The Investigation Of Fatigue Behavior Of Friction Stir Welded Aluminum Alloy's Heat Affected Zone

Friction stir Welding (FSW) is a solid-state joining process which based on the friction welding, This new welding technique with uniqueness and superiority is of major in the welding of aluminium alloys since it can improve the joint properties. The main content of research include microstructure and mechanical properties of the joint, temperature field of welding and welding equipments etc. So far, the research of fatigue properties of welded joint has been at the primary stage. The effect of processing parameter, welding defect and microstructure etc .on fatigue resistance and life time for friction stir welding was discussed, these conclusions offer important reference for extension and application of this technology, and also establishing rational and effective criterion for evaluation in future. In this paper, we choosed four type (2024,6082,7050,7075) of aluminum alloys as study objects. Under different processing parameter, we tested the life time of welded joint with different fatigue test parameters and under different loading modes. A lot of sample datas were obtained in order to make certain the relationship between fatigue properties and heat-affected zone.FSW made a sound joint without any voids or distortion and the microstructure of the welded joint was formally divided into three zones: weld nugget, thermo-mechanically affected zone(TMAZ) and heat affected zone(HAZ). The tendency for micro-hardness of welding line zone showed high-low-high-low-high . The low-water mark of hardness has been emerged at HAZ with coarse microstructure and obvious ruckbildung. FSW joint showed a good performance for fatigue properties. Fatigue specimens mainly ruptured at center-root of welding line and heat-affected zone of obvious ruckbildung. FSW joint of aluminum alloys 7050 and 7075 both showed cyclic softening characteristics; fracture is typical fatigue fracture surface. The crack emerged at the defect of surface and internal of joint, and continuously extented under the effect of adscititious cyclic strain, the fracture mode of the welding line is ductile/brittle mixed fracture.The microstructure of HAZ has a obvious effect on high-cycle fatigue properties of FSW joint for aluminum alloys 2024, high-cycle fatigue properties of FSW joint show a better performance under lower tool rotation speed. The effect of the microstructure of HAZ on high-cycle fatigue properties of welded joint for aluminum alloys 6082 is little. The microstructure of HAZ has a great effect on low-cycle fatigue properties for aluminum alloys 7050 and 7075. Fine grains are beneficial to fatigue properties.Based on series of proving tests, the results show that if the welding parameters are optimal, good appearance and fine-uniform grain structure of FSW welded joint are achieved. The fine and uniform grain structure, especially the microstructure of heat-affected zone has a great effect on fatigue properties of joint, this not only can provide a reference for fatigue properties of FSW welded joint, but also has great practical value.