Abstract: A 2219-T6 aluminum alloy plate was welded by friction stir welding, and the morphology, microstructure and mechanical properties of the joint were investigated correspondingly. Through analyzing the flowing pattern of the plastically deformed metal in the nugget zone, the strength weakening of the joint were analyzed and summarized. The results show that the welded nugget is divided into three different regions according to the flowing pattern of plastic metal, and a turbulent flowing region is produced by unbalanced suction and extrusion of plastic metal at the front side of joint. This region with weak mechanical properties is a channel for the back flow of plastic metal. The weakening strength is due to the plastic deformation induced damage of the metal, which resulted in the formation of defects and the mutational interface. Moreover, grains in the turbulent flowing region presented large thermal instability. After the recrystallization heat treatment, the joint can restore the ductility, which will improve the mechanical properties of the joint.