Microstructure characteristics and corrosion behavior of refill friction stir spot welded 7050 aluminum alloy

Refill friction stir spot welding (RFSSW) of high strength aluminum alloy has a wide application prospect. However, there are few studies between the microstructure and macroscopic corrosion behavior, causing a significant impediment in corrosion protection of the joint. Therefore, the microstructure and corrosion behavior of 7050 aluminum alloy RFSSW joint was investigated in this paper. Due to the difference of microstructure, the joint can be divided into five zones, namely stir zone (SZ), thermo-mechanically affected zone (TMAZ), heat affected zone-I (HAZ-I), heat affected zone-II (HAZ-II) and base metal (BM). Therein, the grain size and the micron-sized second phase particles of SZ were fine, while other areas were coarse. And they were mainly Al2CuMg and Al7Cu2Fe. After intergranular corrosion and exfoliation corrosion tests, it was indicated that the grain morphology and second phase particles did not significantly affect the corrosion susceptibility, but the continuous degree of intergranular nano-precipitates was the key factor. A coefficient r was defined to quantitatively characterize the continuous degree of intergranular nano-precipitates. The HAZ-II and TMAZ exhibited high corrosion sensitivity with a high value of r, while HAZ-I and SZ exhibited low corrosion sensitivity with a low r. The corrosion mechanism was also briefly discussed.