Synergistic effects of ultrasonic vibration and self-riveting on the microstructure and mechanical properties of aluminum/steel friction stir lap welded joints

This study innovatively employs the “ultrasonic vibration + self-riveting” (UR) hybrid technique for friction stir lap welding (FSLW) of 2.5 mm thick 6061-T6 aluminum alloy to 1.8 mm thick Q235 steel, systematically investigating its effects on joint quality, microstructure, and mechanical properties. Results demonstrate that the UR technique macroscopically enhances weld surface quality, increases the interfacial bonding area, and optimizing the hook structures. The synergistic effect significantly improves the filling of pre-punched holes and the strength of mechanical interlocking. Microscopically, ultrasonic vibration eliminates interfacial defects, refines the grain size in the weld nugget zone (to 5.0 μm), and effectively suppresses excessive growth of Al/Fe interfacial intermetallic compounds (IMCs). The core finding reveals UR's synergistic strengthening mechanism: ultrasonic vibration optimizes material flow, while self-riveting reinforces mechanical bonding. Their synergy forms a unique spatially interlocked “metallurgical bonding - mechanical interlocking” hybrid structure at the interface. This “metallurgical-mechanical synergy” mode surpasses the strength limits of traditional single-mechanism approaches, significantly enhancing joint performance. This research elucidates the joining mechanism for dissimilar materials, proposes universally applicable synergistic strengthening principles and a solid-state joining model, providing an innovative and efficient technical solution for lightweight manufacturing.