Investigation On Fretting Damage Of Self-Piercing Riveted Joints In Aluminium Alloy 5052

Self-piercing riveting(SPR)is a new mechanical fasten joining approach,which is suitable for point-joining of sheet materials.Comparing with conventional riveting technique,SPR method doesn't need to pre-hole and can achieve onceformed stamping and riveting during a SPR process,and it is a green manufacturing technology.Therefore,SPR has a potentional to become a crucial technology for joining the sheets in lightweight alloys and it will be wi dely applied in the industry fields of automobile,aeronautics and astronautics,etc.In this paper,SPR joints in aluminium alloy 5052 was the main research object.According to experimental testing results,the relation between fatigue failure mechanism of the joints and fretting damage was analyzed.Fretting damage mechanism of the joints was investigated by microscopic observation method and fretting damage theory.The specific work and results as follows:(1)Fatigue tests of SPR joints in lightweight alloys were performed.Using scanning electronic microscope(SEM)to study the fatigue failure mechanism of the joints for understanding the relation between fatigue failure of the joints and fretting damage.Basing on fretting damage degree of the joints to investigate the changing reason of fatigue failure modes of the joints was caused by the changing of sheet thickness,materials and fatigue loads.The results reveal that fretting fatigue failure positions and fretting damage areas of the joints are coincident: the joints fail in the pierced sheet through rivet head,which is caused by the fretting damage in the area between the pierced sheet and rivet head(area A);the joints fracture in the pierced sheet near rivet head,which is caused by the fretting damage in the riveting area between two sheets(area B);the joints crack in the locked sheet through the button and rivet,which is caused by the fretting damage in the area between locked sheet and rivet shank(area C).Increasing sheet thickness can relieve the damage degree in area B,but the damage degree in area C is aggravated,thus the fatigue failure mode of the joints transforms from the pierced sheet to the locked sheet.The damage degree in area B is relieved during the sheet materials change from AA5052 to TA1,which results in the fatigue failure mode transforms from the pierced sheet fracture to rivet crack.Decreasing fatigue load can aggravate fretting damage degree in area A and this lead to the fatigue failure mode of TA1 joints with 1 mm-1 mm thickness transforms from the locked sheet to pierced sheet;it is also found that fretting initiates at the area of sheet width and propagates to the area of sheet length.(2)Fretting areas of the joints were observed by a SEM.According to fretting damage mechanism,crack initiation,propagation and surface wear of fretting areas in the joints were investigated.Also,the competition between wear and crack damage mechanism was elaborated.The results show that the length,quantity,primary and secondary,propagation direction,the angle between propagation and fretting direction of the fretting cracks,and wear features of granular,flake-like and mixed debris,pit,crack and delamination on the fretting surfaces,which are all impacted by the factors of cyclic life,fatigue load,fretting position and sheet materials,etc.Fretting areas of the joints are the stress concentration positions and this can easily cause micro-crack nucleation;extensive cold solder joints on the fretting surfaces are subjected to minor amplitude and reciprocating wear,which will result in micro-crack initiation;during a fretting process,a few micro-cracks perpendicular to fretting surface will propagate along depth direction and become macro-cracks,but most of the cracks will disappear.The initiation and propagation of Micro-crack,the formation and early extension of fatigue crack are affected by fretting force;latter extension of fatigue crack is influenced by fatigue stress.The competition between wear and crack propagation on the fretting surface of the joints will decide initiation point and propagation direction of fretting cracks.(3)Fretting third body of the joints was observed by an X-ray energy dispersive spectrum with point-scanning,line-scanning and surface-scanning for investigating the element composition,content and distribution of the third body in the joints.The influence of the third body and oxidation on fretting damage degree of the joints was also elaborated basing on fretting theory.It can be found from EDS point-scannig that element compositions of the third body in the contact surface between two sheets is simpler than that of the third body in the contact surface between rivets and sheets;it can be seen from EDS line-scanning that the main fatigue failure position of the joints occurs in the serevest fretting domain;it can be obtained from EDS surface-scanning that the severest fretting damage phenomenon exists in the surface of rivet leg,oxidation exists all surface of rivet shank,and sheet materials are stripped to rivet surface during the fretting process.The function of the third body and oxidation to the fretting damage of the joints is dual and complex,which can both accelerate and release the damage degree of fretting contact surfac es in the joints.(4)Static and fatigue tests of the joints were performed to research the influence of sheet and rivet parameters on the mechanical performance of the joints,combining load-displacement curves of the joints to study the static failure modes and damage degrees of the joints,the influence of sheet and rivet parameters on the fatigue failure modes of the joints were also investigated.The results display that the static strength of the joints can be enhanced efficiently by increasing sheet thickness,strength and rivet quantity,but augmenting sheet width just slightly improves the static strength of the joints.The fatigue property of the joints can be improved by increasing increasing sheet thickness,width,strength and rivet quantity,but the increasing extent is related to the fatigue load.Increasing sheet thickness,width and rivet quantity does not change the static failure mode of the joints,but alters the damage degree of the joints,and the static failure mode of the joints is the pierced sheet and rivet seperates from the locked sheet.According to the extension trend of the failure phase in the load-displacement of the joints can infer the static failure mode of the joints,and its extension level can speculate the static damage degree of the joints.The fatigue failure mode of the joints is impacted by increasing sheet width and rivet quantity but increasing sheet thickness results in the fatigue failure modes of the joints transform from the pierced sheet fracture to the locked sheet fracture,increasing sheet strength causes the fatigue failure modes of the joints change from sheet fracture to rivet crack.