Study On The Fatigue Strength Of FSW Butt Joints With Defects

Friction stir welding(FSW),as a solid-state welding technology,is widely used in non-ferrous metal welding due to its advantages of no hot cracks,small deformation and high fatigue strength.During the welding process forming,two kinds of common defects,kissing bond and tunnel,are prone to occur due to the inherent property of the process.At present,there are relatively few researches on the fatigue performance of FSW joints with such defects.In this paper,the fatigue properties of AA5083 FSW butt joint with kissing bond defect and AA6082-T6 FSW butt joint with tunnels defect were studied,The main work of the paper is as follows:1.The hardness distribution on the weld cross-section of the two specimens was measured by Vickers micro-hardness tester.The influence of FSW technology on the hardness of the welded joints of aluminum alloy plate was analyzed by the hardness distribution profile.Compared the hardness of the two kinds of aluminum alloy joints.Fatigue tests were carried out for two kinds of specimens on MTS809 tester.According to the fatigue life,the S-N curves of the welded joints with different sizes of tunnels defect and kiss depth are established.The influence of defects on the fatigue properties of welded joints was discussed by the nominal stress-life curves of different tensile specimens;The fracture surface was scanned by SEM and the size of the joint defects were measured to observe the crack initiation area and analyze the fracture process.The SEM results were combined with the nominal stress-life curve to analyze the influence of the geometric size of the defects on the fatigue performance of the specimens.2.According to the virtual notch approach in IIW recommendations,two kinds of specimens were modeled and analyzed with the finite element software ABAQUS.According to the stress distribution of the defect area,the influence of the defect location and size on the mechanical properties of the joint was determined.The notch stress method was used to predict the fatigue life of the two joints and compared with the experimental life.The results showed that the stress concentration is easy to occur in the defect areas such as kiss and tunnel,and the location of the defect also has a great influence on the stress distribution.3.Based on the measured hardness,the stress-strain curves in different regions of the joint were estimated.The elastic-plastic stress-strain analysis of AA6082-T6 specimen was carried out,and the influence of plastic deformation on the stress distribution in the defect region of the joint was compared and analyzed;The local stress-strain method was used to predict the fatigue life of the specimen,and the influence of plastic deformation on the fatigue performance of the joint was further discussed;The stress-strain curves estimated by the lowest and highest values of hardness in different regions were applied to the models respectively to study the influence of hardness on the fatigue properties of welded joints with defects;The applicability of IIW virtual notch standard modeling combined with local stress-strain method for life prediction of such welded joints was discussed.For FSW joints with defects,the prediction results of notch stress method combined with virtual notch recommended by IIW are obviously better than that of local stress-strain method.