| Friction stir welding technology as a solid-state welding technology has solved the problem of connection of light metals such as aluminum alloy.With the deepening of research on friction stir welding lap welds are attracting more attention.Asymmetrical hook-shaped defects on both sides of the lap joint cannot be avoided,which directly affects the static tensile strength and fatigue strength of the lap joint.As a new welding process,the current research on the fatigue strength of friction stir welding lap joints is not deep enough,and there is still a lack of deep understanding of the fatigue behavior of lap joints.In this paper,the AA2024-T351 aluminium alloy friction stir welding lap joint is taken as the research object.Through the test analysis and simulation analysis,the fatigue problem is discussed.The specific research contents of the thesis are as follows:1.The metallographic observation and hardness test of the weld cross section of the friction stir welding lap joint were carried out,and the changes of the metallographic structure in each section of the weld cross section caused by the welding process and the defects in the weld joint were analyzed and measured.The hardness distribution of the weld area of the joint is sought,and the hardness distribution of the weld area after welding of the friction stir welded joint is sought.The lap joint was subjected to a static tensile test to determine its mechanical properties.The fatigue test of lap joints under different load levels was designed by static tensile results to investigate the relationship between load level and fatigue life.The fatigue fracture locations at different load levels were observed and the effect of load levels on the fracture mode of the lap joint was analyzed.The fatigue fractures under different fatigue loads were scanned by electron microscopy,and the fracture characteristics of different fractures were analyzed.2.A crack propagation model based on extended finite method is established for the friction stir welding lap joint.The static tensile of the lap joint is simulated.The mechanical properties of the lap joint during static stretching are analyzed.Observing the crack tip stress field in crack propagation,performing initial crack tip element stress analysis,crack propagation path analysis.Two crack propagation models of lap joints without pre-cracking were used to observe the crack initiation and propagation of the lap joint after static stretching.3.A two-dimensional finite element model of lap joint is established.The multi-load step method is used to simulate the tensile state of the lap joint under cyclic loading.The stress-strain state of the lap joint stress concentration region is analyzed,and the finite element analysis is performed under cyclic force.Cyclic stress-strain data for stress-concentrated areas.The fatigue life of the lap joint is predicted by the SWT fatigue damage formula,and compared with the test life.The results show that the SWT fatigue damage formula can predict the low cycle fatigue life of the friction stir lap joint,but the high cycle fatigue life.The forecast is not ideal.The deflection angle model of the hook-shaped defects on different forward sides was established,and the life of the lap joint was predicted and analyzed. |
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