Study On Very High Cycle Fatigue Of Aluminum Alloy FSW Welding Joint

With the rapid development of the aerospace industry,high strength aluminum alloy which has light weight and good toughness plays a more important role. However, high strength aluminum alloy by melting welding has a lot weld defects such as voids and cracks. Therefore the potential industrial application is very low.Self-developed TJU-HJ-I Ultrasonic fatigue test device is used to test the fatigue performance of 7050 aluminum alloy base material and welded joint by FSW in this paper. The test results show that both base material and welded joint specimens will happen fatigue fracture after 107 cycle, and the S-N curve continuous decline in a straight line and no fatigue limit could be found. Compared to large size and small size of the welded joint fatigue test results, the fatigue performance of large size specimens is slightly lower than small size specimens. And it means that the scale effect influences the fatigue performance.Observation of fatigue fracture shows that, the fatigue fracture morphology of base material is smoother than that of weld joints, three areas of the fatigue fracture morphology could be clear distinguished. All base material specimens crack start on the specimen surface, and welded specimen crack start on the surface or sub-surface defects. And there is no "fish eye". Through weld test of same stress, positive correlation between fatigue life and fatigue crack propagation region could be founded.Analyze structure characteristics and hardness of different weld parts. Statistics fracture position of welded specimens, and the fatigue fracture occurs mostly in HAZ, a few specimens failed in NZ which have large weld defects. The surface defects such as lack of fusion will cause Stress concentration greatly and the fatigue performance of this kind specimens is very poor. The fatigue performance of the specimens with inclusion defects is associated with the size and location of inclusions. The greater size, the location which is more close to the surface, the fatigue performance is poor.