The effect of electromagnetic stirring during solidification on the structure of Al-Si alloys

A series of Al-Si alloy ingots were directionally solidified vertically downwards in a rectangular cavity with varying conditions of bulk liquid flow obtained with a controllable electromagnetic stirrer. Examination of their macrostructures showed that the columnar-equiaxed transition was promoted both by increasing bulk liquid velocity and increasing silicon content. Temperature measurements during solidification showed that the electromagnetic stirring resulted in the rapid removal of bulk liquid superheat and therefore the impact of a relatively cold liquid on the solidification front. Fluid flow modelling showed that bulk liquid velocities of up to 0.25 m a s(-1) were obtained. Fragmentation of the solidification front was the most probable mechanism for the promotion of the columnar-equiaxed transition with high velocity flows. However, with low velocity flows, such as were obtained with natural convection or a limited application of electromagnetic stirring, the columnar-equiaxed transition was probably significantly influenced by heterogeneous nucleation effects. Evidence for this came From comparing the behaviour of alloys prepared using aluminium and silicon of different levels of purity.