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In this study, we estimated the equilibrium bubble size of acoustic cavitation in a molten metal, which is basic information in ultrasonic casting. For this, the bubble shape instability of acoustic cavitation in the melt was numerically investigated by solving the Keller–Miksis equation and dynamic equation of the distortion amplitude. The acoustic cavitation bubbles are more stable in aluminum and magnesium melts than in water, and the parametric instability mainly determines the bubble stability at 20–160 kHz in molten metals. However, the afterbounce instability does not significantly affect the bubble stability in molten metals owing to the small number of bubble oscillations after the first rapid compression, and the distortion amplitude cannot grow significantly after the first compression. The bubbles in the melt become more unstable with an increase in the ultrasonic frequency owing to the corresponding increase in the bubble wall velocity. Through this stability analysis, we can estimate that the stable bubble size in the aluminum or magnesium melt is approximately three or four times larger than that in water at the same ultrasonic pressure amplitude.

It is well known that ultrasonic cavitation causes a steady flow termed acoustic streaming. In the present study, the velocity of acoustic streaming in water and molten aluminum is measured. The method is based on the measurement of oscillation frequency of Karman vortices around a cylinder immersed into liquid. For the case of acoustic streaming in molten metal, such measurements were performed for the first time. Four types of experiments were conducted in the present study: (1) Particle Image Velocimetry (PIV) measurement in a water bath to measure the acoustic streaming velocity visually, (2) frequency measurement of Karman vortices generated around a cylinder in water, and (3) in aluminum melt, and (4) cavitation intensity measurements in molten aluminum. Based on the measurement results (1) and (2), the Strouhal number for acoustic streaming was determined. Then, using the same Strouhal number and measuring oscillation frequency of Karman vortices in aluminum melt, the acoustic streaming velocity was measured. The velocity of acoustic streaming was found to be independent of amplitude of sonotrode tip oscillation both in water and aluminum melt. This can be explained by the effect of acoustic shielding and liquid density.

A mathematical model has been developed for single bubble motion and mass transfer for aluminum degassing under a compound field of ultrasonic and rotating flow, and it showed good agreement with experimental data. The effects of the ultrasonic frequency and acoustic pressure on the bubble size, bubble trajectory, and mass transfer rate between the bubble and melt were investigated. The ultrasonic frequency and acoustic pressure had little effect on the bubble trajectory, and its curve radius gradually increased with increasing distance. The retention time of the bubble in the compound field was much greater than that in a single rotating flow field, which is conducive for improving the purification efficiency. Under the experimental conditions of 450 rpm rotation speed and 1.1 m/s gas flow rate, the optimal parameters are a frequency of 50 kHz and an acoustic pressure of 100 kPa.