Prof. Dr.-Ing. Arne Röttger

associated member

New manufacturing technologies and materials
Bergische Universität Wuppertal


  • A novel powder-metallurgical eco-friendly recycling process for tool steel grinding sludge
    Großwendt, F. and Bürk, V. and Kopanka, B. and Jäger, S. and Pollak, S. and Leich, L. and Röttger, A. and Petermann, M. and Weber, S.
    Journal of Cleaner Production 392 (2023)
    view abstract10.1016/j.jclepro.2023.136329
  • Description of a new concept for the development of adapted hot-work tool steels for laser-powder bed fusion
    Röttger, A. and Boes, J. and Großwendt, F. and Weber, S.
    Additive Manufacturing 61 (2023)
    view abstract10.1016/j.addma.2022.103292
  • The behavior of cutting discs for mechanized tunneling under cyclic loading conditions
    Brackmann, L. and Röttger, A. and Treppmann, M. and Weber, S.
    Tunnelling and Underground Space Technology 137 (2023)
    view abstract10.1016/j.tust.2023.105151
  • Tribological characteristics of sintered martensitic stainless steels by nano-scratch and nanoindentation tests
    Farayibi, P.K. and Hankel, J. and van gen Hassend, F. and Blüm, M. and Weber, S. and Röttger, A.
    Wear 512-513 (2023)
    view abstract10.1016/j.wear.2022.204547
  • Influence of a Partial Substitution of Co by Fe on the Phase Stability and Fatigue Behavior of a CoCrWC Hard Alloy at Room Temperature
    Brackmann, L. and Schuppener, J. and Röttger, A. and Weber, S.
    Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science 53 (2022)
    The deformation-induced phase transition from fcc to hcp causes local embrittlement of the metal matrix in Cobalt-base alloys, facilitating subcritical crack growth under cyclic loading and reducing fatigue resistance. Our approach to increasing the fatigue life of Co-based hard alloys is to suppress the phase transition from fcc to hcp by an alloy modification that increases the stacking fault energy (SFE) of the metal matrix. Therefore, we substitute various contents (15, 25, and 35 mass pct) of Co by Fe and analyze the effect on the fatigue life and resistance against subcritical crack growth. Subcritical crack growth in the specimens takes place in a cyclic load test. The proceeding crack growth and the occurrence of phase transformations are monitored by scanning electron microscope (SEM) investigations and electron backscatter diffraction (EBSD). We determined an SFE of 35 mJ/m2 at an iron content of 35 mass pct, which leads to a change of the main deformation mechanism from deformation-induced martensitic transformation to deformation twinning. Analysis of cyclically loaded specimens revealed that the resistance against subcritical crack growth in the metal matrix is facilitated with increasing Fe content, leading to a significant increase in fatigue life. © 2022, The Author(s).
    view abstract10.1007/s11661-022-06700-7
  • Influence of Heat Treatment Parameters on the Carbide Morphology of PM High-Speed Steel HS 6-5-3-8
    Disch, N. and Benito, S. and Röttger, A. and Weber, S.
    Steel Research International (2022)
    view abstract10.1002/srin.202200755
  • Investigation of the potential of topology optimization in additive manufacturing using the example of components subject to bending stress [Untersuchung des Potenzials der Topologieoptimierung in der additiven Fertigung am Beispiel von biegebeanspruchten Bauteilen]
    Jantos, D.R. and Röttger, A. and Junker, P.
    Materialwissenschaft und Werkstofftechnik 53 (2022)
    In this application-oriented work, we examine the performance of topology-optimized structures as compared to the reference I-beam. We make use of the thermodynamic topology optimization based on a linear elastic compliance minimization, i. e. minimization of the elastic strain energy of the whole structure. We investigate, how the optimization of the rather theoretical strain energy influences the efficiency of more practical measurements, i. e. the force-displacement response at the loading points and the maximum tolerable force. For this purpose, starting from a cuboid design space with the boundary conditions of a 3-point and 4-point bending stress, the geometry with constant volume was optimized. The topology-optimized bending beams were subsequently produced by stereolithography and mechanically tested with respect to the previously defined boundary conditions. In order to avoid a falsification of results due to internal sample defects, all samples were previously examined with the aid of computer tomography with regard to the defects in the volume. As a general result, the topology-optimized bending beams can bear a higher load in the experiment, which shows the usefulness of the coupling of additive manufacturing and topology optimization methods without any special constraints or enhancements regarding the manufacturing process within the optimization. © 2022 The Authors. Materialwissenschaft und Werkstofftechnik published by Wiley-VCH GmbH.
    view abstract10.1002/mawe.202200013
  • Microstructure and Properties of a Novel Carbon-Martensitic Hot Work Tool Steel Processed by Laser Additive Manufacturing without Preheating
    Boes, J. and Röttger, A. and Mutke, C. and Escher, C. and Weber, S.
    Steel Research International (2022)
    Laser additive manufacturing (LAM) techniques, such as laser-powder bed fusion (L-PBF) or laser-directed energy deposition (L-DED), allow for the production of complex-shaped parts by either the local melting of a metallic powder bed by a laser beam (L-PBF) or a local application and laser beam melting of powder material by a nozzle (L-DED). In the case of carbon-martensitic tool steels, their cold crack susceptibility limits their LAM processability and is usually counteracted by substrate preheating. As preheating can increase the oxygen take-up of the powder and alter the part microstructure, it can be disadvantageous for part quality and powder reusability. In this study, it is investigated a carbon-martensitic steel designed for the production of parts with low crack density by LAM without preheating, focusing on the microstructure and hardness of the L-PBF- and L-DED-manufactured steel. The steel can be LAM-processed without preheating, resulting in specimens with low crack densities and martensitic microstructure with retained austenite. The hardness of the as-built material (L-PBF: 542HV30 and L-DED: 623HV30) is increased by quenching and tempering up to 693HV30. Direct tempering of the as-built specimen without previous quenching leads to a shift of the secondary hardness maximum from 500 to 530 °C. © 2022 The Authors. Steel Research International published by Wiley-VCH GmbH.
    view abstract10.1002/srin.202200439
  • NdFeB Magnets with Well-Pronounced Anisotropic Magnetic Properties Made by Electric Current-Assisted Sintering
    Prasad Mishra, T. and Leich, L. and Krengel, M. and Weber, S. and Röttger, A. and Bram, M.
    Advanced Engineering Materials (2022)
    Electric current-assisted sintering (ECAS) technologies are highly promising for processing of NdFeB magnets. Due to the combination of direct Joule heating and application of external load, even powders, whose particle size distribution and morphology are not optimum for conventional powder processing like melt-spun powders or magnet scrap, can be easily sintered to high densities. A systematic study is done to demonstrate the potential of field-assisted sintering technique/spark plasma sintering (FAST/SPS) and flash spark plasma sintering (flash SPS) for sintering of NdFeB powders. Melt-spun, commercial NdFeB powder (Magnequench MQU-F) is used as starting material. Its platelet-like shape makes this powder extremely difficult to sinter by conventional methods. This study clearly reveals that especially in the case of flash SPS application of external pressure in combination with short cycle times enables to achieve well-pronounced anisotropic magnetic properties without the need of subsequent upset forging. Optimized flash SPS parameters are applied to NdFeB magnet scrap with broad particle size distribution, demonstrating the general potential of ECAS technologies for recycling of waste magnet materials. Finally, the results are benchmarked with respect to established NdFeB processing technologies and electrodischarge sintering (EDS), another promising ECAS technology with very short cycling time. © 2022 The Authors. Advanced Engineering Materials published by Wiley-VCH GmbH.
    view abstract10.1002/adem.202201027
  • Normal Indentation of Rock Specimens with a Blunt Tool: Role of Specimen Size and Indenter Geometry
    Yang, H. and Renner, J. and Brackmann, L. and Röttger, A.
    Rock Mechanics and Rock Engineering 55 (2022)
    Indentation testing has been widely used in laboratory environments to investigate the processes leading to rock fragmentation in drilling, mechanized tunneling, and mining. Rock specimens for laboratory testing are limited to finite size, potentially causing size effects that have to be accounted for when transferring results to in situ applications. We present an integrated experimental and theoretical investigation of the specimen size effect in indentation testing (a) to address the limited understanding of its causes and the lack of tools to analyze tests on variable specimen sizes and (b) to identify to what extent an indenter mimicking the shape of a cutter on a tunneling machine can be approximated by a conventional indenter geometry. We performed indentation tests on cylindrical specimens of a porous sandstone with aspect ratios (diameter/height) ranging from 0.3 to 1.7, using a blunt-truncated indenter and monitoring the fracturing process by the acoustic emission technique. A damage zone, enclosing a zone of crushed grains immediately below the indenter tip, forms and grows due to tool penetration. Eventually, all specimens failed as a result of the propagation of a sub-vertical fracture, initiated close to peak indentation pressure. Peak force, its corresponding penetration depth, and peak indentation pressure increase with specimen size, more significantly with specimen diameter than with height. We developed a semi-analytical model based on cavity-expansion theory and linear elastic fracture mechanics for the formation of the damage zone and the nucleation and propagation of the macroscopic vertical fracture, respectively, whose predictions are in good agreement with our experimental data. The observed increases of peak indentation pressure with specimen size can be explained by the effect of the free surfaces on damage zone growth rather than on fracture propagation. The model permits evaluating the specimen size effect through the ratio between two geometrical parameters, specimen diameter and tip width of the truncated indenter, which has to be larger than around 102 for the size effect to be insignificant. The model permits upscaling of experimental results to in situ conditions based on geometrical indenter parameters and commonly used material parameters. © 2022, The Author(s), under exclusive licence to Springer-Verlag GmbH Austria, part of Springer Nature.
    view abstract10.1007/s00603-021-02732-4
  • Processability of a Hot Work Tool Steel Powder Mixture in Laser-Based Powder Bed Fusion
    Hantke, N. and Großwendt, F. and Strauch, A. and Fechte-Heinen, R. and Röttger, A. and Theisen, W. and Weber, S. and Sehrt, J.T.
    Materials 15 (2022)
    Powder bed fusion of metals using a laser beam system (PBF-LB/M) of highly complex and filigree parts made of tool steels is becoming more important for many industrial applications and scientific investigations. To achieve high density and sufficient chemical homogeneity, pre-alloyed gas-atomized spherical powder feedstock is used. For high-performance materials such as tool steels, the number of commercially available starting powders is limited due to the susceptibility to crack formation in carbon-bearing steels. Furthermore, scientific alloy development in combination with gas-atomization is a cost-intensive process which requires high experimental effort. To overcome these drawbacks, this investigation describes the adaption of a hot work tool steel for crack-free PBFLB/M-fabrication without any preheating as well as an alternative alloying strategy which implies the individual admixing of low-cost aspherical elemental powders and ferroalloy particles with gas-atomized pure iron powder. It is shown that the PBF-LB/M-fabrication of this powder mixture is technically feasible, even though the partly irregular-shaped powder particles reduce the flowability and the laser reflectance compared to a gas-atomized reference powder. Moreover, some high-melting alloying ingredients of the admixed powder remain unmolten within the microstructure. To analyze the laser energy input in detail, the second part of the investigation focuses on the characterization of the individual laser light reflectance of the admixed alloy, the gas-atomized reference powder and the individual alloying elements and ferroalloys. © 2022 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstract10.3390/ma15072658
  • Processing of a Martensitic Tool Steel by Wire-Arc Additive Manufacturing
    Ziesing, U. and Lentz, J. and Röttger, A. and Theisen, W. and Weber, S.
    Materials 15 (2022)
    view abstract10.3390/ma15217408
  • Quantification of methods used in field metallography using the example of quality assurance measures for a circular economy for high-alloy steels [Quantifizierung von Methoden der Ambulanten Metallographie an qualitätssichernden Maßnahmen einer zirkulären Wertschöpfung hochlegierter Stähle]
    Kronenberg, P. and Wieczorek, L. and Weber, S.L. and Röttger, A.
    Praktische Metallographie/Practical Metallography 59 (2022)
    This study aims to develop a method for on-site metallography, enabling the characterization of carbide banding in cold-work steels via cellulose acetate film replication. It will be demonstrated that for this purpose, it is sufficient to grind the sample surface using P1500 mesh SiC abrasive paper and etch it with V2A etchant or nitric acid for 7 minutes. By sample preparation and etching, the matrix of the parent material is sufficiently removed for the carbides to leave a "negative"impression on the film. This negative replica can then be studied under reflected light microscope, enabling the characterization of carbide banding. © 2022 Walter de Gruyter GmbH, Berlin/Boston, Germany.
    view abstract10.1515/pm-2022-0034
  • Supersolidus Liquid Phase Sintering and Heat Treatment on Atomic Diffusion Additive Manufacturing Produced Ledeburitic Cold Work Tool Steel [Supersolidus-Flüssigphasensintern und Wärmebehandlung an Atomic Diffusion Additive Manufacturing hergestelltem ledeburitischen Kaltarbeitsstahl]
    Wieczorek, L. and Katzwinkel, T. and Blüm, M. and Löwer, M. and Röttger, A.
    HTM - Journal of Heat Treatment and Materials 77 (2022)
    In this work, the possibility of manufacturing complex-shaped components from a carbon-martensitic hardenable cold-work steel (1.2379; X153CrMoV12; D2) is investigated. For this purpose, cube-shaped samples with an edge length of 10 mm were produced using the fused-filament fabrication process, which were post-compacted after solvent debinding by supersolidus liquid-phase sintering. Using the knowledge of liquid phase volume content as a function of temperature, supersolidus liquid phase sintering experiments were performed. The microstructure formation process was characterized by electron microscopy and X-ray diffraction. The microstructure and hardness of the processed samples were compared in the heat-treated condition with the properties of the same steel 1.2379 (X153CrMoV12; D2) in the as-cast, deformed and heat-treated condition. The results demonstrate effective post-densificationc close to theoretical density of cold-work tool steel samples fabricated by fused-filamet fabrication using supersolidus liquid-phase sintering at 1280 °C. The defect-free microstructure in the heat-treated state is characterized by a martensitic matrix and eutectic Cr-rich M7 C3 and small amounts of V-rich MC carbides. The hardness of the annealed Supersolidus liquid phase sintering samples are 681 ± 5 HV10, which is above the level of the reference material 1.2379 (629 ± 7 HV10) in the as-cast, formed and heat-treated condition. © 2022 L. Wieczorek, T. Katzwinkel, M. Blüm, M. Löwer, A. Röttger, publiziert von De Gruyter.
    view abstract10.1515/htm-2022-1019
  • Additive manufacturing of a carbon-martensitic hot-work tool steel using a powder mixture – Microstructure, post-processing, mechanical properties
    Großwendt, F. and Röttger, A. and Strauch, A. and Chehreh, A. and Uhlenwinkel, V. and Fechte-Heinen, R. and Walther, F. and Weber, S. and Theisen, W.
    Materials Science and Engineering A 827 (2021)
    This work examines the processing of a hot-work tool steel using laser-based powder bed fusion of metals (PBF-LB/M). The hot-work tool steel was produced using a low-cost powder mixture consisting of pure iron and other elemental powders as well as ferroalloys. Furthermore, a prealloyed starting powder with the same nominal chemical composition as the powder mixture was produced by inert-gas atomization. Besides, a reference steel was produced by casting to compare the microstructures and mechanical properties resulting from the different processing routes. The first step examined the application of a chemically homogeneous and dense layer of the powder mixture prior to PBF-LB/M densification. In addition to evaluate suitable process parameters for PBF-LB/M processing of the starting materials, the microstructure formation was comprehensively examined using electron microscopy and the processes adapted to it. To eliminate defects (cracks, pores) and chemical inhomogeneities, thermal posttreatments, namely supersolidus liquid phase heat-treatment (SLPHT) and hot isostatic pressing (HIP) were performed. Suitable heat-treatment parameters were evaluated. Finally, the obtained microstructures and the associated properties of the post-processed PBF-LB/M samples were compared with those in the reference states. As a main result, it was possible to achieve full redensification and simultaneous chemical homogenization of the PBF-LB/M-processed powder mixture by SLPHT post-processing. The hardness of the additively manufactured and SLPHT-post-processed specimens exceeds that of the cast reference. © 2021 Elsevier B.V.
    view abstract10.1016/j.msea.2021.142038
  • Comparison of Hilbert Transform and Complex Demodulation for Defect Identification in Cutting Discs using Vibration-Based Feature Extraction
    Priebe, S. and Brackmann, L. and Alabd-Allah, A. and Butt, S. and Röttger, A. and Meschke, G. and Mueller, I.
    Lecture Notes in Civil Engineering 127 (2021)
    This paper presents a novel concept for vibration-based feature extraction to identify damages in cutting discs of Tunnel Boring Machines (TBM). Defect frequencies resulting from repeated interaction of rock and disc defects are analysed. The data set is represented by the normal force acting on the edge of a cutting disc and the rock. Two different methods, the Hilbert transform and the complex demodulation, are used to generate the envelope of the time series, which was used to analyse whether the signal shows a feature representing an existing defect in the frequency domain. For the first proof of concept two numerical models were used - a multi-body system and a peridynamics 3D model simulating time series of normal forces. With both models, the linear motion of the disc on a rock sample with constant velocity was simulated. An experimental setup, mechanically similar to the simulations, was used in two experiments for further comparison. All implemented defects could be detected using vibration data of forces and one of the proposed data analysis techniques. © 2021, Springer Nature Switzerland AG.
    view abstract10.1007/978-3-030-64594-6_55
  • Comparison of the processability and influence on the microstructure of different starting powder blends for laser powder bed fusion of a fe3.5 si1.5 c alloy
    Strauch, A.L. and Uhlenwinkel, V. and Steinbacher, M. and Großwendt, F. and Röttger, A. and Chehreh, A.B. and Walther, F. and Fechte-Heinen, R.
    Metals 11 (2021)
    This paper examines different blends of starting materials for alloy development in the laser powder bed fusion (LPBF) process. By using blends of individual elemental, ferroalloy and carbide powders instead of a pre-alloyed gas-atomized starting powder, elaborate gas-atomization processes for the production of individual starting powders with varying alloy compositions can be omitted. In this work the model alloy Fe3.5 Si1.5 C is produced by LPBF from different blends of pure elemental, binary and ternary powders. Three powder blends were processed. The base material for all powder blends is a commercial gas-atomized Fe powder. In the first blend this Fe powder is admixed with SiC, in the second with the ternary raw alloy FeSiC and in the third with FeSi and FeC. After characterizing the powder properties and performing LPBF parameter studies for each powder blend, the microstructures and the mechanical properties of the LPBF-manufactured samples were analyzed. Therefore, investigations were carried out by scanning electron microscopy, wave length dispersive x-ray spectroscopy and micro hardness testing. It was shown that the admixed SiC dissolves completely during LPBF. But the obtained microstructure consisting of bainite, martensite, ferrite and retained austenite is inhomogeneous. The use of the lower melting ferroalloys FeSi and FeC as well as the ternary ferroalloy FeSiC leads to an increased chemical homogeneity after LPBF-processing. However, the particle size of the used components plays a decisive role for the dissolution behavior in LPBF. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstract10.3390/met11071107
  • Impact of the allowed compositional range of additively manufactured 316l stainless steel on processability and material properties
    Großwendt, F. and Becker, L. and Röttger, A. and Chehreh, A.B. and Strauch, A.L. and Uhlenwinkel, V. and Lentz, J. and Walther, F. and Fechte-Heinen, R. and Weber, S. and Theisen, W.
    Materials 14 (2021)
    This work aims to show the impact of the allowed chemical composition range of AISI 316L stainless steel on its processability in additive manufacturing and on the resulting part properties. ASTM A276 allows the chromium and nickel contents in 316L stainless steel to be set between 16 and 18 mass%, respectively, 10 and 14 mass%. Nevertheless, the allowed compositional range impacts the microstructure formation in additive manufacturing and thus the properties of the manufactured components. Therefore, this influence is analyzed using three different starting powders. Two starting powders are laboratory alloys, one containing the maximum allowed chromium content and the other one containing the maximum nickel content. The third material is a commercial powder with the chemical composition set in the middle ground of the allowed compositional range. The materials were processed by laser-based powder bed fusion (PBF-LB/M). The powder characteristics, the microstructure and defect formation, the corrosion resistance, and the mechanical properties were investigated as a function of the chemical composition of the powders used. As a main result, solid-state cracking could be observed in samples additively manufactured from the starting powder containing the maximum nickel content. This is related to a fully austenitic solidification, which occurs because of the low chromium to nickel equivalent ratio. These cracks reduce the corrosion resistance as well as the elongation at fracture of the additively manufactured material that possesses a low chromium to nickel equivalent ratio of 1.0. A limitation of the nickel equivalent of the 316L type steel is suggested for PBF-LB/M production. Based on the knowledge obtained, a more detailed specification of the chemical composition of the type 316L stainless steel is recommended so that this steel can be PBF-LB/M processed to defect-free components with the desired mechanical and chemical properties. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstract10.3390/ma14154074
  • Improved process efficiency in laser-based powder bed fusion of nanoparticle coated maraging tool steel powder
    Pannitz, O. and Großwendt, F. and Lüddecke, A. and Kwade, A. and Röttger, A. and Sehrt, J.T.
    Materials 14 (2021)
    Research and development in the field of metal-based additive manufacturing are advancing steadily every year. In order to increase the efficiency of powder bed fusion of metals using a laser beam system (PBF LB/M), machine manufacturers have implemented extensive optimizations with regard to the laser systems and build volumes. However, the optimization of metallic powder materials using nanoparticle additives enables an additional improvement of the laser–material interaction. In this work, tool steel 1.2709 powder was coated with silicon carbide (SiC), few-layer graphene (FLG), and iron oxide black (IOB) on a nanometer scale. Subsequently, the feedstock material and the modified powder materials were analyzed concerning the reflectance of the laser radiation and processed by PBF-LB/M in a systematic and consistent procedure to evaluate the impact of the nano-additivation on the process efficiency and mechanical properties. As a result, an increased build rate is achieved, exhibiting a relative density of 99.9% for FLG/1.2709 due to a decreased reflectance of this modified powder material. Furthermore, FLG/1.2709 provides hardness values after precipitation hardening with only aging comparable to the original 1.2709 material and is higher than the SiC- and IOB-coated material. Additionally, the IOB coating tends to promote oxide‐formation and lack‐of‐fusion defects. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstract10.3390/ma14133465
  • Potential of the Recycling of Grinding Sludge by various Powder Metallurgical Processes
    Jäger, S. and Weber, S. and Röttger, A.
    Procedia CIRP 104 (2021)
    The metalworking industry produces a large amount of waste material by machining. In contrast to conventional machining, the waste material generated during a grinding process is not purely metallic, but consists of abrasives, water, lubricants and metal chips. This mixture is called grinding sludge. In terms of ecological and economical aspects, the recycling of these waste materials is very promising. By separating the individual components of the grinding sludge, the recycling potential of the individual components becomes visible. This study focuses on the recycling of the metallic part. For this purpose, various powder metallurgical processes were performed. The produced samples and their properties were then compared with samples made of conventional PM powder. © 2021 Elsevier B.V.. All rights reserved.
    view abstract10.1016/j.procir.2021.11.150
  • Processing of a newly developed nitrogen-alloyed ferritic-austenitic stainless steel by laser powder bed fusion – Microstructure and properties
    Becker, L. and Röttger, A. and Boes, J. and Weber, S. and Theisen, W.
    Additive Manufacturing 46 (2021)
    In this work, a novel alloy design of a stainless steel with a ferritic-austenitic microstructure is derived for PBF-LB/M (powder bed fusion-laser beam/metal). The alloy was developed based on X2CrNiMo17-12-2 steel, for which an austenite volume content of approx. 54 vol% in the PBF-LB/M state was achieved using a reduced Ni equivalent. Partial substitution of Ni by Mn increases the N solubility of the alloy. By melting and further gas-atomizing this melt in an N2 atmosphere, an N content of 0.27 mass% was set in the produced steel powder. This leads to both high strength and high corrosion resistance of the PBF-LB/M-processed steel. However, microstructural investigations in the PBF-LB/M state confirm a microstructure consisting of ferrite, austenite, and Mo- and Cr-rich nitrides of M2N type. The nitrides were not completely eliminated by a subsequent heat treatment of the PBF-LB/M samples. As a result of the solution annealing, the microstructure approaches the thermodynamic equilibrium so that the austenite volume content increases from 54.2 vol% to 92.7 vol%. The higher Cr and N contents result in a higher corrosion resistance of the investigated steel compared to PBF-LB/M-processed X2CrNiMo17-12-2, regarded as the reference material. In addition, the measured strengths are significantly higher due to the larger amounts of austenite/ferrite interfaces and the N-induced solid-solution strengthening effect compared to X2CrNiMo17-12-2. © 2021 Elsevier B.V.
    view abstract10.1016/j.addma.2021.102185
  • Relevance of processing parameters for grain growth of metal halide perovskites with nanoimprint
    Mayer, A. and Haeger, T. and Runkel, M. and Rond, J. and Staabs, J. and van gen Hassend, F. and Röttger, A. and Görrn, P. and Riedl, T. and Scheer, H.-C.
    Applied Physics A: Materials Science and Processing 127 (2021)
    The quality and the stability of devices prepared from polycrystalline layers of organic–inorganic perovskites highly depend on the grain sizes prevailing. Tuning of the grain size is either done during layer preparation or in a post-processing step. Our investigation refers to thermal imprint as the post-processing step to induce grain growth in perovskite layers, offering the additional benefit of providing a flat surface for multi-layer devices. The material studied is MAPbBr3; we investigate grain growth at a pressure of 100 bar and temperatures of up to 150 °C, a temperature range where the pressurized stamp is beneficial to avoid thermal degradation. Grain coarsening develops in a self-similar way, featuring a log-normal grain size distribution; categories like ‘normal’ or ‘secondary’ growth are less applicable as the layers feature a preferential orientation already before imprint-induced grain growth. The experiments are simulated with a capillary-based growth law; the respective parameters are determined experimentally, with an activation energy of Q ≈ 0.3 eV. It turns out that with imprint as well the main parameter relevant to grain growth is temperature; to induce grain growth in MAPbBr3 within a reasonable processing time a temperature of 120 °C and beyond is advised. An analysis of the mechanical situation during imprint indicates a dominance of thermal stress. The minimization of elastic energy and surface energy together favours the development of grains with (100)-orientation in MaPbBr3 layers. Furthermore, the experiments indicate that the purity of the materials used for layer preparation is a major factor to achieve large grains; however, a diligent and always similar preparation of the layer is equally important as it defines the pureness of the resulting perovskite layer, intimately connected with its capability to grow. The results are not only of interest to assess the potential of a layer with respect to grain growth when specific temperatures and times are chosen; they also help to rate the long-term stability of a layer under temperature loading, e.g. during the operation of a device. © 2021, The Author(s).
    view abstract10.1007/s00339-021-04830-0
  • Study on the Austemperability of Thin-wall Ductile Cast Iron Produced by High-Pressure Die-casting
    van gen Hassend, F. and Ninnemann, L. and Töberich, F. and Breuckmann, M. and Röttger, A. and Weber, S.
    Journal of Materials Engineering and Performance (2021)
    The production of thin-wall ductile iron (TWDI) by high-pressure die-casting (HPDC) is complex because of several metallurgical and microstructural challenges. The present work aims to evaluate the austemperability of components (4 mm thickness) produced by HPDC process. The graphitization kinetics, the pearlite formation during continuous cooling, and the effect of austempering on the evolution of the ausferritic microstructure were investigated using dilatometric tests, microstructural analysis as well as Vickers hardness tests and tensile tests. Results show that components exhibit a brittle behavior because of white structures, small shrinkage cavities, and microporosity in the as-cast condition. Graphitization at 1100 °C allows rapid formation of small graphite particles within a short time (40 s). The critical cooling time (t8/5) to avoid the formation of pearlite upon cooling was found to be 5 s at a martensite start temperature of 193 ± 14 °C. Austempering at 360 °C for 40 min results in an ausferritic microstructure with stable carbon-enriched austenite which provides a high hardness (355 ± 4 HV10) and tensile strength (Rm = 709 ± 65 MPa). The results represent main criteria regarding the producibility of die-casted TWDI, which are helpful for future alloy and heat treatment design. © 2021, The Author(s).
    view abstract10.1007/s11665-021-06252-8
  • Subcritical crack growth in hard alloys under cyclic loading
    Brackmann, L. and Röttger, A. and Weber, S. and Theisen, W.
    Fatigue and Fracture of Engineering Materials and Structures 44 (2021)
    In this work, the subcritical crack growth in Fe-, Ni- and Co-base hard alloys was investigated. Specimens were cyclically loaded in the pressure threshold range until a ring crack resulted as a failure criterion. Crack propagation along with the individual microstructural constituents and the associated resistance of the individual materials to crack propagation was investigated by scanning electron microscopy and by the methods adapted to it. For the Ni-base alloys, the formation of a closed ring fracture occurred after the lowest load cycle number, followed by the Co- and Fe-base alloys. Almost no crack deflection by the hard phases was detected in the Ni-base alloys. The higher number of loading cycles to produce a closed crack ring in the Fe-base alloys is attributed to the pronounced crack deflection by the hard phases and to the higher matrix strength. Besides, phase transformations were registered in front of the crack tip of the Co- and the partially austenitic Fe-base alloy. This phase transformation counteracts crack formation in the case of the Fe-base matrix but promotes crack propagation in the Co-base alloy. © 2020 The Authors. Fatigue & Fracture of Engineering Materials & Structures published by John Wiley & Sons Ltd
    view abstract10.1111/ffe.13363
  • Comprehensive investigation of the microstructure-property relationship of differently manufactured Co–Cr–C alloys at room and elevated temperature
    Krell, J. and Röttger, A. and Theisen, W.
    Wear 444-445 (2020)
    The purpose of this study was to investigate the influence of the microstructure on sliding wear and hardness of four different Co–Cr–C alloys at room and elevated temperature. Different microstructures were produced by applying three different processes. The hardness, hot hardness and wear loss at room temperature of these alloys correlate strongly with the carbide volume content. In sliding wear tests against an Al2O3 ball, abrasive wear occurs at room temperature. The size or geometric arrangement of the carbides or metal matrix plays a minor role at room temperature. At 600 °C the wear behaviour changes due to the softening matrix. In alloys with small free matrix path lengths, the highest wear rates occur due to micro-fatigue and micro-cracking. In hypoeutectic alloys with a high free matrix path length, the carbides lose their effectiveness due to the lack of support by the matrix. In these alloys, wear is dominated by the properties of the matrix. A hypereutectic casting alloy with large primary carbides shows the best wear results, as the carbides support themselves due to their size and retain their wear-reducing effect. © 2019 Elsevier B.V.
    view abstract10.1016/j.wear.2019.203138
  • Electro-discharge sintering of nanocrystalline NdFeB magnets: process parameters, microstructure, and the resulting magnetic properties
    Leich, L. and Röttger, A. and Kuchenbecker, R. and Theisen, W.
    Journal of Materials Science: Materials in Electronics 31 (2020)
    This study investigates the compaction of nanocrystalline NdFeB magnet powder by electro-discharge sintering (EDS). On this account, process parameters, microstructure, and the associated magnetic properties of the EDS-densified nanocrystalline NdFeB specimens were investigated by varying the discharge energy EEDS and compression load pEDS. Although optimized process parameters could be evaluated, three different microstructures (fully densified zone, insufficiently densified zone, and melted zone) are present in the EDS-compacted specimens. Thereby, volume fractions of these formed three different microstructures determine the resulting mechanical and magnetic properties of the specimens. For all specimens, the intrinsic coercivity Hc,J deteriorates with increasing discharge energy, as the generated Joule heat leads to microstructural changes (grain growth, dissolution of magnetic phases), which reduces the magnetic properties. The compression load has less influence on the coercivity Hc,J, as it only affects the initial resistance of the pre-compacted powder loose. The residual induction Br deteriorates with increasing the discharge energy due to microstructural changes. An increase in the compression load pEDS results in an increase in the specimens’ density and thus promotes the residual induction Br. © 2020, The Author(s).
    view abstract10.1007/s10854-020-04562-6
  • Gas atomization and laser additive manufacturing of nitrogen-alloyed martensitic stainless steel
    Boes, J. and Röttger, A. and Theisen, W. and Cui, C. and Uhlenwinkel, V. and Schulz, A. and Zoch, H.-W. and Stern, F. and Tenkamp, J. and Walther, F.
    Additive Manufacturing 34 (2020)
    Nitrogen as an alloying element can improve the corrosion resistance and the mechanical properties of stainless steels. Therefore, nitrogen-alloyed martensitic stainless steels, such as X30CrMoN151, have been developed in recent decades and conventional processing of this steel by casting or powder metallurgy is well understood. However, only very few attempts to process nitrogen-alloyed martensitically hardenable stainless steels containing more than 0.2 mass-% of carbon by laser powder bed fusion (L-PBF) have been reported so far. In this study, X30CrMoN15-1 steel powder has been produced from quasi nitrogen-free X30CrMo15-1 steel by gas atomization using N2 as the process gas to introduce nitrogen into the steel. The gas-atomized powder was characterized in terms of nitrogen content, particle size distribution, particle morphology, and flow properties. The powder was then processed by L-PBF under an N2 gas atmosphere, and microstructural investigations were performed on the L-PBF-built samples using scanning electron microscopy and X-ray computed tomography. Additionally, a first impression of the mechanical properties of the L-PBF-built steel in the as-built and quenched and tempered condition was obtained by means of fatigue tests. It was shown that a nitrogen content of 0.16 mass-% could be introduced into the steel during gas atomization. The resulting powder was successfully processed by means of L-PBF, and specimens with a high density were produced. During fatigue testing, a large amount of retained austenite in the as-built condition resulted in a greater damage tolerance of the specimens compared to the heat-treated condition. © 2020 Elsevier B.V.
    view abstract10.1016/j.addma.2020.101379
  • Influence of Pore Characteristics on Anisotropic Mechanical Behavior of Laser Powder Bed Fusion–Manufactured Metal by Micromechanical Modeling
    R. G. Prasad, M. and Biswas, A. and Geenen, K. and Amin, W. and Gao, S. and Lian, J. and Röttger, A. and Vajragupta, N. and Hartmaier, A.
    Advanced Engineering Materials 22 (2020)
    In recent times, additive manufacturing (AM) has proven to be an indispensable technique for processing complex 3D parts because of the versatility and ease of fabrication it offers. However, the generated microstructures show a high degree of complexity due to the complex solidification process of the melt pool. In this study, micromechanical modeling is applied to gain deeper insight into the influence of defects on plasticity and damage of 316L stainless steel specimens produced by a laser powder bed fusion (L-PBF) process. With the statistical data obtained from microstructure characterization, the complex AM microstructures are modeled by a synthetic microstructure generation tool. A damage model in combination with an element deletion technique is implemented into a nonlocal crystal plasticity model to describe anisotropic mechanical behavior, including damage evolution. The element deletion technique is applied to effectively model the growth and coalescence of microstructural pores as described by a damage parameter. Numerical simulations show that the shape of the pores not only affects the yielding and hardening behavior but also influences the porosity evolution itself. © 2020 The Authors. Published by Wiley-VCH GmbH
    view abstract10.1002/adem.202000641
  • Influence of powder nitriding on the mechanical behavior of laser-powder bed fusion processed tool steel X30CrMo7-2
    Stern, F. and Grabienski, F. and Walther, F. and Boes, J. and Röttger, A. and Theisen, W.
    Materialpruefung/Materials Testing 62 (2020)
    Additive manufacturing allows for the production of highly complex structures due to its layer-wise local melting of powder material. For this reason, this technique has a high potential for manufacturing extremely lightweight components potential. However, laser based additive manufacturing is still restricted due to the limited amount of processable alloys, especially Fe-based materials. A main object in current research is to expand the varieties for steel that may be used. Additionally, the modification and optimization of steel powder is seen as an interesting aspect for improving the material properties of additively manufactured parts. In this work, secondary hardenable martensitic tool steel X30CrMo7-2 is investigated, starting from the raw powder which is enriched with nitrogen by gas nitriding and subsequently characterized to ensure the usability of the modified powder for laser-powder bed fusion. In a next step, the raw and nitrided powder are used to generate cylindrical specimens to allow for further analysis of the microstructure and for a mechanical characterization of compression behavior. Moreover, a variety of heat treatments is carried out. The higher content of nitrogen leads to an increase in porosity. However, the addition of nitrogen causes an increase in hardness and in the compressive yield point, especially after heat treatment. After tempering, compressive yield stress is increased from 1,111 MPa to 1990 MPa, while for conventional material it is slightly reduced from 1,316 MPa to 1225 MPa. © Carl Hanser Verlag GmbH & Co. KG
    view abstract10.3139/120.111446
  • Influence of precipitation hardening on the high-temperature sliding wear resistance of an aluminium alloyed iron-nickel base alloy
    Krell, J. and Röttger, A. and Ziesing, U. and Theisen, W.
    Tribology International 148 (2020)
    This work investigates the effects of precipitation hardening on hot hardness and high-temperature sliding wear resistance of an iron-nickel base alloy. Three variants of a carbide rich alloy containing 0, 2 and 3 wt.-% aluminium were manufactured and aged for 24 h at 650, 700 and 750 °C. Hot hardness (20–800 °C) and sliding wear tests (600 °C) were conducted for each condition. The addition of aluminium has little effect on the type or volume content of the carbides. Ageing caused the precipitation of NiAl in the aluminium-containing alloys. The precipitation-hardened samples show higher hot hardness and better hot wear resistance. The lower wear loss can mainly be attributed to the improved support of the carbides by the precipitation strengthened matrix. © 2020
    view abstract10.1016/j.triboint.2020.106342
  • Laser additive manufacturing of hot work tool steel by means of a starting powder containing partly spherical pure elements and ferroalloys
    Taruttis, A. and Hardes, C. and Röttger, A. and Uhlenwinkel, V. and Chehreh, A.B. and Theisen, W. and Walther, F. and Zoch, H.W.
    Procedia CIRP 94 (2020)
    Until now, additive manufacturing of high-performance materials such as martensitic hardenable tool steels is rarely investigated. This work addresses the introduction of an alternative alloying strategy for hot work tool steel powder, provided for laser powder bed fusion (L-PBF). The focus is on the question whether a powder mixture of spherical iron powder mixed with mechanically crushed ferroalloy particles can be processed by L-PBF, instead of using cost-intensive pre-alloyed gas-atomized powder, and to investigate the material properties associated with it. The particle morphology, packing density and flowability of this L-PBF powder feedstock is compared to gas-atomized spherical pre-alloyed steel powder and the results are correlated to the defect density, the resulting microstructure and the chemical homogeneity. Finally the resulting surface hardness is compared to a conventionally casted material as a reference state. It shows that the L-PBF fabrication of high-dense parts by means of both starting powders is technically feasible. Even though the alternative alloying concept promotes local chemical inhomogeneities within the microstructure, the overall porosity and the appearance of micro cracks are reduced. © 2020 The Authors. Published by Elsevier B.V.
    view abstract10.1016/j.procir.2020.09.010
  • Microstructure and mechanical properties of 316L austenitic stainless steel processed by different SLM devices
    Röttger, A. and Boes, J. and Theisen, W. and Thiele, M. and Esen, C. and Edelmann, A. and Hellmann, R.
    International Journal of Advanced Manufacturing Technology 108 (2020)
    In this work, we examined the influence of different types of selective laser melting (SLM) devices on the microstructure and the associated material properties of austenitic 316L stainless steel. Specimens were built using powder from the same powder batch on four different SLM machines. For the specimen build-up, optimized parameter sets were used, as provided by the manufacturers for each individual SLM machine. The resulting microstructure was investigated by means of scanning electron microscopy, which revealed that the different samples possess similar microstructures. Differences between the microstructures were found in terms of porosity, which significantly influences the material properties. Additionally, the build-up direction of the specimens was found to have a strong influence on the mechanical properties. Thus, the defect density defines the material’s properties so that the ascertained characteristic values were used to determine a Weibull modulus for the corresponding values in dependence on the build-up direction. Based on these findings, characteristic averages of the mechanical properties were determined for the SLM-manufactured samples, which can subsequently be used as reference parameters for designing industrially manufactured components. © 2020, The Author(s).
    view abstract10.1007/s00170-020-05371-1
  • Microstructure and properties of high-strength C + N austenitic stainless steel processed by laser powder bed fusion
    Boes, J. and Röttger, A. and Theisen, W.
    Additive Manufacturing 32 (2020)
    In the developing field of laser powder bed fusion (L-PBF), austenitic stainless steels, such as AISI 316L, have gained great importance owing to their excellent processability. However, the moderate strength of these steels limits their applicability. This can be counteracted by the use of nitrogen as an alloying element to improve both strength and corrosion resistance. In this work, nitrogen-alloyed high-strength austenitic stainless steel X40MnCrMoN19-18-1 was processed by L-PBF, and the resulting microstructural and mechanical properties were investigated. The same material was also processed by hot isostatic pressing (HIP), which was used as a reference state. In the L-PBF process, argon and nitrogen were used as process gases to investigate the influence of process atmosphere on the microstructure and on changes in the chemical composition during processing. The results show a minor decrease in the nitrogen content of the steel after L-PBF, independently of the process gas, whereby argon resulted in a slightly higher specimen density. The microstructure after L-PBF processing contained small precipitates that could be removed by a short solution-annealing treatment. The tensile properties of the L-PBF-built steel are comparable to those of the steel produced by hot isostatic pressing in terms of ultimate tensile strength, but had lower elongation to fracture values. The ductility of the material was enhanced by solution annealing without significant impairment of the ultimate tensile strength. This work demonstrates that nitrogen-alloyed stainless steels can be processed by means of L-PBF and can extend the variety of appropriate steels towards applications with high requirements for the material strength and chemical resistance. © 2020 Elsevier B.V.
    view abstract10.1016/j.addma.2020.101081
  • Microstructures, Heat Treatment, and Properties of Boron-Alloyed Tool Steels
    Lentz, J. and Röttger, A. and Theisen, W.
    Steel Research International 91 (2020)
    To enable the development of novel Fe–C–B–Cr and Fe–C–B–Cr–Mo cold work tool steels, the microstructures and hardness-tempering behaviors of hypoeutectic laboratory melts are investigated. The results show that increasing Cr content enhances the thermodynamic stability of the ultrahard M2B borides. The formation of carboborides is suppressed by adjusting the B/(C + B) ratio, Cr content, and austenitization temperature. A secondary hardenability at 500 °C is achieved by Mo addition. In addition, Mo stabilizes the M23(C,B)6 phase and at higher contents the M3B2 boride. Based on these investigations, Fe0.4C1B–Cr alloys are designed which, inspired by the microstructure of the steel X153CrMoV12-1, feature a α′-Fe hardenable matrix but 15 vol% of eutectic M2B borides instead of M7C3 for wear protection. The Fe0.4C1B–Cr steels are produced by casting and hot rolling as well as powder metallurgy and hot isostatic pressing. The (tribo-) mechanical properties are investigated and compared with X153CrMoV12-1. Fracture toughness, bending strength, wear resistance, and hardness of the novel Fe0.4C1B–Cr alloys are found to be similar or superior to the steel X153CrMoV12-1, at decreased material cost. © 2019 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/srin.201900416
  • Processing of X65MoCrWV3-2 Cold Work Tool Steel by Laser Powder Bed Fusion
    Boes, J. and Röttger, A. and Theisen, W.
    Steel Research International 91 (2020)
    Laser powder bed fusion (L-PBF) of forming tools has become of major interest in the tooling industry because of the high geometrical flexibility of this process. During L-PBF, a metallic powder bed is melted selectively by a laser beam, enabling the layer-wise manufacturing of parts from 3D computer-aided design data. The process is characterized by a locally and temporally unsteady heat flow in the solidified part and in the melt pool, causing nonequilibrium solidification and phase transformations. In addition, rapid heating and cooling occur, promoting the formation of microstructural defects, cold cracks, and distortion. Because of the high tendency to form cold cracks, processing of martensitic tool steels is still a challenging task. Tool steel X65MoCrWV3-2 is processed by L-PBF and the resulting microstructure and the associated local properties are investigated by microhardness measurements, nanoindentation, and scanning electron microscopy. It is gathered from the investigations that regions of different microstructures and mechanical properties on both micro- and macroscale are present in the L-PBF-densified steel. The different microstructures and properties are the result of the alternating heat insert at different temperature regimes, forming heat-affected zones in which the tempering processes are triggered and strongly varying properties are generated. © 2019 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/srin.201900445
  • A Concept for the Estimation of Soil-Tool Abrasive Wear Using ASTM-G65 Test Data
    Hoormazdi, G. and Küpferle, J. and Röttger, A. and Theisen, W. and Hackl, K.
    International Journal of Civil Engineering 17 (2019)
    In this paper, a new procedure for the prediction of soil-tool abrasive wear is presented which drastically reduces the duration and, therefore, the cost of simulations in comparison to conventional 3D wear modeling. The goal is to extend the experimental data from a single scratch test to the wear of mixtures by means of equations obtained from discrete element method (DEM) simulations and geometric relations. We are predicting abrasive wear with a combination of numerical and experimental approaches taking two shapes of particles into account. Single wear is quantified by measuring the width of scratch induced by a single quartz particle. Geometrical relations together with the particle’s microscopic picture are used to find the depth of scratch. DEM mixture simulations result in equations for the number of contacts and normal contact forces. Finally, the wear rate is calculated for a specific soil sample as an example to clarify the developed prediction procedure. The DEM simulations are performed using PFC 3 D code for both a homogeneous soil sample and a mixture of two different soils. We are specially investigating a relation to predict the abrasive wear caused by a mixture of particles. © 2018, Iran University of Science and Technology.
    view abstract10.1007/s40999-018-0333-9
  • Cavitation erosion resistance of 316L austenitic steel processed by selective laser melting (SLM)
    Hardes, C. and Pöhl, F. and Röttger, A. and Thiele, M. and Theisen, W. and Esen, C.
    Additive Manufacturing 29 (2019)
    Every SLM-fabricated component typically possesses a process-specific microstructure that fundamentally differs from any conventionally fabricated specimen. This publication addresses the evaluation of microstructure-related influencing factors on the resistance against cavitation erosion. We exemplarily compared the findings to a cast and hot rolled reference sample. Due to careful adjustment of the process parameters, the overall cavitation erosion resistance of both SLM-processed and conventionally fabricated 316L are very much alike in the investigated case. The incubation period of intact surface areas is improved by the greater hardness and yield strength of the SLM specimen, which is attributable to an increased dislocation density and a smaller grain size. Nevertheless, processing and powder feeding during SLM-fabrication occasionally results in microstructural defects, at which pronounced mass loss during cavitation was registered. X-ray measurements of the residual stresses reveal the development of severe compressive stresses that emerge after a few seconds of cavitation. This compressive stress state delays the immediate propagation of SLM-inherent micro cracks. Moreover, investigations of the microstructure in combination with examination of the ongoing surface deformation highlighted the emergence of coarse grains that grew towards the temperature gradient. This effect leads to a temporarily high surface roughness, local stress concentrations and an increased probability of cavitation impacts. Furthermore, parallel cracks appear perpendicular to the scan tracks that are traced back to formerly protruded slip bands. © 2019 Elsevier B.V.
    view abstract10.1016/j.addma.2019.100786
  • Chromium-nickel-alloys for wear application at elevated temperature
    Krell, J. and Röttger, A. and Theisen, W.
    Wear 432-433 (2019)
    Chromium alloys are a promising class of materials for high temperature application. In this work we investigated the microstructure, hot hardness and sliding wear resistance against an Al2O3 counter-body of three Chromium-Nickel (CrNi) alloys. The microstructure of the investigated alloys consists of a Ni-rich and a Cr-rich phase and can be manipulated by heat treatments. In the solid solution annealed state the room temperature (RT) hardness of CrNi alloys exceeds 800 HV10. The most potent strengthening mechanism in this system is the super saturation of the Cr-rich phase with Ni. Consequently, solid solution annealed specimen experience a strong decline in hardness with the onset of diffusion and precipitation of Ni-rich precipitates within the Cr-rich phase. Aged specimens display a lower hardness at RT but show a lower relative decline in hardness at temperatures above 600 °C. Alloys with higher contents of Ni show a better performance at high temperature sliding wear due to the formation of stable mechanically mixed layers (MML). © 2019 Elsevier B.V.
    view abstract10.1016/j.wear.2019.05.039
  • Microstructure, mechanical, and tribological properties of M3:2 high-speed steel processed by selective laser melting, hot-isostatic pressing, and casting
    Geenen, K. and Röttger, A. and Feld, F. and Theisen, W.
    Additive Manufacturing 28 (2019)
    In this work, the influence of different manufacturing techniques of M3:2 high-speed steel on the resulting microstructure and the associated material properties was investigated. Therefore, microstructure as well as the mechanical and tribological properties of cast steel (with subsequent hot-forming) and steel powder processed by two techniques: hot-isostatic pressing (HIP) and selective laser melting (SLM) were compared. A detailed SLM parameter analysis revealed that the porosity of SLM specimens can be decreased towards a smaller point distance and a longer exposure time (high energy input). A rise in preheating temperature is associated with a reduction in the crack density or the complete avoidance of cracks. In this context, the high-speed steel showed outstanding densification behavior by SLM, even though this steel is considered to be hardly processable by SLM due to its high content of carbon and hard phase-forming elements. In addition, the reusability of steel powder for SLM processing was investigated. The results indicated that multiple reuse is possible, but only in combination with powder processing (mechanical sieving) after each SLM cycle. The microstructure of SLM-densified high-speed steel consists of a cellular, fine dendritic subgrain segregation structure (submicro level) that is not significantly affected by preheating the base plate. The mechanical and tribological properties were examined in relation to the manufacturing technique and the subsequent heat treatment. Our investigations revealed promising behavior with respect to hardness tempering (position of the secondary hardness peak) and tribology of the M3:2 steel processed by SLM compared to the HIP and cast conditions. © 2019
    view abstract10.1016/j.addma.2019.05.028
  • Phase Formation during Heat Treatment of Zn- and ZnAlMg-Coated Steels at 400 and 750 °C
    Windmann, M. and Barthen, M. and Opitz, T. and Hahn, I. and Röttger, A. and Theisen, W.
    Steel Research International (2019)
    In contrast to a cold-forming process, a tempered forming process is able to deform high-strength steel used for manufacturing automotive bodyworks in a more economic manner. Cold-formed steel sheets are commonly coated with a Zn or ZnAlMg layer for cathodic corrosion protection. The tempering process would lead to diffusion processes at the steel/coating interface, which is accompanied by the formation of new phases in the coatings. This publication focuses on phase formation in Zn and ZnAlMg coatings on steel sheets, which are heat-treated at 400 and 750 °C. the authors find that the pure Zn coating remains in the solid state and transforms into the intermetallic δ phase (FeZn 10 ) during heat treatment at 400 °C. The coating melts during heating to 750 °C, but remains in the solid state after transformation into the Γ phase (Fe 4 Zn 9 ) and α-Fe. In the ZnAlMg coating, minor iron diffusion occurs at a temperature of 400 °C. Within a dwell time of 600 s, intermetallic Fe–Zn phases are not formed. During heat treatment at 750 °C, phase formation in the ZnAlMg coating is very similar to that in the pure Zn coating, during which Γ (Fe 4 Zn 9 ) and α-Fe are formed. © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/srin.201800588
  • Processing of gas-nitrided AISI 316L steel powder by laser powder bed fusion – Microstructure and properties
    Boes, J. and Röttger, A. and Becker, L. and Theisen, W.
    Additive Manufacturing 30 (2019)
    This work investigated the processing of high nitrogen-alloyed austenitic stainless steels by laser powder bed fusion (L-PBF). Prior to L-PBF processing, the AISI 316 L steel powder was nitrided at a temperature of 675°C in a 3 bar nitrogen atmosphere, thus achieving a N content of 0.58 mass-%. By mixing nitrided 316 L powder with untreated 316 L powder, two different powder mixtures were obtained with 0.065 mass-% and 0.27 mass-% nitrogen, respectively. After nitriding and mixing, the powder was characterized in terms of its flow properties and chemical composition. The nitrided steel powder was then processed by L-PBF, and the microstructure as well as the chemical composition were investigated by means of scanning electron microscopy and carrier gas hot extraction. It was shown that nitriding of steel powders in an N2 atmosphere can be used to significantly increase the nitrogen content of the powder without impairing its flow properties. With increasing nitrogen content of the powder, the porosity within the L-PBF built specimens increased. However, both the yield strength and the tensile strength were greatly improved without a marked reduction in the elongation at fracture of the respective steels. This work shows that nitrogen-alloyed austenitic stainless steels can be processed by L-PBF and the mechanical properties can be improved. © 2019 Elsevier B.V.
    view abstract10.1016/j.addma.2019.100836
  • Recycling of NdFeB Magnets by Electrodischarge Sintering—Microstructure, Magnetic, and Mechanical Properties
    Leich, L. and Röttger, A. and Krengel, M. and Theisen, W.
    Journal of Sustainable Metallurgy 5 (2019)
    In this work, we investigate the feasibility of recycling NdFeB magnets by means of electrodischarge sintering (EDS). We crushed, sintered, and hot-deformed NdFeB magnets in a jaw crusher, and the NdFeB fragments were further compacted to a round shape by EDS. The EDS technique is a fast and energy-saving compaction process for powders with sufficient electrical conductivity. The current is discharged from capacitors into a loose powder that has been precompacted by Cu punches into a ceramic die, thus resulting in fully dense magnets. In this study, we investigated the apparent density, particle size distribution, oxygen content, and morphology of the crushed powder. In addition, the microstructure, compressive strength, and the magnetic properties of the EDS-densified samples were examined. For all specimens, the energy product decreases with the increasing discharge energy during EDS processing and the increasing oxygen content of the initial powder. Furthermore, high apparent densities together with large particle sizes promote EDS densification of NdFeB magnets. The applied EDS parameters led to the formation of three different microstructures (insufficiently densified zone, fully densified zone, and remelted zone) along the cross section of the EDS-densified specimens. These volume fractions of the different microstructural constituents during the EDS process and the powder characteristics (oxygen content, morphology, etc.) determine the resulting mechanical and magnetic properties of the specimens. © 2019, The Minerals, Metals & Materials Society.
    view abstract10.1007/s40831-018-0204-2
  • Time-Dependent Evolution of Microstructure and Mechanical Properties of Mortar
    Röttger, A. and Youn-Čale, B.-Y. and Küpferle, J. and Lentz, J. and Breitenbücher, R. and Theisen, W.
    International Journal of Civil Engineering 17 (2019)
    This study investigates the evolution of the microstructure and mechanical properties of mortar. Mortar samples consisting of Portland cement CEM I42.5 R (~ 60 vol% of quartz sand 0/2 mm, w/c-ratio of 0.5) were prepared and stored according to EN 1015. After 1, 2, 7, 14 and 28 days, the samples were oven-dried until constant weight as well as vacuum-dried. The microstructure of the mortar samples was investigated using scanning electron microscopy. Phase analysis was performed using X-ray diffraction, allowing the description of the crystalline phase evolution during hardening. Mechanical properties were evaluated using nanoindentation. Based on the nanoindentation results, the effective Young’s modulus was calculated using the model by Hashin and Shtrikman. The moduli calculated based on the values of the nanoindentation experiments were compared to the Young’s modulus determined in compression experiments. The results show that the Young’s modulus determined by the nanoindentation and compression test describes a degressive curve progression. The studies show a correlation between the results from nanoindentation tests and the mechanical properties obtained from the compression tests. Therefore, the microstructural evolution of mortar, including the influence of pores on Young’s modulus, must be taken into account to estimate the macroproperties from the nanoindentation tests. © 2018, Iran University of Science and Technology.
    view abstract10.1007/s40999-018-0305-0
  • Densification of nanocrystalline NdFeB magnets processed by electro-discharge sintering – Microstructure, magnetic, and mechanical properties
    Leich, L. and Röttger, A. and Theisen, W. and Krengel, M.
    Journal of Magnetism and Magnetic Materials 460 (2018)
    This work investigates the densification process of nanocrystalline NdFeB powder by electro-discharge sintering (EDS) and the associated magnetic properties. The EDS technique is used as a fast and energy-saving compaction process for metal powders. A large current is discharged from capacitors into a pre-compacted loose powder, thus resulting in complete compaction. In this study, the microstructure, magnetic, and mechanical properties of the compacted, hard magnetic NdFeB specimens were investigated under variation of the energy EEDS and compression load pEDS. For all specimens, the intrinsic coercivity HcJ decreases on increasing the discharge energy. However, the compaction load has apparently no influence on the coercivity HcJ, whereas the residual induction Br decreases only with increasing discharge energy. An increase in the compression load pEDS causes an increase in the specimens’ density and thus promotes residual induction Br. The applied EDS parameters led to the formation of three different microstructures (insufficiently densified zone, fully densified zone, and remelted zone) along the cross-section of the EDS-densified specimens. Volume fractions of the three different microstructures that form during the EDS process determine the resulting mechanical and magnetic properties of the specimens. © 2018 Elsevier B.V.
    view abstract10.1016/j.jmmm.2018.04.035
  • Enhancement of hardness, modulus and fracture toughness of the tetragonal (Fe,Cr)2B and orthorhombic (Cr,Fe)2B phases with addition of Cr
    Lentz, J. and Röttger, A. and Großwendt, F. and Theisen, W.
    Materials and Design 156 (2018)
    This study analyzes the influence of Cr content on hardness H, elastic modulus E and fracture toughness KIC of the M2B boride by means of nanoindentation experiments. Additionally, properties of the Fe3(C,B) phase are determined. Samples of the M2B phase are casted and microstructurally characterized by means of scanning electron microscopy, energy dispersive spectroscopy and X-ray diffraction. At a Cr content higher than 14.7 atom% the M2B phase transforms from tetragonal into orthorhombic structure. The tetragonal M2B type possesses an optimum of H (21 ± 1 GPa), E (373 ± 6) GPa and KIC (3.5 ± 0.7 MPam) at 4–5 atom% Cr. The hardness, modulus and toughness of the orthorhombic M2B phase increase with Cr content and reach values of H = 27 ± 0.7 GPa, E = 473 ± 9 of and KIC = 3.26 ± 0.8 MPam at maximal investigated Cr content of 55 atom%. The hardness of the M2B phases decreases around 2.3–3.2 GPa as a function of indentation depth, which is known as the indentation size effect. Hardness and fracture toughness of M2B phase outperform conventionally used M7C3 carbides and are similar to MC-carbides. Findings can be used in novel alloying approaches in order to optimize the performance and reduce cost of tool steels. © 2018 Elsevier Ltd
    view abstract10.1016/j.matdes.2018.06.040
  • General investigations on processing tool steel X40CrMoV5-1 with selective laser melting
    Krell, J. and Röttger, A. and Geenen, K. and Theisen, W.
    Journal of Materials Processing Technology 255 (2018)
    The X40CrMoV5-1 (H13) hot work tool steel was densified by selective laser melting (SLM) using different laser parameters and preheating temperatures. The porosity and crack densities of the processed specimen were determined, the resulting microstructure characterized, tempering hardness diagrams recorded and the reusability of the powder assessed. The X40CrMoV5-1 steel showed a good densification behaviour. Relative densities of above 99.5% were obtained. After SLM densification, the specimen showed a fine-grained microstructure, with a cellular arrangement consisting of ferrite and austenite. Although the microstructure did not change with preheating temperature, a decrease in crack density could be observed for higher preheating temperatures. By combining microstructural observations with some simulations, a new model describing the microstructural evolution of SLM-densified X40CrMoV5-1 is suggested. The peak in secondary hardness after tempering SLM-densified X40CrMoV5-1 was observed at higher temperatures compared to the cast reference steel in the same heat treatment condition. © 2018 Elsevier B.V.
    view abstract10.1016/j.jmatprotec.2018.01.012
  • Hardness and modulus of Fe2B, Fe3(C,B), and Fe23(C,B)6 borides and carboborides in the Fe-C-B system
    Lentz, J. and Röttger, A. and Theisen, W.
    Materials Characterization 135 (2018)
    This work provides a comparative and comprehensive study of the indentation hardness and indentation modulus of iron-rich borides and carboborides of types Fe2B, Fe3(C,B), and Fe23(C,B)6. In addition, the hardness and elastic modulus of Cr-rich M7C are investigated for comparative purposes. We investigated the impact of increasing B content and indentation size effect (ISE). The phases of interest were stabilized in cast Fe-C-B alloys that varied with respect to the B / (B + C) ratio and heat treatment. The resulting microstructures were characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and wavelength X-ray spectroscopy (WDS). Dynamic in-situ nanoindentation experiments based on the method of continuous stiffness measurement (CSM) were coupled to SEM and EBSD investigations to determine the mechanical properties of the individual borides and carboborides as a function of the indentation depth. The results were compared to values obtained for the Cr-rich M7C3 carbide. It was found that the hardness of the B-rich Fe3(C,B) phase is considerably higher than pure Fe3C and increases with increasing B content. The ISE was present in all investigated phases, and the hardness decreased as a function of indentation depth. The hardness at infinite indentation depth H0 was estimated according to the model of Nix and Gao. The Fe2B phase was found to be the hardest phase (H0 = 19.04 GPa), followed by M7C3 (H0 = 16.43 GPa), Fe3(C,B) (H0 = 11.18 to 12.24 GPa), and Fe23(C,B)6 (H0 = 10.39 GPa). © 2017 Elsevier Inc.
    view abstract10.1016/j.matchar.2017.11.012
  • Influence of the slurry-stabilized tunnel face on shield TBM tool wear regarding the soil mechanical changes – Experimental evidence of changes in the tribological system
    Küpferle, J. and Zizka, Z. and Schoesser, B. and Röttger, A. and Alber, M. and Thewes, M. and Theisen, W.
    Tunnelling and Underground Space Technology 74 (2018)
    The wear of slurry shield TBM excavation tools is a challenging topic that has been discussed over many years, Due to the complexity of the tribological system that defines the tool wear, no sufficient approach to determine the wear in a specific tunneling project has been developed up to now. The elementary step to display the application-oriented tribological system is one of the main issues regarding laboratory scale test methods. Therefore, the RUB Tunneling Device has been developed at the Ruhr-University Bochum (Germany). With this apparatus, several influencing factors regarding the tribological system of a TBM-tool can be analyzed. A unique feature of the device is the experimental simulation of a slurry shield excavation, including a realistic tunnel face support. This paper focuses on the experimental simulation of slurry shield excavation and the influence of face support on tool wear. Changes in the soil mechanical properties due to slurry penetration at the tunnel face are regarded and correlated with the tribological system of a slurry shield excavation. It is proven that the tribological system, and thus the tool wear, changes significantly due to slurry injection. © 2018 Elsevier Ltd
    view abstract10.1016/j.tust.2018.01.011
  • Interaction between laser radiation and metallic powder of 316L austenitic steel during selective laser melting
    Mutke, C. and Geenen, K. and Röttger, A. and Theisen, W.
    Materials Characterization 145 (2018)
    During selective laser melting (SLM), a complex heat state develops that leads to a characteristic crystallography and microstructure of the processed materials. Depending on the geometry of the processed part, most scan tracks of a new layer, so-called hatches, are located above a dense substrate or already solidified structures whereas others are located above loose powder. This is, inter alia, the case for overhanging structures. Attributable to the lower thermal conductivity of loose powder, temperature gradients and cooling rates of the melt pool differentiate in these areas, resulting in a different microstructural build-up. In this work, the microstructure and the crystallographic orientation of grade 316L austenitic stainless steel processed by SLM was investigated to understand the interaction between the laser radiation and the metallic powder during SLM-processing and to investigate the remelting of a track on a previous SLM-densified track. Single and multiple tracks on a loose bulk powder substrate, as well as single tracks on a dense substrate plate, were generated. A parameter study revealed that high energy densities are necessary to build continuous tracks on a loose bulk powder substrate. In addition, the amount of adhered particles, which are sintered on the fully melted and solidified tracks, is determined in comparison to the melted powder because the sintered particles strongly influence the surface roughness. To understand the microstructure development and, particularly, the influence of adjacent hatches during SLM-processing, investigations on the resulting microstructure and crystallographic orientation of a single track and two connected multiple tracks were carried out. During SLM processing of the tracks, the substrate plate and the solidified structures influence the temperature gradient and cooling rate of the melt pool, thus directionally solidified and elongated grains occur. Furthermore, the solidification is characterized by an epitaxial growth due to a distinct thermal gradient between the melt pool and the surrounding elements. © 2018 Elsevier Inc.
    view abstract10.1016/j.matchar.2018.08.061
  • Microstructure and mechanical properties of X65MoCrWV3-2 cold-work tool steel produced by selective laser melting
    Boes, J. and Röttger, A. and Mutke, C. and Escher, C. and Theisen, W.
    Additive Manufacturing 23 (2018)
    In this study, martensitic cold-work tool steel X65MoCrWV3-2 was processed by selective laser melting (SLM) by varying the laser scanning parameters and baseplate preheating temperatures. Porosity as well as crack density of the SLM-densified steel were determined by quantitative image analysis. The resulting microstructure and the associated local mechanical properties were characterized, and the hardness-tempering behavior of the SLM-densified steel was compared to the behavior of the conventionally manufactured X65MoCrWV3-2 steel in the cast and hot-formed condition. Regardless of the preheating temperature, SLM-densified X65MoCrWV3-2 possesses a porosity of less than 0.5 vol.-%. The crack density was reduced significantly by means of a higher preheating temperature. The microstructure after SLM densification shows a fine, equiaxed cellular-dendritic subgrain structure, superimposed by lath- or needle-like martensite. The martensite morphology appeared to be finer at a lower preheating temperature, whereas the observed subgrain structure did not seem to be influenced by the preheating temperatures. Microhardness measurements indicated tempering effects in first solidified layers caused by the densification of subsequently deposited layers. Peak hardness after tempering of the SLM-densified steel was found to be higher compared to the maximum hardness in the X65MoCrWV3-2 steel in the cast condition. © 2018 Elsevier B.V.
    view abstract10.1016/j.addma.2018.08.005
  • Mn-Alloyed High-Strength Steels with a Reduced Austenitization Temperature: Thermodynamic Calculations and Experimental Investigations
    Windmann, M. and Opitz, T. and Klein, S. and Röttger, A. and Theisen, W.
    Steel Research International 89 (2018)
    High-strength steels (e.g., 1.5528–22MnB5), processed by direct press-hardening, are widely used for security-relevant structures in automotive bodyworks. In this study, the austenitization temperature AC3 of the steel 22MnB5 (approx. 840 °C) is decreased to enable a reduction in the heat-treatment temperature. Thermodynamic calculations using the CALPHAD method are used to assess the effect of alloying elements on the α–γ transformation temperatures. On this account, 22MnB5 steel is alloyed with 6 to 9.5 mass% manganese, which decreases the α–γ transformation temperature to 744 °C. Simultaneously, the martensite finish temperature decreases below room temperature, which is accompanied by the presence of retained austenite after hardening. Furthermore, ϵ-martensite is formed. High Mn-alloyed steel 22MnB5 (9.5 mass% Mn, AC3 = 744 °C) possesses a high strength of Rm = 1618 MPa, similar to the initial material 22MnB5. Elongation-to-fracture decreases to A5 = 3.5% due to the formation of ϵ-martensite. The material strength of the steel alloyed with 6 mass% manganese (AC3 = 808 °C) strongly increases to Rm = 1975 MPa as a result of α-martensite and solid-solution strengthening by the element manganese. This steel possesses a higher elongation-to-fracture of A5 = 7%. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/srin.201800166
  • Tribological analysis of the TBM tool wear in soil from the view of material science: Tribologische Analyse des Abbauwerkzeugverschleißes von Tunnelvortriebsmaschinen in Lockergestein aus werkstofftechnischer Sicht
    Küpferle, J. and Röttger, A. and Theisen, W. and Alber, M.
    Geomechanik und Tunnelbau 11 (2018)
    Excessive wear to excavation tools leads to unplanned downtime, which results in additional costs. To predict tool wear during the planning phase of a project, an analysis of the acting tribological system is essential. The different influential factors and interactions have to be considered. The abrasive surface degradation of tools is investigated on a microstructural scale with nano-scratch experiments. Material-dependent variables like hard phase content and size are discussed in terms of the resulting wear-resistance. Using the RUB Tunnelling Device, abrasive surface degradation can be investigated on a more global scale and correlated with the acting tribological system, and the different tribological system components and influential factors can be considered. Copyright © 2018 Ernst & Sohn Verlag für Architektur und technische Wissenschaften GmbH & Co. KG, Berlin
    view abstract10.1002/geot.201700066
  • Adaptation of TiC hard particles properties and morphology in metal matrix composites by refractory elements
    Mohr, A. and Röttger, A. and Theisen, W.
    Key Engineering Materials 742 KEM (2017)
    High mechanical loads, corrosion, and abrasion decrease the lifetime of tools. One way to increase the wear resistance of tool materials can be achieved by adding hard particles to the metal matrix such as titanium carbide, which protect the softer metal matrix against abrasive particles. This material concept is designated as metal matrix composite (MMC). Ferro-Titanit® is such MMC material, possessing high wear and a simultaneously high corrosion resistance, for which reason this material is used in the polymers industry. The material concept is based on a corrosion-resistant Fe-base matrix with up to 45 vol% titanium carbide (TiC) as a hard particle addition to improve the wear resistance against abrasion. These TiC hard particles must be adapted to the present tribological system in terms of hardness, size and morphology. This study shows how the size and morphology of TiC hard particles can be influenced by the refractory element niobium (Nb). Therefore, the element Nb was added with 2 and 4 mass% to the soft-martensitic Ferro-Titanit® Grade Nikro128. The investigated materials were compacted by sintering, and the densified microstructure was further characterized by scanning electron microscopy (SEM), energy dispersive spectrometry (EDX), and optical image analyses. Furthermore, microstructure and properties of the compacted Nb-alloyed samples were compared to the reference material Nikro128. The results show that the addition of Nb influences the morphology, size and chemical composition of the TiC hard particle. These changes in the hard phase characteristics also influence the materials properties. It was shown that the phase niobium carbide (NbC) is formed around the TiC during the densification process, leading to a change in morphology and size of the TiC. © 2017 Trans Tech Publications, Switzerland.
    view abstract10.4028/
  • Characterization of recycled TiC and its influence on the microstructural, tribological, and corrosion properties of a TiC-reinforced metal matrix composites
    Mohr, A. and Röttger, A. and Theisen, W.
    Journal of Composite Materials 51 (2017)
    Ferro-Titanit® is a metal matrix composite (MMC) with a high wear and corrosion resistance. It contains TiC as hard particles on account of their high hardness, good corrosion resistance, and low density. This wear- and corrosion-resistant material is amenable to machining in the soft-annealed state, which gives rise to chips containing a large amount of the expensive TiC hard particles. Due to the cost of TiC, there is great interest in recycling the TiC from these chips so that it can be reused in the production of further Ferro-Titanit® materials. In this study, the recycled TiC [(Ti,X)C] is investigated with regard to morphology, particle size, chemical composition, and phases, and the results were compared to industrially produced TiC. In the next step, the (Ti,X)C was reused in the production of new Ferro-Titanit®. The Ferro-Titanit® reinforced with (Ti,X)C was also characterized with respect to microstructure, wear behavior, and corrosion resistance. Our investigations identified a change in the chemical composition of the TiC as a result of diffusion processes and a decrease in TiC particle size with respect to the initial state. The change in morphology and size of TiC during the recycling process influences the microstructure and the material behavior of the MMC containing recycled TiC. © 2017, © The Author(s) 2017.
    view abstract10.1177/0021998317692032
  • Corrosion behavior of 316L austenitic steel processed by selective laser melting, hot-isostatic pressing, and casting
    Geenen, K. and Röttger, A. and Theisen, W.
    Materials and Corrosion 64 (2017)
    This study investigated the corrosion behavior of grade 316L austenitic steel processed by casting, hot isostatic pressing (HIP), selective laser melting (SLM), and SLM+HIP. Electrochemical results showed that the SLM-densified specimen exhibited poorer corrosion resistance than specimens processed by casting and hot isostatic pressing in solution-annealed condition. Microstructural investigations revealed that the SLM-densified specimen had a fine-grained microstructure but comparatively higher porosity, which negatively influenced corrosion resistance. Additional HIP treatment further worsened corrosion resistance. The HIP process does not significantly reduce porosity compared to the SLM process, which can be attributed to the argon atmosphere used when manufacturing the SLM samples. Nevertheless, it was possible to reduce the crack density via HIP treatment and the formerly lamellar oxides underwent spheroidization. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/maco.201609210
  • Excavation tool concepts for TBMs – Understanding the material-dependent response to abrasive wear
    Küpferle, J. and Röttger, A. and Theisen, W.
    Tunnelling and Underground Space Technology 68 (2017)
    Wear of cutting tools for tunneling applications can lead to decreased advance rates and unscheduled downtimes that are associated with increased tunneling times and project costs. During the planning phase, wear of tools and their associated lifetime and replacement times are estimated on the basis of the ground that is to be excavated. However, from the viewpoint of materials technology, this procedure is insufficient because it is essential to take account of the interactions between tool material, ground, and the acting wear mechanisms on the microscopic scale, such as abrasion, fatigue, or forced fracture. The respective tool materials feature different tribomechanical properties and thus different wear mechanisms and rates that depend on the ground to be mined. Low wear rates can only be achieved using an optimized tool material concept that is adapted to the acting ground and the associated tribological system. This requires a comprehensive understanding of the wear behavior of the respective materials. This article focuses on the different, commonly used tool concepts and their microstructure. Interactions of the microstructure of these materials with the abrasive particles and the associated microwear mechanisms are analyzed. The results provide a deeper understanding of the wear process of excavation tools depending on the respective tool and the material concept. The discussed correlations are illustrated by results from the RUB Tunneling Device and nanoscratch tests, which are used to map the tribological TBM tool system on the macroscopic and microscopic scales. © 2017 Elsevier Ltd
    view abstract10.1016/j.tust.2017.05.013
  • Fatigue and surface spalling of cemented carbides under cyclic impact loading – Evaluation of the mechanical properties with respect to microstructural processes
    Küpferle, J. and Röttger, A. and Theisen, W.
    Wear 390-391 (2017)
    Cemented carbides used in excavation tools have to feature particular mechanical properties to withstand impact loads and abrasive degradation processes. It is assumed that a high fracture toughness is important to counteract brittle material failure due to forced rupture (supercritical loads) or material fatigue caused by subcritical impacts or cyclic loads. The fatigue-induced failure mechanism on a microstructural scale (evolution of fracture) in cemented carbides is discussed controversially in the literature. We are thus focusing on fatigue-initiated surface degradation of cemented carbide grades during ground excavation. Various cemented carbide grades were loaded with cyclic subcritical impact loads that lead to microstructural damage. The crack path was analyzed by scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). The number of endured load cycles is further correlated with the microstructural composition and the resulting mechanical properties of the tested grades. The results demonstrate that the commonly used correlation between resistance against fatigue load and fracture toughness KIC does not seem to be suitable for describing subcritical crack growth in cemented carbides due to cyclic loads that lead to surface spalling and microfatigue. The cemented carbide grades with the highest KIC values do not possess the highest resistance against surface spalling and thus the highest fatigue resistance. The influence of the microstructural properties (dWC, VCo, λCo) on the results has been shown. Furthermore, the correlation between fatigue sensitivity and crack path fractions was analyzed. © 2017 Elsevier B.V.
    view abstract10.1016/j.wear.2017.07.002
  • Investigation of the tribological behaviour of HS6-5-3 type tool steels during high-temperature sliding wear
    Walter, M. and Egels, G. and Boes, J. and Röttger, A. and Theisen, W.
    HTM - Journal of Heat Treatment and Materials 72 (2017)
    The wear behaviour of work roll materials is an important issue during the hot rolling process of metals. For this reason, the present study investigates the mechanical properties and the sliding wear behaviour of HS6-5-3 type high-speed steels (HSS) at elevated temperatures. Influences on the performance of HSS discussed are the microstructural constitution (as-cast and electro-slag remelted condition), the heat-treatment condition and the overall tribological system (C60 carbon steel and X5CrNi18-10 stainless steel counter-body materials). The results of the study show, how mechanical and tribological properties of HSS depend on these aspects and how a modification of the processing route can lead to improved high-temperature properties of HSS. As a main aspect the investigations show that, the formation of tribochemical wear layers during high-temperature sliding wear needs to be regarded. Tribochemical wear layers dominate the high-temperature wear behavior of steels. Therefore, the formation, characteristics and stability of tribochemical wear layers are analysed [1]. Copyright © 2017 Carl Hanser Verlag GmbH & Co. KG.
    view abstract10.3139/105.110315
  • Manufacturing of hard composite materials on Fe-base with oxide particles
    Brust, S. and Röttger, A. and Kimm, J. and Usta, E. and Theisen, W.
    Key Engineering Materials 742 KEM (2017)
    Metal matrix composites (MMC) are often applied to tool surfaces to increase resistance to wear and tear. However, some matrix and particle materials such as Ni, Co, WC or TiC are expensive and partly classified as critical elements. With respect to tribo-mechanical properties, Fe-alloys reinforced with oxide particles are promising compound materials to produce wear-resistant MMC with low-cost and readily available materials. However, thus far the technical application of such MMCs is limited due to poor wettability of the oxides by Fe-base melts and an associated weak bonding between the oxide particles and the metal matrix phases. In this work two novel production techniques (namely pre-metallization and active sintering) are introduced, which improve the wettability and interfacial reactions between both materials and therefore enable supersolidus liquid-phase sintering (SLPS) of the MMC. For the first technique the oxide particles are pre-metallized by depositing a thin film of TiN on the surfaces. The second technique is called active sintering. For this technique the alloy design is adapted from active brazing, so that wettability of the oxide particles by the alloy-melt is increased. The resulting effects of these techniques are investigated using wetting and sintering experiments, and are analyzed with respect to the developed microstructures and interfacial reactions between the oxide particles and the metallic phases. © 2017 Trans Tech Publications, Switzerland.
    view abstract10.4028/
  • Mechanical properties of AlXFeY intermetallics in Al-base coatings on steel 22MnB5 and resulting wear mechanisms at press-hardening tool steel surfaces
    Windmann, M. and Röttger, A. and Hahn, I. and Theisen, W.
    Surface and Coatings Technology 321 (2017)
    Hard and brittle intermetallic AlXFeY phases formed in the Al-base coating applied on high-strength steel 22MnB5 promote strong wear of press-hardening tools during forming and quenching (approx. 800–100 °C). In this study, bulk materials of the intermetallic phases Al13Fe4, Al5Fe2, Al2Fe, and AlFe were produced by remelting stoichiometric powder mixtures. These were then used for mechanical and wear investigations. We found that the dominating wear mechanisms on the tool steel surfaces are strongly influenced by the temperature and depend on the mechanical properties of the respective intermetallic phases. Phases of type Al13Fe4, Al5Fe2, and Al2Fe possess a high hardness of 850–1090 HV0.5 and a low fracture toughness of 0.9–1.6 MPa √ m at room temperature, whereas the AlFe phase has a much lower hardness (520 HV0.5) and a higher fracture toughness (26 MPa √ m ). The hardness of all phases decreases with increasing temperature. At high temperatures (500–800 °C), the intermetallic phases lead mainly to adhesive wear of the tool steel surfaces. At lower temperatures, also abrasive wear occurs due to delamination of hard and brittle intermetallic particles. We found that abrasive wear of the tool steel surface could be decreased by adapting the phase composition in the Al-base coating. © 2017
    view abstract10.1016/j.surfcoat.2017.04.075
  • Microstructure and mechanical properties of the heat-affected zone in laser-welded/brazed steel 22MnB5–AA6016 aluminum/AZ31 magnesium alloy
    Windmann, M. and Röttger, A. and Kügler, H. and Theisen, W.
    Journal of Materials Processing Technology 247 (2017)
    The martensitic microstructure of the steel 22MnB5 was tempered during laser welding/brazing. The strength of the HAZ greatly decreased from 1500 MPa to 800–1100 MPa, depending on the heat input. The lowest strength always occurred in the area with the highest heat input directly beside the welding zone. The strength of the aluminum alloy was slightly reduced from 233 MPa to 212 MPa. The strongest decrease in the strength did occur in the area with a critical temperature range of 400–500 °C due to the coarsening of GP zones. The short heat input in the laser welding/brazing process did not lead to a significant change in the material strength and microstructure in the HAZ of the AZ31 magnesium alloy. © 2017 Elsevier B.V.
    view abstract10.1016/j.jmatprotec.2017.04.008
  • An investigation of the tribological behaviour of high-speed tool steels at elevated temperatures
    Walter, M. and Egels, G. and Boes, J. and Röttger, A. and Theisen, W.
    HTM - Journal of Heat Treatment and Materials 71 (2016)
    The tribological behaviour of work roll materials is a key issue during hot rolling process of metals. The characteristics of the material (hardness and wear resistance) at elevated temperatures are of great interest for many industrial applications. The study investigates the mechanical properties and the sliding wear behaviour of HS 6-5-3-5 (HSS) high-speed tool steel, which is a common work roll material of the intermediate and finishing stands of hot rolling manufacturing lines. Experimental analysis focuses on the mechanical properties of steel HS 6-5-3-5 at elevated temperatures and on the microstructural surface changes of this material during metallic sliding wear. The results give an overview about the absolute hardness value of HS 6-5-3-5 at elevated temperatures and its evolution at constant operating temperatures. To conclude interdependencies between mechanical properties, microstructure and wear behaviour at elevated temperatures, results are discussed and connected with wear investigations. Findings reveal that high temperature wear behaviour is mainly dependent on the formation of a tribochemical wear layer on the wear bodies' surfaces. Layers suppress direct metallic contact and change the characteristics of the tribological system. Discussed issues of high temperature sliding wear are the formation and stability of tribochemical wear layers, their connection to and support by the bulk material, as well as the fracturing and damage of the layer-bulk-material compound. © Carl Hanser Verlag GmbH & Co. KG.
    view abstract10.3139/105.110290
  • Comparison of microstructure and mechanical properties of 316 L austenitic steel processed by selective laser melting with hot-isostatic pressed and cast material
    Röttger, A. and Geenen, K. and Windmann, M. and Binner, F. and Theisen, W.
    Materials Science and Engineering A 678 (2016)
    Besides the chemical composition, the manufacturing route primarily determines a material's properties. In this work, the influence of the manufacturing process of the 316 L grade austenitic steel on the microstructure and the resulting material properties were investigated. Thus, the microstructure and mechanical properties of cast and solution annealed, as well as steel powder densified by hot-isostatic pressing (HIP), selective laser melting (SLM) and SLM+HIP, were compared. A SLM parameter study illustrates that the porosity of SLM-densified specimens can be reduced with direction of a higher exposure time and a smaller point distance. With an additional treatment by HIP, the porosity scarcely changes, while cracks are reduced. The mechanical properties were investigated depending on the manufacturing process, and the influence of the sample build up by SLM was examined. High mechanical values have been obtained; in particular, the yield strength in the SLM-densified condition is much higher than in cast or HIP condition, as a result of the smaller grain size. © 2016 Elsevier B.V.
    view abstract10.1016/j.msea.2016.10.012
  • High-temperature stability and interfacial reactions of Ti and TiN thin films on Al2O3 and ZrO2
    Brust, S. and Röttger, A. and Theisen, W.
    Surface and Coatings Technology 307 (2016)
    Metallic thin films are used in many applications to modify ceramic surfaces. However, during subsequent processing, chemical interactions may change the properties of the coating. In addition, differences in thermal expansion can lead to delamination of the coating. In this study, titanium and titanium nitride thin films were deposited via physical and chemical vapor deposition (PVD and CVD, respectively) on alumina- and yttria-stabilised zirconia substrates, before being heat-treated at 1200 °C or 1500 °C in static argon atmosphere and analysed via SEM, EDS and XRD to investigate the effect of temperature on the thin films. It was shown that the chemical interactions between TiN and both Al2O3 and ZrO2 are weak. However, partial delamination of the TiN coating on alumina was observed after both annealing temperatures. The TiN coating on zirconia remained adherent. In contrast, the Ti coatings underwent a transformation to cubic TiO on both oxide substrates. This was due to partial reduction of the ZrO2 to ZrO2 − x and dissolution of the Al2O3, which leads to a Ti3Al0.9O1.1 interlayer. The TiO coating which formed remained adherent on the alumina at both annealing temperatures, but delaminated from the ZrO2 substrate after annealing at 1500 °C. © 2016 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2016.08.060
  • Laser beam welding of magnesium to coated high-strength steel 22MnB5
    Windmann, M. and Röttger, A. and Kügler, H. and Theisen, W.
    International Journal of Advanced Manufacturing Technology 87 (2016)
    Magnesium alloy AZ31 was laser-welded to AlSi10Fe3-coated high-strength steel 22MnB5. The surface of the 22MnB5 steel sheet was treated by sandblasting before welding. Laser welding of magnesium with steel was found to be a welding-brazing process, due to the large difference in the melting temperatures of steel and magnesium. The liquid magnesium was wetted on the solid steel surface, which thus was a brazing process on the steel side. AZ61 magnesium welding filler was used to improve adhesion of the liquid magnesium to the steel surface. Furthermore, the suitability of using a flux was determined. Welded steel/magnesium joints with a high tensile strength of 2680–3090 N (178–213 MPa) were produced using a flux and inductive preheating of the conditioned steel surface. Under tensile loading, the welded joints did not fail at the steel/magnesium interface, but in the AZ61 magnesium welding filler. Metallurgical bonding at the magnesium/steel interface was attributed to the formation of a thin Al-Fe-rich layer, which is due to the alloyed aluminum (6 mass%) in the magnesium welding filler. © 2016, Springer-Verlag London.
    view abstract10.1007/s00170-016-8682-5
  • Mechanism of the Fe3(B,C) and Fe23(C,B)6 solid-state transformation in the hypoeutectic region of the Fe-C-B system
    Lentz, J. and Röttger, A. and Theisen, W.
    Acta Materialia 119 (2016)
    This study investigates the microstructural mechanisms involved in the solid-state transformation of the Fe3(B,C) → Fe23(C,B)6 phases in the hypoeutectic region of the iron-carbon-boron (Fe-C-B) system. We analyzed the influence of different initial microstructural characteristics on the Fe3(B,C) → Fe23(C,B)6 transformation with regards to the matrix phase, matrix C content, B/(C + B) ratio, and agglomeration of the parental Fe3(B,C) phase. We performed thermodynamic calculations using the CALPHAD method, validated by laboratory melts with varying B/(B + C) ratios. These laboratory melts were then microstructurally characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and wavelength-dispersive X-ray spectroscopy (WDS). We particularly focused on solid-state transformation of borides and carboborides of type M3(C,B) and M23(C,B)6 in the hypoeutectic region of the ternary system Fe-C-B, investigated via both in situ and ex situ XRD measurements. It was found that the solid-state transformations are influenced by enriched B inside the eutectic structure, a result of solidification. This increased B content is not reduced in solid state due to the kinetic limitations of B and C inside the hard-phase structure. Thus phase stability is subject to local equilibria depending on the local C and B concentration of the hard-phase structure. In this process the Fe23(C,B)6 phase also forms a shell-like structure surrounding the Fe3(B,C) and Fe2B phases. © 2016 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2016.08.009
  • Removal of oxides and brittle coating constituents at the surface of coated hot-forming 22MnB5 steel for a laser welding process with aluminum alloys
    Windmann, M. and Röttger, A. and Kügler, H. and Theisen, W.
    Surface and Coatings Technology 285 (2016)
    The surface of a press-hardened steel 22MnB5 coated with Al-base (AlSi10Fe3) and Zn-base (ZnNi10) was conditioned by a pulse laser and by sandblasting to remove undesirable oxides and brittle phases. Oxides formed on coating surfaces counteract the wettability of welding filler during a welding or brazing process. Furthermore, welding and brazing joints of 22MnB5 coated with aluminum alloys failed along the brittle intermetallic phases in the coating under a low mechanical load. Treated 22MnB5 surfaces were analyzed microscopically, and the phase compositions were investigated by synchrotron diffraction measurements. It was found that brittle phases could be locally removed by laser ablation; however, high laser energies led to remelting and oxidation of the coating surface. In contrast, sandblasting homogenously removed oxides and brittle intermetallic phases. Surface-treated 22MnB5 steel sheets were joined to AA6016 aluminum sheets by laser welding, and the strength of the weldment was determined by tensile tests. The measured mechanical strength of the aluminum/steel joints was 210-230. MPa. Failure of the weldments under tensile loading occurred within the aluminum sheet, away from the steel surface/welding filler interface if brittle coating components and oxides were removed homogenously. © 2015 Published by Elsevier B.V.
    view abstract10.1016/j.surfcoat.2015.11.037
  • The RUB Tunneling Device - A newly developed test method to analyze and determine the wear of excavation tools in soils
    Küpferle, J. and Röttger, A. and Theisen, W. and Alber, M.
    Tunnelling and Underground Space Technology 59 (2016)
    The efficiency of a tunneling project is mainly associated with the abrasivity of the acting soil and the wear resistance of the cutting tools. Heavy wear can dull the cutting tool, which negatively affects the penetration rate and therefore the efficiency of a tunneling process. Completely worn tools with a short service life have to be replaced by newer ones. This circumstance results in unplanned machine shutdowns and higher maintenance costs. It is thus of high economic as well as technical interest to obtain a deeper understanding of soil/cutting tool interactions during tunneling. To meet this challenge, a large number of different testing devices to estimate the abrasivity of soils have been developed within the last two decades. An innovative and promising experimental setup is presented in this work. A horizontal implementation offers the possibility of simulating a tunneling process as well as the tribological system of a TBM tool. The interactions between all system components can be mapped and analyzed in detail. This method offers a unique setup, which allows wear prediction of TBM tools in a homogeneous soil with project-specific parameters (soil composition/condition, soil mechanics, tool material and machine/tunneling data). © 2016 Elsevier Ltd.
    view abstract10.1016/j.tust.2016.06.006
  • Assessment of the LCPC abrasiveness test from the view of material science
    Küpferle, J. and Röttger, A. and Alber, M. and Theisen, W.
    Geomechanik und Tunnelbau 8 (2015)
    The abrasiveness of crushed rock is determined in the design phase of a tunnelling project to estimate the wear on excavation and boring tools, using for example the LCPC abrasiveness test. Considered from the point of view of material science, there are problems with the validity of such an index value since important tribological factors, like for example the internal structure of the sample impeller (size, phase composition) are not considered in the current testing standard (AFNOR P18-579). In this investigation, LCPC tests were performed with impellers of various steels of the same and different hardnesses against two abrasives (Mohs hardness 7, 9) in order to determine the influence of the internal structure and the associated tribo-mechanical properties on the LCPC index value (A<inf>BR</inf>). In addition to the material hardness required by the standard (60 to 75 HRB ≈ 105 to 140 HV 10 according to DIN EN ISO 18265-2013), sample materials with higher hardness values were also used in order to extend the basis of the LCPC abrasiveness index to these practically applicable material groups. The results make clear that awareness has to be raised regarding the materials used for the LCPC test. An abrasive can be classified differently just by using various materials of the same hardness. In order to obtain reliable and reproducible index values for the abrasiveness of crushed rock, it is necessary to consider the influence of materials in future recommendations for the LCPC. © 2015 Ernst & Sohn Verlag für Architektur und technische Wissenschaften GmbH & Co. KG, Berlin.
    view abstract10.1002/geot.201500002
  • Boron-alloyed Fe-Cr-C-B tool steels - Thermodynamic calculations and experimental validation
    Röttger, A. and Lentz, J. and Theisen, W.
    Materials and Design 88 (2015)
    This study focuses on the development of boron-alloyed tool steels. The influence of Cr additions from 0 to 10mass% on microstructural changes were investigated for a constant metalloid content (C+B=2.4mass%). In the first step, thermodynamic calculations were performed to map the quaternary Fe-Cr-C-B system. In the second step, thermodynamic calculations were validated with laboratory melts that were investigated with respect to the microstructure and phase composition. The results of thermodynamic calculations correspond to real material behavior of Fe-Cr-C-B alloys. Furthermore, the influence of chromium on hard phase formation was investigated by means of phase analysis methods, X-ray diffraction (XRD), and energy dispersive spectrometry (EDS). Nanoindentation was used to determine hard phase properties (hardness, Young's modulus). It was shown that chromium promotes the formation of M2B-type borides. An increase in the Cr content within the M2B phase led to a transformation from the tetragonal structure into an orthorhombic structure. This transformation is accompanied by an increase in hardness and in the Young's modulus. In contrast, Cr also promotes the formation of Cr-rich carboborides of type M23(C,B)6. However, an increased Cr content within the M23(C,B)6 phase is not associated with an increase in hardness or elastic modulus. © 2015 Published by Elsevier Ltd.
    view abstract10.1016/j.matdes.2015.08.157
  • Laser beam welding of aluminum to Al-base coated high-strength steel 22MnB5
    Windmann, M. and Röttger, A. and Kügler, H. and Theisen, W. and Vollertsen, F.
    Journal of Materials Processing Technology 217 (2015)
    The microstructure of aluminum-Al-coated steel laser beam welding joints was analyzed with respect to the welding energy. Quantitative and qualitative analysis of the welding microstructure were used to measure the weld width as well as the thickness of the resulting intermetallic layer at the 22MnB5/aluminum interface in relation to the welding parameters. Weldability of Al-coated steel could be improved by removing brittle coating particles and oxides on the steel surface by sandblasting. Adhesion of aluminum filler material to the 22MnB5 steel sheet could be enhanced by inductive preheating of the steel surface during laser welding. This produced welded 22MnB5/aluminum joints that exhibited a linear mechanical resistance of 220 MPa and which failed away from the brittle intermetallic layer on the aluminum side under a tensile load. The shear strength of the intermetallic layer on the 22MnB5/aluminum interface was evaluated to 74 ± 21 MPa. © 2014 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.jmatprotec.2014.10.026
  • Solidification and phase formation of alloys in the hypoeutectic region of the Fe-C-B system
    Lentz, J. and Röttger, A. and Theisen, W.
    Acta Materialia 99 (2015)
    In this work, alloys from the hypoeutectic iron-rich region of the iron-carbon-boron (Fe-C-B) system were investigated with respect to the solidification and the phase formation. Laboratory melts with a constant carbon content of 0.6 mass% and boron contents of 0.2 mass%, 0.6 mass%, and 1.8 mass% were fabricated and metallographically examined. In addition the microstructures were investigated by CALPHAD method in the state of equilibrium and by multiphase-field (MPF) method to reproduce the non-equilibrium process of the technical solidification. The results were analyzed with respect to the effect of boron on the solidification paths, microstructural crystallization processes as well as the morphological and chemical characteristics of the solidified phases. The investigated alloys undergo primary crystallization of austenite (γ-Fe). Due to the low solubility of B in the primary phase γ-Fe, B is strongly segregated in the melt and the solidification paths are deviated to high B contents. Therefore, as the B content increases, the eutectic solidification sequence starts with the B-rich Fe<inf>2</inf>B phase and continues with the formation of the B-rich Fe<inf>3</inf>(B,C) phase in the latter process. The B content of the melt thus decreases during the eutectic reaction, and the eutectic Fe<inf>3</inf>(B,C) phase exhibits a decreasing B gradient in the direction of growth. Consequently, the low-melting phase of the Fe-C-B system is the Fe<inf>3</inf>(B,C) phase with a low B content and a composition closest to its low-melting B content of 14.10 at.% B. Increasing B/(C + B) ratios of the alloy composition raise the average B content of the Fe<inf>3</inf>(B,C) phase (up to >20 at.% B) and hence at the same time increase the solidus temperature of the alloy. These findings revealed consistency with experimental results for chemical composition (WDX), phase analysis (diffraction with synchrotron radiation, EBSD), and thermal analysis (DTA). © 2015, Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2015.07.037
  • Formation of intermetallic phases in Al-coated hot-stamped 22MnB5 sheets in terms of coating thickness and Si content
    Windmann, M. and Röttger, A. and Theisen, W.
    Surface and Coatings Technology 246 (2014)
    AlSiFe coatings with differing thicknesses and Si contents were applied to steel sheets by hot dipping. The steel sheets were austenitized at TAUS=920°C for different dwell times and then quenched in water. Phase formation as a function of coating thickness and Si content at the steel substrate/coating interface was investigated by ex-situ phase analysis with synchrotron radiation and by electron backscatter diffraction (EBSD). X-ray diffraction (XRD) and EBSD investigations confirmed the formation of AlFe-rich intermetallics at the steel/coating interface as a result of a strong diffusion of the elements Al and Fe. Within the first minute, Fe diffusion into the partially melted Al-base coatings promotes the formation of intermetallics of type Al8Fe2Si, Al13Fe4, and Al5Fe2. After the coating has transformed completely into Al-Fe intermetallics, Al diffusion into the steel substrate becomes more pronounced, thus reducing the Al content in the Al-Fe intermetallics and promoting formation of the phases of type Al2Fe and AlFe in the coating and formation of an Al-rich bcc layer in the steel substrate. The transformation kinetics of the resulting Al-, Fe-rich intermetallics are influenced by the coating thickness and the chemical composition of the Al-base coating. On the one hand, faster saturation of Fe in the Al-base coating is promoted by a shorter diffusion path and therefore by a thinner coating thickness. Otherwise, Si influences the diffusivity of the elements Al and Fe in the Al-, Fe-rich intermetallics and promotes the formation of Si-richer intermetallics, which then act as nuclei for Fe-richer intermetallics. © 2014 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2014.02.056
  • Recycling of metallic chips by electro-discharge sintering
    Mohr, A. and Röttger, A. and Windmann, M. and Theisen, W.
    Materialwissenschaft und Werkstofftechnik 45 (2014)
    Composite material Ferro-Titanit® is produced powder-metallurgical by Deutsche Edelstahlwerke GmbH (DEW) and is commonly used for wear and corrosion resistant component parts. Materials properties can be attributed to the microstructure which consists of a corrosion resistant metallic matrix and a huge amount of approx. 50 vol.% of hard Ti-monocarbides. Although Ferro-Titanit® possesses a high amount of hard particles, the material can be machined by turning and drilling in solution annealed condition. Due to the alloying content (Mo, Cr, TiC) of Ferro-Titanit®, there is a high motivation to recover those elements by a recycling process of the chips, thus expensive and limited resources can be saved. On idea of a recycling process can be found in the redensification of those chips by electro discharge sintering (EDS). In this work, chips of the material Ferro-Titanit® were densified by EDS technique and the resulting microstructure was investigated by optical and scanning electron microscopy. Furthermore, microstructure and hardness of the EDS densified specimens was discussed with regard to the microstructure of conventionally sintered Ferro-Titanit®-samples in laboratory conditions. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/mawe.201400266
  • Phase formation at the interface between a boron alloyed steel substrate and an Al-rich coating
    Windmann, M. and Röttger, A. and Theisen, W.
    Surface and Coatings Technology 226 (2013)
    Al-base coating (AlSi10Fe3) was applied to a steel substrate (22MnB5) by hot dipping. The coated steel substrates were austenitized at 920. °C for several dwells, and phase formation at the steel/coating interface was investigated by means of ex-situ phase analysis with synchrotron radiation and EBSD. Phase identification by EBSD and XRD confirmed the formation of Al-rich intermetallics during austenitization. Increasing the dwell time led to Fe diffusion into the Al-base coating as well as Al diffusion into the substrate. As a result of the diffusion processes, Al-rich intermetallics in the coating transformed to more Fe-rich intermetallics. Simultaneously, Al diffusion into the substrate changed the microstructure of the steel substrate near the coating interface. Formation of FeAl intermetallics and thus the mechanical properties of the AlSi10Fe3 coating can be influenced by heat treatment. Higher austenitization temperatures and longer dwell times support the formation of more ductile FeAl intermetallics but also lead to grain growth; thus having a negative effect on the mechanical properties of the steel. © 2013 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2013.03.045
  • HVOF spraying of Fe-based MMC coatings with in situ formation of hard particles by hot isostatic pressing
    Röttger, A. and Weber, S.L. and Theisen, W. and Rajasekaran, B. and Vaßen, R.
    Journal of Thermal Spray Technology 21 (2012)
    Thick (2-3 mm) Fe-base coatings with admixed ferrotitanium (Fe 30Ti 70) were applied to austenitic steel by a high-velocity oxy-fuel process (HVOF). Hot-isostatic pressing (HIP) was carried out to the decrease porosity and to increase the material strength, wear resistance, and adhesive bond strength of the deposited coating to the substrate material. SEM and XRD investigations confirmed the formation of hard titanium carbide (TiC) particles during HIP treatment as a result of strong carbon diffusion out of the metal matrix and into the Fe 30Ti 70 particles. The mechanical and wear properties of the densified coatings were investigated by means of shear tests, hardness measurements, and abrasive wear tests. A comparison of the coatings in the as-sprayed and the HIPed state showed a large increase in the wear resistance due to in situ TiC formation. © ASM International.
    view abstract10.1007/s11666-012-9736-3
  • Influence of post-treatment on the microstructural evolution of thermally sprayed Fe-base MMC containing TiC and Cr 3C 2
    Röttger, A. and Weber, S.L. and Theisen, W.
    Surface and Coatings Technology 209 (2012)
    In this study, ledeburitic cold-work tool steel X220CrVMo13-4 (AISI D7) was admixed with TiC and Cr 3C 2 and the resulting feedstock powder was deposited on an austenitic substrate X5CrNiMo17-12-2 (AISI 316L) by the High Velocity Oxy Fuel (HVOF) process. The coatings exhibited low porosity and good adhesion to the substrate material. Further improvement of the material and tribological properties of the deposited coatings was achieved by hot isostatic pressing (HIP) and heat treatments. HIP treatment, which was carried out without encapsulation, led to a fully dense MMC coating. Further heat treatment to secondary peak hardness of the ledeburitic cold-work tool steel increased the hardness and thus the tribological properties. Diffusion reactions and the embedding behavior of the hard particles in the steel matrix were investigated with respect to the post-treatment processes by means of diffusion calculations and microstructural examinations. The results indicated that diffusion processes at the hard particle/metal matrix interface exert a strong influence on the embedding behavior of TiC and Cr 3C 2 particles in the coating microstructure. During HIP and heat treatment, new phases with a larger amount of metallic bonding were formed, thus improving particle adhesion to the metal matrix. © 2012 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2012.08.003
  • Supersolidus liquid-phase sintering of ultrahigh-boron high-carbon steels for wear-protection applications
    Röttger, A. and Weber, S. and Theisen, W.
    Materials Science and Engineering A 532 (2012)
    Powder metallurgy (PM) represents an alternative to conventional casting processes for the production of wear-resistant materials. PM hard alloys for wear-protection applications feature both higher strength and fracture toughness compared to cast hard alloys due to their more finely grained microstructure. However, densification by hot-isostatic pressing (HIP), the conventional PM-compaction method, is relatively expensive and thus partially counteracts low-cost processing. To increase the economic efficiency of the processing route, supersolidus liquid-phase sintering (SLPS) was investigated. In addition, expensive Ni- and Co-base hard alloys were substituted by boron-rich Fe-base hard-facing alloys.In this study, three ultrahigh-boron hard-facing alloy powders were densified by SLPS and HIP. The sintering temperatures were optimized by means of sintering experiments that were supported by thermodynamic calculations. Both densification states were investigated and compared with respect to the microstructure and the tribological and mechanical properties of the compacted hard-facing alloys. It was shown that the mechanical and tribological properties are strongly influenced by the microstructure. Although the microstructure is affected by the chemical composition, it can also be adapted by the densification process. SLPS-densified hard-facing alloys have a coarse microstructure that imparts not only a high wear resistance but also a detrimental effect on the mechanical properties. © 2011 Elsevier B.V.
    view abstract10.1016/j.msea.2011.10.118
  • Diffusion and phase transformation at the interface between an austenitic substrate and a thermally sprayed coating of ledeburitic cold-work tool steel
    Röttger, A. and Weber, S. and Theisen, W. and Rajasekeran, B. and Vaßen, R.
    Steel Research International 82 (2011)
    An overlay coating against wear or corrosion on components is required for various technical applications. Thermal spraying is a well-established and near-net-shape deposition method. In this work, high-velocity oxy-fuel spraying of two different ledeburitic cold-work tool steels was employed to produce wear-resistant Fe-base coatings on a stainless steel substrate. This work focuses on the investigation of diffusion processes across the coating/substrate interface. Specimens were heat-treated for different dwell times and then analyzed by means of EBSD, XRD, OM, as well as SEM. Results of phase formation and diffusion profiles were compared with equilibrium and diffusion calculations obtained with ThermoCALC® and DICTRA®. The influence of diffusion processes across the coating/substrate interface on the mechanical properties, such as adhesive bond strength and hardness, was investigated by shear tests and microhardness profiles. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/srin.201000199
  • Hot Direct Extrusion of Abrasion Resistant Fe-Base Metal Matrix Composites-Interface Characterization and Mechanical Properties of Co-Extruded Layered Structures
    Silva, P.A. and Weber, S. and Karlsohn, M. and Röttger, A. and Theisen, W. and Reimers, W. and Pyzalla, A.R.
    Friction, Wear and Wear Protection: International Symposium on Friction, Wear and Wear Protection 2008 Aachen, Germany (2011)
    view abstract10.1002/9783527628513.ch90
  • Hot Direct Extrusion of Abrasion Resistant Fe-Base Metal Matrix Composites-Microstructure and Wear Properties
    Karlsohn, M. and Röttger, A. and Silva, P.A. and Weber, S. and Pyzalla, R. and Reimers, W. and Theisen, W.
    Friction, Wear and Wear Protection: International Symposium on Friction, Wear and Wear Protection 2008 Aachen, Germany (2011)
    This paper discusses hot direct extrusion as a novel method to produce long cylindrical rods of a metal-matrix composite (MMC) and co-extruded rods of a low alloyed steel substrate cladded with a MMC layer. The excellent abrasive wear resistance of MMC is based on hard phase and iron powder mixtures, which can be tailored to the respective abrasive environment. This production process combines sintering of capsuled powder blends and subsequent pressing of the capsules through an extrusion die to produce an (almost) completely densified MMC. In contrast to HIP cladding (hot isostatic pressing), the standard method of producing powder metallurgical components (PM) and PM-layers, the simpler capsule technique is less expensive. The extruded MMC were investigated with respect to their microstructure, wear properties and the microstructural and mechanical properties of the interface of substrate and MMC layer. © 2009 Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstract10.1002/9783527628513.ch17
  • Mechanical properties of thermally sprayed Fe based coatings
    Röttger, A. and Weber, S. and Theisen, W. and Rajasekeran, B. and Vassen, R.
    Materials Science and Technology 27 (2011)
    An additional coating against wear or corrosion on component parts is required for many applications. These coatings protect the substrate material against external influences, thus increasing the economic lifetime of the component. Coating processes such as build-up welding and thermal spraying are well established and commonly used. The thermal spray process, in particular, permits deposition of metals, ceramics, or cermets materials to produce near net shape coatings on complex surface geometries. However, commonly used coating materials suffer from high raw material costs, thus decreasing the cost effectiveness of the coating process. Fe based materials are low priced and possess noteworthy mechanical properties; they thus provide the possibility of substituting the expensive Ni and Co based materials commonly used for thermal spray processes. In this work, 2 mm thick high velocity oxyfuel sprayed Fe based coatings in the as sprayed and thermally sprayed and hot isostatic pressed condition were investigated with respect to their mechanical and wear properties. Additionally, the fracture surface was investigated by scanning electron microscopy to characterise the fracture behaviour. It could be demonstrated that the substrate and the heat treatment have the greatest impact on the shear strength of thermally sprayed cold work tool steel. It is shown that the substrate materials as well as the heat treatment are promoting diffusion processes across the interface between the coating and the substrate. Hence, a material integrated bond is formed. The microstructures of the thermally sprayed coatings become more important regarding the mechanisms of failure of the four point bending tests. The material strength is influenced by quenching and tempering and the specimen deflection is influenced by diffusion reactions induced by hot isostatic pressing treatment. The thermally sprayed coatings in the as sprayed condition feature the highest wear resistance due to their hardness. © 2011 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute.
    view abstract10.1179/1743284710Y.0000000002
  • Wear protection by Fe-B-C hard phases
    Berns, H. and Saltykova, A. and Röttger, A. and Heger, D.
    Steel Research International 82 (2011)
    Four as-cast iron alloys with (mass%) 1B + 3C, 2B + 2C, 3B + 1C and 4B + 1C were investigated in respect to their microstructure by optical and scanning electron microscopy with EDX and ESBD and by microprobe analysis. The microhardness of eutectic Fe3(C,B) increased with the B/C ratio and raised the resistance to scratching by Flint particles. The low melting range of the castings was used for the powder metallurgical production of a metal matrix composite by liquid phase sintering of admixed hard particles in an Fe-B-C base material. Abrasive wear tests showed that the eutectic carborides in the base material raised the wear resistance even more than the admixed particles. Copyright © 2011 WILEY-VCH Verlag GmbH &amp; Co. KGaA, Weinheim.
    view abstract10.1002/srin.201000255
  • Coating of high-alloyed, ledeburitic cold work tool steel applied by HVOF spraying
    Rajasekaran, B. and Mauer, G. and Vassen, R. and Röttger, A. and Weber, S. and Theisen, W.
    Journal of Thermal Spray Technology 19 (2010)
    This study demonstrates the processing of a cold work tool steel (X220CrVMo13-4) coating using HVOF spraying. The coating formation was analyzed based on microstructure, phase, hardness, porosity, oxidation, and adhesion characteristics. An online diagnostic tool was utilized to find out the in-flight characteristics of powder such as temperature and velocity during the coating process to identify the influencing parameters to achieve dense cold work tool steel coatings with low oxidation. The influence of powder size, process parameters, and in-flight characteristics on the formation of cold work tool steel coatings was demonstrated. The results indicated that thick and dense cold work tool steel coatings with low oxidation can be obtained by the selection of appropriate powder size and process parameters. © ASM International.
    view abstract10.1007/s11666-009-9456-5
  • Development of cold work tool steel based-MMC coating using HVOF spraying and its HIP densification behaviour
    Rajasekaran, B. and Mauer, G. and Vaßen, R. and Röttger, A. and Weber, S. and Theisen, W.
    Surface and Coatings Technology 204 (2010)
    The aim of the present study is to develop a Fe-based metal matrix composite (MMC) coating using high velocity oxy-fuel spraying (HVOF) process. A ledeburitic high alloyed cold work tool steel (X220CrVMo13-4) and NbC with an average size of 2μm at different volume fractions have been considered as metal matrix and hard particles respectively. MMC coatings were deposited on austenitic stainless substrates and the coatings were subsequently densified by hot isostatic pressing (HIP) with and without encapsulation. Microstructural analysis of the as-sprayed and HIPed coatings were characterized by SEM and XRD methods. Results showed that the feedstock preparation involving fine NbC was an influencing factor on the coating deposition. A relatively homogeneous dispersion of fine NbC up to 30. vol.% in cold work tool steel matrix was possible using optimized HVOF spraying. Besides, HVOF spraying and its subsequent HIP treatment induced significant microstructural and phase changes in the MMC coatings. The study showed the potential of HVOF spraying for the development of steel based MMC coatings and its subsequent densification can be achieved by HIP process with and without encapsulation. © 2010 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2010.05.001
  • Thick tool steel coatings using HVOF spraying for wear resistance applications
    Rajasekaran, B. and Mauer, G. and Vaßen, R. and Röttger, A. and Weber, S. and Theisen, W.
    Surface and Coatings Technology 205 (2010)
    This paper deals with the processing of thick cold work tool steel coating using high velocity oxy-fuel (HVOF) spraying process. A full factorial experimental design was established to identify the influencing process parameters on the formation of dense coating with low oxidation. Microstructural analysis of the coating was carried out using optical, SEM and XRD techniques. Cold work tool steel coatings with a thickness up to 2 mm were developed on bond coated low carbon steel substrates for wear resistance evaluation. A pin on disc test was performed to examine the wear resistance of thick cold work tool steel coatings on different types and sizes of abrasive papers. The wear results were compared with the wear resistance of a standard high speed steel pin. The abrasive wear resistance of cold work tool steel coated pins was found to be superior against soft and fine abrasive papers than the standard high speed steel. Besides, the performance of the coated pins against hard and coarser abrasive papers was found to be similar to standard high speed steel. The study showed the potential of HVOF spraying on the development of thick cold work tool steel coatings for wear resistance applications. © 2010 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2010.09.041
  • alloys

  • laser sintering

  • mechanical properties

  • microstructure

  • steel

  • tribology

  • wear and friction

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