Research On Fractal Analyze And Simulation On Microstructure Of TB8 Alloy Under Hot Deformation

With the development of hot deformation technique and the higher requiring to product quality, it is necessary to know microstructure evolution of products during hot deformation comprehensive and thoroughly. Therefore, analysis and simulation on the evolution of microstructure has been a researching hotspot in recent years. It has important theory significance and utility. Grain shape is one of the important indexes to characterize microstructure. However, classical stereology has some difficulties in describing irregular shapes of grain quantificationally. The use of fractal geometry offers a powerful instrument for solving the problem. In this study, the hot deformation behavior and dynamic recrystallization rule of TB8 alloy were systemic studied. The fractal theory was applied to analyse the microstructure during hot deformation for the first time. On the basis of kinetic theory, a fractal simulation model was established to simulate the microstructure evolution during dynamic recrystallization process. The major content can be summarized as follows:Hot compression test of TB8 alloy were performed on Gleele-1500. Hot deformation behavior was studied and the model of flow stress was obtained by hyperbolic-sine-type Arrhenuis equation. This model can describe the flow stress during hot deformation accurately and become a predeterminate condition of FE numerical simulation.The thermal-mechanical coupled rigid-viscoplastic finite element simulation was carried adopting the model of flow stress of TB8. Simulation result show good agreement with experiment. Strain and displacement during hot deformation were calculated which provided an essential precondition for microstructure simulation and proved the correctness of flow stress model.In order to improve precision of the microstructure simulation, accurate microstructure predicting model is needed. In this paper, dynamic recrystallization rules of TB8 alloy were studied, size model and kinetic model of dynamic recrystallization were established by experience formula. The results show that this method is able to predict the variation of microstructural parameters with technique parameters successfully. It not only creates necessary prerequisite for improving the accuracy of microstructure simulation, but also lays a scientific foundation for determining reasonable hot forming process.To perform fractal simulation for microstructure, the fractal theory was applied to analyze the recrystallized microstructure during hot deformation. Based on the computer image treatment, Slit Island Method and Box Counting Method were applied to calculate the fractal dimension of recrystallized microstructure and deformed microstructure. It is shown that microstructure presents typical fractal character, and fractal dimension can be used to describe its degree of irregularity. With decreasing strain rate and increasing deformation temperature, the fractal dimension of recrystallization grain decreases while grain size increases. At the same time, the variety of deformation temperature and strain rate results in the changes of grain amount in unit volume, recrystallization grain size, the size and shape of original grains to a certain extent, which influences the fractal dimension of deformed microstructure consequently. Owning to the nonlinear and complexity of relationship between fractal dimension and deformed parameter, the predicting model was established with a three-layer feed-forward artificial neural network with a back-propagation learning rule for the first time. It provides more scientific theory foundation for the quantitative study of deformed microstructure and accurate simulation of microstructural evolution.Based on the fractal simulation method—Cellular Automaton method, efficient simulation on normal 2D grain growth was accomplished, the simulated microstructure was similar to actual alloy. Analysis shows that the grain shape by simulation presents fractal character and simulation results accords with the kinetic rule of grain growth. So the initial microstructure by this method is rational and effective.In order to reflect kinetic mechanism of DRX microstructural evolution and show the evolutional process truly, dynamic fractal simulation of recrystallized microstructure was realized by incorporating the coupled thermomechanical rigid-viscoplastic FE model and the kinetic model of microstructure into Cellular Automaton model. Compared with the experimental results, it shows that the simulated microstructures are similar to the actual ones. The fractal dimensions and their variety tendency agree well with the actual results too. So the whole shapes of dynamic recrystallized microstructure can be displayed accurately. The simulated results describe the kinetic rule of DRX well. The study shows that: the thermomechanical rigid-viscoplastic finite element simulation, kinetic model of dynamic recrystallization, Cellular Automaton model of grain growth and microstructural evolution model by combining them are reliable and stable. It has important practical meanings for optimizing the deformation process and improving the microstructure and capability of product.