On the columnar to equiaxed transition in small ingots

Abstract

The columnar to equiaxed transition (CET) in small ingots of, aluminum alloys was found to occur more easily for alloys with a larger value of the constitutional supercooling parameter (−mCo(1-k)/k). The CET was found to be completely suppressed by increases in the mold temperature by preheating before casting. These results are discussed in terms of the model proposed by Burden and Hunt that the CET occurs by the effect of the thermal gradient, arising from the slow, solidification of equiaxed dendrites, which increases the undercooling of the columnar dendrites. The application of the model due to Burden and Hunt is shown to require, the use of the ‘big bang’ model for equiaxed nucleation on pouring. A higher density of the nuclei, that grow into equiaxed grains, formed by pouring with lower superheat and into a cold mold, gives a higher thermal gradient immediately in front of the growing columnar grains. Other evidence in favor of the model is briefly discussed.