Influence of paint baking conditions on the mechanical performance of thin aluminum-steel blanks

This work investigates the influence of the automotive paint baking (PB) process, a heat treatment that is typically applied to assembled and dip-coated car bodies-in-white (BIW), on the basic mechanical performance of lightweight aluminum-steel blanks. For this purpose, 1.2 mm-thin sheets of AW-6016-T4 aluminum alloy were butt-welded-brazed to 0.8 mm-thin sheets of DC04 zinc-coated steel using the single-sided Cold Metal Transfer (CMT) process and an AA-4020 aluminum alloy filler. The aluminum-steel blanks were subjected to a post-weld heat treatment (PWHT) for 20 min at 185±2 °C, simulating industrial paint baking conditions. Samples for microstructure analysis, hardness testing and quasi-static and cyclic tensile testing were prepared from the blanks. Under all loading conditions, fracture of the as-welded and heat-treated samples primarily occurred in the aluminum-based weld. However, fracture in the steel sheet occurred in cases of low porosity or high reinforcement of the weld. The heat treatment slightly improved the quasi-static tensile and low-cycle fatigue performance, but it decreased the high-cycle fatigue performance. Moreover, it reduced the hardness mismatch between different zones, causing more homogeneous properties across the joints. Nevertheless, the primary failure mode of the blanks did not change, as the influence of the weld porosity was more significant than the influence of the heat treatment. The results provide a fundamental understanding of the bake hardening response, reveal insights into the porosity-related failure behavior under quasi-static and cyclic tensile loadings, and highlight the importance for avoiding weld defects to exploit the load bearing potential of butt-welded-brazed aluminum-steel blanks.