基于XIDS技术铝合金凝固过程中气孔形成行为及相关凝固理论的研究

In this project, studies are focused on porosity formation during solidification of aluminum alloys and its corresponding solidification theory. By XIDS (micro-focus X-ray imaging and directional solidification) technology, the evolution of porosities during solidification under conditions of different alloy compositions, melt properties and solidification variables will be observed in real time and recorded automatically. An computer automatic quantitative analysis software self-developed will be used to characterize the time-dependant properties of each porosity, then the average nucleation temperature, growth velocity and nucleation rate of porosities are obtained. Thus, the physics process of porosity formation kinetics (on nucelation rate and growth velocity) will be discussed especially in the view of point of the interaction between nucleation and growth of porosities. At very low solidification velocity, the relationship between the profile parameters of growing porosity, after encountering the solidifying interface of liquid/solid, and the hydrogen content, orignal curvature radius of porosity and solidification velocity will be investigated using the modified XIDS technology. Based on a great amount of experiments, the profile growth equation of porosity will be modeled and solved theoretically, and then the cooperative growth modes between porosity and solid will be constructed. Based on having highlights in the porosity formation kinetics and cooperative growth between porosity and solid, the corresponding solidification theory will be developed, focused on solute redistribution in liquid, solid and gas tri-phases system and on growth of gas phase. To carry out this project is contributed to understand the physics process of porosity formation during solidification of aluminum alloys, to develop the corresponding theory of solidification in liquid, solid and gas tri-phases system, and to support numerical simulation and prediction of porosity formation in experiment and in theory. Thus it is very important and necessary in science and practice.

本项目是针对铝合金凝固过程中气孔形成行为及相关凝固理论开展研究工作的。通过XIDS(射线成像定向凝固) 技术实时观察、采集气孔在不同合金性质、熔体性质和凝固条件下的演化过程，利用气孔特征计算机自动定量分析处理技术描述每个气孔特征参数的时序量值，获得不同条件下气孔的平均形核温度、气孔的长大速度及形核率，着重从气孔形核与生长交互作用的角度探寻气孔形成动力学规律及其内在本质；采用改进的XIDS 技术考察低凝固速度下气孔遭遇液固界面后气孔形貌参数与熔体中氢含量、气孔初始曲率半径和凝固速度间的关系，并在大量实验研究基础上，通过理论建模与求解，获得气孔生长外形方程，构建气孔与固相协同生长模型。在此基础上，拓展液/固/气三相体系相关凝固理论。本项目研究将为气孔形成机理的认识、液/固/气三相体系相关凝固理论的拓展、气孔形成的数值模拟与预测打下了坚实的实验与理论基础，具有重要的科学意义和实践指导价值。

采用XIDS技术实时观察了在不同熔体状态和凝固条件下A356合金中气孔形成行为，得到了气孔的平均形核温度，平均生长速率。对完全凝固样品进行X射线探伤，定量特征孔洞的性质(孔体积分数、数密度、大孔个数)及其随凝固过程的变化。.研究发现：气孔生长速率随局域液体温度降低而降低，而且扰动剧烈。这种扰动是因气孔形核与生长对氢供给竞争引起气孔形核和气孔生长的相互抑制，从而导致氢溶质过饱和度的扰动。同样原因也导致孔体积分数和孔密度在凝固过程中存在扰动。.在相同的凝固条件下，氢含量对孔洞形成的影响比较显著。搅拌熔体和加入Sr都增加了熔体中氧化夹杂水平， 更重要地是增加了更高活性的气孔形核基底数，导致了气孔形核温度的显著升高。这些在高温区形核的气孔的快速生长继而抑制了局域新孔的的形核行为。因此，搅拌熔体和加入Sr导致了合金中孔密度的明显降低，这与以往的研究结果不同。在A356和近共晶Al-Si合金中都发现熔体中的Sr能扩散到氧化夹杂上形成松散的氧化物。这增强了这些夹杂物作为气孔形核基底的活性，导致气孔形核温度的显著升高。Sr的加入增强了孔体积分数、孔数密度沿凝固长度的扰动。在高氢低杂的合金中增加凝固速度提高了凝固前沿液体中氢的过饱和度，这促进了气孔在非常靠近凝固前沿处形核，因而气孔的形核温度显著降低。增加凝固速度导致氢的过饱和度增加从而增大了生长速率。由于减少了生长时间，随凝固速度的增加，最终孔的尺寸减小。增加凝固速度使得气孔的生长速率的扰动更剧烈。在高氢低杂的熔体条件下，提高凝固速度使孔密度略微增加、孔隙率略有下降，影响甚微，但却使大孔个数显著降低。.利用XIDS实时观察了近共晶铝硅合金中与固液界面协同生长的气泡形貌演变过程。孔洞的形貌演变取决于固相凝固速度和气泡生长速度的竞争结果。凝固速度对气泡与固相协同生长有重要的影响。结合气泡周围氢浓度的热力学计算，建立了气泡与固相稳态协同生长的数学模型，并提出了简单判据。基于三种模型：液相充分扩散固相无扩散、液相有限扩散无对流和液相有限扩散有对流，考察氢溶质在液/固两相体系的溶质再分配，发现只有第三种模型与XIDS实时观察的气孔形成行为相吻合。假定距凝固始端l处液体有一个直径为10微米的气孔，发现只有在固相分数大于临界固相分数时，气泡开始生长，此时氢溶质分配则发生在固\液\气三相中，并导出了气泡生长方程和整体液体的氢浓度方程。