In situ synchrotron study of sintering of gas-atomized Ti-6Al-4 V powders using concomitant micro-tomography and X-ray diffraction: Effect of particle size and interstitials on densification and phase transformation kinetics

Compared to the well-established powder bed fusion techniques, sinter-based additive manufacturing of titanium alloys remains extremely challenging. This technique involves three steps: (i) shaping of a part composed of metallic powders bound with polymeric binder (ii) debinding (iii) sintering. One main issue is that densification during the solid sintering is promoted by small powder particles whereas the latter have a high propensity to carbon and oxygen uptakes from the binder, which are detrimental to ductility of titanium parts. In this article, we report a unique in-depth characterization of solid sintering of titanium powders using in situ coupled micro-computed tomography (µCT) and X-Ray diffraction under synchrotron radiation at high temperature, combined with in situ environmental scanning electron microscopy (HT-eSEM). Evolution of global porosity, pore size distribution and interconnectivity as well as allotropic titanium phase transformation and precipitation of second-phase precipitates (titanium carbides) were determined, allowing a discussion on the densification/phase transformation relationship. This multi-scale in situ study of solid sintering was used to identify the effect of powder particle size on the contamination / densification trade-off. Carbon/oxygen uptakes clearly increase the α-to-β transus but do not affect significantly the final porosity of the sintered parts, which argues for a secondary role of the β phase on the sintering kinetics of titanium alloys. Reducing the powder particle size has a tremendous effect on both the densification kinetics and the final pores structure.