Recycling of gamma titanium aluminide scrap from investment casting operations

Abstract

In investment casting processes for TiAl alloys, a substantial amount of the original raw material does not end-up as a final product but is instead solidified in runners and feeders of the casting system or as a skull in the crucible. Because of the high prices for the virgin alloys, there is a strong interest in remelting this scrap. This is a challenging task due to the tremendous oxygen affinity of titanium alloys. The contact of the melt with shell mould and crucible material leads to oxygen pickup during casting. Up to now no technique exists to avoid this effect, thus any recycling process for TiAl casting scrap potentially needs a deoxidation technique. Within the IMPRESS project, experimental proof for the feasibility of a novel treatment has been obtained in a series of tests and results are presented in this paper. Casting scrap from pilot-trials has been consolidated via classic vacuum induction melting (VIM) in special ceramic crucibles and then successfully deoxidized by pressure electroslag remelting (PESR) under reactive slags. Further refining by vacuum arc remelting (VAR) finally leads to removal of dissolved reducing agent and impurities like non-metallic-inclusions.