Effect of tool pin length on interfacial behaviors and mechanical properties of friction stir-welded lap joints of 430/304 stainless steels

In this study, the effect of tool pin length on friction stir lap welding of 430 and 304 stainless steels has been investigated. The thermal history of interface was measured by thermocouple. The interfacial macro- and micro-structures under different tool pin lengths were analyzed via optical microscope and scanning electron microscope. Also, tensile shear tests were carried out to evaluate the strength of the lap joint. The results show that increasing the pin length from 0.5 to 1.3 mm increases the peak temperature of the interface, which has a significant effect on diffusion layer formation and hook geometry. The fracture mode of the joints shifts from shear fracture to tensile fracture with the increase of the pin insertion depth. When the pin length is 1.0 mm, i.e., a slight penetration into the 304 plate, the joint presents a maximum failure load of 2.86 kN. The findings reveal that the effective top plate thickness ( T eff ) used to describe the hook geometry is an important factor determining the joint strength and fracture mode.