Effect of post-welding aging on the microstructure and mechanical properties of high-strength Al-6.7Zn-1.8Mg-0.2Cu aluminum alloy joints prepared by friction stir welding

Friction stir welding (FSW) is used to weld Al-6.7Zn-1.8Mg-0.2Cu aluminum alloy sheet with 3.2 mm thickness, and the welded joint is subjected to post-welding aging heat treatment, and that is artificial ageing (AA) 120 ℃ × 24 h. The welded and aged joints are studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM), hardness test and tensile test. The results show that the tensile strength of the joint is 470 MPa, the welding coefficient is 80.0%, and the elongation is 10.1%. AA could effectively improve the tensile strength of the joint, which is 525 MPa, 89.6% of the base material (BM). Compared to as-welded joint increases 11.8%. And the elongation is 8.5%. In the FSW process, the precipitated phases in thermo-mechanically affected zone (TMAZ) and nugget zone (NZ) are dissolved and then precipitated after cooling, in which the precipitated phase is mainly η' phase, and the dissolution of GP zone (Guinier-Preston zone) and coarsening of η' phase mainly occurred in the heat-affected zone (HAZ) , and the precipitated phase gradually changed from dissolution to coarsening under different heat input. After welding ageing, GP zone precipitates and GP zone transforms to η' phase, and the existing η' phase at HAZ transforms into coarse η phase. The hardness curve of the joint is obviously W-shaped, the fracture of the FSW joint is in the NZ, and the fracture of the post-welding aging joint is in the HAZ.