Mechanism of effect of stable cavitation on dendrite growth in ultrasonic field

Abstract

Ultrasonic waves used in liquid alloys can produce refined grain structures, which mainly contributes to ultrasonic cavitation and acoustic streaming. According to the bubble lifetime and whether they are fragmented into “daughter” bubbles, acoustic cavitation can be divided into transient cavitation and stable cavitation. Compared with the transient cavitation, the interaction between stable cavitation bubbles and solidifying alloys have been rarely investigated previously . In this work, the effect of stable cavitation on the dendritic growth of succinonitrile (SCN)-8.3% (mole fraction) water organic transparent alloy is systematically investigated by high-speed digital image technique and numerical simulation. It is found that when the bubble migration direction is consistent with that of dendritic growth, the periodic high pressure generated in bubble oscillation process increases the local undercooling, speeding up the dendrites growth effectively. Meanwhile, the concentrated stress inside dendrites induced by the linearly oscillation of cavitation bubble can break up dendrites into fragments. Specifically, if there exist stable cavitation bubbles suspended around the liquid-solid interface, periodically alternating flow field and high shear force in their surrounding liquid phase is produced. As a result, the nearby dendritic fragments will be attracted to those bubbles and then transformed into spherical grains.