Effect of viscous layer on the mechanical properties and fracture mechanism of friction stir butt welded Al/Steel joints

Friction stir welding (FSW) of aluminum (Al)/steel has been extensively studied. However, there is limited research on the material flow and deformation behavior between FSW tools, Al base materials as well as steel base materials. In this paper, Al alloy was joined to steel by FSW and an in-depth study of the viscous layer was conducted. The study revealed that the different regions of the stirring zone had inconsistent material flows and microstructures. Furthermore, according to the characteristics of material flow and deformation in the distinct regions, the formation process of the unique "C-shaped" bonding interface was summarized. In addition, the study of exit hole revealed the presence of a viscous layer on the steel surface consisting of plastic Al. The formation process of this viscous layer and its role in forming welded joints were investigated. The viscous layer has a critical impact on joint performance. Different microstructures of the viscous layer led to various crack propagation paths, which ultimately resulted in different joint failure mechanisms, thus affecting the joint properties.