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

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 100 mm min−1. At a joining speed of 50 mm min−1, the morphology of the interface was a flat and simple interfacial reaction layer whose thickness was less than 1 μm. 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 25 mm min−1, the interface consisted of a macroscopically mixed and laminated structure approximately 300 μm 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.