| Hollow shaft has the advantages of light weight and small inertia.Its weight reduction has a significant effect on energy conservation and emission reduction of vehicles,and even on improving the motor response performance of engines and transmissions.At present,the production of hollow shaft is mainly the re-machining of solid shaft,which has low production efficiency,low material utilization and high processing cost.Cross-Wedge Rolling(CWR)process has the advantages of high production efficiency,high material utilization and good product microstructure and properties.At present,there are few studies on the CWR of hollow shaft.In this paper,the key problems of precision forming and microstructure of CWR hollow shaft with mandrel were systematically studied.Firstly,the typical material 20CrMoH steel for hollow spindle was selected as research object.The high-temperature compression behavior of the material was obtained through hot compression experiment.The microstructure evolution law was studied through grain static growth experiment and dynamic recrystallization experiment.A unified viscoplastic constitutive model of 20CrMoH steel was established by using internal variable method.The material constants of the model were solved by genetic algorithm.After evaluation,the correlation coefficient of the constitutive model is 0.9861 and the average absolute relative error is 4.16%.The constitutive model can better predict the hot deformation behavior and microstructure.The unified constitutive model was compiled into DEFORM-3D Finite Element(FE)software through secondary development,and a multi-field coupling FE model of CWR hollow shaft with mandrel was established.The model was verified by CWR hollow shaft experiment.The maximum error of forming size is 9.6%,and the average grain size error is 12.1%.The result shows that the FE model can better predict the geometric dimension and microstructure of hollow shaft in CWR.Based on the FE model,the deformation law and microstructure evolution law of hollow shaft in the CWR with mandrel were studied.The results show that under the joint effect of die and mandrel,the equivalent strain of hollow shaft is mainly distributed in the inner and outer layers of the rolled-part,while the deformation of the middle layer is small,resulting in the distribution of grain size of hollow shaft,which is small inside and outside and large in the middle.When the reduction ratio is 0.3,the relative mandrel diameter is 0.7 and the initial relative wall thickness is 0.52,the inner and outer grain size of the rolled piece is 32?m and the middle layer is 35 ?m.Increasing the reduction ratio and mandrel diameter and reducing the initial wall thickness can improve the overall deformation of the rolled-part and improve the uniformity of microstructure along the radial direction.When rolling the hollow shaft with variable inner diameter,it is found that the mandrel step hinders the axial flow of the metal in the inner hole,resulting in the mandrel being subjected to axial force in the forming process.Therefore,the mandrel axial fixing device should be designed to prevent its axial movement.Secondly,the mechanism of co-rolling of outer diameter and inner hole of hollow shaft was analyzed by theoretical analysis and FE simulation,and the expression of instantaneous diameter reduction of outer diameter and inner hole was established.The results show that the diameter reduction and wall reduction of hollow shaft occur under the joint effect of the outer rolling zone(die and rolled-part)and the inner rolling zone(rolled-part and mandrel),and the inner diameter of the hollow shaft decreases gradually with the diameter reduction of the outer diameter.However,due to the wall reduction,the metal accumulates on one side of the deformation zone and extends on the other side,which increases the inner diameter and outer diameter of the hollow shaft circumferentially at the same time,resulting in expanding rolling of outer and inner diameter.Therefore,the CWR of hollow shaft with mandrel is a process of collaborative diameter reduction.The precision forming of inner and outer diameter requires the mutual cooperation of die and mandrel,so that the finishing effect of die on rolled piece should be greater than that of expanding rolling.The mathematical model of outer step shaping-curve of hollow shaft in CWR with mandrel was established by the volume balance method,and the accuracy of the model was verified by experiments.The results show that the axial deviation range of the right angle step of the hollow shaft formed by the shaping-curve is 0.3 mm,which shows that the model can realize the accurate forming of the outer step of the hollow shaft.The influence of process parameters on the shaping-curve was analyzed by FE simulation.The results show that the length of the shaping-curve of hollow shaft is shorter than that of solid shaft,and the length of the shaping-curve shortens with the increase of mandrel diameter and initial wall thickness.The dimensional accuracy of inner hole of hollow shaft rolled-part was analyzed by FE and experiment,the reason of dimensional fluctuation was revealed,and the effects of process parameters on inner hole roundness,knifing position expansion,and spiral groove of inner hole were studied.The results show that the fluctuation of inner hole size is caused by the uneven metal flow in each rolling stage.When the axial flow of metal is insufficient and the tangential flow is serious,the rolled piece is easy to lose roundness.The hole expansion at the knifing position is caused by the significant shortage of metal radial and axial flow in the knifing area of the rolled-part.The experimental results show that the modified knifing-tip can effectively solve the expansion problem at the knifing position and reduce the axial size fluctuation of the hole in the knifing area from 0.7 mm to 0.2 mm.The forming law of inner hole step was analyzed through simulation and experiment.The results show that the design angle of inner hole step should be less than 60° during forward rolling,axial dislocation and helix will appear during reverse rolling,and the design angle of inner hole step should be less than 30°.Finally,the scaled down hollow spindle of automobile gearbox were rolled by the CWR process with mandrel.Through the above research on the forming theory and law,the die and mandrel parameters were designed,and the hollow spindle with good forming quality and performance were obtained.The relative roundness error is within 5.3%,the axial size fluctuation of inner hole is within 3 mm,and the grain size after rolling is 4 ?m?25 ?m.Each outer step can be accurately formed.In conclusion,through the combination of theoretical analysis and modeling,FE simulation and experiment,this paper studied the key scientific and engineering problems of the CWR hollow shaft with mandrel,such as cooperative rolling mechanism,outer step precision forming,inner hole dimension accuracy control and microstructure evolution law,which enriches the forming theory of the CWR hollow shaft with mandrel.It provides a theoretical reference for die and mandrel parameter selection and process parameter design of the CWR hollow shaft with mandrel. |
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