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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. [ABSTRACT FROM AUTHOR]

Electric current‐assisted sintering (ECAS) is a promising powder consolidation technique that can achieve short‐term sintering with high heating rates. Currently, main methods of performing ECAS are indirect heating of the powder compact in a conductive tool or direct heating with current flowing through the powder compact. Various influencing factors have been identified to explain the rapid densification during ECAS, such as ultrahigh heating rates, extra‐high temperatures, and electric field. However, the key consolidation‐enhancing factor is still under debate. This study aims at understanding the role of heating rate on the enhanced densification during ECAS of 8 mol% Y2O3‐stabilized ZrO2 (8YSZ) by experimental and numerical methods. Two different heating modes, ultrafast high‐temperature sintering (UHS, indirect heating) and flash sintering (FS, direct heating), are studied. The novel UHS technique is successfully applied to consolidate the 8YSZ samples. Additionally, finite element methods (FEM) combined with a constitutive model is adopted to predict the densification and grain growth. Furthermore, a comparison of UHS and FS is performed to investigate the thermal effect (heating rate) and athermal effect (electric field) individually. The results indicate that the high heating rate is the key factor of the rapid densification during UHS and FS of 8YSZ. [ABSTRACT FROM AUTHOR]

As a promising sintering technique, flash sintering utilizes high electric fields to achieve rapid densification at low furnace temperatures. Various factors can influence the densification rate during flash sintering, such as ultrahigh heating rates, extra‐high sample temperatures, and electric field. However, the determining factor of the densification rate and the key mechanism during densification are still under debate. Herein, the densification and grain growth kinetic during flash sintering of 8 mol% Y2O3‐stabilized ZrO2 (8YSZ) is studied experimentally and numerically using finite element method (FEM). The roles of Joule heating and heating rate on the densification are investigated by comparing flash sintering with conventional sintering. An apparently smaller activation energy for the material transport resulting in densification is obtained by flash sintering (Qd =424 kJ mol−1) compared to the conventional sintering (Qd = 691 kJ mol−1). In addition, a constitutive model is implemented to study both the densification and the grain growth during flash and conventional sintering. Furthermore, the effect of electrical polarity on the density and the grain size evolution during flash sintering of 8YSZ is also investigated. The simulation results of average density and grain size inhomogeneity agree well with the experimental data. [ABSTRACT FROM AUTHOR]

The laser powder bed fusion (LPBF) technology has been involved in the tooling industry to produce tools with complex geometry and integrated functions. However, tool steels with high carbon content tend to crack due to the thermal stresses during the LPBF process. One solution is increasing the powder bed temperature to avoid large thermal gradients. In the present study, the influence of the preheating temperature on microstructure and corresponding hardness is systematically investigated. With the help of time–temperature–transformation diagram, the phase evolution during the LPBF process is systematically explained. AISI M50 samples are produced by LPBF from room temperature to a preheating temperature of 650 °C. Higher preheating temperatures shift the optimal laser parameter window to lower volume energy densities. A cellular/dendritic microstructure formed during the rapid solidification with retained austenite is located at the interdendritic regions. Moreover, a high preheating temperature reduces the retained austenite fraction, specifically from 39% without preheating to 7.6% at 650 °C preheating temperature. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Laser additive manufacturing enables the one‐step fabrication of complex parts. However, pores and carbide networks, which are not avoidable from the laser powder‐bed fusion (LPBF) process, deteriorate the fatigue strength significantly. Hot isostatic pressing (HIP) with integrated heat treatment is a powerful post‐treatment that densifies the material and modifies the microstructure. Herein, AISI M3:2 samples are produced by LPBF and then are either austenitized, quenched, and tempered in a HIP unit under pressure or are only hardened and tempered in a vacuum oven. The corresponding microstructure is analyzed by optical microscopy, scanning electron microscopy using energy‐dispersive X‐ray spectroscopy, and X‐ray diffraction. The fatigue strength is determined by rotation bending tests. Fracture surfaces are observed under scanning electron microscopy for failure analysis. While both post‐treatments lead to similar microstructure, the fatigue strength is significantly improved by the HIP process. [ABSTRACT FROM AUTHOR]

Tool steels are known to be difficult to process by laser powder bed fusion as they were not designed to fit the welding or additive manufacturing (AM) process route. Hot cracks and cold cracks are known to occur in such alloys. To gain insights on limits of alloying contents regarding primary carbides in the melt, powder blends based on AISI H13 with additions of Cr3C2 carbides are processed by laser powder bed alloying (LPBA) herein. The LPBA process is evaluated based on demixing phenomena observed in the macroscopic chemical composition of the specimen. In‐depth microstructural analyses are carried out to understand the metallurgical behavior of the new alloy and are accompanied by Calphad modelling. Hardness tests characterize the alloy's suitability as a novel tool steel. The alloy's hot cracking tendency is reduced by systematic additions of Cr3C2 to AISI H13 which narrow the solidification interval. Hardness tests after martensitic hardening without annealing have shown the applicability of the new alloy as AM tool steel. Apparent chemical gradients over the build plate demonstrate that further research is needed to improve the technology of LPBA for processing these novel alloys. [ABSTRACT FROM AUTHOR]

Tool steels being processed by laser powder bed fusion (LPBF) receive increasing attention in research. High hardness and low toughness lead to a challenging crack susceptibility of these alloys. Latest advancements in process technology have shown promising results toward crack‐free build‐up of additive manufactured tools. Detailed microstructure analysis is carried out to understand the mechanisms of solidification as an influencing factor for cracking. These findings can provide insights toward an alloy optimization regarding a higher intrinsic crack resistance while maintaining the typical mechanical properties of tool steels. PMHS 6–5–3–8 high‐speed steel shows directional dendritic solidification with carbides mainly precipitating in enriched interdendritic areas. Increasing the alloying content by additions of PMHS 7–7–7–11 leads to a shift toward a less‐pronounced texture. The positive effect of texture reduction is superimposed by a highly increased crack formation due to carbide networks. [ABSTRACT FROM AUTHOR]

This study aims to understand the effect of the electrical field on microstructure evolution during field‐assisted sintering or spark plasma sintering (FAST/SPS) of 10 mol% gadolinium‐doped ceria (GDC) with experimental and numerical methods. The novelty of this study has been the observation of enhanced grain growth in the region closer to the anode, even under FAST/SPS conditions with electrical fields less than 5 V/cm. The grain growth kinetics, including determination of activation energy and grain‐boundary mobility, were analyzed along the cross section of the samples for different temperatures and dwell periods. With an increase in distance from the anode, reduction in the activation energy for grain growth and grain‐boundary mobility was observed. These observations attributed to the attraction of oxygen ions to the anode region under an electrical field with an increase in defects along the grain boundaries. Thereby an increase in the grain‐boundary mobility and larger grains in that region were observed. A homogenous microstructure was observed in a case where the current did not flow through the sample. Furthermore, a numerical strategy has also been developed to simulate this behavior in addition to heat generation, heat transfer, and densification using Finite Element Methods (FEM) simulations. The simulation results provided an insight into the presence of a potential difference across the cross section of the samples. The simulation results were also in good agreement with the experimental observations. [ABSTRACT FROM AUTHOR]

The microstructure and mechanical properties of high‐speed steel AISI M50 (80MoCrV42‐16, Mat. Nr. 1.3551), produced by laser powder bed fusion (LPBF), are analyzed. The mechanical properties in hardened and tempered condition are characterized by hardness, fatigue strength, and toughness and compared with the properties of conventionally produced samples. Moreover, the effects of an additional posttreatment by hot isostatic pressing (HIP) on the microstructure and mechanical properties are investigated. Dilatometric testing is used to investigate the influence of the different initial microstructures on hardening. All heat‐treated samples expose a fine martensitic microstructure with high hardness. The conventionally produced samples show a band‐like orientation of carbides due to the production by vacuum induction melting and vacuum arc remelting followed by a hot working process. This carbide structure is bypassed by the rapid cooling in the LPBF process. The LPBF samples show a comparable hardness after hardening and tempering to the conventionally produced material. In heat‐treated state, the LPBF samples show a low fatigue strength. Posttreatment by HIP included in the heat‐treatment chain significantly increases the fatigue strength. Nevertheless, the fatigue strength is still lower compared with the reference material. Both LPBF grades show a low toughness compared with the reference material. [ABSTRACT FROM AUTHOR]

The field of additive manufacturing (AM), and especially laser powder‐bed fusion (LPBF), is constantly growing. Process windows for a large variety of materials are already developed. Nevertheless, some materials are still difficult to manufacture with LPBF. One of these materials is the tungsten carbide/cobalt‐based hard metal (WC–Co), which is conventionally produced by powder metallurgy including liquid‐phase sintering. Most approaches to manufacture WC–Co with LPBF show a high porosity, undesirable phases in the microstructure, and inhomogeneous carbide distribution. However, the production of WC–Co cutting tools by LPBF will offer some benefits such as production of geometrically optimized inner cooling channels or optimized geometry of the cutting edges. Herein, WC with 17 wt% Co is processed by LPBF with a powder‐bed heating of 900 °C. Afterwards microstructure, density, and hardness are determined. In addition, X‐ray diffraction (XRD) analysis is performed to determine the phase composition. To investigate the edge‐holding properties of LPBF‐manufactured WC–Co cutting tools, stock removal tests are conducted on three different workpiece materials. [ABSTRACT FROM AUTHOR]

Dense and gastight dead-end tubular Ba0.5Sr0.5Co0.8Fe0.2O3-δ ( BSCF) membranes were fabricated by thermoplastic extrusion. Optimal feedstocks for the extrusion of dead-end tubular structures were obtained for a feedstock consisting of 89.83 wt% BSCF powder, 5.63 wt% ethylene vinyl acetate ( EVA), 3.56 wt% paraffin wax ( PW), and 0.98 wt% stearic acid ( SA). Thermogravimetric analysis ( TGA) was used to determine the thermal characteristics of the feedstock and to characterize the amount of binder removed during the wicking step. Straight and round dead-end tubular membranes with wall thickness of 0.5 mm could be achieved after sintering. The gastightness test showed that the membranes were leak-tight up to a pressure of 35 bar. [ABSTRACT FROM AUTHOR]

The perovskite Ba0.5Sr0.5Co0.8Fe0.2O3− δ (BSCF) is a mixed ionic electronic conductive ceramic and can be used as oxygen transport membrane (OTM) material for oxygen supply in high-temperature applications. For BSCF, reactive brazing in air atmosphere (RAB) is the only technical applicable solution for a heat resistant and gas tight sealing to metals. As a consequence of brazing BSCF to the heat resistant austenitic steel AISI 314 a change in microstructure in the BSCF appears at the interface due to reactions with the copper oxide (CuO) from the AgCu braze alloy. After aging for 500 h at 850 °C in air atmosphere, a further change in the microstructure appears, which is characterized by an additional increase of porosity in the interface area between ceramic and braze. Also, a needle shaped phase at the grain boundaries in the ceramic can be observed. This kind of structure is known as product phase of a transformation of cubic BSCF into hexagonal perovskite. Mechanical tests with the BSCF/AISI 314-joints show a decrease of the room temperature strength after aging. [ABSTRACT FROM AUTHOR]

Reactive air brazing (RAB) is a promising and economic method for joining ceramic to metal as well as ceramic to ceramic. Hybrid joints gain importance in the field of challenging applications such as gas separation or solid oxide fuel cells. While the ceramic partner is often directly chosen due to its functionality, the metallic partner needs to be chosen carefully in order to avoid high residual stresses due to the differences in the thermal expansion coefficients. This leads to the restriction of the usage of a significant number of potential joining partners. Ceramic/metals components with unadapted thermal expansion behavior (Ba0.5Sr0.5Co0.8Fe0.2O3-δ/Crofer 22 H and 3YSZ/AISI 314) were joined by RAB. Cu-containing as well as Cu-free Ag-based filler metals were used. Analysis of the cross-section of brazed samples showed that the microstructure of the samples depends especially on the used filler metal. The melting process as well as the exothermic reactions leading to a distinct reaction zone could be observed in the differential scanning calorimetric measurements. While the microstructure of the joints is not significantly influenced by the base material, the strength of the BSCF/Crofer 22 H specimens is nearly four times lower than the strength of the specimens with the adapted thermal expansion coefficients (BSCF/AISI 314 and 3YSZ/Crofer 22 H). The reasons are residual stresses caused by the different thermal expansion coefficients. Finite element simulations assuming a visco-plastic material model show that an amount of stress can be reduced by relaxation within the silver based braze. But the volume of the high residual stresses is still larger in the BSCF/Crofer 22 H joints than that of the BSCF/AISI 314 joint resulting in a lower fracture strength. [ABSTRACT FROM AUTHOR]