Microstructure evolution and corrosion behavior of refill friction stir spot welding joint for dissimilar Al alloys

The dissimilar 2B06 and 7B04 Al alloy joints were prepared by refill friction stir spot welding (RFSSW), and the microstructural evolution and corrosion behavior of the joints were investigated. Based on microstructural analysis, the welded joints exhibit distinct microstructural zones, including the stir zone (SZ), thermomechanically affected zone (TMAZ), and heat-affected zone (HAZ). The grain size of each zone is in the order of HAZ > TMAZ > SZ. Notably, the TMAZ and HAZ contain significantly larger secondary-phase particles compared to the SZ, with particle size in the HAZ increasing at higher rotational speeds. Electrochemical tests indicate that corrosion susceptibility follows the sequence of HAZ > TMAZ > SZ > BM, with greater sensitivity observed at increased rotational speeds. Post-corrosion mechanical performance degradation primarily arises from crevice corrosion at joint overlaps, but not from the changes in the microstructure.