THE EFFECT OF ELECTROMAGNETIC STIRRING DURING SOLIDIFICATION ON THE STRUCTURE OF AL-SI ALLOYS

A series of Al-Si alloy ingots were directionally solidified verticall y downwards in a rectangular cavity with varying conditions of bulk li quid flow obtained with a controllable electromagnetic stirrer. Examin ation of their macrostructures showed that the columnar-equiaxed trans ition was promoted both by increasing bulk liquid velocity and increas ing silicon content. Temperature measurements during solidification sh owed that the electromagnetic stirring resulted in the rapid removal o f bulk liquid superheat and therefore the impact of a relatively cold liquid on the solidification front. Fluid flow modelling showed that b ulk liquid velocities of up to 0.25 m a s(-1) were obtained. Fragmenta tion of the solidification front was the most probable mechanism for t he promotion of the columnar-equiaxed transition with high velocity fl ows. However, with low velocity flows, such as were obtained with natu ral convection or a limited application of electromagnetic stirring, t he columnar-equiaxed transition was probably significantly influenced by heterogeneous nucleation effects. Evidence for this came From compa ring the behaviour of alloys prepared using aluminium and silicon of d ifferent levels of purity.