The grain refinement of Mg alloy subjected to dual-frequency ultrasonic melt treatment: A physical and numerical simulation

Abstract

Ultrasonic melt processing of magnesium (Mg) alloys has received widespread attention. However, the cavitation behavior and microstructure evolution are difficult to be directly observed in high temperature melts. In this work, single-frequency ultrasonic field (SUF) and dual-frequency ultrasonic field (DUF) were introduced into succinonitrile (SCN) melt and real-time images of dendrite growth and evolution were captured to explore the regulation mechanism of DUF on melt solidification structure. Numerical simulation and corresponding experiments were performed to investigate the acoustic pressure distribution and cavitation area of DUF in Mg alloy melt. Ultrasonic treatment increased the SCN dendrite growth rate and refined the solidification microstructure, and a higher efficiency was achieved by DUF when compared to SUF when the total electric power was the same. DUF decreased sound pressure attenuation and enlarged cavitation area. A result of this improvement was that its grain refinement efficiency was 13.8% and 25.6% higher than SUF at 15 kHz and 20 kHz, respectively. The input power ratio plays a crucial part in improving the grain refinement efficiency of DUF. While ensuring the symmetrical distribution of cavitation area, the grain refinement efficiency can be significantly optimized by appropriately increasing the power share of 15 kHz ultrasound is optimal at a power ratio of 2:1.