Numerical Simulation Of Microstructure Evolution During Solidification Of Twin-Roll Casting Process

Twin-roll casting technology is considered as one of the frontier technologies in the field of metallurgy and material processing for its characteristic of near net shape and rapid solidification, which can not only improve the product efficiency and reduce the cost, but also improve the quality of the casting strips. Meanwhile, this technology can also be used to manufacture the product which can hardly be produced by the conventional process. Due to the complicate relationships among the processing parameter in the twin-roll casting process, it will be time consuming, high cost and sometimes aimless of the experimental investigation. This problem can be solved by using the computer assisted simulation though which the relationships among the processing parameters can be easily obtained as well as the effect of the processing parameters on the micro structure of the casting strips and the stability of the casting process. The law which is obtained from these simulations has important theoretical significance and can be used to optimize the casting process and improve the quality of the casting strips.Based on the characteristic of vertical twin-roll casting process, the mathematical model of the evolution of the solidification microstructure was established in this paper. The simulation of the solidification microstructure evolution of the casting strips was achieved by the combination of the Cellular Automaton (CA) techniques and the mathematical model. The main work of this paper is as follows:(1) The finite element method has been used to analyze the three-dimensional transport phenomena in the pool region of the twin-roll strip casting process. Meshing method and smart-sizing algorithm were used to adapt the shape of the molten pool. The simulations of temperature and flow field of magnesium alloy AZ31B were performed to examine the effect of the process parameters such as the roll speed, pouring temperature, height of the molten pool on the velocity and temperature distributions in the molten pool during the twin-roll casting process.(2) A modified mathematical model for dendritic growth which is based on solute diffusion was established and a modified KGT model for the growth of dendrite tip was also developed in order to account for the effects of fluid flow in front of solid/liquid interface. The numerical method of the dendritic growth model was also provided based on the Cellular Automaton method.(3) A micro area which is attached to the surface of the casting roller is selected as the domain of the microstructure evolution simulation. The temperature field of the micro domain is obtained by the interpolation of the simulating result of macro temperature field in the molten pool and the calculation of solute diffusion, the nucleation and growth of grains is carried out in the micro scale directly. The position of the simulation domain changes with the rotation of the roller, thus the macro/micro coupling simulation of the solidification microstructure evolution during the twin-roll casting process can be realized.(4) Based on the mathematical model and numerical method developed in this paper, a computer program was developed for the microstructure evolution during the twin-roll casting process, which is capable for the visual simulation of both the dendritic growth and the formation of grain structure.(5) Based on the developed computer program, the single equiaxed dendritic growth and the multiple equiaxed dendritic growth of magnesium alloy AZ31B were performed. The simulated morphology and microstructure showed good agreement with the experiment results. Then the microstructures of the casting strips under various conditions were calculated to obtain the low of the effect of processing parameters on the solidification microstructure.(6) Based on the developed computer program, the single equiaxed dendritic growth and columnar dendritic growth in directional solidification were performed to study the behavior of the silicon steel with3wt%Si during the solidification process. The columnar-to-equiaxed transition was also simulated and the effects of processing parameters such as cooling rate and temperature gradient on the competition growth between columnar and equiaxed grains were analyzed. Then the microstructures of the casting silicon steel strips at different melt superheats were simulated, the results showed good agreement with the experimental ones. The effects of processing parameters on the solidification microstructure were also studied by the calculation of microstructure at various casting conditions, which can provide the theoretical basis for the optimization of the twin-roll casting process. Finally, the phenomenon that the columnar grains of the casting strips are tilted in the direction of casting was simulated by the combination of the modified KGT model and the "decentered quadrilateral" CA growth algorithm, and good agreement was found between the simulated and experimental results.