Microstructures and mechanical properties of friction stir welded lap joints of commercially pure titanium and 304 stainless steel

Abstract

Friction stir welding was performed to accomplish dissimilar lap joining of commercially pure titanium (CP-Ti) to 304 stainless steel (SUS304). The joining speed was varied from 25 to 100mmmin−1. At a joining speed of 50mmmin−1, the morphology of the interface was a flat and simple interfacial reaction layer whose thickness was less than 1μm. The reaction layer consisted of four layers: β-Ti (+ ω-Ti), Ti2Ni, FeTi+Fe2Ti, and σ-FeCr, listed in order from the CP-Ti side to the SUS304 side. At a joining speed of 25mmmin−1, the interface consisted of a macroscopically mixed and laminated structure approximately 300μm thick consisting of multiple reaction layers. During the tensile shear test, joint fractures occurred in the CP-Ti base material at every joining speed. However, during the peel test, joint fractures occurred at the joint interface.