Coalescence of equiaxed grains during solidification

The tendency of low-concentration alloys to develop hot tears during solidification is strongly related to the ability of the primary solid phase to form a coherent network, which can withstand significant tensile or shear stresses. Coalescence, i.e. the transformation of two impinging solidification fronts into a solid bridge (or grain boundary), has been simulated for a low-concentration, grain-refined aluminium alloy. Based on a front-tracking microsegregation model and a sharp solid–liquid interface hypothesis, the globulitic grain structure of the alloy has been approximated in two dimensions by a Voronoitessellation with a limited number of straight segments representing the grain boundaries. Near the end of solidification, the liquid film between two neighbouring grains is undercooled by an amount proportional to the grain boundary energy, which is itself a function of the relative misorientation of the grains. As main results, the model can predict, during cooling, the thickness and the spatial repartition of the intergranular liquid films, the fraction of bridged grain boundaries and the gradual transformation of a continuous interdendritic liquid film into a fully coherent solid.