Ultrasonic Melt Processing: Progress, Applications, and Future Directions.

Ultrasonic melt processing (UMP) has garnered significant attention from both academic and industrial communities as a promising solution to critical challenges in the metal casting industry. This technique offers a clean, environmentally friendly, and energy-efficient approach to improving melt quality and achieving structural refinement. However, due to the opaque nature of metals, understanding the fundamental mechanisms governing the interactions among ultrasonic bubbles, acoustic streaming, and the melt remains still challenging. This review traces the evolution of UMP research, from its inception in the mid-20th century to recent advancements, with particular emphasis on the application of state-of-the-art synchrotron X-ray imaging and computational modeling. These approaches have been instrumental in unraveling the complex, multiscale dynamics occurring across both temporal and spatial scales. Key findings in various metallic alloy systems are critically reviewed, focusing on new insights into cavitation bubbles, acoustic streaming, and the interactions of growing solid phases in different alloys. Additionally, the review discusses the resulting phenomena, including grain refinement, fragmentation, and the mitigation of solidification defects, in detail. The review concludes by identifying critical research gaps and emerging trends, underscoring the indispensable role of in situ studies and robust theoretical frameworks in advancing UMP. These developments are poised to reshape the future of innovation in materials science and engineering.