Prof. Dr.-Ing. Jan T. Sehrt

Hybrid Additive Manufacturing
Ruhr-Universität Bochum

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  • 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
  • Investigation of the in situ thermal conductivity and absorption behavior of nanocomposite powder materials in laser powder bed fusion processes
    Pannitz, O. and Lüddecke, A. and Kwade, A. and Sehrt, J.T.
    Materials and Design 201 (2021)
    One of the AM processes for direct manufacturing of metallic components is powder bed fusion of metals using a laser beam system (PBF-LB/M), in which metallic powders are molten and solidified in a layer upon layer manner by a focused laser beam. In recent years, the focus has initially been on increasing the efficiency of the systems itself. However, the modification of standard AM metal alloys using nanoparticles offers the possibility to improve the PBF-LB/M-process concerning its process efficiency and actual densification and thermal conductivity. In this work, a methodology for an in-situ investigation of the thermal conductivity as well as the reflectance behavior of metallic powder materials during the PBF-LB/M-process is established. The powder material stainless steel 1.4404 was coated with different nanoparticles (few-layer graphene (FLG), silicon carbide (SiC)) and processed in a standardized build process. As a result, the reflectance rate of all modified materials could be increased. Besides, the thermal conductivity of the material is attested to be a decisive and influencing factor for the quality of the final component. Thus, an improved relative density was achieved using the FLG/1.4404 and SiC/1.4404 (1 vol%) due to the increased thermal conductivity of the material. Also significant defects in the cross section were visible at SiC/1.4404 (4 vol%). © 2021 The Authors
    view abstract10.1016/j.matdes.2021.109530
  • Powder properties and flowability measurements of tailored nanocomposites for powder bed fusion applications
    Lüddecke, A. and Pannitz, O. and Zetzener, H. and Sehrt, J.T. and Kwade, A.
    Materials and Design 202 (2021)
    The modification of metal alloy powders by coating with nanoparticles offers the possibility to improve additive manufacturing processes, in particular the powder bed fusion of metals with laser beams (PBF/LB-M), from the material side of view. Subsequently, component qualities including mechanical properties and microstructural characteristics could be improved. Furthermore, the modification enables improved energy utilization due to an increase in laser absorption. In this work three commercial additive manufacturing powders, namely stainless steel (1.4404), tool steel (1.2709), and aluminum alloy (3.2381) were coated with three different nanoparticles (Silicon carbide (SiC), few layer graphene (FLG), and iron oxide black (IOB) to increase the laser light absorption in the PBF/LB/M process, mechanical properties, and flowability of the powders. The coating was conducted within a fluidized bed system, resulting in homogeneous coatings. This study demonstrates, that well scalable processes i.e. stirred media milling and fluidized bed coating have the potential to improve the commercial AM powders regarding their bulk density, flowability, and energy absorption, which is a crucial step towards an improvement in the efficiency of the whole PBF process. Overall important information and relations were gathered to transfer them to the real powder deposition process in future work. © 2021
    view abstract10.1016/j.matdes.2021.109536
  • Transferability of process parameters in laser powder bed fusion processes for an energy and cost efficient manufacturing
    Pannitz, O. and Sehrt, J.T.
    Sustainability (Switzerland) 12 (2020)
    In the past decade, the sales of metal additive manufacturing systems have increased intensely. In particular, PBF-LB/M systems (powder bed fusion of metals using a laser-based system) represent a technology of great industrial interest, in which metallic powders are molten and solidified layer upon layer by a focused laser beam. This leads to a simultaneous increase in demand for metallic powder materials. Due to adjusted process parameters of PBF-LB/M systems, the powder is usually procured by the system's manufacturer. The requirement and freedom to process different feedstocks in a reproducible quality and the economic and ecological factors involved are reasons to have a closer look at the differences between the quality of the provided metallic powders. Besides, different feedstock materials require different energy inputs, allowing a sustainable process control to be established. In this work, powder quality of stainless steel 1.4404 and the effects during the processing of metallic powders that are nominally the same were analyzed and the influence on the build process followed by the final part quality was investigated. Thus, a correlation between morphology, particle size distribution, absorptivity, flowability, and densification depending on process parameters was demonstrated. Optimized exposure parameters to ensure a more sustainable and energy and cost-efficient manufacturing process were determined. © 2020 by the authors.
    view abstract10.3390/su12041565
  • Additive manufacturing of soft magnetic permalloy from Fe and Ni powders: Control of magnetic anisotropy
    Schönrath, H. and Spasova, M. and Kilian, S.O. and Meckenstock, R. and Witt, G. and Sehrt, J.T. and Farle, M.
    Journal of Magnetism and Magnetic Materials 478 (2019)
    The influence of the process parameters in Laser Beam Melting (LBM) on the element distribution and magnetic properties of permalloy (Ni 78.5 Fe 21.5 ) is studied. Iron and nickel powders are mixed in the respective proportions to build twenty-five permalloy samples. The process parameters for each sample are varied to achieve different volume energy densities. An increase of the saturation magnetization M S up to 14% of the samples with respect to the initial powder blend is found. For a volume energy density of 428 [Formula presented] we detect a stripe-like segregation of iron and nickel in the uppermost layer. In the volume a homogeneous element distribution is found. The segregation at the surface leads to a sizable uniaxial magnetic anisotropy. When using parameter combinations resulting in similar volume energy densities, we observe different surface morphologies depending on scan speed and laser power. The implications for creating tailored magnetic anisotropy directions in Fe-Ni soft magnets are discussed. © 2019 Elsevier B.V.
    view abstract10.1016/j.jmmm.2018.11.084
  • Flow characteristics of porous metal structures for specified permeability manufactured by laser beam melting technology
    Benra, F.-K. and Dohmen, H.J. and Clauss, S. and Sehrt, J.T. and Witt, G.
    ASME International Mechanical Engineering Congress and Exposition, Proceedings (IMECE) 2A (2014)
    The characteristic additive build-up at the laser beam melting technology provides the opportunity to freeform porous and defined structures at specific areas in one part. By adjusting the process parameters specific characteristics of the manufactured part such as density, permeability, pore size, porosity and shear strength can be realized. The manufacturing process of a test body is described in detail. The permeability of the manufactured parts is investigated experimentally. In addition a numerical model is build and the flow structure inside of the test body is illustrated. The numerically obtained results are compared to the experimentally obtained results. To show the advantages of this technology for future applications a numerical model of a porous blade surrounded by a hot gas flow and cooled from inside of the porous structure is investigated. The results show that the method to define the characteristics during the laser beam melting process has to be optimized. Copyright © 2014 by ASME.
    view abstract10.1115/IMECE2014-39672
  • Molecular dynamics and experimental study of conformation change of poly(N -isopropylacrylamide) hydrogels in mixtures of water and methanol
    Walter, J. and Sehrt, J. and Vrabec, J. and Hasse, H.
    Journal of Physical Chemistry B 116 (2012)
    The conformation transition of poly(N-isopropylacrylamide) hydrogel as a function of the methanol mole fraction in water/methanol mixtures is studied both experimentally and by atomistic molecular dynamics simulation with explicit solvents. The composition range in which the conformation transition of the hydrogel occurs is determined experimentally at 268.15, 298.15, and 313.15 K. In these experiments, cononsolvency, i.e., collapse at intermediate methanol concentrations while the hydrogel is swollen in both pure solvents, is observed at 268.15 and 298.15 K. The composition range in which cononsolvency is present does not significantly depend on the amount of cross-linker. The conformation transition of the hydrogel is caused by the conformation transition of the polymer chains of its backbone. Therefore, conformation changes of single backbone polymer chains are studied by massively parallel molecular dynamics simulations. The hydrogel backbone polymer is described with the force field OPLS-AA, water with the SPC/E model, and methanol with the model of the GROMOS-96 force field. During simulation, the mean radius of gyration of the polymer chains is monitored. The conformation of the polymer chains is studied at 268, 298, and 330 K as a function of the methanol mole fraction. Cononsolvency is observed at 268 and 298 K, which is in agreement with the present experiments. The structure of the solvent around the hydrogel backbone polymer is analyzed using H-bond statistics and visualization. It is found that cononsolvency is caused by the fact that the methanol molecules strongly attach to the hydrogels backbone polymer, mainly with their hydroxyl group. This leads to the effect that the hydrophobic methyl groups of methanol are oriented toward the bulk solvent. The hydrogel+solvent shell hence appears hydrophobic and collapses in water-rich solvents. As more methanol is present in the solvent, the effect disappears again. © 2012 American Chemical Society.
    view abstract10.1021/jp212357n
  • additive manufacturing

  • laser beam melting

  • laser sintering

  • porous materials

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