Taking the spur cylindrical gear for truck wheel reducer as the research object, a new compound forming process scheme of continuous “cold extrusion + cold shaping” of spur cylindrical gear is proposed, and its key technologies are studied and analyzed. The influence of cold extrusion process parameters on the forming quality of spur gears is analyzed, and the multi-objective optimization design of cold extrusion process parameters is carried out by response surface method. The influence law of cold shaping mode and cold shaping quantity on forming accuracy is analyzed, and the reasonable cold shaping mode and quantity are determined. The stress distribution of cold extrusion combined die under preload and working state is analyzed, and the structural parameters of cold extrusion combined die are optimized by the combination of Kriging model and particle swarm optimization algorithm. Finally, the process experiment of continuous “cold extrusion + cold shaping” compound forming process of spur cylindrical gear is carried out, and the precision, metallographic structure and hardness of spur cylindrical gear formed by the compound process are tested. The main conclusions are as follows:
① Aiming at the problems of poor forming quality and low forming accuracy of spur gear produced by cold extrusion in the original process, the cold extrusion finite element model of spur gear under the original forming process scheme is established. The numerical simulation results are quantitatively compared with the actual production results. The maximum error between the two is 6.8%, and the coincidence is high. By analyzing the metal flow and stress-strain distribution in the cold extrusion process of spur gear, it is concluded that the main causes of poor quality and low precision of formed gear are insufficient back pressure during cold extrusion and elastic recovery of cold extrusion gear. Therefore, a new compound forming process of straight tooth cylindrical gear through continuous “cold extrusion + cold shaping” is proposed.
② The influence law of the main process parameters of cold extrusion on the forming quality of spur gear is studied and analyzed. The combination of numerical simulation and experimental design is proposed. Taking the response surface method as the optimization method and the forming quality and forming load as the optimization objective, the process parameters are optimized and the optimal combination of process parameters is obtained.
1) The die angle affects the forming quality of spur gear. The larger the die angle is, the greater the tooth top collapse angle and end face convexity are. This is because the die angle affects the flow velocity of the inner and outer layers of the blank. The larger the die angle is, the greater the difference in the flow velocity of the inner and outer layers of metal is. The faster the inner layer of metal flows, the faster the outer layer of metal flows, and the more obvious the front-end angle collapse of spur gear is. However, the forming load decreases with the increase of die angle.
2) With the increase of splitting angle thickness, the resistance of metal axial flow becomes stronger, the flow speed of outer metal becomes slower, and the axial flow speed difference of inner and outer metal becomes larger. Therefore, the amount of tooth top collapse angle and end face convexity increase with the increase of splitting angle thickness, and the forming load increases with the increase of splitting angle thickness.
3) The length of sizing band is one of the key parameters to ensure the cold extrusion quality and forming accuracy of spur gears. If the sizing belt is too short, the plastic deformation occurs outside the constraint area of the die, resulting in the reduction of the forming quality and accuracy of spur gears. If the sizing belt is too long, it will cause the surface strain of the extrusion and the defects of die sticking.
4) Based on the response surface method, three target response tooth top collapse angle, end face convexity and forming load are established respectively. For the second-order response models of design variables tooth top fillet, die angle, splitting angle thickness and sizing belt length, the fitting degrees of the regression models are 98.36%, 99.01% and 99.06% respectively, It can better describe the response of three objective functions to design variables.
5) Using MATLAB software to further optimize the experimental parameters, the optimal cold extrusion process parameters of large module spur gear are obtained: the fillet of tooth top is 1.6mm, the die angle is 40 ℃, the thickness of splitting angle is 2mm, and the length of sizing belt is 15mm. Error analysis and production practice show that the optimal combination of process parameters can effectively reduce the collapse angle of tooth top, end face convexity and forming load.
③ Based on the actual measured data of cold extrusion spur gear, the error model is established, the influence laws of different cold shaping methods and shaping quantities on metal flow and spur gear accuracy in the cold shaping process of spur gear are analyzed, and the reasonable cold shaping methods and shaping quantities are determined.
1) During the whole tooth shaping, the tooth surface deformation area is large, the tooth surface deformation is uneven, the stress at the tooth top of the tooth concave die is large, and the gear accuracy after shaping is grade 10; When only shaping the tooth surface, the deformation area of the tooth surface is small, the deformation of the tooth surface is uniform, the stress on the tooth top of the tooth profile die is small, and the gear accuracy is improved to grade 7 after shaping; The shaping of tooth surface and tooth root is between the two. After shaping, the accuracy of spur gear is improved to grade 9.
2) When the shaping amount is less than 0.1mm, the shaping effect is very limited, and even only elastic deformation occurs. The shaping amount is in the range of 0.1 ~ 0.2mm. After the cold extrusion straight tooth cylindrical gear passes through the shaping die, only slight plastic deformation occurs on the tooth surfaces on both sides, the tooth surface deformation is uniform, the tooth profile accuracy and tooth direction accuracy are the highest, and theoretically it can reach grade 8 straight tooth cylindrical gear accuracy. When the shaping amount is greater than 0.25mm, the tooth surface has undergone severe plastic deformation, which can not achieve the purpose of shaping. The total deviation of tooth profile and the total deviation of helix decrease with the increase of the length of die sizing belt, and reach stability when the length of sizing belt is greater than 10mm. The deformation heat has an obvious influence on the forming accuracy of spur gears. The deformation heat will increase the tooth surface deviation and reduce the accuracy.
3) The process experiment is carried out by using the cold shaping method with bilateral shaping amount of 0.15mm, shaping die sizing belt length of 10mm and only shaping tooth surface. The test results show that the tooth profile accuracy of spur cylindrical gear is improved from Grade 8 to grade 7, and the tooth direction accuracy is improved from grade 10 to grade 8. The cold shaping process can significantly improve the accuracy of cold extruded spur cylindrical gear.
④ The stress distribution of the combined die for cold extrusion of spur gears under preload and working conditions is analyzed. The structural parameters of the combined die are optimized by the combination of Kriging model and particle swarm optimization algorithm, and applied to production practice.
1) The finite element model for stress analysis of combined die in the stable forming stage of spur gear is established. The numerical simulation results show that the combined die designed by empirical method can meet the requirements of process design under preload and working state. However, due to the conservative selection of structural size and interference of combined die, the die core is in a state of great compressive stress, Although it can ensure normal operation, the die structure is too large, resulting in the waste of expensive die materials.
2) In order to reduce the equivalent stress on the inner wall of the die core, the structural parameters of the cold extrusion combined die for spur gear were optimized. The Kriging model is used to establish the approximate model between the structural parameters of the combined die and the maximum equivalent stress of the die core, and the particle swarm optimization algorithm is used to find the optimal solution in the feasible solution space to obtain the optimal diameter ratio and interference coefficient of the combined die.
3) By comparing the structural parameters and numerical results of the combined die designed by the empirical method and the optimal design, it is found that the size of the combined die designed by the optimal design is less than the design size of the empirical method, and the overall size after optimization is only 68.75% of the design size of the empirical method. In the pre tightening and working states, the maximum equivalent stress of the inner wall of the optimized mold core is less than the empirical method, and there is no circumferential tensile stress in both States, which can avoid the failure of cemented carbide mold core due to tensile stress.
4) The optimized combined die is used for actual production, and the service life of the die is about 150000 pieces. Compared with the combined die designed by the original empirical method, the service life remains unchanged, and the die keeps teeth and does not crack during use. The optimized combined die can effectively reduce the overall size of the combined die and save die materials. It provides a quantitative determination method and basis for the design of combined die for cold extrusion forming of tooth parts.
⑤ The process experiment of continuous “cold extrusion + cold shaping” compound forming process of spur cylindrical gear is carried out, and the precision, metallographic structure and hardness of spur cylindrical gear formed by the compound process are tested.
1) According to the measurement of the compound formed spur cylindrical gear, the minimum diameters of the upper and lower tooth tops are 74.40mm and 73.86mm respectively, which meet the requirements of subsequent turning outer circle. The outer convex of the lower end face is about 2mm and the concave of the upper end face is about 2mm, which is better than the spur cylindrical gear processed under the original process. The accuracy test of spur gear shows that the maximum total deviation of tooth profile is 12.2 μ m. The tooth profile accuracy grade of GB / t10095 is grade 6; The maximum total deviation of helix is 26.6 μ m. The accuracy grade of GB / t10095 tooth direction is grade 8, so the large module spur gear for truck can be processed and produced by compound forming process.
2) The metallographic and hardness tests of the compound formed spur gear show that the surface metal has a large amount of extrusion deformation, the ferrite grains are broken or distorted in varying degrees, the cold work hardening phenomenon is obvious, and the hardness value of the surface metal is large; The deformation of the core metal is small, the ferrite grains are extruded and deformed, but the grain boundary remains, the granular carbides gather at the grain boundary, and the hardness of the core metal is low.