Thermal stability during annealing of friction stir welded aluminum sheet produced by constrained groove pressing

The severely deformed aluminum sheets with strain of 2.32 were successfully joined by friction stir welding (FSW) process. Welding was carried out at various rotation speeds of 600, 800, and 1000 rpm at traveling speed of 200 mm/min corresponding to various revolution pitches of 0.33, 0.25, and 0.20 mm/r, respectively. In order to investigate the thermal stability of the joints, annealing was carried out at temperature of 200 °C for 1 h. Microstructure examinations revealed that severely deformed samples were quite unstable upon FSW process. However, during annealing it was found that in contrast to regions of heat affected zone (HAZ) and thermo mechanical affected zone (TMAZ) where recrystallization and grain growth occurred, grain growth did not mainly occur at the stir zone showing the thermal stability of this area. Also, the mechanical properties of joints in as-welded and annealed conditions were examined. The microhardness measurements and transverse tensile tests were carried out. It was observed that all joints were fractured from TMAZ and stir zone interface.

► The effect of annealing after FSW of severely deformed Al-sheets is discussed.
► Recrystallization at stir zone with low heat input is completed after annealing.
► Recrystallization and grain growth in HAZ and TMAZ are intensified after annealing.
► Degree of strength reduction versus rotation speed is decreased after annealing.