Fracture mechanisms of Al/steel resistance spot welds in lap shear test

Abstract

This study investigated failure modes, mechanical strength and fracture mechanism of Al/steel resistance spot welds in lap shear test. Three major fracture mechanisms and failure modes were identified: semi-brittle or brittle fracture in the intermetallic compound (IMC) layer which led to interfacial failure, ductile fracture in the aluminum fusion zone (FZ) which led to thickness failure, and ductile fracture in the aluminum heat affected zone (HAZ) which led to the engineering preferred button pullout failure. The failure mode and mechanical strength of the weld depended highly on the dimension and properties of the IMC layer, aluminum FZ, and aluminum HAZ. The IMC thickness was the dominant factor of the failure mode and mechanical properties. The button pullout failure or thickness failure was related to thin IMC thickness of no more than 3μm. Meanwhile, increasing the Al nugget size or steel bulge height helped to increase lap shear peak load and promote button pullout failure. Thick IMC layer with thickness more than 3μm tended to cause interfacial failure in the IMC layer, even with large Al nugget size and severe steel bulging.