Friction stir spot welding between a 1?5 mm 6061 Al-T6 sheet and a 1?5 mm Cu sheet was conducted. The effects of the following parameters on the lap shear weld strength were investigated: tool pin length, shoulder plunge depth, weld time and tool rotation speed. With rotation speeds of 2000 rev min 21 and a 2?60 mm tool pin length, weld strengths just over 2000 N were produced. Welds made at 1000 rev min 21 with 1?83 or 2?60 mm tool pin lengths produced weak welds, with strengths below 900 N. Stronger welds had a Cu ring extruded upward from the lower Cu sheet into the upper Al sheet, which promoted bonding and interlocking between the sheets. The interface between this Cu ring and the surrounding aluminium was free of Al/Cu intermetallics. The weld strength appeared driven by the aluminium layer thickness between the tip of the Cu ring and the keyhole with the strength increasing by 2833 N mm 21 of aluminium layer thickness.