Evolution of microstructure and mechanical properties in 7A52 aluminum alloy thick-plate welded joints with ultrasonic vibration applied to different weld layers

Ultrasonic vibration was selectively introduced into different weld layers of multi-layer gas metal arc welded 7A52 aluminum alloy thick plates to alleviate weld defects, distortion, residual stress concentration, and property heterogeneity. The effects of layer-specific ultrasonic assistance on weld quality, residual stress distribution, microstructure evolution, and mechanical performance were systematically investigated. Applying ultrasound to all weld layers produced the best overall response. The plate flatness decreased to 0.663 mm, the average residual stress in the x-direction near-weld zone was markedly reduced, the average grain size in the first-layer weld zone was refined to 10.47 μm, and the tensile strengths reached 249.20 MPa and 254.17 MPa on Sides A and B, respectively. The corresponding impact absorption energies were 17.7 J, 18.2 J, and 17.4 J at three sampling locations. Applying ultrasound to the first four layers was effective for deformation control and impact toughness improvement, whereas applying ultrasound to the last two layers was more beneficial for reducing the residual stress gradient and improving stress uniformity. These results demonstrate that layer-dependent ultrasonic assistance is an effective strategy for balancing defect suppression, stress regulation, and property uniformity in thick-plate aluminum alloy welding.