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Microstructural evolution in titanium alloys under additive manufacturing conditions
Saville, Alec Ian
Saville, Alec Ian
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2022
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Saville_mines_0052E_12503.pdf
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Saville_mines_0052E_316/DED_Ti-6Al-4V_Video.mp4
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2024-04-22
Abstract
Titanium alloys are an attractive class of materials for use in metallic additive manufacturing (AM), the layer-by-layer deposition of metal feedstock into a computer designed geometry. This is due to the inherent freedom of design, ability to produce components on demand, and vastly reduced waste associated with AM over traditional manufacturing processes. This makes metallic AM desirable for the biomedical, aerospace, and related industries. However, despite success in producing components via metallic AM, the properties and performance of the titanium metal itself often fails to meet specifications. This is due to a lack of microstructural control both during the solidification, and from uncontrolled solid state transformations of titanium alloys at lower temperatures during the build process. In this thesis, the microstructural evolution of titanium alloys under additive manufacturing conditions is explored to better understand how build parameters, alloy selection, and other factors influence the as-built microstructures of titanium components. Without this understanding, manufacturers of AM titanium components will continually risk producing components that fall out of specification, preventing the use of metallic AM to realize new technologies and designs. Three alloy systems were explored in this work. The first was Ti-6Al-4V (wt %), a ubiquitous alloy in the biomedical and aerospace industries. PBF-EB and DED builds of Ti-6Al-4V were explored to understand how changes to thermal conditions altered both the solidification and solid state microstructural evolution. These investigations found considerable variations in microstructure can result even from changes to scan strategy or the implementation of a build pause, leading to considerably different material performance and the evolution of significant cracking/other defects. Unique crystallographic textures were also discovered, indicating both qualitative estimates on what solid state microstructure formed from the build process and estimates on if the as-solidified grain size was coarse or fine. Both the PBF-EB and DED builds required extensive use of neutron diffraction to understand microstructural evolution. As a result, a chapter was dedicated in this work to documenting how to successfully process neutron diffraction data in a systematic and repeatable fashion using the Rietveld refinement program known as MAUD. Microstructural evolution of other alloys such as binary eutectoid Ti-Cu and the metastable β-Ti alloy was also investigated. Unlike Ti-6Al-4V where the directional as-solidified microstructure is maintained throughout the build process, it was discovered Ti-Cu can effectively erase the as-solidified microstructure through the formation of new parent grains. This is possible due to a solid state eutectoid transformation intrinsic to the Ti-Cu system, analogous to that observed in the Fe-Fe3C system. Such a transformation pathway opens the doors for unique microstructural control using the thermal cycles intrinsic to AM, and the ability to produce near-born-ready components with refined microstructures directly from the build process. Reconstruction of the parent β-Ti grain size for Ti-Cu was also attempted to elucidate information about the high temperature parent phase. Successful reconstruction was achieved with α' martensitic microstructures, but further work is required to successfully reconstruct the more commonly observed eutectoid α-Ti + Ti2Cu microstructure. The insights gained during reconstruction of both the Ti-6Al-4V and Ti-Cu microstructures was lastly applied to Ti-1023 to reconstruct partially transformed TRIP microstructures. Such a process was done to understand how the orientation of parent β-Ti orientations influences the activation of different deformation mechanisms, and how these could be best tailored for use in build processes as Ti-1023 gains further interest for use in AM. The insight collected in this work acts as part of the foundation for producing titanium AM components with controlled microstructures in structural and higher-risk applications. The lack of microstructural control currently possible with existing AM production routes greatly limits the impact this manufacturing process can have. The knowledge developed here helps fill this gap, and provide mechanistic insight into the microstructural evolution of titanium alloys, unique crystallographic textures to rapidly qualify as-build microstructures, and avenues for future advancement with alloys tailored to take advantage of the unique thermal conditions in metallic AM. EBSD data processing scripts and related scripts utilized in this work are included for readers at https://github.com/Savillian/Doc-Scripts.
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