Corrosion Behaviors of CMT Spot-Joined AA6022-T4 to Galvanized DC03

The mixed use of aluminum and steel is increasing in lightweight automotive bodies. The cold metal transfer (CMT) arc spot welding of aluminum alloy to steel has recently been developed. However, the electrochemical potential difference of dissimilar materials will inevitably degrade weld performance. Thus, this study investigated the corrosion behavior of CMT spot-welded 1-mm AA6022-T4 sheet to 1.5-mm hot dipped galvanized (HDG) low-carbon steel, DC03. The open circuit potential (OCP) of the base materials and varying positions within the welds was measured, and the results showed that the OCP of the zinc coating on the steel substrate and zinc-rich zone at the weld root were the most negative and likely to corrode. Salt spray exposure was conducted specifically on the welds with and without e-coating. The results indicated a similar fracture mode after 20 cycles as with the as-welded condition, i.e., fracture in the weld metal, though with a slightly reduced lap-shear strength. After an exposure of 63 cycles, the weld strength decreased nearly 38%, and the fracture mode transitioned to interfacial fracture due to the corroded Fe-Al IMC layer, whereas postweld e-coating of the CMT Al-steel joints provided a high level of corrosion protection.