This study is aimed at examining the influence of ultrasonic vibration on the
in-situ formation of stable oxides, Al
2O
3 and MgAl
2O
4, from SiO
2 particles added as an oxygen source in melt stirring. The preheated SiO
2 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 SiO
2 particles are not transferred into the molten Al by melt stirring without ultrasonic vibration. With ultrasonic vibration, the SiO
2 particles, which have been transferred into the molten Al due to the improved wettability, change to stable Al
2O
3. The transferred SiO
2 particles into the molten Al-Mg alloy change to MgAl
2O
4 and Al
2O
3. 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 SiO
2 to MgAl
2O
4 and Al
2O
3. The stable oxides are rapidly formed due to the fast diffusion of Al and Mg through the cracks. The exothermic reactions of MgAl
2O
4 and Al
2O
3 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.
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