Ultrasonic Sensing of Solid-Liquid Interface during Directional Solidification of Aluminum

This paper presents a newly developed high temperature ultrasonic sensor and its application to in situ observation of aluminum (Al) alloy during solidification and melting in temperature up to 800C. The ultrasonic sensor mainly consists of a conventional piezoelectric transducer, a titanium buffer rod as an acoustic waveguide and an air cooling pipe. The length is approximately 300 mm. This ultrasonic sensor, because of the unique characteristics of titanium, provides not only good acoustic coupling to molten aluminum but also good corrosion resistance. Pulse echo measurements in the molten aluminum are performed using the sensor at 5 or 2.25 MHz. A clear reflected echo from a stable solid-liquid interface of the aluminum has been successfully monitored. Furthermore, the reflected echo from the growing interface during unidirectional solidification has also been monitored. The growth rates of the interface for Al-Si alloy were determined to be 0.12 mm/s from the monitored echo.