Ti powder compacts can be sintered using a variety of processes, including pressureless sintering under vacuum or inert gas, as well as pressure-assisted consolidation techniques such as vacuum hot pressing (VHP) and HIP. Of course, the microstructure of the sintered Ti alloy depends on the specific process and the processing parameters employed, as well as the feedstock powder.
We will focus on the effects of sintering processes on porosity, purity, and microstructure (grain size and morphology), and analyse how these three factors affect the mechanical properties of sintered Ti alloys. It should be noted that this review does not delve into the sintering mechanisms or kinetics of sintering, but rather the focus is on the effects of PM processing on the microstructure and properties of Ti alloys.
Issues of residual porosity and purity were the primary focus of the earliest research into Ti powder metallurgy. Impurity elements in Ti alloys include: oxygen, chlorine, carbon, and nitrogen. A significant advance in the BE approach has been achieved with the use of low chloride Ti powder [127]. Chlorine or chloride contents as low as 200 ppm, which is typical for inexpensive Ti powders, have been reported to adversely affect the densification process during sintering [128]. Profound improvement in the as-sintered density has been achieved simply by using powder with less than 10 ppm chlorine [129]. Although the effects of chlorine, carbon, and nitrogen are important, they can generally be controlled during Ti sponge and powder production processes. Additionally, sponge can be produced with very low oxygen content. Nonetheless, oxygen content is the most challenging problem during powder processing and sintering.
As with other reactive metals, a native oxide layer is often present on the surface of titanium powder particles. However, this oxide layer, which tends to retard sintering of aluminium and magnesium alloys, can dissolve at elevated temperatures and does not impede the sintering of Ti [130]. Mo et al. reported that the oxide layer is effectively removed at approximately 700°C [131]. It should be noted that there is some disagreement in the literature regarding the exact temperature, though it is consistently reported that this effect occurs below 1000°C [132,133]. Ti also exhibits comparatively strong diffusion bonding at relatively moderate temperatures. In fact, solid state diffusion bonding at a temperature below the β-transus (995°C for Grade 5 Ti–6Al–4V [134,135]) has been used by Rolls Royce to join Ti sheet in the production of turbine engine blades [136]. For these reasons, Ti readily densifies at temperatures above 1200°C, though residual porosity remains when sintered using conventional pressureless sintering processes.
Regarding the issue of porosity, it has been demonstrated that the residual porosity may be effectively closed via pressure-assisted consolidation (e.g. HIP) or thermomechanical processing (TMP, e.g. forging) after sintering [129,137,138]. However, incorporating these energy-intensive post-processing steps drives up cost [4].
Additionally, refining the as-sintered microstructure produced by traditional sintering processes is almost impossible because of the lack of stored energy to drive recrystallisation. This is particularly true for α + β Ti alloys (e.g. Ti–6Al–4V), which have a tendency to form coarse lamellar microstructures during sintering. The coarse microstructures are detrimental to mechanical properties, especially the fatigue strength, but recent breakthroughs have identified mechanisms to reduce the residual porosity and refine the microstructure of as-sintered Ti–6Al–4V (section ‘Sintering of TiH2‘).
This article will highlight the microstructure as a function of the process, laying the groundwork for examining the relationships between processing, microstructure, and properties in the subsequent sections.