Abstract
To address difficulties in treating large volumes of liquid metal with ultrasound, a fundamental study of acoustic cavitation in liquid aluminium, expressed in an experimentally validated numerical model, is presented in this paper. To improve the understanding of the cavitation process, a non-linear acoustic model is validated against reference water pressure measurements from acoustic waves produced by an immersed horn. A high-order method is used to discretize the wave equation in both space and time. These discretized equations are coupled to the Rayleigh-Plesset equation using two different time scales to couple the bubble and flow scales, resulting in a stable, fast, and reasonably accurate method for the prediction of acoustic pressures in cavitating liquids. This method is then applied to the context of treatment of liquid aluminium, where it predicts that the most intense cavitation activity is localised below the vibrating horn and estimates the acoustic decay below the sonotrode with reasonable qualitative agreement with experimental data.
Highlights
Significant improvements in the quality and properties of metallic materials are observed when treating them near their liquidus temperature [1,2,3]: the beneficial effects of the treatment include the degassing of dissolved gases, improved wetting, activating inclusions by cleaning the solid-liquid interface, enhancing nucleation, and refining the structure of the solidified metal [1, 4]
The presence of bubbles significantly alters the ultrasonic wave propagation in the melt, and this influence must be accurately quantified to understand the effect of the acoustic field on a volume of treated metal
Following the comparison with water, the aluminium sonication case is presented with qualitative comparison with experimental data
Summary
Significant improvements in the quality and properties of metallic materials are observed when treating them near their liquidus temperature [1,2,3]: the beneficial effects of the treatment include the degassing of dissolved gases, improved wetting, activating inclusions by cleaning the solid-liquid interface, enhancing nucleation, and refining the structure of the solidified metal [1, 4] These improvements are primarily attributed to acoustic cavitation [5]; the term “cavitation” here follows the definition of Neppiras [6] and is restricted to cases involving the formation, expansion, pulsation, and collapse of existing cavities and bubble nuclei. The presence of bubbles significantly alters the ultrasonic wave propagation in the melt, and this influence must be accurately quantified to understand the effect of the acoustic field on a volume of treated metal In this endeavour, we are proposing a macroscopic cavitation model coupled with a high-order acoustic solver, using reference experiments in water [14] for validation
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