Spark plasma sintering of commercial and development titanium alloy powders

Abstract

Emerging lower cost titanium metal powder produced via an electrolytic method has been fully consolidated using spark plasma sintering (SPS) generating microstructures comparable to those observed in Ti–6Al–4V PM product. This is the first time powder from an alternative titanium extraction method has been processed via SPS and it is benchmarked with commercial alloys (CP–Ti, Ti–6Al–4V, and Ti–5Al–5V–5Mo–3Cr). The effect of powder feedstock size, morphology, and alloy chemistry on the consolidated density and microstructure is presented. Through a design of experiments (DoE) type approach the effect of SPS processing conditions on these alloy powders, including maximum sintering temperature, pressure, heating rate, and dwell time were investigated. The SPS process is found to be largely insensitive to feedstock size and morphology, although very large or highly porous powder particles are more difficult to fully consolidate. The maximum sintering temperature and pressure have the largest contribution to achieving full consolidation, with higher pressures and temperatures increasing the final density. Increasing heating rate increases the final grain size, despite less time being spent at the higher temperature and it is thought this is due to bypassing the traditional first phase of sintering. This paper shows that SPS is a viable step for a low-cost manufacturing route, for example to produce preform billets to be finished with a one-step forging operation, especially when combined with the possibility of lower cost powder. In the long-term, SPS will allow a significant reduction in the processing cost, contributing to an increased usage of titanium powder feedstock for a range of applications. This is reinforced by the successful large scale production of a 5 kg SPS Ti-6-4 billet, demonstrating the potential industrial scalability of the process, particularly for the aerospace industry.