Powder Metallurgy

The AlFeCoNiCrTi high-entropy alloy (HEA) powders were prepared by high-energy ball milling. The ultrafine-grained WC–HEA and WC–Co-cemented carbides were fabricated through planetary ball milling and heat-pressure sintering. The microstructures and properties of the sintered alloys were compared using scanning electron microscope, X-ray diffraction, mechanical property testing and electrochemical testing. It has been shown that the AlFeCoNiCrTi HEA can be used as a binder for the ultrafine-grained WC-based cemented carbide. The WC–HEA-cemented carbide has better performances than the WC–Co-cemented carbide. The suitable contents of HEA can inhibit the WC grain growth and improve the mechanical properties and corrosion resistance of cemented carbides.

The rate-dependence of hardmetal powder pressing in cutting insert production is investigated experimentally and numerically. In the latter case, the finite element method is relied upon using a continuum mechanics approach. In particular, possible rate-dependency due to creep deformation and rate-dependent friction is discussed with the experimental investigation focusing mainly on dimensional changes during sintering but also pressing forces. The results indicate that rate-dependent frictional effects are the dominating feature and accordingly, it can be argued that for the metal powders investigated here, creep deformations do not have to be accounted for in the constitutive description at the timescales relevant for powder pressing and when the shape after sintering is concerned. For the present powder, the apparent frictional effect decreases at higher pressing rates. Additional details of the friction behavior are studied comparing finite element simulations with experiments.

Spray-drying process was selected to agglomerate ball milled NiCoCrAlY–Al₂O₃ composite powders. The effect of the starting alloy powder size on the morphology of composite powder was studied. The parameters of milling were optimised by orthogonal experiment to improve the powder's flowability and apparent density. Then the optimised powder was sprayed by air plasma spray to prepare NiCoCrAlY–Al₂O₃ composite coating. The results showed that the size distribution of starting particles decided the deformation of alloy particles and the characteristics of agglomerated powders eventually. With the decreasing size range of the starting alloy particles, the sphericity of agglomerated powders increased. The optimised milling parameters were as follows: solid content, 60 wt-%; BPR, 4:1; the rotating speed, 350 rev min⁻¹; and milling time, 5 h. And the contribution of solid content was the largest. The Al₂O₃ splats showed good adhesion with alloy matrix when the composite powder melted in good condition.

Titanium and titanium matrix composites, reinforced with TiB₂ particles, have been synthesised by the spark plasma sintering method at 1050°C under 50 MPa pressure, using mixtures of 2.4 wt.-% TiB₂ and 97.6 wt.-% Ti powders. The changes in microstructural features and mechanical properties were investigated. XRD results and SEM observations confirm the formation of TiB whiskers as a result of the reaction between Ti and TiB₂. However, some unreacted TiB₂ particles have remained in the composite owing to the incomplete chemical reaction between matrix and additives. The measured mechanical properties demonstrate that the increase in hardness and tensile strength with TiB₂ addition is mainly attributed to the generation of TiB whiskers, increase in relative density and decrease in grain size, while the reduction in bending strength is possibly due to the plastic restraint imposed on the matrix by the TiB whiskers and unreacted TiB₂ particles.

Nanocrystalline composites of Cu_90−xCo₁₀Ni _x (x = 0, 10, 20 wt-%) alloys reinforced with 7.5 wt-% of intermetallic SmCo₅ particles were prepared by several processing steps. The production of these alloys started with the synthesis of Cu_90−xCo₁₀Ni _x powder alloys by mechanical alloying of pure Cu, Co and Ni in a planetary mill for 30 h at 250 rev min⁻¹. As a second step, intermetallic SmCo₅ particles were added, and additional milling was performed for 10 h. The resulting powders of both milling processes were characterised by means of an X-ray diffraction method, a scanning electron microscopy and an electron probe microanalysis. As a final fabrication step, Cu_90−xCo₁₀Ni _x –7.5% SmCo₅ alloy powders were consolidated by hot pressing under 88 MPa for 75 min at 750°C in an argon atmosphere. The resulting compacts are reinforced by the precipitation of Co(Ni) during consolidation, and the presence of second-phase particles of oxides and intermetallic SmCo₅.

Surface chemistry of the titanium powder has particularly growing interest due to the increasing application of titanium components prepared by powder metallurgy, in particular metal injection moulding and additive manufacturing. Due to the high chemical activity, number of titanium oxides, calcium and complex Ca–Ti–oxides can be expected on the component/medical implant surface, depending on powder and component manufacturing and post-treatment, but are very difficult to analyse due to the lack of the experimental data and analysis methodology. Therefore, a methodology for the analysis of the surface chemistry of the Ti-powder by XPS utilising internal standard reference was developed. The obtained methodology was used for the surface analysis of titanium powder and identification of its surface oxide composition. The results show that the powder surface is covered by TiO₂ layer in the form of rutile with a thickness of 4.4 nm. Carbon and nitrogen impurities were also found present on the powder surface.

Powder metallurgy (PM) steel components are generally considered ‘difficult’ to machine when compared with their wrought counterparts. Thus, efficient and cost effective machining strategies have to be devised. This paper summarises the results obtained by utilising ultrasounds to improve the machining behaviour of copper and sinter-hardenable PM steels. The effect of ultrasonic assisted drilling on tool life with and without admixed MnS is discussed as well. These results are compared to conventional machining methods and show that, for FC-0208 PM copper steels, tool life is increased by 180% and can be several thousand per cents higher if used in combination with admixed MnS. At the same time, the use of ultrasounds permitted to increase productivity by 86%, owing to the possibility of using higher feed rates. Similarly, for FLNC-4408 pre-alloyed PM steels, tool life was increased by 130% and productivity was 60% higher than when no ultrasounds were used.

PM steels with density higher than 7.5 g cm⁻³ were prepared from binder-treated powder, and the effects of alloying elements and microstructure on dynamic performance were studied. Although the addition of 1.5 wt-% Cu improves the tensile strength, it impairs toughness of the alloy by the formation of low-toughness pearlite. The 0.5 wt-% Cr addition promotes the formation of Ni-rich martensite and bainite at the expense of pearlite, which is beneficial to strengthen and homogenise the microstructure. The 0.5Cr alloy exhibits improved tensile strength and fatigue strength without sacrificing impact toughness, which exhibits high potential to be used for dynamic and cyclic loading applications. The tensile strength and fatigue endurance strength under 10⁷ cycles of 0.5Cr alloy achieve 1350 and 562 MPa respectively, and impact energy is as high as 21.1 J. The enhanced fatigue life is attributed to the high density, smaller pore size, rounder pores and composite-like microstructure.

In this study, aluminium matrix hybrid composites (AMHCs) containing 10 wt-%SiC microparticles and wt-%TiB₂ (  = 1, 3 and 5) nanoparticles were fabricated by powder metallurgy. The effect of TiB₂ content on microstructure and mechanical properties were examined by a combination of scanning electron microscopy, transmission electron microscopy and tension tests. The results showed that mechanical properties of the hybrid composites were improved with increasing the content of TiB₂ nanoparticles. Compared to aluminium matrix composites reinforced with mono SiC microparticles, the ultimate tensile strength and yield strength of the AMHCs with additional 5 wt-% TiB₂ nanoparticles were increased by 64 and 23%, respectively. Based on the fractographs, the relevant fracture mechanism was also discussed.

In the present study, the effects of microwave sintering (MWS) on microstructural, tribological and corrosion properties of Cu_0.7Ni_0.3-5 wt-% Al₂O₃ nanocomposites were evaluated. The Cu and Ni powders along with Al₂O₃ nanopowder were mixed in a high-energy ball mill. The XRD patterns indicating formation of Cu_0.7Ni_0.3 solid solution alloy during mechanical alloying process. The mechanically alloyed powders were compacted by coldpress, and sintered samples at 200 and 300°C for 30 and 60 min by means of conventional sintering and MWS. Tribological properties of sintered samples were evaluated by ball-on-disk wear test. For MWS, results indicate that increasing the MWS temperature decreased the wear rate about 30%. Corrosion behaviour of nanocomposites were studied by cyclic potentiodynamic polarisation in 1 M NaOH solution and was found that corrosion current density of microwave sintered sample was decreased almost 35 times of that for conventional sintered sample.