Multiscale modeling of the solidification microstructure evolution in the presence of ultrasonic stirring

Abstract

Ultrasonic treatment (UST) was studied to improve the quality of cast ingots as well as to control the solidification microstructure evolution. Ultrasonically-induced cavitation consists of the formation of small cavities (bubbles) in the molten metal followed by their growth, pulsation and collapse. These cavities are created by the tensile stresses that are produced by acoustic waves in the rarefaction phase. The cavitation threshold pressure for nucleation of the bubbles may decrease with increasing the amount of dissolved gases and especially with the amount of inclusions in the melt. A UST model was developed to predict the ultrasonic cavitation and acoustic streaming. The developed UST modeling approach is based on the numerical solution of Lilley model (that is founded on Lighthills's acoustic analogy), fluid flow, and heat transfer equations, and mesoscopic modeling of the grain structure. The UST model was applied to study the solidification of Al-based alloys) under the presence of ultrasound. It is found that the predicted ultrasonic cavitation region is relatively small, the acoustic streaming is strong and thus the created/survived bubbles/nuclei are transported into the bulk liquid quickly. The predicted grain size under UST condition is at least one order of magnitude lower than that without UST, which is in excellent agreement with the experimental data.