A Study On Numerical Simulation Of Microstructure And Shrinkage Cavity Formation During Solidification Process Of Casting

Numerical simulation of the microstructure formation during casting solidification is currently an active area of research frontiers in foundry technology. In this thesis, the simulation of the microstructure formation of spheroidal graphite (SG) iron casting was carried out as a main research content, which was financed by Natural Science Foundation of China. At the same time, research has been done on the prediction of shrinkage cavity and porosity of SG iron based on the micro-modeling. In addition, the quantitative prediction of shrinkage cavity and porosity of steel casting has been further studied.SG cast iron is a very important engineering material. The processing parameters have great effects on the formation of the microstructures and as-cast mechanical properties of SG iron. If the microstructures and mechanical properties of SG iron can be predicted and optimized prior to the production, it will greatly facilitate casting design and quality control. The influences of processing factors on the formation of the microstructures of SG iron were conducted by experiments. And then the relations between microstructures and mechanical properties were obtained. Based on experiments, the mathematical models of the microstructure formation of SG iron from solidification down to room temperature were established according to the theories of solidification kinetics, among which the diffusion-interface-controlled growth model of graphite spherulite was first proposed. According to the solidification kinetics model, a 3D finite difference code was completed by the C language to simulate the formation of microstructures of SG iron. The impingement factors during the grain growth were determined by the calculation, and the additivity of phases was theoretically proved as well. The microstructure formation of SG iron specimens was simulated and the as-cast mechanical properties were predicted. Good agreement was obtained through comparing the calculated results with those of experimentsThe mathematical models describing the volume changes during the solidification of SG iron were established based on the micro-modeling (MM). A new method for the quantitative prediction of the shrinkage cavity and porosity of SG iron was proposed, in which the simulation of the formation of microstructure and the prediction of shrinkage defects during the solidification of SG iron were first coupled in a single model. Using the MM method, the simulation of the formation of microstructure and prediction of shrinkage defects could be simultaneously carried out. The shrinkage cavity and porosity formation of T-shaped SG iron samples was calculated by MM method. The calculated results were compared with the experimental results as well as with the simulation results of Dynamic Expansion Contraction Accumulation Method (DECAM) . It was shown that the predicted size and distribution of shrinkage cavity and porosity by MM method were more accuratethan that of DECAM. However, the predicted shape of shrinkage cavity by both MM and DECAM was identical and both were in good agreement with the measured results.Experiments of the formation of shrinkage cavity and porosity of steel castings were conducted by taking account of both the metallurgical and processing factors. On the basis of the experiments, a 3D quantitative method for prediction of the formation of shrinkage cavity of steel castings so called Equivalent Liquid Surface DescendingMethod (ELSDM) was proposed. The shrinkage porosity was predicted by using G/Vr method. In ELSDM method, the boundary condition could be reset during the calculation. The volume rendering method was first used to the postprocessing of the predicted results. Based on the calculation with ELSDM, 0.8 was proposed as the critical value ofG/Vr and it was not affected by the metallurgical and processing factors.A large number of calculations have been carried out to verify the simulation results of shrinkage cavity and porosity formation of steel castings as well as microstructure formation of SG iron castings in practical...