Friction stir spot welding (FSSW) was performed for lap joining of 1.1 mm-thick 6022 aluminum alloy sheets. The FSSW was carried out by using tools with three different probe bottom shapes (flat bottom, cross groove,cross stage) for several plunge depths which were all less than the top sheet thickness. Static joint strength was evaluated by cross tensile tests. Results showed that compared with flat bottom probe, cross groove and cross stage probes promoted the material flow to form larger periphery stir zones. Hook feature appeared obviously for the cross groove and cross stage probes at the plunge depth of 1.0 mm. When the plunge depth was 0.8 mm, higher fracture load was obtained for the joints welded by the cross groove and cross stage probes than that obtained by the flat bottom probe. This is attributed to their fracture paths passing through the well-grown stir zone which were produced for the cross groove and cross stage probes. However, the hook at the interface significantly reduced the local top sheet thickness for the cross stage specimen in the plunge depth of 1.0 mm. This provided the preferential fracture site and caused a large fracture load reduction.

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