Effect of post-weld heat treatment on the microstructure and plastic deformation behavior of friction stir welded 2024

To improve the plasticity of friction stir welded joints for plastic processing applications, post-weld heat treatment (PWHT) of 2024-O aluminum alloy friction stir welding joints was carried out at annealing temperatures from 250 °C to 450 °C with an interval of 50 °C for 2 h, followed by cooling to 200 °C in the furnace. The effect of PWHT on the microstructure and plastic deformation behavior of the joints was investigated. It was found that the fine-equiaxed grains are stable and retained in the nugget of the joints even after annealing at 450 °C for 2 h. However, the grains in the thermo-mechanically affected zone (TMAZ) of the joints become coarse and equiaxed as annealing temperature increases. The plastic deformation of as-welded joint is very heterogeneous. In contrast, the plastic deformation of PWHT joint is relatively homogeneous by both the nugget and the base material showing large deformation. The decrease in elongation of as-welded joints is completely recovered by PWHT. The high ductility of the joint is mainly attributed to the retention of the fine-equiaxed grains in the nugget during PWHT.

► Post-weld annealing treatment (PWAT) is used to improving the plasticity of FSW joints for manufacturing lightweight structures.
► The maximum elongation of the PWAT joint can reach 160% that of the as-welded joint.
► No abnormal grain growth (AGG) was observed even the annealing temperature up to 450 °C.
► The fine-equiaxed grains are retained in the nugget, and the joint show excellent plasticity.