The influence of polymer content and sintering temperature on yttria face-coat moulds for TiAl casting

Yttria is an important primary coat material in investment shells for casting extremely reactive TiAl alloys and polymer is usually added to slurry to improve strength of the shell. In this investigation, systems that vary the polymer content through 0%, 6% and 30% were produced and the samples were sintered at 1000 °C, 1200 °C and 1400 °C. The results suggest that polymer content and sintering temperature appear to have little effect on the hardened alpha layer of the cast alloy, which is elevated by near-surface oxygen content. Silica from the backing coat is seen to travel through the primary coat and diffuse into the alloy regardless of the shell system. Firing temperatures above 1200 °C increase the shell strength undesirably but the friability of the primary coat decreases as firing temperature increases. Higher mould permeability was found in the mould containing higher polymer level and this would be advantageous for complete mould filling.