Progress in Hot Isostatic Pressing Technology of Titanium Alloy Powder

The research status and application of powder metallurgy (PM) titanium alloys in connection with near net shape forming technology using hot isostatic pressing (HIP) are reviewed in this paper. A brief summary of historic developments with production of clean prealloyed powder and the use of computer simulation techniques in powder densification as important milestones is presented first. The bulk of the paper is concerned with progress made in the last 15 years, especially in the last decade, citing examples from the authors' group. Four types of alloys are covered: a cryogenic titanium alloy, Ti-5Al-2.5Sn with extra-low interstitial (ELI), which is used to make impeller for hydrogen pump of rocket engine, a high temperature titanium alloy Ti55, which is intended for long term service at 550 degrees C in engine applications, and two Ti-Al based intermetallic compounds including both gamma-TiAl and an orthorhombic alloy based on Ti2AlNb. Comparisons in mechanical property were made between the PM alloys and their wrought and cast versions wherever possible. Key issues influencing densification, such as powder size segregation and gas pores in large powders, variation in powder surface oxygen content with powder store time, oxygen layer on gamma-TiAl powder surface due to abnormally high fraction of the alpha(2)-Ti3Al phase as a result of rapid solidification of the powder, were discussed. The final section is dedicated to finite element modelling of powder densification, taking into account such factors as tooling design and stress shielding effect during HIPing. Future research directions are suggested in the summary section.