Research And Application Of Hydroforming Technology For Exhaust Pipe Based On SCR System

Hydroforming is a plastic processing technology in which gas,liquid or solid particles are used as the pressure transmitting medium to transfer pressure load to the material and exceed its yield strength to cause plastic deformation until the wall adhered to the die cavity.The exhaust pipe of SCR system produced by hydroforming is thin wall,smooth,high strength and long service life.The technology can form exhaust pipes with special complex spatial axis and cross-section shape,meet the performance requirements of exhaust flow field,and achieve the purpose of fully mixing urea and exhaust gas in the exhaust pipe.Double-walled hollow exhaust pipe can be realized by hydroforming,which can fully meet the heat preservation requirements of catalyst reduction.Its heat preservation ability,stiffness and fatigue strength are much higher than those of traditional welded exhaust pipe.However,the problems of high defect rate and uncontrolled caused by wrinkle and crack are remained to solve.In this paper,each forming stage of SCR exhaust tube hydroforming technology are analyzed.Based on the theory of metal plastic flow,it is concluded that the wall thickness of the tube decreases during the free bulging stage and the phenomenon of active material feed occurs;The maximum stress position is the tangent point between the convex plane and the corner in the truing forming stage of the convex platform;The relationship between stress at any point of hump and geometric model is established.The middle position of hump at any time is the required maximum axial thrust point,and the relationship between the maximum axial feeding force,bulging pressure and geometric parameters of hump is obtained.The wall thickness distribution and forming limit diagram of 119A0-SCR exhaust pipe with hump and bump characteristics were obtained by hydroforming simulation.The forming scheme C(?=14mm) has high forming quality,low cost and high feasibility,which provides theoretical support for subsequent hydroforming experiments.The process optimization and finite element simulation of 0211-SCR exhaust pipe were carried out.It was determined that the pre-forming process with back-cushion die cavity could obviously solve the problem of tube blank offset and related defects during free bulging stage.The Thickness diagram and Forming Limit Diagram obtained from the simulation of hydroforming with hump and boss were used to determine the hydroforming plan C,which could improve the forming quality and obtain the ideal mold plan,providing theoretical support for the subsequent relevant experiments.The internal high pressure forming defects of SCR exhaust pipe are classified into three categories: wrinkling,cracking and hump forming defects.Design die for optimized hydroforming process of SCR exhaust pipe and carry out the hydroforming experiment used as the produced die.The 119A0-SCR and 0211-SCR exhaust pipes were respectively tested by hydroforming,and the 119A0 exhaust pipes were cut by wire-electrode cutting.The wall thickness measurement results of the cut section are compared with the simulation analysis,and the actual forming results are consistent with the simulation analysis results.It is verified that the wrinkling on the inside of curved arc can be solved obviously by optimizing the hydroforming process of 0211 SCR exhaust pipe.