during a combined tube spinning and bending process
    Becker, C. and Tekkaya, A.E.
    Key Engineering Materials 651-653 (2015)
    During a tube bending process the wall thickness distribution plays an important role concerning the process limits. Especially the wall thinning at the extrados of the tube is crucial. The wall thickness in a combined tube bending and tube spinning process will be analyzed. Therefore, possible stages of complexity are presented to show the possibilities of such a process combination. Based on this the interactions between the bending and spinning process on the wall thickness distribution will be discussed. Finally, a diagram will show how to adjust the wall thickness at the extrados of the tube only by adapting the tube spinning process. © (2015) Trans Tech Publications, Switzerland.
    view abstract10.4028/www.scientific.net/KEM.651-653.1614
  • Writing a good review for the Journal of Materials Processing Technology
    Allwood, J.M. and Tekkaya, A.E.
    Journal of Materials Processing Technology 215 (2015)
    view abstract10.1016/j.jmatprotec.2014.08.027
  • An experimental and numerical investigation of different shear test configurations for sheet metal characterization
    Yin, Q. and Zillmann, B. and Suttner, S. and Gerstein, G. and Biasutti, M. and Tekkaya, A.E. and Wagner, M.F.-X. and Merklein, M. and Schaper, M. and Halle, T. and Brosius, A.
    International Journal of Solids and Structures 51 (2014)
    Simple shear tests are widely used for material characterization especially for sheet metals to achieve large deformations without plastic instability. This work describes three different shear tests for sheet metals in order to enhance the knowledge of the material behavior under shear conditions. The test setups are different in terms of the specimen geometry and the fixtures. A shear test setup as proposed by Miyauchi, according to the ASTM standard sample, as well as an in-plane torsion test are compared in this study. A detailed analysis of the experimental strain distribution measured by digital image correlation is discussed for each test. Finite element simulations are carried out to evaluate the effect of specimen geometries on the stress distributions in the shear zones. The experimental macroscopic flow stress vs. strain behavior shows no significant influence of the specimen geometry when similar strain measurements and evaluation schemes are used. Minor differences in terms of the stress distribution in the shear zone can be detected in the numerical results. This work attempts to give a unique overview and a detailed study of the most commonly used shear tests for sheet metal characterization. It also provides information on the applicability of each test for the observation of the material behavior under shear stress with a view to material modeling for finite element simulations. © 2013 Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.ijsolstr.2013.12.006
  • Analysis of the potential of incremental stress superposition on air bending
    Weinrich, A. and Becker, C. and Maevus, F. and Chatti, S. and Tekkaya, A.E.
    Key Engineering Materials 622-623 (2014)
    Springback and limited forming limits of modern high strength steels are a big challenge in manufacturing engineering. Both aspects are crucial in sheet metal bending processes. Different modifications of the air bending process have already been developed in order to reduce springback and also to increase the forming limits of materials. A new method (the incremental stress superposition on air bending) has been developed. Studies of this new process alternative show a positive effect on the springback behavior. In order to investigate the potential of this process a comparison with other already established bending processes have been carried out. A possible process control to extend the forming limits has also been investigated. © (2014) Trans Tech Publications, Switzerland.
    view abstract10.4028/www.scientific.net/KEM.622-623.1173
  • Analytical methodology for the process and joint design of form-fit joining by die-less hydroforming
    Weddeling, C. and Gies, S. and Khalifa, N.B. and Tekkaya, A.E.
    ASME 2014 International Manufacturing Science and Engineering Conference, MSEC 2014 Collocated with the JSME 2014 International Conference on Materials and Processing and the 42nd North American Manufacturing Research Conference 2 (2014)
    In modern lightweight concepts, for example in automotive engineering, structures are increasingly composed of several dissimilar materials. Due to the different material properties of the joining partners, conventional and widely used joining techniques often reach their technological limits when applied in the manufacturing of such multi-material structures. This leads to an increasing demand for appropriate joining technologies, like joining by die-less hydroforming (DHF) for connecting tubular workpieces. The present work introduces an analytical model to determine the achievable joint strength of this connection type. This approach, taking into account the material parameters as well as the groove and tube geometry, is based on a membrane analysis with constant wall thickness. Additionally, bending stresses and friction are considered locally. Besides a fundamental understanding of the load transfer mechanism, this analytic approach allows a reliable joining zone design. To validate the model, experimental investigations using aluminum specimens were performed. Copyright © 2014 by ASME.
    view abstract10.1115/MSEC2014-3955
  • Analytical prediction of roughness after ball burnishing of thermally coated surfaces
    Hiegemann, L. and Weddeling, C. and Khalifa, N.B. and Tekkaya, A.E.
    Procedia Engineering 81 (2014)
    An analytical model to predict the roughness of a thermally sprayed coating after a ball burnishing process is presented. Based on the observation of a roughness peak, a correlation between the leveling during rolling and the surface pressure under the rolling ball as well as a material parameter is derived. Ball burnishing experiments on thermally coated surfaces were carried out to verify the model. A good correspondence between the experimental data and the calculated values is presented. © 2014 The Authors. Published by Elsevier Ltd.
    view abstract10.1016/j.proeng.2014.10.257
  • Application of the bending theory on square-hollow sections made from high-strength steel with a changing angle of the bending plane
    Hudovernik, M. and Kosel, F. and Staupendahl, D. and Tekkaya, A.E. and Kuzman, K.
    Journal of Materials Processing Technology 214 (2014)
    The bending behaviour of thin-walled profiles made from high-strength steels with respect to a changing angle of the bending plane or, in other words, the rotation of the section geometry on the longitudinal axis of the profile, has not yet been fully characterized. The investigations presented in this paper lead to an improvement of the description for bending square hollow sections under unified and constant loading conditions, and contribute to the general understanding of such bending problems. The methodological approach is based on analytic, numerical, and experimental analysis. The analytical formulation is primarily built on the principle of a simplified cantilever beam model. Bending curvatures are assumed to be generated with constant radiuses of curvatures. The change of the angle of the section, with respect to the direction of bending, is applied before bending and remains unchanged throughout the process. In this way, the effects of a changing angle with regard to the direction of bending are analyzed for several constant curvatures and angles of 2D bent profiles. With a clear understanding of the 2D bending of high strength profiles, the same principles can also be used incrementally for analyses of 3D bending. The analytical theory is tested with an emphasis on using profiles with high-strength material properties compared to profiles made from standard low-carbon steel, by using the innovative torque superposed spatial (TSS) bending method. The results are supported by FE models generated with the Abaqus numerical FEM tool and verified with the results of actual experiments. © 2014 Elsevier B.V.
    view abstract10.1016/j.jmatprotec.2014.05.016
  • Bending of tailored blanks using elastic tools
    Weinrich, A. and Becker, C. and Maevus, F. and Chatti, S. and Tekkaya, A. E.
    Advanced Materials Research 1018 (2014)
    Tailored blanks are suitable for the manufacture of lightweight structures due to the loadoptimised design. However, the forming of tailored blanks is problematic because of the varying properties. Especially springback is a main challenge to focus on. An innovative process will be discussed which concentrates on the air bending process of sheet metal extended by a local application of a stress superposition to reduce springback. © (2014) Trans Tech Publications, Switzerland.
    view abstract10.4028/www.scientific.net/AMR.1018.301
  • Bridge design influences on the pressure conditions in the welding chamber for porthole die extrusion
    Gagliardi, F. and Schwane, M. and Citrea, T. and Haase, M. and Khalifa, N.B. and Tekkaya, A.E.
    Key Engineering Materials 622-623 (2014)
    Porthole die extrusion of lightweight alloys is used for the production of profiles, which may have complex cross section geometries. The mechanical properties of these profiles are deeply affected by the seam welds, which are generated in hollow profiles along the whole length. The seam welds result from the rejoining of the material streams in the welding chamber of the porthole die. The joining phase and hence the seam weld quality are strongly influenced by the temperature and the pressure conditions in the welding chamber. Those process conditions can be adjusted by a proper die design. In this work, the focus lies on the feeder section of the extrusion die, which consists of a set of bridges, whose shapes influence the material entry in the welding chamber. A numerical study was carried out to investigate different bridge shapes with regard to the pressure inside the welding chamber and the punch load. Subsequently, the volume of the bridge was fixed to isolate and better investigate the influence of the shape. It was observed that bridge designs leading to higher flow distortion cause higher pressure decrement along the welding plane and, consequently, degradation of the welding conditions. © (2014) Trans Tech Publications, Switzerland.
    view abstract10.4028/www.scientific.net/KEM.622-623.87
  • Characterization of fracture loci in metal forming
    Martins, P.A.F. and Bay, N. and Tekkaya, A.E. and Atkins, A.G.
    International Journal of Mechanical Sciences 83 (2014)
    Fracture in metal forming can occur in three different modes: (i) tensile; (ii) in-plane shear; and (iii) out-of-plane shear (respectively the same as modes I, II and III of fracture mechanics). The circumstances under which each mode will occur are identified in terms of plastic flow and microstructural ductile damage by means of an analytical framework to characterize fracture loci under plane stress conditions that also takes anisotropy into consideration. Experimental results retrieved from the literature give support to the presentation and show that plastic flow and failure in sheet forming results from competition between modes I and II whereas in bulk forming fracture results from competition between modes I and III. © 2014 Elsevier Ltd.
    view abstract10.1016/j.ijmecsci.2014.04.003
  • Composite hot extrusion of functional elements
    Schwane, M. and Dahnke, C. and Haase, M. and Ben Khalifa, N. and Tekkaya, A. E.
    Advanced Materials Research 1018 (2014)
    The composite extrusion process, which is investigated at the Institute of Forming Technology and Lightweight Construction (IUL), allows the combination of different materials within an aluminium profile. In contrast to the useage of reinforcing elements, this paper focuses on the embedding of functional elements, such as isolated electric conductors. Results of the experimental and numerical investigations are presented within this paper. © (2014) Trans Tech Publications, Switzerland.
    view abstract10.4028/www.scientific.net/AMR.1018.223
  • Dry shearing of micro-alloyed steels
    Steinbach, F. and Güner, A. and Tekkaya, A.E.
    Key Engineering Materials 622-623 (2014)
    In the production process of sheet metal parts, oil is widely used as lubricant, not only in sheet metal forming but also in shearing and blanking. Due to environment, health and cost reasons, the absence of lubricants is an aim for future production as it has initiated for machining in the last years. For lubricant-free shearing, it has to be known if there is an influence on the process itself when using oil or not. To find this out, experiments are carried out with a small testing device installed in a tensile testing machine and a blanking tool installed in a servo press. With the small device it is possible to make a piercing process with a circle punch of 16 mm diameter. The blanking tool produces a larger cut part with different holes and open cuts. Without lubricant, there is no difference in the maximum shearing force for the small device while the stripping force is higher and the cut edge zones differs slightly. Using oil or not has a small effect on the force using the blanking tool. © (2014) Trans Tech Publications, Switzerland.
    view abstract10.4028/www.scientific.net/KEM.622-623.1058
  • Effect of cold forming and heat treatment on size and shape changes
    Nadolski, D. and Schulz, A. and Hoffmann, F. and Zoch, H.-W. and Hänisch, S. and Ossenkemper, S. and Haase, M. and Tekkaya, A.E.
    HTM - Journal of Heat Treatment and Materials 69 (2014)
    Dimensionally accurate workpieces can be produced resource-conserving and economically by metal forming of steel at low temperatures. Especially when components are needed on a constant high quality level in large quantities, such as in the automotive industry, cold forming processes often provide the optimal solution. If there are high demands on the strength and wear resistance of the components in service, the parts must be heat treated after cold forming. Beside the desired property changes also size and shape changes are caused by the heat treatment. The residual stresses that are introduced by cold forming introduce a significant distortion potential, since they are released during the heat treatment. In this paper the size and shape changes of cold extruded C53G steel (SAE 1055) shafts due to inductive heat treatment are investigated. Using the method of design of experiments (DoE) it was possible to identify significant factors from the cold forming and the inductive heat treatment controlling the part's distortion. Additionally, by means of residual stress measurements evidence for the different shape change mechanisms of the cold formed shafts could be obtained. A direct comparison of the size and shape changes of cold formed and machined shafts due to the same induction hardening process demonstrates an explicit influence of the manufacturing process on distortion. © 2014 Carl Hanser Verlag GmbH & Co. KG.
    view abstract10.3139/105.110240
  • Effects of weld line in deep drawing of tailor welded blanks of high strength steels
    Mennecart, T. and Güner, A. and Khalifa, N.B. and Tekkaya, A.E.
    Key Engineering Materials 611-612 (2014)
    Due to the increase of lightweight design in car bodies, there is a raise in use of tailored welded blanks (TWB). With these blanks it is possible to strengthen the car body where it is necessary. This can lead to less weight. In the case of TWB, there is a weld line, which influences the deep drawing behavior significantly during forming. In the presented results two different high strength steels (HCT980X and HCT600X) are welded together. One forming operation is performed, in which the weld line is positioned differently. The results show the influence of the weld line on the forming behavior which is realized by the comparison of deep drawn monolithic parts with the deep drawn tailored welded blanks. While the monolithic parts could be formed without failure, the forming of tailored welded blanks was accompanied by cracks in dependency to the weld line orientation and the applied load in this region. The results also show that the failure occurs in the base material and that the weld line is not damaged by the applied load. After the characterization of the base materials and the weld material, a numerical modelling of the whole TWB could be realized in this work. Two different ways of modelling techniques of the weld line are compared and the necessity of the consideration of the weld line properties is demonstrated. Furthermore, in consideration of the weld line properties in the FE-Model, it is possible to show that the weld line resists the forming operation without failure. © 2014 Trans Tech Publications, Switzerland.
    view abstract10.4028/www.scientific.net/KEM.611-612.955
  • Electrically driven plasma via vaporization of metallic conductors: A novel tool for joining tubular workpieces
    Vivek, A. and Weeldeling, C. and Hahn, M. and Daehn, G.S. and Tekkaya, A.E.
    Procedia CIRP 18 (2014)
    Electrically driven rapid vaporization of thin aluminum wires is an innovative forming process. In this manuscript, joining of tubular workpieces by expansion using this technique is investigated. The goal is to identify the influence of major process parameters, like input energy and discharge frequency on the workpiece deformation, the forming velocity, and the resulting joint strength. Therefore, free-forming as well as joining experiments were conducted. During these experiments, thin aluminum wires were intentionally vaporized in the center of aluminum tubes. The pressure was transmitted from the region of vaporization to the inside wall of the tubes via an elastomer medium. In the free forming experiments it was found that, at the same charging energy level, a decrease of the discharge's rise time leads to an increase of the maximum velocity. Therefore, the discharge frequency was found to have a positive effect on process efficiency. The joining experiments led to strong interference-fit connections, whose strength increased proportionately with increasing input energy. © 2014 Elsevier B.V.
    view abstract10.1016/j.procir.2014.06.108
  • Experimental and numerical analysis of dry shearing of aluminum 6082
    Steinbach, F. and Chen, L. and Güner, A. and Tekkaya, A. E.
    Advanced Materials Research 1018 (2014)
    In shearing and blanking lubrication is widely used. For future production one aim is to decrease the use of oil or to do without such lubricants. For this purpose it is interesting, if there is an influence of using lubricants on the cut edge quality. Therefore numerical analysis and experiments are carried out with a blanking tool, installed in a servo press, to analyze the cut edge quality of shearing of aluminum 6082-T6. Using FEA with Lemaitre’s damage model, different friction coefficients for modelling different kinds of lubrication are used for modelling a process of punching a circular hole with a punch of 16 mm diameter. In the experiments the cut edge quality and the maximum shearing force do not differ significantly. With a friction coefficient of 0.2 it is possible to give the right cut edge quality by using the damage model of Lemaitre. © (2014) Trans Tech Publications, Switzerland.
    view abstract10.4028/www.scientific.net/AMR.1018.261
  • Extending the flexibility in the composite extrusion process
    Dahnke, C. and Pietzka, D. and Haase, M. and Tekkaya, A.E.
    Procedia CIRP 18 (2014)
    The research in the field of composite extrusion led to a fundamental understanding and characterization of the process in the last years. Based on the gained knowledge, this paper focuses on the possibilities to increase the flexibility of the composite extrusion process i.e. to manufacture profiles with functional properties graded over the length, profiles with improved mechanical properties and profiles with functional properties. © 2014 Elsevier B.V.
    view abstract10.1016/j.procir.2014.06.103
  • Extrusion benchmark 2013 - Experimental analysis of mandrel deflection, local temperature and pressure in extrusion dies
    Selvaggio, A. and Kloppenborg, T. and Schwane, M. and Hölker, R. and Jäger, A. and Donati, L. and Tomesani, L. and Tekkaya, A.E.
    Key Engineering Materials 585 (2014)
    A bridge die was designed for the simultaneous extrusion of two rectangular profiles and used in a strictly monitored aluminum extrusion process. Experimental investigations aimed at the measurement of the mandrel deflection, the local die temperature, and the pressure inside the welding chamber by means of special measurement equipment. AA6082 alloy was used as extrusion material. The influence of the extrusion speed on the aforementioned objectives is reported. The experiments were repeated at least three times under the same conditions in order to achieve a statistical validation of the acquired data. These data are provided as reference for the 2013 edition of the Extrusion Benchmark. © (2014) Trans Tech Publications, Switzerland.
    view abstract10.4028/www.scientific.net/KEM.585.13
  • Extrusion of multi-material components
    Khalifa, N.B. and Foydl, A. and Pietzka, D. and Jäger, A. and Tekkaya, A.E.
    ASME 2014 International Manufacturing Science and Engineering Conference, MSEC 2014 Collocated with the JSME 2014 International Conference on Materials and Processing and the 42nd North American Manufacturing Research Conference 2 (2014)
    Three innovative extrusion processes for the manufacture of multi-material parts are discussed: co-extrusion of discontinuously steel reinforced aluminum profiles, composite extrusion of continuously steel wire reinforced profiles and composite rod extrusion. In the first two processes the embedded steel elements are not deformable while by composite rod extrusion both materials are deformable. By means of experimental and numerical analysis, the parameters that mainly influence the reinforcement ratio, the extrusion force as well as the material distribution are analyzed. On the basis of this, analytical approaches are deduced to describe the process limits for the technologies. The paper closes with examples of applications regarding the lightweight requirements as well as functional integrations by forming multi-materials. Copyright © 2014 by ASME.
    view abstract10.1115/MSEC2014-4197
  • Extrusion of profiles with variable wall thickness
    Selvaggio, A. and Haase, M. and Khalifa, N.B. and Tekkaya, A.E.
    Procedia CIRP 18 (2014)
    The paper describes new developments in extrusion processes for the reduction of process chains and the improvement of the flexibility of the extrusion process itself. Hereby, the presented and discussed technology is the extrusion of profiles with variable cross-sections. Besides the process principle and advantages, several results of experimental investigations are given in order to show the motivation for fundamental and application-oriented research in the field of extrusion technology. First results on the extrusion of profiles with variable cross-section are shown in this paper. Here, the focus was placed on the development of dies for the extrusion of open and hollow profiles with variable wall thickness. © 2014 Published by Elsevier B.V.
    view abstract10.1016/j.procir.2014.06.100
  • Finite element based determination and optimization of seam weld positions in porthole die extrusion of double hollow profile with asymmetric cross section
    Schwane, M. and Kloppenborg, T. and Ben Khalifa, N. and Jäger, A. and Tekkaya, A.E.
    Key Engineering Materials 585 (2014)
    Finite elemente analysis (FEA) allows to reduce development time during the die design stage as well as costly extrusion trials with prototypes. Therefore, it is essential that FEA computation provides reliable results. Among other output quantities such as temperature, load, or die stress, the prediction of material flow is one of the most essential ones. Especially in porthole dies, the material flow can be very complex and thus the position of the seam welds in the profile may be uncertain. In this study the particle tracing method was utilized to determine and finally adjust the seam weld positions in a double hollow profile with varying wall thicknesses over the cross section. The seam weld positions resulting from the original die design were determined by Eulerian FEA computation in the first step. Subsequently, the seam weld positions were adjusted by changing the die geometry. The simulation results were verified by means of extrusion tests, which were conducted under industrial conditions. In addition, Lagrangian and Eulerian FEA was utilized to analyze the evolution of the seam weld positions by evaluation of material flow as well as pressure distribution during the transient initial stage and the steady-state stage of the extrusion process. It was demonstrated that steady state process simulation and the particle tracing method can be used for the prediction of seam weld positions in complex hollow cross sections. © (2014) Trans Tech Publications, Switzerland.
    view abstract10.4028/www.scientific.net/KEM.585.95
  • Formability limits by fracture in sheet metal forming
    Isik, K. and Silva, M.B. and Tekkaya, A.E. and Martins, P.A.F.
    Journal of Materials Processing Technology 214 (2014)
    The aim of this paper is twofold: first, to revisit the forming limit diagram (FLD) in the light of fundamental concepts of plasticity, damage and ductile fracture mechanics and, second, to propose a new experimental methodology to determine the formability limits by fracture in sheet metal forming. The first objective makes use of the theory of plasticity applied to proportional strain loading paths, under plane stress conditions, to analyze the fracture forming limit line (FFL) and to introduce the shear fracture forming limit line (SFFL). The second objective makes use of single point incremental forming (SPIF), torsion and plane shear tests to determine the experimental values of the in-plane strains at the onset of fracture. Results show that the proposed methodology provides an easy and efficient procedure to characterize the formability limits by fracture in sheet metal forming. In particular, the paper shows that the FFL determined by means of tensile and conventional sheet formability tests is identical to that determined from SPIF tests on conical and pyramidal truncated specimens. The new proposed approach is expected to have impact in the established methodologies to outline the formability limits on the basis of the forming limit curves (FLC's) at the onset of necking. © 2014 Elsevier B.V.
    view abstract10.1016/j.jmatprotec.2014.02.026
  • Forming limit extension of high-strength steels in bending processes
    El Budamusi, M. and Weinrich, A. and Becker, C. and Chatti, S. and Tekkaya, A.E.
    Key Engineering Materials 611-612 (2014)
    A conventional air bending process, which is applied with a hydrostatic pressure in the bending zone, is presented. This was done by using an elastomer tool. The advantage of this method is that the flexibility of air bending is maintained by reducing the springback while the forming limits are extended. Experiments are conducted to investigate the influence of the elastomer tool. Furthermore, the geometry of the elastomer tool was analyzed by means of FEM simulations in order to investigate the influence on springback and bending force. The investigation leads to a reduction of the process forces by minimizing the springback and to an extension of the forming limits. © 2014 Trans Tech Publications, Switzerland.
    view abstract10.4028/www.scientific.net/KEM.611-612.1110
  • Forming of lightweight metal components: Need for new technologies
    Tekkaya, A.E. and Ben Khalifa, N. and Grzancic, G. and Hölker, R.
    Procedia Engineering 81 (2014)
    Contributing to lightweight design in the field of metal forming, four different strategies are introduced - The material, component, functional, and conditional lightweight strategy. Their main objective is the reduction of a component's weight and the saving of our available resources. Regarding each approach, innovative forming technologies are presented and their contribution to lightweight success is depicted. On the one hand, this article covers conventional processes like e.g. extrusion, which are used for processing hybrid lightweight materials. On the other hand, new forming technologies are introduced to serve lightweight requirements like e.g. load adaption. Finally, the importance of numerical damage modeling regarding lightweight design is shown. © 2014 The Authors. Published by Elsevier Ltd.
    view abstract10.1016/j.proeng.2014.09.125
  • Fundamentals of the incremental tube forming process
    Becker, C. and Tekkaya, A.E. and Kleiner, M.
    CIRP Annals - Manufacturing Technology 63 (2014)
    The manufacture of bent tubes made of high-strength materials requires high bending loads, which lead to large springback and eventually distortion of the cross-section. The incremental tube forming process allows significant reduction of the bending moment. This is achieved by combining the continuous bending process with an incremental tube spinning process. The paper describes an analytical model to predict the bending moment reduction as a function of the superposed spinning process parameters. Experimental results are provided to validate the theoretical results. This model allows the design and optimization of the incremental tube forming process with low springback. © 2014 CIRP.
    view abstract10.1016/j.cirp.2014.03.009
  • Hybrid processes in manufacturing
    Lauwers, B. and Klocke, F. and Klink, A. and Tekkaya, A.E. and Neugebauer, R. and McIntosh, D.
    CIRP Annals - Manufacturing Technology 63 (2014)
    Hybrid manufacturing processes are based on the simultaneous and controlled interaction of process mechanisms and/or energy sources/tools having a significant effect on the process performance. These processes have a large influence on the processing/manufacturing characteristics resulting in higher machinability, reductions of process forces and tool wear, etc. Due to the combined action of processes, it also has an important - and most of the time - positive effect on the surface integrity of machined parts. This paper gives a definition and classification of hybrid processes, followed by a description of principles and future perspectives, benefits on productivity, effects on surface quality and applications of common hybrid processes. © 2014 CIRP.
    view abstract10.1016/j.cirp.2014.05.003
  • In-situ measurement of loading stresses with X-ray diffraction for yield locus determination
    Güner, A. and Zillmann, B. and Lampke, T. and Tekkaya, A.E.
    International Journal of Automotive Technology 15 (2014)
    The application of the X-ray diffraction method is introduced to solve the problem of inhomogeneous deformation fields in the specimens used for sheet metal characterization. In this method, strains are measured on one side of a specimen with optical measurement systems. On the other side, loading stresses on a specimen are captured with an X-ray diffractometer mounted on a universal testing machine. By this way, the whole stress-strain history of a material point is tracked during testing. The method was first applied to uniaxial tension tests, whereby the applicability of the theory of stress factors and effective X-ray elastic constants were tested. The relaxation behavior of a sheet material which shows itself as stress drops during in-situ experimentation was characterized and compensated by a visco-plastic material model for different stress states. The proposed method was applied to characterize aluminum alloy AA5182 under plane strain tension and shear conditions and the results were compared with the conventionally obtained yield locus. Numerical analyses of a workpiece with the Vegter and Yld2000-2D material models show that the enriched yield locus definition with accurate plane strain tension and shear stresses captures the experimentally obtained surface strains more precisely. © 2014 The Korean Society of Automotive Engineers and Springer-Verlag Berlin Heidelberg.
    view abstract10.1007/s12239-014-0031-9
  • Increased productivity in hot aluminum extrusion by using extrusion dies with inner cooling channels manufactured by rapid tooling
    Hölker, R. and Haase, M. and Khalifa, N.B. and Tekkaya, A.E.
    Key Engineering Materials 611-612 (2014)
    The influence of local inner die cooling on the heat balance in hot aluminum extrusion was investigated. For the manufacturing of the die with cooling channels close to the forming zone, the layer-laminated manufacturing method was applied. The new tooling technology was applied in order to decrease the profile's exit temperature and to avoid thermally induced surface defects with the aim to raise the productivity in hot aluminum extrusion processes. Numerical and experimental investigations revealed that, while maintaining the exit temperature of the extrudate, a distinct increase of the production speed up to 300 % can be realized, while the extrusion force increases only slightly. An effect on the profiles microstructure was also detected. By applying die cooling, grain coarsening can be significantly limited or even be avoided. © 2014 Trans Tech Publications, Switzerland.
    view abstract10.4028/www.scientific.net/KEM.611-612.981
  • Innovative machine design for incremental profile forming
    Grzancic, G. and Becker, C. and Hermes, M. and Tekkaya, A.E.
    Key Engineering Materials 622-623 (2014)
    The Incremental Profile Forming process (IPF) is a new method to manufacture tubes and profiles with variable cross-section design along the centre-line of the profile. The innovative process design enables the combination of high workpiece complexity and high process flexibility. For this reason, a machine concept was developed and finally a prototype realized. The new machine consists of eight numerically controlled axes, which allow the processing of thin-walled tubes and profiles with a maximum diameter of 80 mm. The design of the machine combined with the control system leads to a forming technology with a high degree of flexibility, which is an advantage of the process. Depending on the final workpiece shape the forming process proceeds in several steps and can therefore be considered as an incremental forming process. Furthermore, the tool concept supports both a kinematical and a form-closed forming process. © (2014) Trans Tech Publications, Switzerland.
    view abstract10.4028/www.scientific.net/KEM.622-623.413
  • Investigations on the manufacturability of thin press hardened steel components
    Georgiadis, G. and Tekkaya, A.E. and Weigert, P. and Weiher, J. and Kurz, H.
    Procedia CIRP 18 (2014)
    In the recent years, the automotive industry is focusing on the reduction of the vehicles weight, so as to minimize the CO2emissions, while improving the crashworthiness levels. In order to achieve a further body-in-white weight reduction and exploit the potential for lightweight construction of the hot-dip aluminized press hardening manganese-boron steel, further research on the design and development of a process chain for the production of thin hot stamped components is carried out. For this purpose the impact of different blank thickness-dependent process parameters on the component properties is determined through both simulation and experimental analysis. The temperature profile of the heating process is determined for different blank thicknesses and the development of the diffusion layer between the AlSi-coating and the base material is examined. With respect to the transfer process of the thin austenitized blanks from the roller hearth furnace into the forming tool, the time slot is determined via the analysis of the corresponding time-temperature curves. In addition, different simulation models are developed, aiming at the validation and optimization of the transfer process. In order to further investigate the manufacturability of thin press hardened components, the simulation of a hot stamping process is carried out. The developed models are verified by an optical 3D forming analysis of the hot stamped components. As a conclusion of the current investigations, thin hot stamped components can be manufactured only under the precondition that the process is optimally designed and the process chain properly adjusted. © 2014 Elsevier B.V.
    view abstract10.1016/j.procir.2014.06.110
  • Magnetic pulse welding by electromagnetic compression: Determination of the impact velocity
    Lueg-Althoff, J. and Lorenz, A. and Gies, S. and Weddeling, C. and Goebel, G. and Tekkaya, A.E. and Beyer, E.
    Advanced Materials Research 966-967 (2014)
    The implementation of multi-material concepts and the manufacturing of modern lightweight structures, for example in automotive engineering, require appropriate joining technologies. The ability to join dissimilar materials without additional mechanical elements, chemical binders, or adverse influences of heat on the joining partners is key in reaching the desired weight reduction in engineering structures. The Magnetic Pulse Welding (MPW) process meets these demands, making it a viable alternative to conventional thermal welding and mechanical joining processes. The present paper focuses on the analytical determination of the impact velocity as one of the key parameters of MPW processes. On the basis of experimentally recorded data concerning the course of the discharge current and geometrical parameters of the welding setup, the respective velocity is determined. A comparison with measurement data gained by Photon Doppler Velocimetry is performed. © (2014) Trans Tech Publications, Switzerland.
    view abstract10.4028/www.scientific.net/AMR.966-967.489
  • Manufacturing of steel-reinforced aluminum parts by co-extrusion and subsequent forging
    Behrens, B.-A. and Tekkaya, A.E. and Kosch, K.-G. and Foydl, A. and Kammler, M. and Jäger, A.
    Key Engineering Materials 585 (2014)
    The processes of manufacturing continuously and discontinuously steel-reinforced aluminum profiles by means of co-extrusion and subsequent forging were examined. In the co-extrusion and subsequent forging of discontinuously reinforced parts, influences of the reinforcing elements on forming behavior and material bonding for both processes were investigated. It was shown that forming temperature as well as ram speed have no influence on joining quality and forming behavior of the reinforcing elements in the co-extrusion of continuously reinforced profiles. The analyses of the joining zone between the composite partners revealed that a good connection of the two materials could be achieved. © (2014) Trans Tech Publications, Switzerland.
    view abstract10.4028/www.scientific.net/KEM.585.149
  • Mechanics of sheet-bulk indentation
    Sieczkarek, P. and Isik, K. and Ben Khalifa, N. and Martins, P.A.F. and Tekkaya, A.E.
    Journal of Materials Processing Technology 214 (2014)
    The purpose of this paper is twofold: first, it aims to characterize plastic flow and ductile fracture in sheet-bulk indentation and, second, it proposes a closed-form analytical framework that can be easily applied to estimate the through-thickness pressure and force that needs to be applied by a flat compression punch as a function of the geometry, the mechanical properties of the blanks and the friction along the blank-tool interfaces. The methodology combines experiments with properly designed tool systems, which facilitate or constrain material to flow sideways (in the direction of the length), and analytical developments build upon the upper bound method for upsetting, transition to die filling and die filling of sheet-bulk compression by a flat punch. Experimental work with aluminium EN AW-1050A shows that depending on the geometry of the punch, the physics of sheet-bulk indentation may exclusively involve plastic flow or may result from a combination of plastic flow and fracture to detach surfaces from the neighbouring regions of the blank through controlled crack propagation. Results also show that the mechanics of sheet-bulk indentation can be easily and effectively analyzed by means of sheet-bulk compression under plane strain conditions. © 2014 Elsevier B.V.
    view abstract10.1016/j.jmatprotec.2014.05.018
  • Modeling and finite element simulation of loading-path-dependent hardening in sheet metals during forming
    Clausmeyer, T. and Güner, A. and Tekkaya, A.E. and Levkovitch, V. and Svendsen, B.
    International Journal of Plasticity 63 (2014)
    A recent material model considering the evolution of plastic anisotropy in interstitial free steels is validated for the forming process of the channel die, a complex part. In the model the evolution of the intra-granular microstructure is represented by tensor-valued internal variables. The model accounts for the cross hardening behavior observed in rheological tests of interstitial free steels. A novel cross hardening indicator which is directly derived from the constitutive model is proposed. This cross hardening indicator is a quantitative measure for the occurrence of cross hardening in the forming process of complex parts. A correlation between the occurrence of cross hardening and larger values of the stored (elastic) energy is observed. The influence of cross hardening on the forming process is investigated, in particular, the drawing forces and the geometric deviations due to springback. The influence of cross hardening on the forming process of the channel die geometry is small. The influence of cross hardening on the more complex S-Rail geometry is larger due to larger plastic deformation and more severe loading path changes. The concept of the proposed transient hardening indicator should be applicable to other models for the evolution of plastic anisotropy. A possible use of the cross hardening indicator would be the efficient choice of the material model in the context of sheet metal forming simulations. © 2014 Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.ijplas.2014.01.011
  • Plastic flow and failure in single point incremental forming of PVC sheets
    Alkas Yonan, S. and Silva, M.B. and Martins, P.A.F. and Tekkaya, A.E.
    Express Polymer Letters 8 (2014)
    This paper presents an innovative and effective methodology to characterize plastic flow and failure in single point incremental forming (SPIF) of polymers that allows determining the stresses and the accumulated values of ductile damage directly from the experimental values of strain at various positions over the deformed polymer sheets. The approach traces the deformation path of material elements in conical and pyramidal SPIF parts, undergoing linear strain loading paths from beginning until failure, and is built upon the generalization of the analytical framework conditions assumed by Glover et al. [1] to the pressure-sensitive yield surfaces of polymers under incompressible, non-associated, plastic flow. Experimentation in conventional and multi-stage SPIF of Polyvinylchloride (PVC) sheets confirms the effectiveness of the proposed methodology and demonstrates that standard non-coupled damage models currently utilized in sheet metal forming are inapplicable to describe failure in polymers. Instead fracture forming limit lines (FFL's) should be employed. © BME-PT.
    view abstract10.3144/expresspolymlett.2014.34
  • Recycling of aluminum chips by hot extrusion with subsequent cold extrusion
    Haase, M. and Tekkaya, A.E.
    Procedia Engineering 81 (2014)
    In this paper, the direct conversion of AA6060 aluminum alloy machining chips into finished products by hot extrusion with subsequent cold extrusion is investigated. For hot extrusion, two different types of extrusion dies, a conventional flat-face die and an experimental die, are used. The experimental die combines the process of equal channel angular pressing with the process of hot extrusion in a single die, which increases the strain and pressure affecting the chips during extrusion, both critical factors for achieving sound chip bonding. Subsequently, the chip-based extrudates are machined to fabricate chip-based preforms for the cold extrusion experiments. In order to investigate different processing routes, forward rod extrusion and backward can extrusion trials were conducted. In all steps, cast material was processed similar to the chips as a reference. The results showed that the quality of the chip-based finished parts strongly depends on the bonding quality between the individual chips, determined during the hot extrusion process. © 2014 The Authors. Published by Elsevier Ltd.
    view abstract10.1016/j.proeng.2014.10.055
  • Residual stresses in continuously reinforced composite profiles with symmetric cross sections
    Schwane, M. and Kloppenborg, T. and Haase, M. and Khalifa, N.B. and Tekkaya, A.E.
    Procedia CIRP 18 (2014)
    Composite profiles composed of aluminum base material and steel reinforcing elements manufactured by bar extrusion exhibit residual stresses. These stresses occur due to the different thermal shrinkage of the materials during cooling of the profile. In this paper, the residual stresses are investigated by means of analytical and numerical analyses. The influence of the reinforcing volume and the material behavior, i.e. the plastification of the base material, is discussed. Furthermore, the residual stresses in symmetric profile cross sections with multiple reinforcings are examined and compared to the stresses in a simple composite rod with a centric reinforcing. © 2014 Elsevier B.V.
    view abstract10.1016/j.procir.2014.06.119
  • Simulation of composite hot extrusion with high reinforcing Volumes
    Schwane, M. and Citrea, T. and Dahnke, C. and Haase, M. and Khalifa, N.B. and Tekkaya, A.E.
    Procedia Engineering 81 (2014)
    Experimental results, which indicate a significant influence of the reinforcing elements on the material flow during composite extrusion with high reinforcing volumes, are presented. In order to analyze the process numerically, finite element simulations with models taking into account the reinforcing elements were carried out. The results are discussed with regard to the material flow and the load of the reinforcing elements. © 2014 The Authors. Published by Elsevier Ltd.
    view abstract10.1016/j.proeng.2014.10.108
  • Testing and modelling of material behaviour and formability in sheet metal forming
    Bruschi, S. and Altan, T. and Banabic, D. and Bariani, P.F. and Brosius, A. and Cao, J. and Ghiotti, A. and Khraisheh, M. and Merklein, M. and Tekkaya, A.E.
    CIRP Annals - Manufacturing Technology 63 (2014)
    The paper deals with the testing and modelling of metals response when subjected to sheet forming operations. The focus is both on the modelling of hardening behaviour and yield criteria and on the description of the sheet metal formability limits. Within this scope, the paper provides a critical review of the models available today for predicting the material behaviour at both industrial and scientific level, and the tests needed to identify the models' material parameters. The most recent advances in the field are also presented and discussed with particular emphasis on the challenges the sheet metal forming community is now facing. © 2014 CIRP.
    view abstract10.1016/j.cirp.2014.05.005
  • Thermal loads of working coils in electromagnetic sheet metal forming
    Gies, S. and Löbbe, C. and Weddeling, C. and Tekkaya, A.E.
    Journal of Materials Processing Technology 214 (2014)
    One basic problem of electromagnetic forming is the limited tool life. Besides the mechanical loads especially thermal loads acting on the tool coil affect its lifetime. In electromagnetic forming, about 50% of the deployed electrical energy is lost because of joule heating in the working coil. In case of high volume production, an accumulation of this heat promotes failure of the coil and reduces the coil lifetime. Despite this importance of the thermal loads only insufficient information about the coil temperature and its influencing parameters is available. Focus of this paper is on the determination of the temperature distribution in case of long-term discharge sequences. Experimental investigations using an infrared camera were performed to measure the coil surface temperature. Numerical process simulation is used to gather information about the temperature inside the working coil. The results prove that the coil reaches an equilibrium temperature after several discharges. For the analyzed range of input power the maximum coil surface temperature and the maximum coil winding temperature reached values of 92 °C and 178 °C, respectively. These temperatures exceed the weakening temperature of most reinforcement and insulation materials. The derived knowledge about the parameters influencing the coil temperature can be used for an improved process design to avoid thermal overstressing of the coil. A comparison of experiments with and without workpiece deformation revealed that the temperature in case of prevented deformation is always higher, and thus, represents an upper bound for the coil temperature. © 2014 Elsevier B.V.
    view abstract10.1016/j.jmatprotec.2014.05.005
  • Tool design induced anisotropic flow behavior of hot extruded aluminum profiles
    Ossenkemper, S. and Haase, M. and Soyarslan, C. and Jäger, A. and Tekkaya, A.E.
    Key Engineering Materials 585 (2014)
    The hot extrusion process may lead to frequently observed textures in the profiles, like fiber structures in longitudinal direction. Aluminum profiles (AA6060) were extruded with different tool types (flat-face dies and modified porthole dies). Compression tests of cylindrical specimens, which were machined out of these profiles likewise in extrusion direction, were conducted to examine possible effects of the in-plane anisotropy in lateral direction. A hardness distribution over the cross section of the specimens was measured. It was found that dependent on tool design and profile geometry, the specimens developed preferred lateral flow directions during upsetting. Simulations of the upsetting test, with assigned Hill parameters to consider anisotropy of the material, showed, that this anisotropy, not the local hardness nonuniformity, is the main reason for the detected plastic flow properties. © (2014) Trans Tech Publications, Switzerland.
    view abstract10.4028/www.scientific.net/KEM.585.131
  • Uniform pressure electromagnetic actuator - An innovative tool for magnetic pulse welding
    Weddeling, C. and Hahn, M. and Daehn, G.S. and Tekkaya, A.E.
    Procedia CIRP 18 (2014)
    The uniform pressure electromagnetic actuator (UPEA) is an innovative tool design for electromagnetic forming applications. In this article its suitability for magnetic pulse welding is demonstrated. To facilitate the process design, a simple mathematical model based on analytical equations describing the electromagnetic behavior of the system and the mechanical behavior of the workpieces under impulse loads is presented in this manuscript. The goal of the model is to predict the workpiece velocity considering the input energy, equipment and setup characteristics as well as mechanical properties of the workpiece. To validate the model, experimental analyses with aluminum-to-aluminum joints were conducted. © 2014 Elsevier B.V.
    view abstract10.1016/j.procir.2014.06.124
  • What students use: Results of a survey on media use among engineering students
    May, D. and Lensing, K. and Tekkaya, A.E. and Grosch, M. and Berbuir, U. and Petermann, M.
    Proceedings - Frontiers in Education Conference, FIE 2015-February (2014)
    Nowadays, university students are facing a large number of highly diverse media, including conventional books as well as online-based mobile applications - all used to support learning. Especially the internet with its connected social media services or e-learning possibilities induced significant changes in society and in the landscape of higher education during the last years and still do so. The four universities RWTH Aachen University, Ruhr-University Bochum, TU Dortmund University, and the Karlsruhe Institute of Technology conducted an exploratory student survey on media and information use, in order to expand the empirical database on that topic. A special focus was laid on mobile learning. In this context the survey asked for hardware and software the students are using and for those situations where they already got in contact with any kind of mobile learning - e.g. by using special apps for learning or because they were asked by their teachers to use a mobile device. The results of the survey elucidate that the use of online media and especially social media as well as mobile devices in higher education are in need to be promoted in future. Furthermore, it reveals demands for action in the field of media competency concerning students as well as teachers. © 2014 IEEE.
    view abstract10.1109/FIE.2014.7044002
  • 3D Numerical Analysis of 2D Profile Bending with the Torque Superposed Spatial Bending Method
    Hudovernik, M. and Staupendahl, D. and Gharbi, M. and Hermes, M. and Tekkaya, A. E. and Kuzman, K. and Slabe, J. M.
    Strojniski Vestnik-journal of Mechanical Engineering 59 (2013)
    Engineering research, in the field of light weight design, is strongly oriented towards the development of new high strength materials and innovative forming methods, capable of withstanding limitations with regard to the wide variety of technological and economical aspects. Cost effective lightweight construction, in addition to the reduction of energy/material consumption and overall reduction of weight, also strongly depends on stability, continuity and robustness of production processes. Kinematic solutions for the production of spatial designed structures, in terms of 3D bending of profiles and tubes, show great potential in an increase of efficiency in the field of light weight design. The Torque Superposed Spatial bending method - TSS, developed at the Institute of Forming Technology and Lightweight Construction, Technische Universitat Dortmund, represents an innovative, robust and cost effective technical solution for 2D and 3D bending of tubes and profiles and offers a wide spectrum of capabilities, such as process continuity, parameter adaptation and flexibility for spatial bending of profiles with arbitrary cross-sections. In this paper, an introduction to 3D numerical analysis of the 2D profile bending method using TSS method is introduced and presented. The first objective of the work is to establish validity of the numerical model for the bending parameters, such as the bending force and bending momentum. Secondly, further investigations of the state of stresses and strains during load and unload conditions were performed. These are important for any further analysis and understanding of spring-back, residual stresses and cross section deformation of the profiles. The numerical simulations are performed with the use Abaqus software code, with elastic plastic material characteristics, and are, for the purpose of validation, compared to experimental data.
    view abstract10.5545/sv-jme.2012.483
  • A numerical study on intended and unintended failure mechanisms in blanking of sandwich plates
    Chen, L. and Soyarslan, C. and Tekkaya, A.E.
    AIP Conference Proceedings 1532 (2013)
    Metal-polymer-metal sandwich plates are widely used in the automotive and aerospace industry. As for different applications the sandwich plates can be divided into two types. They are sound-damping laminates with a polymer core much thinner than the metallic faces and low-density laminates with a core thickness of approximately 40-60% of the total thickness. One frequent process step in production of parts made of these plates is the blanking process whose hereditary effects draw the limits of further forming stages or service performance and life; e.g. the failure of the adhesive in the thermoplastic polymer interface affects the sound-damping efficiency intensively. With this motivation, we present FE simulation of an axi-symmetric blanking process of steel/polyethylene/steel sound-damping laminates. The mechanical behavior of the metallic layers was characterized by finite strain rate independent elasto-plasticity where progressive material deterioration and fracture are given account for using continuum damage mechanics (CDM). This material model is made accessible via implementations as VUMAT subroutines for ABAQUS/Explicit. Possible failure of the thermoplastic polymer which may lead to delamination of the metallic layers is modeled using ABAQUS built-in cohesive zone elements. The results show that existing intended and unintended failure modes, e.g. blanking of the metallic and thermoplastic polymer constituents as well as failure of polymer layer under shear and compression, can be effectively studied with the proposed framework for process enhancement. As a future work, a damage coupled nonlinear visco-elastic constitutive model will be devised for the simulation of the thermoplastic layer in low-density laminates. © 2013 AIP Publishing LLC.
    view abstract10.1063/1.4806909
  • A simple finite strain non-linear visco-plastic model for thermoplastics and its application to the simulation of incremental cold forming of polyvinylchloride (PVC)
    Alkas Yonan, S. and Soyarslan, C. and Haupt, P. and Kwiatkowski, L. and Tekkaya, A.E.
    International Journal of Mechanical Sciences 66 (2013)
    This paper introduces a finite strain extension of a non-linear visco-plastic material model, previously proposed by the authors, and its application to the finite element simulation of incremental cold forming processes of thermoplastics, demonstrated on PVC. Preserving the original structure of the model, its finite strain extension does not rely on any presumed kinematic split, either multiplicative or additive, among elastic and inelastic parts. It uses a systematic replacement of the strain and stress tensors and their rates by their respective spatial counterparts. A deviatoric Oldroyd rate is introduced to preserve the objectivity as well as the deviatoricity of the integration of the rate forms of deviatoric tensors. To cope with the incremental loading paths within the process, where through-thickness variations of the variables gain importance, the material model is posed in 3D formulation. The developed model is implemented as an ABAQUS®/UMAT subroutine and used in the simulations following parameter identification studies. The numerical results are compared with analogous experimental ones to evaluate the performance of the material model where PVC sheets of three different thicknesses are formed incrementally with path controlled tool force monitoring. The investigations have the following consequences: the deformation-limited homogeneous stress-strain portion at uni-axial tensile tests, which is generally used in parameter identification of the constitutive model, is not able to reflect the post necking regime and its extrapolation ends up with a stiffer response with much less retained strains. Once a semi-inverse parameter identification is followed by taking into account the overall experimental outputs, one ends up with a considerable improvement in the tool force, geometry and the wall thickness predictions. Nevertheless, these improvements are inversely proportional with the sheet thickness where the local indentation effects (strains and stresses) become larger. © 2012 Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.ijmecsci.2012.11.007
  • Achieving high strains in sheet metal characterization using the in-plane torsion test
    Yin, Q. and Kolbe, J. and Haupt, M. and Tekkaya, A.E.
    Key Engineering Materials 554-557 (2013)
    The in-plane torsion test is used to determine plastic flow curves for sheet metals. Very high strains of up to an equivalent strain of 1.0 can be measured since there are no edge effects in a plane torsion specimen. In combination with optical strain measurement, an efficient evaluation method for this test was developed. However, the achievable strain varies for each material. The slippage between the inner clamps and the specimen was found to be one main limiting effect. In order to improve the clamping capability, different surface corrugations are applied at the inner clamping tool. Four sheet materials, DC06, DP600, AA6016, and AA5182 are selected for testing this new clamping setup. While the flow curve of DC06 is determined until a strain of 1.0 and above, such high values cannot be achieved for the other materials. It can be shown that the measurable strain can be increased by the choice of the surface corrugation features at the inner clamping. For the DP steel and the aluminum alloys, the flow curve can be determined until equivalent plastic strains of 0.5 to 0.6, which is also a significant improvement compared to many other sheet metal testing methods. Copyright © 2013 Trans Tech Publications Ltd.
    view abstract10.4028/www.scientific.net/KEM.554-557.77
  • Composite extrusion of thin aluminum profiles with high reinforcing volume
    Pietzka, D. and Khalifa, N.B. and Gerke, S. and Tekkaya, E.
    Key Engineering Materials 554-557 (2013)
    The combination of different materials within aluminum profiles offers significant potential for increasing the mechanical properties as well as the functionality [1]. Direct extrusion using special porthole dies which feed elements in terms of continuous wires through bridges into the aluminum base material flow, was studied to manufacture continuous reinforced profiles. To achieve an essential advantage of the technology for lightweight applications a high reinforcing volume of aluminum profiles is targeted. A comparatively high reinforcing volume can be reached either by a high number of reinforcing elements or through a reduction of the profile wall thickness. A high number of reinforcing elements leads to a small distance between the single elements in the profile cross-section. The paper will show the results of an experimental and numerical analysis carried out to determine the minimum distance between the reinforcing elements as well as the minimum allowable profile thickness. In the trials different arrangements of the elements in the profile cross-section and profile thicknesses were considered. Main parameters which have an influence on the process stability were analyzed and a process window for the manufacture of thin profiles with high reinforcing volume was deduced. Copyright © 2013 Trans Tech Publications Ltd.
    view abstract10.4028/www.scientific.net/KEM.554-557.801
  • Controlling heat balance in hot aluminum extrusion by additive manufactured extrusion dies with conformal cooling channels
    Hölker, R. and Jäger, A. and Ben Khalifa, N. and Tekkaya, A.E.
    International Journal of Precision Engineering and Manufacturing 14 (2013)
    To prevent overheating of the workpiece material, an extrusion die with integrated local cooling was designed and manufactured by selective laser melting (SLM) as an additive manufacturing technology. The major advantage of SLM is the geometric freedom of the components that can be manufactured, which has been used to produce a die with integrated multidirectional channels for a cooling medium and the integration of thermocouples for temperature measurement. To analyze the influence of the die cooling on the heat balance in hot extrusion, extrusion trials at different ram speeds and billet preheating temperatures with and without applying die cooling were performed. Compressed air was used as coolant. At lower ram speeds, a significant reduction of the profile's exit temperature in hot aluminum extrusion was achieved without causing an excessive rising of the extrusion force. At higher production speeds, surface defects in the shape of stripes of rough surfaces occurred but could be prevented by applying internal die cooling. Due to focusing of the heat exchange on the area of the die bearings, only a little influence of the cooling on the microstructure can be observed. © 2013 Korean Society for Precision Engineering and Springer-Verlag Berlin Heidelberg.
    view abstract10.1007/s12541-013-0200-1
  • Effect of die design on the welding quality during solid state recycling of AA6060 chips by hot extrusion
    Güley, V. and Güzel, A. and Jäger, A. and Ben Khalifa, N. and Tekkaya, A.E. and Misiolek, W.Z.
    Materials Science and Engineering A 574 (2013)
    Solid state recycling of aluminum chips by hot extrusion is a novel processing technique, which utilizes remarkably lower energies compared to conventional recycling by remelting. The mechanical properties of the extruded profiles can be improved by optimizing the effect of extrusion die design on the welding quality of machining chips. The chips were extruded through two dies of different design to produce solid rectangular profiles. One of the dies was a flat-face die, which represents a conventional extrusion die design for production of solid aluminum profiles. The second die was a porthole die typically used for complex hollow and semi hollow aluminum profiles. AA6060 chips were compacted at room temperature into billets and hot-extruded at approximately 500 °C to aluminum profiles. The microstructure and the mechanical properties of the profiles extruded through the flat-face and porthole dies were compared. The extrusion through the porthole die resulted in a much better welding of the chips and revealed more than 80% higher ductility compared to the profiles extruded through a flat-face die. The welding quality of the chips was studied using a two-step analytical approach: a criterion for the breaking of the oxide layers and an index for the welding quality. These analytical approaches were implemented with the help of subroutines in the FEM code, in which the results of the simulations were compared and confirmed by the experimental results. © 2013 Elsevier B.V.
    view abstract10.1016/j.msea.2013.03.010
  • Friction analysis of thermally sprayed coatings finished by ball burnishing and grinding
    Tekkaya, A.E. and Kleiner, M. and Biermann, D. and Hiegemann, L. and Rausch, S. and Franzen, V. and Kwiatkowski, L. and Kersting, P.
    Production Engineering 7 (2013)
    Thermally sprayed coatings offer a promising approach as efficient method to increase the wear-resistance of sheet metal forming tools. However, the roughness of thermally sprayed surfaces is quite high. The use of these coatings for deep drawing tools results in poor sheet surface qualities and low drawing ratios. Because it is suspected that high friction is the reason for the low drawability, hard metal coatings (WC-12Co), deposited by high velocity oxygen fuel flame-spraying, were machined by grinding and ball burnishing to improve their friction behavior and the accuracy of the tool shape. The investigation was conducted by plane strip drawing tests. Strips of high strength steel were mated with these novel and effective coatings at different normal contact pressures and drawing velocities. Uncoated friction elements made of C60 steel were considered as reference during the analysis. The results revealed that coated but unmachined friction elements showed high friction values, which led to scratch marks on the sheet surface after drawing. Applying the finishing processes, the friction coefficient could be reduced significantly. Additionally, deep drawing tests were carried out to determine the drawing ratio for coated, unmachined as well as for processed, coated dies. Thermally sprayed and ball burnished as well as thermally sprayed and ground coatings are feasible for deep drawing. Due to the post treatment, the drawing ratio β = 1.8 was increased to 2.0. This is consistent to the results of the friction tests. © 2013 German Academic Society for Production Engineering (WGP).
    view abstract10.1007/s11740-013-0485-z
  • Fundamentals for controlling thickness and surface quality during dieless necking-in of tubes by spinning
    Kwiatkowski, L. and Tekkaya, A.E. and Kleiner, M.
    CIRP Annals - Manufacturing Technology 62 (2013)
    Dieless necking-in by spinning is a highly flexible process to manufacture tubular parts with variable cross-sections and nearly arbitrary contours. However, the thickness distribution of such products is influenced primarily by the toolpath. Based on analytical models this study introduces the fundamentals to control the tube thickness. Two principal tool movements are identified causing different deformation modes: shear-necking, which leads to thickening, and stretch-necking, which leads to thinning. Based on an additional model for the surface quality general criteria are derived to setup basic process parameters. The developed approach is validated by various experiments. © 2013 CIRP.
    view abstract10.1016/j.cirp.2013.03.054
  • Grain size evolution simulation in aluminium alloys AA 6082 and AA 7020 during hot forward extrusion process
    Foydl, A. and Segatori, A. and Ben Khalifa, N. and Donati, L. and Brosius, A. and Tomesani, L. and Tekkaya, A.E.
    Materials Science and Technology (United Kingdom) 29 (2013)
    The present paper investigates the grain size evolution in aluminium alloys AA 6082 and AA 7020 during hot forward extrusion process. The aim of the present work is the definition and implementation of a predictive algorithm that is able to compute the evolution of the grain shape during the process within the finite element method code Deform. Extrusion experiments were performed at two levels: at reduced scale for investigating and identifying the predictive equations and at industrial scale for validating the developed algorithm. At small scale extrusion, a complete factorial plan was performed for two alloys at three different temperatures, three extrusion ratios and two ram speeds: the discards and extrudates from the experiments were quenched immediately in order to avoid any potential recrystallisation, hence allowing measurements of transitional processing steps. At the industrial scale, instead, the 7020 alloy was extruded with two different die designs, thus producing a 20 mm diameter round bar under different extrusion ratios and strain paths. Finite element simulations were initially validated over visioplastic investigations in order to establish an accurate computation of the material flow, then experimental and numerical results were coupled, thus allowing the definition of the grain evolution model that was successfully integrated and validated on industrial scale trials. © 2013 Institute of Materials, Minerals and Mining.
    view abstract10.1179/1743284712Y.0000000132
  • Groove filling characteristics and strength of form-fit joints produced by die-less hydroforming
    Gies, S. and Weddeling, C. and Kwiatkowski, L. and Tekkaya, A.E.
    Key Engineering Materials 554-557 (2013)
    The manufacturing of modern lightweight structures and the implementation of multimaterial concepts, for example in automotive engineering, entails appropriate joining technologies. The absence of additional connection elements or filling materials as well as the possibility to join dissimilar metals are basic requirements in this field of application to reach the aspired weight reduction. In case of tubular joints the die-less hydroforming process meets these demands and thus makes it an interesting alternative to conventional welding and riveting processes. The present work focuses on form fit joints produced by die-less hydroforming. It provides a verification of a previously presented analytical approach that allows the calculation of the working fluid pressure which is required to bulge the tube material into the groove of the outer joining partner. For that purpose, the groove filling characteristics of joined specimens with different groove geometries are analyzed. Here both joining partners were made of the aluminum alloy EN AW-6060. Additionally the connection strength of the joined specimens is determined using tensile tests. The results prove that the groove angle is the most important factor on the connection strength and that it can be used for an ordinal comparison of different groove geometries. Copyright © 2013 Trans Tech Publications Ltd.
    view abstract10.4028/www.scientific.net/KEM.554-557.671
  • In-Situ Stress Analysis with X-Ray Diffraction for Yield Locus Characterization of Sheet Metals
    Guner, A. and Zillmann, B. and Lampke, T. and Tekkaya, A. E.
    Numisheet 2014: The 9th International Conference and Workshop on Numerical Simulation of 3d Sheet Metal Forming Processes: Part a Benchmark Problems and Results and Part B General Papers 1567 (2013)
    A main problem in the field of sheet metal characterization is the inhomogeneous plastic deformation in the gauge regions of specimens which causes the analytically calculated stresses to differ from the sought state of stress acting in the middle of the gauge region. To overcome this problem, application of X-Ray diffraction is analyzed. For that purpose a mobile X-ray diffractometer and an optical strain measurement system are mounted on a universal tensile testing machine. This enables the recording of the whole strain and stress history of a material point. The method is applied to uniaxial tension tests, plane strain tension tests and shear tests to characterize the interstitial free steel alloy DC06. The applicability of the concepts of stress factors is verified by uniaxial tension tests. The experimentally obtained values are compared with the theoretical values calculated with crystal elasticity models utilizing the orientation distribution functions (ODF). The relaxation problem is addressed which shows itself as drops in the stress values with the strain kept at a constant level. This drop is analyzed with elasto-viscoplastic material models to correct the measured stresses. Results show that the XRD is applicable to measure the stresses in sheet metals with preferred orientation. The obtained yield locus is expressed with the Yld2000-2D material model and an industry oriented workpiece is analyzed numerically. The comparison of the strain distribution on the workpiece verifies the identified material parameters.
    view abstract10.1063/1.4850059
  • Incremental Analysis of Springback and Kinematic Hardening by the Variation of Tension During Deep Drawing
    ul Hassan, H. and Guner, A. and Ben Khalifa, N. and Tekkaya, A. E.
    Numisheet 2014: The 9th International Conference and Workshop on Numerical Simulation of 3d Sheet Metal Forming Processes: Part a Benchmark Problems and Results and Part B General Papers 1567 (2013)
    Springback is considered as one of the major problems in sheet metal forming. It leads to assembly defects and cause a huge amount of cost for tool modifications. In this work a tool for incremental analysis of springback analysis has been presented. Development of springback with punch travel has been analyzed for the simple U draw-bend geometry, tunnel geometry with open base and modified tunnel geometry with closed base and variable flange height. The effect of tension variation in the sheet with punch travel has been considered as the steering parameter for the springback and various profiles of varying tension are studied, which would generate different tensile forces in sheet. It is found that the tension in the part in the last quarter of punch travel has a profound effect on the springback reduction as compared to the traditionally applied constant BHF. Two selected kinematic hardening models, namely Yoshida-Uemori(YU) model and Armstrong-Frederick(AF) model are used to study the coupled effects of tension and material hardening.
    view abstract10.1063/1.4850070
  • Innovative tools to improve incremental bulk forming processes
    Sieczkarek, P. and Kwiatkowski, L. and Tekkaya, A.E. and Krebs, E. and Kersting, P. and Tillmann, W. and Herper, J.
    Key Engineering Materials 554-557 (2013)
    Sheet-bulk metal forming is an innovative process with a high potential to generate load-adapted parts with high precision. Bulk forming processes of sheet metals especially require high process forces, resulting in an intense contact pressure and, thus, in a very high abrasive and adhesive wear. As a method to reduce or avoid these common wear phenomena, even hardened or coated tool surfaces are not sufficient. The objective of this paper is to show an improvement of the tool resistance during an incremental forming process by an adapted tool design and the application of structured tool surfaces combined with coatings. For the tool surface the structure of the scarabaeus beetle serves as the basis for a bionic structure. This structure was manufactured by micromilling. Despite the high hardness of the tool material and the complex geometry of the forming tools, very precise patterns were machined successfully using ball-end milling cutters. The combination of bionic structures with coating techniques like physical vapor deposition (PVD) on plasma nitrided tool surfaces is very promising. In this work, the influence of process parameters (workpiece material, lubrication, tool design, stepwise infeed) on the tool resistance during the forming operation was analyzed experimentally. The results of the optimized forming tools were compared to conventional, unstructured, uncoated, and only plasma nitrided forming tools. The different tools were applied to 2 mm thick metal sheets made of aluminum (AlMg3) and steel (nonalloy quality steel DC04). As a result, the process forces could be reduced by a modified shape and surface of the tools. Thus, the lifetime of the tools can be enhanced. Copyright © 2013 Trans Tech Publications Ltd.
    view abstract10.4028/www.scientific.net/KEM.554-557.1490
  • Inverse identification of CDM model parameters for DP1000 steel sheets using a hybrid experimental-numerical methodology spanning various stress triaxiality ratios
    Yue, Z.M. and Soyarslan, C. and Badreddine, H. and Saanouni, K. and Tekkaya, A.E.
    Key Engineering Materials 554-557 (2013)
    A hybrid experimental-numerical methodology is presented for the identification of the model parameters regarding a mixed hardening anisotropic finite plasticity fully coupled with isotropic ductile damage in which the micro-crack closure effect is accounted for, for DP1000 steel sheets. The experimental tests involve tensile tests with smooth and pre-notched specimens and shear tests using recently proposed specimen [16]. These tests cover stress triaxiality ratios lying between 0 (pure shear) and 1 / 3 (plane strain). To neutralize machine stiffness effects, displacements of the chosen material surface pixels are kept track of using the digital image correlation system ARAMIS, where recorded inputs are synchronized with force measurements. Advanced constitutive equations fully coupled with ductile damage implemented into ABQUS/Explicit using a user defined material subroutine VUMAT are used. 3D hexahedral elements (rather than thin shells elements) are used to model the tests and the identification methodology combines the FEM using the VUMAT together with experimental results using an appropriate inverse method in framework of MATLAB. The validity of the material model and transferability of its parameters are checked using tests involving complex strain paths. Copyright © 2013 Trans Tech Publications Ltd.
    view abstract10.4028/www.scientific.net/KEM.554-557.2103
  • JMPT in different countries
    Tekkaya, A. E. and Allwood, J.M.
    Journal of Materials Processing Technology 213 (2013)
    view abstract10.1016/S0924-0136(12)00320-2
  • Joining by plastic deformation
    Mori, K.-I. and Bay, N. and Fratini, L. and Micari, F. and Tekkaya, A.E.
    CIRP Annals - Manufacturing Technology 62 (2013)
    As the scale and complexity of products such as aircraft and cars increase, demand for new functional processes to join mechanical parts grows. The use of plastic deformation for joining parts potentially offers improved accuracy, reliability and environmental safety as well as creating opportunities to design new products through joining dissimilar materials. This paper aims to provide an overview of the state of the art in such joining processes, including cold welding, friction stir welding, self-pierce riveting, mechanical clinching and joining by forming. The paper includes description of the mechanism of joint formation, and analysis of joint performance and applicability. © 2013 CIRP.
    view abstract10.1016/j.cirp.2013.05.004
  • MiniLABs drop in and become fascinated by engineering experiments
    Pleul, C. and Hermes, M. and Chatti, S. and Tekkaya, A.E.
    IEEE Global Engineering Education Conference, EDUCON (2013)
    Laboratory experiments play a significant role in engineering education. The main concern of the described hands-on miniLABs initiative (as a work in progress) is lowering the hurdles in order to provide engineering students with an informal and straightforward access to experiments carried out in labs of the IUL at TU Dortmund University. miniLABs will offer students different, short and voluntary hands-on lab sessions, consisting of two different modes and different aspects related to manufacturing technology in the field of forming processes. In small teams, students can get in touch with practical engineering activities in the fields of present scientific research, either to study a certain phenomenon or to look at a wider engineering context. Based on the framework of experiential learning, miniLABs tries to foster the shift from teaching to deep learning. Finally, this initiative aims to inspire young students for real and hands-on engineering experiments and to contribute to the science education of these young and future engineers. © 2013 IEEE.
    view abstract10.1109/EduCon.2013.6530181
  • Modeling approach for the determination of material flow and welding conditions in porthole die extrusion with gas pocket formation
    Schwane, M. and Gagliardi, F. and Jäger, A. and Ben Khalifa, N. and Tekkaya, A.E.
    Key Engineering Materials 554-557 (2013)
    The material flow in porthole dies is of crucial importance with regard to the seam weld quality in aluminum extrusion. Thus, experimental as well as numerical investigations on the effect of die geometry on the material flow were conducted. The experimental tests were performed on a 10 MN laboratory extrusion press. During the experimental trials, the extrusion ratio was varied by means of exchangeable die plates. Since the modular die allows removal of the aluminum in the welding chamber as well as in the feeders after the process, the material flow could be inspected in detail. The experimental results were used to improve the accuracy of FEA simulations, which were also conducted by commercial software. An attempt was made to improve the result quality of Eulerian FEA model regarding the simulation of an extrusion process with a gas pocket in the welding chamber. The influence of the modeling approach on the predicted material flow and on the contact pressure was analyzed and finally linked to the seam weld quality. Copyright © 2013 Trans Tech Publications Ltd.
    view abstract10.4028/www.scientific.net/KEM.554-557.787
  • Novel five-axis forming press for the incremental sheet-bulk metal forming
    Sieczkarek, P. and Kwiatkowski, L. and Ben Khalifa, N. and Tekkaya, A.E.
    Key Engineering Materials 554-557 (2013)
    The incremental procedure of sheet-bulk metal forming was classified into two different forming sequences, the discrete and the continuous. Based on these two groups, a movement matrix was developed, which captures required kinematic motions to manufacture a variety of functional components. With the objective of producing near-net-shape workpiece geometries within the Collaborative Research Centre TR73 - sheet-bulk metal forming, the required positioning accuracies of conventional metal forming machines exceed the current state of the art. Therefore, a suitable machine concept was developed and realized. This new machine represents a unique prototype for a flexible application of bulk forming operations to 2 - 3 mm sheets with five motion axes. During continuous forming, such as rolling, and also during simultaneous operations, increased lateral forces prevail. The machine was provided with a high stiffness. That enables a positioning accuracy which, also under load and at rest, correlates the high demands of the sheetbulk metal forming within a range of ±0.01 mm. Copyright © 2013 Trans Tech Publications Ltd.
    view abstract10.4028/www.scientific.net/KEM.554-557.1478
  • Numerical investigation of the incremental tube forming process
    Becker, C. and Isik, K. and Bayraktar, A. and Chatti, S. and Hermes, M. and Soyarslan, C. and Tekkaya, A. E.
    Key Engineering Materials 554-557 (2013)
    As a response to the recent years' growing demand for innovation in manufacturing processes towards lightweight design in several industrial sectors, a new process, called Incremental Tube Forming (ITF), and a corresponding machine layout have been developed. ITF is a process to manufacture bent tubes with varying cross-sections. During ITF a tube is clamped in a feeding device, which transports the tube through a spinning tool, where the diameter reduction takes place. This stage is followed by a superposed bending process without suppressing continuous feeding. This combination leads to various advantages such as improved tool life with reduced tool forces and improved product accuracy (e.g. springback behavior), as it is shown in various experimental works. This paper presents a complementary numerical treatment of the process using FEA. For this purpose, a 3D model is constructed using ABAQUS/Explicit, where the tube is modeled with conventional shell elements with uniformly reduced integration to avoid shear and membrane locking (S4R), whereas the spinning rolls are modeled as discrete rigid. With this model, the influences of process parameters, such as diameter reduction ratio and tool geometry, are investigated. This helps not only to gain a deeper understanding of the process but also to interpret already gathered experimental data with better precision and, thus establishing a basis for further improvement and optimization of this fairly new process. Copyright © 2013 Trans Tech Publications Ltd.
    view abstract10.4028/www.scientific.net/KEM.554-557.664
  • Prediction of surface roughness due to spinning in the incremental tube forming process
    Becker, C. and Quintana, G. and Hermes, M. and Cavallini, B. and Tekkaya, A.E.
    Production Engineering 7 (2013)
    Incremental tube forming (ITF) is a new process allowing a flexible manufacturing of 2D and 3D bent tubes with load-optimized cross sections by means of the combination of the procedures spinning and bending. The aim of this paper is to acquire an in-depth process understanding concerning the surface roughness. This paper focuses on the spinning process operation of the ITF process. The influence of the spinning roll geometry and the process parameters on the theoretical surface roughness is studied in detail. Crest height h and roughness average parameter Ra are formulated as function of process parameters and spinning roll geometry. Also, a fishbone diagram with the parameters influencing the tube surface characteristics is provided. Experiments are performed to quantify the divergences of the equations. The theoretical approach can be used to understand the incremental tube forming process in more detail. © 2012 German Academic Society for Production Engineering (WGP).
    view abstract10.1007/s11740-012-0424-4
  • Recycling of Aluminum Chips by Hot Extrusion
    Haase, M. and Jager, A. and Tekkaya, A. E.
    New Production Technologies in Aerospace Industry (2013)
    view abstract10.1007/978-3-319-01964-2_26
  • Simulation of electromagnetic forming of a cross-shaped cup by means of a viscoplasticity model coupled with damage at finite strains
    Kiliclar, Y. and Demir, O.K. and Vladimirov, I.N. and Kwiatkowski, L. and Reese, S. and Tekkaya, A.E.
    Key Engineering Materials 554-557 (2013)
    In the field of sheet metal forming conventional forming processes are well established. However, a quasi-static forming process combined with a high speed forming process can enhance the forming limits of a single one. In this paper, the investigation of the process chain quasi-static deep drawing - electromagnetic forming by means of a new coupled damage-viscoplasticity model for large deformations is performed. The finite strain constitutive model, used in the finite element simulation combines nonlinear kinematic and isotropic hardening and is derived in a thermodynamically consistent setting. This anisotropic viscoplastic model is based on the multiplicative decomposition of the deformation gradient in the context of hyperelasticity. The kinematic hardening component represents a continuum extension of the classical rheological model of Armstrong-Frederick kinematic hardening. The coupling of damage and plasticity is carried out in a constitutive manner according to the effective stress concept. The constitutive equations of the material model are integrated in an explicit manner and implemented as a user material subroutine in the commercial finite element package LS-DYNA with the electromagnetical module. The aim of the work is to show the increasing formability of the sheet by combining quasistatic deep drawing processes with high speed electromagnetic forming. Copyright © 2013 Trans Tech Publications Ltd.
    view abstract10.4028/www.scientific.net/KEM.554-557.2363
  • A combined experimental-numerical investigation of ductile fracture in bending of a class of ferritic-martensitic steel
    Soyarslan, C. and Malekipour Gharbi, M. and Tekkaya, A.E.
    International Journal of Solids and Structures 49 (2012)
    We present a combined experimental-numerical study on fracture initiation at the convex surface and its propagation during bending of a class of ferritic-martensitic steel. On the experimental side, so-called free bending experiments are conducted on DP1000 steel sheets until fracture, realizing optical and scanning electron microscopy analyses on the post mortem specimens for fracture characterization. A blended Mode I - Mode II fracture pattern, which is driven by cavitation at non-metallic inclusions as well as martensitic islands and resultant softening-based intense strain localization, is observed. Phenomena like crack zig-zagging and crack alternation at the bend apex along the bending axis are introduced and discussed. On the numerical side, based on this physical motivation, the process is simulated in 2D plane strain and 3D, using Gurson's dilatant plasticity model with a recent shear modification, strain-based void nucleation, and coalescence effects. The effect of certain material parameters (initial porosity, damage at coalescence and failure, shear modification term, etc.), plane strain constraint and mesh size on the localization and the fracture behavior are investigated in detail. © 2012 Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.ijsolstr.2012.03.009
  • A cyclic twin bridge shear test for the identification of kinematic hardening parameters
    Yin, Q. and Soyarslan, C. and Güner, A. and Brosius, A. and Tekkaya, A.E.
    International Journal of Mechanical Sciences 59 (2012)
    A twin bridge cyclic shear test with in-plane torsion is proposed for the identification of kinematic hardening parameters for metallic sheets. Besides its simplicity, noteworthy advantages of the test are (a) reduced loads on the experimental device as compared to a one-sided shear test, (b) identical orientation of the principal stresses with respect to the rolling direction in both of the shear bridges, e.g. which cannot be realized by the classical Miyauchi shear test, and (c) no premature termination by instability mechanisms such as buckling or necking. Two main disadvantages appear to be (a) a preclusion of the use of analytically solved initial value problem in parameter identification due to a diffusivity of the plastic region around the shear bridges, (b) smeared out anisotropic material response proportional with the width of the shear bridge. As a remedy for the former, an inverse parameter identification methodology is used to determine the hardening parameters using an objective function devising the measured moment and rotation angle. For the latter, an optimum shear bridge width is selected which also minimizes the edge effect where a shear equilibrium is not possible. A combined non-linear isotropic and kinematic hardening model respectively based on Voce and Armstrong-Frederick is selected as the material model which is implemented as a VUMAT subroutine for ABAQUS/EXPLICIT. Strain-controlled tests are conducted using three different classes of steel sheet materials, namely a mild steel DC06, a dual phase steel DP600 and a Transformation Induced Plasticity steel TRIP700. These tests with one cycle including a forward shearing and a reverse shearing phase merely focus on the Bauschinger effect. Variations with different stress and strain based loading cycles for phenomena like shakedown, ratcheting, mean stress relaxation, cyclic hardening and softening are not explored and left beyond the scope of the current study. The results, besides showing the applicability of the test to the kinematic hardening parameter identification purposes, also show that the Armstrong-Frederick model falls short to capture the cyclic response of the selected materials, especially advanced high strength steels DP600 and TRIP700. © 2012 Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.ijmecsci.2012.02.008
  • A new method for determining dynamic grain structure evolution during hot aluminum extrusion
    Güzel, A. and Jäger, A. and Parvizian, F. and Lambers, H.-G. and Tekkaya, A.E. and Svendsen, B. and Maier, H.J.
    Journal of Materials Processing Technology 212 (2012)
    In this paper, a new method for analyzing the microstructure evolution of aluminum during deformation at elevated temperatures by extrusion is presented, which is entirely separated from secondary restoration effects viz. static recrystallization and grain growth. In order to observe the development of grains and their orientation under severe plastic deformation, a small-scale forward extrusion setup was designed which allows quenching the extrusion butt together with the die and the container immediately after extrusion to preserve the grain structure evolved during the deformation. The forming path and the forming history of a selected material point were calculated by numerical simulation. The evolution of the microstructure along the forming path was analyzed using electron backscatter diffraction. A database for the development of physically based phenomenological models for predicting and simulating the evolution of microstructure during the hot deformation of EN AW-6082 alloy is provided. © 2011 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.jmatprotec.2011.09.018
  • An experimental and numerical investigation on polymer melt injected sheet metal forming
    Hussain, M.M. and Trompeter, M. and Witulski, Jö. and Tekkaya, A.E.
    Journal of Manufacturing Science and Engineering, Transactions of the ASME 134 (2012)
    Polymer injection forming (PIF) is a recent advancement in manufacturing of plastic- metal hybrid products. It is a combination of injection molding and sheet metal forming in which the molten polymer additionally serves as a pressure medium. This paper presents fundamental investigations on the use of polymer melt as a pressure medium in sheet metal forming. The development of forming pressure and localized blank temperature as a result of polymer injection is analyzed. The experiments are performed using aluminum alloy. The experiments comprise the bulging of a (free form) dome and stretch forming of a cup using thermoplastic polypropylene as a pressure medium. A simple approach is presented to model a combined process. An incremental lagrangian formulation is used to describe the polymer flow and sheet metal deformation. The Newtonian behavior of the polymer melt is modeled as rigid-viscoplastic medium. Standard finite element coupling is used to model the mutual metal-polymer interaction. The simulation results in the form of forming pressure profile, localized blank temperature profile, formed shape, and strain distribution are presented and validated with experimentally obtained results. © 2012 American Society of Mechanical Engineers.
    view abstract10.1115/1.4006117
  • An experimental study on the groove design for joints produced by hydraulic expansion considering axial or torque load
    Marré, M. and Rautenberg, J. and Tekkaya, A.E. and Zabel, A. and Biermann, D. and Wojciechowski, J. and Przybylski, W.
    Materials and Manufacturing Processes 27 (2012)
    This article presents research work on the influence of the design characteristics of the joint partner elements and especially of grooving and pocketing on the tensile and torsional strength of tubular joints produced by hydraulic expansion when tubes are made of aluminum EN AW-6060. In general, joining of tubular elements can be performed by different methods, but internal expansion presents an interesting alternative to other methods like welding and mechanical forming processes. Commonly, hydraulic expansion is used for the manufacturing of heat exchangers. As a result, time effective and resistant joints must be produced in particular when applying hydraulic expansion in the manufacturing of lightweight structures. © Taylor and Francis Group, LLC.
    view abstract10.1080/10426914.2011.593232
  • Analytic prediction of the process parameters for form-fit joining by die-less hydroforming
    Gies, S. and Weddeling, C. and Marré, M. and Kwiatkowski, L. and Tekkaya, A.E.
    Key Engineering Materials 504-506 (2012)
    The Commission of the European Communities aims for a reduction of new car CO 2 emissions of 120 grams per kilometer in 2012. As a result of the omnipresent efforts of the automotive industry to hit these tighter emission standards innovative lightweight strategies, e.g. the use of lightweight materials are developed. This entails new joining techniques that are appropriated to the new lightweight materials. The die-less hydroforming process is a joining method for tubular joints that meets the new demands of lightweight strategies. Since there is no need for any additional connection elements or filling material, it is an interesting alternative to conventional welding and riveting processes. The present paper describes the basic principle of the die-less hydroforming joining technology with a special focus on form-fit connections. An analytical model, based on the membrane theory with an additional local consideration of bending stresses is developed. This analytic approach can be used to calculate the working fluid pressure, required to bulge the tube material into the groove of the outer joining partner. Taking into account the material parameters as well as the groove and tube geometry, this model allows a reliable process design. Additionally, validation of the model by experimental investigations will be provided. © (2012) Trans Tech Publications.
    view abstract10.4028/www.scientific.net/KEM.504-506.393
  • Bulk forming of sheet metal
    Merklein, M. and Allwood, J.M. and Behrens, B.-A. and Brosius, A. and Hagenah, H. and Kuzman, K. and Mori, K. and Tekkaya, A.E. and Weckenmann, A.
    CIRP Annals - Manufacturing Technology 61 (2012)
    Ever increasing demands on functional integration of high strength light weight products leads to the development of a new class of manufacturing processes. The application of bulk forming processes to sheet or plate semi-finished products, sometimes in combination with conventional sheet forming processes creates new products with the requested properties. The paper defines this new class of sheet-bulk metal forming processes, gives an overview of the existing processes belonging to this class, highlights the tooling aspects as well as the resulting product properties and presents a short summary of the relevant work that has been done towards modeling and simulation. © 2012 CIRP.
    view abstract10.1016/j.cirp.2012.05.007
  • Characterization of anisotropy of sheet metals employing inhomogeneous strain fields for Yld2000-2D yield function
    Güner, A. and Soyarslan, C. and Brosius, A. and Tekkaya, A.E.
    International Journal of Solids and Structures 49 (2012)
    A method to include the distribution of strains in the identification of the planar anisotropy of sheet metals is proposed. The method includes the optical measurement of strains on a flat specimen with a varying cross-section and an inverse parameter identification scheme which minimizes the differences between the numerical simulation results and the experimental measurements by using Levenberg-Marquardt algorithm. The main advantage is the reduction of the needed number of material tests especially for complex material models, under the assumption of negligible kinematic hardening. The utilized specimen geometry covers a deformation state between uniaxial tension and plane strain tension cases. In order to supply additional information to the inverse scheme, the equi-biaxial stress state obtained from layer compression test is also included in the definition of the objective function. The anisotropy of the sheet is modeled with the Yld2000-2D model which is implemented as a VUMAT subroutine for ABAQUS-Explicit. Numerical tests point out that the orientation of the specimen defines the quality of the found yield loci. The proposed method is applied to characterize the commercial aluminum alloy AA6016-T4 and the obtained material parameters are used to analyze a deep drawn car hood geometry. The results show that the use of the strain distribution is a crucial point in identification of the planar anisotropy. The yield loci obtained with the proposed method are in accordance with the conventionally obtained yield stresses and r-values. © 2012 Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.ijsolstr.2012.05.001
  • Determination of the flow curve at high strain rates using electromagnetic punch stretching
    Chu, Y.Y. and Lee, R.S. and Psyk, V. and Tekkaya, A.E.
    Journal of Materials Processing Technology 212 (2012)
    A simple new method is proposed and applied to determine the flow curve of aluminum alloy 1100-O at high strain rates. A high-speed camera was used to record the free flying process, from which the retrieved images were used to characterize the impacting velocity. The determined flow curve was established by combining the effective stress retrieved from the simulation and the effective strain measured from the deformed workpiece. Moreover, an iteration procedure was utilized to improve the accuracy of the determined flow curve. Using the determined flow curve to simulate the forming process, the simulated deformation performed good agreement with the experimental result, where the deviation of effective strain could be reduced from 17.9% to 6.74%. Besides, the effective strains reached in these high rate forming experiments exceed the effective strain at failure determined in a quasi-static tensile test. The material could be deformed to the effective strain of 0.56 without any fracture. © 2012 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.jmatprotec.2012.01.017
  • Dynamic forming limits and numerical optimization of combined quasi-static and impulse metal forming
    Taebi, F. and Demir, O.K. and Stiemer, M. and Psyk, V. and Kwiatkowski, L. and Brosius, A. and Blum, H. and Tekkaya, A.E.
    Computational Materials Science 54 (2012)
    Subject of this work is the incorporation of forming limits in the numerical optimization of technological forming processes for sheet metal. Forming processes with non-linear load paths and strongly varying strain-rate, such as, e.g., combinations of deep drawing and electromagnetic forming are of particular interest. While in the latter impulse forming process inertial forces play a significant role, the first one is of quasi-static nature such that inertial forces may be neglected. Although classical forming limit diagrams provide an easily accessible method for the prediction of forming limits, they cannot be applied in situations involving pulsed loading along non-linear strain paths. Hence, they are extended to forming limit surfaces here. The target function to be minimized is computed via finite-element simulation. To avoid a large number of simulations, an interior point method is employed as optimization method. In this algorithm, forming limits appear via a logarithmic barrier function, which has to be computed sufficiently fast. The optimization algorithm is exemplarily applied to an identification problem for a two-stage forming process. © 2011 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.commatsci.2011.10.008
  • Effect of porthole design and welding chamber dimensions on material flow and weld deformability of extruded aluminium profiles
    Donati, L. and Ben Khalifa, N. and Tomesani, L. and Tekkaya, A.E.
    Key Engineering Materials 504-506 (2012)
    Aim of the work is to investigate different strategies in balancing material flow during direct extrusion through porthole dies. Two AA6082 hollow profiles were simultaneously extruded by a single die with different portholes extrusion ratio, dissimilar welding chambers and different bearing lengths. A strict process control was realized by measuring thermal conditions in the die by means of 6 thermocouples and on the profiles by a self calibrating pyrometer for aluminum alloy applications. Several billets were extruded at different ram speeds (2 to 7 mm/sec) and the effect of die design on surface quality, profile lengths and thermal field was recorded. The profiles were then sectioned and the position of the seam welds in the profiles identified and compared also with the profiles tip. © (2012) Trans Tech Publications.
    view abstract10.4028/www.scientific.net/KEM.504-506.523
  • Experimental and numerical analysis of material flow in porthole die extrusion
    Kloppenborg, T. and Schwane, M. and Khalifa, N.B. and Tekkaya, A.E. and Brosius, A.
    Key Engineering Materials 491 (2012)
    The design of porthole dies for aluminum extrusion processes is very complex. For the accurate design, fundamental knowledge about material flow is of major importance. To gain these information, numerical methods are increasingly utilized. The accuracy of the simulation results depends mainly on the precision of the used boundary conditions in the model. Therefore, visioplastic analyses of the material flow inside a porthole die are presented in this paper. A special modular tool concept was developed to prepare and visualize the material flow inside the process. The results of the experimental analysis were used for the verification of numerical results which were calculated with the commercial software codes Deform3D and HyperXtrude. © (2012) Trans Tech Publications.
    view abstract10.4028/www.scientific.net/KEM.491.97
  • Extrusion benchmark 2011: Evaluation of different design strategies on process conditions, die deflection and seam weld quality in hollow profiles
    Selvaggio, A. and Segatori, A. and Guzel, A. and Donati, L. and Tomesani, L. and Tekkaya, A.E.
    Key Engineering Materials 491 (2012)
    In the paper experimental investigations aimed at allowing a detailed and accurate comparison of different FEM codes were presented and discussed. Two hollow profiles within the same die were characterized by different thicknesses within the profile, two welding chambers and critical tongues (one fully supported and one partially supported). The material flow balance was performed by means of feeder size and position on a profile and by means of bearings on the other one. Accurate monitoring of process parameters was carried out by using a self-calibrating pyrometer for profile temperature, six thermocouples for die thermal monitoring, a laser velocitymeter for profile speed and two laser sensors for die deflection on critical tongues. AA6082 alloy was used as deforming material, while H-11 hot-work tool steel was selected for the die material. The experiments were repeated at least three times under the same conditions in order to provide a nearly steady state statistical distribution of the acquired data. These are used as a reference for the 2011 edition of the extrusion benchmark. © (2012) Trans Tech Publications.
    view abstract10.4028/www.scientific.net/KEM.491.1
  • FlexDie: A flexible tooling concept for incremental sheet forming
    Sebastiani, G. and Wawrosch, A. and Franzen, V. and Brosius, A. and Tekkaya, A.E.
    Key Engineering Materials 504-506 (2012)
    Forming tasks in Sheet Metal Prototyping are currently a balancing act between part flexibility and accuracy. In view of Asymmetric Incremental Sheet Forming (AISF), the part support is the decisive factor: Die-based processes such as TPIF are restricted to the given geometry of the part. On the other hand, the die-less variant (SPIF) is prone to a much more complex process-layout - once a similar accuracy needs to be obtained. Consequently, this requires a flexible die concept, supporting the part in the critical zones only. Within this article we meet this challenge by introducing the configurable tooling concept "FlexDie" [1]. This support tool comprises a construction kit for skeleton dies allowing for an adjustment of its geometry to almost any desired shape. Based on the solar cooker benchmark by Jeswiet et al. [2], we show both the tooling-concept and the feasibility. The latter we discuss, based on the quality features geometric accuracy as well as surface quality. Both features are assessed with respect to the forming results obtained by use of a full-die. The accuracy resulting by applying the FlexDie is only slightly inferior to the parts formed by use of a full-die. However, the FlexDie allows for simple optimization of both, die and part geometry. In addition, compensation strategies by adapting the toolpath are still possible. In summary, the results show the feasibility of the FlexDie concept for industrial ISF tasks - even at very low production volumes.© (2012) Trans Tech Publications.
    view abstract10.4028/www.scientific.net/KEM.504-506.883
  • High quality extrudates from aluminum chips by new billet compaction and deformation routes
    Misiolek, W.Z. and Haase, M. and Ben Khalifa, N. and Tekkaya, A.E. and Kleiner, M.
    CIRP Annals - Manufacturing Technology 61 (2012)
    The effects of different billet preparation techniques as well as selection of various deformation routes and their influence on the final mechanical properties in chip extrusion was studied. The AA6060 chips were compacted into billets using various techniques and then extruded through the flat-face, porthole and ECAP dies to create different deformation routes. The microstructures and the mechanical properties of the chip extruded profiles were compared to cast billets extruded through the flat-face die under the same conditions. The proposed technology shows very promising results in terms of energy savings and production of the high quality engineered aluminum profiles. © 2012 CIRP.
    view abstract10.1016/j.cirp.2012.03.113
  • Improved tool surfaces for incremental bulk forming processes of sheet metals
    Sieczkarek, P. and Kwiatkowski, L. and Tekkaya, A.E. and Krebs, E. and Biermann, D. and Tillmann, W. and Herper, J.
    Key Engineering Materials 504-506 (2012)
    Sheet-bulk metal forming is a process used to manufacture load-adapted parts with high precision. However, bulk forming of sheet metals requires high forces, and thus tools applied for the operational demand have to withstand very high contact pressures, which lead to high wear and abrasion. The usage of conventional techniques like hardening and coating in order to reinforce the surface resistance are not sufficient enough in this case. In this paper, the tool resistance is improved by applying filigree bionic structures, especially structures adapted from the Scarabaeus beetle to the tool's surface. The structures are realized by micromilling. Despite the high hardness of the tool material, very precise patterns are machined successfully using commercially available ball-end milling cutters. The nature-adapted surface patterns are combined with techniques like plasma nitriding and PVD-coating, leading to a multilayer coating system. The effect of process parameters on the resistance of the tools is analyzed experimentally and compared to a conventional, unstructured, uncoated, only plasma nitrided forming tool. Therefore, the tools are used for an incremental bulk forming process on 2 mm thick metal sheets made of aluminum. The results show that the developed methodology is feasible to reduce the process forces and to improve the durability of the tools.© (2012) Trans Tech Publications.
    view abstract10.4028/www.scientific.net/KEM.504-506.975
  • Improving mechanical properties of chip-based aluminum extrudates by integrated extrusion and equal channel angular pressing (iECAP)
    Haase, M. and Ben Khalifa, N. and Tekkaya, A.E. and Misiolek, W.Z.
    Materials Science and Engineering A 539 (2012)
    In order to improve the mechanical properties of profiles extruded from aluminum chips, a four turn equal channel angular pressing tool was integrated into an extrusion die (iECAP die). AA6060 aluminum alloy turning chips were cold pre-compacted to chip-based billets and hot extruded through the iECAP die on a conventional forward extrusion press. Mechanical properties and microstructure of the chip-based billets extruded through the iECAP die were investigated and compared to those extruded through a conventional flat-face die and a porthole die. To evaluate the performance of the iECAP processed chip-based profiles, conventional cast billets were extruded through the flat-face die as a reference material. To investigate the influence of temperature on mechanical properties and microstructure of chip-based profiles, the extrusion was performed at 450. °C and 550. °C.Tensile tests revealed superior mechanical properties of the chip-based billets extruded through the iECAP die in comparison to chip-based billets extruded through the flat-face and the porthole die as well as to cast billets extruded through the flat-face die. © 2012 Elsevier B.V.
    view abstract10.1016/j.msea.2012.01.081
  • Lightweight construction by means of profiles
    Chatti, S. and Pietzka, D. and Khalifa, N.B. and Jäger, A. and Selvaggio, A. and Tekkaya, A.E.
    Key Engineering Materials 504-506 (2012)
    This paper shows some product and process developments at the Institute of Forming Technology and Lightweight Construction of the TU Dortmund University supporting the lightweight construction. It presents the manufacturing of lightweight profiles by hot extrusion and their benefits as well as their design, material, and manufacturing potential for lightweight construction. Examples of process extensions in hot extrusion like curved profile extrusion, twisted profile extrusion and manufacturing of functional graded profiles and profiles with variable crosssections during extrusion are shown. These procedures allow a flexible change of the profile geometry or contour in longitudinal axis and, therefore, support the shape lightweight construction. Other extensions like composite profile extrusion and energy efficient extrusion of profiles from scrap materials like chips support the material lightweight construction. The manufacturing and use of these profiles allow the realisation of diverse lightweight construction principles and promise to become a pillar of lightweight construction in future. © (2012) Trans Tech Publications.
    view abstract10.4028/www.scientific.net/KEM.504-506.369
  • Manufacturing of steel-reinforced aluminum products by combining hot extrusion and closed-die forging
    Foydl, A. and Pfeiffer, I. and Kammler, M. and Pietzka, D. and Matthias, T. and Jäger, A. and Tekkaya, A.E. and Behrens, B.-A.
    Key Engineering Materials 504-506 (2012)
    Aluminum extrudates reinforced with steel elements represent an innovative material concept for lightweight structures. Investigations on the process chain of extrusion and subsequent die forging to produce steel-reinforced aluminum parts were carried out. For the investigation the aluminum alloys AA6060 and AA6082 were used as workpiece material. Regarding the extrusion step the position of reinforcement elements with different shapes and their adhesion to the matrix in discontinuously-reinforced, semi-finished aluminum profiles was in focus. Sometimes defects were formed near the reinforcement elements and the nature of such defects was characterized. For the investigations on the forging step extruded profiles reinforced with steel wires were used. Finite element analyses were carried out in order to predict the position of the wires in the forged parts depending on their initial position in the extrusions. Furthermore, the flow behavior of the wires inside the forging part was investigated. © (2012) Trans Tech Publications.
    view abstract10.4028/www.scientific.net/KEM.504-506.481
  • New concepts for cooling of extrusion dies manufactured by rapid tooling
    Hölker, R. and Jäger, A. and Khalifa, N.B. and Tekkaya, A.E.
    Key Engineering Materials 491 (2012)
    To prevent local overheating of the workpiece material in hot aluminum extrusion the influence of die cooling was investigated. Numerical simulations of extrusion revealed an advantage of the die bearing cooling, which can be accomplished by locating the cooling channels close to the die/bearing surface. Since the fabrication of especially geometric complex cooling channels located near the die surface is not possible by conventional manufacturing technologies, the technology of rapid tooling was introduced into hot aluminum extrusion and experimentally tested. Cooling channels near to the bearings show promising results allowing extensions of extrusion limits, especially the extrusion speed and therefore productivity. © (2012) Trans Tech Publications.
    view abstract10.4028/www.scientific.net/KEM.491.223
  • Numerical approach for the evaluation of seam welding criteria in extrusion processes
    Schwane, M. and Kloppenborg, T. and Reeb, A. and Ben Khalifa, N. and Brosius, A. and Weidenmann, K.A. and Tekkaya, A.E.
    Key Engineering Materials 504-506 (2012)
    The accurate simulation and the optimization of extrusion processes can be a helpful technique to ensure producibility of complex aluminum profiles, for example for the automobile industry. Currently, the die designing is based on expert's knowledge and cost-intensive prototyping. The paper deals with numerical investigations based on finite element simulations as well as experimental investigations of an industrial extrusion process. A newly developed method for longitudinal seam weld prediction is applied to analyze the position of the longitudinal welding line and the welding quality. © (2012) Trans Tech Publications.
    view abstract10.4028/www.scientific.net/KEM.504-506.517
  • Processing of new solar absorbers in steel design based on partially cold roll bonded hybrid semi-finished parts
    Steinbach, F. and Koch, L. and Hermann, M. and Witulski, J. and Tekkaya, A.E.
    Key Engineering Materials 504-506 (2012)
    A new approach of manufacturing solar absorbers is presented with which it is possible to design the channel geometry as a quasi-fractal structure. The so called FracThermR structure (developed by Fraunhofer Institute for Solar Energy Systems) aims at reducing the pressure drop and at a uniform flow distribution. To manufacture these new absorbers, a fast, nearly continuous production process was developed which consists of partial cold roll bonding and subsequent hydroforming, similar to symmetric hydroforming of sheet metal pairs. With the help of a small demonstrator the feasibility of the hydroforming process is shown and the process chain is validated. © (2012) Trans Tech Publications.
    view abstract10.4028/www.scientific.net/KEM.504-506.137
  • The non-hydrostatic response of polymer melts as a pressure medium in sheet metal forming
    Tekkaya, A.E. and Hussain, M.M. and Witulski, J.
    Production Engineering 6 (2012)
    The polymer injection forming process is a recent invention for producing plastic-metal hybrids. It is a combination of injection molding and sheet metal hydroforming process in which polymer melt serves as a pressure medium. This paper presents the experimental investigations on the non-Newtonian nature of thermoplastic melt as pressure medium. The objective of this work is to identify the presence of non-hydrostatic pressure distribution within the cavity and its influence on the final shape of the formed sheet metal component. Experiments are conducted with center-gated injection mold under varying processing conditions. The development of localized cavity pressure during the process is recorded and evaluated against the final shape of formed sheet metal. It has been observed that higher injection rate, higher injection temperature, and higher melt flow index of the processed polymer is necessary for the uniform pressure distribution and subsequently uniform forming of the sheet metal. © 2012 German Academic Society for Production Engineering (WGP).
    view abstract10.1007/s11740-012-0392-8
  • Wear analysis of rolled and ground iron-based-alloy-coatings with and without hard material reinforcement
    Tillmann, W. and Luo, W. and Tekkaya, A.E. and Franzen, V.
    Materialwissenschaft und Werkstofftechnik 43 (2012)
    In this research work the wear behavior of thermal sprayed wear resistant coatings, which are finished by incremental roller burnishing and by grinding in order to smooth the surface, are analyzed by means of the Pin-on-Disc test. Two different arc sprayed coatings WSC-FeCSiMn and FeCrBSiMn are compared to each other. At first the microstructure of the smoothed coatings were characterized by investigation of the topography and morphology. After that the wear behavior was analyzed with two different counterparts made of stainless steel and ceramic. In order to determine the different wear mechanisms the wear traces have been investigated by scanning electron microscope. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/mawe.201200852
  • Writing a review paper
    Allwood, J.M. and Tekkaya, A.E.
    Journal of Materials Processing Technology 212 (2012)
    view abstract10.1016/j.jmatprotec.2011.09.003
  • A new shear test for sheet metal characterization
    Brosius, A. and Yin, Q. and Güner, A. and Tekkaya, A.E.
    Steel Research International 82 (2011)
    In order to predict plastic material behavior for sheet forming processes by finite element simulation, shear tests are useful to identify material parameters. Since the existing shear test setups have certain disadvantages, a new twin bridge torsion shear test is proposed. Stress and strain calculation is derived from the presented geometrical features. The clamping situation and the shear gauge dimensions are investigated to evaluate the quality of the obtained flow curves. It is shown that this test specimen is suitable to determine anisotropic yield behavior and to characterize prestrained specimen, for instance due to cold rolling, when the yield locus is shifted by a backstress tensor. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/srin.201000163
  • A viscoplastic material model based on overstress for the simulation of incremental sheet forming of thermoplastics
    Yonan, S.A. and Haupt, P. and Kwiatkowski, L. and Franzen, V. and Brosius, A. and Tekkaya, A.E.
    AIP Conference Proceedings 1353 (2011)
    This article introduces a phenomenological material model developed for simulation of viscoplastic behavior of thermoplastics during cold forming. The model presented has no yield surface and is based on the existence of an equilibrium stress and a non-linear strain-rate dependent overstress. Within the first approach the hardening and softening are neglected. The results of specific material characterization tests on three thermoplastics (HDPE, PVC and PC) and the model response in comparison to the experiments are presented. A good accordance between model predictions and experimental results for HDPE and sufficient match for PVC and PC are observed. It is intended to apply the constitutive model to the simulation of incremental sheet forming processes (ISF). © 2011 American Institute of Physics.
    view abstract10.1063/1.3589614
  • Accurate deep drawing simulation by combining analytical approaches
    Cwiekala, T. and Brosius, A. and Tekkaya, A.E.
    International Journal of Mechanical Sciences 53 (2011)
    The basic contribution of this work is the description of the development of an analytical simulation method for deep drawing processes. By considering multiple deformation steps, this method takes time dependent process parameters and non-linear deformation paths into account. Contrary to existing analytical approaches, this method allows an accurate strain prediction and, thus, a prediction of formability. Compared to numerical onestep solvers, the developed method is much faster, and due to a better consideration of deformation paths, also a higher accuracy is reached in simulating axisymmetric and prismatic parts. Due to its efficient combination of computation speed and accuracy, this method allows an application in fast process optimizations or online process control systems, where existing approaches are either too slow in case of numerical simulation or too inaccurate in case of analytical simulation. © 2011 Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.ijmecsci.2011.02.007
  • Advanced extrusion processes
    Ben Khalifa, N. and Selvaggio, A. and Pietzka, D. and Haase, M. and Tekkaya, A.E.
    Materials Research Innovations 15 (2011)
    This paper focuses on new extrusion processes for the improvement of product quality, the reduction of process chains and the saving of our available resources. The presented and discussed technologies cover the extrusion of reinforced profiles, curved profiles and twisted profiles. Besides, the process principles and advantages, several results from numerical and experimental investigations are given in order to show the motivation for fundamental and application oriented research in the field of extrusion technology. © W. S. Maney & Son Ltd. 2011.
    view abstract10.1179/143307511X12858957676074
  • An experimental and numerical assessment of sheet-bulk formability of mild steel DC04
    Soyarslan, C. and Fassmann, D.P.F. and Plugge, B. and Isik, K. and Kwiatkowski, L. and Schaper, M. and Brosius, A. and Tekkaya, A.E.
    Journal of Manufacturing Science and Engineering, Transactions of the ASME 133 (2011)
    This paper presents investigations on development of a new way of teeth-forming, which is related to sheet-bulk metal forming, with application of incremental bulk forming process to sheets. For this purpose, a combined experimental-numerical study on damage assessment in sheet-bulk forming of DC04 is presented. Using scanning electron microscope (SEM) and glow discharge optical emission spectrometry (GDOS), a combined quantitativequalitative metallurgical survey is carried out on undeformed specimens to illuminate microstructural aspects in the context of nonmetallic inclusion content, distribution and size which act as prime failure factors. These surveys are extended to monitor ductile damage accumulation with cavitation at different stages of the incremental sheet indentation process over certain sections. An anticipated failure mode is captured where formability is limited by severe macro-cracking preceded by localization with void sheeting. To this end, using a developed VUMAT subroutine for the micromechanically based Gurson damage model which is recently enhanced for shear fracture, the processes are simulated in ABAQUSExplicit and comparisons with experiments are provided. The results support the requirement of integrating powerful coupled accumulative damage models in the virtual process design procedure for sheet-bulk metal forming. This requirement also arises from distinct features of these class of processes from conventional sheet metal forming processes which preclude use of forming limit curves. © 2011 American Society of Mechanical Engineers.
    view abstract10.1115/1.4004852
  • Characterization of initial anisotropy of sheet metals employing inhomogeneous strain fields
    Güner, A. and Soyarslan, C. and Brosius, A. and Tekkaya, A.E.
    AIP Conference Proceedings 1383 (2011)
    The realistic finite element simulation of sheet metal forming processes necessitates the accurate characterization of plastic behavior of the sheet. For this purpose different experiments and specimen geometries are used in the field of sheet metal characterization to supply information to the increasing number of sophisticated constitutive models demanding many parameters. This work proposes an inverse procedure that reduces the number of needed experiments by utilizing the distribution of strains in the identification of the initial yield locus of sheet metals. The procedure includes the optical measurement of strains on a flat specimen with a varying cross-section. The geometry of the specimen allows obtaining a distribution of strains rather than a uniform deformation. The measured deformation field is compared with the numerical results which use the YLD2000-2D yield criterion and the difference is tried to be minimized in an inverse framework using the Levenberg-Marquardt algorithm. The proposed method is applied to the commercial aluminum alloy AA6016 by varying the specimen orientation with respect to the rolling direction. The results show that the use of the strain distribution in the objective function is a crucial point in identification of the yield locus. The inversely obtained yield loci are in accordance with the conventionally obtained yield stresses and r-values and it is showed that the proposed specimen and the method can successfully characterize the initial anisotropy of the sheet metals in the first quadrant by using one tensile experiment and one layer compression test. © 2011 American Institute of Physics.
    view abstract10.1063/1.3623681
  • Co-extrusion of discontinuously, non-centric steel-reinforced aluminum
    Foydl, A. and Haase, M. and Khalifa, N.B. and Tekkaya, A.E.
    AIP Conference Proceedings 1353 (2011)
    The process of manufacturing discontinuously non-centric steel reinforced aluminum by means of co-extrusion has been examined. By this process semi-finished reinforced profiles can be fabricated for further treatment through forging techniques. Therefore, steel reinforcement elements consisting of E295GC were inserted into conventional aluminum billets and co-extruded into two different solid profiles; a rectangle one by an extrusion ratio of 10.1:1 and a round one by 4.8:1. The used aluminum alloy is EN AW-6060. The billet temperature as well as the ram speed were varied to investigate their influence on the position of the reinforcement elements inside the strand. The measurement was done by a video measurement system, called Optomess A250, after milling off the strand. The distances between the elements in longitudinal direction were nearly constant, apart from the rear part of the strand. The same was observed for the distance of the steel elements to the profile edge. This due to the inhomogeneous material flow in the transverse weld, related to the billet-to-billet extrusion. The rotation of the reinforcement elements occurs because the elements flow nearby the shear zone. Further, micrographs were made to investigate the embedding situation and the grain size distribution. The embedding of the reinforcement elements were good in the solid round profile, but in the rectangle profile were found some kind of air pocket. The grain size of the aluminum alloy close to the steel elements is much smaller than in the other parts of the solid round profile. © 2011 American Institute of Physics.
    view abstract10.1063/1.3589555
  • Comparison of Different FEM Codes Approach for Extrusion Process Analysis
    Donati, L. and Khalifa, N.B. and Tomesani, L. and Tekkaya, A. E.
    Supplemental Proceedings: Materials Fabrication, Properties, Characterization, and Modeling 2 (2011)
    The simulation of the extrusion process by means of FE codes has been applied in a great number of papers available in literature but its application in everyday production was limited due to several factors like computational times, user's skills as well as prediction accuracy. Indeed, the inner complexity of the process, characterized by extremely high deformations, strain rates and heat exchange phenomena, has lead only in the last few year commercial FE codes to gain sufficient accurate solving capabilities. In the paper, five FEM codes based on different approaches were applied in the simulation of the same experiment: the results were compared in term of set-up times, computational time as well as process prediction accuracy. Process load, profile speeds, die and profile temperatures were accurately monitored during the experiment in order to realize an effective comparison of the different FEM codes approaches. © 2011 The Minerals, Metals & Materials Society.
    view abstract10.1002/9781118062142.ch31
  • Electromagnetic forming - A review
    Psyk, V. and Risch, D. and Kinsey, B.L. and Tekkaya, A.E. and Kleiner, M.
    Journal of Materials Processing Technology 211 (2011)
    Electromagnetic forming is an impulse or high-speed forming technology using pulsed magnetic field to apply Lorentz' forces to workpieces preferably made of a highly electrically conductive material without mechanical contact and without a working medium. Thus hollow profiles can be compressed or expanded and flat or three-dimensionally preformed sheet metal can be shaped and joined as well as cutting operations can be performed. Due to extremely high velocities and strain rates in comparison to conventional quasistatic processes, forming limits can be extended for several materials. In this article, the state of the art of electromagnetic forming is reviewed considering:basic research work regarding the process principle, significant parameters on the acting loads, the resulting workpiece deformation, and their interactions, and the energy transfer during the process;application-oriented research work and applications in the field of forming, joining, cutting, and process combinations including electromagnetic forming incorporated into conventional forming technologies. Moreover, research on the material behavior at the process specific high strain rates and on the equipment applied for electromagnetic forming is regarded. On the basis of this survey it is described why electromagnetic forming has not been widely initiated in industrial manufacturing processes up to now. Fields and topics where further research is required are identified and prospects for future industrial implementation of the process are given. © 2010 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.jmatprotec.2010.12.012
  • High wear resistant deep drawing tools made of coated polymers
    Witulski, J. and Trompeter, M. and Tekkaya, A.E. and Kleiner, M.
    CIRP Annals - Manufacturing Technology 60 (2011)
    A methodology for the rapid production of drawing tools with high wear resistance to form free-form shaped sheet metal parts is developed. Hard material shells are thermally sprayed on an original mould and supported by a polymer. The bonded shell is removed from the original mould and acts as the surface of the forming tool. Deep drawing experiments show that the wear resistance of these hybrid tools is adequate to form high-strength steels. The tools are a suitable alternative to existing tool systems for the intended use in small up to medium size productions. © 2011 CIRP.
    view abstract10.1016/j.cirp.2011.03.149
  • Impulse forming
    Tekkaya, A.E. and Allwood, J.M.
    Journal of Materials Processing Technology 211 (2011)
    view abstract10.1016/j.jmatprotec.2011.01.020
  • Influence of groove characteristics on strength of form-fit joints
    Weddeling, C. and Woodward, S.T. and Marré, M. and Nellesen, J. and Psyk, V. and Tekkaya, A.E. and Tillmann, W.
    Journal of Materials Processing Technology 211 (2011)
    Joining by electromagnetic forming (EMF) is an innovative method to connect, e.g. extruded aluminum profiles to lightweight frame structures without heating or penetrating the profile. This article describes the joining of extruded aluminum profiles by electromagnetic forming, taking into account the process characteristics and the joint design. Forming is initiated by a magnetic impulse of high energy density, such that material with high electrical conductivity is deformed by Lorentz forces. With this high-speed forming process high-strength joints can be manufactured as interference fits, form fits, impact welded joints or a combination of these types. The focus of this paper is on form-fit joints of extruded aluminum profiles for lightweight frame structures. Based on fundamental technological investigations the parameters which specifically affect the strength of the joints were identified and analyzed. Throughout these experiments the groove geometry was varied size and shape wise. The influence of the acting magnetic pressure and the charging energy on the joint strength was also analyzed for the various groove geometries. © 2010 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.jmatprotec.2010.08.004
  • Innovative machine concepts for 3D bending of tubes and profiles
    Hermes, M. and Staupendahl, D. and Becker, C. and Tekkaya, A.E.
    Key Engineering Materials 473 (2011)
    The paper deals with two new processes and developed special machines for profile and tube bending. The first process is a new roll-based machine for three-dimensional bending of profiles with symmetrical and asymmetrical cross-sections that has been developed. Compared to conventional processes like stretch bending, the advantage of Torque Superposed Spatial (TSS) Bending is the kinematic adjustment of the bending contour, leading to higher flexibility and cost efficiency especially in small batch production. The second process is the new process of Incremental Tube Forming (ITF). This process is based on a combination of a spinning process and kinematic free form bending of tubular semi-finished products. It is suitable for bending tubes two- and three-dimensionally to arbitrary contours and for manufacturing tailored tubes. The combined spinning and bending process leads to low bending forces with the possibility of a significant springback reduction. © (2011) Trans Tech Publications.
    view abstract10.4028/www.scientific.net/KEM.473.37
  • Inverse method for identification of initial yield locus of sheet metals utilizing inhomogeneous deformation fields
    Güner, A. and Yin, Q. and Soyarslan, C. and Brosius, A. and Tekkaya, A.E.
    International Journal of Material Forming 4 (2011)
    Accurate finite element simulation of sheet metal forming processes requires among others accurate description of plastic behaviour of materials. This is achieved by utilization of sophisticated yield criteria having several material parameters. This work proposes a procedure which makes use of the distribution of strains to identify the initial yield locus of sheet metals by the help of inverse analysis. For this purpose a flat specimen having a varying cross-section is introduced, which is capable of revealing different deformation states in one test. Numerical simulations are performed with 2 representative materials for steel and aluminium, using the material model Yld2000-2d. The results of these simulations are treated as experimentally obtained results and with the inverse methods it is tried to obtain the given yield locus. The relation between the supplied input and the outcome of the inverse algorithm is studied by examining different objective function definitions. The numerical studies show that inclusion of the strain distribution in the definition of objective function is a key issue in identification of the yield locus. The orientation of the specimen with respect to the rolling direction also determines the amount and quality of the information used for parameter identification. Consequently the circumstances, under which the inverse method can predict the initial yield locus, are defined. © 2010 Springer-Verlag France.
    view abstract10.1007/s12289-010-1009-4
  • Investigation and prediction of grain texture evolution in AA6082
    Segatori, A. and Foydl, A. and Donati, L. and Khalifa, N.B. and Brosius, A. and Tomesani, L. and Tekkaya, A.E.
    AIP Conference Proceedings 1353 (2011)
    Extrusion applications require a strict control of the mechanical proprieties of the extrudates, in particular when undergoing severe loading conditions like in the transportation sector. Profile mechanical properties directly depend on its microstructure and texture, which are the result of multiple mechanisms based on precipitation mechanism or on grain shape evolution (grain refinement, recrystallizations, recovery and grain growth). In this direction, predicting the final profile microstructure under specific process parameters in the die design stage is of great relevance. The present study involved experimental activity on grain size measurements of profile and butt during interrupted direct extrusion of an AA6082 round profile. The grain size measurements were coupled with simulation results in order to regress analytical models based on effective strain, strain rate and temperature. Finally, the developed model was implemented in the numerical code by means of a subroutine that can be used as microstructure prediction tool. © 2011 American Institute of Physics.
    view abstract10.1063/1.3589556
  • Investigation of forming strategies to set up mechanical properties of parts made by incremental sheet bulk rolling
    Plugge, B. and Schunck, S. and Kwiatkowski, L. and Brosius, A. and Tekkaya, A.E.
    AIP Conference Proceedings 1353 (2011)
    Load-adapted parts with an increasing number of functions become more and more interesting in order to reduce the weight of all kinds of mechanical constructions. Such parts require varying mechanical properties and towards they are cost-expensive. One approach to reduce the costs is the application of cheap semi-finished parts. To process such parts, especially in thickness direction, bulk-forming operations are requested. This leads to high forming forces. A feasible approach to reduce the forces is the application of incremental forming techniques. In this paper an incremental rolling process is presented. The sequential order of forming operations during incremental rolling allows an individual adjustment of mechanical and the geometrical properties. In the presented study a sheet with a thickness of 2 mm made of mild steel is formed using a roller ball with a diameter of 13 mm. Main objective of the investigation is to manufacture parts with equal values for thickness but different values of the local surface hardness. The investigation is supported by Finite-Element-Analysis (FEA) to determine the distribution of the strains over the part's thickness. The results show that it is possible to manufacture parts with the same thickness and different surface hardness by applying different forming strategies. Keeping the entire process parameters constant but rolling the part alternating at both surfaces will result in a higher hardness on the surface in comparison to a one-sided rolling. © 2011 American Institute of Physics.
    view abstract10.1063/1.3589514
  • Machines and tools for sheet-bulk metal forming
    Merklein, M. and Tekkaya, A.E. and Brosius, A. and Opel, S. and Kwiatkowski, L. and Plugge, B. and Schunck, S.
    Key Engineering Materials 473 (2011)
    The demand on closely-tolerated and complex functional components in the automotive sector, like e.g. synchronizer rings, leads to the development of a new process-class named "sheet-bulk metal forming". Within this technology bulk metal forming operations are applied on sheet metals. In the following two novel approaches considering machines and tools for sheet-bulk metal forming are presented. The first approach aims on a technology based on rolling, which is suitable for mass production. The second one is an incremental forming solution for low batch production. Both machine concepts allow the application of different forming strategies to manufacture individual tailored semi-finished products in term of a pre-distribution of material. These products feature variable sheet thicknesses and mechanical properties, which can be adapted to their case of application. Depending on the individual batch size, the blanks can be finished to functional parts by subsequent forming processes like deep drawing and upsetting, extrusion or incremental forming. In this paper the case of an incremental tooth-forming is mainly considered. Forming sequences and resulting loads are modeled and calculated by finite elements simulations for all discussed processes to serve as a basis for the design and dimensioning of the machine components and forming tools. © (2011) Trans Tech Publications.
    view abstract10.4028/www.scientific.net/KEM.473.91
  • Modeling of dynamic microstructure evolution of en AW-6082 alloy during hot forward extrusion
    Parvizian, F. and Güzel, A. and Jäger, A. and Lambers, H.-G. and Svendsen, B. and Tekkaya, A.E. and Maier, H.J.
    Computational Materials Science 50 (2011)
    The aim of this work is to present briefly a model for predicting and simulating the evolution of microstructure, in particular the evolution of grains, during hot forming processes of aluminum alloy EN AW-6082 and give a comparison with the experimental results. The model is a physically motivated phenomenological model based on internal state dependent variables. The microstructure evolution is a temperature dependent process and is simulated in a fully coupled thermo-mechanical process by help of Finite Element software Abaqus. The results are compared and verified with experimental results obtained by EBSD measurement of a small-scale extrusion process established for scientific purposes. The simulation results are in reasonable agreement with experiment. © 2010 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.commatsci.2010.12.009
  • Newest developments on the manufacture of helical profiles by hot extrusion
    Khalifa, N.B. and Tekkaya, A.E.
    ASME 2011 International Manufacturing Science and Engineering Conference, MSEC 2011 1 (2011)
    The paper presents a new innovative direct extrusion process, Helical Profile Extrusion (HPE), which increases the flexibility of aluminum profile manufacturing processes. The application fields of such profiles can be seen in screw rotors for compressors and pumps. The investigations concentrate on experimental and numerical analyses by 3D-FEM simulations to analyze the influence of friction on the material flow in the extrusion die in order to find out the optimal parameters with reference to the twisting angle and contour accuracy. By means of FEM, the profile shape could be optimized by modifying the die design. The numerical results were validated by experiments. For these investigations, a common aluminum alloy AA6060 was used. The accuracy of the profile contour could be improved significantly. However, increasing the twist angle is limited due to geometrical aspects. Copyright © 2010 by ASME.
    view abstract10.1115/MSEC2011-50126
  • On the study of constitutive parameter identification of advanced yield criteria
    Aydin, M.-S. and Canpolat, A. and Gerlach, J. and Kessler, L. and Tekkaya, A.E.
    Key Engineering Materials 473 (2011)
    Recently, an alternative inverse-analysis approach was proposed to obtain the material parameters of the advanced yield criteria by employing tensile and cup drawing tests [1]. In this paper, the applicability of this strategy will be investigated for a mild steel grade by means of cruciform, plane strain tension and hydraulic bulge tests. Other than this, the impact of the strain rate on the hydraulic bulge tests will be one another aspect of this work. © (2011) Trans Tech Publications.
    view abstract10.4028/www.scientific.net/KEM.473.452
  • Platform for e-Learning and Telemetric Experimentation (PeTEX). Tele-operated laboratories for production engineering education
    Terkowsky, C. and Pleul, C. and Jahnke, I. and Tekkaya, A.E.
    2011 IEEE Global Engineering Education Conference, EDUCON 2011 (2011)
    The development of tele-operated experimentation and its provision to distance learners opens new dimensions of knowledge acquisition, particularly where experiments are the core elements of engineering education. The finalized EU-funded project PeTEX-Platform for e-Learning and Telemetric Experimentation has developed a prototype of an e-learning platform based on Moodle for the design and implementation of educational and training programs in the field of manufacturing engineering. The principle goal of this project was to establish individual and group oriented learning for different target groups like students and professional workers within a platform-system able to sustain a multi-country learning community. Hence, an educational model was designed which integrates the tele-operated experimentation platform with teaching content and learning activities in order to support a successful learning walkthrough for different target groups. © 2011 IEEE.
    view abstract10.1109/EDUCON.2011.5773181
  • Process design for the manufacturing of magnetic pulse welded joints
    Psyk, V. and Gershteyn, G. and Barlage, B. and Weddeling, C. and Albuja, B. and Brosius, A. and Tekkaya, A.E. and Bach, F.-W.
    Key Engineering Materials 473 (2011)
    When manufacturing joints of dissimilar materials thermal technologies as welding reach their limits. Impact welding by electromagnetic forming is a promising alternative because undesired heating of the parts and related disadvantages are avoided. In this process impact parameters need to be adjusted to each specific joining task, but cannot be settled directly. Thus, a two-step methodology is suggested for the process design: First the influence of the impact parameters and the surface preparation on the joint properties is investigated using a model experiment. Joint properties are characterized by metallographic investigations. Parallel to this, the influence of the adjustable process parameters and the equipment on the workpiece acceleration and the impact properties is analyzed. Then the results of both investigation paths are combined and conclusions regarding a target-oriented adjusting of the impact parameters via the process parameters are drawn. In the paper first results considering the model experiment and the analysis of the electromagnetic expansion process are presented and joints manufactured by electromagnetic expansion are characterized. © (2011) Trans Tech Publications.
    view abstract10.4028/www.scientific.net/KEM.473.243
  • Production of low-volume aviation components using disposable electromagnetic actuators
    Woodward, S. and Weddeling, C. and Daehn, G. and Psyk, V. and Carson, B. and Tekkaya, A.E.
    Journal of Materials Processing Technology 211 (2011)
    Electromagnetic forming is commonly used to produce high strain rates to improve the formability of sheet metal. The objective of this paper is to discuss the feasibility of the use of disposable actuators during electromagnetic forming of two aluminum components: a very simple part with a one-dimensional curve, and a realistic part whose main feature is a convex flange with two joggles. The main forming complications after the parts were formed using conventional methods were the presence of wrinkles and excessive springback. The goal of this work is to use large controlled electromagnetic impulses to minimize the springback of these components from a rough-formed shape. The optimum test protocols for electromagnetic calibration of the components were determined by optimizing parameters such as actuator design, tool material, press force, and capacitor discharge energy. The use of disposable actuators for electromagnetic calibration of the parts showed significant reductions in springback compared to the parts which were only preformed using conventional techniques (hydroforming and rubber pad forming). Springback was decreased in the curved component by up to 87%. The wrinkles were eliminated on the flanged component, the joggles were formed properly, and the average bending angle of the part was improved from 95.3° to 90.3°, very near the target bending angle of 90°. This study demonstrates that these techniques can be used to improve current sheet metal production processes. © 2010 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.jmatprotec.2010.07.020
  • Strategies for springback compensation regarding process robustness
    Gösling, M. and Kracker, H. and Brosius, A. and Kuhnt, S. and Tekkaya, A.E.
    Production Engineering 5 (2011)
    In this article, strategies which compensate geometrical deviations caused by springback are discussed using finite element simulations and statistical modelling techniques. First of all the ability to predict springback using a finite element simulation model is analysed. For that purpose numerical predictions and experiments are compared with each other regarding the amount of springback. In a next step, different strategies for compensating springback such as a modification of stress condition, component stiffness and tool geometry are introduced. On the basis of finite element simulations these different compensation strategies are illustrated for a stretch bending process and experimentally checked for an example. Finally springback simulations are compared regarding their robustness against noise variables such as friction and material properties. Thereby a method based on statistical prediction models is introduced which allows for an accurate approximation of the springback distribution with less numerical effort in comparison to a classical Monte-Carlo method. © 2010 German Academic Society for Production Engineering (WGP).
    view abstract10.1007/s11740-010-0251-4
  • Tele-operated laboratories for online production engineering education platform for e-learning and telemetric experimentation (PeTEX)
    Terkowsky, C. and Pleul, C. and Jahnke, I. and Tekkaya, A.E.
    International Journal of Online Engineering 7 (2011)
    The development of tele-operated experimentation and its provision to distance learners opens new dimensions for experience-based scientific and engineering education, particularly where experiments are the core elements of teaching and learning. The finalized EU-funded project PeTEX-Platform for e-Learning and Telemetric Experimentation has developed a prototype of an e-learning platform based on the learning and content management system Moodle for the design and implementation of educational and training applications in the field of production engineering. The principle goal of this project was to establish individual and group oriented learning for different target groups like students and professional workers within a platform-system capable to serve a multi-lingual learning community. Hence, an educational model was designed which integrates the tele-operated experimentation platform with teaching content and learning activities in order to support a successful learning walkthrough for different target groups.
    view abstract10.3991/ijoe.v7iS1.1725
  • The editorial board of the journal of materials processing technology
    Tekkaya, A.E. and Allwood, J.M.
    Journal of Materials Processing Technology 211 (2011)
    view abstract10.1016/j.jmatprotec.2010.10.007
  • The effect of extrusion ratio and material flow on the mechanical properties of aluminum profiles solid state recycled from 6060 aluminum alloy chips
    Güley, V. and Ben Khalifa, N. and Tekkaya, A.E.
    AIP Conference Proceedings 1353 (2011)
    This paper presents the effect of the extrusion ratio (ER) on the mechanical properties of aluminum profiles solid state recycled from 6060 aluminum chips by hot extrusion. The chips were extruded through three different dies. The tensile test results and microstructures were discussed in comparison with the conventional profiles extruded with the same dies under the same extrusion conditions. The conventional profiles extruded with these three different ERs revealed almost the same mechanical properties. The profiles extruded from chips with an ER of 68:1 showed an approx. 20% higher strength and ductility compared to the profiles of an ER of 34:1, while the exerted pressure was not sufficient for welding the chips in case of an ER of 10:1. It was also shown that, by adjusting the material flow through the die, it was possible to recycle aluminum chips even at ERs as low as 10:1. © 2011 American Institute of Physics.
    view abstract10.1063/1.3589746
  • Thermally sprayed coatings as effective tool surfaces in sheet metal forming applications
    Franzen, V. and Witulski, J. and Brosius, A. and Trompeter, M. and Tekkaya, A.E.
    Journal of Thermal Spray Technology 20 (2011)
    Two approaches to produce wear-resistant effective surfaces for deep drawing tools by thermal arc wire spraying of hard materials are presented. Arc wire spraying is a very economic coating technique due to a high deposition rate. The coated surface is very rough compared to that of conventional sheet metal forming tools. In the first approach, the coated surface is smoothed in a subsequent CNC-based incremental roller burnishing process. In this process, the surface asperities on the surface are flattened, and the roughness is significantly reduced. In the second approach, the hard material coatings are not sprayed directly on the tool but on a negative mould. Afterward, the rough "as-sprayed" side of the coating is backfilled with a polymer. The bonded hard metal shell is removed from the negative mould and acts as the surface of the hybrid sheet metal forming tool. Sheet metal forming experiments using tools based on these two approaches demonstrate that they are suitable to form high-strength steels. Owing to a conventional body of steel or cast iron, the first approach is suitable for large batch sizes. The application of the second approach lies within the range of small up to medium batch size productions. © 2011 ASM International.
    view abstract10.1007/s11666-011-9627-z
  • Thermo-mechanical processing of aluminum profiles by integrated electromagnetic compression subsequent to hot extrusion
    Jäger, A. and Risch, D. and Tekkaya, A.E.
    Journal of Materials Processing Technology 211 (2011)
    In this paper, a strategy for the thermo-mechanical processing of aluminum profiles by subsequent electromagnetic forming and heat treatment is given. A tool coil for electromagnetic compression was positioned behind the die exit and coaxially to the extrudate in order to reduce the workpiece cross section locally. Additionally, a counter die in the shape of a mandrel was mounted to the mandrel of a porthole extrusion die which extended into the tool coil. Besides achieving a more defined geometry in comparison to a free forming operation, by this also the geometrical complexity of locally compressed areas can be achieved. © 2010 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.jmatprotec.2010.06.016
  • Velocity effects in metal forming and machining processes
    Neugebauer, R. and Bouzakis, K.-D. and Denkena, B. and Klocke, F. and Sterzing, A. and Tekkaya, A.E. and Wertheim, R.
    CIRP Annals - Manufacturing Technology 60 (2011)
    Velocity is probably the most important parameter in manufacturing, influencing performance, cost, productivity, energy and resources efficiency as well as safety and environmental issues. This paper presents basic phenomena as well as other important effects which are linked to velocity as a process parameter. In addition, applications, for example superplastic forming or high speed cutting, which have been founded on uncommon process velocities are discussed in the context of technological developments which have taken place over the past several years. © 2011 CIRP.
    view abstract10.1016/j.cirp.2011.05.001
  • Yield locus evolution and constitutive parameter identification using plane strain tension and tensile tests
    Aydin, M.-S. and Gerlach, J. and Kessler, L. and Tekkaya, A.E.
    Journal of Materials Processing Technology 211 (2011)
    An inverse calibration strategy to determine the constitutive parameters of phenomenological advanced yield criteria in a convenient and economical manner is presented. The studies on the shape of the yield loci for various steel types revealed that their work contours exhibit almost no evolution in the vicinity of the equibiaxial tension after roughly 4% equivalent strain. In other words, the ratio between balanced biaxial and uniaxial stresses of the yield locus reaches a saturation value after undergoing some deformation. Accordingly, the balanced biaxial stresses can be associated with the tensile flow stresses in the rolling direction by means of a constant factor, which can be generalized for different steel families. Based on this, an alternative inverse-analysis strategy using the tensile and plane strain tension tests will be proposed and validated within this work. Cup drawing tests have been applied to assess the accuracy of the optimized yield loci for different strain paths. © 2011 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.jmatprotec.2011.06.018
  • A damage coupled orthotropic finite plasticity model for sheet metal forming: CDM approach
    Soyarslan, C. and Tekkaya, A.E.
    Computational Materials Science 48 (2010)
    A framework for orthotropic finite plasticity coupled with a Lemaitre type isotropic ductile damage is presented in a thermodynamically sound setting for sheet metal forming. The finite plasticity utilizes Green-Naghdi type additive decomposition in the logarithmic Lagrangean strain space, which allows adaptation of the return mapping schemes of damage coupled infinitesimal plasticity. Hydrostatic stress state and principal stress state dependent unilateral damage evolutionary conditions are efficiently formulated without need for repetitive tensor transformations, taking advantage of the eigenvalue equivalence in between the stress measure conjugate to the logarithmic Lagrangean strain and the Kirchhoff stress. For the sake of completeness a Perzyna type viscous regularization is also elaborated. A three-step, staggered local integration algorithm, composed of elastic prediction, plastic correction and damage deterioration, is performed for return mapping at integration points. To this end, the framework is implemented as a VUMAT subroutine for ABAQUS/Explicit and used in a set of simulations. Besides proving the applicability range of the methodology, the outcomes show that Lemaitre model, once enhanced with unilateral damage evolutionary conditions, gives physically meaningful results in cup drawing simulations. © 2010 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.commatsci.2009.12.022
  • A review on hot stamping
    Karbasian, H. and Tekkaya, A.E.
    Journal of Materials Processing Technology 210 (2010)
    The production of high strength steel components with desired properties by hot stamping (also called press hardening) requires a profound knowledge and control of the forming procedures. In this way, the final part properties become predictable and adjustable on the basis of the different process parameters and their interaction. In addition to parameters of conventional cold forming, thermal and microstructural parameters complicate the description of mechanical phenomena during hot stamping, which are essential for the explanation of all physical phenomena of this forming method. In this article, the state of the art in the thermal, mechanical, microstructural, and technological fields of hot stamping are reviewed. The investigations of all process sequences, from heating of the blank to hot stamping and subsequent further processes, are described. The survey of existing works has revealed several gaps in the fields of forming-dependent phase transformation, continuous flow behavior during the whole process, correlation between mechanical and geometrical part properties, and industrial application of some advanced processes. The review aims at providing an insight into the forming procedure backgrounds and shows the great potential for further investigations and innovation in the field of hot sheet metal forming. © 2010 Published by Elsevier B.V.
    view abstract10.1016/j.jmatprotec.2010.07.019
  • Accurate welding line prediction in extrusion processes
    Kloppenborg, T. and Ben Khalifa, N. and Tekkaya, A.E.
    Key Engineering Materials 424 (2010)
    In contrast to conventional extrusion processes, where a lot of research is done on in the welding quality, in composite extrusion, research is investigated into the welding line positioning. As a result of the process principle, the reinforcing elements are embedded into the longitudinal welding line. Hence, an undefined material flow inside the welding chamber induces reinforcement deflection, which can lead to reduced mechanical properties, as momentum of inertia. Therefore and to reduce costly experimental investigations, a new method of an automated numerical welding line prediction was developed. The results form HyperXtrude finite element calculations are used for special particle tracing simulations to predict the welding line in the profile cross section accurately. The procedures of segmentation and characteristic extraction are presented to approximate the welding line by cubic spline functions. The method was fully programmed in the Java program language, and works well for all HyperXtrude process models consisting of tetrahedral elements. © (2010) Trans Tech Publications.
    view abstract10.4028/www.scientific.net/KEM.424.87
  • Advances on hot extrusion and simulation of light alloys: Preface
    Tekkaya, A.E. and Khalifa, N.B.
    Key Engineering Materials 424 (2010)
    view abstract10.4028/www.scientific.net/KEM.424.-4
  • An assessment of the grain structure evolution during hot forward extrusion of aluminum alloy 7020
    Foydl, A. and Ben Khalifa, N. and Brosius, A. and Tekkaya, A.E.
    Key Engineering Materials 424 (2010)
    The current investigation is concerned with the grain structure evolution in an Al-Zn alloy (EN AW-7020) during the hot forward extrusion process. In order to analyze that, a miniature hot forward extrusion setup was designed which allows the quenching of the extrusion butt immediately after extrusion. In order to gain a better understanding of the process, the shape of the deformed grains was analyzed and the process was simulated. The shape of these grains was indentified in two directions in the different grain zones, e.g. dead metal zone and shear zone. The FE simulations showing the different grain zones were also illustrated. Simulation results and the micrographs were quite promising to find parameters for simulation models in order to predict grain sizes with the method presented in the current research work. © (2010) Trans Tech Publications.
    view abstract10.4028/www.scientific.net/KEM.424.35
  • Classification of reviewers and papers for the Journal of Materials Processing Technology
    Allwood, J.M. and Tekkaya, A.E.
    Journal of Materials Processing Technology 210 (2010)
    view abstract10.1016/j.jmatprotec.2009.09.015
  • Comparison of different FEM code approaches in the simulation of the die deflection during aluminium extrusion
    Donati, L. and Khalifa, N.B. and Tomesani, L. and Tekkaya, A.E.
    International Journal of Material Forming 3 (2010)
    In this paper a multihole die producing two overlapped U-shape profiles was used in order to analyze the die deflection during the extrusion process thus allowing the benchmarking of different FEM codes. The two die openings were differently designed in order to realize different die deflection (fully supported and partially supported). Profile lengths, die and profile temperatures, process load and die deflections were used as benchmarking parameters for FEM comparison. A summary of the different FEM codes approaches, of the computational times of the main outputs and their comparison with experimental results are presented. A detailed discussion for all output parameters is realized in order to understand the potentials and limits of each code. Finally a discussion on future perspective for FE code application and designing guides is reported. © 2010 Springer-Verlag France.
    view abstract10.1007/s12289-010-0785-1
  • Direct recycling of 1050 aluminum alloy scrap material mixed with 6060 aluminum alloy chips by hot extrusion
    Güley, V. and Ben Khalifa, N. and Tekkaya, A.E.
    International Journal of Material Forming 3 (2010)
    In this paper, a method for the direct recycling of aluminum scrap by hot extrusion is investigated. 1050 aluminum alloy material in the form of pins remained as scrap after a lateral extrusion process and was mixed with 6060 aluminum alloy chips resulting from a turning operation. Contrary to the conventional method of re-melting aluminum scrap to produce secondary aluminum, this aluminum scrap+chip mixture was cold compacted into billets and hot extruded at 500°C to full rectangular profiles. The extruded profiles were examined by tensile tests and microstructural investigations and compared to the conventionally extruded profiles from as-cast material. It was shown that not only the aluminum chips but also the aluminum scrap material can be recycled directly by hot extrusion, which requires only ~10% of the energy required for recycling by re-melting. Also, the profiles extruded from billets containing 1050 aluminum alloy scrap with and without cooling lubricant were compared and a deteriorating effect was determined. © 2010 Springer-Verlag France.
    view abstract10.1007/s12289-010-0904-z
  • Erratum to: On the reduction of gloss property of organic coated sheet metals after forming
    Pham, H.-D. and Trompeter, M. and Tekkaya, A.E.
    Production Engineering 4 (2010)
    Organic coated sheet metals (OCSM) are widely used in many industrial applications such as automobile or electrical appliances because of their corrosion resistance and satisfaction of ecological requirements. Nevertheless, original functions of the very thin coating layer in OCSM products may be altered after being formed which consequently leads to a reduction of the product's quality. This paper deals with the reduction of gloss property of OCSM products. A forming limit diagram (FLD), a forming limit diagram of the coating layer (FLDC) of OCSM and a forming diagram for the prediction of gloss reduction are determined by using Nakajima tests. The Nakajima tests are performed with predefined strain states of the specimen. The experimental results show the dependency of the FLDC curve and the gloss reduction on both the equivalent strain and the strain ratio. Deep-drawing tests prove the applicability of the determined forming diagrams to predict coating fracture and gloss properties of OCSM after sheet metal forming and, thus, their potential as a tool for the process design for OCSM products. © 2010 German Academic Society for Production Engineering (WGP).
    view abstract10.1007/s11740-010-0262-1
  • Erratum: A damage coupled orthotropic finite plasticity model for sheet metal forming: CDM approach (Computational Materials Science (2010) 48 (150-165))
    Soyarslan, C. and Tekkaya, A.E.
    Computational Materials Science 48 (2010)
    view abstract10.1016/j.commatsci.2010.03.047
  • Experimental investigation of tool path strategies for incremental necking-in
    Kwiatkowski, L. and Melsheimer, O. and Wenzel, S. and Kunert, J. and Tekkaya, A.E.
    International Journal of Material Forming 3 (2010)
    Incremental necking-in is a forming process to achieve a diameter reduction of tubular parts. Conventionally an inside mandrel is used which gives its shape to the part. In order to increase the flexibility of the process, necking-in is performed without any mandrel in this investigation. The objective of this paper is to deliver a qualitative insight into the effects of the tool motion on the properties of the parts which are manufactured by this incremental forming process. Using a conventional spinning machine, tubular parts are necked by applying six different tool path strategies. The investigation is focused on determining the geometrical properties of the produced parts. With statistically planned screening experiments, including linear models for the process effects, the influence of several tool paths on the workpiece quality is analyzed. The results show a high dependency of the chosen tool path on the reached value of the inclined area as well as the parts elongation. All observed effects are explained in a qualitative manner. Additionally, undesired deformation of the part contour can be observed. Using the given results, a further tool path is developed, which allows a compensation of those effects. © 2010 Springer-Verlag France.
    view abstract10.1007/s12289-010-0930-x
  • Extrusion benchmark 2009 experimental analysis of deflection in extrusion dies
    Pietzka, D. and Khalifa, N.B. and Donati, L. and Tomesani, L. and Tekkaya, A.E.
    Key Engineering Materials 424 (2010)
    In this paper experimental investigations aimed at measuring the die deformations during aluminum extrusion process is presented and discussed. A two-holes die generating two U-shape profiles with different supporting legs was produced and tested under strictly monitored conditions. The influence of die deformation on the speed, temperature distribution and distortion of the two profiles is reported and analyzed. AA6082 alloy was used as deforming material while H-13 hotwork tool steel was selected as die material. The experiments were repeated at least three times in the same conditions in order to achieve a statistical distribution of the acquired data: such data are then used as a reference for the 2009 edition of the extrusion benchmark. © (2010) Trans Tech Publications.
    view abstract10.4028/www.scientific.net/KEM.424.19
  • Finishing of thermally sprayed tool coatings for sheet metal forming operations by roller burnishing
    Franzen, V. and Trompeter, M. and Brosius, A. and Tekkaya, A.E.
    International Journal of Material Forming 3 (2010)
    Processing high strength sheet metal materials causes high tribological loads to the surface of forming tools. To increase the wear resistance and therewith the service time of tools, surface coatings containing hard materials are applied by thermal spraying. In the initial "as sprayed" state, the coatings show a rough surface and porous structure, which is not suitable for sheet forming tools. This article describes the finishing of the coatings by subsequent roller burnishing. By the rolling process surface asperities of the coating are flattened and the contact area between the finished surface and the sheet is increased. Furthermore, residual compressive stresses in the surface zone are generated by the rolling process. The tribological properties of the coated and finished surfaces are analyzed in strip drawing tests with uncoated DP600 sheets varying the contact pressure and drawing velocity. It is shown that the process parameters of the roller burnishing process have a strong influence on the surface topology of the friction elements and their tribological properties. The coated and finished friction elements are compared to conventional grinded steel friction elements, made of C60. © 2010 Springer-Verlag France.
    view abstract10.1007/s12289-010-0728-x
  • Finite deformation plasticity coupled with isotropic damage: Formulation in principal axes and applications
    Soyarslan, C. and Tekkaya, A.E.
    Finite Elements in Analysis and Design 46 (2010)
    A local, isotropic damage coupled hyperelasticplastic framework is formulated in principal axes. It is shown that, in a functional setting, treatment of many damage growth models, including those originated from phenomenological models (with formal thermodynamical derivations), micro-mechanics or fracture criteria, proposed in the literature, is possible. As a model problem, a Lemaitre-variant damage model with quasi-unilateral damage evolutionary forms is given with special emphasis on the feasibility of formulations in principal axes. To this end, closed form expression for the inelastic tangent moduli, consistent with the linearization of the closest point projection algorithm, is derived. It is shown that, generally, even in the absence of quasi-unilateral damage evolutionary conditions, the consistent tangent moduli are unsymmetric. The model is implemented as a user defined material subroutine (UMAT) for ABAQUS/Standard. The predictive capability of the selected model problem is studied through axi-symmetric application problems involving forward extrusion of a cylindrical billet, upsetting of a tapered specimen and tension of a notched specimen, in which characteristic failure mechanisms are observed. © 2010 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.finel.2010.03.006
  • Friction model selection in FEM simulations of aluminium extrusion
    Donati, L. and Tomesani, L. and Schikorra, M. and Ben Khalifa, N. and Tekkaya, A.E.
    International Journal of Surface Science and Engineering 4 (2010)
    Visioplastic analyses with the rod technique were performed in the extrusion of AA6060 alloy at different processing conditions in order to measure the friction effect at the billet-container interface. During the trials an accurate monitoring of the relevant process variables such as punch force and temperatures was performed in order to validate FEM simulations. Different FE codes were used to carry out the simulations: Deform, HyperXtrude, and Superform. A particular attention was given on evaluating the several coefficients of the available friction models by comparing the FEM results with experimental results. Copyright © 2010 Inderscience Enterprises Ltd.
    view abstract10.1504/IJSURFSE.2010.029627
  • Joining of lightweight frame structures by die-less hydroforming
    Marré, M. and Gies, S. and Maevus, F. and Tekkaya, A.E.
    International Journal of Material Forming 3 (2010)
    A successful approach to achieve a reduction of a car's total weight is the implementation of lightweight strategies in the design process, e. g. using lightweight materials. An interesting alternative to conventional welding and riveting processes is joining by die-less hydroforming. This work describes an analytical model which can be used to calculate the strengths of these joints, taking into account the material parameters, joint geometry and process parameters. Additionally, validation of the model by both finite element simulations and experiments will be provided. Furthermore, investigations were carried out to implement the described methodology for a multi-joint used in a space frame structure. © 2010 Springer-Verlag France.
    view abstract10.1007/s12289-010-0946-2
  • Numerical material flow optimization of a multi-hole extrusion process
    Kloppenborg, T. and Brosius, A. and Tekkaya, A.E.
    Advanced Materials Research 83-86 (2010)
    The decrease of the bearing length in the aluminum extrusion processes results in an increase of the material flow and offers, through this, the possibility for correction and optimization. This study presents a simulation-based optimization technique which uses this effect for optimizing the material flow in a direct multi-hole extrusion process. First the extrusion process was numerically calculated to simulate the production of three rectangular profiles with equal cross sections. Here, the die orifices were arranged at various distances to the die centre, which lead to different profile exit speeds. Based on the initial numerical calculation, an automated optimization of the bearing length with the adaptive-response-surface-method was set up to achieve uniform exit speeds for all profiles. Finally, an experimental verification carried out to show the influence of the optimized die design. © (2010) Trans Tech Publications.
    view abstract10.4028/www.scientific.net/AMR.83-86.826
  • On the reduction of the gloss property of organic coated sheet metal after forming
    Tekkaya, A.E. and Trompeter, M. and Pham, H.-D.
    Production Engineering (2010)
    Organic coated sheet metals (OCSM) are widely used in many industrial applications such as the automobile industry or in electrical appliances because of their high corrosion resistance and satisfaction of ecological requirements. However, original functions of the very thin coating layer in OCSM products may change during forming, which leads to a reduction of the product's quality. This paper deals with the reduction of the gloss property of OCSM products. To this end, first a forming limit diagram (FLD) and a fracture limit diagram of the coating layer (FLDC) of OCSM are established by using Nakajima tests. Subsequently, the Nakajima tests are performed at a predefined strain in order to evaluate the change of the gloss property depending on the strain states. The loss of gloss as a function of plastic strain is used as a reference. For the verification of this reference, deep-drawing tests are carried out and the change of the gloss property on the surface is analyzed. The obtained results are in good agreement with the reference, so it can be applied for process design to predict the gloss reduction of OCSM products. © 2010 German Academic Society for Production Engineering (WGP).
    view abstract10.1007/s11740-010-0215-8
  • Simulation of the quench sensitivity of the aluminum alloy 6082
    Güzel, A. and Jäger, A. and Ben Khalifa, N. and Tekkaya, A.E.
    Key Engineering Materials 424 (2010)
    A method for the numerical estimation of the final hardness distribution of heat treated aluminum alloys was developed and implemented into a commercial finite element (FE) tool. Jominy end-quench tests were carried out in order to determine the quench sensitivity of the aluminum alloy EN AW-6082. The hardness distribution of the alloy after end-quenching was related to the corresponding cooling rates. The derived relation was tested for an industrial application by investigating the local heat treatment of a prototype crash absorbing structure. Numerical estimations were validated with experimental measurements. Effectiveness of the derived method and possible improvements were discussed. © (2010) Trans Tech Publications.
    view abstract10.4028/www.scientific.net/KEM.424.51
  • Simulation of tube wrinkling in electromagnetic compression
    Demir, O.K. and Psyk, V. and Tekkaya, A.E.
    Production Engineering 4 (2010)
    Wrinkle formation at electromagnetic tube compression was simulated using finite element (FE) method. Three-dimensional (3D) calculations were performed using a staggered coupling scheme between the electromagnetic and structural sides of the problem. Introducing the tube's contour imperfections into the FE model makes the simulation of the wrinkle formation possible. The results show good correspondence with the experiments. © 2010 German Academic Society for Production Engineering (WGP).
    view abstract10.1007/s11740-010-0243-4
  • Textured surfaces for deep drawing tools by rolling
    Franzen, V. and Witulski, J. and Brosius, A. and Trompeter, M. and Tekkaya, A.E.
    International Journal of Machine Tools and Manufacture 50 (2010)
    In this paper, macroscopic textured tool surfaces manufactured by rolling are investigated. Focus is on selective adjustment of friction by local texturing of tool areas to influence the material flow during deep drawing operations. Flat strip drawing tests were performed using friction elements with open textures. The texturing influences the friction conditions and affects the material properties of the stripes. The use of these surfaces results in a significant increase in friction, which allows an additional control of the material flow during sheet drawing operations. The main mechanisms for increased drawing forces are elastic deformation near the area of the texture and local plastic deformation on the sheet surface. Using strips made of mild steel, the texturing leads to an increased roughness of the sheet metal surface and, in the case of high surface pressure, to plastic deformations of the strips. Compared to conventional measures like draw beads, rolled-textured surfaces allow to retard the material flow during sheet drawing operation without excessive strain hardening in the sheet material. © 2010 Elsevier Ltd.
    view abstract10.1016/j.ijmachtools.2010.08.001
  • The new TSS bending process: 3D bending of profiles with arbitrary cross-sections
    Chatti, S. and Hermes, M. and Tekkaya, A.E. and Kleiner, M.
    CIRP Annals - Manufacturing Technology 59 (2010)
    A new roll-based process and machine for three-dimensional bending of profiles with symmetrical and asymmetrical cross-sections have been developed. Compared to conventional processes like stretch bending, the advantage of the Torque Superposed Spatial (TSS) bending is the kinematic adjustment of the bending contour, leading to higher flexibility and cost efficiency, especially in small batch production. To define the spatial geometry of the workpiece, a torque is superposed to the bending moment. Results of the analytical and numerical investigations concerning the mechanics of deformation and the machine parameters of the new process are presented. © 2010 CIRP.
    view abstract10.1016/j.cirp.2010.03.017
  • Tooling concepts to speed up incremental sheet forming
    Kwiatkowski, L. and Urban, M. and Sebastiani, G. and Tekkaya, A.E.
    Production Engineering 4 (2010)
    A major drawback of asymmetric incremental sheet forming (AISF) is the long cycle time. AISF is known in general as forming of a sheet metal by only one small forming zone. The developed concepts presented in this paper aim at decreasing the main process time by applying several forming zones on the part through multiple tools working in parallel. By the use of a systematic design process, four major structural tooling concepts were invented. Those variants were compared to each other by a cost-utility analysis. As a result, a prototype with two tools was produced to fasten the AISF process. © German Academic Society for Production Engineering (WGP) 2009.
    view abstract10.1007/s11740-009-0206-9
  • Towards the flexible and near-net-shape production of three-dimensionally curved extrusion profiles
    Schneider, M. and Koehler, G. and Becker, D. and Selvaggio, A. and Tekkaya, A. E. and Munzinger, C. and Schulze, V. and Kleiner, M.
    Production Engineering 4 (2010)
    The present paper describes the new developments in continuous extrusion of curved profiles by a flexible production method which is composed of a short process chain starting with the extrusion press, a deflection tool, one robot for the flying cutting of the profile and another robot for supporting and handling the profile. Because of the flexibility of this complete system according to the profile curvature it is convenient for smallvolume production. In order to reach high profile accuracy all kinematic systems in the process chain have to be synchronized with the profile speed caused by the extrusion process. The results of the investigation of different existing synchronization methods identify the need for an additional measuring system to compensate the large deviation of the cut profile length. By adjusting parameters of the synchronization the accuracy of the profile length could be strongly improved. Furthermore influencing parameters like die deformation during the extrusion process are measured and combined with the results of the cut profile length over the process time. © German Academic Society for Production Engineering (WGP) 2010.
    view abstract10.1007/s11740-010-0246-1
  • deep drawing

  • extrusion

  • finite element method

  • manufacture

  • mechanical properties

  • metal forming

  • tensile testing

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