The effect of stable cavitation on dendrite growth within ultrasonic field

Abstract

Applying ultrasonic waves in liquid alloys produces refined grain structures, which is mainly contributed to ultrasonic cavitation and acoustic streaming. According to the bubbles lifetime and whether are fragmented into "daughter" bubbles, acoustic cavitation can be divided into transient cavitation and stable cavitation. As compared with the transient cavitation, the interaction between stable cavitation bubbles and solidifying alloys have been rarely reported in previous literatures. In this work, the effect of stable cavitation on the dendritic growth of succinonitrile (SCN)-8.3mol.% water organic transparent alloy is systematically investigated by high-speed digital image technique and numerical simulation. It is firstly found that when the bubble migration direction was consistent with that of dendritic growth, the periodic high pressure generated during bubble oscillation process increased the local undercooling, promoting the dendrites growth velocity effectively. Meanwhile, the concentrated stress inside dendrites induced by the linearly oscillation of cavitation bubble could break up dendrites into fragments. Specifically, if there were stable cavitation bubbles suspended around the liquid-solid interface, periodically alternating flow field and high shear force in their surrounding liquid phase was produced. As a result, the nearby dendritic fragments would be attracted to those bubbles and then transformed into spherical grains.