Repair of through holes in aluminum alloy plates using synergistically double-sided probeless friction stir spot welding

This study for the first time used synergistically double-sided probeless friction stir spot welding (SDP-FSSW) to repair the hole defects in a die-cast aluminum alloy ADC12 joints. Compared with conventional FSW repair methods, SDP-FSSW enables symmetric material flow from both sides without a probe, thereby promoting efficient defect closure while avoiding residual keyhole defects. Experiments showed that complete filling of holes could be achieved for joints (repaired zone) welded at a rotation speed of 1500 rpm, dwell time of 4 s, and plunge depth of 0.3 mm. When the rotation speed or dwell time increased, the flash height increased as well, while joint thickness decreased, and the grains refined in the joint. At low heat-input parameters, repair relies on fine-grain filling, while at high heat-input parameters, it depends mainly on material flow. The hardness distribution of the joint shifts from “W” to “U” shape as the heat input increases. The increase in rotation speed or dwell time produced lower joint hardness.