Influence of TiO2 on alpha case reaction of Al2O3 mould in Ti investment casting

Experiments were conducted to evaluate the alpha case reaction layer of Ti casting with four different mould materials: Al2O3, Al2O3+Ti, Al2O3+TiO2, and TiO2. Al2O3+Ti moulds were manufactured by a reaction between Al2O3 and Ti powder in air or vacuum. The results show that the Al2O3 mold exhibits an alpha case layer of similar to 350 mu m. The microstructural characterisation and surface hardness profiles of castings with Al2O3+Ti moulds indicated that the alpha case thickness decreased significantly to similar to 45 mu m. This difference was due to the presence of TiO2, which formed a TiO intermediate phase that acted as a diffusion barrier and hindered the alpha case reaction at the casting surface. In addition, Al2O3+TiO2 moulds were ineffective in alpha case reduction due to the unacceptable network between the TiO2 and binder.