Effect Of Pre-corrosion On Mechanical Properties Of Sandwich Structure Self-piercing Riveted Joint

In order to improve the quality of China’s economic development and build an innovative country,energy conservation and emission reduction and automobile lightweight have become important targets for the development of China’s automotive industry.The self-pierce riveting(SPR)technology has been gradually applied to the mechanical industry because of its wide application range,easy integration with the robot arm,and excellent mechanical performance of the SPR samples.In order to optimize the performance of SPR joints,and to study the load bearing reliability of lightweight SPR joints under extreme conditions.Firstly,the influence of foam metal inter-layer on the forming characteristics of SPR joint was analyzed.Secondly,the corrosion resistance of the sandwich structure SPR joint with AA5052 aluminum alloy as the substrate in the salt bath periodic infiltration environment was investigated,and the static properties of the pre-corroded joints were analyzed.Then,the fatigue properties of specimens with different pre-corrosion time were tested,and the influence of pre-corrosion on the fatigue life of SPR joint was explored.At the same time,the observation of fatigue fracture was carried out to study the microscopic fatigue failure forms and fretting damage characteristics of joints under the influence of pre-corrosion.The main study contents and conclusions are summarized as follows:1.The SPR equipment was used to carry out the riveting tests of the lightweight sandwich structure SPR joints.The joint was made of foam metal as the interlayer,and the AA5052 aluminum alloy sheet was used as the substrate.The forming characteristics of sandwich structure SPR joint was explored by the microscopic observation and analysis,and the pre-corrosion test of the sandwich structure SPR joint was carried out by using the periodical immersed corrosion test.The results indicated that the properties and thickness of the foam metal sandwich can significantly affect the forming quality of the joint.During the riveting process,thefoam metal interlayer reduced the local pressure of the contact surface between the rivet and the plate.Under the high-speed impact along the axial direction,the foam metal near the riveting point expanded in the horizontal direction,which locally promoted the rivet foot to expand along the circumference with the plastic deformation of the sheet material,and increased rivet flaring of the joint relatively.2.Static tests were carried out to study the static failure loads,failure displacements,shear stiffness and energy absorption properties of various types of SPR joints.Firstly,the influence of foamed nickel inter-layer on the mechanical properties of the joint was analyzed.Secondly,the corrosion characteristics and mechanical properties of the sandwich structure SPR joints under different pre-corrosion time were investigated,and the failure modes of various joints were compared and analyzed,which provided reference for the structural optimization of the SPR joint.At the same time,it also provided a theoretical basis for the design and strength evaluation of joints under extreme environmental conditions.The research showed that the reasonable configuration of foam metal inter-layer can enhance the tensile shear properties of the joint.Under the influence of pre-corrosion,the static tensile strength of sandwich structure SPR joints were closely related to the corrosion mechanism of the inner lock zone of the sample during each corrosion period.3.The fatigue tests of SPR joints with different pre-corrosion time were carried out,the F-N curves of each sample were fitted,and the fatigue life of various samples was compared and analyzed.Meanwhile,the pre-corrosion influence coefficient C of SPR joint was proposed to describe the influence of pre-corrosion damage on fatigue life of the specimen.The results showed that the structure of SPR joint was easier to fail under the common influence of pre-corrosion damage and fatigue loading,and the fatigue life and fatigue limit of the joint decreased with the increase of pre-corrosion time.Under the same pre-corrosion time,the C value decreased with the increase of the loading level.At this time,the corrosive damage and mechanical damage inside the joint have a competitive mechanism.4.Through the microscopic analysis of the fatigue fracture of each joint,the fatigue failure characteristics of the joint under the influence of pre-corrosion were explored,and the fretting damage behavior of the specimen was analyzed.During theperiod,the fatigue crack propagation and the fracture deflection mechanism were studied.The results showed that in the medium and long fatigue life zone,multiple fatigue sources will be generated in the pre-corroded joints and the crack propagation will be accelerated,at this time,the corrosion pit on the lower plate is the dominant factor for fatigue crack initiation and propagation.Pre-corrosion damage changed the orientation of the grains near the secondary phase particles of aluminum alloy,which caused the orientation of the crack propagation subsurface of the lower plate to change with the orientation of the grain.Therefore,the crack propagation paths were inclined and distorted locally,and finally many step surfaces with different heights on the fatigue section can be observed macroscopically.