Motion law of cavitation bubbles in ultrasonic field and mechanism of their interaction with dendrites

Ultrasound treatment (UST) has been demonstrated to be an effective approach to refining the microstructures of metallic alloys during solidification. The cavitation-induced fragmentation is considered as the major mechanism for grain refinement in the recent study, but the interaction between dynamic bubble motion and dendrite behaviour has been rarely investigated previously. In this work, the dynamic behaviour of cavitation bubbles and their interactions with succinonitrile (SCN)-2% (mole fraction) water organic transparent alloy are systematically investigated by high-speed digital image technique and numerical simulation. It is found that increasing the driving pressure transforms the bubble oscillation mode from volume oscillation to splitting oscillation, which significantly enhances the transient pressure and flow strength in the liquid. When a dendrite exists below the bubble, the fracture mode of the secondary branch undergoes a transition from high peripheral fatigue fracture to low peripheral fatigue fracture and to overload fracture with the increase of the driving acoustic pressure, and the fracture period is shortened in the form of a power function trend. The closer the bubble is to the dendrite, the longeritudinal radius of the bubble is gradually larger than the transverse radius during compression, and with the bubble shrink time increasing, the minimum bubble volume decreases. In addition, the decrease in distance between bubbles and dendrites leads to a significant reduction in the maximum pressure generated by bubble collapse, while the maximum flow rate shows a trend of first increasing and then decreasing. When the root radius of the secondary branch decreases or its length increases, the number of fatigue fracture cycles of the secondary branch decreases significantly. The calculated bubble expansion and contraction and secondary dendrite rupture processes are basically consistent with the experimental results, which indicates that the model constructed in this work can accurately predict the bubble motion and its interaction with dendrite in ultrasonic field.