| Large shafts play an important role in large-elongated axial parts manufacturing and die-forging billets preforming,which are widely used on transportation,engineering machinery,marine ships,national defense,and military industry.At present,the production of large shafts is mostly completed by HighSpeed Forging,which leads to low production efficiency,high material consumption,poor forming quality and so on.Besides,about 30%of large shaft parts are formed by Radial Forging.However,the Radial Forging equipment is imported and controlled by other countries,which leads to high equipment cost.Therefore,a novel Flexible Skew Rolling(FSR)process is proposed by combining the motion relation of Skew Rolling and the deformation characteristics of CrossWedge Rolling.Since each of the FSR rollers has three degrees of freedom(circumferential rotating,radial rotating,and radial feeding),the FSR process can be divided into four stages:radial rolling,rollers inclining,skew rolling,and rollers leveling.Therefore,the FSR process can produce various shafts with the same rollers by programming different movements.In this study,in order to develop the FSR process,its key technologies were studied systematically and four kinds of shafts were FSR rolled in laboratory.(1)Based on the spatial transmission theory,a rolling theory for flexible skew rolling including the kinematics relationship,geometric relationship and rolling condition was established.By analyzing the forward and backward slipping between roller and shaft,the FSR conjugate kinematics model was established and the analytical formulas of rotation radius,contact area,axial speed and theoretical sizing length were deduced.The irregular elliptical section of the deformation zone was analyzed geometrically,and the mathematical model describing the shape of the deformation zone was established.In addition,the conditional equations of circumferential rotating and axial feeding were deduced respectively by analyzing the stress state of rolled piece.The theoretical analysis shows that:with the increase of area reduction,the theoretical sizing length increases,the contact surface increases and the discharge speed decreases.With the increase of forming angle,the discharge speed increases,and the theoretical sizing length and the equivalent contact area change slightly;With the increase of skewing angle,the theoretical sizing length,equivalent contact area and discharge speed increase.(2)By conducting uniaxial compression experiment,uniaxial tensile fracture experiment and dynamic recrystallization experiment under different deformation conditions at high temperature of LZ50 steel,a microstructure evolution model coupled with dislocation density,recrystallization percentage and grain size and a hybrid fracture criterion considering stress triaxiality,temperature and strain rate were established.The established microstructure evolution model and hybrid fracture criterion were embedded into Simufact.Forming software through secondary development technology,then a thermal-force-microstructure-damage FE model of FSR process has been established in this study.The reliability of the FE model was verified by a ?60 mm stepped shaft FSR experiment.The maximum error of forming size is 2.2%and the average grain size error is 15.5%,which shows that the FE results are reliable.(3)Through the numerical simulation and FSR experiment,the deformation mechanism,rolling conditions and forming defects of the FSR process were studied.The numerical simulation results show that:the shaft was knifed by radial cross rolling and stretched by skew rolling.The outer metal drives the inner metal to axial flow,and temperature dropping and rolling force is small during the whole process.The effects of process parameters on forming conditions were analyzed through regular experiments,which revealed that the large area reduction,large forming angle and small skewing angle are not conducive to the steady-state rolling which may led to the central cracking because the rolling piece has a serious axial slipping then undergo a periodic repeated rolling.There are some forming defects in FSR shafts,such as central cracks,concavity,surface threads and knurled step.The coMParative experiment shows that the depth of concavity was reduced by 27.56 mm when using 400 cone-angle billet,and the height of surface threads is reduced by 0.26 mm when using 15°unloading circular chamfer.(4)Through the established multi-field FE model,the evolution law of LZ50 steel microstructure during FSR rolling was revealed,and the effects of process parameters on grain size were analyzed.The FE results show that the FSR grain of the rolled piece is distributed in a ring,and the outer grain refined greater than that of the inner.The FSR grain size decreases with the increase of area reduction,sizing length,forming angle and the decrease of skewing angle.The microstructure uniformity after rolling is improved with the increase of area reduction,sizing length,forming angle and skewing angle.In detail,the area reduction has a significant effect on the grain size.When the initial grain size is 74.76 ?m,the rolling temperature is 105?,the sizing length is 25 mm,the forming angle is 20°,the skewing angle is 8°,and the area reduction is 65.97%,the average grain size of the rolled cross section is 25.25?m m and its standard deviation is 0.6,which shows that FSR process can significantly refine the grain and have good microstructure uniformity.(5)The formation mechanism of FSR central cracking was analyzed by experiment and FE method.On the one hand,the center of workpiece undergoes two tensile stresses and one compressive stress and the tensile stress is greater than compressive stress.On the other hand,the material temperature at the center is higher than that of the outer layer,and therefore the center of the workpiece is most likely to form a cracked hole.In view of the stress triaxiality in the deformation zone is a hybrid distribution,a hybrid fracture criterion considering stress triaxiality,temperature and strain rate was established,and the prediction results of established fracture criterion are consistent with the actual crack morphology.The effects of process parameter on central cracking were studied by FE analysis.The results show that the radial stress is the maximum tensile stress under small area reduction and the axial stress is the maximum tensile stress under large area reduction so that the central of the rolled piece undergo a tensile stress state.In a certain range,the central damage increases with the increase of area reduction,first increases and then remains unchanged with the increase of sizing length,first decreases and then increases with the increase of forming angle,and first decreases and then increases with the increase of skewing angle.(6)The application exploration of FSR process was conducted in laboratory by designing a FSR experimental equipment and its multifunctional system to manufacture four typical shafts which have different shapes.The FSR rolling paths for four typical shafts were designed and calculated and then programmed into the FSR mill.The material of the four typical shafts is LZ50 and diameter is 60 mm.By conducting experiments,four FSR shafts with different shapes were rolled successfully.There is no central crack in the rolled piece,and the outer grain is refined into 22.23 ?m from 76.45?m and the inner grain is refined into 38.94?m from 83.47 ?m.After the heat treatment of two times normalizing and one time tempering,the tensile strength of the rolled-surface material is 729.42 MPa and the elongation is 23.11%,which can meet the technical requirements of TB 2945-1999.In order to develop the novel FSR process,its key technologies including rolling theory,forming process,microstructure evolution and central cracking were studied systematically in this paper.Finally,various FSR shafts were obtained and the FSR process was developed,which will be of great engineering significance for enriching the forming theory of large shafts. |
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