Formation mechanism and fracture behavior in extra-filling refill friction stir spot weld for Al–Cu–Mg aluminum alloy

Abstract

Joint quality of conventional refill friction stir spot welding (RFSSW) is sensitive to welding parameters, and the welding parameters are difficult to control. Thus, deep surface indentation, large hook angle, and void usually occur within the RFSSW joint. An extra-filling RFSSW method is proposed in accordance with defect characteristic. The additional filling method, which was implemented through feed by sidewall, aims to compensate for the material loss within the RFSSW joint. The effect of the filling material on the formation characteristics, triangle void, and mechanical properties of the joint is investigated. Results show that the filling material was stacked in the top region of the stir zone (SZ). Furthermore, the surface indentation of the welding spot, hook angle, and bonding ligament thickness of extra-filling RFSSW joint were reduced, and triangle void at the bottom of the sleeve-affected zone was eliminated. Thus, tensile-shear load of extra-filling RFSSW joint was higher than that of conventional RFSSW joint. This study reveals the filling mechanism of triangle void and fracture behavior of RFSSW joints simultaneously. The squeezing effect of filling material makes more material in the SZ and the lap interface outside the SZ fill the triangle void. The filling of triangle void increases joint net-section area and decreases the stress concentration, and the samples fracture along the bonding ligament.