Multi-dendrite fragmentation dynamics in acoustic flow with oscillating cavitation bubbles revealed in situ by synchrotron X-ray radiography.

We used synchrotron X-ray radiography to study in-situ and in real-time the fragmentation dynamics of multiple dendritic microstructures in a Ga-40wt.% In alloy under ultrasound. A dedicated U-shaped quartz tube was designed and used as the sample holder to create well-controlled solidification conditions in a relatively large field of view suitable for imaging acoustic flow, cavitation bubbles, and their dynamic interactions with growing multiple dendrites. Without the presence of oscillating cavitation bubbles, the acoustic flow is able to bend multiple dendrites and then fragment them through cyclic fatigue mechanism over tens to hundreds of seconds. The local swirling flow, formed due to geometric constraints or flow obstacles, can trap and encircle multiple dendrites within the swirl, resulting in 3-4 times higher dendrite fragmentation efficiency than the linear streaming flow. The oscillating cavitation bubbles are more energetic and effective in causing mechanical fatigue fragmentation of dendrites, which often occur at the roots of dendrite arms in a time scale of ∼ 10 ms. Dendrite main trunks may also be fragmented by cavitation bubbles in a much longer time scale (i.e., a few seconds). The real-time observations have unambiguously revealed the fragmentation efficiency of multiple dendrites caused by different flow types and cavitation bubbles in the ultrasound processing conditions that are often found in industrial operations.