Bernard Tougas

This study investigates direct powder forging (DPF) as a new approach for near-net-shape processing of titanium alloys using a coarse particle size distribution (PSD) between 90 and 250 μm. This route was utilised to takes advantage of DPF’s enclosed nature to make near-net-shape components with conventional forging equipment, making it attractive and viable even for reactive powder such as titanium. In this study, the uncompacted Ti-6Al-4V ELI powder was sealed under vacuum in a stainless-steel canister and hot forged in air to produce a fully dense titanium femoral stem. After the final forging stage, the excess material in the flash region was cut, which efficiently released the canister, revealing the forged part with minimal surface contamination. The as-forged microstructure comprises coarse β grains with a martensitic structure. The subsequent annealing was able to generate a fine and homogenous lamellar microstructure with mechanical properties that respects the surgical implant standard, showing that DPF offers significant potential for forged titanium parts. Therefore, the DPF process provides a suitable alternative to produce titanium components using basic equipment, making it more available to the industry.

In this paper, we report the first hydrogenation (activation) of a 1.2Ti-0.8Fe alloy synthesized by induction melting (9 kg ingot). The alloy presented a three-phase structure composed of a main TiFe phase, a secondary Ti₂Fe phase and a Ti-rich BCC phase. The alloy required cold rolling to achieve activation at room temperature. However, it did so with good kinetics, reaching saturation (2.6 wt.% H) in about 6 h. After activation, the phases identified were TiFe, Ti₂FeH_x and an FCC phase. The Ti₂FeH_x and FCC are the stable hydrides formed by the secondary Ti₂Fe and BCC phases, respectively. The stoichiometry of the Ti₂FeH_x was calculated to be between x = 3.2–4.75. As the microstructure obtained by an industrial-scale synthesis method (induction melting) may be different than the one obtained by laboratory-scale method (arc melting), a small 3 g sample of Ti_1.2Fe_0.8 was synthesized by arc melting. The lab-scale sample activated (2 wt.% H in ~12 h) without the need for cold rolling. The phases identified for the lab-scale sample matched those found for the induction-melted sample. The phase fractions differed between the samples; the lab-scale sample presented a lower abundance and a finer distribution of the secondary phases. This explains the difference in the kinetics and H capacity. Based on these results it can be concluded that the alloy of composition, 1.2Ti-0.8Fe, can absorb hydrogen without the need for a heat treatment, and that finer microstructures have a strong influence on the activation kinetics regardless of the secondary phases’ phase fractions.

The study of powder metallurgy processing methods for titanium represents a promising avenue that can respond to a growing demand. This work reports the feasibility of direct powder forging (DPF) as a method to process large spherical Ti-6Al-4V powder into wrought products with noteworthy properties and physical characteristics. Direct powder forging is a thermomechanical process that imparts uniaxial loading to an enclosed and uncompacted powder to produce parts of various sizes and shapes. Stainless steel canisters were filled with prealloyed Ti-6Al-4V powder and consolidated through a multi-step open-die forging and rolling process into wrought DPF bars. After DPF, annealing was performed in the upper α+β phase. The results show that full consolidation was achieved and higher mechanical properties than the Ti-6Al-4V grade F-23 requirements in annealed conditions were obtained. The results also show that direct powder forging of spherical titanium powder could produce wrought mill products and exhibit some potential for further investigation for industrial applications.

In this paper, we report the effect of adding Zr + V or Zr + V + Mn to TiFe alloy on microstructure and hydrogen storage properties. The addition of only V was not enough to produce a minimum amount of secondary phase and, therefore, the first hydrogenation at room temperature under a hydrogen pressure of 20 bars was impossible. When 2 wt.% Zr + 2 wt.% V or 2 wt.% Zr + 2 wt.% V + 2 wt.% Mn is added to TiFe, the alloy shows a finely distributed Ti₂Fe-like secondary phase. These alloys presented a fast first hydrogenation and a high capacity. The rate-limiting step was found to be 3D growth, diffusion controlled with decreasing interface velocity. This is consistent with the hypothesis that the fast reaction is likely to be the presence of Ti₂Fe-like secondary phases that act as a gateway for hydrogen.

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.