Dynamic behavior of a hydrogen gas bubble during ultrasonic treatment of molten and solidifying Al-Cu alloys

摘要

In this paper, mathematical models are developed to demonstrate the mechanisms of the ultrasonic grain refinement of a solidifying melt and the ultrasonic degassing of a gassy liquid. In the first part, a mathematical model of grain refinement is developed to simulate the transient dynamic behavior of a hydrogen gas bubble present in the mushy region of a solidifying aluminium-3.4 wt pct copper alloy melt under various ultrasonic pressure fields. The results show that the pressure generated in the melt, surrounding the bubble, is sufficiently high to fracture the dendrite arms and produce nuclei for equiaxed crystal growth. In the second part of this study, a mathematical model of degassing is presented to simulate the dynamic behavior of a stable hydrogen gas bubble inside a molten aluminium-3.4 wt pct copper alloy under various ultrasonic pressure fields. The results show that, by the process of rectified diffusion, large gas bubbles can be formed rapidly which can easily float to the surface under the action of the buoyancy force. Also, an aqueous experimental work has been carried out to simulate the growth of an air bubble by the process of rectified diffusion. The experimental results for an air bubble growth are compared to the results of the corresponding mathematical model which show an overall reasonable agreement between the experiments and the predictions.