Reactions at the mould/metal interface play an important role in determining the quality of investment castings. They are particularly critical in the case of magnesium alloys cast in industrial plaster moulds. In this work, reactions of molten magnesium alloy with plaster mould were studied. First the potential interactions with mould materials (including gases) were examined using thermodynamic considerations. Then thin sheets of AZ91D magnesium alloy were cast in industrial plaster moulds using vacuum assistance: the surface of sheets and plaster mould were characterised. The analysis of reaction products indicates that magnesium vapours diffuse through the plaster and reduce the silica present in the investment material according to the following reaction: 4Mg + SiO2=2MgO + Mg2Si. The extent of reactions is controlled by mould temperature and thickness of castings.
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