The study of the oxidation of a Ti-45Al-2Nb-2Mn-0.2B alloy in the investment casting using an yttria face coat mould

Investment casting is an economical method to manufacture near net-shape metal components. Due to the very high thermal and chemical inertness, yttria has been widely used as the mould face coat material for the investment casting of titanium alloy for many years. An investigation was undertaken to study the oxidation behaviour of TiAl alloy during casting in a mould using pure yttria as the face coat. This research shows that the TiAl alloy was still oxidized in the mould during casting when using yttria as the face coat. During high temperature casting, the yttria in the face coat was dissolved by high temperature molten metal flow. The oxygen from the yttria face coat diffused into TiAl and interacts with TiAl to form different microstructure and phases (e.g. precipitates such as oxygen enriched Ti₃Al and Al ₂O₃ phases). Meanwhile, the dissolved yttrium was then re-precipitated at the metal interfacial area as yttria inclusions after metal cool down.