Influence of TiO 2 on alpha case reaction of Al 2 O 3 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 ∼350 μm. The microstructural characterisation and surface hardness profiles of castings with Al₂O₃+Ti moulds indicated that the alpha case thickness decreased significantly to ∼45 μm. 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.