In situ synchrotron radiography and spectrum analysis of transient cavitation bubbles in molten aluminium alloy

Abstract

The melt processing of conventional and advanced metallic materials with high-intensity ultrasonic vibrations significantly improves the quality and properties of molten metals during their solidification. These improvements are primarily attributed to ultrasonic cavitation: the creation, growth, pulsation, and collapse of bubbles in the liquid. However, the development of practical applications is limited by the lack of fundamental knowledge on the dynamics of the cavitation bubbles; it is very difficult to directly observe ultrasonic cavitation using conventional techniques in molten metals due their high temperature and opaqueness. In this study, an in situ synchrotron radiography experiment was performed to investigate bubble dynamics in an Al-10 wt.% Cu alloy under an external ultrasound field at 30 kHz. Radiographs with an exposure time of 78 ms were collected continuously during the sonication of molten alloys at temperatures of 660±10 °C. To the best of our knowledge, this is the first time that transient cavitation bubbles have been observed in liquid aluminium. Quantification of bubble parameters such as average size and time of collapse were evaluated from radiographs using advanced image analysis. Additionally, broadband noise associated with the acoustic emissions from shock waves of transient cavitation bubbles and estimation of the real-time acoustic pressure at the driving frequency were assessed using an advanced high-temperature cavitometer in separate bulk experiments.