Microstructure and tensile properties of a friction stir welded Al–Mg–Si alloy

Abstract

Abstract The present paper describes correlations between microstructure and tensile properties of a friction stir (FS) welded Al–Mg–Si alloy (6061–T6 Al). Mechanical tests were performed to determine the hardness and tensile stress–strain properties for the base material and for the nugget (or stirred) zone in the FS weld. The microstructure in the nugget zone was characterized using a weak-beam technique, high-resolution electron microscopy, and scanning transmission electron microscopy. Reduction of flow stress in the nugget zone was caused by thermal softening of the base material during the friction stir welding (FSW) process. Fine needle-like precipitates, dislocation segments, and fine lattice defects were observed in the base material, while only isolated long dislocations were found in the nugget zone. These results suggest that dynamic recrystallization occurs during FSW. Deterioration of tensile properties in the nugget zone was caused by the annihilation of lattice defects formed during the thermal refining process. The strain-hardening rate of the nugget zone was higher than that of the base material, and this was caused by a simple dislocation multiplication due to the dynamic recrystallization of the base material.