Stress-level dependency of creep ageing behavior for the nugget zone in a friction stir welded Al–Cu–Li alloy

Abstract

In this study, the creep response and microstructure evolution for the nugget zone (NZ) of a friction stir welded Al–Cu–Li alloy under different stress levels are systematically investigated. Compared to the base material (BM), the creep strain and steady creep strain rate of the NZ increase rapidly with enhancing the applied stress. The total creep strain of NZ is noticeably higher than that of BM under the same stress. The creep mechanism of NZ changes from diffusion creep to dislocation climbing creep with the increase of stress. After creep ageing, both NZ and BM samples have a significant enhancement in the strength. However, the elongation at fracture of creep-aged NZ sample falls sharply for higher creep aging stress. The fracture mechanism of creep-aged NZ sample transforms from the mixed fracture to brittle intergranular fracture with the increase of the stress. The average length of T1 precipitates of creep-aged NZ sample gradually increases with rising the stress. Meanwhile, the average length of T1 precipitates of NZ is significantly larger than that of BM under the same stress. It has been demonstrated that, during the creep-ageing process, high external stress could cause the formation of wide PFZ and lower the ductility of NZ. These results provide important theoretical support for the creep aging forming of large-size Al–Cu–Li alloy components contained the friction stir welding joint.