Effect of low-intensity ultrasound on grain refinement and heterogeneous nucleation mechanism of 2219 Al alloy

The ultrasonic cavitation and acoustic streaming have long been regarded as the dominant mechanisms for refining the solidification microstructure of Aluminum (Al) alloys. This work investigated the effects of low-intensity ultrasound on the solidification microstructure of 2219 Al alloy by setting an ultrasonic application angle of 15° and with the different depths (30 mm, 70 mm, and 110 mm). The experimental results show that low-intensity ultrasound can also achieve a significant refining effect on the microstructure. Comparative analysis of solidified microstructures across multiple samples revealed, for the first time, that low-intensity ultrasound refines grain morphology primarily through enhanced heterogeneous nucleation. By establishing a theoretical model between acoustic intensity ($I_e$) and heterogeneous nucleation energy ($\Delta G^{\ast }$), the required low-intensity acoustic pressure amplitude ($P_a^{\prime }$) for heterogeneous nucleation was determined. The calculation results are in good agreement with the experimental conclusions, thereby proposing a new mechanism for ultrasound to improve solidification microstructures. This work demonstrates that the ultrasonic cavitation and acoustic streaming are not necessary conditions for refining grain structures. Low-intensity ultrasound can also promote the refinement of Al alloy grain structures when satisfying the critical nucleation acoustic pressure conditions.