Dissimilar lap joining of 304 stainless steel to CP-Ti employing friction stir welding

► Lap joining of 304 SS to CP-Ti by FSW using the former as lap top member is novel. ► A new designed tool with double-shoulder was employed to plasticize both lap members. ► Complex microstructures were presented at joint interface. ► Features like vortices of bimetallic weld and mechanical interlock are interesting. ► A greater shear strength value was achieved using 304 SS as the top member.