Overcoming property degradation in 6061-T6 aluminum alloy: Crack repair by friction stir processing and tailored heat treatments

Repairing cracks in high-strength aluminum alloys, such as AA6061-T6, is challenging due to the poor repair quality and mechanical property degradation (i.e., strength reduction) often experienced with conventional repair techniques (i.e., fusion-based). This study investigates the use of the solid-state friction stir processing (FSP) for crack repairing in 6061-T6 aluminum alloy, followed by two post-FSP heat treatments aimed at enhancing repair quality and restoring material strength. Mechanical properties, microstructure evolution, and crystallographic analyses demonstrated that FSP alone improved strength to 199.2 MPa (62.8 % recovery) and ductility to 24.8 %, though properties remained below the as-received condition due to dissolution of strengthening precipitates. Post-FSP artificial aging (FSP + Aged) increased strength to 221.1 MPa but reduced ductility to 14.3 %. The most effective route, FSP + Solutionized + Aged, achieved 293.7 MPa (92.7 % recovery) with a uniform precipitate distribution and near-complete hardness recovery, though ductility dropped to 9 %. Microstructural observations revealed fine equiaxed grains in the stir zone, while fracture analysis showed a transition from ductile to mixed brittle-ductile behavior, confirming a strength-ductility trade-off. These findings demonstrate that combining FSP with solutionizing and aging can restore near-original strength, but challenges remain in balancing ductility. The study highlights the importance of tailoring post-FSP heat treatments for effective structural repair of precipitation-hardened alloys.