Effect of Yttria Mould Coating on the Investment Casting of AZ91D-1 wt% CaO Magnesium Alloy

The main objective of the present research is to develop a technique to inhibit the mould–metal interaction in the investment casting of magnesium (Mg) alloys. The use of yttria layers (Y2O3) as an interface mould coating to manufacture thin-walled samples of AZ91D-1 wt% CaO alloy was investigated and the reaction mechanisms were exploited. Additive manufacturing models with thin walls (2 mm thick) were designed to study the casting behaviour of the melt in ceramic moulds (plaster), for both coated and uncoated models. Additionally, the optimal processing parameters were taken into account using a vacuum induction furnace under a flowing argon atmosphere to prevent the ignition of the molten Mg. Regarding uncoated models tests, a considerable content of reaction products was revealed, resulting in large defects all over the castings. In these samples, new phases have been formed around the surface and the matrix due to the mould–metal reactions. On the other hand, the interaction between the liquid metal and the plaster was prevented in those ceramic moulds where yttria was used as an interface protective refractory, which led to a cleaner surface and a more uniform microstructure, improving the effectiveness of this process in the manufacture of Mg structures.