2009 Volume 81 Issue 10 Pages 469-474
Ultrasonic vibration is applied to various molten metal processes for such purposes as (1) improving wettability, (2) promoting liquid adhesion at vibrating surface and (3) promoting sono-solidification like grain refinement. The present study is focused on sono-solidification with acoustic cavitation in hypereutectic Al-18mass%Si alloy. An equilibrium microstructure composed of primary silicon and coupled eutectic α-Al/Si phases is generally observed in Al-18mass%Si alloy. However, non-equilibrium α-Al solid solution grains develop along with equilibrium phases in the sono-solidification. That is, during the sono-solidification of Al-18mass%Si alloy, non-equilibrium α-Al grains crystallize near the ultrasonic radiator before reaching the eutectic temperature of 577℃, in addition to the refined primary silicon grains. Ultrasound in molten Al-Si alloys exhibits two outstanding behaviors of cavitation bubbling and acoustic streaming, which cause the crystallization of α-Al grains in hypereutectic Al-Si alloy. It is known that high pressures of over 1GPa generated by collapse of cavitation bubbles lead not only to increase in the eutectic temperature, but also higher silicon content at the eutectic point in Al-Si alloy. With chemical analyses of EPMA, non-equilibrium α-Al grains are characterized by higher silicon content compared with that of primary α-Al grains in hypoeutectic Al-7masst%Si alloy. Consequently, non-equilibrium α-Al nuclei are crystallized at collapsed cavitation bubble sites before reaching the eutectic temperature and they are dispersed throughout the billet due to acoustic streaming.
