Detailed characterizations of microstructure evolution, corrosion behavior and mechanical properties of refill friction stir spot welded 2219 aluminum alloy

2.0 mm thick 2219-O (upper plate) and 8.0 mm thick 2219-C10S (lower plate) were successfully fabricated using refill friction stir spot welding (RFSSW), and the microstructure evolution, corrosion behavior and mechanical properties of the joints were investigated. The results indicated that the stir zone (SZ) is characterized by refined and equiaxed grains due to dynamic recrystallization, whose grain size gradually increases, and the texture intensity decreases with the increase of the distance from the upper plate top surface. Tangled dislocations and dislocation cells are observed in the SZ, while rod-like precipitates with 20 nm–100 nm length are found in the base material (BM) of the lower plate. Also, numerous Al₂Cu particles are observed in the upper plate BM, while only a few particles are aggregated in the lower plate BM. However, Al₂Cu particles are fragmented and uniformly distributed in the SZ. The hardness of the upper plate and the lower plate is approximate 50 HV and 160 HV, respectively. The hardness increases gradually from top to bottom of the cross-section. The self-corrosion potential and corrosion current density of the SZ are −0.45 V and 5.62 × 10⁻⁵ A/cm², respectively. In comparison, those of the BM of the upper plate are −0.49 V and 3.16 × 10⁻³ A/cm², and those of the lower plate BM are −0.68 V and 7.94 × 10⁻³ A/cm². The electrochemical experiment indicated that the SZ presents excellent corrosion resistance, followed by upper plate BM and then lower plate BM. The corroded area has mainly occurred at the lower plate BM when the joint cross-section is immersed in the EXCO solution for one hour. However, upper and lower plate BMs are eroded seriously when the immersion time increases to two hours. Nevertheless, no obvious corrosion can be observed in the SZ. The mechanical analysis indicated that the tensile load decreases from 7.25 to 4.10 kN, and the tear load decreases from 3.20 to 1.78 kN after being immersed in the 3.5% NaCl solution for 48 h.