Research On The Forming And Microstructure Evolution Of Hollow Shafts By Flexible Skew Rolling

The hollow shaft has have the advantages of lightweight,and higher safety with the convenient of flaw detection,which has become the trend of world transportation development.The hollow shaft production methods include solid shafts drilling,extrusion process,and wedge cross rolling process.The production of large shafts mainly depends on drilling after forging,which not only wastes materials but also increases manufacturing difficulty.Extrusion process requires multiple forming,extrusion pressure is high,the molds have a short life span due to the difficult service conditions.The wedge cross-rolling process has a large die size and high cost,which is not suitable for the economic production of small batches of hollow shafts.The flexible skew rolling can realize the rolling of different sizes and shapes of shaft parts by using a pair of rolls,which reduces the cost of special dies and makes the process adjustment easy.In this paper,the key problems such as forming mechanism,Diameter fluctuation and microstructure evolution of hollow shaft by flexible skew rolling with mandrel are studied.Firstly,the deformation behavior of 34CrNiMo6 steel at different strains from 1223K to 1373K,and strain rates of 0.1 s-1-10s-1 was obtained by high temperature compression experiments,and and the microstructure evolution of grain growth under thermal load and grain refinement under deformation load was revealed by grain growth experiments and dynamic recrystallization experiments.Based on experimental stress-strain curves and grain size variation data,a set of internal variable constitutive equations considering grain size variation was established,where the material constants are calibrated by a genetic optimization algorithm.Comparing the predicted curve with the experimental value,the correlation coefficient is 0.9959,and the average relative error is 3.6%.It can be concluded that the established constitutive equation can accurately represent the hot deformation behavior and grain size change of 34CrNiMo6 steel.The numerical simulation model of the flexible skew rolling hollow shaft with mandrel is established to understand the metal flow law,stress and strain evolution,rolling force and temperature field changes during the forming process.The mathematical equation for the change of outer diameter dimension and the mathematical expression for the height of rolling of the mandrel to the inner hole are established in the defomation process.The metal flow of the rolled part after the integrated action of the roll and mandrel is analyzed from the theoretical point of view,and the maximum flattening amount of the rolled part during the deformation process was investigated.The change of the wall thickness after the co-deformation of the shaft diameter and the shaft hole was quantitatively analyzed,and the evolution model of the wall thickness of the skew rolled hollow shaft with mandrel was established.The effects of mandrel diameter,swing angle,billet wall thickness and reduction ratio on the dimensional accuracy of shaft diameter,the dimensional accuracy of shaft hole and spiral mark depth were summarized by experiments.The causes of the deviation of the shaft diameter and shaft hole of the hollow shaft,the location and formation mechanism of the spiral mark defect are analyzed from the perspective of metal flow and stress by the numerical simulation model.The surface quality control method of flexible skew rolling process of hollow shaft with mandrel is proposed,and the deviation of shaft diameter and the depth of spiral marks can be improved significantly by lengthening the sizing section and increasing the unloading angle.It is verified that setting the unloading rounding after the sizing section can significantly reduce the depth of spiral marks,and increasing the length of sizing section can not only effectively improve the deviation of shaft diameter and hole forming,but also effectively reduce the spiral marks defects.When the length of the sizing section reaches 60mm,the shaft diameter deviation is basically eliminated.With the assistance of user-defined subroutine interface of a commercial finite element software Simufact,the internal variable constitutive model of 34CrNiMo6 steel is compiled in Fortran language and imported into the numerical simulation model of flexible skew rolling hollow shaft with mandrel,and the forming process of hollow shaft with multi-field coupling such as thermal-mechanicalmicrostructure is carried out.The grain size of the rolled piece is significantly refined.Comparing the experimental and simulation results,the error of the average grain size is less than 10%,which confirms that the internal variable constitutive equation describing the grain size evolution established in this paper is suitable for the numerical simulation of flexible skew rolling hollow shaft with mandrel.The influence of the swing angle,mandrel diameter,wall thickness and reduction ratio on the grain size distribution of the hollow shaft was predicted.With the increase of the diameter of the mandrel and the ratio,the degree of thinning of the wall thickness increases,the deformation of the workpiece is more adequate and the grain size distribution is more uniform.A response surface model with shaft diameter,shaft hole deviation and helical trace depth as the optimization targets was established,and the interaction of rolling parameters on the forming of hollow shafts was investigated.The optimized process parameters were obtained and rolling experiments were conducted,the relative errors of the outer diameter and inner diameter are both less than 10%,the microstructure at different positions of the rolled piece shows that the grain size is obviously refined,which proves that the flexible skew-rolled hollow shaft with mandrel can be accurately formed.In conlusion,a study on the forming of hollow shaft with mandrel is carried out based on the flexible skew rolling process,and made a preliminary discussion on the forming precision of outter diameter,inner hole,forming defects and grain size evolution.The relevant results can provide guidance for the forming of large hollow shaft parts by flexible skew rolling.