Newly developed hot isostatic pressing (HIP) units with integrated rapid cooling systems that are capable of operating at gas pressures up to 200 MPa provide the opportunity to densify and harden the metallic materials in a single step. For the application of these advanced HIP units in producing steels with defined microstructure, a crucial factor is the austenite grain size, which strongly influences the phase transformation during cooling and is effectively affected by the pressure during HIP. To study the effects of pressure, temperature and holding time on the growth of austenite grains and subsequently the final microstructure, a series of interrupted HIP experiments on hot work steel AISI H13 were conducted using such an advanced HIP unit. The austenite grain growth during HIP was analysed by metallographic microstructural investigations after rapid cooling. Thereafter, the effect of the austenite grain size on the phase transformation was studied using quenching dilatometry. Following this, the relationship between austenite grain size and martensitic/bainitic phase transformation temperature was determined, which indirectly indicates the influence of temperature, pressure and holding time on the final microstructure. By implementing this relationship into numerical models, the effect of HIP parameters on the microstructure of large-scale components, particularly those that experience large temperature gradients during the heating and cooling stages, can be predicted in the future.

In situ alloying via laser-based powder bed fusion of metals (PBF-LB/M) has gained attention as an alternative to prealloyed powders. In this study, H13 tool steel is in situ alloyed with titanium carbides (TiC) in amounts up to 30 wt.%. This approach offers the advantage that the carbides melt only partially during processing, preventing excessive carbon enrichment of the steel matrix. Using suitable process parameters, samples with up to 15 wt.% TiC were successfully fabricated with relative density above 99.6 vol.%. The resulting microstructure consists of coarse, undissolved and fine, reprecipitated carbides, leading to grain refinement and an isotropic microstructure. By varying the scanning speed, the energy input and thus the degree of carbide dissolution can be controlled. As-built hardness values for powder mixtures with 10 wt.% TiC reach 590 HV30, which is higher than the maximum hardness achieved through subsequent hardening. The secondary hardness peak is reached with direct tempering for 4 h at 500 degrees C with 659 HV30.

During metal binder jetting (MBJ), powder bed density critically governs binder migration and influences the final properties of the components. Heterogeneous powder bed density distribution in a single powder spreading process, possibly arising from both powder characteristics and spreading conditions, can lead to non-uniform binder saturation patterns. For process optimization in practical manufacturing, adjusting the spreading process is often more efficient than altering the powder that may have been chosen to meet final property requirements. However, the influence of process-controlled variations in powder bed density on binder migration has not yet been sufficiently addressed, and independent studies of powder spreading or binder deposition are not sufficient to capture the sequential effects. To fill this gap, this study integrated powder spreading simulations under varying spreading conditions using the discrete element method (DEM) with binder deposition simulations by computational fluid dynamics (CFD), in which the spread stainless steel 17-4PH powder beds served as the initial condition of binder deposition. The results of single-droplet simulations reveal that increasing the powder bed density initially promotes lateral binder spreading and suppresses vertical binder penetration, however, it ultimately results in a deep penetration. When extended to the more realistic multiple-droplet simulation, decreasing the droplet spacing was found to delay the local equilibrium saturation but promote the binder migration. A series of innovative single-layer printing experiments adopting the corresponding settings in the simulations confirms the validity of the developed workflow.

The advantages of nitrogen as an alloying element in austenitic stainless steels has been recognized for decades. However, its solubility varies across steel phases, with the highest in the austenitic solid state. Recent studies show that Si3N4 and X2CrNi18-9 (AISI 304L) powder mixtures enable solid-state alloy formation. During hot isostatic pressing (HIP), Si3N4 dissolves, allowing nitrogen diffusion into the austenitic matrix, forming high-nitrogen steel (HNS). This study examines the mechanical properties of HNS produced via powder metallurgy HIP and additive manufacturing using a shell-core scanning strategy, followed by HIP densification. Processing routes significantly affect microstructure and nitrogen content. Clear trends are found between hardness, tensile strength, and high-cycle fatigue strength, influenced by processing conditions and nitrogen levels. Additionally, Si3N4 particle size distribution impacts microstructure, with larger particles reducing grain size and enhancing strength. A linear relationship is observed between nitrogen content and both hardness and tensile strength.

Metal Binder Jetting (MBJ) has attracted increasing attention due to its capability to produce parts with complex geometries. However, a major challenge is the anisotropic sintering shrinkage caused by interlayer pores in green parts. In this study, we aimed to overcome this challenge by developing numerical models to understand the potential formation mechanisms of the interlayer pores during printing of 17-4PH and the subsequent microstructural evolution during sintering. A numerical workflow combining Computational Fluid Dynamics and Discrete Element Method was developed to first study the powder-binder interactions in a single-layer powder bed. Based on the understanding of the effects of binder impact and migration, the initial anisotropic microstructure was generated for sintering modeling based on the kinetic Monte Carlo Potts model. In this way, the simulated microstructural evolution showed a good agreement with the characterized real sintered parts.

Additive manufacturing (AM) of metal components has developed rapidly throughout the last decade. Sinter-based processes hereby offer the implementation of AM within existing powder metallurgical process routes, utilizing similar powders and sintering furnaces. While processes such as binder-jetting or fused-filament-fabrication have disadvantages such as limited resolution accuracy, lithography-based metal manufacturing (LMM) enables the production of filigree, complex structures with low surface roughness. However, layer-by-layer powder deposition induces anisotropic shrinkage during the subsequent sintering process.

Laser beam powder bed fusion of metals (PBF-LB|M) provides exceptional geometric freedom, but the variety of processable materials remains limited. In particular, the precise control of specific microstructures, like coarse carbides for better resistance against abrasive wear, continues to present challenges in research. In this study, hot work tool steel powder was mixed with high-speed steel (HSS) powder and titanium carbides (TiC) to develop an in-situ alloyed carbide-rich cold work tool steel.Thermodynamic calculations were performed to design an alloy system leading to a promising microstructure. During processing, TiC partially dissolved, resulting in coarse undissolved carbides and reprecipitated primary carbides. Carbide-forming elements like molybdenum (Mo), vanadium (V) and tungsten (W) are added as HSS powder to enhance the alloy´s potential for secondary hardening. The influence of PBF-LB|M process parameters on resulting carbides were analysed. In-depth microstructure analyses were conducted using scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD).

Nowadays, the fabrication of multi-material components with desired geometries and tailored microstructural properties is drawing continuous attention. By laminating or additive manufacturing and subsequent co-sintering, multi-material composites that combine a wide range of favorable properties can be produced. Examples are metal-ceramic laminates and multi-ceramic composites, which not only open the spectrum of material applications but also provide a higher degree of flexibility than single materials. However, undesired deformation that triggers delamination, curvature, cracks or even catastrophic failure frequently occurs during the co-sintering process. To solve these issues, a thermo-mechanical model that predicts densification, deformation, and delamination of multi-material components along the entire co-sintering process was developed. A multi-ceramic composite and a metal-ceramic laminate were selected to test and validate the developed model by experimental investigations. The developed model is proven to effectively describe the deformation, curvature, and stress distribution in the studied material combinations.

Powder metallurgical components reveal a high dimensional accuracy while offering a cost efficient and sustainable production. The inherent porosity contributes to a notable weight reduction but also affects the strength and durability. Numerous studies examined the effect of the porosity and the size and shape of individual pores on the fatigue behaviour. However, its effect on yielding behaviour is rarely reported. In this work, uniaxial compaction and dilatometry are used to produce cylindrical samples with different sintering states and densities. Microstructural analyses are performed to evaluate the effect of different sintering conditions on pore morphology and subsequently derive two-dimensional finite element models to virtually assess the effect of the pore morphology and density on strength. The results are compared with corresponding Rastagaev compression tests that are conducted with identical cylindrical samples. Additional three-dimensional simulations are performed based on μ-CT measurements to evaluate the difference between two- and three-dimensional simulation models.

Following the development of hot isostatic pressing (HIP) with integrated rapid cooling technology, it is now possible to combine the consolidation of encapsulated powder and subsequent heat treatment in a single step. Therefore, a simulation method combining discrete element (DE-) and finite element (FE-) simulations was developed to predict both the deformation and the microstructure in HIP with integrated heat treatment. For HIP, both types of simulations were performed in the first step to predict the initial powder filling density distribution prior to HIP and the final capsule shape of the components after HIP. The FE-simulation was also used for capsule design to achieve near-net-shape production of complex shaped components. Additionally, the FE-simulation was applied to foresee the microstructural evolution during the cooling stage. The agreement between the experimental and simulated results validated the method is able to digitalize the manufacturing route and reduce the development time in industrial applications.

Additive manufacturing of metallic materials by laser powder bed fusion enables the production of complex geometries with a degree of design freedom that far exceeds the limits of conventional manufacturing processes. Nevertheless, long process times and process-induced defects constrain the feasibility of large-scale production and restrict component reliability. One potential avenue for enhancing the productivity of laser powder bed fusion (PBF-LB) while concurrently mitigating the impact of defects on the material properties is to accept increased porosity during PBF-LB and to combine the process with hot isostatic pressing (HIP) in a so-called shell-core approach. The objective of the present study is to examine the influence of the process atmosphere on the impact toughness of DSS 2205 duplex stainless steel manufactured via PBF-LB and HIP for the case of high initial porosities to further advance the applicability of the shell-core approach. For that purpose, impact toughness tests were conducted on samples manufactured by PBF-LB under argon and nitrogen with initial porosities of 10 % and 20 % which were subjected to a HIP treatment followed by a pressure-less solution annealing and quenching. The specimens that were processed under argon showed an increase in argon pick-up with higher initial porosity, which prevented effective HIP post-densification and retained residual porosity, resulting in a significant deterioration of impact toughness at elevated temperatures. In contrast, the specimens produced under nitrogen exhibited only minor binding defects after HIP, with only a slight influence on impact toughness.

Laser additive manufacturing is a promising technology for producing tools with complex geometry. The tools for hot working applications typically require high thermal conductivity to reduce thermal stresses. However, the relationship between thermal conductivity and the unique microstructure created by Laser Powder Bed Fusion (LPBF) is not yet fully understood.In this study, we investigated the thermal conductivity of AISI M50 in the as-built and different heat-treated conditions. As-built samples exhibited anisotropy and low thermal conductivity. The low thermal conductivity can be improved after heat treatments by modifying the microstructure. Moreover, the influence of each microstructural characteristic was interpreted. Thereby, a correlation between thermal conductivity and microstructure can be addressed. With this knowledge, we proposed a suitable heat treatment concept for improving the thermal conductivity of additively manufactured tool steels.

Duplex stainless steels (DSSs) exhibit excellent mechanical properties due to their austenitic-ferritic microstructure, which, for example, leads to higher impact toughness compared to fully ferritic steels. The phase equilibrium is dependent on solidification conditions and chemical composition, with elemental nitrogen stabilizing the austenitic phase. To better understand the influence of pro-cess gases during processing by laser-based powder bed fusion (PBF-LB) on microstructure and mechanical properties, in this study, DSS AISI 318LN was processed with PBF-LB with different shielding atmospheres (argon and nitrogen) and subsequently, some of the samples were hot-isostatically post-densified before final heat treatment. The impact toughness of the different condi-tions was tested and complemented by microstructural and fractographic analysis. To completely exclude the influence of process gases, samples were also built up by electron-beam powder bed fusion (PBF-EB) under vacuum as a reference and subjected to the same post-processing strategies for comparison with the samples produced by PBF-LB.

Today, there is a high demand and increasing trend for multifunctional composite materials and components due to the combination of a wide range of favorable properties. However, undesired deformation leading to delamination, curvature, cracks, or even complete fracture frequently occurs during the co-sintering process of metal-ceramic laminates (MCLs). To solve these issues, experimental work highly relies on the time-consuming trial-and-error principle. This work aims to develop a thermo-mechanical-metallurgical model capable of predicting the densification, deformation, and delamination of MCLs during the co-sintering process. It is found that the developed model successfully considers the inhomogeneous relative density distribution and phase transformation of the steel tape. In addition, a thin interlayer formed by a mixed slurry in the MCLs is modeled as cohesive elements to simulate the delamination process. The developed model is systematically validated by the experimental measurements and observations in terms of densification, deformation, and delamination during the co-sintering process of the investigated laminate variants.

Shrinkage during the sintering of powder compacts depends on numerous parameters, including green body characteristics such as particle size and green density. These parameters are also decisive for the initial microstructure and its evolution during sintering. In this study, a novel experimental setup is used to quantify the time-dependent microstructural evolution in water-atomised Astaloy 85Mo powder. Green bodies with different particle sizes and density levels were polished on the top surface and then subjected to an interrupted sintering procedure in a quenching dilatometer. Intermediate examinations of the microstructure by scanning electron microscopy revealed the pore morphology and the thermally etched austenite grain size. It was found that pore rounding relies solely on the local curvature only, whereas neck growth is in good agreement with analytical models. An increase in diffusivity was found on the macroscale and on the microscale due to the pre-deformation of the particles.

Additive manufacturing processes continue to grow in popularity. Hereby, metal binder jetting (MBJ) has a high potential for series production of highly complex parts with added value compared to other production technologies. Applications already exist in thermal management, filtering, or fluid distribution. As with beam-based additive manufacturing processes, the materials available for MBJ are still limited. Especially in the area of nickel-free stainless steels; for example, for medical applications, there are hardly any options available. Therefore, this study investigates a newly developed nickel-free stainless steel specifically designed for the MBJ process. Particular attention is paid to the microstructure and the mechanical properties such as hardness and tensile strength. In addition, the investigation focuses on the potential of hot isostatic pressing (HIP) after sintering to favorably influence the microstructure and the mechanical properties. The results show that in the as-sintered state, a maximum porosity of 2.8% is present, which can be completely removed by hot isostatic post-densification. HIP post-densification improves both the yield strength and the tensile strength by about 10%. The increase in elongation at break is around 50%.

The separation of oxygen from air by means of inorganic ceramic membranes requires gas-tight ceramic–metal joints that enable reliable permeation operation in the oxygen partial pressure gradient at 850 °C. Reactive air brazing is a promising method to solve this challenge. However, reactive air brazed BSCF membranes suffer from a significant strength degradation that is caused by unhindered diffusion from the metal component during aging. In this study, we investigated how diffusion layers applied on the austenitic steel AISI 314 influence the bending strength of BSCF-Ag3CuO-AISI314 joints after aging. Three different approaches were compared as diffusion barriers: (1) aluminizing via pack cementation, (2) spray coating with NiCoCrAlReY, and (3) spray coating with NiCoCrAlReY and an additional 7YSZ top layer. Coated steel components were brazed to bending bars and aged for 1000 h at 850 °C in air prior to four-point bending and subsequent macroscopic as well microscopic analyses. In particular, coating with NiCoCrAlReY showed low-defect microstructures. The characteristic joint strength was raised from 17 MPa to 35 MPa after 1000 h aging at 850 °C. In addition, the dominant delamination fracture between the steel and the mixed oxide layer, observed in the reference series with uncoated steel, could be replaced by mixed and ceramic fractures of higher strength. The effect of residual joint stresses on the crack formation and path is analyzed and discussed. Chromium poisoning could no longer be detected in the BSCF, and interdiffusion through the braze was effectively reduced. Since the strength degradation of reactive air brazed joints is mainly caused by the metallic joining partner, the findings on the effect of the diffusion barriers in BSCF joints might be transferred to numerous other joining systems.

Additive manufacturing by laser‐based powder bed fusion of metals (PBF‐LB/M) enables the production of complex shaped components. High‐carbon tool steels tend to cracking during PBF‐LB/M due to internal stresses caused by the rapid solidification. Expensive atomization and long lead times for powder generate high costs in this processing route. In situ alloying during PBF‐LB/M of powder blends from conventionally available powders enables a more flexible approach of alloy design. For industrial use, the mechanical properties of in situ alloyed parts must be comparable to those of conventionally manufactured parts. In some cutting and forming applications, high wear resistance and corrosion resistance are required simultaneously. High alloyed cold work tool steels with sufficient chromium solved in the metal matrix fulfill these demands. Herein, AISI H13 is modified by Cr3C2 and elemental Cr to suit these requirements. Two novel alloys are modeled thermodynamically and processed by PBF‐LB/M. In‐depth microstructural investigations by backscatter electron imaging and diffraction in combination with abrasive wear tests and potentiodynamic polarization curves allow microstructure property correlations for different heat‐treated conditions. Partial crack‐free processing, hardenability, formation of Cr‐rich carbides, and residual Cr‐rich inclusions are observed and their influence on the wear and corrosion resistance is discussed.

The powder metallurgical (PM) process chain stands out by its ability to produce precise components at low cost. However, the inherent porosity of PM components, which has a particular impact on fatigue behavior, is crucial for components such as gears. Hence, cold rolling is commonly applied to densify the surface of sintered components. This induced densification can be modelled by a constitutive law introduced by Gurson, Tvergaard and Needleman. In this work, a modified GTN model was derived to simulate the densification behavior of Astaloy 85Mo sintered steel. The stress-strain-behavior of sintered samples with different densities was deduced from compression tests according to Rastagaev. A synthesized description of the plasticity of the dense material was then combined with the densification behavior during compression to obtain a density-dependent GTN model. The model was validated by comparison with experimental data on the densification during sizing and cold isostatic pressing of sintered samples.

The uniaxial viscosity is an important material property that has to be determined in order to understand the densification of a material during the sintering process. It depends simultaneously on temperature, relative density and grain size which complicates its determination. To determine its dependence on these three factors, bending creep tests are performed on pre-sintered 3Y-TZP specimens with different relative densities and grain sizes up to the corresponding pre-sintering temperatures. The uniaxial viscosity of 3Y-TZP specimens is calculated after obtaining the deflection rates. It is found that the uniaxial viscosity decreases with temperature according to the Arrhenius equation. In addition, the tetragonal to cubic phase transformation at temperature above 1300 degrees C leads to a decrease of the activation energy. Apart from that, the contribution of densification and grain growth to the increase of the uniaxial viscosity is quantitatively determined. When grain boundary diffusion dominates, the contribution of grain growth is up to 1.8 times that of densification to the uniaxial viscosity. Furthermore, it is found that Rahaman's model best fits the normalized uniaxial viscosity. At the end, an evolution profile of the uniaxial viscosity for polycrystalline materials during the sintering process is proposed to facilitate the analysis of the sintering shrinkage rate.