Influence of interfacial structure on the fracture behavior of friction stir spot welded dissimilar joints

Abstract

Dissimilar joints generally involve the formation of a non-uniform interfacial reaction layer, which affects the joint strength significantly. However, a clear understanding of the effect of interfacial local strength on the macroscopic fracture behavior of the dissimilar joints is lacking in the existing literature. Here, we demonstrate the influence of interfacial reaction layer on the fracture behavior of the friction stir spot welded dissimilar joints between galvanized steel and aluminum alloy through the microscale evaluation of interfacial strength in addition to the conventional macroscale evaluations. Macroscale evaluations showed that the rim region of the joint, which includes a thin reaction layer comprising of both intermetallic compound (IMC) with several hundred nm thickness and Zn-concentrated area partly including voids derived from the discharge of galvanized layer, primarily resists the fracture in tensile shear and cross tension tests. Microscale cantilever beam bending tests showed that the interfacial strength increases with the decrease of reaction layer thickness except in the case of voids. Our study highlights the significance of rim region in spot welding, which has been indicated only by macroscale evaluations.