Study On The Fatigue Crack Propagation And Fracture Characteristic Of Friction Stir Welded AA7075-T6

Friction Stir Welding (FSW), as a new solid state welding technology, has possessed many significant advantages in light alloys, such as Al alloys. There are many researches have been carried out on the welding parameters, joints structure and welding defects, but the subjects on the fatigue properties of joints is seldom, especially in domestic. Building a reasonable fatigue assessment norm is a necessary condition for the popularization and application of FSW.In this thesis, through the metallographical tests of the FS-welded AA7075-T6 and discovered that the welding joint was composed of Weld Nugget (WN), Thermal Mechanical Affected Zone (TMAZ) and Heat Affected Zone (HAZ). The Weld Nugget was made up of fine equiaxial recrystallized grains and the grains in the TMAZ have been elongated. And through the hardness tests we found that hardness profile demonstrated as the"W", the hardness value in the WN is the second high and reach to 160HV, the lowest value is in the advancing side of HAZ in the joint and close to 100HV. The hardness varied as the microstrcuture varied in the joints.In this thesis, we studied the fatigue properties of FSW joints through the fatigue crack propagation tests in different places of the FS-welded AA7075-T6. The fatigue tests indicated that the FCP rate in the retreating side of the HAZ was the lowest and the highest was in the region perpendicular to the weld line. In the lowâ–³K regime, the FCP rate in WN was lower than it in the advancing side of the HAZ; but in the highâ–³K regime, the situation was contrary. Compared with the standard of IIW, the FCP rates in all regions were lower than the data recommended in IIW and it can concluded that the fatigue properties of FS-welded AA7075-T6. Then we discussed the factors affected the fatigue crack propagation in the FSW Al alloys, including the base material properties, residual stress, welding parameters, joints shape, welding defects and joints surface. Among these, residual stress and welding defects were the key factors and we can do something to improve the resistance to the fatigue crack on the two factors.We also observed the fatigue fracture of the FS-weld AA7075-T6 using the SEM and found that fatigue fracture in the Weld Nugget was smooth and fatigue fracture in other areas were rather crude. The fracture mode in the WN was always intergranular failure and the fracture modes in other areas were intergranular failure in the beginning and intracrystalline failure in the later. And we can't find fatigue striation in the WN, but we discovered it in HAZ and perpendicular to the welding zone. This is in relation to the stress condition in the specimens and the properties of the base metal.