Investigation on microstructure and localized corrosion behavior in the stir zone of dissimilar friction-stir-welded AA2024/7075 joint

Friction stir welding (FSW) is an effective welding technique to realize the joining of dissimilar aluminum alloys. The microstructural heterogeneities induced by FSW across the joints could have curial implication for the corrosion performance of the joints. In this research, the microstructure and localized corrosion behavior of shoulder interface zone (SIZ), vortex zone (VZ), bottom zone (BZ) and bottom interface zone (BIZ) in the stir zone (SZ) of dissimilar FSW AA2024/AA7075 joint was systematically investigated through detailed microstructural characterization and relevant corrosion tests. The results indicated that plentiful of Cu-rich constituent particles are formed on AA2024 side and the areas near the interface on both sides, and corrosion originates from these regions. Grain size has little influence on corrosion behavior of the SZ, while the local regions with higher stored energy are more sensitive and liable to corrosion. The sequence of mixing degree of materials in the four regions of the SZ is: BZ > VZ > SIZ > BIZ, which is in contrast to the order of corrosion rate. Galvanic corrosion is detected in the SIZ and BIZ, and sufficient mixing of materials significantly weakens the galvanic corrosion, resulting in higher corrosion resistance in the BZ.