Microporosity Formation in Alloy Solidification

这项研究的目的是促进对合金凝固过程中微孔形成的基本了解。这项研究的一个最重要的特点是，它只关注可以直接观察到成核、界面、生长形态和微观流动的微观尺度(微米量级)。以往对微孔形成的研究关注的是较大的长度尺度，并没有解决实际的固体和孔隙微结构。这一研究将为孔的成核和生长速率、孔形成过程中固-液-气系统中的微观流动和溶质场以及孔与微结构之间的相互作用提供有价值的知识。这项研究将通过结合使用新的建模、数值模拟和实验技术来完成。相场方法将被用来模拟复杂的相变和输运现象，包括多组分热力学、曲率效应和对流。将采用变形有限元网格和高效并行求解器等现代数值技术来求解模型方程。这些实验将在高分辨率显微镜下使用Hele-Shaw池中的透明模型合金。在实验中，将仔细控制初始气体含量和凝固条件。模型预测将通过实验测量得到验证。这项工作几乎在每一个金属铸造过程中都有应用。由于铸件在汽车和航空航天工业中的使用急剧增加，微疏松问题重新引起了人们的关注。只有对显微疏松进行详细的预测，才能帮助预防和评估其对铸件强度和疲劳寿命的影响。通过拟议项目获得的知识将通过继续与铸造行业合作、开发用于铸造模拟软件的改进的微气孔模型以及对学生进行教育来实现。从更根本的角度来看，该项目将推进一个很大程度上未知的研究领域，涉及具有相变的微尺度、多组分、多相系统。这样的系统不仅在金属铸造中很重要，而且在其他材料加工操作中也很重要，在自然界中，在生物体内也是如此。

The objective of this research is to advance fundamental understanding of microporosity formation during solidification of alloys. The single most important feature of the research is that it focuses solely on the microscopic scale (on the order of microns) where nucleation, interfaces, growth morphologies, and micro-flows can be directly observed. Previous studies of microporosity formation have been concerned with larger length scales and have not resolved the actual solid and pore microstructures. The study will provide valuable knowledge of pore nucleation and growth rates, microscopic flow and solute fields in the solid-liquid-gas system present during pore formation, and the interactions between the pores and the evolving microstructure. The research will be accomplished through the combined use of novel modeling, numerical simulation, and experimental techniques. A phase-field method will be developed to model the complex phase transformation and transport phenomena present, including multi-component thermodynamics, curvature effects, and convection. Modern numerical techniques, such as deforming finite element meshes and highly efficient parallel solvers, will be employed to solve the model equations. The experiments will use transparent model alloys inside a Hele-Shaw cell under a high-resolution microscope. In the experiments, the initial gas content and the solidification conditions will be carefully controlled. The model predictions will be validated by the experimental measurements.The work finds application in nearly every metal casting process. The problem of microporosity is of renewed interest because of the dramatically increased use of castings in the automotive and aerospace industries. Only the detailed prediction of microporosity can aid in its prevention and in assessing its influence on the strength and fatigue life of cast components. The transfer of the knowledge obtained through the proposed project will take place through continued collaboration with the casting industry, the development of improved microporosity models for use in casting simulation software, and the education of students. From a more fundamental point of view, the project will advance a largely uncharted area of research that is concerned with micro-scale, multi-component, multi-phase systems with phase change. Such systems are important not only in metal casting, but also in other materials processing operations, in nature, and in living organisms.