In Situ Observation of Fragmentation of Primary Crystals by Ultrasonic Cavitation in Water

Ultrasonic melt processing is a promising technique for microstructural refinement in castings. Several mechanisms have been proposed for the observed effects, including cavitation-induced nucleation, activation of substrates and fragmentation. Until now, however, real-time experimental observations which could clarify any of the above mechanisms are very limited. For the first time we directly observed the fragmentation of primary crystals formed in alloys by ultrasonic cavitation. The primary crystals were extracted from real Al alloys and subjected to ultrasonic processing in water with in situ high-speed filming. The recordings of fragmentation of the primary crystals allowed us to observe the different mechanisms of fragmentation, depending on the mechanical properties and morphology of the primary crystals. The collapse of cavitation bubbles in water is less violent than that in liquid aluminum due to the lower cavitation threshold, viscosity and surface tension. Therefore the fragmentation mechanisms for the primary crystals observed in water should also be present for the same primary crystals in the more violent cavitation situation in liquid aluminum.