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An Al2O3–Y2O3 based face coat material was researched by adding Al2O3 into yttria in different concentrations to reduce the mould cost whilst retaining the face coat chemical inertness. The face coat compositions and sintering properties at different temperatures were investigated using X-ray diffraction, friability and dilatometers. Meanwhile, the chemical inertness of the face coats was identified using a sessile drop test method to interact with a Ti–45Al–2Mn–2Nb–0·2TiB alloy. The results show that at different firing temperatures, a series of phase transformations have taken place between Y2O3 and Al2O3, resulting in different face coat compositions and sintering properties. However, the chemical inertness of the face coat against the molten TiAl alloy was not influenced by the phase transformations, and it was dependent on the amount of Al2O3 added to the face coat. In this experiment, no interaction was observed between the Al2O3–Y2O3 face coat material and TiAl alloy when Al2O3 additions we...

Yttria is widely used as a key material in the manufacture of the mould face coat for the investment casting of Ti or titanium aluminide alloys, as it has the required thermal and chemical inertness. Because of the high melting temperature of yttria, moulds manufactured using an yttria face coat need high sintering temperatures to achieve a good surface finish, which increases the production cost of the mould. In this research, small amounts of sintering aids, B2O3, YF3, Al2O3 and ZrO2, were added into a simple yttria face coat and sintered at temperatures from 1000 to 1400°C to investigate the influence on the sintering properties of yttria. These experiments found that the sintering additive B2O3 showed the best performance in enhancing yttria face coat sintering over the test temperatures. Sintering aids such as YF3 only slightly altered the sintering properties of yttria at temperatures around 1000–1200°C, and powders such as YAZ were seen to be more suitable for sintering at temperatures arou...

Post-heat treatment recrystallisation is a significant cause of scrap in single crystal component production. This defect could be prevented by a reduction in the mechanical strength of the shell after the metal has solidified. Addition of a zirconia layer, to produce large cracks around grain boundaries and among the large grains during phase transformations, was considered as a method of reducing sintered strength. In this study, two types of zirconia stucco, monoclinic and partially stabilised, were introduced into the shell. The effect of the incorporation of a zirconia layer on the microstructure and mechanical properties of the shell were investigated. Shell with a monoclinic zirconia additional layer showed no reduction in the green strength but a better performance in high temperature deformation tolerance, maintaining shell performance during wax removal and metal casting. The shell strength after alloy solidification is reduced, suggesting that a reduction in recrystallisation may be pos...