| Aiming at the problem of cracking during the internal high pressure forming of the front pipe of the automobile exhaust system,this paper carried out the mechanical modeling of the high pressure forming in the front pipe of the automobile exhaust system,the simulation and optimization of the internal high pressure forming process parameters,The research of forming test verification and other contents has guiding significance for the actual production of high pressure forming in the front tube.Firstly,the mechanical model of the front pipe of the automobile exhaust system during the internal high pressure forming process was established,and the stress conditions and forming laws of the different stages in the internal high pressure forming process were obtained.The high-pressure forming process in the exhaust pipe is divided into three stages: the initial forming stage,the forming stage,and the shaping stage.The pressure in the initial forming stage is relatively low,and the pipe is mainly subjected to the axial feed pressure at the shaft end,and the wall thickness of each forming zone increases.In the early stage of forming,the hump forming zone expands outwards and feeds the material axially at the same time because the internal pressure It is still small,the axial feed force plays a leading role in deformation,and the hump wall thickness increases.In the later stage of forming,the hump continues to expand outward,and the internal pressure plays a leading role in deformation.The axial feed effect is weakened,and the wall thickness is reduced.The annular boss forming area belongs to free expansion forming during the forming stage,and the wall thickness is reduced;during the shaping stage,the axial feed is stopped,and the hump and the intermediate annular boss are die-molded under an increased internal pressure,which belongs to free expansion forming and has a wall thickness reduce.Secondly,the effects of various GTN damage parameters on the tensile stress-strain curve of 439 stainless steel were verified by finite element simulation,and the GTN damage model was established by the inverse finite element method.Among them,the initial hole volume fraction and critical hole volume fraction mainly affect the material’s fracture strain point.The larger the value,the more forward the fracture point;the nucleation volume fraction not only affects the fracture point,but also affects the rate of decline of the bearing capacity after the material reaches the yield limit The larger the value,the more forward the fracture point,and the faster the decline rate;the volume fraction of the failure hole mainly affects the decline rate of the bearing capacity after the material has a macro-crack.The larger the value,the slower the decline.In the end,it was obtained that the cause of high-pressure forming cracks in the front tube was that the hump forming area did not obtain sufficient axial feed during the expansion and deformation,the axial stress was reduced,the hoop stress was increased during the expansion deformation,and the pipe wall was affected by the ring.It becomes thinner in the tensile direction,and the initial holes in the material begin to grow and aggregate,reaching the volume fraction of the failed holes,macroscopic cracks are generated,and finally cracking is caused.The best loading path is to feed axially under low pressure to form beneficial folds in the hump forming area,and then increase the pressure to flatten the folds to avoid local thinning caused by dead wrinkles or stress concentration caused by excessive compression of the folds,and finally at the hump.When approaching forming,stop the axial feed and raise the pressure to the highest level,so that the tube blank is completely formed.Through the combination of finite element simulation and experimental verification,the process parameters of high-pressure forming in the front pipe of the automobile exhaust system are formed: the feed amount is 14mm;the loading path is divided into three stages,the pressure in the first stage is 0-6MPa,and the pressure in the second stage Pressure 6-30 MPa,three stages 30-80 MPa;annealing process parameters are 650 ℃;air cooling after 2 hours of heat preservation,test results are: hump thinning rate of exhaust pipe before exhaust 6.7%,thinning rate of middle annular boss 13.3%,in line with technology Claim. |
