Effects of Post Weld Heat Treatments (PWHT) on Friction Stir Welded AA2219-T87 Joints

Friction Stir Welding (FSW) offers significantly better performance on aluminum alloy joints compared to the conventional fusion arc welding techniques; however, plastic deformation, visco-plastic flow of metals, and complex non-uniform heating cycles during FSW processes, result in dissolution of alloying elements, intrinsic microstructural changes, and post-weld residual stress development. As a consequence, about 30% reduction in ultimate strength (UTS) and 60% reduction in yield strength (YS) were observed in defect-free, as-welded AA2219-T87 joints. PWHT is a common practice to refine grain-coarsened microstructures which removes or redistributes post-weld residual stresses; and improves mechanical properties of heat-treatable welded aluminum alloys by precipitation hardening. An extensive experimental program was undertaken on PWHT of FS-welded AA2219-T87 to obtain optimum PWHT conditions and improvement of the tensile properties. Artificial age-hardening (AH) helped in the precipitation of supersaturated alloying elements produced around weld nugget area during the welding process. As a result, an average 20% improvement in YS and 5% improvements in UTS was observed in age-hardened (AH-170°C-18h) specimens as compared to AW specimens. To achieve full benefit of PWHT, solution-treatment followed by age-hardening (STAH) was performed on FS-welded AA2219-T87 specimens. Solution-treatment (ST) helps in the grain refinement and formation of supersaturated precipitates in aluminum alloys. Age-hardening of ST specimens help in the precipitation of alloying elements around grain boundaries and strengthen the specimens. Optimum aging period is important to achieve better mechanical properties. For FS-welded AA2219-T87 peak aging time was 5 hours at 170°C. STAH-170°C -5h treated specimens showed about 78% JE based on UTS, 61% JE based on yield strength, and 36% JE based on tensile toughness values of base metal.