Fretting Wear Mechanism And Fatigue Life Prediction Of DP780 And AA6061 Sheet Self-piercing Riveted Joint

The steel-aluminum hybrid structure is widely used in automobile body-in-white manufacturing because of its excellent mechanical properties of both steel and aluminum sheets.Self-piercing riveting(SPR)has the characteristics of being able to joint dissimilar sheets,high riveting efficiency and reliable quality.It has become an alternative technology for spot welding and is applied to the jointing of steel and aluminum sheets.In this paper,the high-strength steel(1.26 mm DP780)and aluminum alloy(2 mm AA6061)thin-sheet SPR joints under different overlap sequences were taken as the research object.Their static properties,fatigue performance,fretting wear and life prediction was explored.The specific research contents and results are as follows:1.Static performance of DP780 and AA6061 self-piercing riveted jointsThe hardness of the rivet legs of the two joints was measured by a micro hardness tester,and it is found that the work hardening caused by the plastic deformation of the rivet legs is mainly concentrated in the contact area between the rivet legs and the both sheets.The hardened area and maximum hardness value of AA6061/DP780 rivet legs are greater than DP780/AA6061 rivet legs.The tensile-shear test of the two groups of joints found that the DP780/AA6061 joints are larger than the AA6061/DP780 joints in terms of static strength,stiffness and energy absorption value,while the DP780/AA6061 joints have more excellent static mechanical properties.2.Fatigue performance of DP780 and AA6061 self-piercing riveted jointsBased on the three-parameter S-N method,the fatigue data obtained by the test were fitted,and the fatigue limits of DP780/AA6061 and AA6061/DP780 self-piercing riveted joints are approximately 1.37 k N and 1.87 k N,respectively,and the corresponding fatigue limit and static strength ratio are 19.30% 35.28%.The joint fracture dissipation work obtained by integrating the hysteresis loop shows that the AA6061/DP780 joint needs more work from the outside to overcome the internal damping of the joint to make the joint fatigue failure.The fatigue performance of the AA6061/DP780 joint is better than the DP780/AA6061 joint.Combined with the macro-failure mode and micrograph of the fracture,the DP780/AA6061 cracks initiate in the contact area between the locked sheet and the rivet tip,and the AA6061/DP780 cracks in the contact area between the rivet legs and the locked sheet.3.Fretting wear analysis of DP780 and AA6061 self-piercing riveted jointsThe fretting wear of the two sets of joints is mainly distributed between the overlapping area of the pierced and locked sheets,and between the rivet legs and the contact area of the locked sheet.By the means of SEM and EDS analysis methods,it was found that fretting wear occurs near the fatigue crack.Fretting wear causes fatigue cracks to occur between the rivet legs and the locked sheet of the two sets of joints.There is a competitive relationship between the two sets of fatigue cracks.The breakage of locked sheet of the DP780/AA6061 joint is caused by the fretting wear of the locked sheet and the rivet tip,and the breakage of the rivets of the AA6061/DP780 joint is caused by the fretting wear of the rivet leg and the locked sheet.4.DP780/AA6061 fatigue life prediction of self-piercing riveting jointAccording to the linear elastic fracture mechanics method,based on the correlation theory of global stress intensity factor and local stress intensity factor,the Paris formula is used to predict the fatigue life of DP780/AA6061 self-piercing riveted joints using Zhang-Lins and Pook-Linf models.The life prediction value in the mid-load phase is in good agreement with the test value.However,in the low-load phase,the number of cycles in the crack initiation phase is ignored,and the obtained fatigue life prediction value differs greatly from the test value.After the predicted life of the model in the low-load stage is corrected,the fatigue prediction life of the Pook-Linkf model is mostly dispersed within a factor of 3 error factor,and the fatigue prediction life of the Zhang-Links model is mostly dispersed within the error factor of 2.5 factor,The Zhang-Links' life prediction model is superior to the that of Pook-Linkf.The Zhang-Lins' model has a higher accuracy in predicting the fatigue life of 1.26 mm DP780-2 mm AA6061 self-piercing riveted joints.