ULTRASONIC STIRRING EFFECTS ON THE MICROSTRUCTURE OF A356 ALLOYS

摘要

Ultrasonic treatment has significant effects on the solidified microstructure, which includes grain structure, distribution of inclusions, refinement of secondary phases, etc. The primary causes are due to ultrasonic cavitation, acoustic streaming and movement of dislocations associated with propagation of ultrasound waves in media. However, the mechanism of how those effects happen are not fully understood and quantified. In this study, molten A356 alloy was treated with high power ultrasonic then cast into a metal mold at relatively high superheats, as encountered in standard foundry practice. The parameters of the ultrasonic stirring technology (UST) were determined using a UST software previously developed and validated. The UST analysis tool is capable to model acoustic streaming and cavitation. The ultrasonically-stirred A356 alloy shows superior microstructure characteristics with very low micro-porosity levels. The microstructure, secondary dendrite arm spacing (SDAS) and eutectic spacing of the A356 alloy processed with and without UST were also discussed.