Influence of accelerated corrosion on Al/steel RSW joints by in situ compression tests

Abstract

The present work investigates the correlations between galvanic corrosion, intermetallic compound (IMC) formation, and the input of welding energy with respect to the initiation and propagation of micro-cracks in micropillars of resistant spot welding (RSW) joints between aluminum (Al) and steel. The present results indicate that, in the high welding energy region, Al was corroded first after 26 cycles of corrosion, and Al5Fe2 IMC was corroded after 72 cycles of corrosion due to its high corrosion potential predicted by the calculations based on density functional theory (DFT) in terms of the Nernst equation. In comparison with the high welding energy region, less corrosion was observed in the middle welding energy region due to the thinner Al5Fe2 IMC layer, a lower amount of Al13Fe4 IMC in the Al matrix, and lower residual stress. Mechanical properties at different locations after various corrosion conditions were obtained using in situ compression tests, including the stress-strain responses and the strain rate sensitivity. The micropillars from the high welding energy region have a higher average yielding stress due to the thicker IMC layer than those from the middle welding energy region. The yielding stress decreases gradually with increasing corrosion cycles. Three conditions for crack initiation and propagation have been identified: firstly, for pillars from the high welding energy region before corrosion or the middle welding energy region (before or under the minimal corrosion conditions), the cracks initiate within the IMC layer; secondly, for pillars from the high welding energy region after 26 cycles of corrosion (under the moderate corrosion condition), cracks propagate at the Al/IMC interface; finally, for pillars from the high welding energy region after 72 cycles of corrosion (under the severe corrosion condition), the cracks initiate at the IMC/steel interface.