Powder Metallurgy

A new shrinkage kinetic equation accounting for the extension of the interparticle contact areas is proposed. The mass transport mechanism responsible for shrinkage is bulk diffusivity. Shrinkage increases on increasing the internal neck radius that represents the geometrical activity in sintering of prior cold compacted powders.

Hot isostatic pressing (HIP) is an ideal fabrication technique for products with complex structure, but also a reliable post-treatment method. The individual effect of the HIP or HIP post-treatment (HIPPT) on the mechanical properties of powder compacts have been investigated, but little attention has been paid to their combined effect. In this study, the effect of HIPPT on the densification, microstructure and mechanical properties of HIPed powder compacts were investigated. The relative density shows a regional increase after HIPPT. The interlayer spacing of lamellar α phase is increased, equiaxial grains are coarsened and high-angle grains are decreased. Moreover, an obvious increase in elongation is detected, which demonstrates that HIPPT is beneficial to improve the properties of HIPed powder compacts.

A novel low-cost gas atomisation technology producing spherical titanium powder (wire induction heating gas atomisation, WIGA) has been developed for additive manufacturing. Combined with the gas atomisation principle, the characteristics of WIGA were analysed. The effects of the gas pressure, metal temperature and the wire-feeding speed on the particle size of the titanium powder were studied. The results indicated that the decreases in mass median particle diameter (D₅₀) and the increases in efficiency of fine size powders occurred with the increase in gas atomisation pressure and melting temperature and with the decrease of wire-feed speed. The optimum parameters are that the main gas pressure (P₀) is 4.0 MPa, the degree of superheat of the metal melt is 350°C and the wire-feed speed is 50 mm s⁻¹. On the condition, the D₅₀ of titanium powder was 40.2 μm and powder morphology was spherical. Satellites rarely existed on the surface of particles.

A new multi-functional binder with adhesive and lubricant effect was designed, and the thixotropic properties of the binder components and the resulting binder, the adhesive effect and compaction behaviour of the binder-treated powder were investigated. The results demonstrate that the thixotropic components in the binder are polyethylene wax and microcrystalline wax. With the increase of polyethylene wax content, the critical solid–liquid shear stress of binders decreases gradually. The main lubrication components in the low-pressure and high-pressure stages during the compaction process are found to be polyethylene wax and microcrystalline wax, respectively. The fine metal or graphite additives are effectively bonded on the surface of iron particles and filled in the pits. The green density reaches 7.25 g cm⁻³ at 600 MPa when the content of polyethylene wax is 30%. From 400 to 750 MPa, the content of polyethylene wax corresponding to the minimum ejection pressure decreases from 20 to 0%.

Nickel-based K417G superalloy powder is the main material used in laser additive manufacturing (LAM) of aerospace engine turbine blades. In this study, K417G powder was prepared by the vacuum induction melting gas atomisation method (VIGA). Powder characteristics and microstructure were measured, and the properties of fabricated samples were studied. The results show that the powder mean particle size D₅₀ was 74 μm, flowability was 16.6 s/50 g, apparent density was 4.78 g cm^–3, and the oxygen content of the powder was 0.015%. Powder and LAM sample microstructure consisted of γ, γ′, and carbide. The average microhardness was 410 HV, tensile strength was 1080 MPa, yield strength was 828 MPa and elongation was 13.52%. K417G superalloy powder prepared by VIGA method has potential for application in laser additive manufacturing.

The equiatomic ratio CrMnFeCoNi high entropy alloy (HEA) was prepared by mechanical alloying (MA) and spark plasma sintering. This paper reports the behaviour of MA, the phase formation, microstructure and mechanical properties of CrMnFeCoNi HEA. With the increase of milling time, solid solution with single FCC phase was gradually formed. The single FCC phase remained as matrix after SPS at 1373 K and 50 MPa. Ultrafine-grained microstructure and good mechanical properties were obtained: At room temperature, the as-sintered bulks exhibit an excellent combination of high compressive strength (2390 MPa) and high fracture strain (47%).

Ti–Nb β alloys are a promising alternative as an implant material due to their good properties and low Young's modulus, compared to other Ti-alloys currently employed as biomaterials. In this study, three materials of the Ti–Nb and Ti–Nb–Fe systems were produced by powder metallurgy techniques starting from TiH₂ (TH) powder. Several sintering cycles were employed to evaluate the H₂ elimination and the effect of sintering temperature on densification and fraction of β-Ti phase. Also, the influence of alloying element size using two kinds of Fe powder was evaluated. The highest loss of H₂ was achieved by decreasing heating rate at the temperature range of hydride decomposition. SEM images and XRD results show mainly a β-Ti phase for TH–40Nb and TH–5Fe–25Nb samples. The TH–12Nb sample shows (α + β) microstructure. Fe addition with smaller particle size seems to improve the diffusion of Nb into Ti which promotes a higher β-phase fraction and sample homogeneity.

This work is devoted to the preparation of alloys based on intermetallic compounds in the Ti–Al–Si system by powder metallurgy using mechanical alloying and the spark plasma sintering (SPS) method. The aim was to describe the formation of intermetallic phases during mechanical alloying of TiAl15Si15 (wt-%) alloy and to consolidate the powder prepared by optimised conditions. Phase composition, microstructure and hardness of compacted alloy were determined. Four hours of mechanical alloying is sufficient time for preparation of pure elements free material composed only of intermetallic phases. After consolidation, the TiAl15Si15 alloy has a homogeneous structure composed of silicide (Ti₅Si₃) in aluminide (TiAl) matrix. The hardness of the material reaches 865 ± 42 HV 5.

AlCrFeCoC high entropy alloy was synthesised through mechanical alloying and spark plasma sintering. The milling time had a strong influence on the particles shape and structure and consequently on microstructural and mechanical evolution of the material after sintering. The material's microstructure after spark plasma sintering contained FCC and BCC phases as well as ordered BCC and C₂₃C₆ carbide. The material's strength increased with increasing the milling time because of the finer microstructure and phases formation evolution.