Friction surfacing assisted refilled friction stir spot welding of AA6061 alloy and Q235 steel

The AA6061 alloy and Q235 steel were well joined by refilled friction stir spot welding without stirring steel, as a 1 mm thick coating was deposited on the steel by friction surfacing before welding. The coating was composed of highly refined grains. The materials experienced shear deformation in the stir zones and the extrusion in hook and thermo-mechanical affected zones, resulting different predominant textures in the weld. A Fe-rich layer was depicted from the interface due to the enhanced atomic diffusion, and the layer's thickness increased as the sleeve plunged deeper during welding. Al-Fe intermetallics, including Al13Fe4, Al5Fe(2), formed in the Fe-rich layer, since the solubility of Fe in Al was far lower than the measured content. The optimal shear strength reached 7.31 kN in this study. The welds fractured along the weld boundary and the Al/steel interface. The fracture initiated at the edge of the gap between AA6061 plate and coating, and propagated along the weld boundaries, resulting in the shear dimples and the faceted appearance. However, the spot weld with 2.9 mm penetration was fractured along the Al/steel interface mainly, which was attributed to the over formed intermetallics at the interface.