Research On Precision Forging Process And Microstructure Of Half Axle Flange Of Differential Mechanism

With the emergence of environmental and energy crises,such as the increase of industrial sewage,excessive emissions of automobile exhaust and resource depletion,energy conservation,emission reduction and sustainable development have become the consensus of the people all over the world.As an industry with relatively high energy consumption and pollution,automobile parts manufacturing industry faces the dual challenges of rapid development,energy conservation and emission reduction.According to the development trends of automobile parts manufacturing industries at home and abroad,precision forging forming technology with low consumption,high efficiency and material saving advantages will become the most important one.Generally,the hollow thin-walled parts of automobile require multi-process precision forming,since they have complex structures,many fine structural features and large diameter ratio.If the deformation amount and shape of the blank in each process are not designed properly,it would lead to the formation of defects and poor dimensional accuracy.Moreover,due to the difference of forming temperature and deformation degree,the microstructure distribution of blank becomes uneven during precision forging.Currently,there are few researches on precision forming of automobile hollow thin-walled parts,and some critical technical problems and basic scientific issues have not been solved.Based on above background,this paper took the typical hollow thin-walled part,half axle flange of differential mechanism,as the study object.Based on the numerical simulation and precision forging experiments,the hot deformation behavior of materials,the optimization design of preforming shape and process parameters and the microstructure evolution were studied.The purpose of this study is to provide theoretical guidance for precision forging of hollow thin-walled parts,improve forgings quality,reduce production cost furtherly,and realize the automobile parts precision forging with material saving,low energy consumption and high production efficiency,which has great significance and value in theory and engineering applications.The main work and conclusions of this study are listed as follows:(1)The isothermal compression experiment was carried out under different forming temperature and strain rate to study the heat flow behavior of 40Cr steel,which is a common material of automobile parts.Based on the friction-corrected flow stress and strain data,the strain compensated Arrhenius model,the dynamic recrystallization equation based on Avrami model and dynamic recrystallization grain size equation were constructed.The high strain rate is beneficial to obtaining uniform grain structure,however,with the increase of temperature,the number of recrystallized grains increased gradually,and the growth of grains becomes increasingly obvious.Comparing the experimental and simulated results,it was found that both of the two results agreed well with each other.It was also demonstrated that the constitutive equation and dynamic recrystallization equations could provide accurate basic data for the following numerical simulation of hot forming.(2)Based on the constitutive equation and recrystallization equations of 40Cr steel,the numerical model of the initial process scheme of half axle flange was established.The simulated results showed that the forming defects existed on the forging part,such as un-filling and folding.According to the causes of defects,metal flow law and volume distribution principle of the preforming,the shape and size of the preforming were optimized.Several preforming schemes were designed based on the process of reverse extrusion,forward extrusion and compound extrusion.After simulation of schemes,the optimal preforming shape and size were obtained.After optimization,the number of forming process was reduced from three to two,which shortened the manufacturing route and improved the production efficiency.Moreover,the form'ing defects were avoided effectively.The microstructure characteristics of different zones of the billet,preforging and forging were analyzed.It was found that the banded segregation structure of the billet was effectively improved after preforming and final forging.However,the inhomogeneity of microstructure still existed in different locations of the half axle flange forgings.(3)In order to realize coordination control of the dimension and property of the half axle flange,the multi-objective optimization design of the process parameters was carried out.Four process parameters including friction factor,billet temperature,extrusion speed and die temperature were selected as the design variable factors,and three indexes including dynamic recrystallization volume fraction,die load and die wear were determined as optimized objectives.The 29 parameter combination schemes were designed by Box-Behnken method,and the response surface models of optimized objectives were established according to the simulation results of the 29 schemes.The influence of variable factors on the response surface and its contour lines were analyzed,and it was found that the influence of billet temperature on the optimized objects was the most obvious one,while the interacting relationship of extrusion speed and friction factor was a bit obvious one,and the influence of die temperature on optimized objects was the least obvious one.Combined with the particle swarm optimization algorithm,the integrated functions of the three objectives were optimized in the feasible variable space.The optimal combination of process parameters was the friction factor was 0.34,billet temperature was 1030?,extrusion speed was 35 mm/s and die temperature was 293 ?.After the validation of optimum combination of process parameters,it was found that the recrystallization volume fraction of the forgings was increased by 9.56%,and both of the maximum load and wear of the die were reduced correspondingly.