In this study, Al-5083 and St-12 alloy sheets were friction stir lap welded at different travel (7-23 cm min(-1)) and rotation (750-1125 rev min(-1)) speeds of the welding tool. The welded joints were characterised by various methods including shear tensile and Vickers microhardness tests, optical and scanning electron microscopies and X-ray diffraction analysis. The results showed that the weld zone defects decreased, and the joint strength also improved significantly with the reducing tool travel speed from 23 to 7 cm min(-1). The travel speed of 11 cm min(-1) was an optimum speed within the travel speeds investigated in this work. Additionally, raising the tool rotation speed enhanced the joint strength slightly. The microhardness results confirmed the formation of intermetallic phase layer with the hardness of similar to 335 HV at the swirl layered structure.