Ti-6Al-4V and stainless steel 316L have been processed by selective laser melting under similar conditions, and their microstructures and mechanical behaviours have been compared in details. Under the investigated conditions, Ti-6Al-4V exhibits a more complex behaviour than stainless steel 316L with respect to the occurrence of microstructural and mechanical anisotropy. Moreover, Ti-6Al-4V appears more sensitive to the build-up of internal stresses when compared with stainless steel 316L, whereas stainless steel 316L appears more prone to the formation of 'lack of melting' defects. This correlates nicely with the difference in thermal conductivity between the two materials. Thermal conductivity was shown to increase strongly with increasing temperature and the thermophysical properties appeared to be influenced by variations in the initial metallurgical state.
Room temperature tensile ductility is an important property of titanium (Ti) and titanium alloys for structural applications. This article reviews the dependency of tensile ductility on oxygen for α-Ti, (α+β)-Ti and β-Ti alloys fabricated via traditional ingot metallurgy (IM), powder metallurgy (PM) and additive manufacturing (AM) or three-dimensional printing methods and recent advances in understanding the effect of oxygen on ductility. Seven mechanisms have been discussed based on case studies of individual titanium materials reported in literature. The dependency of ductility on oxygen is determined by both the composition and microstructure of the titanium alloy. For Ti-6Al-4V (wt-%), as sintered Ti-6Al-4V shows a critical oxygen level of about 0·33 wt-% while additively manufactured Ti-6Al-4V exhibits different critical levels ranging from about 0·22% to well above 0·4% depending on microstructure. Rare earth (RE) elements are effective scavengers of oxygen in titanium materials even just with a small addition (e.g. 0·1 wt-%), irrespective of the manufacturing method (IM, PM and AM). High cycle fatigue experiments revealed no initiation of fatigue cracks from the resulting RE oxide particles over the size range from submicrometres to a few micrometres. A small addition of RE elements offers a practical and affordable approach to mitigating the detrimental effect of oxygen on ductility.
In this study, aluminium matrix hybrid composites (AMHCs) containing 10 wt-%SiC microparticles and wt-%TiB 2 ( = 1, 3 and 5) nanoparticles were fabricated by powder metallurgy. The effect of TiB 2 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 TiB 2 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-% TiB 2 nanoparticles were increased by 64 and 23%, respectively. Based on the fractographs, the relevant fracture mechanism was also discussed.
High-entropy alloys (HEAs) have emerged as a new alloy system, with many attractive properties. Among the fabrication routes, mechanical alloying followed by sintering, have been widely used. However, sintering mechanisms of HEA powders have not yet been fully understood. This work attempts to understand the sintering kinetics of CoCrFeNiMn HEA powders. A comparative study has been done on CoCrFeNiMn alloy powders on as-milled and annealed conditions, which revealed different sintering behaviours. Decreasing densification rates with increasing activation energies were observed through dilatometry after the annealing treatment. Combined diffusion coefficients and activation energies analysed through the sintering models indicate that during sintering of the as-milled powder, mass transport occurred through several modes. On the other hand, the annealed CoCrFeNiMn alloy powder (which was nearly single phase) clearly reveals volume diffusion as the controlling mechanism during sintering. It was characterised with large activation energy.
Spherical 24CrNiMo alloy steel powder used for selective laser melting (SLM) fabricating high-speed train brake disc was prepared by the vacuum induction melting gas atomisation (VIGA) method. Powder morphology, particle size, flowability and microstructure were measured. Part properties fabricated by SLM were investigated via some modern analysis method. The experimental results showed that powder mean particle size D 50 was 75 μm, flowability was 16.69 s/50 g and apparent density was 4.71 g cm −3 . 24CrNiMo alloy steel specimen microstructures prepared by SLM consisted of proeutectoid ferrite and granular bainite. Average microhardness was 346 HV, tensile strength was 1223 MPa, extensibility was 13.1% and the product of strength and elongation was 16.1 GPa%. 24CrNiMo alloy steel powder prepared by the VIGA method had good laser printability and huge potential application value for SLM-fabricated brake disc.
An Fe-35 wt-%Mn alloy, aimed to be used as a metallic degradable biomaterial for stent applications, was prepared via a powder metallurgy route. The effects of processing conditions on the microstructure, mechanical properties, magnetic susceptibility and corrosion behaviour were investigated and the results were compared to those of the SS316L alloy, a gold standard for stent applications. The Fe35Mn alloy was found to be essentially austenitic with fine MnO particles aligned along the rolling direction. The alloy is ductile with a strength approaching that of wrought SS316L. It exhibits antiferromagnetic behaviour and its magnetic susceptibility is not altered by plastic deformation, providing an excellent MRI compatibility. Its corrosion rate was evaluated in a modified Hank's solution, and found superior to that of pure iron (slow in vivo degradation rate). In conclusion, the mechanical, magnetic and corrosion characteristics of the Fe35Mn alloy are considered suitable for further development of a new class of degradable metallic biomaterials.
Lately high-entropy alloys (HEAs) have been the topic of extensive research, as these materials are promising candidates for many challenging applications, as for example tools, moulds and functional coatings. In contrast to conventional alloys, HEAs consist of five or more principal elements, each having a concentration between 5 and 35 at.-%. Against expectations, HEAs show a rather simple microstructure consisting preferentially of cubic phases. Due to this microstructure, HEAs show promising properties, e.g. in terms of high-temperature stability, high strength and ductility. Within this research, a single-phase CoCrFeMnNi HEA was produced by powder metallurgy (PM). In contrast to conventional metallurgy, PM offers a lot of advantages, e.g. good material efficiency and high shape complexity. Gas atomised powder was used and selected PM methods are presented (e.g. pressureless sintering, spark plasma sintering, additive manufacturing (EBM)). The process methods were evaluated by characterising the material properties (density, microstructure, mechanical properties) of the compacted and sintered samples.