Acoustic streaming and cavitation effect on microstructure evolution of Nb-Si-Ti alloy treated by ultrasound

Abstract

Heat and mass transfer play a critical role in grain refinement by ultrasound treatment. In order to reveal the mechanism of ultrasonic in Nb-Si molten metal, a 3-D model was simulated on the acoustic, temperature and flow fields with different acoustic frequencies and displacement amplitudes by Comsol Multiphysics. Simulation results show that a new harmonic wave appears with frequency increasing, and the acoustic streaming effect is significant at 20 kHz. As the displacement amplitude increases from 10 μm to 20 μm, the maximum velocity varies from 0.01 m/s to 0.03 m/s. The distributions of three fields are dependent on frequency, and the displacement amplitude determines acoustic pressure value and fluid velocity. The displacement amplitude has a positive effect on the acoustic streaming effect improvement in the melt. In the UST ingots with different displacement amplitudes, the size of the broken dendrites decreases from 57.5 μm to 10 μm with the displacement amplitude lower than 16 μm, by 82.6%. γ-Nb5Si3 phase increases and the primary Nbss phases are elongated, coarsened and spheroidized with the increase of the displacement amplitudes. The simulation results of the flow directions can be verified by the elongated Nbss phase. The acoustic streaming effect promotes heat and mass transfer and the sluggish eutectoid reaction in Nb-Si-Ti alloy during the ultrasound treatment.