Origin of the Equiaxed Zone in Small Ingots

It is deduced by thermal analysis that no entirely new crystals are nucleated in the interior of an ingot at any time after pouring. By comparing the conditions for columnar growth in ingots and in unidirectional solidification experiments, it is concluded that in the ingot, sufficient constitutional supercooling for nucleation might occur if it were not prevented by the presence of crystals in the liquid. At low pouring temperatures, most of the equiaxed grains are nucleated as separate crystals at the time of pouring. When the superheat is higher, the main process for the origin of the equiaxed zone appears to be a multiplication mechanism operating by local remelting. Dynamic procedures used during unidirectional solidification and conventional ingot freezing permit one to conclude that the main mechanism operating to refine the equiaxed structure, as a function of imposed disturbances ( mechanical or forced fluid flow) is a multiplication mechanism, most likely the process of detachment of dendritic arms by remelting.