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

增材制造可以使工业在远程站点、空间或战场上按需生产零件，以最小的库存、交货时间和工具成本。它还可以使研究人员探索新的材料成分，从而产生定制的新特性。为了确保激光熔化（增材制造工艺之一）过程中部件的质量并缩短交货时间，能够评估过程中动态高热梯度下材料的微观结构变化以及过程后结构材料在静、动载荷下的强度是至关重要的。该奖项支持基础研究，使建模和仿真方法能够实现现实预测，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增材工艺以低成本设计小批量新材料提供了基础。它还有助于理解激光熔化过程中各种材料的行为。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训练机会。研究目标是：(1)了解激光熔化非平衡凝固机理；(2)确定多层结构导致的非均匀循环热历史对微观组织变化的影响；(3)建立激光熔化后的微观组织与材料性能之间的关系。为了实现第一个目标，将构建热力学有限元分析来模拟激光熔化的材料添加过程，建立基于计算热历史的相场方法来计算合金相场的随时间增长，并在中碳钢上进行单道和多层激光熔化实验。将计算所得的高热梯度与实验观察所得的溶质俘获现象相关联，揭示非平衡凝固机理。为了实现第二个目标，采用相场方法比较了单道次和多层激光熔化过程下的微观组织演变，并通过实验进行了验证。由于材料点的不同热历史，将获得在晶粒尺寸、相组成和分布方面的微观结构变化。为实现第三个目标，将根据获得的材料微观结构建立估算强度的分析模型，并根据沿材料最弱点和路径产生裂纹时应用的最小能量原理估算疲劳裂纹起裂寿命。

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.