Refill friction stir spot welding of dissimilar aluminum alloy and AlSi coated steel

AA5754 Al alloy and coated steel were successfully joined by refill friction stir spot welding through the eutectic Al-Si coating. The overlap shear strength increased with plunge depth and reached a high strength plateau which was attributed to maximum bonding size and strong Al/AlSi coating joint interface. The AlSi coating in the as-received coated steel consisted of AlSi eutectic phase and two main intermetallic phases (Al7Fe2Si and Al5Fe2(Si)) with an average width of 8.5 μm and around thickness of 1 μm, respectively. The former played a key role in dissimilar welding, the latter effectively restrained the formation of additional IMCs and the growth of exiting IMCs during welding. Microstructural evolution showed the high strength of joints is also associated with fine silicon particles, elimination of defects and control of intermetallics at the joint interface. Fracture of full bonding welds initiated from the partial metallurgical bonds (PMBs) or kissing bonds, then propagated through the AlSi coating and AlSi/Al7Fe2Si interface, final fracture occurred at Al7Fe2Si/Al5Fe2(Si)/steel substrate interfaces.