Ultrasonic modification of A356-Al2O3 composites

In this work, the dry sliding wear behavior of ultrasonic-modified A356 composite was studied. The main factors in this work are the times and frequencies of ultrasound applied to the composite melt, which were in the range of 5-15 min and 10-50 Hz, respectively. It is indicated that there is an effective range of time and frequency within which decreasing the grain size and also uniformity in the distribution of the eutectic silicon phase, Al2O3 reinforcement particles and solidification shrinkage pores can be achieved. Frequencies up to 50 Hz during a time longer than 10-15 min yielded the proper conditions in this case. With these conditions of structural modification, an optimum decrease in the grain size with less directionality of the columnar or dendritic grains is seen. A decrease in the grain size and more uniformity in the microstructure gives rise to decreases in the wear rate at a given load. At higher loads, the oxide debris is expected to become better compacted to form a transfer layer, spread over a larger area of the sliding surface, and thus its removal causes greater wear compared with that without removal of wear debris. The results indicate that the frequency is more effective in the reduction procedure of the grain size and density than the time.