Improving Fabrication and Functionality in Metal Additive Manufacturing: From Defect Formation to Fatigue Performance

170 pagesThis dissertation is composed of four published papers in the field of metal additive manufacturing. These papers examine the formation and mechanical effects of build defects through computational and experimental investigation. A major challenge in process modeling of AM fabrication is the discrepancy in spatial and temporal scales. Previously, accurate and efficient thermal modeling was considered computationally infeasible on the component-scale using traditional numerical methods (i.e. finite element or spectral methods). The first paper presented here introduces a novel and computational efficient simulation technique. This technique combines the geometrical robustness of traditional numerical approaches with the computational speed and accuracy of analytic solutions, making realistic component-scale simulations possible. Critically, and unlike other proposed methods, this technique does not coarsen the physical problem in either time or space. The theory and methodology of this novel simulation method are presented and verified by simulating a single scan on an orthogonal geometry with simple boundary conditions prescribed. The second paper presented in this dissertation builds on this foundation by modeling a real metal AM component. In this work, the model's capabilities were extended to include: physical boundary conditions, complex geometries, multi-layer simulations, residual heat, and realistic build parameters (e.g. complex scanning strategy of thousands of layers). With the resulting thermal field, a criteria for defect prediction was created. The resulting software was shown capable of simulating the fabrication process in a fraction of the component build time. At the end of Chapter 3, the deployment of this software on the International Space Station during a collaborative project with Hewlett Packard Enterprise and NASA is briefly discussed. Currently, component certification and reliability are two of the primary obstacles limiting the application of metal AM to fracture critical applications. In the third paper presented in this dissertation, a key contribution to this uncertainty is discovered and the causes identified. This study investigates the observed variations in defect populations across the build plate in components fabricated with identical build parameters. The asymmetries of the powder bed fusion process were investigated to explain this spatial gradient in fabrication quality. Through the use of computational fluid dynamic simulations, the effects of the shielding gas flow were explored. The results of this work highlight the importance of a homogeneous and laminar velocity field for consistent fabrication quality. The above three papers focus primarily on predicting or reducing the formation of build defects during metal AM fabrication. Although this is critical work towards the goal of improved and reliable fabrication quality, currently, some degree of build defects are inherent in components fabricated by additive processes. In the final paper presented in this dissertation, mitigating the negative mechanical effects of defects through post-processing are considered within the context of fatigue performance in metal AM components. Specifically the effects of hot isostatic pressing (HIP) on the fatigue response in surface finished Ti-6Al-4V specimens is examined. These specimens were intentionally fabricated with varying initial material states (to include defect population). The dependence of fatigue performance on defect populations was examined after ASTM recommended standard HIP treatment. The conclusions in this paper contradict much of the published literature, suggesting a different mechanism governing fatigue performance in HIP'd Ti-6Al-4V. Furthermore, the findings of this work show that the popular ``weakest-link" modeling approaches, which utilize extreme value statistics to map defects to fatigue performance, are not mechanistically appropriate for HIP'd Ti-6Al-4V. Finally, alternative treatments to the ASTM recommended HIP schedule were tested. The results show that a low-temperature high-pressure HIP treatment produces a considerable increase in performance and is recommended for fatigue critical applications