Analysis Of Metal Flow In Open Punch Rotary Backward Extrusion

With the increasing development of industry and manufacturing,people are demanding lighter weight for various parts and components,and at the same time they have more stringent performance requirements,especially in the military and aerospace fields in order to achieve both lightweight and high-performance requirements.Many researchers have made a lot of attempts on the forming methods of metal components.Among them,the rotary back-extrusion technology proposed for cup-shaped parts and tube-shaped parts has proven not only to effectively solve the disadvantage of large extrusion force in traditional back-extrusion,but also to effectively improve the the anisotropy of cup-shaped parts in some aspects.However,the research on the material deformation law under the rotating reverse extrusion process is rarely seen in China.Therefore,this paper focuses on the analysis and research of metal flow deformation behavior and defect forming process caused by uneven deformation in the process of rotary backward extrusion.Taking AZ80 magnesium alloy as an example,through orthogonal test,the influence of various factors on its forming quality and deformation degree is analyzed,so as to obtain the optimal forming parameters.Firstly,in the process of expounding the forming principle of rotary backward extrusion with open punch,the stress of the material in the die cavity is analyzed,and the deformation area is divided according to the deformation characteristics of different parts of the blank.Secondly,based on the assumption of axisymmetric flow function,the velocity components in each direction of metal flow in each region of cylindrical coordinate system are calculated,and the theoretical model of circumferential torsion angle of rotary backward extrusion is established,which provides a theoretical basis for the discussion of metal flow and deformation rules of rotary backward extrusion.The finite element simulation method is used to analyze the changing laws of the metal flow velocity in various parts of the billet during the rotating back extrusion process,and explore the influence of different forming parameters(friction coefficient,punch rotation angular velocity and punch axial loading speed)on the end face of the billet and the metal flow inside the billet.The influence of speed and material deformation law is compared and analyzed through physical test and simulation results.Studies have shown that there are periodic fluctuations in the radial flow velocity and circumferential flow velocity of the material on the end face of the blank.The metal flow velocity inside the blank gradually decreases from the top to the bottom,and it can be clearly seen that the direct influence zone of the punch is concentrated on the top part of the blank.The curved surface extension factor and relative torsion angle are introduced to characterize the radial deformation and circumferential torsion of the blank.The results show that the increase in the friction coefficient will lead to a corresponding increase in the extension and torsion of the material,and the stress concentration will become more obvious..When the rotational angular velocity is constant,as the axial loading speed increases,the degree of material extension does not change significantly,but the degree of torsion is greatly reduced.Increasing the rotational angular velocity will reduce the extent of material extension,but the degree of torsion will greatly increase,and the unevenness of deformation will also increase significantly.The research shows that the radial flow velocity and circumferential flow velocity of the slab end face material have periodic fluctuation changes.The metal flow velocity inside the blank gradually decreases from the top to the bottom,and it can be clearly seen that the direct influence zone of the punch is concentrated on the top part of the blank.The surface extension ratio and relative torsion angle are introduced to characterize the degree of radial deformation and circumferential torsion of the blank.The results show that an increase in the friction coefficient will lead to a corresponding increase in the degree of surface extension and circumferential torsion of the material,and stress concentration also It becomes more obvious.When the rotational angular velocity is fixed,as the axial loading speed increases,the degree of extension of the material curved surface does not change significantly,but the degree of circumferential torsion is greatly reduced.Increasing the rotational angular velocity will result in a decrease in the extension of the material surface,but the relative torsion will greatly increase,but at the same time the unevenness of deformation will gradually increase.Finally,by observing the defects on the inner wall of the AZ80 magnesium alloy cup-shaped parts,the defects can be divided into two types,namely folding defects and convex defects,and it is known through the step-by-step study of the defect formation process in the finite element simulation.The main reason for the formation of defects is the mismatch between the axial loading speed and the rotation speed.Through the orthogonal experiment method,the rotary back extrusion process is set to simulate experiments under different process parameters,and the multi-index analysis is carried out according to the experimental results,and the optimal forming parameters are obtained,so that the molded part can obtain a greater degree of deformation and a lower deformation.Eliminate or minimize internal defects at the same time of uniformity to improve product quality.Through comparison,it is known that when the axial loading speed is 0.2mm/s and the punch rotation angular velocity is 0.05rad/s,the forming quality of the cup-shaped parts is better.In order to minimize the stress concentration and eliminate folding defects,the graphite sheet was wrapped around the punch during the experiment to minimize the friction between the punch and the blank.It is verified by physical experiment that the forming quality of the formed part obtained under this parameter is better,the inner wall has no folding defects,and less convex defects.This verifies the above research results and has a certain guiding effect on actual production.