Friction Stir Welding process introduces a degree of deformation in the material that is related to process parameters. They directly regulate the heat input of welding process and then the morphology and the microstructure characteristics. In the present work, an investigation was carried out on 6082T6 butt joints obtained by Friction Stir Welding process. The microstructure and mechanical characterizations were performed before and after a post-welding treatment at 535°C for 2 hours and cold deformation. The thermal stability of fine recrystallized grains in the nugget zone depends on process parameters and post-welding heat treatment and it has unusual consequences on mechanical properties. In fact abnormal grain growth, that occurs in the nugget zone, increases ductility of the joints and homogenizes the hardness profile of the whole joint.