Grain refinement of an Al-2Cu alloy using ultrasonic treatment was investigated numerically. A finite element model coupling fluid flow and heat transfer was developed and validated by comparing the results of both numerical simulations and physical experiments. The model successfully describes hydrodynamic fields generated by ultrasonic treatment and its influence on heat transfer. The simulations were used to study the influence of the duration of ultrasonic treatment and the associated acoustic streaming on convection and the resulting temperature distribution. It was revealed that a relatively cold sonotrode applied during ultrasonic treatment for up to 4 minutes created a casting environment that promoted crystal nucleation and enabled their growth and survival during transport of these grains into the bulk of the melt by strong convection. The enhanced convection established a low temperature gradient throughout the melt which favours the formation of an equiaxed grain structure. Therefore, the convection induced by acoustic streaming plays a critical role in facilitating nucleation, growth, and transport of grains.

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The evolution of the grain structure of an Al-2Cu alloy is investigated numerically during its solidification while being subjected to ultrasonic treatment. A CAFE (Cellular Automation Finite Element) model coupling fluid flow, heat transfer, nucleation, and grain growth is developed and validated by comparing the results of both numerical simulations and physical experiments. The model successfully describes hydrodynamic fields generated by ultrasonic treatment and their influence on microstructural evolution. This validated model is then used to study the influence of the duration of ultrasonic treatment on temperature distribution and grain structure. A predominantly equiaxed grain structure developed due to the low temperature gradient throughout the melt is a result of the enhanced convection. These results highlight the critical role played by the convection induced by acoustic streaming during casting, that is, facilitating nucleation and grain growth.

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