Study On Microstructures And Mechanical Properties Of Friction Stir Welding Joints Of6082-T6Aluminum Alloy

The6082-T6aluminum alloy is characterized by high specific strength, excellent corrosion resistance and extrusion performance, and has been widely used in railway vehicles industries. The porosities, hot cracks and low joint strength are main problems for the fusion welding of aluminum alloys. As a solid state joining process, the friction stir welding (FSW) has been shown to be suitable for joining aluminum alloys. At present, there are relatively few studies concerning with FSW6082-T6aluminum alloy in the world. Therefore, in this paper, the temperature distribution, microstructure evolution and the mechanical properties of6082-T6aluminum alloy FSW joints, and the effects of FSW parameters were investigated. In addition, a FSW heat source model was established based on the experiment results and the simulation of temperature distribution during FSW was conducted by solving the model in ANSYS software. The simulated results were verified by the measured one.The experimental results show that the farther from the weld center line, the higher the peak temperature of welding thermal cycle, and its effects on microstructure and mechanical properties of joints are more obvious. The6082-T6aluminum alloy FSW joint is composed of nugget zone (NZ), thermal-mechanical affected zone (TMAZ), heat affected zone (HAZ) and base metal (BM). The microstructures of the BM are mainly α-Al phase and β"(Mg5Si6) precipitates. The optical microstructure within NZ was fine α-Al equated grains and GP zones were also observed by means of TEM. TMAZ is characterized as elongated grains which contain a high density of dislocations, and the existence of these dislocations increases the TMAZ hardness. The grains of HAZ are seriously coarsened and the original β" precipitates transformed into β’. The micro-hardness tests show that the hardness distribution of FSW joints approximates to the W-type and the minimum hardness is located at TMAZ/HAZ on advancing side. The micro-hardness in the NZ top is higher than that in its bottom. The distribution trend of the joint micro-hardness values is the basic symmetry on both sides, but themicro-hardness values of the retreating side are higher than those of the advancing side.The results of tensile tests showed that the tensile strength of FSW joints are higher than70%of those of BM and the joints mainly break at TMAZ/HAZ on advancing side.Results show that the welding speed and rotation speed of the pin have a certaineffect on the temperature distribution, microstructure and mechanical properties of thejoints. The decreasing of the welding speed and the increasing of the rotation speed canresult in the increasing of the weld heat input and the grain coarsening for NZ, TMAZ andHAZ. With increasing heat input, the hardness value of TMAZ/HAZ has a decreasedtrend. Under the conditions of this experiment, the appropriate increase of welding speedand the decrease of rotation speed can be beneficial to the improvement of the jointmechanical properties.The FSW temperature distribution was simulated by using ANSYS software. Thesimulated welding thermal cycles are close to the measured those. The research resultsprovide necessary theoretical foundation and experimental data for the parameteroptimization of FSW and the improvement of joint properties.