Forming mechanism and mechanical properties of dissimilar friction stir lap welds of 304 austenitic stainless steel to a Ti6Al4V alloy

A TC4 titanium alloy plate was lap joined to a 304 austenitic stainless steel plate via friction stir welding. The microstructures at the lap joint interface were intensively examined by means of energy dispersive X-ray spectroscopy and transmission electron microscopy analysis, and the mechanical properties of the lap joint were evaluated by a microhardness and shear tensile test. The results show swirling-like stir zones of TC4 and 304 SS are formed along the interface, where an aprox. 200 nm interface layer composed of TiFe and TiFe2 intermetallic compound is dispersed at the high rotating speed of the tool. In addition, the high hardness value is discontinuous distribution at the interface, and the shear strength of the commercially TC4 plate and 304 SS FSW lap joint can reach 7507 N at a rotational speed of 600 rpm, a welding speed of 30 mm/min, and a press amount of 0.4 mm.