Effect of ultrasonic melt treatment conditions on melt quality of Al–Mg alloy

Abstract

In this study, the effect and mechanism of ultrasonic melt treatment (UST) were studied to achieve high-quality molten Al–Mg alloy. The purpose was to analyze the change in the cleanliness of molten aluminum according to UST process conditions such as ultrasonic power, treatment duration, and stabilization duration for molten Al–Mg alloy, and to find the optimal ultrasonic process conditions to achieve high-quality molten aluminum. UST results were compared to the effect of the Gas Bubbling Filtration (GBF) process, which is a general molten aluminum treatment process. Density index, bifilm index, porosity, and X-ray CT analysis were used for quality evaluation of melt cleanliness. UST was performed at a frequency of 19.7 kHz and two outputs of 600 W and 1200 W after maintaining 1 kg of molten aluminum at 800 °C for 10 min. The UST treatment durations were 1, 3, and 5 min, the stabilization durations after treatment were 0, 10, and 20 min. Then the quality of the molten aluminum was measured. The GBF treatment was applied for 1, 5, 10, or 30 min under impeller rotation speed of 300 rpm and gas flow rate of 1 L/min. During UST, the melt quality improved as the ultrasonic power increased and the treatment duration increased. To secure the high melt quality, UST could obtain the required degassing effect after 3 min of treatment, whereas GBF treatment took 10 min. The bubble size generated by UST is smaller than that by GBF, so UST has higher mas transfer coefficient of hydrogen than GBF and hydrogen in the melt could be removed with only a short UST treatment. UST was superior to GBF immediately after treatment, however, the melt quality deteriorated during the stabilization after UST. It caused from the unstable oxide film and a relatively rapid high-temperature oxidation, so appropriate molten metal management and rapid post-processing are necessary after UST treatment.