Study On The Microstructure Evolution During Radial-Axial Ring Rolling Of IN718:Modeling And Simulation

IN718 alloy,serving as key mechanical components in aircraft engine,rocket and nuclear reactor,have been often employed in extreme conditions of high stress and cyclic loading at elevated temperatures,so a high performance of IN718 alloy ring is needed.The radial-axial ring rolling technology is the most common forming process of IN718 ring.Dynamic recrystallization during ring rolling process,metadynamic recrystallization in deformation retention period and the growth of grain are the main microstructure evolution during ring rolling.However,radial-axial ring rolling is an extremely complex dynamic rolling process with non-uniform local deformation and non-uniform temperature distribution,making the microstructure control difficult.As a result,structure defects such as coarse grains and mixed crystal may occur.This thesis focuses on engineering science problems and technical difficulties corresponding to the microstructure evolution during the radial-axial large ring rolling process.Firstly,the hot compression test,deformation annealing experiments and static thermal insulation test were conducted to investigate the high temperature deformation behavior,dynamic and meta-dynamic recrystallization behaviors,and static grain growth of IN718 alloy,respectively.Based on the hot deformation behavior and microstructure evolution mechanism revealed by experiments,a set of unified constitutive model based on physical variable of IN718 alloy was established to depict the flow stress and microstructure evaluation during the high temperature deformation and annealing process after deformation.The interaction among dislocation density,dynamic recrystallization and grain size as well as the effect of microstructure on flow stress can be described by the established model.In addition,considering the pinning effect of 8-phase particles,the static model of grain growth in dissolve and precipitation zone of ?-phase was built.The proposed model has good prediction in flow stress and recrystallization fraction during the high temperature deformation and annealing process.The correlation coefficient and the mean absolute relative error of flow stress and recrystallization fraction were 0.994,5.82%and 0.988,7.54%,respectively.Besides,this model still has good prediction in transient deformation condition.Based on the stable rolling condition,the coordinated radial-axial feeding equation was proposed.Moreover,the reasonable ranges of the rotational speed of the drive roll,cone roll together with the withdrawing speed and the diameter increasing speed of the workpiece were determined.Based on the radial returned algorithm,the multi-field coupling FE model of the axial-radial ring rolling process of IN718 alloy with adaptive controlled rollers realized by the user defined subroutines,VUMAT and VUAMP,in Abaqus software was established.The experiment of ring rolling of IN718 was carried out and according to the comparison of the experimental and simulation results,the established FE model can predict the diameter of the workpiece,rolling force,the recrystallization fraction and the grain size effectively during the whole ring rolling process.The effects of process parameters such as the diameters of mandrel and the drive roll,heating temperature,the rolling ratio and the diameter increasing speed on microstructure evolution during the ring rolling process were investigated.It is beneficial for the middle area to recrystallize and even the microstructure by increasing the mandrel diameter.Increasing the drive roll can make the deformation at the outer layer larger,while at the middle smaller,which can increase the average grain size and make the distribution more non-uniform.When the heating temperature is 1020 or 1040 0C,the grain grows rapidly and the initial grain size increases,which may increase the average grain size and make the distribution heterogeneous.Therefore,1000 ? is considered as the best deformation temperature.When the rolling ratio is small(1.333,1.429),the recrystallization occurred only at the outer layer,which make the grain distributed non-uniform.When the rolling ratio is large(1.546,1.667),the grain distributes homogeneous.Enlarging the diameter increasing speed is beneficial for the recrystallization in the middle area,resulting in the refinement of the grain and uniform distribution in the whole workpiece.The response surface models for the average and standard deviation of the IN718 ring grain size were established.The influences of mandrel and drive roll diameter,diameter increase speed of the ring and rolling ratio on the average and standard deviation of the IN718 ring grain size were investigated.A multi-objective genetic algorithms NSGA-II was employed to optimize the rolling process parameters to minimize the mean and standard deviation of the ring grain size.The optimized drive roll diameter,diameter increases speed of the ring and rolling ratio are 735 mm,7.5 mm/s and 1.667,respectively.There are multiple sets of solutions of the mandrel diameter which range in 158 and 200 mm.The optimization results were verified by finite element simulation and the simulated results agree well with the response surface model.Compared with the simulation results of rolling tests,the optimized mean and standard deviation of the ring grain size decrease.