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 MgZn2 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.

Argon degassing, ultrasonic degassing and a novel ultrasonic argon degassing treatment were applied for the hydrogen removal of AZ91 magnesium alloy. The hydrogen concentration, microstructures and mechanical properties have also been investigated. AZ91 alloys contains a high hydrogen concentration. The mechanical properties of the as-cast alloy are much improved using degassing process, which should be mainly attributed to the hydrogen removal. Among the three degassing process, the ultrasonic argon treatment is a high efficient process both for hydrogen removal and microstructure refining. One hand, ultrasonic wave could break up the purged argon bubble to improve the degassing efficiency of these bubbles. On the other hand, ultrasound could also generate many cavitation bubbles in the melt, which should account for the microstructure refinement. The ultrasonic argon treatment involves dynamics between the ascending argon bubbles and ultrasonic effects, such as cavitation and streaming, etc.

In this work, the role of ultrasound in hydrogen removal and microstructure refinement by the ultrasonic argon degassing has been fully investigated by the experimental work in water and AZ91-0.4Ca magnesium melt, respectively. Ultrasound is able to break up argon gas into numbers of small bubbles and drive them diving deeply to the bottom of water, which are responsible for the efficient degassing regime of ultrasonic argon process. The argon flowrate plays a dominant role in promoting hydrogen removal effect. Meanwhile, the increasing argon flowrate can suppress the microstructure refinement, due to the subdued ultrasonic cavitation under a large argon flowrate. Mechanical properties of AZ91-0.4Ca alloy can be much promoted by the ultrasonic argon degassing process. Ultrasound is the key to achieve not only efficient degassing regime, but also microstructure refinement as well as mechanical properties promotion.