Effect of the punch-nose radii on the stress state of core deformation zone in shearing-extruding trimming technology*

Ming DENG，Qiu YUAN，Lin LYU

Chongqing Mold Engineering Technology Research Center，Chongqing University of Technology，Chongqing 400054，China

1.Introduction

In order to improve the local precision profile of plate parts，the shearing-extruding trimming technology is adopted to perform combined process on profiles with finishing allowance，by using the"finishing cut+plastic extrusion”combined processing method．Consequently high-precision dimensions and high finish profiles could be obtained．The basic process is shown in Figure 1［1］．Both the formation of new surface and the separation of chip occur in the core deformation zone，whose stress state directly affects whether there will be tears in the newly formed surface and the quality of the surface．Being a critical process parameterofshearing-extruding trimming technology，punch-nose angle will directly affect the stress state of the core deformation zone［2］．On the basis of theoretical analysis and experiment，this paper studied how the punch-nose radii affects the stress state of core deformation zone，with the aid of the finite element method．Reasonable regularity was obtained and it provides a reference for shearing-extruding trimming technology for the practical applications．

Figure 1．Basic process of shearing-extruding trimming technology

2.Analysis of the material stress state in the deformation zone

Figure 2 shows the external forces acting on the materialin shearing-extruding trimming process，which consists of finish trimming force and friction force f1．Due to the punch-nose angle，the finish trimming force could be decomposed into two components force F11and F12，respectively，along the axial and radial direction．That is to say，punch-nose angle provides vertical and radial extrusion on materials．The material also receives the pressure force F2and friction force f2from binder surface together with the support force F3and friction force f3from the die．All of these forces make the material in the hydrostatic stress state．

Figure 2．Deformation model under external forces and the stress state of one point in the core deformation zone

Take a point O from the core deformation zone．Its stress state is shown in Figure 2．It could be seen that the core deformation material is in a compression state，and the hydrostatic stress could be represented as shown in Eq．1:

Where，σx，σy，and σz，are the normal stresses，which are caused by F12，F(xiàn)11and the constraining force on the material by mold，respectively．The magnitude of the compressive stress depends on the magnitude of the F11and F12，which are affected by the punch radius．Therefore，the punch-nose angle is the key technical parameter to improve the compressive stress of core deformation zone，and the extrusion of punch-nose angle will obviously improve the quality of forming surface．

3.Influences of punch-nose angle on the stress state of core deformation zone

3.1.Experiment materials and methods

20 carbon steel plates with the thickness of 3．2 mm were used as the materials for finite element simulation and physical experiment．Punch-nose angle was the only variable parameter，with the condition that the trimming allowance was 0．5 mm，while the die clearance was 0．01 mm and die profile radius was set to 0 mm．The experiment was conducted by crank shaft press of 350 kN．

Numerical simulation and experiment were accomplished in this project．In the numerical simulationwith Deform-3D software， Normalized C＆L Model［3-6］ was chosen as the criteria of ductile fracture and 1．54［7-8］was selected as the threshold value for 20 carbon steel material．Eight different points were selected from the core deformation zone under different punch profile radius．The mean values of stress in core deformation zone were calculated to investigate the effects of the punch radius on the stress state of core deformation zone．

3.2.Results and analysis of the simulation and experiment

By changing the size of the punch radius，several different sets of data were obtained．The numerical simulation results and corresponding experimental results are shown in Table 1．

Table 1．The mean stress distribution and the experiment results under different punch radius

In Table 1，when the punch radius is 0．02 mm，it could be seen from the simulation model of forming surface that the finished surface is uneven．The yellow zone is the area which receives too much tensile stress．This tensile stress is actually beyond the threshold of the material，and it will result in tear phenomenon．The simulation results are consistent with the experimental results．The experimental specimen forming surface was rough with tear phenomenon．In stable finish trimming stage and chip separation stage，the mean stress in the core deformation zone are characterized by tensile stress，which is prone to tear．The fracture form mainly belongs to the shear fracture．

When the punch radius is 0．1 mm，tensile stress of the deformation zone in chip separation stage reaches the maximum value of 336．375 MPa．The surface tear phenomenon is most likely to happen at this time．Corresponding experimental results also show that when the punch radius is 0．1 mm，the tear proportion of forming surface is larger than others，which verifies the above simulation result．This suggests that the greater tensile stress in chip separation stage，the more serious of the forming surface tear phenomenon．

When the punch radius is 0．3 mm，tensile stress in chip separation stage is obviously smaller than that with the punch radius of 0．01 mm．It also can be seen from the experimental results that the proportion of euphotic belt was increased，while some tear phenomena still exist．

When the punch radius is 0．5 mm，the stress of core deformation zone is compressive stress both in the stable finish trimming stage and the chip separation stage．Experimental results show that high-precision dimensions and high finish profiles could be obtained on the surface of the specimen，with very small collapse angle and relatively better surface quality．This suggests that this punch radius is big enough to provide large compressive stress，ensuring the forming surface in the separation stage will not be teared．

According to the data in Table 1，when the punch radius is 0．02 mm，0．1 mm，0．3 mm and 0．5 mm，respectively，the mean stress values of core deformation zone are 290 MPa，-100 MPa，-323．125 MPa and-473．875 MPa，respectively in the stable finish trimming stage，and 159．4 MPa，326．375 MPa，79 MPa and-115．475 MPa，respectively in Chip separation stage．According to the simulation results，the relationships between the punch radius and the mean stress value of core deformation zone under two different states are shown in Figure 3．

Figure 3．Relationship between punch radius and mean stress

In Figure 3，along with the increase of the punch radius in the stable finish trimming stage，the mean stress of the core deformation zone gradually turns from tensile stress into compressive stress，which increases with the increase of the punch radius．In chip separation stage，when punch radius is 0．1 mm，the maximum tensile stress could be reached，which might be resulted from the friction force on the material caused by punch radius．The bigger the punch radius is，the greater the friction on the material surface．Sequently，the greater the tensile stress on the material is．At that time，the compressive stress provided by the punch radius is so small that the tensile stress takes the lead position．Afterwards，with the increase of punch radius，the tensile stress gets decreased gradually．A three-way compressive stress state could be achieved with the punch radius of 0．5 mm．It also can be seen from the experimental result that，the proportion of euphotic zone in the forming surface increases as the punch radius gets greater．The forming surface could achieve high-precision dimensions and high finish profiles under the condition of a 0．5 mm punch radius．

4.Conclusion

1)With the finishing process proceeding，compressive stress could be decreased rapidly in the fracture separation phase，while tensile stress can even appear．This is the primary cause for the occurrence of the fracture．Big enough compression stress should be provided in order to delay the occurrence of the fracture．Punch radius provides a big compressive stress on deformation zone in shearing-extruding trim-ming．It is an important parameter that affects the stress state of the core deformation zone．

2)Within a certain range，the bigger the punch profile radius is，the greater the compressive stress is provided and the better the forming quality is．When punch radius is too small to provide enough compressive stress，the shaping surface basically belongs to the shearing fracture，instead of the plastic deformation．

3)With the selected process parameters as in this paper，when punch radius is 0．5 mm，the forming surface could achieve high-precision dimensions and high finish profiles，with small collapse angle and better surface quality．

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