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Modeling Material Microstructure and Fatigue Life of Metal Components Produced by Laser Melting Additive Process
There has been a long-standing need in the marketplace for the economic production of small lots of components that have complex geometry. A potential solution is additive manufacturing (AM). AM is a manufacturing process that adds material bottom-up. It has the distinct advantages of low preparation cost and high geometric creation capability. Components fabricated via AM are now being selectively used for less-demanding applications in motor vehicles, consumer products, medical products, aerospace devices, and even some military projects.
For engineering applications, high value-added components require consistency in the fatigue properties. However, components fabricated by AM have large variation in the fatigue properties compared to those by conventional manufacturing processes. To alleviate unpredictable catastrophic failures of components, it is essential to study and predict fatigue life. Previous study reported that fatigue crack initiation process accounts for a large portion of fatigue life, especially for low loading amplitude and high cycle fatigue. However, this major portion of fatigue life prediction is mostly ignored by main stream researchers working on fatigue modeling. For industrial applications, engineers often specify a lower stress condition to obtain a higher safety factor. Under these circumstances, fatigue crack initiation becomes even more important, so it is essential to further study of crack initiation.
The objective of this research is to develop a fatigue crack initiation model for metal components produced by AM. To improve life prediction accuracy, the model must incorporate the effect of different microstructures, which are typically produced by AM due to a large number of repetitive cycles of re-heating and re-cooling processes. To fulfill this objective, the tasks are separated into three studies: (1) developing a temperature model to simulate temperature history, (2) modeling the component’s microstructure for the potential crack initiation zone, and (3) developing a fatigue crack initiation model for life estimation. A summary of each task is provided in the following.
First, the role of temperature model is to understand the mechanism that leads to the variation of microstructures. The existing temperature models are computationally expensive to obtain an accurate prediction of the temperature history due to repetitive heating and cooling. The main reason is that these models considered entire boundary conditions of all the material points. In this section, we proposed and employed the concept of effective computation zone, which can save the computational time significantly for AM process.
Second, it is critical to include the effect of microstructure in the fatigue life model since the microstructure variation at different locations within the real AM component is large. The grain size variation is modeled by using representative volume element, which is defined as a volume of heterogeneous material that is sufficiently large to be statistically representative of the real component’s microstructure. Regarding phase transformation, a continuous cooling transformation (CCT) diagram is a useful tool that can be used with a thermal model for microstructure design and manufacturing process control. However, traditional CCT diagrams are developed based on slow and monotonic cooling processes such as furnace cooling and air cooling, which are greatly different from the repetitive heating and cooling processes in AM. In this study, a new general methodology is presented to create CCT diagrams for materials fabricated by AM. We showed that the effect of the segmented duration within the critical temperature range, which induced precipitate formation, could be cumulative.
Third, the existing fatigue crack initiation life model has poor accuracy. One of the reasons for the poor accuracy is the coefficients change due to the variation in microstructure is not accounted for. In this section, a semi-empirical fatigue crack initiation model is presented. The important coefficients include maximum persistent slipband width, energy efficiency coefficient, resolved shear stress and plastic slip rate per cycle. These coefficients are modeled and determined as a function of microstructure, which can improve the accuracy of life estimation.
The contribution of this study is to provide a new engineering tool for designing the melting AM process based on scientific research. With this tool, the fundamental mechanism contributing to a large variation of the fatigue life of the metal components made by AM process can be understood, attributed, predicted and improved. The seemly ‘stochastic’ nature of fatigue life of the AM components can be changed to be more deterministic and predictable. This approach represents a major advance in fatigue research on AM materials. The model developed is considered as a tool for research, design, and control for laser-based AM process applications.