Cavitation phenomena in ultrasonic casting and their industrial application

Although the effect of ultrasonic vibrations on the structure of solidifying metals has been known for long time, the practical application of ultrasound to casting technology still remains a big challenge. Ultrasonic casting exploits cavitation in molten metal to disperse particles of grain refiners or to break dendrites during solidification. Therefore, care must be taken to control the passage of melt through the cavitation zone. There is still, however, a lack of data in this area. The present study consisted of two parts. In the first one, intensity and spectral characteristics of cavitation noise generated during radiation of high-intense ultrasonic vibrations into water and molten aluminum alloys were investigated by using a high temperature cavitometer. Based on these data, a measure for evaluating the cavitation intensity was established and verified for relatively low and high vibration amplitudes. The second part presents results on the application of ultrasonic vibrations to a DC caster to refine the primary silicon grains of a model Al-17Si-0.01P alloy during the casting of 178-mm billets. High amplitude ultrasonic vibrations were radiated into a specially designed hot-top unit of a DC caster to allow a better control of the melt flow through the cavitation zone as compared, for example, to ultrasonic treatment in launder. It was shown that refinement effect of ultrasonic vibrations and structure uniformity can be significantly improved by optimizing the amplitudes of horn tip vibration and horn position in the unit.