The influence of bulk liquid natural convection on the formation of the equiaxed regions in AlCu and AlSi alloys

A series of ingots of AlSi alloys were solidified vertically downwards to promote convection in the bulk liquid. The ingots solidified with a columnar region above a region of equiaxed grains, the extent of which was greatest in those alloys in which the solid precipitated during solidification was buoyant with respect to the bulk liquid. Time—temperature relationships recorded during solidification, and fluid flow modelling, indicated that convection caused considerable fluid flow in the bulk liquid, but there was little evidence to suggest that this had led to the formation of the equiaxed region by fragmentation of the columnar front. Rather, in ingots of Al-24wt.%Cu, microanalysis suggested that the equiaxed regions formed by a process of heterogeneous nucleation. Similarly, the time—temperature relationships obtained from the equiaxed regions of the AlSi alloy ingots suggested that, while fragmentation may have been a factor in the formation of the equiaxed region, it was not the whole explanation.