In this paper

solidification of pure aluminum melt weighing 1.5 kg under power ultrasonic treatment（UST） was tested. And the effect of UST on solidification structure of aluminum melt with different purity was investigated. Besides

the grain refining effects of mechanical stirring and UST were compared. The results show that the UST was able to refine both the 99.7% pure aluminum melt

and the 99.992% high purity aluminum melt. Compared to the mechanical stirring

the UST can bring out more homogeneous structure without casting defects. Furthermore

the mechanisms of the grain refinement by UST were analyzed. The cavitations stimulated the heterogeneous nucleation at the ultrasonic resonator

and the acoustic flows promoted the dispersion and the precipitation of the nuclei. These two processes were together considered as the main reason why the equiaxed zone was formed.