Effects of semi-solid structure on interface formation of dissimilar aluminum to galvanized steel welds produced by load-controlled Refill Friction Stir Spot Welding


Refill Friction Stir Spot Welding (Refill FSSW) allows welding dissimilar materials providing excellent bonding between both parts. On this work, a multi-scale analysis of load-controlled Refill FSSW was performed to analyze dissimilar AA6016-T4 and Zn-coated DX56D steel joints. These were produced using optimized process parameters and analyzed in both as-welded and bake-hardened conditions. During the process, fusion and subsequent dispersion of Zn favored the formation of a semi-solid structure characterized by an intense microsegregation. Therefore, incipient melting of Zn-rich phase followed by eutectic reaction was observed. The presence of liquid phases along the grain boundaries led to a complex relationship between mechanical properties, microstructure and processing variables. Joints with enhanced mechanical properties were produced by limiting the growth of intermetallic compounds (IMC) at the interface, which coupled with stir zone (SZ) strengthening due to Zn dispersion, led to less pronounceable secondary bending effects. The bake hardening process was also found to have a substantial influence on diffusion-dependent mechanisms and, consequently, on the final performance of the welded joint. The results highlighted a great potential of load-controlled Refill FSSW for producing high-strength dissimilar joints in short cycles, which is desirable for applications in the automotive industry.
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