Refinement of Microstructures for Aluminum Piston through Ultrasonic Melt Treatment

The effects of ultrasonic melt treatment on the microstructures of aluminum piston were examined at five observation parts hav-ing different cooling rates. The microstructure of aluminum piston consisted of primary Si, eutectic Si, and various types of intermetallic compounds. Regardless of cooling rate, the ultrasonic melt treatment transformed dendritic eutectic cells to equiaxed eutectic cells and it decreased the sizes of eutectic Si and intermetallic compounds that exist at eutectic cell boundaries. In the absence of ultrasonic treatment, coarse primary Si particles were severely segregated and its size was increased with decreasing the cooling rate. The ultrasonic treatment decreased the size of primary Si particles from 25.5~31.0 µ m to 17.6~23.1 µ m, depending on the cooling rate. In the presence of ultrasonic treatment, relatively fine primary Si particles were homogeneously distributed throughout the piston. In addition, the ultrasonic treatment increased the population density and area fraction of fine primary Si particles.