AZ31B alloy ingots were prepared by applying ultrasonic vibration on the top of the melt with different powers and water-cooling at the bottom of the stainless steel crucible.Quantitative metallographical analysis software was employed to calculate the grain size of primary α-Mg phase in the solidification structure.The results reveal that with increasing in ultrasonic powers

the nucleus numbers are increased as a result of the enhancement of cavitation-induced nucleation.Meanwhile

with accelerating in acoustic streaming

the diffusion of the latent heat and the solute at the roots of secondary dendrite arm and the dispersion of dendrite-broken are improved

which is responsible for the transformation of α-Mg phase from dendrite into rosette and granular structure.With applying ultrasonic power of 600W

the average grain size of the α-Mg phase can be dropped to 93μm

while it becomes coarse with 800W power because of the heating effects of ultrasonic more than the dissipation of the latent heat.