【作者】 李文珍；

【导师】 柳百成；

【作者基本信息】 清华大学 ， 铸造， 1995， 博士

【摘要】 铸件凝固过程中微观组织形成的数值模拟研究是铸造学科前沿的研究课题。本文根据国家自然科学基金重点项目的要求,选择球墨铸铁件微观组织形成模拟作为主要研究方向。同时,从微观模拟的角度研究了球铁件缩孔缩松预测。此外,对铸钢件缩孔缩松的定量预测进行了较深入的研究。 球墨铸铁作为一种重要的工程材料,其微观组织形成和铸态机械性能与工艺因素有很大关系,如果能在设计阶段就对其微观组织形成和性能进行模拟预测和优化,具有很重要的工程意义。本文首先通过实验研究了球铁微观组织形成与工艺因素的关系,测定了试样的铸态机械性能,得到了微观组织与机械性能关系的数学模型。在实验基础上,根据结晶动力学理论,建立了球铁凝固过程各阶段微观组织形成的数学模型,其中首次提出了石墨球生长的扩散-界面控制模型。根据结晶动力学模型,用C语言实现了球铁微观组织形成的计算机模拟。通过模拟计算确定了晶粒长大过程中的碰撞因子,并从理论上给出了各相形成的相加性证明。模拟了试验铸件的微观组织及机械性能并与实测数据进行了比较,结果表明两者吻合较好。 基于球铁微观模拟的结晶动力学模型,建立了球铁凝固过程中的体积变化模型,提出并实现了定量预测球铁缩孔缩松形成的一种新方法-微观模拟法。该方法首次将微观模拟和缩孔缩松预测耦合,可同时进行微观组织形成模拟和缩孔缩松预测。对T形球铁试块的缩孔缩松形成进行了模拟计算并与用收缩膨胀动态叠加法模拟的结果进行了对比。结果表明,用微观模拟法预测的缩孔缩松的体积大小和分布,其精度高于收缩膨胀动态叠加法。而在缩孔缩松的形状方面,两者预测结果一致并与实测吻合。 综合考虑冶金和工艺因素的影响,进行了铸钢件缩孔缩松形成的试验研究。在实验基础上,提出了铸钢件缩孔形成的三维定量模拟法-等效液面下降法。采用G/2^1/R判据预测缩松。该方法在计算过程中,可对边界条件进行重新设置,并首次采用体绘制技术进行缩孔缩松预测结果的后处理。通过对试验铸件的模拟计算,确定了G/2^1/R的临界值为0.8。在本文的研究条件下,该临界值与冶金和工艺因素无关。 对实际生产中的球铁件微观组织特别是铸钢件缩孔缩松形成进行了大量模拟计算及生产验证。结果表明,用本文所开发的软件可以对现行工艺设计方案进行评价,对新的工艺方案进行优化设计,并已在生产实际上取得显著的经济和社会效益。更多还原

【Abstract】 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更多还原