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Protrusion friction stir spot welding: a simple novel method to produce dissimilar joints of galvanized steel/aluminum sheets with high mechanical performance

In this study, protrusion friction stir spot welding (PFSSW) as a simple novel method was applied using pinless tools and a specially designed protrusion on the surface of the backing anvil to weld aluminum and galvanized steel sheets with 1 mm thickness. Welding process was carried out at tool rotational speeds of 1600, 2000 and 2500 rpm and dwelling times of 3, 6 and 9 s. Microstructure and mechanical properties were used to evaluate the produced welds. The welds produced by PFSSW illustrated uniform and smooth surface appearance (free keyhole) with very low burs content compared to conventional FSSW due to the placing of the steel sheet on the aluminum sheet and presence of the protrusion. Completely metallurgical bonding and continuous structure of intermetallic compounds (IMCs) layer at the interface was obtained in the PFSSW. Dynamic recrystallization and presence of pearlite and non-equilibrium phases were main factors of the grain refining in the SZ while phases content increased with increase in the processing parameters. All the produced welds by PFSSW were failed by the CF mode and welding samples at 1600 rpm exhibited more peak load and failure energy than other conditions due to more joint width and lower IMCs layer thickness. The current process was able to produce dissimilar joints with superior mechanical performance compared to other techniques.