Dissimilar materials welding between A6061 and SPC270 thin sheets was performed through friction stir lap welding and the effects of welding tool shape and clearance were investigated. The welding process was carried out on the 3-axis vertical machining center without tilt angle, using welding tools with flat or scroll shoulder surfaces. The highest tensile shear strength of 208.2 MPa using the flat shoulder was achieved under tool rotation speed of 2000 rpm and welding speed of 500 mm/min. Groove-like defects were found at the advancing side of this welding joint, which was thought to be caused by the deficiency of plastic material flow. Scroll shoulder improved the plastic material flow of aluminum alloys and weld surface without defects. However, cavities were formed in joints welding with a clearance of 0.1 mm near the interface between A6061 and SPC270. Reduction of clearance suppressed the formation of cavities and tensile shear strength of 217.9 MPa was achieved with the clearance of 0 mm. The welding interface was observed by SEM and TEM for microstructure investigation. Welding interface temperature during FSLW was measured and the factors for welding strength variation were also discussed in this paper.

Friction stir lap welding (FSLW) between SPC270 steel sheet and A6061 aluminum alloy sheet were investigated with the effect of tilt angle and plunge depth of the welding tool. Tilt angle of 3 degree improved the material flow and suppressed generation of defect in the weld. The weld strength with tilt angle increased with the plunge depth of the welding tool and achieved maximum value of 212 MPa at 0.15 mm of plunge depth. From the TEM observation, uniform reaction layer of 450 nm was formed on the weld interface. On the weld interface, probe rotation and movement fabricated weld bead like structure on the steel surface and formed hook like structure on the cross section. Although the weld without tilt angle had a groove like defect, the weld strength was 208 MPa at 0.06mm of plunge depth.The weld interface was composed of laminated structures with different composition of aluminum and iron with 400 nm in each layer. This achieved by the mixing and pressurization by the probe on the weld interface.

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