The influence of mould pre-heat temperature and casting size on the interaction between a Ti–46Al–8Nb–1B alloy and the mould comprising an Al2O3 face coat

• During casting, oxygen from Al2O3 face coat can harden TiAl interfacial area.
• Si from the backup coat was detected on the metal surface after casting.
• High mould pre-heat temperatures can cause heavier metal/shell interactions.
• Large metal cast dimensions can lead to a server interaction between TiAl and shell.

Alumina has been used as a mould face coat material for investment casting titanium alloys for many years; the interaction between the Al2O3 and the alloys are exemplified by a hardened layer formed at the alloy interface. In this research, the interaction between the Al2O3 face coat and a Ti–46Al–8Nb–1B alloy was investigated by modelling the cooling profile of the mould during casting. The interaction between the mould and metal was identified from the hardened layer thickness and metal/shell interface microstructural changes. Results showed that a high mould pre-heat temperature and large bar diameter will enhance the interaction between the shell and the TiAl alloy. During the casting process, not only was oxygen observed penetrating into the metal through the decomposition of the face coat materials, but also silicon from the backup coat was found to interact with the metal during the casting process.