The in situ observations of solidification and melting of an aluminum-silicon alloy (Al-12.6%Si) using an ultrasonic waveguide sensor are presented. The ultrasonic sensor consists of a conventional piezoelectric ultrasonic transducer, a cooling system and a titanium (Ti) rod as an ultrasonic waveguide. The sustainability of the Ti rod in the molten Al alloy is investigated by immersion tests for 1, 4, 8 and 16 h at 800 degrees C. The formation of a layer consisting of globular TiAl3, disperse AlSi2Ti and alpha-Al phases has been observed at the interface between the Ti and the At alloy. Ultrasonic pulse-echo measurements of the Al alloy during solidification and melting have been performed using the ultrasonic sensor in temperature range from 200 to 800 degrees C. The longitudinal wave velocity of the Al alloy shows a rapid and significant change from about 3900 to 5600 m/s around the eutectic point. An attempt to measure a solid/liquid interface of the Al alloy has been made at frequency of 2.25 MHz. The reflected echo from the interface undergoing directional solidification has been observed. The position and growth rate of the interface have also been determined from the reflected echo.
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