THE INFLUENCE OF BULK LIQUID NATURAL-CONVECTION ON THE FORMATION OF THE EQUIAXED REGIONS IN ALCU AND AL-SI ALLOYS

A series of ingots of Al-Cu and Al-Si alloys were solidified verticall y downwards to promote convection in the bulk liquid. The ingots solid ified with a columnar region above a region of equiaxed grains, the ex tent of which was greatest in those alloys in which the solid precipit ated during solidification was buoyant with respect to the bulk liquid . Time-temperature relationships recorded during solidification, and f luid flow modelling, indicated that convection caused considerable flu id flow in the bulk liquid, but then was little evidence to suggest th at this had led to the formation of the equiaxed region by fragmentati on of the columnar front. Rather, in ingots of Al-24wt.%Cu, microanaly sis suggested that the equiaxed regions formed by a process of heterog eneous nucleation. Similarly, the time-temperature relationships obtai ned from the equiaxed regions of the Al-Si alloy ingots suggested that , while fragmentation may have been a factor in the formation of the e quiaxed region, it was not the whole explanation.