Compared with other dual-frequency acoustic applications, melt-treatment with dual-frequency ultrasound was less researched, especially in magnesium field. In this present work, traditional single-frequency ultrasonic field (SUF) treatment and dual-frequency ultrasonic field (DUF) treatment were used to refine the as-cast microstructure and improve the mechanical properties of the ZK60 (Mg-Zn-Zr) magnesium alloy. The influences of DUF on the microstructure evolution and mechanical properties were systematically investigated, and the cavitation bubble's dynamic behaviors were investigated by numerical simulation. α-Mg grains and second phases were dramatically refined by introduced ultrasound, and DUF showed higher refinement efficiency than SUF. The DUF treatment promoted the formation of small α-Mg globular grains and changed the distribution and morphology of MgZn phases. Mechanical properties of the as-cast alloy were much promoted with DUF. Yield strength, ultimate tensile strength and elongation increased to 153MPa, 239MPa and 13.9% respectively after 1400W DUF treatment, which were 30.8%, 42.3% and 58.0% higher than the values obtained from untreated samples and 20.5%, 20.7% and 30.0% higher than 1200W SUF treated samples. The DUF can generate more and larger cavitation bubbles, and make more bubbles into instantaneous bubbles, improving refinement efficiency.

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