Abnormal Grain Growth and Strain Behavior of Friction Stir Welding Joint for 6061 Aluminum Alloy

The abnormal grain growth (AGG) and strain behavior of friction stir welding (FSW) joints for a 6061 aluminum alloy subjected to various solution treatments was investigated using electron backscattering diffraction. The deformation storage energy derived from mechanical stirring was retained during solution treatment through strain inheritance. As a result of strain inheritance and grain size, exceedingly coarse grains possessing low strain and grain-orientation spread (GOS) values were formed by the abnormal growth of fine equiaxed grains possessing low strain and GOS values in the nugget zone (NZ). Owing to severe grain size inhomogeneity, AGG in the NZ of the FSW joints significantly decreased the mechanical properties and led to cleavage fracture of the tensile fracture surfaces.