Simulation Of Microstructure Evolution During Discontinuous Hot Forging Processes Using Cellular Automaton Method

The prediction and control of microstructure evolution in heavy forgings during discontinuoushot deformation play a key role in improving the mechanical properties of heavy forgings. Heavyforging process usually consists of multi-pass and each pass consists of multiple reductions, so it is atypical process of incremental bulk forming. During multi-pass deformation, different parts of theforging at different time will stay in two states: deformation and non-deforamtion. Different statesinclude different mechanisms. Both incomplete and complete dynamic recrystallization (DRX) occurin the deformation state. In non-deforamtion state, incomplete DRX will result in meta-dynamicrecrystallization (MDRX) and static recrystallization (SRX). Complete DRX will result in graingrowth. The complicated recrystallization behavior leads to the non-uniform distribution of storedenergy among individual grains and within grain interiors. However, it is difficult to conceive atraditional phenomenological model to describe the complicated recrystallization processes.Therefore, it is highly necessary to develop a new algorithm that better reflects the physical behaviorobserved in heavy forging production. According to the micro-and macroscopic relationship among"strain, dislocation density, DRX and flow stress", a mesoscale Celullar Automon (CA) model isdeveloped to predict the microstructure evolution for typical heavy forged materials during heavyforging production, during which DRX, SRX and MDRX may occur. The main research content is asfollows:The compression tests are conducted with Gleeble-3500thermo-mechaincal simulator to studythe DRX behavior and rheological regularity of LP and HP-IP rotor steels at various temperaturesand strain rates. Based on trandional stress-dislocation relation and kinematics of DRX, the flowstress constitutive equations of the work hardening-dynamical recovery period and dynamicalrecrystallization period are established to accurately describe the relationships of the flow stress,strain rate and temperature of LP and HP-IP rotor steels. More importantly, the necessaryexperimental data for the estabilishment of the CA model is abtained by conducting these physicsexperiments.The CA model based on the curvature-driven mechanism, thermodynamic driving mechanismand lowest energy principle has been developed to simulate normal grain growth during high-temperature austenitizing for LP rotor steel. In this model, the morphology, grain size distribution,topological aspects and local kinetics of austenite grain growth are annlyzed under differenttemperatures and calculation time, i.e.CAS.The DRX-CA model coupling foundamental metallurgical principles has been developed topredict microstructure evolution during DRX in heavy forged materials. The evolution of dislocationdensity, DRX nucleation and grain growth are under consideration. At the same time, the effects ofalloying elements and the second-phase particles on the recyrstallization nucleation and growth arealso taken into account in the developed DRX-CA model. In order to examine the prediction performance, DRX behavior of LP rotor steel is simulated under different strains, deformationtemperatures, strain rates and initial grain sizes.The effect of deformation on grain surface area per unit volume and edge length per unit volumeis another issure in DRX. During deformation, the dislocation density increases gradually and whenit reaches a critical density, new nucleus will appear on the grain boundaries and grow with themodel of equiaxed growth. This growth relies on the difference of dislocation density between thedynamic recrystallized grains and deformed grains. When the dislocation density in the new dynamicrecrystallized grains reaches the critical density again, the next round of DRX occurs. Thecompression only occurs on the grains that have not completed the DRX in each round. To describethe compression effect more accurately, an update CA model is propsed in which a cellularcoordinate system and a material coordinate system are establishe separately. The cellular coordinatesystem remains unchangeable, but the material coordinate system and the corresponding grainboundary shape will change with deformation. The simulation results show that the topologydeformation has an accelerated affect on the rate of DRX. The results also indicate that the averagegrain size exhibits a slight decreas because of the effect of topology deformation on therecrystallized grain growth. The average grain size is closer to the experimental results using thetopological CA model with a comparison of the conventional CA simulation.On the basis of the topological CA model, a mesoscale CA model is established to simulatemicrostructure evolution during discontinuous hot deformation for LP rotor steel. The model has thecapability of tracking the deformation history of each cell with dislocation density being an internalvariable, and accurately describes even complex recrystallization process. To examine the validity ofthe developed CA model, we attempted to apply the mesoscale simulation method to four-pass hotdefroamtion for LP rotor steel. The multi-scale simulation platform for prediction the microstructureevolution during discontinuous hot deformation is established by coupling FEM method and CAmethod. Bsed on the established simulation platform for microstructure evolution, numericalsimulations for stretching process with multi-stroke and multi-pass are performed. It proveides anovey way for investigating the microstructure evolution during the complicated recrystallizationprocesses observed in heavy forging production on a multi-scale.Research results show that the impact of the law of temperature, strain rate and intial grain sizeon grain evolution is revealed by CA simulation. Using the developed CA model, the relationshipbetween the flow stress, volume fraction recrystallized and recrystallized grain size and thethermomechanical parameters is established. By comparison of the simulated results by the CAmodel with the flow stress and metallographs from the experiment, it is obviously seen that thesimulation results closely resemble the experimental results. It is proved that the developed CAmodel accurately reflects the interaction mechanism between micro and macro evolution. Therefore,this model can then be used to simulate the microstructure evolution during the discontinuousforming processes. Multi-scale digital simulation can be achieved to predicte the evolution of themacro-physical fields and mesoscopic grain during discontinuous hot forging by coupling mesoscopic CA model with macro-scale finite element analysis. It shows that, based on the multi-scale simulation platform, the optimum forging method can be designed.