Study On The Fatigue Performance Assessment And Defect Analysis Of Friction Stir Welded Joints Of 7075 Al Alloy

Frcition Stir Welding(FSW), as a solid state welding technology, has behaved many significant advantages in welding field of Al and Al alloys. That researching the influnce of the welding technology, welding flaws, joints structure and tempreature field distribution on the fatigue strength and life can provide important reference to build a reseonable fatigue assessment norm.In the scope of this thesis, through the metallographical and microhardness tests in different areas of 7075-T6 Al alloy joints, the results show that the welds achieved finer equiaial recrystallized grain structure than the base material. The microhardness variation was consistent with that of microstructure in weld transverse section. The welds gained a higher hardness, and the minimum hardness occurred in the Heat Affected Zone(HAZ). In the interface between the HAZ and Thermal Mechanical Affected Zone(TMAZ), the hardness value decreased significantly.In this paper, series of mechanical property tests of 7075-T6 Al alloy joints have been proceeded. The tensile test results showed that the fracture position occurred in the softened zone between welds and HAZ or HAZ, and the tensile strength reached the 88 percent of base material. Fatigue crack propagation rate test results showed that the da/dN of HAZ and welds were all higher than that of the base material in the whole fatigue life area. This difference behaved more significantly in the higher stress intersity factor range(ΔK). The fatigue S-N test results showed that the friction stir welded jionts of 7075-T6 Al alloy achieved higher fatigue strength than that of the correspoonding fusion welded joints.A three-dimensional finite element numerical simulation was firstly performed on the friction stir welded joints of 7075-T6 Al alloys. The results of finite element analysis showed that the temperature of weld surface varied between 437℃～490℃, and the temperature field gradient in front of stir head was bigger than that in the back of stir head. The steady-state temperature of weld center varied between 470℃～490℃and the temperature of HAZ fluctuated between 335℃～385℃. The temperature of weld retrived side was about 10℃higher than that of the weld advanced side and the temperature difference between the weld surface and bottom was about 15℃.The defect analysis of friction stir welded joint of 7075-T6 Al alloy demonstrated that the surface maching marks can easily become the resorce of fatigue cracks. The lip and flash had great influence on the fatigue performance of welds. The'onion ring'flaws have some influence on the whole life of joints because they provided conjunctive path for the initial two cracks and on the other hand they also have influence on the fracture toughness property of welded joints. The'zigzag'flaws in the weld center can significantly decreased the fatigue strength of friction stir welded joints and this type of flaw formed by the initial oxide layer in the butt face and the surface of welds contacted with the stir shoulder.