The effect of a paint bake treatment on joint performance in friction stir spot welding AA6111-T4 sheet using a pinless tool

It has been previously shown that it is possible to produce friction stir spot welds (FSSW) in thin (1 mm thick) aluminium sheet, when using a tool without a probe, which has the significant advantage of not leaving a keyhole in the weld. The aim of this paper was to investigate the effect of a post-weld paint-bake thermal cycle on the performance and precipitation behaviour of FSSW joints produced using a pinless tool, in the heat-treatable Al automotive alloy AA6111-T4, with rapid, industrially relevant, weld cycle times. It has been demonstrated that strong joints could be obtained within <1 s, that exhibited a nugget pull-out failure mode, and the weld strength increased further after the paint bake treatment. After the paint-bake thermal cycle the weld zone was found to be considerably harder than the parent material and this behaviour has been attributed to the shorter delay time between welding and artificial ageing, relative to the longer pre-natural ageing time experienced by the parent sheet. With a rapid (<1 s) weld cycle, no hardness minima were observed in the heat affected zone (HAZ) after the paint-bake treatment.