Three sleeve structures (conventional sleeve, threaded sleeve, grooved sleeve) were used for refill friction stir spot welding (RFSSW) of 2A12-T42 aluminium alloy to investigate the effect of sleeve geometry on hook formation and mechanical properties of the joints. The high-pressure area generated during the plunge stage was responsible for the hook formation. Threads and grooves sleeves not only enhanced the mixing effect but inhibited the hook formation by breaking up the alclad layer or promoting the downward material flow, respectively. The welds using a grooved sleeve had the lowest hook height and highest tensile-shear failure load. As the hook height decreased, the upward tendency of crack propagation was suppressed, which confined the propagation paths within the regions of higher hardness.
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