Mechanisms of equiaxed grain formation have been studied. A review of the literature concluded that, depending on the material and casting conditions, one or more of the five proposed mechanisms can become operative. For the aluminium - silicon and aluminium - copper alloy systems investigated in this study and cast under a variety of casting conditions, it is shown that in most casting situations, the dominant mechanisms of equiaxed crystal formation are the wall mechanisms and constitutional undercooling. In the wall mechanisms, equiaxed grains are nucleated due to thermal undercooling provided by the mould walls. Nucleation occurs repeatedly on or near the mould walls, with convection and turbulence acting to disperse equiaxed crystals throughout the remainder of the casting. In constitutional undercooling, nucleation occurs in the liquid ahead of a growing dendrite. Rejection of solute during growth establishes a solute concentration gradient ahead of the dendrite tip. This changes the freezing temperature of the liquid ahead of the dendrite, restricting growth and initiating a nucleation event in the liquid. The process continues through repeated nucleation and growth of successive crystals. The proportion of nuclei generated by each mechanism depends most strongly on the amount of undercooling generated by the mould walls, and on the chemistry of the alloy. In this case alloy chemistry refers to the amount and type of both nucleant particles and solute.
The grain size transition in aluminium - silicon alloys has also been examined. The transition is observed as a rapid increase in grain size with addition of solute, after an initial decrease, and it is most likely produced by a decrease in nucleant potency due to poisoning of nucleants. It is shown that decreases in grain size prior to the transition are the result of increases in constitutional undercooling with initial increases in solute content. Subsequently, increasing latent heat generation acts to slow the rate of reduction in grain size with increases in solute content. When this occurs in the aluminium - silicon system, the effects of a reduction in nucleant potency become apparent, and the grain size increases. The mechanism of poisoning remains unclear but the degree of poisoning appears to be directly proportional to the silicon content of the alloy.