Microstructure Evolution and Properties of Friction Stir Welding Joint for 6082-T6 Aluminum Alloy

To investigate the evolution of grain morphologies, grain boundary features, and dislocation structure in upper part of nugget zone during friction stir welding of 6082-T6 aluminum alloys, electron backscattered diffraction and transmission electron microscope techniques were employed. The results showed that the metal on retreating side of the weld zone produced thermoplastic deformation during the welding process. The substructure of the lamellar dislocation wall formed at retreating side. Motivated by the welding thermal cycle, geometric dynamic recrystallization occurred in the substructure of the lamellar grains, leading to the lamellar array of grains. Discontinuous dynamic recrystallization occurred in the lamellar grain, leading the grain to be arranged in a "necklace" form. Simultaneously, the coarse grain of base material was refined. Due to the inhomogenous distribution of stress during the welding process, the alternating presence of large grain and small grain regions on advancing side took place. During the process of welding joint tensile, the fracture results from dislocation piling-up at the Mg2Si interface in large grains with a (111)[110] orientation. Thus, the advancing side becomes the weakest part of the mechanical properties in the welding joint, the tensile strength of the joint was 275MPa with a break elongation of 6%.