Microstructure Evolution and Mechanical Property of Ultrasonicated Al–7 Pct Si Alloy Under Coupled Modulation with Grain Refiner

Twenty kilohertz power ultrasounds with 14, 18, and 22 μm amplitudes together with 0, 0.05, and 0.1 wt pct Al–5Ti–B grain refiners were introduced into the solidification process of liquid Al–7 Pct Si alloy. The combination of power ultrasound and grain refiner reduced the average α-Al dendrites size to 8.6 pct of that during static solidification, which was far superior to the refining effect of only using power ultrasound or adding grain refiner alone. The acoustic field characteristics versus refiner content were in situ determined, based on which their coupled effects on solidification process were further explored. On one hand, both the transient and stable cavitation were enhanced by TiB₂ particles produced by grain refiner, with the former significantly increased local undercooling in alloy melt to promote the nucleation rate for α-Al phase. On the other hand, the flow field uniformly dispersed TiB₂ particles within liquid alloy and their wettability was further improved by stable cavitation, which provided more potential heterogeneous nucleation sites. Their synergistic effect ultimately led to three orders of magnitude increase in the nucleation rate of α-Al phase. As compared with the statically solidified microstructure, the cooperative regulation of ultrasound and refiner resulted in simultaneous improvement of tensile strength and elongation rate by 1.23 and 1.77 times.