Showing papers by "Carolin Körner published in 2023"
1 citations
TL;DR: In this article , an alternative approach for modeling nucleation in PBF of metals is introduced, based on observations reported from different researchers in the field of PBF, a heuristic approach is applied to consider new grains within a cellular automata-based crystal growth model for PBF.
Abstract: In powder bed fusion (PBF) of metals, energy of a laser or electron beam is utilized to form near‐net‐shaped parts from a powder bed. A very promising application of PBF lies in the direct control of the resulting microstructure by adjusting process parameters. Especially the intentional tilting of grains in one certain direction offers a complete new field of activity for additively produced parts specially designed for a given load case. Nucleation is essential for utilization of this effect. Herein, an alternative approach for modeling nucleation in PBF of metals is introduced. The model is not intended to cover every physical effect of this extreme complex phenomenon. Based on observations reported from different researchers in the field of PBF, a heuristic approach is applied to consider new grains within a cellular automata (CA)‐based crystal growth model for PBF. Utilizing the implicit capability of the CA to model competitive growth of grains, this approach does not require any additional computational capacities. The numerical calculations are carried out using a message–passing interface (MPI) parallelization on a high‐performance computing (HPC) cluster located at the Regional Research Center Erlangen. The numerical results are validated by means of experiments and electron backscatter diffractometric measurements.
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TL;DR: In this paper , a multiscale and multi-purpose simulation framework is introduced to investigate selective beam melting processes for metallic cellular structures along the complete process chain, up to the topology optimization of components made of cellular materials.
TL;DR: In this article , the variations of specimen weight, microstructure and hardness of nitrided layer as well as the developed nitriding products were investigated as a function of treatment temperature and time.
Abstract: Internal nitriding behavior of Mo-15 at.% Ti and Mo- 5/15 at.% V alloys were compared in a temperature range from 700 °C to 1100 °C using Forming gas (95% N2 and 5% H2) for nitriding times of 10 h and 15 h. The variations of specimen weight, microstructure and hardness of nitrided layer as well as the developed nitriding products were investigated as a function of treatment temperature and time. VN and TiN nitrides with a minor amount of TiO2 oxide are formed on the surface of Mo-V and Mo-Ti alloys, respectively. The maximum nitrided depths of around 14 μm are achieved for both Mo-15 Ti/V alloys after nitriding at 1100 °C for 10 h. Larger hardness values are measured on nitrided layer of Mo-Ti in comparison to that of Mo-V at the same treatment temperature. According to X-ray diffraction assessment, at moderate temperature of 900 °C, the diffraction of Mo matrix and VN nitrides is recognized as an incoherent diffraction, whereas coherent diffraction by matrix and nitrides is realized for Mo and TiN. In general, higher affinity of nitride formation for V is observed at lower temperature of 700 °C as compared to Ti in a Mo matrix, which is attributed to the stronger interaction of V and N in Mo matrix given by calculating corresponding interaction parameters. The difference in stress relieving and coarsening behavior were also observed in matrices containing VN and TiN, which is argued with regards to the dissimilar impacts of VN and TiN on precipitation hardening of Mo matrix at investigated temperatures. These results imply that the previously reported brittleness of Mo-based alloys containing Ti can be improved at moderate temperatures by replacing Ti with other nitride forming elements such as V, regardless of N concertation in Mo matrix.
TL;DR: In this article , the martensitic phase transition behavior of the as-built NiTi parts and powder is analyzed using Differential Scanning Calorimetry (DSC) analysis.
TL;DR: In this article , a complex structure consisting of three conic shapes with narrow cylinders in between hindering heat flux was used to demonstrate the effects of different thermal regimes on the part density, microstructure and mechanical properties aided by finite element simulations as well as by thermography and X-ray computed tomography measurements.
Abstract: One major advantage of additive manufacturing is the high freedom of design, which supports the fabrication of complex structures. However, geometrical features such as combined massive volumes and cellular structures in such parts can lead to an uneven heat distribution during processing, resulting in different material properties throughout the part. In this study, we demonstrate these effects, using a complex structure consisting of three conic shapes with narrow cylinders in between hindering heat flux. We manufacture the parts via powder bed fusion of Ti6Al4V by applying a laser beam (PBF-LB/M) as well as an electron beam (PBF-EB). We investigate the impact of the different thermal regimes on the part density, microstructure and mechanical properties aided by finite element simulations as well as by thermography and X-ray computed tomography measurements. Both simulations and thermography show an increase in inter-layer temperature with increasing part radius, subsequently leading to heat accumulation along the build direction. While the geometry and thermal history have a minor influence on the relative density of the parts, the microstructure is greatly affected by the thermal history in PBF-LB/M. The acicular martensitic structure in the narrow parts is decomposed into a mix of tempered lath-like martensite and an ultrafine α + β microstructure with increasing part radius. The EBM part exhibits a lamellar α + β microstructure for both the cylindric and conic structures. The different microstructures directly influence the hardness of the parts. For the PBF-LB part, the hardness ranges between 400 HV0.5 in the narrow sections and a maximum hardness of 450 HV0.5 in the broader sections, while the PBF-EB part exhibits hardness values between 280 and 380 HV0.5.
TL;DR: In this article , the powder smoke phenomenon and its development are investigated using an ELectron-Optical (ELO) monitoring system, which collects backscattered electrons during PBF-EB in similarity to scanning electron microscopy.
Abstract: Electron Beam Powder Bed Fusion (PBF-EB) is an additive manufacturing technique that produces customized components using an electron beam under vacuum conditions. A so-called smoke phenomenon might occur during the process, leading to an explosion-like powder spreading in the vacuum chamber, which is catastrophic for the build process. This phenomenon happens rapidly and it is hard to be observed from its initial stage. In this study, the powder smoke phenomenon and its development are investigated using an ELectron-Optical (ELO) monitoring system, which collects backscattered electrons during PBF-EB in similarity to scanning electron microscopy. In comparison to the optical process monitoring tool based on high-speed camera, the ELO system shows very promising results for real-time smoke detection and possibly prevention.
TL;DR: In this paper, the authors used high-throughput thermal simulation for the identification of fundamental relationships between process parameters, processing conditions, and the resulting melt pool geometry in the quasi-stationary state of line-based hatching strategies in PBF-EB.
Abstract: The reliable and repeatable fabrication of complex geometries with predetermined homogeneous properties is still a major challenge in electron beam powder bed fusion (PBF-EB). Although previous research identified a variety of process parameter–property relationships, the underlying end-to-end approach, which directly relates process parameters to material properties, omits the underlying thermal conditions. Since the local properties are governed by the local thermal conditions of the melt pool, the end-to-end approach is insufficient to transfer predetermined properties to complex geometries and different processing conditions. This work utilizes high-throughput thermal simulation for the identification of fundamental relationships between process parameters, processing conditions, and the resulting melt pool geometry in the quasi-stationary state of line-based hatching strategies in PBF-EB. Through a comprehensive study of over 25,000 parameter combinations, including beam power, velocity, line offset, preheating temperature, and beam diameter, process parameter-melt pool relationships are established, processing boundaries are identified, and guidelines for the selection of process parameters to the achieve desired properties under different processing conditions are derived.
TL;DR: In this paper , the impact of the fine powder fraction (0 wt.% to 15 wt%) blended to a coarse H11 powder in the preform on thermal conductivity, Vickers hardness and tensile strength was elucidated.
Abstract: Spontaneous infiltration of a porous preform by a metallic melt provides the potential of generating metal matrix composites (MMCs) with tailored combinations of material properties at low cost. The bulk of tool inserts for injection molding must sustain high mechanical and thermal loads and simultaneously exhibit high thermal conductivity for efficient temperature control of the mold insert. To fulfill these contradictory requirements, AISI H11 tool steel preforms were infiltrated by liquid copper. The impact of the fine powder fraction (0 wt.% to 15 wt.%) blended to a coarse H11 powder in the preform on thermal conductivity, Vickers hardness and tensile strength was elucidated. The thermal conductivity of the composites could be enhanced by a factor of 1.84 (15 wt.% fine powder) and 2.67 (0 wt.% fine powder) with respect to the sintered H11 tool steel. By adding 15 wt.% fine powder to the coarse host powder, the tensile strength and Vickers hardness of the copper-infiltrated steel were 1066.3 ± 108.7 MPa and 366 ± 24 HV1, respectively, whereas the H11 tool steel yielded 1368.5 ± 89.3 MPa and 403 ± 17 HV1, respectively. Based on the results obtained, an appropriate particle size distribution (PSD) may be selected for preform preparation according with the requirements of a future mold insert.
TL;DR: In this article , a physically based ray tracing model is proposed to rationalize the effect of detector positioning on image contrast development and masking, and a validated method to compute build surface height gradients directly from experimentally recorded electron-optical images of a multi-detector system is presented.
Abstract: The recent success of the process monitoring method Electron Optical Imaging, applied in the additive manufacturing process Electron Beam Powder Bed Fusion, necessitates a clear understanding of the underlying image formation process. Newly developed multi-detector systems enable the reconstruction of the build surface topography in-situ but add complexity to the method. This work presents a physically based raytracing model, which rationalises the effect of detector positioning on image contrast development and masking. The model correctly describes the effect of multiple scattering events on vacuum chamber walls or heat shields and represents, therefore, a predictive tool for designing future detector systems. Most importantly, this work provides a validated method to compute build surface height gradients directly from experimentally recorded electron-optical images of a multi-detector system without any calibration steps. The computed surface height gradients can be used subsequently as input of normal integration algorithms aiming at the in-situ reconstruction of the build surface topography.
TL;DR: In this paper , the effect of Cu nanoparticles on the mechanical properties of a novel Fe-based α/α′/α″ superalloy was studied, and it was shown that a configuration of α-matrix and intermetallic α−/α−phases forming an interpenetrating network is superior to a state with isolated precipitates.
Abstract: Introducing Cu nanoparticles is an effective mechanism for strengthening and toughening Fe‐based materials such as ultra‐high‐strength steels. Herein, the effect of Cu on the mechanical properties of a novel Fe‐based α/α′/α″ superalloy is studied. Compared to a Cu‐free reference alloy, nanoindentation reveals an increase in hardness, which was associated with the formation of Cu nanoparticles. Both alloys show room temperature (RT) compressive plastic strain at maximum stress greater than 8%, irrespective of the heat‐treatment. At RT and at 750 °C, the Cu‐containing alloy exhibits a slightly higher strength, but the heat treatment has a more significant impact: A configuration of α‐matrix and intermetallic α′/α″‐phases forming an interpenetrating network is superior to a state with isolated precipitates. This difference vanishes in monotonic creep experiments, and under the same conditions, the Cu‐containing alloy exhibits a twice as high creep rate despite a slightly higher precipitate fraction. This is linked to a higher lattice misfit and faster‐coarsening kinetics. Post‐mortem transmission electron microscopy analysis of the creep‐deformed specimens identifies dislocation bypass as the dominant deformation mechanism. However, the presence of <010>{110} dislocations in the interfacial networks and evidence of dislocation activity within α′/α″ precipitates suggest the occurrence of shearing events.
TL;DR: In this article , the microstructural and mechanical characteristics of PBF-EB processed 4th generation titanium aluminides (TiAl) alloy TNM were determined. And the failure always takes place in the weaker microstructure (tensile: FL; creep: NL + γ).
TL;DR: In this article , the extents of material contrast using ELO are investigated, focusing mainly on identifying powder contamination, and it is shown that an ELO detector is capable of distinguishing a single 100 μm foreign powder particle, during an PBF-EB process, if the backscattering coefficient of the inclusion is sufficiently higher than its surroundings.
Abstract: Electron Beam Powder Bed Fusion (PBF-EB) is an Additive Manufacturing (AM) method that utilizes an electron beam to melt and consolidate metal powder. The beam, combined with a backscattered electron detector, enables advanced process monitoring, a method termed Electron Optical Imaging (ELO). ELO is already known to provide great topographical information, but its capabilities regarding material contrast are less studied. In this article the extents of material contrast using ELO are investigated, focusing mainly on identifying powder contamination. It will be shown that an ELO detector is capable of distinguishing a single 100 μm foreign powder particle, during an PBF-EB process, if the backscattering coefficient of the inclusion is sufficiently higher than its surroundings. Additionally, it is investigated how the material contrast can be used for material characterization. A mathematical framework is provided to describe the relationship between the signal intensity in the detector and the effective atomic number Zeff of the imaged alloy. The approach is verified with empirical data from twelve different materials, demonstrating that the effective atomic number of an alloy can be predicted to within one atomic number from its ELO intensity.
TL;DR: In this article , an analysis of powder bed fusion (PBF) samples made of Ti64 and PA12 was performed and different evaluation methods for the CT-measured values were analyzed and recommendations on a procedure for measuring internal structures were given.
Abstract: X-ray-computed tomography (CT) is today’s gold standard for the non-destructive evaluation of internal component defects such as cracks and porosity. Using automated standardized evaluation algorithms, an analysis can be performed without knowledge of the shape, location, or size of the defects. Both the measurement and the evaluation are based on the fact that the component has no internal structures or cavities. However, additive manufacturing (AM) and hybrid subtractive procedures offer the possibility of integrating internal structures directly during the building process. The examination of powder bed fusion (PBF) samples made of Ti64 and PA12 showed that the standardized evaluation methods were not able to identify internal structures correctly. Different evaluation methods for the CT-measured values were analyzed and recommendations on a procedure for measuring internal structures are given.
TL;DR: In this paper , different hatching strategies are applied to realize specific patterns of molten material alternating with non-molten powder particles, and the magnetic performance of the specimens is characterized by determining hysteresis loops (B-H curves), power losses and maximum magnetic flux density at frequencies between 50 Hz and 1000 Hz.
Abstract: Fe93.5 Si6.5 (wt.%) soft magnetic materials in toroidal shape are additively manufactured by means of Electron Beam Powder Bed Fusion (PBF-EB). Different hatching strategies are applied to realize specific patterns of molten material alternating with non-molten powder particles. The specimens produced using different hatching strategies show identical relative densities but various structural features resulting in different magnetic properties. The magnetic performance of the specimens is characterized by determining hysteresis loops (B-H curves), power losses and maximum magnetic flux density at frequencies between 50 Hz and 1000 Hz. At constant mass, the different structures induced by using various hatching strategies have a strong influence on the hysteresis losses. These losses can be significantly reduced by applying a targeted structure design. The modified specimens show superior magnetic properties at sub-kHz compared to some soft magnetic materials fabricated by means of conventional methods and Laser Powder Bed Fusion (PBF-L). This article is protected by copyright. All rights reserved.
TL;DR: In this article , an equiaxed or columnar cell morphology can be obtained, exhibiting a plate-like or an interconnected network of the (Cr,Mo) reinforcement phase which is embedded in a NiAl matrix.
Abstract: By increasing the density of interfaces in NiAl-CrMo in-situ composites, the mechanical properties can be significantly improved compared to conventionally cast material. The refined microstructure is achieved by manufacturing through electron beam powder bed fusion (PBF-EB). By varying the process parameters, an equiaxed or columnar cell morphology can be obtained, exhibiting a plate-like or an interconnected network of the (Cr,Mo) reinforcement phase which is embedded in a NiAl matrix. The microstructure of the different cell morphologies is investigated in detail using scanning electron microscope (SEM), transmission electron microscopy (TEM) and atom probe tomography (APT). For both morphologies, the mechanical properties at elevated temperatures are analyzed by compression and creep experiments parallel and perpendicular to the building direction. In comparison to cast NiAl and NiAl-(Cr,Mo), the yield strength of the PBF-EB fabricated specimens is significantly improved at temperatures up to 1027 °C. While the columnar morphology exhibits the best improved mechanical properties at high temperatures, the equiaxial morphology shows nearly ideal isotropic mechanical behavior, which is a substantial advantage over directionally solidified material. This article is protected by copyright. All rights reserved.