Ultrasonic in-situ observation of alumina particles in molten aluminum at temperature up to 800• C is presented. A focused ultrasonic sensor is employed for the high temperature measurement with a high spatial resolution. The sensor mainly consists of a conventional piezoelectric transducer, a taper-shaped clad buffer rod as a waveguide and a cooling system. Martensitic stainless steel is used as a material for the buffer rod and an acoustic lens is fabricated at the probe end of the rod. In order to examine the focusing ability of the acoustic lens, the acoustic field near the focusing zone is numerically evaluated by a finite difference method. Using the developed focused ultrasonic sensor, backscattered echoes from alumina particles of 160 µm suspended in the molten aluminum at 800• C have been observed clearly in pulse echo mode at 10 MHz. The effect of the size and the aggregation condition of the particle on the backscattered echoes has also been examined.

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 C. The formation of a layer consisting of globular TiAl 3 , disperse AlSi 2 Ti and -Al phases has been observed at the interface between the Ti and the Al 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 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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A high temperature ultrasonic sensor for detecting inclusion particles in molten metals is presented. The sensor consists of a conventional piezoelectric transducer and a focused waveguide, and provides high spatial resolution measurements in molten metals in a pulseecho mode at 10 MHz. A taper-shaped clad buffer rod is used as the waveguide. In order to design the waveguide, elastic wave propagation in the rod is evaluated by a numerical simulation using a fmite difference method. Focusing ability of the acouStic lens fabricated at the end of the rod is also examined numerically. Using the developed sensor, backscattered echoes from alumina particles of 160 pn suspended in molten aluminum at 1073 K have clearly been observed.