Effect of cooling rates on as-cast microstructures of Mg-9Al-xSi (x=1, 3) alloys

The effects of cooling rates corresponding to different diameters of the steel mould and laser surface melting (LSM) on the as-cast microstructures of Mg-9Al-xSi (x=1, 3) (mass fraction, %) alloys were investigated by XRD and OM. The results show that obvious refinement of the alloy microstructure is obtained with increasing cooling rate by conventional ingot metallurgy. However, no evident modified morphologies of both dendritic primary Mg2Si and Chinese script eutectic Mg2Si in the Mg-Al-Si alloy occurs. Surprisingly, the morphologies of Mg2Si phases within the laser-melted Mg-Al-Si alloy transform drastically from both coarse Chinese script shape for the eutectic Mg2Si and dendrite for the primary Mg2Si to fine spherical particles with an average size of about 3 μm due to the rapid cooling of the melted layer, and the Mg2Si particulates distribute more uniformly in the α-Mg matrix.