Prof. Dr.-Ing. Gerd Witt

Institue of Product Engineering
University of Duisburg-Essen

Author IDs

  • Laser powder bed fusion of Cu-Ti-Zr-Ni bulk metallic glasses in the Vit101 alloy system
    Frey, M. and Wegner, J. and Barreto, E.S. and Ruschel, L. and Neuber, N. and Adam, B. and Riegler, S.S. and Jiang, H.-R. and Witt, G. and Ellendt, N. and Uhlenwinkel, V. and Kleszczynski, S. and Busch, R.
    Additive Manufacturing 66 (2023)
    view abstract10.1016/j.addma.2023.103467
  • Influence of process gases containing helium on the laser beam melting process [Einfluss heliumhaltiger Prozess-gase auf den Laser-Strahlschmelz-prozess]
    Deckers, T. and Ammann, T. and Forêt, P. and Dubiez-Le-goff, S. and Zissel, K. and Witt, G.
    Zeitschrift Kunststofftechnik/Journal of Plastics Technology 117 (2022)
    Influence of Process Gases Containing Helium on the Laser Beam Melting Process. Can the process gas play a key role in optimizing the PBF-LB/M process (layer thickness, scan speed, processability of new materials, etc.)? This article provide insights into the current state of research at Linde GmbH regarding he-lium-containing process gases and presenting the novel process gas ADDvance® Laser230. Due to its composition, the gas mixture allows to significantly improve process productivity and stability. © 2022 Walter de Gruyter GmbH, Berlin/Boston, Germany.
    view abstract10.1515/zwf-2022-1094
  • Cavity vat photopolymerisation for additive manufacturing of polymer-composite 3D objects
    Bachmann, J. and Obst, P. and Knorr, L. and Schmölzer, S. and Fruhmann, G. and Witt, G. and Osswald, T. and Wudy, K. and Hinrichsen, O.
    Communications Materials 2 (2021)
    Vat photopolymerisation describes resin-based additive manufacturing processes in which ultraviolet light is used to layer-wise solidify liquid resin into a desired 3D shape. If the starting resin is a dual-curing formulation the object is also thermally cured to attain its final properties, obtaining either an elastomer or a thermoset. Here, we introduce cavity vat photopolymerisation, in which one photopolymer resin produces a composite material of an elastomer and thermoset. Cavities of any geometry are purposefully designed in the solid object and then filled with liquid resin during printing due to negative pressure. Thermal curing then solidifies the resin in the cavities into an elastomer, forming a distinct interface held together by strong covalent bonds. Hybrid specimens indicate improved damping, reduced fragmentation upon fracture and increased local elasticity, and we suggest several hard-shell/soft-core applications that might benefit. © 2021, The Author(s).
    view abstract10.1038/s43246-021-00211-5
  • Control of residual oxygen of the process atmosphere during laser-powder bed fusion processing of Ti-6Al-4V
    Pauzon, C. and Dietrich, K. and Forêt, P. and Dubiez-Le Goff, S. and Hryha, E. and Witt, G.
    Additive Manufacturing 38 (2021)
    The effect of the residual oxygen concentration in the process atmosphere during laser-powder bed fusion (L-PBF) of Ti-6Al-4V was investigated, using an external oxygen monitoring system equipped with two types of oxygen sensors typically used in L-PBF hardware: a lambda probe and an electrochemical oxygen sensor. The recordings of the oxygen variations during L-PBF highlighted that the electrochemical sensor is more reliable than the lambda probe, whose signal showed a maximum deviation of about 700 ppm O2 after 7 h, attributed to its sensitivity to hydrogen present in the system. The study revealed that proper monitoring of the oxygen in the laboratory scale L-PBF system used is necessary to limit oxygen and nitrogen pick-ups by the built material. Concentrations as high as 2200 ppm O2 and 500 ppm N2 in the Ti-6Al-4V part built under standard conditions were measured, compared to maximum levels of 1800 ppm O2 and 250 ppm N2 with the external oxygen control. In addition, the findings underline the critical effect of the component design, such as the high aspect ratio columns or the lattice structures, on the heat accumulation in case of Ti-6Al-4V, leading to enhanced oxygen and nitrogen pick-up, as high as 600 ppm O2 and 150 ppm N2 difference between the bottom and top of the cylindrical samples of 70 mm height used in this study. The determination of tensile properties of samples built at different heights put in evidence the detrimental effect of the oxygen increase with build height on the ductility, which decreased from 12% to below 6% between the bottom and top positions. This work highlights that the possible presence of impurities in the L-PBF atmosphere can have harmful impact on the properties of Ti-6Al-4V components, which can be mitigated adjusting the oxygen control system. © 2020 The Authors
    view abstract10.1016/j.addma.2020.101765
  • Experimental and numerical thermal analysis of the laser powder bed fusion process using in situ temperature measurements of geometric primitives
    Schnell, N. and Schoeler, M. and Witt, G. and Kleszczynski, S.
    Materials and Design 209 (2021)
    Laser powder bed fusion (PBF-LB/M) is a potent technology for manufacturing demanding geometries using innovative materials. The complex thermal conditions during the process are nontrivial to describe and have a significant impact on final material properties. Therefore, these conditions are analyzed using an experimental setup based on thermocouples embedded into the substrate plate close to the substrate-part interface of the respective sample. The in situ data allows for an in-depth investigation of correlations between core process parameters (laser power, scan velocity, exposed area) and the temperature progression at the base of the part. The alternative view on the conditions during the process enables a novel analytical description of the thermal history. Additionally, a macroscopic FEM-model is presented. It is calibrated and validated through the empirical data of geometric primitives to emphasize the added value of the setup as a calibration tool for thermal simulations. © 2021 The Authors
    view abstract10.1016/j.matdes.2021.109946
  • Failure criterion for PA 12 multi-jet fusion additive manufactured parts
    Osswald, P.V. and Obst, P. and Mazzei Capote, G.A. and Friedrich, M. and Rietzel, D. and Witt, G.
    Additive Manufacturing 37 (2021)
    Offering the possibility of producing complex geometries in a compressed product development cycle, it comes as no surprise that additive manufacturing (AM) techniques have become attractive to multiple industries, including the automotive and aerospace segments. Unfortunately, the ubiquitous stratified build approach used by these technologies is responsible for the pain point that hinders their adoption in production of parts that will be subjected to complex loads: the junction of adjacent layers tends to have subpar mechanical properties when compared to those of the bulk material, and thus, assessing the structural integrity of an AM part becomes difficult. In the advent of the industrialization of series production of AM parts for the automotive industry, the necessity to understand and predict how and why AM parts fail under complex stress states becomes of paramount importance. This paper applies a failure criterion for materials with anisotropic properties with stress interactions, to predict failure of multi-jet fusion (MJF) parts manufactured using polyamide 12 powder. The results are compared to the failure surfaces of Selective Laser Sintering (SLS) components. Special test specimens were designed, produced, and tested to measure failure under tensile, compressive, shear, and combined loading scenarios. The results show that much like SLS, MJF parts have a notable difference in tensile and compressive strengths. Unlike SLS however, MJF parts do not exhibit a strong interaction between stresses when under combined loading. The experimental data shows an excellent fit with the failure criterion, precisely capturing the strength behavior of MJF printed parts under complex loading conditions. Of great interest in this study is that the stress interactions with MJF parts were determined to be negligible when compared to SLS specimens, which emphasizes the fact that when performing stress analyses, each one of these powder-based additive manufacturing techniques must be treated differently. © 2020 Elsevier B.V.
    view abstract10.1016/j.addma.2020.101668
  • Impact of single structural voids on fatigue properties of AISI 316L manufactured by laser powder bed fusion
    Kotzem, D. and Kleszczynski, S. and Stern, F. and Elspaß, A. and Tenkamp, J. and Witt, G. and Walther, F.
    International Journal of Fatigue 148 (2021)
    The laser powder bed fusion of metals (PBF-LB/M) process is already exploited in several industrial applications. The process itself allows to introduce artificial defects which can later be characterized by their influence on the resulting mechanical properties. In this study, the influence of isolated single structural defects (0.3 mm ≤ √area ≤ 1.5 mm) on the fatigue properties is discussed and the √area-parameter model is applied. The obtained results show that the investigated material is highly defect tolerant as artificial defects with √area = 0.3 mm are not crack initiating. Specimens with a defect of √area = 1.0–1.5 mm clearly show crack initiation and propagation starting from the defect, and a fatigue strength estimation tends to be more conservative. © 2021 Elsevier Ltd
    view abstract10.1016/j.ijfatigue.2021.106207
  • Influence assessment of artificial defects on the fatigue behavior of additively manufactured stainless steel 316LVM
    Stern, F. and Grabowski, J. and Elspaß, A. and Kotzem, D. and Kleszczynski, S. and Witt, G. and Walther, F.
    Procedia Structural Integrity 37 (2021)
    The laser powder bed fusion of metals (PBF-LB/M) is one of the most promising techniques to realize lightweight optimized parts and structures. Possible design elements are internal cooling channels or topology optimized geometries. However, not only does the process suffer of instabilities causing pores and lack-of-fusion defects but also a low surface quality. A further knowledge about these defects and their influence on the mechanical behavior are needed to use additively manufactured parts in structural relevant applications. In this work, the austenitic stainless steel 316LVM (X2CrNiMo18-15-3) has been processed by PBF-LB/M. In total, four different batches were manufactured with either no intended porosity or specific cubic defects ranging between 0.3 and 1.5 mm edge length. The fatigue behavior was evaluated at stress ratio R = -1 up to 1E7 cycles. The fracture surface was analyzed by scanning electron microscopy and the relationship between artificial defect size and fatigue strength was investigated by Kitagawa-Takahashi (KT) diagram and its modification by El Haddad's intrinsic crack length. The results show that the KT-diagram underestimates the fatigue strength of the investigated steel indicating a high defect tolerance and possible hardening mechanisms during cyclic loading such as possible nano-twinning. An influence of the entrapped process gas could also play a role. As long as this is unclear, the models can only be used conservatively as the full potential of the PBF-LB/M steel cannot be fully exploited. © 2022 The Authors.
    view abstract10.1016/j.prostr.2022.01.071
  • Influence of powder characteristics on the structural and the mechanical properties of additively manufactured Zr-based bulk metallic glass
    Wegner, J. and Frey, M. and Piechotta, M. and Neuber, N. and Adam, B. and Platt, S. and Ruschel, L. and Schnell, N. and Riegler, S.S. and Jiang, H.-R. and Witt, G. and Busch, R. and Kleszczynski, S.
    Materials and Design 209 (2021)
    Additive manufacturing of Zr-based bulk metallic glasses (BMGs) is subject to growing scientific and industrial attention. Laser-based powder bed fusion of metals (PBF-LB/M) becomes a key technology to overcome current restrictions of size and geometry in the manufacturing of BMGs. For industrial application, further knowledge about defect formation, such as porosity and crystallization, is mandatory to develop processing strategies and suitable quality assurance. In this context, the influence of the particle size distribution, oxygen contamination, and applied process parameters during the PBF-LB/M of the glass-forming alloy AMZ4 (in at.% Zr59.3Cu28.8Al10.4Nb1.5) on the structural and mechanical properties were evaluated. It was found that the addition of SiO2 flow aid to the feedstock is suitable to increase flowability without impeding fabrication of the amorphous material. Furthermore, the processing of partially crystalline powder particles into amorphous samples is demonstrated. It indicates that today's high effort producing amorphous powders and thus the production costs can be reduced. Flexural bending tests and high-energy synchrotron X-ray diffraction reveal that the powder feedstock's oxygen content is crucial for the amorphization, embrittlement, and flexural strength of PBF-LB/M processed Zr-based BMGs. © 2021
    view abstract10.1016/j.matdes.2021.109976
  • Investigation of the Roughness Influence on the Absorption Behavior of additively manufactured Metals by the Laser Speckle Photometry
    Dang, D. and Elspas, A. and Cikalova, U. and Kleszczynski, S. and Bendjus, B. and Witt, G.
    Proceedings of the International Spring Seminar on Electronics Technology 2021-May (2021)
    The Laser Powder Bed Fusion of Metals (LPBF-M) is one of the most important methods in the additive manufacturing. This process can be used to produce components with a high degree of complexity and design freedom as well as with material density. Unfortunately, hundreds of factors influence the quality of the processes and thus the material characteristics which limits the reproducibility and the economic viability. Therefore, quality control during process, as well as the testing of material properties afterward, is one of the key development fields. This paper presents the applicability of the Laser Speckle Photometry (LSP) for determination of the surface topology in additive manufacturing of metals. The LSP is a non-destructive testing method which examines optical interference patterns bases on the speckle phenomena for the defect detection on surfaces. For this purpose, samples with a specific pore structure in the near-surface zone which influences the surface characteristics were manufactured. With the LSP, the surfaces were measured to verify a correlation between the roughness and the LSP signal. Depended on the surface roughness different absorption behaviors of the fabricated specimens were determined during external laser excitation as a part of LSP measurement and simultaneously measured temperature. © 2021 IEEE.
    view abstract10.1109/ISSE51996.2021.9467607
  • Mitigating oxygen pick-up during laser powder bed fusion of Ti-6Al-4V by limiting heat accumulation
    Pauzon, C. and Dietrich, K. and Forêt, P. and Hryha, E. and Witt, G.
    Materials Letters 288 (2021)
    The dissolution of oxygen in Ti-6Al-4V during laser powder bed fusion (L-PBF) is a limitation for the final ductility of the produced components and a challenge for the end-users. In the present work, the effect of the residual oxygen in the process atmosphere of a laboratory scale L-PBF machine, as well as the role of heat accumulation, are studied. It was shown that oxygen content in the as-built Ti-6Al-4V is determined by the size of the scanned area and build time. The heat accumulation aspect was investigated by adjusting the inter-layer time (ILT), by increasing the recoating time or the number of produced parts. The results showed that oxygen pick-up could be limited by reducing residual oxygen level in the atmosphere or heat accumulation. A 400 ppm O2 reduction measured at the top of a 70 mm column was achieved by increasing the ILT manually by 4.5 s, and a 1200 ppm O2 reduction by increasing the scanned area by 7 times. By doing so, the hardness at full height was reduced by approximately 30 HV10. It is shown that design features characterised by high aspect ratio can absorb significant amount of oxygen resulting in increased brittleness. © 2021 The Author(s)
    view abstract10.1016/j.matlet.2021.129365
  • Modeling Fused Filament Fabrication using Artificial Neural Networks
    Oehlmann, P. and Osswald, P. and Blanco, J.C. and Friedrich, M. and Rietzel, D. and Witt, G.
    Production Engineering (2021)
    With industries pushing towards digitalized production, adaption to expectations and increasing requirements for modern applications, has brought additive manufacturing (AM) to the forefront of Industry 4.0. In fact, AM is a main accelerator for digital production with its possibilities in structural design, such as topology optimization, production flexibility, customization, product development, to name a few. Fused Filament Fabrication (FFF) is a widespread and practical tool for rapid prototyping that also demonstrates the importance of AM technologies through its accessibility to the general public by creating cost effective desktop solutions. An increasing integration of systems in an intelligent production environment also enables the generation of large-scale data to be used for process monitoring and process control. Deep learning as a form of artificial intelligence (AI) and more specifically, a method of machine learning (ML) is ideal for handling big data. This study uses a trained artificial neural network (ANN) model as a digital shadow to predict the force within the nozzle of an FFF printer using filament speed and nozzle temperatures as input data. After the ANN model was tested using data from a theoretical model it was implemented to predict the behavior using real-time printer data. For this purpose, an FFF printer was equipped with sensors that collect real time printer data during the printing process. The ANN model reflected the kinematics of melting and flow predicted by models currently available for various speeds of printing. The model allows for a deeper understanding of the influencing process parameters which ultimately results in the determination of the optimum combination of process speed and print quality. © 2021, The Author(s).
    view abstract10.1007/s11740-021-01020-y
  • Speeding up Additive Manufacturing by Means of Forming for Sheet Components with Core Structures
    Rosenthal, S. and Hahn, M. and Tekkaya, A.E. and Platt, S. and Kleszczynski, S. and Witt, G.
    International Journal of Precision Engineering and Manufacturing - Green Technology (2021)
    A new process combination route consisting of additive manufacturing (AM) with a subsequent forming operation is proposed. The process route has the opportunity to increase the efficiency of the AM process route up to 360%. Stainless steel 316L sheets with different core structures (similar to sandwich sheets) are produced by AM, characterized, and formed in a die bending operation. The bending characteristics of this novel semi-finished product can be accurately predicted in a numerical simulation. The new process route is discussed in detail and compared to conventional AM parts in terms of the production efficiency. © 2021, The Author(s).
    view abstract10.1007/s40684-021-00384-x
  • Thermoplastic forming of additively manufactured Zr-based bulk metallic glass: A processing route for surface finishing of complex structures
    Frey, M. and Wegner, J. and Neuber, N. and Reiplinger, B. and Bochtler, B. and Adam, B. and Ruschel, L. and Riegler, S.S. and Jiang, H.-R. and Kleszczynski, S. and Witt, G. and Busch, R.
    Materials and Design 198 (2021)
    Additive manufacturing of bulk metallic glasses (BMGs) through laser powder bed fusion (LPBF) has drawn growing interest in the last years, especially concerning industry-relevant alloys based on iron or zirconium. The process-inherent high cooling rates and localized melting pools allow to overcome geometrical restrictions given for the production of BMGs by classical casting routes. Yet, the achievable surface qualities are still limited, making an adequate post-processing necessary. In this work, we report on applying thermoplastic forming on LPBF-formed parts for the first time to decrease surface roughness and imprint finely structured surface patterns without the need for complex abrasive machining. This BMG-specific post-processing approach allows to functionalize surface areas on highly complex LPBF-formed specimens, which could be of interest especially for medical or jewelry applications. © 2020 The Authors
    view abstract10.1016/j.matdes.2020.109368
  • Calibration approach for reliable in-situ process monitoring of multi-optic selective laser melting
    Alberts, D. and Standfuß, M. and Schwarze, D. and Witt, G.
    Procedia CIRP 94 (2020)
    Additive Manufacturing is on the threshold of full industrial use in various leading industries. This is due to the continuous development of machine systems regarding availability, stability, reproducibility and repeatability alongside their ever-increasing productivity. To achieve high quality parts, process variables must be monitored during production, for example with a Melt Pool Monitoring (MPM) system to measure their characteristics. Initial and long-term stable calibration of the monitoring system must be ensured with proper alignment, positioning and the size of the measuring surface itself. To secure direct comparison of multioptics within a machine as well as transferability from machine to machine, a quantitative intensity calibration must also be carried out. This paper provides a new calibration approach with measuring equipment and tools including a Measurement System Analysis (MSA), exploring possibilities and limitations in terms of absolute accuracy, repeatability and reproducibility as well as workflows for the measurement equipment and machine calibration. © 2020 The Authors. Published by Elsevier B.V.
    view abstract10.1016/j.procir.2020.09.155
  • Diamond-impregnated 316L metal matrix composites fabricated by powder bed fusion with laser beam - Influences of the energy input on the microstructural properties
    Wegner, J. and Fehr, A. and Platt, S. and Kleszczynski, S. and Witt, G. and Tillmann, W.
    Diamond and Related Materials 109 (2020)
    Fabricating diamond metal matrix composites (DMMCs) by means of powder bed fusion of metals using a laser beam (PBF-LB/M) is a new approach to extensively expand the spectrum of geometrical freedom for diamond tools. However, it must be borne in mind that the temperature input has a significant influence on the diamond condition since graphitizations are likely to occur. Therefore, it was analyzed how varying volume energy densities and substrate heating affect the microstructure and the densification of a 316 L stainless steel matrix, which was impregnated with 5 vol.-% Ni-coated diamonds. With regard to the densification, it was shown that an elevated substrate temperature (473 K) allowed to apply reduced volume energy densities and reduce stress-induced cracking. Thus, a relative density of 99.5% could be achieved. Furthermore, decreasing the volume energy density avoided graphitizations of the diamonds. Cr and Fe contents of the matrix material dissolved at the Ni-coated diamond surface revealing a diamond-metal interaction. A longer heat flux generally supported these diffusion processes at the interfaces. Finally, it became obvious that increased laser powers resulted in a higher densification, while low scan speeds and laser powers are desirable to foster diffusion and to avoid graphitizations. © 2020 Elsevier B.V.
    view abstract10.1016/j.diamond.2020.108040
  • Influence of process gas during powder bed fusion with laser beam of Zr-based bulk metallic glasses
    Wegner, J. and Frey, M. and Kleszczynski, S. and Busch, R. and Witt, G.
    Procedia CIRP 94 (2020)
    Zr-based bulk metallic glasses offer a unique combination of hardness, high strength, and high elastic limits. Yet, manufacturable size and complexity are limited due to the required cooling rates. Short laser-material interaction times together with layer-wise and selective energy input allows the laser powder bed fusion process to largely overcome those restrictions. Still, the complex process-material interactions inhere numerous uncertainties. In the present work, additively manufactured Zr-based bulk metallic glasses produced under three different process gases are investigated by calorimetry, x-ray diffraction, and bending tests. A strong dependence between the thermophysical properties, flexural strength, and the applied atmosphere is found. © 2020 The Authors. Published by Elsevier B.V.
    view abstract10.1016/j.procir.2020.09.039
  • Investigation of the influence of exposure time on the dual-curing reaction of RPU 70 during the DLS process and the resulting mechanical part properties
    Obst, P. and Riedelbauch, J. and Oehlmann, P. and Rietzel, D. and Launhardt, M. and Schmölzer, S. and Osswald, T.A. and Witt, G.
    Additive Manufacturing 32 (2020)
    Photo-differential scanning calorimetry (Photo-DSC), standard DSC and tensile testing are used in order to investigate the influence of different exposure times on the dual-curing reaction and mechanical properties of RPU 70 specimens produced by the additive manufacturing technology Digital Light Synthesis (DLS). A comparison of reaction enthalpies measured with a Photo-DSC of different exposure times followed by thermal curing and DSC experiments with different exposed DLS green products shows that the degree of thermal cross-linking of the two-component resin RPU 70 is determined by the preceding cross-linking reaction during photo-polymerization. A shorter exposure time of UV-light enhances the subsequent thermal cross-linking, while increased exposure reduces the effect of thermal curing in the second process step. Tensile testing results of specimen produced with custom-made print profiles on the DLS Carbon M2 with different exposure times show that mechanical characteristics such as tensile strength and elongation at break are strongly dependent on the degree of UV-cross-linking. The mechanical characteristics of DLS parts produced with the RPU 70 resin system can therefore be adapted to specific requirements and applications using these customized print profiles with different exposure times. © 2019 Elsevier B.V.
    view abstract10.1016/j.addma.2019.101002
  • Investigation of the influence of various post-treatment methods on the properties of additive manufactured FDM Nylon 12 samples [Untersuchung des einflusses verschiedener nachbehandlungsmethoden auf die eigenschaften additiv gefertigter FDM Nylon 12 proben]
    Wiedau, L.C. and Hesse, D. and Baumann, J. and Witt, G. and Stommel, M. and Zabel, A. and Kuhlenkötter, B. and Künne, B.
    Zeitschrift Kunststofftechnik/Journal of Plastics Technology 2020 (2020)
    The MERCUR research project Pr-2017-0003 "Analysis of the product development process in the combination of additive and subtractive manufacturing processes for the manufacture of multi-material products" combines different manufacturing and post-treatment processes to connect the respective advantages. The additive fused deposition modeling (FDM) together with the anisotropic component properties resulting from the process are used in this investigation. The influence of chemical and mechanical post-treatment steps on these properties will be investigated. © Carl Hanser Verlag.
    view abstract10.3139/O999.02022020
  • Investigation of the track width-dependent melt pool characteristics during laser-sintering of polyamide 12 in correlation to various focus diameters
    Meyer, L. and Witt, G.
    Progress in Additive Manufacturing 5 (2020)
    Factors such as not only costs, production time, reproducibility, but also the quality of the components are decisive factors in assessing the economic efficiency of a manufacturing process. With additive manufacturing processes, component production is made possible directly from a 3D CAD model. This means that small series and prototypes can already be produced economically today. In this area, the laser-sintering process, in particular, offers great potential for series production due to its high strength values and ductility. With laser-sintering systems that allow an optical widening of the laser focus, a faster exposure of the component and thus a shortening of the building time is possible. We developed a laser-sintering system whose laser focus diameter is adjustable in its cross-sectional area from 0.47 to 2 mm. The goal for the future is to produce large-area components significantly faster by widening the focus diameter, thus making laser-sintering more productive. In this paper, the focus-dependent melt pool formation is examined in correlation to different hatch distances during the laser-sintering of polyamide 12. For this purpose, a test specimen was developed which can display single tracks as well as a multitude of different track widths for all feasible focus level variations. This knowledge is required to determine and investigate the track width-dependent melt pool formation as a function of the focal diameter of the component cross sections. © 2020, The Author(s).
    view abstract10.1007/s40964-020-00126-6
  • LPBF-M manufactured Zr-based bulk metallic glasses coated with magnetron sputtered ZrN films
    Tillmann, W. and Fehr, A. and Wegner, J. and Stangier, D. and Kleszczynski, S. and Witt, G.
    Surface and Coatings Technology 386 (2020)
    The fabrication of Zr-based bulk metallic glasses (BMGs) by means of laser powder bed fusion of metals (LPBF-M) is recently emerging. This production route allows to widely overcome current geometrical restrictions of casting routes while maintaining the amorphous character, which is decisive for the unique mechanical properties, for instance. However, the roughness of the LPBF-M fabricated BMGs is still a challenging property, impeding the application of near-net shaped thin films that modify BMG surfaces, e.g. with respect to wear resistance. Zr59.3Cu28.8Al10.4Nb1.5 (at.%) substrates were manufactured by means of LPBFM, applying various exposure strategies, including laser remelting of the last solidified layer to influence the surface topography. Furthermore, BMG substrates were post-treated by grinding and polishing. Thus, varying degrees of crystallinity as well as surface roughness states were generated to analyze the effect of these characteristics on the microstructural properties of additionally applied magnetron sputtered ZrN films. Substrates that were fabricated with higher energy densities during LPBF-M exhibited (101)-Zr as well as (013)- and (110)-CuZr2 phases, which were accompanied by a decreased surface roughness. It was shown that all films had a crystalline structure on amorphous and partly crystalline BMG surfaces. A decreased surface roughness of the BMG substrates could be directly correlated with a higher hardness and a better adhesion of the ZrN film. © 2020 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2020.125463
  • Material Characterization of Additively Manufactured Metals by Laser Speckle Photometry
    Dang, D. and Elspas, A. and Cikalova, U. and Kleszczynski, S. and Bendjus, B. and Witt, G.
    Proceedings of the International Spring Seminar on Electronics Technology 2020-May (2020)
    In contrast to traditional time-consuming and costly manufacturing processes, additive manufacturing offers an effective production of prototypes. In addition to the numerous process developments of recent years, quality assurance as well as the testing of material properties afterward have now become one of the main requirements. Previous research approaches such as thermography or optical imaging have essential disadvantages, why there is a need for simple and effective solutions. For this purpose, the Laser Speckle Photometry (LSP) as a novel nondestructive approach for evaluating the material quality manufactured by Laser Powder Bed Fusion of Metals (LPBF-M) is presented. For the validation of this testing method exactly defined defective samples were reproducibly fabricated by specific adaptation in the building process. Voids were introduced into the structure, which are not visible on the surface. The simple LSP setup and the adjusted evaluation algorithm, based on the correlation function, are decisive for characterizing different material density states. This paper presents the potential of LSP for the use in LPBF-M processes stated with the first results of the validation. © 2020 IEEE.
    view abstract10.1109/ISSE49702.2020.9120869
  • Optimization of the production processes of powder-based additive manufacturing technologies by means of a machine learning model for the temporal prognosis of the build and cooling phase
    Osswald, P.V. and Mustafa, S.K. and Kaa, C. and Obst, P. and Friedrich, M. and Pfeil, M. and Rietzel, D. and Witt, G.
    Production Engineering 14 (2020)
    The increase in additive manufacturing production volume in recent years has led, not only, to a need for an increased level of productivity during the build job, but also the optimization of other crucial steps in each respective technologies process chain. With HP’s Multi-jet Fusion technology relatively recent entry (2014) into a seemingly stagnant scene of industrial scale powder-based additive manufacturing, it presented itself as an ideal candidate for post-processing optimization using methods of machine learning due to its faster print time. 66% of the Multi-jet Fusion production process, from build job preparation and nesting to delivery of finished parts is comprised of its cooling process step. This cooling process step can take anywhere from 3 to 30 h, depending on a number of factors. While speeding up cooling does not come into question when processing semi-crystalline polymers, such as polyamides, knowing the necessary cooling time with relatively high accuracy, becomes crucial. In this study, a machine learning model was created to predict a significantly more accurate cooling time using a number of build job parameters. The optimized cooling model was trained using the measured cooling times of varying build jobs as output and build job height, number of layers, packing density, number of parts and room temperature as inputs. The machine learning model predicts significantly more accurate cooling times than the manufacturer predictions. Furthermore, as with all machine learning models, it was shown that an increased number of data, resulted in more accurate predictions. The implementation of the optimized cooling model at BMW’s AM production facility leads to increased transparency, leaner production and a higher overall economic viability of AM technologies. © 2020, German Academic Society for Production Engineering (WGP).
    view abstract10.1007/s11740-020-00987-4
  • The influence of oxygen on the chemical composition and mechanical properties of Ti-6Al-4V during laser powder bed fusion (L-PBF)
    Dietrich, K. and Diller, J. and Dubiez-Le Goff, S. and Bauer, D. and Forêt, P. and Witt, G.
    Additive Manufacturing 32 (2020)
    In Laser powder bed fusion (L-PBF), metal powders, sensitive to humidity and oxygen, like AlSi10Mg or Ti-6Al-4 V are used as starting material. Titanium-based materials are influenced by oxygen and nitrogen due to the formation of oxides and nitrides, respectively. During this research, the oxygen concentration in the build chamber was controlled from 2 ppm to 1000 ppm using an external measurement device. Built Ti-6Al-4 V specimens were evaluated regarding their microstructure, hardness, tensile strength, notch toughness, chemical composition and porosity, demonstrating the importance of a stable atmospheric control. It could be shown that an increased oxygen concentration in the shielding gas atmosphere leads to an increase of the ultimate tensile strength by 30 MPa and an increased (188.3 ppm) oxygen concentration in the bulk material. These results were compared to hot isostatic pressed (HIPed) samples to prevent the influence of porosity. In addition, the fatigue behavior was investigated, revealing increasingly resistant samples when oxygen levels in the atmosphere are lower. © 2019
    view abstract10.1016/j.addma.2019.100980
  • Additive manufacturing of soft magnetic permalloy from Fe and Ni powders: Control of magnetic anisotropy
    Schönrath, H. and Spasova, M. and Kilian, S.O. and Meckenstock, R. and Witt, G. and Sehrt, J.T. and Farle, M.
    Journal of Magnetism and Magnetic Materials 478 (2019)
    The influence of the process parameters in Laser Beam Melting (LBM) on the element distribution and magnetic properties of permalloy (Ni 78.5 Fe 21.5 ) is studied. Iron and nickel powders are mixed in the respective proportions to build twenty-five permalloy samples. The process parameters for each sample are varied to achieve different volume energy densities. An increase of the saturation magnetization M S up to 14% of the samples with respect to the initial powder blend is found. For a volume energy density of 428 [Formula presented] we detect a stripe-like segregation of iron and nickel in the uppermost layer. In the volume a homogeneous element distribution is found. The segregation at the surface leads to a sizable uniaxial magnetic anisotropy. When using parameter combinations resulting in similar volume energy densities, we observe different surface morphologies depending on scan speed and laser power. The implications for creating tailored magnetic anisotropy directions in Fe-Ni soft magnets are discussed. © 2019 Elsevier B.V.
    view abstract10.1016/j.jmmm.2018.11.084
  • Adjustment of isotropic part properties in laser sintering based on adapted double laser exposure strategies
    Wegner, A. and Witt, G.
    Optics and Laser Technology 109 (2019)
    Laser sintering of polymers gets more and more importance for small series production. However, laser sintered parts have often an explicit anisotropy of mechanical properties. Due to the layer-wise production, parts show no homogenous morphological structure as known from injection molded parts. Voids appear within the part and are concentrated in the area between two consecutive layers, resulting in reduced bonding. Therefore, mechanical properties and in particular the elongation at break show significantly lower values in the direction of build compared to the properties within the build plane. Sometimes, these effects are an obstacle in the usage of laser sintering for series production of parts. The aim of the experiments was to investigate to what extent an improvement of the layer-to-layer bonding and thus of the characteristic values in the building direction can be achieved by alternative exposure strategies. For this purpose, the influence of adaptive double laser exposure strategies on layer-to-layer bonding and anisotropy was investigated. The influence of different parameter settings for the first and second exposure was investigated. In a further step it was examined to what extent the influence of disturbances on the process, such as the inhomogeneous temperature distribution or cycle time variations can be reduced by applying the developed double laser exposure strategies. For this purpose, a comparison was made between an optimized double laser exposure parameter set and the standard parameter set. © 2018 Elsevier Ltd
    view abstract10.1016/j.optlastec.2018.08.017
  • Analysis of material aging and the influence on the mechanical properties of polyamide 12 in the Multi Jet Fusion process
    Riedelbauch, J. and Rietzel, D. and Witt, G.
    Additive Manufacturing 27 (2019)
    view abstract10.1016/j.addma.2019.03.002
  • 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
  • In situ measurement of part geometries in layer images from laser beam melting processes
    zur Jacobsmühlen, J. and Achterhold, J. and Kleszczynski, S. and Witt, G. and Merhof, D.
    Progress in Additive Manufacturing 4 (2019)
    Laser beam melting (LBM) enables production of three-dimensional parts from metallic powder with very high geometrical complexity and very good mechanical properties. In LBM, a thin layer of metallic powder is deposited onto the build platform and melted by a laser according to the desired part geometry. Until today, the potential of LBM for critical applications such as medical devices and aerospace has not been exploited due to the lack of build stability and quality management. We present an image analysis method, which segments part contours in high-resolution images of LBM-produced layers. Based on the reference contour from 2D slices of the 3D part model and edge-detection results, a graph model is built and segmented using Graph Cuts (min-cut max-flow algorithm). Our method is evaluated on 124 part contours from 5 build jobs with different part geometries. Iterative GrabCut segmentation on nonlinearly smoothed images achieves the best results with a median Jaccard distance of 0.035 (32 % improvement over the reference geometry masks) and a mean contour distance below 2.4 px (36.4 % improvement). © 2018, Springer Nature Switzerland AG.
    view abstract10.1007/s40964-018-0068-9
  • Influence of chemical postprocessing on mechanical properties of laser-sintered polyamide 12 parts
    Wörz, A. and Wiedau, L.C. and Wudy, K. and Wegner, A. and Witt, G. and Drummer, D.
    Journal of Polymer Engineering 39 (2019)
    A limiting factor for industrial usage of laser-sintered parts is the high surface roughness due to the semi-molten or attaching powder particles resulting from tool and pressureless manufacturing. An approach to improve the surface quality is the postprocessing with acids to smoothen the surface as it enables improvement without geometrical restrictions of the parts. The present work deals with the usage of nitric, hydrochloric, and trifluoroacetic acids, and exhibits the influence on the resulting surface morphology, dimensional accuracy, and the mechanical properties. The results exhibit different interaction mechanics and show great differences in the resulting part properties. © 2019 Walter de Gruyter GmbH, Berlin/Boston.
    view abstract10.1515/polyeng-2019-0110
  • Investigation of LBM-processed bimodal powder mixtures of the nickel base alloy HX and WC–Co
    Schaak, C. and Kleszczynski, S. and Tillmann, W. and Witt, G.
    Progress in Additive Manufacturing 4 (2019)
    Laser beam melting (LBM) is an additive manufacturing technology (AM), which enables the production of individual, complex metal parts. The development of AM-specific materials is a necessary step to exploit the full technological potential and should be a useful contribution for the ongoing process of implementing, establishing, and realizing AM processes in industrial fields of application. Theoretically, the LBM process offers the opportunity to process any weldable metal powder feedstock. Since LBM parameters affect the solidification conditions directly, this process is predestinated for future material developments. In the present paper, first research results to process the conventional LBM powder (Hastelloy X; HX) that was blended with a fine WC–Co powder are presented. Due to its high temperature and corrosion resistance, HX is widely used in industrial AM applications. WC–Co was added to improve the wear and mechanical properties. In this study, the blended, bimodal powder feedstock was analyzed, and the influence of the key process parameters on the porosity, morphology, and grain structure was investigated. The aim was to obtain the first basic information concerning the behavior of such powder blends during LBM processing and the resulting material properties. In summary, processing powder blends is possible. Since an increased porosity was determined for a high WC–Co content, the process parameters need to be further improved. In addition, it was determined that the energy input during the powder melting process directly affects the distribution of WC–Co particles within the HX matrix. High-energy inputs lead to a degradation of the WC particles. Low-energy inputs foster a consistent embedding of WC–Co particles within the HX matrix. © 2018, Springer International Publishing AG, part of Springer Nature.
    view abstract10.1007/s40964-018-0058-y
  • Mechanical properties of honeycomb structured zr-based bulk metallic glass specimens fabricated by laser powder bed fusion
    Wegner, J. and Frey, M. and Stiglmair, P. and Kleszczynski, S. and Witt, G. and Busch, R.
    South African Journal of Industrial Engineering 30 (2019)
    Laser powder bed fusion of bulk metallic glasses offers great potential to overcome the existing restrictions of the geometrical size and complexity of bulk metallic glasses in conventional manufacturing routes due to high cooling rates during laser powder bed fusion. Bulk metallic glasses exhibit extraordinary strength, paired with high elasticity. Yet insights into additive manufactured bulk metallic glasses, especially of complex structures, are limited. The present article investigates the mechanical behaviour of Zr-based bulk metallic glasses, fabricated into honeycomb structures through laser powder bed fusion, by performing three-point bending tests. The results reveal a significant increase in specific strength, quasi-plasticity, and high elastic elongation. These structures thus offer great potential for light-weight applications and compliant mechanisms. © 2019, South African Institute of Industrial Engineering. All rights reserved.
    view abstract10.7166/30-3-2265
  • Comparison between the optical properties of injection molded and additive manufactured components
    Kuehn, C. and Mehl, O. and Laumer, T. and Witt, G.
    Procedia CIRP 74 (2018)
    Fused Layer Manufacturing (FLM) is an additive technology based on polymer material extrusion. Due to variations in temperature during the manufacturing process and the resulting stress between the stacked layers, the final parts show anisotropic mechanical properties. One possible approach for their reduction is the immediate local preheating of the surface via laser radiation. At first, our research examines the influence of laser parameters as wavelength, power, velocity and area of impact for the preheating of the surface. In addition, an overview of possible parameter combinations is given based on the selection of raw materials, its colors, thicknesses and the manufacturing process. Initially, the absorption level of the materials regarding the emitted wavelength is detected using a spectrophotometer. Subsequently, preheating tests are conducted with different laser types while the temperature is determined by a thermal camera. The selected laser type is planned to get mounted on a prototype-machine for further in-situ preheating experiments on FLM parts during the manufacturing process. © 2018 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license.
    view abstract10.1016/j.procir.2018.08.106
  • Comparison of long-term properties of laser sintered and injection molded polyamide 12 parts
    Wörz, A. and Wudy, K. and Drummer, D. and Wegner, A. and Witt, G.
    Journal of Polymer Engineering 38 (2018)
    Selective laser sintering (SLS) of polymers is on the edge from a pure prototyping technique to a small-scale production. For this transition, characteristic values such as long-term properties, and thus the degradation mechanism, are crucial factors for enabling a series application. Due to the specific characteristics of SLS parts like porosity and rough surfaces, a direct transfer of known mechanisms and models for injection molded (IM) parts is not or just to a limited extent possible. This leads to the aim of this paper, which is to investigate and compare the degradation behavior of polyamide 12 parts produced by SLS and IM. © 2018 Walter de Gruyter GmbH, Berlin/Boston.
    view abstract10.1515/polyeng-2017-0227
  • Developing laser beam welding parameters for the assembly of steel SLM parts for the automotive industry
    Fieger, T.V. and Sattler, M.F. and Witt, G.
    Rapid Prototyping Journal 24 (2018)
    Purpose: This paper aims to identify issues with joining selective laser melting (SLM) steels with conventional cold rolled steels through remote laser beam welding. Design/methodology/approach: A novel approach for substituting conventional cold rolled metal sheets with SLM metal sheets, made of 316L and 18-Ni 300, is presented. The characteristics of the interaction of wrought and SLM materials are described, and joining benchmark parameters are presented and compared to known existing joining results. Finally, the joints are assessed in line with automotive specifications. This research also addresses the importance of joining technologies for the implementation of SLM as a full-fledged manufacturing technology for the automotive industry. Findings: New parameter ranges for laser beam welding of SLM steels are defined. Research limitations/implications: This research is limited to the examined steels and the used machines, parameters and equipment. Practical implications: The presented benchmark parameters are expected to be useful for designers, product developers and machine operators. Originality/value: Little knowledge is available about the behavior of SLM materials and their suitability for assembly processes. Novel information about SLM steels and their interaction with conventionally produced steel sheets is presented. © 2018, Emerald Publishing Limited.
    view abstract10.1108/RPJ-12-2016-0204
  • Inconel 625 lattice structures manufactured by selective laser melting (SLM): Mechanical properties, deformation and failure modes
    Leary, M. and Mazur, M. and Williams, H. and Yang, E. and Alghamdi, A. and Lozanovski, B. and Zhang, X. and Shidid, D. and Farahbod-Sternahl, L. and Witt, G. and Kelbassa, I. and Choong, P. and Qian, M. and Brandt, M.
    Materials and Design 157 (2018)
    Additive Manufacture (AM) enables the fabrication of highly complex lattice structures with exceptional engineering properties. Inconel is a technically useful material in that it provides high resistance to oxidisation, creep and loss of mechanical properties at elevated temperatures. The combination of Inconel material properties and the geometric freedom of AM provides a unique opportunity for the fabrication of engineered structures with exceptional strength and stiffness at elevated temperatures, as for example is required for high temperature turbomachinery. Despite the associated technical opportunities, there exists no design data on the mechanical response, deformation characteristics and failure modes of AM Inconel 625 lattice structures. This research provides a comprehensive reference for the mechanical response of Inconel 625 lattice structures fabricated by Selective Laser Melting (SLM). Furthermore, the high ductility of Inconel 625 lattice enables novel insight into the structural mechanics of AM lattice, and the associated deformation photography provides a reference for the validation and verification of numerical models of AM lattice behaviour. © 2018 Elsevier Ltd
    view abstract10.1016/j.matdes.2018.06.010
  • Comparison of long-term properties of laser sintered and injection molded polyamide 12 parts
    Wörz, A. and Wudy, K. and Drummer, D. and Wegner, A. and Witt, G.
    Journal of Polymer Engineering (2017)
    Selective laser sintering (SLS) of polymers is on the edge from a pure prototyping technique to a small-scale production. For this transition, characteristic values such as long-term properties, and thus the degradation mechanism, are crucial factors for enabling a series application. Due to the specific characteristics of SLS parts like porosity and rough surfaces, a direct transfer of known mechanisms and models for injection molded (IM) parts is not or just to a limited extent possible. This leads to the aim of this paper, which is to investigate and compare the degradation behavior of polyamide 12 parts produced by SLS and IM. © 2017 Walter de Gruyter GmbH, Berlin/Boston.
    view abstract10.1515/polyeng-2017-0227
  • Compound quality assessment in laser beam melting processes using layer images
    Zur Jacobsmühlen, J. and Kleszczynski, S. and Witt, G. and Merhof, D.
    I2MTC 2017 - 2017 IEEE International Instrumentation and Measurement Technology Conference, Proceedings (2017)
    Laser beam melting (LBM) is an additive manufacturing (AM) technology that allows the layer-based production of geometrically complex parts from metal powder. Non-destructive evaluation of part quality is a pre-requisite for widespread application of this promising technology and an ongoing research topic. In-process measurement of part layer compound quality will be beneficial during optimization of process parameters for best part quality. We inspect the part surface in high resolution layer images acquired after laser exposure. Based on a fast Fourier transform (FFT) we decompose surface images into laser scan lines (oriented component) and underlying powder structure (not oriented component). By computing the signal energy ratio we obtain a measure for the prominence of laser scan lines which is correlated with the compound quality. For 25 parts with varied laser power and scan velocity the signal energy ratio is compared to the computed energy input. A linear model achieves a root-mean-square error (RMSE) of 9.76Jmm-3. Additionally we automatically classify part regions as good/not good and compare our results to a manual selection based on light microscopy. © 2017 IEEE.
    view abstract10.1109/I2MTC.2017.7969870
  • Investigations of aging behaviour for aluminium powders during an atmosphere simulation of the LBM process
    Bauer, D.M. and Schwarzenböck, E. and Ludwig, I. and Schupp, N. and Palm, F. and Witt, G.
    Powder Metallurgy (2017)
    Additive layer manufacturing (ALM) offers for production of parts and components for aeronautical applications potential cost benefits over conventional manufacturing routes. In particular, powder bed processes offer a high degree of design flexibility while enabling weight reduction due to topological optimisation. The quality and properties of the parts are strongly dependent on the powder quality which, in turn, is influenced by handling and storage of the powder. For this reasons an undefined contamination of atomised powder materials by oxygen and hydrogen has to be avoided. Aluminium-silicon powder was aged under atmosphere of different moistures and temperatures for defined duration. The effect of these environments as well as the effect of vacuum drying on the flowability was investigated. The morphology was evaluated by scanning electron microscope. The chemistry including oxygen content of the powder was measured by inductively coupled plasma optical emission spectrometry and hot fusion analysis. © 2017 Institute of Materials, Minerals and Mining Published by Taylor & Francis on behalf of the Institute
    view abstract10.1080/00325899.2017.1288841
  • Isothermal and non-isothermal crystallization kinetics of polyamide 12 used in laser sintering
    Neugebauer, F. and Ploshikhin, V. and Ambrosy, J. and Witt, G.
    Journal of Thermal Analysis and Calorimetry 124 (2016)
    Laser sintering of polymers is a steadily improving additive manufacturing method. Mainly used materials are semicrystalline thermoplastics, and as part of those, polyamide 12 has established most. When semicrystalline polymers cool down from the melt, they exhibit volume shrinkage due to crystallization. This crystallization occurs non-uniformly within the produced parts and thus is responsible for part warpage. Aim of this study was to investigate the crystallization kinetics of an, in laser sintering widely used, polyamide 12-based polymer available by supplier EOS with appellation PA2200. For that purpose, several isothermal and non-isothermal DSC measurements were taken. The isothermal measurements were analyzed according to the theory of Avrami. Furthermore, the parameters of the crystallization model by Nakamura were calibrated, and both conditions were simulated. It was found that the isothermal data are very well describable by the theory of Avrami as well as the model by Nakamura can be used to model both conditions. © 2015, Akadémiai Kiadó, Budapest, Hungary.
    view abstract10.1007/s10973-015-5214-8
  • Quality Control of Laser-Beam-Melted Parts by a Correlation Between Their Mechanical Properties and a Three-Dimensional Surface Analysis
    Grimm, T. and Wiora, G. and Witt, G.
    JOM 69 (2016)
    Good correlations between three-dimensional surface analyses of laser-beam-melted parts of nickel alloy HX and their mechanical properties were found. The surface analyses were performed with a confocal microscope, which offers a more profound surface data basis than a conventional, two-dimensional tactile profilometry. This new approach results in a wide range of three-dimensional surface parameters, which were each evaluated with respect to their feasibility for quality control in additive manufacturing. As a result of an automated surface analysis process by the confocal microscope and an industrial six-axis robot, the results are an innovative approach for quality control in additive manufacturing. © 2016 The Minerals, Metals & Materials Society
    view abstract10.1007/s11837-016-2190-9
  • Robust calibration marker detection in powder bed images from laser beam melting processes
    Zur Jacobsmuhlen, J. and Achterhold, J. and Kleszczynski, S. and Witt, G. and Merhof, D.
    Proceedings of the IEEE International Conference on Industrial Technology 2016-May (2016)
    Laser beam melting (LBM) systems produce parts by melting metal powder according to the sliced 3D geometry using a laser. After each layer, new powder is deposited and the process is repeated. Process monitoring via acquisition and analysis of layer images during the build job is a promising approach to thorough quality control for LBM. Image analysis requires orthographic images, which are usually not available as the camera cannot be placed directly above the build layer due to the position of the laser window. The resulting perspective distortions have to be corrected before analysis. To this end we compute a homography from four circular markers which are «drawn» into the powder bed by the machine's laser and detected in the acquired images. In this work we present a robust method for the automatic detection of calibration markers, which deals with the noise-like powder regions, disconnected lines, visible support structures and blurred image regions. Our homography estimation method minimizes the shape error between transformed circular reference marker shapes and detected elliptical markers yielding an image with correct aspect ratio and minimal distortions. Our method achieves a detection rate of 96.3 % and a spatial detection error of 2.0 pixels (median, 95 %-percentile: 5.17pixels) compared to a manually created ground truth. © 2016 IEEE.
    view abstract10.1109/ICIT.2016.7474873
  • Three-dimensional surface measurement for the quantification of mechanical properties of laser-sintered parts
    Grimm, T. and Witt, G. and Wiora, G.
    Materialpruefung/Materials Testing 58 (2016)
    A good correlation (σ = -0.912) is found between three dimensional surface features and mechanical properties of laser-sintered parts. This study identifies correlating surface parameters and features out of the wide range of existing evaluation methods resulting from the new possibilities of a three dimensional surface measurement in contrast to a conventional tactile profilometry. Therefore, surface analyses were performed using a confocal microscope and tensile tests according to ISO 527-1 were conducted. Especially the motifs dale and volume of islands analyses show a good correlation concerning the tensile strength, the Young's modulus and the elongation at break. The differences with regard to each feature and their influence on the mechanical properties are discussed. © 2016 Carl Hanser Verlag GmbH & Co. KG.
    view abstract10.3139/120.110851
  • Characterization of typical surface effects in additive manufacturing with confocal microscopy
    Grimm, T. and Wiora, G. and Witt, G.
    Surface Topography: Metrology and Properties 3 (2015)
    The surfaces of laser-sintered and laser beam melted parts are characterized by typical surface effects and thus cause difficulties when using the popular tactile profilometry method and conventional two-dimensional surface parameters for analysis. Therefore, this study shows the potential of a new measurement approachby a confocal microscope and three-dimensional surface parameters. Within this study, a special focus is laid on the classification of different surface orientations and on a special issue in laser sintering: orange peel severity. © 2015 IOP Publishing Ltd.
    view abstract10.1088/2051-672X/3/1/014001
  • Detection of elevated regions in surface images from laser beam melting processes
    Zur Jacobsmuhlen, J. and Kleszczynski, S. and Witt, G. and Merhof, D.
    IECON 2015 - 41st Annual Conference of the IEEE Industrial Electronics Society (2015)
    Laser Beam Melting (LBM) is a promising Additive Manufacturing technology that allows the layer-based production of complex metallic components suitable for industrial applications. Widespread application of LBM is hindered by a lack of quality management and process control. Elevated regions in produced layers pose a major risk to process stability as collisions between the powder coating mechanism and the part may occur, which cause damages to either one or even both. We train a classifier-based detector for elevated regions in laser exposure result images. For this purpose we acquire two high resolution layer images: one after laser exposure and another one after powder deposition for the next layer. Ground truth labels for critical regions are obtained from analysis of the latter, where elevated regions are not covered by powder. We compute dense descriptors (HOG, DAISY, LBP) on the surface image after laser exposure and compare their predictive power. The top five descriptor configurations are used to optimize parameters of Random Forest, Support Vector Machine and Stochastic Gradient Descent (SGD) classifiers. We validate the detectors with optimized parameters using cross-validation on 281 images from three build jobs. Using a DAISY descriptor with a SGD classifier we achieve a F1-score of 0.670. The presented method enables detection of elevated regions before powder coating is performed and can be extended to other surface inspection tasks in LBM layer images. Detection results can be used to assess LBM process parameters with respect to process stability during process design and for quality management in production. © 2015 IEEE.
    view abstract10.1109/IECON.2015.7392275
  • Temperature effects on tensile properties of laser sintered polyamide 12
    Neugebauer, F. and Miiller, N. and Ploshikhin, V. and Thiel, S. and Ambrosy, J. and Witt, G.
    Materialpruefung/Materials Testing 57 (2015)
    This paper focuses on a study of temperature effects oil tensile properties of a polyamide 12 based polymer. Specimens for testing were generated using the additive manufacturing process of laser sintering. Tensile tests were conducted according to DIN EN ISO 527 at temperatures between 20°C and 140°C. A Zwick 250N5A was used for the tests. Fine strain up to 0.25 % was measured using a MTS biaxial extensometer. Crosshead displacement was recorded as well and used for generation of stress-strain curves. Within the paper, Young's modulus, Poisson ratio, tensile strength, tensile strain at tensile strength, stress and elongation at break and yielding are reported. The study showed that all analyzed mechanical properties depend significantly on temperature. Above 100°C strain raised up to 100 %, however, it was limited by the traveling limit of the machine. The collected data were embedded in a table-driven elastic plastic material model for finite element simulation of tensile tests. Good agreements of simulations and experiments were obtained at all temperatures.
    view abstract10.3139/120.110756
  • Understanding the decisive thermal procebes in laser sintering of polyamide 12
    Wegner, A. and Witt, G.
    AIP Conference Proceedings 1664 (2015)
    Due to the advancements during the last decade, the laser sintering proceb has achieved a high technical level, which allows parts being used for Rapid Manufacturing applications. However, the procebes still show a poor reproducibility of part quality. Furthermore, proceb interruptions or defective parts still occur regularly. The know-how and expertise needed to avoid these kind of problems is still insufficient. The temperature increase in the powder bed during laser exposure is the driving force in laser sintering of plastics. The resulting part properties strongly depend on the interaction of melt temperature and melt viscosity caused by the laser power input in the powder bed. Additionally, cooling conditions and temperature gradients in the powder bed significantly influence the accuracy and especially the warpage of parts. However, literature provides only little information on these decisive thermal procebes. Therefore, additional information is necebary to improve proceb understanding as well as part properties in laser sintering. In a first approach, a high-speed thermal imaging system is implemented into a LS-machine in order to measure the melt temperatures during and after laser exposure as a function of different proceb parameters. The measured data show significant correlations between temperatures and part properties. It turns out that especially the melt temperature after laser exposure has a strong influence on the resulting part properties. In a second approach, detailed measurements of temperature distributions within the powder bed are performed while using wireleb temperature sensors. In addition, the influence of different heater parameter settings on the cooling conditions is investigated by using a new Advanced Temperature Guiding system with 15 single heater cycles. The results of the study lead to an enhanced understanding of the thermal procebes in laser sintering and enable a significant improvement of procebing conditions. © 2015 AIP Publishing LLC.
    view abstract10.1063/1.4918511
  • Determination of robust material qualities and processing conditions for laser sintering of polyamide 12
    Wegner, A. and Mielicki, C. and Grimm, T. and Gronhoff, B. and Witt, G. and Wortberg, J.
    Polymer Engineering and Science 54 (2014)
    Material aging of Polyamide 12 (Laurinlactam) is a very common problem in laser sintering (LS). For stable process conditions, recycled material used in previous processes should be refreshed with 30-50% virgin powder material. However, even by following these refreshing strategies, material quality drops to an insufficient level after several process cycles which leads to poor part quality showing orange peel or poor mechanical properties when processed. In order to avoid this, a quality assurance system has been established to provide recommendations for robust process conditions and material qualities. A detailed study on aging processes in LS comparing two different machines was performed in order to analyze correlations between material quality, process parameters and part properties. Energy input allowing for robust processing conditions should be in a range between 0.325 and 0.42 J/mm3 showing almost identical values for both machines. Optimal material quality ranges was found to be machine specific, while the lower limit lies between 20 and 25 cm 3/ 10 min for both machines used. Additionally, material aging characteristics in an oven and a LS machine were compared, in order to simulate material aging in the LS process by simple experiments in an oven. POLYM. ENG. SCI., 54:1540-1554, 2014. © 2013 Society of Plastics Engineers © 2013 Society of Plastics Engineers.
    view abstract10.1002/pen.23696
  • Flow characteristics of porous metal structures for specified permeability manufactured by laser beam melting technology
    Benra, F.-K. and Dohmen, H.J. and Clauss, S. and Sehrt, J.T. and Witt, G.
    ASME International Mechanical Engineering Congress and Exposition, Proceedings (IMECE) 2A (2014)
    The characteristic additive build-up at the laser beam melting technology provides the opportunity to freeform porous and defined structures at specific areas in one part. By adjusting the process parameters specific characteristics of the manufactured part such as density, permeability, pore size, porosity and shear strength can be realized. The manufacturing process of a test body is described in detail. The permeability of the manufactured parts is investigated experimentally. In addition a numerical model is build and the flow structure inside of the test body is illustrated. The numerically obtained results are compared to the experimentally obtained results. To show the advantages of this technology for future applications a numerical model of a porous blade surrounded by a hot gas flow and cooled from inside of the porous structure is investigated. The results show that the method to define the characteristics during the laser beam melting process has to be optimized. Copyright © 2014 by ASME.
    view abstract10.1115/IMECE2014-39672
  • Robustness analysis of imaging system for inspection of laser beam melting systems
    Zur Jacobsmuhlen, J. and Kleszczynski, S. and Witt, G. and Merhof, D.
    19th IEEE International Conference on Emerging Technologies and Factory Automation, ETFA 2014 (2014)
    Laser Beam Melting (LBM) is an additive manufacturing process, which enables the layer-based production of complex parts from metal powder, i.e. '3D printing' with metal. In previous publications, we presented a high-resolution imaging system for inspection of LBM processes, which uses a high-resolution camera to acquire images of each powder layer and laser exposure result. The external camera position necessitates perspective correction, which is based on calibration markers which are 'drawn' onto the powder by the LBM system's laser in the first layer. As movements of the powder deposition mechanism cause vibrations, the orientation of the camera may be changed, which would invalidate the calibration results and lead to imprecise measurements or segmentations. To evaluate the effect of these disturbances, we placed calibrations markers in multiple layers and determined the position offset using template matching. We analyze the relative marker drift in three LBM processes and determine the spatial acquisition error. The maximum distance is 4.91 pixels (156.1 μm on the part), while most detected markers deviate by less than 1.5 pixels (46 μm). Compared to the pixel size of 20 μm to 32μm, these deviations are significant and require a repeated calibration in higher layers for valid high-resolution image-based measurements. © 2014 IEEE.
    view abstract10.1109/ETFA.2014.7005262
  • High resolution imaging for inspection of Laser Beam Melting systems
    Jacobsmuhlen, J.Z. and Kleszczynski, S. and Schneider, D. and Witt, G.
    Conference Record - IEEE Instrumentation and Measurement Technology Conference (2013)
    Laser Beam Melting (LBM) allows the fabrication of three-dimensional parts from metallic powder with almost unlimited geometrical complexity and very good mechanical properties. LBM works iteratively: a thin powder layer is deposited onto the build platform which is then melted by a laser according to the desired part geometry. Today, the potential of LBM in application areas such as aerospace or medicine has not yet been exploited due to the lack of process stability and quality management. For that reason, we present a high resolution imaging system for inspection of LBM systems which can be easily integrated into existing machines. A container file stores calibration images and all layer images of one build process (powder and melt result) with corresponding metadata (acquisition and process parameters) for documentation and further analysis. We evaluate the resolving power of our imaging system and show that it is able to inspect the process result on a microscopic scale. Sample images of a part built with varied process parameters are provided, which show that our system can detect topological flaws and is able to inspect the surface quality of built layers. The results can be used for flaw detection and parameter optimization, for example in material qualification. © 2013 IEEE.
    view abstract10.1109/I2MTC.2013.6555507
  • Preprocess-Optimization for Polypropylene Laser Sintered Parts
    Reinhardt, T. and Martha, A. and Witt, G. and Köhler, P.
    Computer-Aided Design and Applications 11 (2013)
    Additive manufacturing delivers the opportunity to manufacture complex geometry with comparable effective effort. Nevertheless, comprehensive information for the sufficient configuration of the process and related parameters are still missing. Here joint researches of the chairs for Manufacturing Technology and Computer Aided Design at the University of Duisburg-Essen were carried out in order to receive detailed information about the influencing factors on part quality for polypropylene laser sintering parts. These experimental results provided the basis for the development of software supported applications for the preprocess optimization. © 2013 Copyright CAD Solutions, LLC.
    view abstract10.1080/16864360.2013.834138
  • Correlation of process parameters and part properties in laser sintering using response surface modeling
    Wegner, A. and Witt, G.
    Laser Assisted Net Shape Engineering 7 (lane 2012) 39 (2012)
    Due to the advancements during the last decade, the laser sintering process has achieved a high technical level, allowing for Rapid Manufacturing in some applications. However, processes still show poor repeatability of part quality, process interruptions or defective parts. The knowledge needed to avoid such problems is still insufficient. Literature provides only few detailed correlations between process parameters and part properties. Therefore, an approach using response surface methodology was chosen to correlate part properties with main influencing factors. Aim of the analyses was to predict and to improve part properties based on an enhanced process understanding. (C) 2012 Published by Elsevier B. V. Selection and/or review under responsibility of Bayerisches Laserzentrum GmbH
    view abstract10.1016/j.phpro.2012.10.064
  • Influence of surface characteristics on fatigue behaviour of laser sintered plastics
    Blattmeier, M. and Witt, G. and Wortberg, J. and Eggert, J. and Toepker, J.
    Rapid Prototyping Journal 18 (2012)
    Purpose - The purpose of this paper is to provide macromechanical insight into the fatigue behaviour of laser sintered parts and to understand the influence of the laser sintered surface structure on this behaviour. Design/methodology/approach - A background on the technological maturity of manufacturing processes and the demand for structural and aesthetic properties of laser sintered plastic products is given. As the contribution of surface structure on part quality was the focus, laser sintered specimens with and without surface finishes, as well as injection moulded specimens were used. The latter simply served as a comparison and was not intended to qualify injection moulding. The study comprises the determination of short-term tensile properties, the load increase method for investigating fracture and deformation behaviours, and fatigue crack propagation analysis. Findings - According to the test results, the contribution of laser sintered surface structures to relevant mechanical properties can be neglected. Under dynamic loading conditions, laser sintered specimens achieved a longer lifetime but showed less deformation capabilities in contrast to injection moulded specimens. In general, laser sintered specimens presented considerable resistance to crack initiation and propagation. Research limitations/implications - Because of the long-term approach of the research, the number of tests conducted per lot was limited. Thus, the effects of different process settings and the reproducibility could not be fully analysed. Practical implications - The studied fatigue behaviour of laser sintered specimens has implications for the functional testing of parts or components, for the product and process design as well as for the general compatibility of laser sintering as a manufacturing technology of end-customer products. Originality/value - The value of this paper lies in the better understanding of deformation and fracture behaviours of laser sintered polymers. © 2012 Emerald Group Publishing Limited.
    view abstract10.1108/13552541211212140
  • 3d printers

  • additive manufacturing

  • injection molding

  • laser beam melting

  • laser sintering

  • mechanical properties

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