Elucidation of interface joining mechanism of aluminum alloy/dual-phase steel friction stir riveting (FSR) joint

The mixed use of aluminum (Al) alloy and high strength steel (HSS) is an effective way to reduce the weight of vehicles, which meanwhile brings great challenges to the traditional spot joining technologies. Three-stage friction stir riveting (FSR) process using a precisely designed semi-hollow rivet was developed for Al-steel joining. By coordination control the rotation and feed movement of the rivet, a “tube-sheet” annular solid-phase welding zone is formed between the semi-hollow rivet and the lower steel sheet, realizing the reliable joining of Al-steel dissimilar sheets combination. The rivet structure was designed to accommodate the trapped Al alloy in the rivet cavity during the friction welding stage, obtaining an inclusion-free friction welding interface which was not only mechanically mixed but also fully interatomic binding. The optimal FSR joint is composed of ferrite softening zone (FSZ), carbide precipitation zone (CPZ), ultrafine martensite zone (UMZ), and fine dual-phase zone (FDPZ). Moreover, due to the circumferential flow of materials during friction welding stage, the dynamic recrystallization (DRX) microstructures of the rivet and steel sheet both have shear texture consistent with {112} and {110} cube texture, which brings the uniformity of microstructure and performance. With the combined contribution of inclusion-free joining interface and the microstructure with uniform cube texture, the optimal FSR joints have the maximum average tensile-sheer strength of 281.1 MPa (90.7%·σ6061-T6) and peak load of 5.29 kN, presenting pull-out fracture mode with good fracture energy absorption.