Collaborative Research: Modeling Material Microstructure Evolution and Fatigue Life of High Strength Metal Components Produced by Laser Melting Additive Process

加法制造可以使工业在偏远地点、太空或战场按需生产零部件，库存、交付时间和工具成本都最低。它还可以使研究人员探索新的材料组成，从而获得定制的新颖性能。为了确保激光熔化(一种添加剂制造工艺)中的零件质量并缩短提前期，关键是能够评估材料在加工过程中的动态高温度梯度下的微观结构变化，以及加工后建筑材料在静态和动态载荷下的强度。该奖项支持基础研究，以支持建模和模拟方法，从而实现现实的预测、工艺设计和优化以及激光熔化添加剂工艺的设备设计。所获得的知识为研究人员和制造商利用激光熔化添加剂工艺设计小批量、低成本的新材料提供了基础。它还有助于理解广泛的材料在激光熔化中的行为。研究成果将加强现有的工程课程，并为研究生提供跨学科的培训机会。研究的目的是：(1)了解激光熔凝的非平衡凝固机理，(2)确定多层结构引起的非均匀循环热历史对组织变化的影响，(3)建立激光熔凝组织与材料性能之间的关系。为了实现第一个目标，我们将建立一个热力有限元分析来模拟激光熔化的材料添加过程，建立一个相场方法来计算基于计算的热历史的合金相场随时间的增长，并对一种中碳钢进行单道和多层激光熔化实验。将计算得到的高温度梯度与实验观察到的溶质捕获现象进行关联，以揭示非平衡凝固机理。为了实现第二个目标，利用相场方法比较了单道次和多层激光熔化过程中的组织演变，并进行了实验验证。不同物质点的热历史会导致显微组织在晶粒度、相组成和分布上的变化。为了实现第三个目标，将根据所获得的材料微观结构建立强度估计的解析模型，并基于最小能量原理估计沿材料最薄弱点和路径产生裂纹时的疲劳裂纹萌生寿命。

Additive manufacturing can enable industry to produce on-demand parts at a remote site, in space, or in a battlefield, with minimal inventory, delivery time, and tooling cost. It can also enable researchers to explore new material compositions leading to customized novel properties. To ensure quality of components in laser melting (one of the additive manufacturing processes) and reduce the lead time, it is critical to be able to evaluate material microstructure changes in response to the dynamic high thermal gradient in the process, and the strength of constructed materials under static and dynamic loads after the process. This award supports fundamental research to enable modeling and simulation methods that allow for realistic predictions, process design and optimization, and equipment design of laser melting additive process. The obtained knowledge provides the foundation for researchers and manufacturers to engineer new materials in small lot size at low cost by using laser melting additive process. It can also contribute to understanding the behavior of a broad range of materials in laser melting. Research results will enhance current engineering courses, and provide cross-disciplinary training opportunities for graduate students. The research objectives are to: (1) acquire knowledge on the mechanism of non-equilibrium solidification in laser melting, (2) determine the effects of non-uniform cyclic thermal history due to multilayer construction on microstructure changes, and (3) establish the relationship between the microstructure resulted from laser melting and the material performances. To achieve the first objective, a thermo-mechanical finite element analysis will be constructed to simulate the material addition process of laser melting, a phase-field approach will be created to calculate the time-dependent growth of alloy phase field based on the computed thermal history, and single-pass and multilayer laser melting experiments will be conducted on a medium carbon steel. The correlation between high thermal gradients from computation and the solute trapping phenomenon from experimental observation will be made to reveal the non-equilibrium solidification mechanism. To achieve the second objective, the microstructure evolutions under both single pass and multilayer laser melting processes are compared using the phase field approach, and verified by experiments. Microstructure variations in terms of grain size, phase composition and distribution will be obtained, resulting from different thermal histories of material points. To achieve the third objective, the analytical models for estimating strengths will be established based on the obtained material microstructure, and the fatigue crack initiation life will be estimated based on the minimum energy principle applied when a crack is created along the weakest material point and path.