Powder Metallurgy

When processing powder metallurgy (PM) steels, the conventional press and sinter route can reach a relative density up to 95%, which is insufficient for applications when dynamic mechanical performance is critical. In this study, a novel route is demonstrated consisting of cold isostatic pressing (CIP) followed by sintering and capsule-free hot isostatic pressing (HIP), allowing to achieve full density PM steels. Water-atomized steel powder admixed with 2 wt.% Ni was subjected to CIP and followed by sintering in 90N₂/10H₂ atmosphere at 1120 and 1250°C, and in vacuum (10⁻² mbar) at 1250 and 1350°C, respectively. At the highest explored CIP pressure of 600 MPa, the three high-temperature sintering runs at 1250°C in 90N₂/10H₂ atmosphere and vacuum, and 1350°C in vacuum resulted in relative density of ∼94% and closed surface pores. This condition with necessary closed porosity then allowed subsequent capsule-free HIP after sintering, resulting in full densification of the components.

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.

Electromagnetic axial powder compaction (EMAPC) uses strong magnetic fields to compact powder metallurgy components at high speeds. Lorentz forces accelerate the punch to compact powder in EMAPC. Thus, high magnetic fields cause powder deformations in microseconds. Therefore, measuring the compact height, magnetic field distribution, and compaction velocity was difficult. No literature has reported EMAPC finite element (FE) modeling. Thus, an LS-DYNA multi-physics solver-based FE 3D model has been developed to study SS316s EMAPC. A cylindrical SS316 sample was simulated for EMAPC at various discharge energies. The powder-compressed sample's final deformation was predicted through simulation. To characterize compacted samples, sintered samples were studied for density, porosity, and microhardness. Compressed samples were microscopically examined using optical microscopy. Increased discharge energy lowers height, increases density, and microhardness. FE analysis can be used to optimize EMAPC process parameters for powder compact density and porosity.

Metal injection molding (MIM) is a net-forming technology for manufacturing miniature metal parts, which can improve the manufacturing accuracy of complex-shaped parts. In this study, aluminum stearate was used to modify the 17-4PH stainless steel powder, the binder was composed of (PMMA) and (PEG) with the volume ratio of 7:3. The effects of aluminum stearate surfactant amount on the feedstock viscosity, water debinding rate and thermal debinding shape were studied. The results showed that the coating of aluminum stearate breaks the agglomeration between the powders and decreases the viscosity of the feedstock. When the addition of aluminum stearate was 0.6wt% of the powder mass, the melt index, density and flexural modulus of the feedstock were 81.5 g/10 min, 5.44 g/cm³, and 1643Mpa, respectively, and the thermal debinding shape retention was the best of all debinded parts. However, too much aluminum stearate makes the feedstocks thicker and the green part weaker.

The current interest in Al-Mg-Sc-Zr alloy fabricated by powder bed fusion-laser beam (PBF-LB) is centered around excellent mechanical property without taking into account its corrosion behavior. In this work, gas-atomized Al-Mg-Sc-Zr alloy powder was manufactured by PBF-LB. The influence of heat treatment on microstructure, phase evolution, mechanical properties, and electrochemical corrosion behaviors were investigated and microstructure–property relationship was established to obtain high-performance alloy. The result suggests that the sample heat treated at 350°C for 4 h possesses highest mechanical properties, but is more susceptible to corrosion. When heat treatment temperature reaches 400°C, the loss of coherency for Al3(Sc, Zr) precipitates decreased the strength, but the Al₆Mn phase is corroded as cathode instead of the α-Al matrix, thereby inhibiting the occurrence of pitting corrosion. The specimen heat treated at 300°C for 4 h exhibited high strength and high elongation as well as superior corrosion resistance.

We investigated the effect of high-energy ball milling (HEBM) on the microstructure and mechanical properties of powders and spark plasma sintered samples of an Al–Zn–Mg–Cu–Si alloy. HEBM produced a nanocrystalline powder with an average grain size of 0.16 μm while increasing the amount of solid solution and the formation of fine amorphous aluminium oxide. The sintered alloy without HEBM consisted of η-Mg(Zn,Cu,Al)₂, T-Mg₃₂(Al,Zn)₄₉, β-Mg₂Si, and Q-Al₅Cu₂Mg₈Si₆ phases. The grain size of the sintered alloy decreased from 2.66 to 0.40 μm due to the application of HEBM. The amorphous aluminium oxide phase in the milled powder was transformed into MgO particles during sintering. The formation of MgO particles caused the depletion of Mg solid solutions, which resulted in the formation of Mg-free phases during sintering. High-energy ball milling (HEBM) improved the microhardness of the sintered alloy from 94 to 134 HV owing to grain refinement and the formation of fine secondary phases and MgO particles.

A gas atomised Ti-6Al-4V powder, classified as out-of-specification for additive manufacturing (AM), was consolidated via Field-Assisted Sintering Technology (FAST). Fully dense 250 mm diameter discs with lamellar or bimodal microstructures were produced by FAST processing either above or below the β-transus temperature. Static and dynamic mechanical properties were assessed by testing full-size specimens in the ‘as-FAST’ condition. Material from both processing conditions exceeded the ASTM Ti-6Al-4V powder metallurgy requirements for yield/tensile strength and elongation. Furthermore, material from the edge of the disc processed below the β-transus temperature meets ASTM requirements for wrought Ti-6Al-4V. Fatigue performance also compared favourably with conventionally processed Ti-6Al-4V. This work establishes that surplus AM powders can be successfully recycled via the one-step FAST process and provides confidence that this ASTM-grade material can be used in a range of applications under both static and dynamic loading, which will improve the sustainability credentials of the AM sector.

Bi–Te-based thermoelectric materials with excellent room-temperature properties are difficult to commercialise because of their low dimensionless figure of merit (ZT). In this study, a powder was synthesised by adding an excess amount of Te to improve its thermoelectric performance at room temperature. The synthesised powder was sintered using spark plasma sintering at various temperatures to examine the effect of temperature-dependent behaviour of Te on the electrical resistivity and Seebeck coefficient. The excess Te-containing powder exhibited excellent electrical conductivity and Seebeck coefficient at high temperatures, and the thermal conductivity was significantly reduced. Bi_0.4Sb_1.6Te_3+x (x = 2) showed a maximum room-temperature ZT of 1.57 at a sintering temperature of 450°C.