In-situ observation of fragmentation of primary crystals by ultrasonic cavitation in water

Abstract

Ultrasonic melt processing is a promising technique for micro-structural refinement in castings. Several mechanisms have been pro-posed 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 mecha-nisms are very limited. For the first time we directly observed the frag-mentation of primary crystals formed in aluminum 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 film-ing. 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 col-lapse of cavitation bubbles in water is less violent than that in liquid alu-minum due to the lower cavitation threshold, viscosity and surface ten-sion. 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.