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

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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.