Density is the single most important property of sintered steels; it controls virtually all other properties. In the present feasibility study, warm compaction in the temperature range of 400 °C to 520 °C was investigated. A special warm compacting tool was designed, which enabled powder preheating and compacting under a protective atmosphere. It showed that density levels of up to >7.6 g cm −3 can be attained with plain iron base powder. Since standard organic lubrication is not possible, graphite was used as a pressing lubricant. It was found that a mix of moderately fine artificial graphite and ultrafine natural graphite resulted in an acceptable combination of lubrication and alloying at least on a laboratory scale, while the higher density of graphite compared to organic lubricants increased the theoretical—and thus also the practically attainable—density.
In the first part of this feasibility study, a warm compaction route for ferrous powders has been described that operates in the 400–520°C range. In this second part, warm compacting studies with various types of unalloyed and alloyed steels were done. It showed that short preheating of the powder batch in the tool cavity and in protective atmosphere is sufficient, and also the time at pressure may be fairly short. However, too high green density may result in cracking during sintering, caused by trapped gases formed during carbothermal reduction processes. When comparing warm compaction with cold compacting at high pressures, a significant effect of the alloying route was observed: for elemental powder mixes, warm compaction showed to be preferable regarding the densities attained. For prealloyed grades, in contrast, high-pressure compaction yielded generally higher green densities, although for special prealloyed powder grades trimmed to maximum compactibility, also warm compaction seems to be eligible.
The influence of powder reuse on binder-jetted green parts, powder characteristics, and sintering densification was investigated for 17-4 PH stainless steel. Reusing overflow powder led to consistent printing results despite a slight humidity pickup. Powder oxidation after curing in ambient air demonstrated decreased powder packing during printing, which was connected to increased powder flowability. In comparison, curing in inert Ar preserved the powder's flowability and oxygen content. The green densities reduced from 4.71 to 4.47 g/cm 3 after 20 build jobs, while the powder's oxygen content increased by 20% due to four curing cycles in ambient air. Dilatometry of the reused samples with lower green densities revealed higher shrinkages and a change in the shrinkage anisotropy. The shrinkage of the reused samples increased mainly in the powder spreading and compaction roller axis direction as opposed to the build direction. Despite higher shrinkage, the density of the reused samples decreased compared to the virgin samples from 91.5% to 88.3% for sintering in Ar, whereas a lower reduction from 99.2% to 98.9% was detected when sintering in H 2 . Samples processed in H 2 exhibited higher sintering densification due to the formation of δ-ferrite, which was not observed for sintering in Ar. The absence of δ-ferrite was caused by a high residual carbon content, above 0.17 wt.%, due to inefficient debinding in Ar. The high carbon content also inhibited martensite formation, as indicated by a low microhardness of 159 HV0.1 compared to 315 HV0.1 for sintering in H 2 .
Lithography-based ceramic manufacturing (LCM) is a capable additive manufacturing (AM) or three-dimensional (3D) printing technology for producing ceramics with high precision and excellent mechanical properties similar to conventionally manufactured components. This AM method is a digital light processing-based vat-polymerization technique that allows the curing of photosensitive ceramic slurry in a layer-wise fashion by irradiating selected pixels (40 µm in the current study) within a layer (with a layer height of 25 µm). Subsequent thermal post-processing of the 3D-printed green parts includes debinding, which removes the organic matrix, and densification of the parts by sintering. Alumina-toughened zirconia (ATZ) offers high mechanical strength, fracture toughness, and biocompatibility, and has recently been adapted for use in LCM technology. ATZ is an excellent material for different load-bearing applications like artificial hip joints or areas where high hardness and wear resistance are needed such as cutting tools. To further improve the ceramic properties of the 3D-printed ATZ components eliminating residual porosity while maintaining a fine and homogeneous microstructure are important characteristics. To achieve this, hot isostatic pressing (HIP) was evaluated to reduce the part porosity and suppress excessive grain growth during sintering. To compare conventional pressureless sintering in air with HIP, the same test specimen geometry (2.1 × 2.5 × 25 mm) was 3D-printed and thermally processed using both conventional sintering and HIP. While the final relative densities for both techniques were greater than 98.5%, the achieved characteristic strength (four-point bending tests) of ATZ produced by HIP (>1200 MPa) was significantly exceeding that of conventional sintering in air (650 MPa).
The CoCrFeNiAl 0.5 high-entropy alloy (HEA) offers excellent properties for aerospace and energy applications, but conventional processing methods face high costs and low efficiency. Direct Current Fast Hot-Pressed Sintering (DC-FHPS), an innovative and rapid powder metallurgy technique, was used to prepare this alloy. The effect of sintering temperature on microstructure, densification, mechanical properties, and corrosion resistance was investigated. HEAs sintered at 950–1000 °C exhibited FCC + BCC dual phases. Higher sintering temperatures transformed the BCC morphology from rod-shaped to ellipsoidal, reduced Gibbs free energy, and improved interfacial bonding. Densification increased with temperature, reaching approximately 99.96% at 1000 °C, and hardness reached approximately 433 HV. The specimen consolidated at 1000 °C showed superior wear resistance and enhanced corrosion resistance. This study establishes a key theoretical foundation for the industrial rapid manufacturing and engineering applications of CoCrFeNiAl 0.5 HEA, demonstrating DC-FHPS as a cost-effective, efficient powder metallurgy route for advanced materials.
Cu–Al–Ni shape memory alloys (SMAs) have been produced by casting and conventional powder metallurgy, but their fabrication by powder injection molding (PIM) has not yet been established. This study explores PIM for Cu–Al–Ni SMAs by developing feedstocks with suitable rheological behavior for defect-free green-part formation. The critical powder volume concentration (CPVC) of an 84Cu–12Al–4Ni (wt.%) powder mixture was determined as 69.7 vol.%, and feedstocks containing 64, 66, and 68 vol.% powder were prepared. All feedstocks exhibited pseudoplastic behavior. Differences in the flow behavior index among the formulations were temperature dependent. The 64 vol.% feedstock exhibited the lowest flow activation energy (11.00 kJ/mol). At 190 °C, it also showed the lowest instability index (0.025) and Casson apparent yield stress (24.96 kPa), together with the highest moldability index (8.4696 × 10 −6 Pa −1 s −1 K −1 ). Accordingly, the 64 vol.% feedstock was identified as the most favorable rheology-based candidate, while the 68 vol.% feedstock showed a narrower processing window because of its proximity to the CPVC. Preliminary injection molding of the 64 vol.% feedstock produced green parts without visible molding defects. Semi-quantitative spatial analysis of the exported EDS maps identified Al as the element with the greatest regional signal variation in both the initial powder mixture and the injected green part. The feedstock and green-part densities were 4.73 and 4.67 g/cm 3 , respectively.
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.
Particle size control in gas atomization is essential for powder metallurgy processing, yet reliable prediction remains difficult due to the coupled influence of melt properties, gas dynamics and atomizer geometry. In this work, a dimensionless empirical model was developed to predict the median particle size (D50) of gas-atomized powders. Twenty-six variables were reduced using Buckingham pi analysis into three groups: melt parameter (Mp), gas parameter (Gp) and design parameter (Dp), which were correlated with dimensionless particle diameter. Copper powder was produced using a pilot-scale close-coupled atomizer under varying gas pressure, superheat temperature and nozzle configuration. A 3D map was plotted along with the 2D contour to locate optimum conditions for the desired powder particle size. An approximate generalised model has been established to resolve and identify the role of the influencing variables. The approach provides a practical tool for selecting atomization conditions to obtain the desired powder size.
Low-pressure powder injection moulding (LPIM) has advanced through developments in feedstock formulation, mould-filling simulation, thermal wick-debinding, and sintering of irregular metallic powders. This review focuses specifically on the wax-based, backbone-free branch of LPIM, in which thermal wick-debinding is the distinctive binder-removal route, as this is where the most recent quantitative progress has occurred. It synthesizes these developments and identifies the gaps that define the next phase of LPIM research. Wax-based binder formulations have been developed for stainless steels, iron, titanium, and Inconel 718, with solid loadings up to 62 vol.% and viscosities below 20 Pa·s. Numerical simulation has evolved from qualitative flow-pattern matching to quantitative in-cavity pressure validation, and current models reproduce experimental pressures with errors of 30%–64% across complex geometries. Thermal wick-debinding, the distinctive debinding route for backbone-free LPIM feedstocks, has been placed on a quantitative footing through a coupled experimental and COMSOL-based study. Sintering studies of irregular water-atomized iron powders, matched to optimized debinding and sintering schedules, have achieved densities and tensile properties comparable to those obtained with spherical gas-atomized feedstocks, supporting a lower-cost route to structural LPIM components. The review concludes with five research opportunities: closing the simulation–experiment gap; systematically studying the green-density–sintered-density relationship that distinguishes LPIM from HPIM, including dimensional accuracy and shrinkage anisotropy; extending to reactive and oxidation-sensitive alloys; integrating with material extrusion additive manufacturing, including SLA-printed and sacrificial mould strategies; and developing sustainable wax recovery, biosourced binders, and reduced-energy debinding routes.
This study investigates the synthesis of the MAX phase Ti 3 SiC 2 using pressureless sintering of Ti-Si-C powder mixtures. The influence of sintering temperature on phase formation and microstructure was systematically analysed through differential thermal analysis (DTA), X-ray diffraction (XRD), scanning electron microscopy (SEM), and nuclear magnetic resonance (NMR) spectroscopy. DTA shows the complex phase transformations in the range 1300–1400 °C. XRD analysis showed that the MAX phase content increased with temperature, reaching 94 wt.% at 1395 °C, with minimal content of secondary phases. SEM confirmed that a clearly defined layered microstructure characteristic of the MAX phase is formed. NMR spectroscopy provided insights into the local atomic environment of titanium, revealing two distinct titanium sites with quadrupole coupling constants of 9.3 MHz and 1.7 MHz for ‘outer’ and ‘inner’ titanium atoms, respectively. These findings demonstrate that sintering temperature critically affects the phase yield but not the intrinsic structural characteristics of Ti 3 SiC 2 .
