Pressureless sintering includes inert gas and vacuum sintering, which are both reported to have been used for sintering Ti. However, vacuum is far more commonly used than inert gas sintering for Ti.
The first experiments aimed at sintering Ti sponge were performed by Kroll in 1937. During these experiments, Kroll used a low-pressure argon atmosphere (
) [139]. In the 1940s, additional investigations into the effect of the sintering atmosphere were performed by Dean et al. [140] with the aim of producing bulk Ti samples with improved ductility. In this study, it was found that helium was a suitable protective gas. However, it was also determined that vacuum was required to remove residual hydrogen, as well as magnesium left from the extractive process.
Vacuum has long been considered the ideal sintering atmosphere for Ti, owing largely to the work performed by Dean et al. [140] in the 1940s. However, any process which requires vacuum is inherently limited to batch processing. For this reason, continuous sintering processes under protective gases have long been investigated. All non-noble protective gases (e.g. nitrogen) react readily with Ti and will compromise the purity and mechanical properties of the alloy if used for sintering. With respect to cost, argon is the only feasible option of the noble gases. In fact, argon sintering has been used commercially by DuPont, dating back to the 1950s, to produce PM Ti components [141]. Additionally, Toyota uses argon sintering to produce PM Ti metal matrix composites [142].
Owing to Ti’s strong gettering of oxygen and nitrogen, commercial grades of argon may result in unacceptable pickup of these elements during sintering. Methods are available for purifying argon in situ, such as flowing the gas over Ti chips or sponge at temperatures over 800°C [143]. Another approach utilises specialised ‘OXYNON’ sintering furnaces produced by Kanto Yakin Kogyo (Japan). These furnaces use a carbon-fiber sintering belt, designed to remove oxygen from argon during sintering [144]. OXYNON furnaces have reportedly been used to continuously sinter Ti since 2002 [130]. Heaney and German reported interesting results in 2004 from a study in which CP-Ti was sintered using a vacuum furnace versus an OXYNON furnace under argon [145]. During the study, identical sintering experiments were performed in each type of furnace and repeated for three different starting powders. It was reported that the samples sintered in the OXYNON furnace had lower oxygen, nitrogen, and carbon content than identical samples sintered in the vacuum furnace. The lower carbon content is a surprising result, considering the fact that the OXYNON furnace employed carbon hardware at high temperatures. However, the samples were separated from the carbon belt with zirconia plates and covered with molybdenum sheets. It was determined that the small amount of carbon introduced by the vacuum oil residue in the vacuum furnace was greater than that picked up from the carbon hardware in the OXYNON furnace. Each sample set had similar density after sintering for both the OXYNON and vacuum-sintered samples. The OXYNON-sintered samples exhibited similar ductility to the vacuum-sintered samples in all but one sample, which had significantly lower ductility. However, the poor elongation of this particular sample was attributed to contamination by sodium-reduced titanium powder next to it in the sintering furnace. The vacuum-sintered samples exhibited consistently higher strength for each sample set by approximately 100 MPa. This fact was not discussed by the authors in the context of the sintering atmosphere, though it could be due to the increased interstitial content of the vacuum-sintered samples.
For BE powders of α + β alloys, temperatures of 1200°C or higher are usually necessary to both facilitate densification and allow for sufficient homogenisation of the alloying elements [146,147]. For α + β alloys, this is well above the β-transus (995°C for Grade 5 Ti–6Al–4V [134,135]). Therefore, at a typical sintering temperature, the microstructure consists entirely of equiaxed β grains on the order of hundreds of microns in diameter and with a homogenous distribution of the alloying elements. As the material is cooled relatively slowly, preferential nucleation of α grains along the prior β grain boundaries results in a continuous layer of ‘grain boundary α’ (αGB). As the material is continually cooled, α grains grow into the bulk of the β grains as colonies of parallel plates with a (110)β||(0001)α Burgers relationship (Figure 15) [134,148,149]. The size of the colonies, as well as the individual α lamellae, is determined by the cooling rate. Faster cooling forces the nucleation of more α grains, which results in a finer microstructure. The high sintering temperatures and relatively slow cooling rate of conventional pressureless sintering, therefore, consistently result in a coarse lamellar microstructure for PM Ti–6Al–4V (Figure 16(a,b)).