Study On Continuous Extrusion Circumferential Expansion Forming Process Of T2 Copper

With the development of high-voltage power,electronics,information and high-speed railway industry,the demand of copper and copper alloy rod,wire rod especially high-precision copper bar is increasing rapidly.The key products of high-precision copper rods include the moving guide rods and static guide rods in high-voltage and ultra-high-voltage vacuum switches,the contactors and conductors of high-voltage electrical appliances and ultra-high-voltage electrical controllers,and high-speed electrified railway devices,etc.For the continuous extrusion circumferential expansion forming technology,the friction between the billet and the extrusion wheel is utilized effectively,the large section products are extruded by using the small section billet,the near-net extrusion forming of the rod can be realized without heating.However,there are more or less internal inclusions and layered defects in the continuous extruded copper rod with large area expansion ratio within a certain depth from the surface.Therefore,it is positive significance to promote the technical progress of continuous extrusion circumferential expansion that reveal the deformation mechanism of continuous extrusion circumferential expansion and determines the relationship between deformation process conditions and layered defects.Based on the TLJ400 continuous extruder,the methods of theoretical calculation,numerical simulation,physical simulation and experimental research are used to study the effect of the area expansion ratio,extrusion wheel speed,flow channel length,die structure and other process parameters on continuous extrusion forming and microstructure of T2copper in this work.The mechanism of layered defects was revealed.The primary studied result is as follows:The mechanical model in the chamber of the continuous extrusion circumferential expansion forming process of copper rod was established by the principal stress method.The relationship between the process parameters and the unit extrusion force at the entrance of the chamber was deduced,and the unit extrusion force calculating formula at the entrance of the cavity was obtained.(?)Aiming at the ultimate load of the abutment,the influence of the process parameters on the unit extrusion force at the entrance of the cavity was analyzed by MATLAB software.The result shows that:the unit extrusion force decreases with the increase of the expanded angle;when the expansion angle isπ/2,the unit extrusion force is lowest.The larger the expanded angle,the smaller the influence of the expansion die length on the unit extrusion force.According to the mechanical model,the limit range of each parameter under the limit load of the abutment can be obtained and proved by experiment.The continuous extrusion circumferential expansion forming process of copper rod is simulated by using DEFORM-3D.The load stroke of the abutment is divided into three stages:feeding stage,filling stage and stable stage.The load in stable stage is about 1.53×10~5N.In the continuous extrusion circumferential expansion forming process,the metal flow velocity reaches the maximum in the right-angle deformation zone,and there is obvious turbulent flow at the abutment.The right-angle deformation zone is the most severely deformed area.The temperature reaches the highest,and the temperature on both sides of the cross section is higher than the middle.The speed of the extrusion wheel has a great influence on the speed in the right-angle deformation zone.When the speed is 2 rpm and 10 rpm,the speed differences on the same section are about 10 mm/s and 100 mm/s respectively.When the area expansion ratio is 1.0,the velocity variation from entrance to exits of the chamber is the largest.When the area expansion ratio is 6.25 and 9.0,the velocity difference between the center and the edge of the maximum expansion section is small,only 2 mm/s.Increasing the length of the flow channel can reduce the velocity gradient of the cavity entrance to the die exit,and improve the uniformity of the metal flow.The optimized C type abutment has the best flow blocking effect,which can effectively improve the turbulent state,and the metal flow velocity difference and temperature difference in the right-angle deformation zone are the smallest.When the expansion angle is 50°and the extrusion angle is 40°,the dead zone between the expansion die and the extrusion die is the smallest,and the metal flow velocity in the cavity is the most uniform.Physical simulation results show that the billet tends to flow upward after passingthe right-angle deformation zone,resulting in uneven flow up and down on the cross section,and eccentricity of the billet at the initial expansion can be seen on the cross section.It can be seen from the longitudinal section that the dead zone in the chamber does not always exist,and the original billet can be gradually replaced with the continuous feeding of new billet.The smaller the area expansion ratio is,the faster the billet flows at the exit of the die and the less layered lines are generated.In the upsetting zone,high-density dislocation is generated,and dislocation lines form dislocation entanglement through movement and interaction.With the increase of deformation,dislocation proliferates and accumulates,and the cell wall composed of dislocation lines flattens out and forms subgrain during the heating process.The elongated grains will be broken into fine recrystallization cores.The large strain rate difference results in the large difference of grain size at different locations in the upsetting zone,adhesion zone and right-angle deformation zone.The metal flow velocity difference at the entrance of the chamber is large,which leads to the violent shear action at the lower edge of the metal,thus forming the layered defect.Moreover,the width of the maximum extension layer in the cavity is the largest,and the layered defect is brought into the product along with the metal flow.The layered defect of the lower edge of the product increases with the increase of the speed of the extrusion wheel.When the rotational speed exceeds 8 rpm,cracking occurs at the lower edge.The layer thickness widens with the increase of area expansion ratio.The die angle directly affects the dead zone in the chamber and has a significant influence on the expansion deformation of copper bar.The larger the Angle of the extended die and the extrusion die,the more layers the product has.Optimizing the structure of the die can improve the layered defects.The arched abutment structure can effectively reduce the layered defects in the chamber entrance,and the use of the arc-shaped die to eliminate the dead zone can effectively reduce the layered defects.Combining the two structures can successfully extrude defect-free copper rod products withΦ50 mm diameter.