Mechanisms of grain formation during ultrasonic solidification of commercial purity magnesium

Abstract

Ultrasonic treatment (UST) during the solidification of commercial purity Mg was investigated over different time-temperature ranges: (i) UST in the liquid region only (ii) during the transition from liquid to solid including the onset of nucleation and (iii) UST initiated before the end of solidification but well after the onset of nucleation of α-Mg for the formation of equiaxed grains. If UST is terminated at the end of the liquid stage or above 650 °C there is a negligible effect on the final grain size. The solidification treatment (case ii) produces incremental grain refinement when UST is applied from 690 to 650 °C for 1 or 2 min, however, from 750 to 650 °C for 3 or 4 min shows only coarse, non-equiaxed grains. Introducing UST well after the onset of solidification produces a region with the finest grains of all treatment ranges investigated. The degree of grain refinement was found to be temperature dependent where low temperature initiation resulted in a higher number density of grains. The mechanism of grain formation during UST is discussed with respect to the processing parameters and thermal conditions driving the formation of equiaxed grains.