Stress-level dependency of creep ageing behavior for friction stir welded Al-Cu alloy

Creep ageing forming (CAF) has been widely used in the aerospace engineering, but how to optimize the processing conditions, especially under complex stress state of the CAF process for large-size components produced by friction-stir welding is still a great challenge to now. In this work, the creep ageing behaviors and underlying microstructure evolution of a thick friction-stir welded Al-Cu alloy plate after CAF process under different stress levels are systematically investigated. The creep strain and the strength of the joint are both significantly increased when the stress is close to the average yield strength of the initial weld joint. The grain size reduces while the local strain and dislocation density increase from top to bottom of the NZ; hence, the bottom layer of the weld joint exhibits higher creep strain and steady-stage creep strain rate during the CAF process. The results reveal that the gradient microstructures sensitive to the stress level effectively govern the creep-ageing performance from the upper to the bottom layer in a thick friction stir welded Al-Cu alloy plate. Rationally increasing the initial dislocation density of the weld joint can both enhance the tensile properties and promote the creep deformation of the weld joint for CAF process.