With applying ultrasonic vibration the refinement mechanism of primary crystals in Al-Si alloy isothermally maintained at a semi-solid state was examined. The mechanical destruction of primary crystals and promoted formation of solidification nucleus have been considered as a refinement mechanism of ultrasonic vibration. It was confirmed that the application of ultrasonic vibration dose not cause the segmentation of grown primary dendrite crystals of the hypoeutectic Al-Si alloy by EBSD analysis. Neither is the case for primary Si crystals in hypereutectic Al-Si alloy. No particle size changes were detected by SEM observation and a coulter counter method of Si powder in water, when the ultrasonic vibration was applied. Refinement of primary crystals by ultrasonic vibration during solidification hardly depends on the mechanical destruction due to vibration energy. It was, therefore, concluded that both the molten metal vibration and the large acoustic stream formed in molten metal are considered to promote the nucleation and mechanical segmentation of primary dendrite crystals immediately after their nucleation.
