ABSTRACTHigh oxygen Ti6Al4V alloys are fabricated by vacuum sintering and hot rolling using hydrogenation-dehydrogenation Ti6Al4V powder and ZrO2 powder as raw materials. The complete dissolution of oxygen and zirconium atoms results in an increase in lattice constant and strength. As-rolled Ti64-ZrO2 sample exhibits a bimodal structure with a fine acicular α phase of around 1 μm. Ti64-0.75 ZrO2 alloy with equivalent oxygen content close to 0.6 wt.-% presents a good combination of the high tensile strength (1180.2 MPa) and sufficient elongation (7.3%). Therefore, the critical oxygen content for the ductile-to-brittle transition of powder metallurgy Ti6Al4V alloy can be improved by suitable preparation methods. The formation of a fine β transformation structure may be one of the reasons for maintaining sufficient ductility.

ABSTRACTTi is well-known for its high strength-to-weight ratio and biocompatibility. It can be alloyed with Zr to improve mechanical properties. Currently, Ti-Zr alloys can be fabricated by powder metallurgy using Ti and Zr powders, although high oxidation reactivity and cost could still be an issue. In this study, premixed TiH2 and ZrH2 powders were used to prepare Ti-Zr binary alloys with different Zr contents of 0–30 mass% ZrH2 via dehydrogenation and sintering by spark plasma sintering (SPS) process. α-Ti and δ-TiH2 phases coexisted in the sintered Ti-Zr alloys, and a characteristic lamellar microstructure was formed. The tensile strength of the Ti-Zr alloys increased with increasing Zr content due to the solid solution effect, grain refinement and the appearance of δ-TiH2 phases, although the elongation was reduced. This study shows that the fabricated Ti-Zr alloys possess controllable mechanical properties, which can be beneficial for biomedical and other engineering applications.

ABSTRACTThe present study investigates the properties and microstructure evolution of MA 6000, a nickel-based alloy produced by mechanical alloying and spark plasma sintering (SPS), a powder processing technique in metallurgy. This study aims to explore the potential of MA 6000 as a high-temperature material for industrial applications. Modified MA 6000 samples of different powder sizes were sintered in a high-vacuum environment at temperatures ranging from 800 to 1100°C. At 800°C, the cohesion between powder particles was not significant, resulting in low-density samples. However, at 1000°C, the samples consisted of many fully sintered regions related to finer powder particles, while no specific morphology was observed at 1050°C. The image quality and inverse pole figure (IQ-IPF) map indicated that the grains were distributed randomly in all sintered samples, and the average distribution of grain size of samples sintered at 1050°C was larger than that of those sintered at 1000°C.

ABSTRACTToday, microfiltration plays the most significant role in removing fine particulate impurities in gas streams used in high-tech industries. However, the current SS316L gas filter has a limitation in enhancing performance due to the trade-off relationship between filterability and permeability. This limitation can be overcome by the geometrical effect of the gas filter that maximises inertial impaction and diffusion interception due to complex flow channels using small flake particles. However, since the smaller the particle size, the yield strength higher, there are few reports of preparing the small flake particles by plastic deformation. This study used a high-energy ball milling process to demonstrate the preparation method for small stainless steel 316L flake particles. Furthermore, the process parameters are systematically optimised to fully transform the spherical particles into thin platelets without fracture and cold welding, and the transformation behaviour is discussed in detail.

ABSTRACTA method to manufacture a porous Cu–Al–Ni shape memory alloy by powder metallurgy using space holders is presented. Two aluminium powders with different particle morphologies were employed to investigate their influence on phase formation, microstructure and mechanical properties. The variation of the relative amount of space holders in the mixture allows to obtain different porosities. Samples prepared with irregular-shaped aluminium powder include both 18R and 2H martensitic phases and exhibit the shape memory effect and pseudoelastic behaviour under uniaxial compression tests. In contrast, the samples made with aluminium flakes present the α phase accompanying the 18R martensitic phase, and do not exhibit the shape memory effect. Both the aluminium flakes flat shape and the higher proportion of aluminium oxide associated with its larger surface area to volume ratio hindered the interdiffusion of the metals, resulting in an aluminium-depleted martensitic phase surrounded by an aluminium oxide-rich layered structure.

ABSTRACTAn apparatus was developed to study the effects of operating parameters on liquid spreading and particle size in centrifugal atomisation. Different areas of aluminium spreading on the surface of the rotary disc were observed, in which the diameter of the ‘Plane area’ showed a linear relationship with the rotational speed. The microscopic morphology of the aluminium powder samples was analysed by scanning electron microscopy (SEM). The variation of particle size distribution curve with rotational speed was explored, and the shape of the curve was changed from ‘Single-peak’ to ‘Double-peak’ when the rotating speed increased to a certain value. The effect of the fragmentation mode on the particle size distribution and median diameter was analysed. The variation of the median diameter of the powder with the rotating speed was obtained. The effect of different disc configurations on the particle size was obtained.

ABSTRACTThis works proposes a methodology for fabricating materials with specific characteristics mimicking human bones. A Ti64 alloy powder was used as the base material and it was mixed with Ag, Ta, TiN and salt particles to obtain different features. A hip-bone like component was fabricated, including a highly porous core of Ti64/25Ta/5Ag and a compact outer shell of Ti64/5Ag that is supposed to improve corrosion and osseointegration. Besides, a harder cover surface in Ti64/10TiN composite should increase the wear resistance. The green component was sintered at 1260°C in argon. Its Young’s modulus was close to the one of bones due to the added porosity, which also provided a permeability close to the one reported for trabecular bones. Tribocorrosion behaviour in simulated body fluid was improved by TiN addition. In conclusion, the proposed processing route was able to produce complex components fulfilling specific features required for human bone replacement.

ABSTRACTHigh layer thicknesses for laser powder bed fusion are promising for productivity increase. However, these are associated with increased process instability, spatter generation and powder degradation, crucial for alloys sensitive to oxygen. The effect of increasing layer thickness from 30 to 60 µm is studied focusing on Ti-6Al-4V spatter formation during LPBF and its characterisation, with scanning and transmission electron microscopy, combustion analysis and X-ray photoelectron spectroscopy. Results indicate that spatters are covered with a uniform Ti-Al-based oxide layer and Al-rich oxide particulates, the thickness of which is about twice that present on virgin powder. The oxygen content was about 60% higher in spatters compared to the virgin powder. The study highlights that increasing the layer thickness to 60 µm permits to reduce the total generation of spatters by ∼40%, while maintaining similar spatter characteristics and static tensile properties. Hence, this allows to increase build rate without compromising process robustness.

Published in Powder Metallurgy (Vol. 66, No. 1, 2023)

ABSTRACTThe aim of this study was to investigate and fabricate a Ti64-bagasse ash (BA) composite using the spark plasma sintering (SPS) method. Accordingly, the samples were fabricated under partial densification. XRD analysis, optical and electron microscopy (SEM), hardness measurement, and bending test were used to study the phases formed, the morphology of powders, microstructure, and mechanical properties of the samples, respectively. The results showed that increasing the volume percentage of bagasse ash affected the mechanical properties of the samples in addition to the microstructure. It was also observed that in the samples containing 5 vol.% bagasse ash, in addition to a significant decrease in the value of elastic modulus, the sample fabricated contained pores with a wall composed of ceramic and metal.

ABSTRACTThis work reports the effect of micro strain of soft magnetic materials obtained by low-energy ball milling. It has been observed that the low energy of the ball milling changes the magnetic domain size, increasing magnetisation, remanence and magnetic permeability of the milled powders as ferromagnetic particles. It is due to the increase of linear defects and the increase of lattice strain. The magnetisation MS increases up to 70% by the accumulation of crystalline defects, although MS decreases with the reduction of particle size, due to the connections of voids between the lines of magnetisation across the crystalline defects obtained by milling. This response is produced by the cold working through the ball milling.

ABSTRACTThe goal of this study was to investigate the mechanical properties and microstructure of Ti-15% Mo alloy fabricated using the mechanical alloying and spark plasma sintering (MA-SPS) method. Accordingly, Ti and Mo powders were milled for different times, including 5, 15, 25, 35, and 45 h, and the SPS technique for sintering under a pressure of 25 MPa at 1100°C was used. The X-ray diffraction (XRD) analysis, optical and electron microscopy (SEM), hardness measurements, and compression testing were used to study the phases formed, the morphology of powders, microstructure, and mechanical properties of the as-prepared samples, respectively. The results revealed that owing to increasing the mechanical milling time, the percentage of the beta phase formed was higher, and in addition, it influenced the microstructure and mechanical properties of the samples fabricated after the sintering process.

ABSTRACTThis work proposes a new neutron absorber, Cu–B composites, with excellent thermal conductivity and good neutron absorbing capability. Mixtures of elemental Cu and B powders with the B contents varying between 10 and 40 at.-% were consolidated into round bars by hot extrusion. The addition of B significantly refined the grain structure, which is related to the suppression of dynamic recrystallisation during hot extrusion and extended recovery during subsequent annealing. Such fine-grained structure in Cu–B composites together with dispersion hardening by B particles contributed to the higher hardness of Cu–B composites. The thermal conductivity of Cu–B composites, being roughly two times higher than that of Al–B4C MMC, decreases with increasing the B contents, which is mainly due to higher volume fraction B particles with lower thermal conductivity, and partly to texture randomisation and refined grain structure in the Cu–B composites.

ABSTRACTThis study aims to compare the flow patterns and in-cavity pressures obtained experimentally and numerically for different conditions. Four feedstocks based on 17-4PH stainless steel powder were fully characterised and implemented as material laws in an Autodesk Moldflow package before to obtain numerical simulations that were then validated using real-scale injections. The flow patterns obtained numerically for the different flat bar mold geometries were in good agreement with the experimental flow patterns, showing an almost perfect fit, whereas for the flow patterns of the complex mold geometry, there were some minor discrepancies. The simulated pressure profiles obtained for different mold geometries, feedstock temperatures, mold temperatures and solid loadings were in good agreement with the experimental pressure profiles in terms of trend and pressure values, with maximum relative differences varying from 30 to 64% depending on particular feedstocks and process parameters.

ABSTRACTComparative results of numerical simulation of the process of hot forging of porous preforms with an axial hole for two deformation schemes are presented: in a die with a flash groove on the inner surface of the forged workpiece cavity and in a closed die. A significant difference in the nature of the evolution of the distribution of deformations and density over the cross-section of the workpiece for the two considered schemes is noted. It is shown that forging in a die with an open volume provides significantly higher degrees of material deformation during forging compared to closed forging, which should lead to an increase in the quality of the structure and final properties of the forged material.

ABSTRACTThe powder metallurgical manufacturing of gears offers a promising opportunity in terms of reducing the noise emission and increasing the power density. Sintered gears weigh less than conventional gears and potentially have a better noise-vibration-harshness behaviour, due to the remaining porosity. However, the potential of sintered gears for highly loaded applications is not fully utilised yet. Six variants of surface densified and case-hardened sintered gears from Astaloy Mo85 are tested to analyse the impact of the densification and case hardening depths on both the tooth root and flank load bearing capacities. Experimental investigations including metallography and computer tomography are carried out to characterise the microstructure. Furthermore, a simulation model is developed to quantitatively describe the residual stress and hardness profiles after the heat treatment. The load bearing capacity was improved by increasing the densification and case hardening depths, where the effect of the case hardening was identified to be predominant.

ABSTRACTAs gas atomisation has been the main method for producing high-performance spherical powders in the past decades, its application in the production of metallic powders has become one of the main research subjects in this field. Since it is challenging to directly observe the atomising gas and to investigate melt flow states by experiments, numerical simulation is attracting increasing interest in studying the gas atomisation process. In this work, various computational fluid dynamics models were implemented to simulate the gas atomisation process. With the models, the droplet breakup, cooling, and solidification within the coupled process were investigated. The final mean particle size was predicted through numerical simulations and compared with the statistics extracted from the gas atomisation process, which shows that a reasonable mass median diameter of the particle can be predicted numerically. The results also show a clear relationship between the breakup trajectory and the resulting particle size.

ABSTRACTIn the present study, crack propagation through the case hardened region of two different PM steels manufactured with different powder size distributions and sintered at different temperatures has been investigated. EBSD was used to study the microstructure before and after case hardening, revealing the relationship between powder particle grains after sintering and prior austenite grains after case hardening. A novel approach was used to achieve short cracks (10–20 µm) with high repeatability. The cracks were then analysed using EBSD and SEM, revealing detailed and novel information about the crack propagation route in the materials. Both tested materials show the same crack propagation behaviour. If a prior grain/prior austenite boundary is present within an angle from the crack initiation site, the cracks will follow the boundary and thus propagate intergranular, suggesting that the preferred route of crack propagation in case hardened sintered steel is along these boundaries.

Published in Powder Metallurgy (Vol. 66, No. 2, 2023)

ABSTRACTA preliminary study has been performed to understand the effect of pressureless sintering on the surface morphology of the AlFeCoNi alloy by the addition of the Si element. This study aims to determine the possibility of achieving the densification of high entropy alloy using the conventional sintering technique. The results indicate that the HEAs have a single-phase BCC structure even with the addition of Si. The thermodynamic simulation (CALPHAD) was used to predict the phase formation. The variation of crystallite sizes and lattice strains caused by sintering temperatures was also discussed. In addition, densification mechanisms occurring with the different sintering temperatures have been discussed. The formation of porosity was observed, however, the density of HEAs improved with increasing sintering temperature. Ultimately, it was suggested that the present HEAs required higher sintering temperatures and a longer time to achieve high density.