Metal-Mould Reactions in casting Aluminium-Lithium Alloys in Sodium-silicate-bonded Sand Moulds

Abstract

Metal-mould reactions in casting Al-Li alloys in sodium-silicate-bonded sand moulds have been studied by the modified Gertsman technique. Molten Al-2.7% Li alloy was poured into a bottom-gated vertical cylindrical mould cavity (150 mm x 50 mm dia) made from no-bake organic-binder-based sands. At the bottom of the mould cavity, a standard AFS three-ram sodium-silicate-bonded sand sample (the test sample) was placed vertically to provide the necessary interface for investigation. After cooling, the reaction products formed at the interface and samples from the bottom portion of the castings were collected for investigation. These were analysed to find Li loss from the casting as a result of metal-mould reactions. The casting was vertically sectioned and visually observed for appearance of blow holes, if any, while the sub-surface was studied for microhardness variation. The as-cast surface and the reaction products were also studied by SEM and X-ray diffraction analysis.

The study reveals that the Li in the molten alloy enters into vigorous chemical reaction with the sodium silicate resulting in the release of metallic sodium and formation of reaction products containing αLithium aluminium meta silicate. Li is thus lost from near the surface of the casting. Probably, the sodium released causes the gas blow holes in the sub-surface of castings due to its high vapour pressure at the working temperature.