Strengthening aluminum alloy welds via ultrasound without reduction in the average grain size

Grains refinement induced by ultrasound was regarded as a main mechanism for improving strength. This study exhibits that other strengthening effects resulting from ultrasound when low power is applied, in the absence of average grain size reduction, have been substantially underestimated. Ultrasound achieves strengthening via micro/nano-scale precipitates, increasing dislocation density, and inhibited intermetallic compounds, notably, the average grain size in the weld shows no decrease, which has rarely been reported. Compared to the condition without ultrasound, an ultimate tensile strength improvement of ∼50% was achieved at a 4 μm ultrasonic amplitude, despite being accompanied by a ∼ 33% increase in the average grain size. It reached ∼131% of the base metal's strength. We also characterized the atomic-scale strain near the Al/nano-scale Si interface and analyzed interface configuration in combination with the results of density functional theory (DFT) calculations. These findings offer an alternative strategy for Si modification and provide a new perspective on strengthening via ultrasound.