Research On Wear Of Through Cold Extrusion Die For Hexagon Stud Based On Variable Friction Model

As an important mechanical fastening and connecting part,hexagon stud has been widely used in component assembly field of automobile,aerospace and so on.The traditional method to manufacture hexagon stud is mainly cutting,which not only destroys metal fiber streamline,but also causes a certain impact on the subsequent connection quality.Cold extrusion,a near net forming method with less cutting and no cutting,possess high material utilization rate and is suitable for mass production,which might be an alternative method to form hexagonal stud.However,deformation process of through-type cold extrusion of hexagonal stud is complex,and the interface contact pressure varies greatly.Meanwhile,the phosphating saponification film on the surface of the part is prone to failure easily.Thus,the key role affecting the cold extrusion of hexagon stud is die wear and life.A certain commercial AISI-1025 hexagon stud was taken as the research object,and through-type cold extrusion forming method was adopted.With the combination of friction test,finite element simulation,secondary program development and cold extrusion experiment,a variable friction model of AISI-1025 material/die interface considering the change of interface contact pressure was established and its application in DEFORM software was realized.The deformation law and mandrel wear mechanism of through-type cold extrusion of hexagonal stud were investigated and the results can be concluded as follows:(1)The experimental material AISI-1025 is compressed by INSTRON-8802 material testing machine:?=622?^0.27.In this paper,extrapolation method is used to eliminate the influence of friction between the end face and the indenter,so as to obtain the true stress-strain curve of AISI-1025.(2)Based on the ring pier diameter test and finite element numerical simulation calibration method,the finite element standard calibration curve of friction coefficient with outer diameter:inner diameter:height=4:2:1 and the variable friction model of aisi-1025 material/die interface considering the change of interface contact pressure are established.(3)With the aid of FORTRAN compiler software and subroutine secondary development of DEFORM software,the application of variable friction model in finite element software is realized,and the effectiveness of the variable friction model is verified by the center pin extrusion experiment.(4)Compared with single blank cold extrusion process,the deformation law of through-type cold extrusion shows discrepancy.In the initial stage of cold extrusion process,the plastic deformation of single blank is mainly concentrated at the entrance of the die,and the blank flows into the die in turn with the extrusion punch descending.While in the initial stage of continuous through cold extrusion process,the plastic deformation is mainly concentrated near the top of the mandrel.When the upper part of the billet fits the mandrel,the lower part of the billet flows into the die in sequence.(5)The mandrel wear in through-type cold extrusion process is more serious than that of single billet cold extrusion.Variable residual height after cold extrusion plays a certain role on the mandrel wear,and the maximum linear wear depth of the mandrel moves down with residual height increasing.