In-situ formation of stable oxides in molten aluminum alloy by ultrasonic stirring

This study is aimed at examining the influence of ultrasonic vibration on the in-situ formation of stable oxides, Al2O3 and MgAl2O4, from SiO2 particles added as an oxygen source in melt stinting. The preheated SiO2 particles were added on a molten Al or Al-Mg alloy surface and stirred at 1073 K in an Ar atmosphere. The ultrasonic vibration was then transmitted to molten metal through a ceramic plate. The SiO2 particles are not transferred into the molten Al by melt stirring without ultrasonic vibration. With ultrasonic vibration, the SiO2 particles, which have been transferred into the molten Al due to the improved wettability, change to stable Al2O3. The transferred SiO2 particles into the molten Al-Mg alloy change to MgAl2O4 and Al2O3. The application of ultrasonic vibration to melt stirring makes not only the wettability improve, but also the cracks in the particles grow, which arise from the volumetric shrinkage accompanied with the change from SiO2 to MgAl2O4 and Al2O3. The stable oxides are rapidly formed due to the fast diffusion of Al and Mg through the cracks. The exothermic reactions of MgAl2O4 and Al2O3 formation lead to a temperature rise in the molten Al-Mg alloy, in which the temperature rise of 99 K is approved by applying the ultrasonic vibration. Consequently, the in-situ reactions are promoted further. Gas defects disappear in the solidified Al alloy containing the stable oxides stirred with ultrasonic vibration, in which the particle distribution is also improved.