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The microstructural change related with the mechanical properties of a friction stir welded 6061 Al alloy has been investigated under various welding conditions. Frictional heat and plastic flow during friction stir welding produced fine and equiaxed grains in the stir zone, macroscopically upset and elongated grains in the thermo-mechanically affected zone caused by dynamic recovery and recrystallization. The heat-affected zone, characterized by coarse precipitates, was formed beside the weld zone. Hardness distribution near the weld zone was strongly related to the behavior of precipitates and dislocation density. Especially, hardness of the SZ at a higher tool rotation speed was higher than that of a lower tool rotation speed due to higher density of spherical shaped re-precipitates. The joint strength was approximately 200 MPa which was lower than that of the base metal, 270 MPa, because softening region was formed around the weld zone.
Lee et al. (Thu,) studied this question.