【作者】 陈晋；

【导师】 孙国雄；

【作者基本信息】 东南大学 ， 材料加工工程， 2005， 博士

【摘要】 凝固过程及其微观组织的研究不仅具有很强的工程应用背景,而且具有重要的理论研究价值,因此长期以来都引起不同领域学者的广泛关注。随着计算机技术的发展,用数值模拟方法来研究凝固过程取得了很大的进展。近十几年来,在凝固温度场等宏观模拟逐渐发展成熟的基础上,微观组织模拟开始成为研究的重点。本文在前人研究的基础上,提出了基于溶质扩散控制的枝晶生长数值模型与算法。这一模型采用胞元自动机方法将固液界面两侧的锐变转化为界面胞元固相份数的渐变,既坚持了尖锐界面的基本假设,又避免了跟踪复杂的固液界面。模型充分考虑了成分过冷、曲率过冷、界面能各向异性以及界面扰动等影响枝晶生长的主要因素。利用这一枝晶生长模型,本文对过冷熔体中枝晶的稳态生长行为进行了模拟计算,并与描述枝晶尖端生长的理论模型取得一致。通过在界面上施加扰动模拟了枝晶的分枝机制及侧枝的竞争生长,并研究了扰动波长和振幅对分枝发达程度的影响,研究结果与界面稳定性理论的分析取得一致。将这一枝晶生长模型进行扩展并应用到了定向凝固组织的模拟中去,通过施加不同的温度梯度和凝固速度,成功模拟了定向凝固条件下的平界面、胞状和枝晶状三种典型界面形貌以及一次臂的分枝和调整机制,并分别研究了温度梯度和凝固速度对一次臂间距的影响,研究结果与理论模型的预测基本一致。将这一枝晶生长模型与基于正态分布的形核模型相结合,直接应用到了晶粒组织的模拟中,避免了传统晶粒组织模拟中对晶粒形状的人为假设,用胞元自动机方法对晶粒组织模拟和枝晶生长模拟进行了整合,使这种方法的模拟能力和适用尺度得到了改进。通过耦合传热计算模拟了金属型中正常凝固的等轴晶组织,并通过施加不同的冷却速度、温度梯度、形核参数模拟了晶粒组织中的柱状—等轴转变,分析了不同工艺因素对等轴晶和柱状晶竞争生长的影响,模拟结果与工艺实践和理论分析一致。更多还原

【Abstract】 Solidification process and microstructures formation during solidification have attracted researchers from different fields for a long time because they have not only profound background of engineering application, but also significant theoretical value. With the development of computer technology, numerical simulation on solidification has made great progress. In the last decade, based on the success of macro simulation on temperature field, microstructure simulation has become the major focus.On the basis of previous researches, a solute diffusion controlled model for dendritic growth is developed and relative numerical method is provided. Cellular automaton method is applied to transform the sharp change at the solid-liquid interface to gradual change of solid fraction in interfacial cells, which avoids direct front tracking while sharp interface hypothesis is still kept. The effects of constitutional undercooling, curvature undercooling, anisotropy of interfacial energy and interface perturbation are considered in this model.The model is applied to simulate steady growth behavior in undercooled melt and the simulated results are in agreement with the prediction of theoretical model for tip growth. Branching mechanism and competition growth of side branches are simulated by imposing perturbation at the interface and the effects of perturbation wavelength and amplitude on the degree of side branch are studied. The results are in agreement with marginal stability theory.The model is extended to simulate constrained growth in the directional solidification. By imposing different combinations of temperature gradient and solidification rate, the typical interface morphology including planar, cellular and dendritic are successfully simulated as well as the branching and adjustment of primary arm. The variation of primary arm spacing with the temperature gradient and solidification rate are simulated respectively and agree with the prediction of theoretical model.Combined with the nucleation model based on normal distribution, the dendritic growth model is directly applied to simulate grain structures, which avoids the artificial geometry hypothesis in traditional grain structures simulation. In this way, simulation of grain structures and dendritic growth is integrated with cellular automaton method, which greatly improves the capability and the modeling scale of this method.Equiaxed grain structure normally solidified in metal mold is simulated with coupling of heat transfer calculation. By imposing different cooling rates, temperature gradients and nucleation parameters, the columnar-to-equiaxed transition is simulated and the effects of processing parameters on the competition growth between columnar and equiaxed grains are analyzed. The results are in agreement with practical process and theoretical analysis.更多还原