In order to study influencing factors in interfacial structure on characteristic of macroscopic strength of A1100/Zn-plated steel friction stir welded dissimilar materials lap joint, fracture process under tensile shear loading was investigated by in-situ observation test conducted inside a scanning electron microscope. Intermetallic phase between Fe and Al was formed at the interface in all joints obtained in the present study. Moreover, in the joint with lower strength, Fe-Zn alloy was also formed interface between the intermetallic layer and Zn-plated steel. Those intermetallic phase and Fe-Zn alloy influenced on the fracture process and strength. In case of the joint with higher strength, change in mechanical property of A1100 due to penetration of Zn into A1100 might enhance plastic deformation and crack deflection. Based on results of the in-situ observation test, appropriate condition for friction stir welding of the dissimilar material joint was proposed by taking into account influencing factors shown.
