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In this paper, two key problems in the cold forging process of gears are ensured: the tooth shape is filled and the forming pressure is reduced. The design idea of ​​the extrusion cavity conforming to the metal flow trajectory is proposed, and the spur gear squeezing composite based on the shunting principle is proposed. The idea of ​​process, the concept of flow curve in fluid mechanics and the theory of equal cross-sectional area change are introduced into the cavity design, and the streamlined extrusion cavity is established. The numerical simulation results show that the forming cavity reduces the fluidity of the metal. Gear forming load, etc., has a positive effect.
1 Extrusion cavity modeling The design of the spur cylindrical gear extrusion cavity is focused on the streamlined variation of the root and the change of the area ratio of the tooth profile. The theory is based on the theory of flow curve surface and the theory of equal area ratio change. The process of modeling is a process from a complex curve system to a surface system to a solid model. The whole idea of ​​modeling is to fit the spline curve from the original value point, then fit the surface from the spline curve, then integrate the partial surface, then get the complete tooth surface through the mirror and array, and finally pass the Boolean operation. A squeeze cavity model is obtained. A gear extrusion cavity model based on flow curve theory and equal area variation theory.
2 Numerical simulation and results analysis of the extrusion cavity After the geometric model of the extrusion cavity was established, the extrusion process was numerically simulated using DEFORM software. The spur gear cold extrusion is completed on the 630t cold forging press. In order to simulate the spur gear cold forging process, the displacement load is used to simulate the static pressure of the press, ignoring the temperature change of the forging during the forming process.
Taking 1/2 of the solid model, the four-node solid element is used for meshing. The convex end surface and the concave mold cavity surface are the target surfaces, the blank surface is the contact surface, and the contact unit is used to simulate the friction between the blank and the convex and concave mold, the friction coefficient is 0.12, and the workpiece material is 20 steel.
The basic parameters of the die extrusion cavity are set as follows: number of teeth z=22; modulus m=2.5; tooth thickness L=10mm head clearance coefficient c=0.25; index circle pressure angle 20α=°; billet magnification radius 06.1=λ The basic parameters of the material are set: plasticity: flow stress Tσσεε=elasticity: Young's modulus 206754; Poisson's ratio 0.3; thermal expansion coefficient 1.2e-005; thermal radiation coefficient 0.7 strain reflects a region of the workpiece under the action of external force Total cumulative deformation. DEFORM uses different colors to represent different degrees of deformation, which is the strain of the gear at different times (40th and 60th incremental steps). It can be clearly seen from the figure that all the cavities in the deformation zone of the extrusion die can be completely filled with metal during the entire extrusion process, and the entire tooth shape is full and smooth; the top of the tooth is full and there is no collapse defect; The rounded corners are rounded and have no folding defects.
During the gear forming process, the equivalent plastic strain distribution of the workpiece changes greatly. It can be seen from the figure that the largest cumulative deformation is distributed in the vicinity of the tooth and the root; in the center of the gear, the cumulative deformation is relatively small. The deformation of the area is most significant at the top of the tooth, and the deformation of the root is small. This is in line with the original intention of the shape of the gear extrusion cavity. The streamlined transition of the root makes the metal strain relatively small.
It is the case where the gears are subjected to deformation forces at different times (40th and 60th incremental steps). Similar to the simulation results of the strain field, the deformation region is also subject to large deformation forces. The area where the gear blank is most stressed is distributed in the vicinity of the tooth profile, and the force at the tooth position is most significant, and the root force is relatively small, which is in line with the original intention of this design. From the perspective of the deformation force, the deformation force at the significant deformation of the tooth profile is basically below 600 MPa, which is less than the strength limit of the current mold material of 25003000 MPa.
3 Optimization of main parameters of extrusion cavity 3.1 Optimization of parameter selection Simulation results show that the gear gap filling performance is good. Therefore, based on the guarantee of the modeling scheme, this paper optimizes the billet magnification radius λ for an important parameter. By selecting different λ values, the extrusion cavities with different shapes and different extrusion modes are obtained, and they are numerically simulated under the same parameter setting, and then the results are comprehensively compared to obtain the optimal modeling parameter values. .
When the billet magnification factor takes different parameters, the shape of the extrusion cavity varies greatly. When the billet magnification factor λ is 1, the cross-sectional area of ​​the gear is the same as the cross-sectional area of ​​the billet. The cold extrusion method is composite extrusion. When the billet magnification factor λ is 1.11, the billet radius is close to the tooth tip of the gear. The radius of the circle, when the billet magnification factor λ exceeds this value, the extrusion method becomes a complete positive extrusion, so the lambda value of 1.12 is the boundary point of the extrusion method from composite extrusion to positive extrusion. When the parameter values ​​are selected, in addition to 1.06, 1.00, 1.10, and 1.20 are respectively taken for λ, and a squeeze cavity model having a large change in shape as shown is obtained. When λ is taken as 1.0, the blank diameter is smaller than the diameter of the gear index circle, which means that the depth of metal flowing from the root to the tooth tip during cold forging is deeper than other cavities. When λ is taken as 1.1, the radius of the blank is slightly smaller than the radius of the tip circle, which is close to positive extrusion. When taking 1.2, the radius of the blank is larger than the radius of the tip circle, and the extrusion method is complete positive extrusion. During the cold forging process, the metal is from the circumferential part of the blank. The central portion of the extrusion chamber flows to the middle to complete the filling of the teeth. Different extrusion methods, material filling ability and mold deformation resistance are also different.
3.2 Analysis of simulation results of different parameter values ​​(1) Simulation results of λ=1.2 and λ=1.0 When λ is 1.2, the extrusion method is complete positive extrusion. The numerical simulation results show that the billet metal flows close to the wall of the extrusion chamber during the whole extrusion process, and the top filling is completed smoothly, but the deformation force of the blank is large. It is the z-direction equivalent load curve of the punch and the die at λ=1.2. In the 60th incremental step (the far right end of the curve), the convex z-direction load is close to 1300kN, and the die z-direction load is close to 900kN, which is basically twice the value of several other simulation results, and the load curve still has Upward trend. It is the stress distribution of the blank when the 40th step is pressed. All the parts in contact with the extrusion cavity are subjected to large forces. Even if the formed crest is smaller in diameter, it is always under stress. The original intention of the precision forging scheme in this paper.
When λ is 1.0, the extrusion method is composite extrusion, the radius of the blank is smaller than the radius of the index circle, and the depth of the radial extrusion is deep. The numerical simulation results show that the deformation force of the blank is not large during the whole extrusion process, but under the action of the reverse force of 450kN, the filling of the tooth shape is not completed, and the tooth tip has obvious defects. The z-direction equivalent load curve of the convex and concave molds when λ=1, the z-direction equivalent load of the convex and concave molds does not exceed 650kN and 180kN, the curve is smooth and the transition is uniform, which is an ideal load curve result.
The above analysis shows that under the positive extrusion condition, the impact and wear of the convex and concave molds are much larger than that of the composite extrusion, indicating that the λ value is not too large, and the blank diameter should not be larger than the top circle. radius. The reason why the top of the tooth is not filled is because the radius of the blank is too small, the depth of the radial extrusion is deep, and the required radial deformation force is large. To complete the filling of the tooth shape, a larger reverse force must be set. To generate large radial deformation forces.
The extrusion cavity with the same blank area and tooth profile area does not achieve a good extrusion effect.
(2) Simulation results of λ=1.06 and λ=1.1 The numerical simulation results when the extrusion cavities λ=1.06 and λ=1.1 are ideal results. The equivalent loads of the convex and concave molds are relatively low. During the whole extrusion process, the blank metal flows close to the wall of the extrusion cavity, the tooth shape is in good condition, the deformation force of the blank is always below 600MPa, and the top of the tooth is pressed against the root of the tooth. Smaller, the shape of the toothed surface of the blank forming part is good.
The pressing cavity of λ=1.1 is much less stressed by the convex and concave die during the extrusion process. From the strain situation, the strain distribution of the two extrusion cavities is similar, the strain after the tooth top is formed is small, and the area where the billet strain is large is concentrated at the root of the tooth. From the stress distribution point of view, similar to the strain distribution, the root region is subjected to a large force, and the extrusion cavity blank of λ=1.06 is less stressed.
4 Extrusion cavity optimization conclusions From the numerical simulation results of the four extrusion cavities, under the same simulation parameters, except for the extrusion cavity with λ=1.0, the cavity of other parameters can smoothly complete the filling of the tooth shape. However, the deformation force used for the blank is not the same, and the impact and wear of the mold also vary greatly.
In theory, when the blank area and the gear cross-sectional area are exactly the same, the numerical simulation results should have good filling ability, minimum deformation force and minimum mold force, but the simulation results show that the extrusion cavity with λ=1.0 failed to complete the tooth. The shape of the filling, and the z-direction equivalent load of the mold is larger than the extrusion cavity of λ=1.06, indicating that the diameter of the blank is too small, the radial extrusion depth of the root to the tooth tip is too long, which is not conducive to The filling of the tooth shape also increases the impact and wear of the mold.
The extrusion cavity simulation results of the other three parameter values ​​finally complete the filling of the tooth shape smoothly, and the deformation force of the blank and the impact load and wear of the die change with the increase of the λ value. In the composite extrusion mode, that is, when λ<1.11, the equivalent load curve of the mold transitions smoothly and is basically kept within a constant value range; while in the positive extrusion mode, the equivalent load curve value of the mold is higher, and The upward trend, the larger the λ, the more obvious the upward trend. For example, when the extrusion cavity with λ is 1.2, the z-direction equivalent load of the die is higher than that of the extrusion cavity with λ of 1.06. The z-direction is equivalent to the load and is still increasing by step 60.
Based on the above analysis, in order to make the simulation results have good tooth filling performance, low blank deformation force and low mold bearing load, the blank magnification factor λ should not be too large and should not be too small. The convex and concave modes of the four cavity simulation results are taken to the highest value of the equivalent load, and they are connected by a spline curve to obtain two curves, a convex load curve and a die load curve as shown in FIG. Basically similar, the abscissa of the lowest point of the curve falls on 1.06, that is, the extrusion cavity with λ of 1.06. The numerical simulation results have the lowest die with impact load and wear, and have good tooth filling performance. The numerical simulation results of the λ of 1.06 extrusion cavity also confirmed this result.
λ=1.06 is the optimal solution for the modeling parameters of the cylindrical spur gear cold extrusion cavity.