Microporosity Formation in Alloy Solidification

本研究的目的是促进对合金凝固过程中微孔隙形成的基本认识。这项研究最重要的一个特点是，它只关注微观尺度（微米量级），在那里可以直接观察到成核、界面、生长形态和微流动。以往对微孔隙形成的研究主要集中在较大的长度尺度上，并没有解决实际的固体和孔隙微观结构问题。该研究将为孔隙成核和生长速率、孔隙形成过程中固液气体系中的微观流动和溶质场以及孔隙与微观结构演变之间的相互作用提供有价值的知识。该研究将通过结合使用新颖的建模、数值模拟和实验技术来完成。将发展相场方法来模拟复杂的相变和传输现象，包括多组分热力学，曲率效应和对流。现代数值技术，如变形有限元网格和高效并行求解器，将被用于求解模型方程。实验将在高分辨率显微镜下的Hele-Shaw细胞中使用透明合金模型。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。这项工作几乎适用于所有的金属铸造工艺。由于汽车和航空航天工业中铸件的使用急剧增加，微孔隙问题重新引起人们的兴趣。只有对微孔隙进行详细的预测，才能预防微孔隙的产生，并评估微孔隙对铸件强度和疲劳寿命的影响。通过拟议项目获得的知识的转移将通过与铸造行业的持续合作，开发用于铸造模拟软件的改进微孔隙模型以及对学生的教育来实现。从更基本的角度来看，该项目将推进一个很大程度上未知的研究领域，即与微尺度、多组分、多相相变系统有关的研究。这种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，在生物体中。

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.