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Microstructure and processing links in beta-titanium during additive manufacturing
Jasien, Christopher
Jasien, Christopher
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2024
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Titanium (Ti) alloys have become attractive for the production of aerospace and defense components, due to their high strength to weight ratios. Yet, high material costs and processing difficulties have limited their use. The recent exploration and application of additive manufacturing (AM) for these products has mitigated many of these issues and provided beneficial microstructural responses. Initial work on AM of Ti has primarily focused on Ti-6Al-4V (wt. %) and other α + β alloys, due to familiarity with the alloy class. However, Ti-6Al-4V can crack when produced by AM techniques because of thermal stresses. For this reason, there has been an increasing desire to move away from α + β alloys to take advantage of the properties and processing advantages that metastable β-Ti alloys can provide. These alloys better accommodate residual stresses, as well as maintain a unique ‘metastable’ phase. This phase metastability is conducive to deformation induced transformations, as well as increased precipitation strengthening during post-build heat treatments.
To further investigate one commercial metastable β-Ti alloy, Ti-10V-2Fe-3Al (wt. %) (Ti-1023), under AM conditions, single and overlapping laser spot-melts and rasters at varying powers and travel speeds were performed on substrates at the Advanced Photon Source at Argonne National Laboratory. Computational tools were used to model the experiments and predict processing conditions, such as solidification rates (Vs) and thermal gradients (Gs). The Vs and Gs were then used in the development of a columnar to equiaxed transition (CET) model to predict grain morphology at various locations in the melt-pools. Post-mortem microscopy of spot-melts and rasters revealed grain morphology predictions matched many of the experimental observations. Microstructural imaging of the Ti-1023 after simulated AM also revealed the presence of martensitic transformation, or transformation induced plasticity (TRIP), in the melt-pools and neighboring substrate. This martensite formed in varying amounts and locations. The extent and location of this phase could be controlled by alterations to the process, such as the introduction of thermal cycling, in addition to aspects like crystallographic orientation and the occurrence of low Schmid factor grains with respect to the {"112" }_"β" ⟨"11" "1" ̅ ⟩_"β" martensite shear system and the presence of small, equiaxed grains that generally reduced the extent of transformation.
With a greater understanding of commercial metastable β-Ti alloys, novel Ti alloys with decreased levels of β-phase stability were investigated. Single and overlapping laser tracks were conducted on Ti-1023, Titan 23, and Titan 27 substrates with single melt tracks, providing information on the as-solidified structure, while overlapping tracks simulated the effect of thermal cycling inherent in a full powder bed fusion-laser beam (PBF-LB) build. Decreasing β-phase stabilities resulted in a shift from deformation- to cooling- induced martensite in the as-solidified and thermally cycled microstructures. Limitations of previously accepted methods to predict the deformation behavior of metastable β-Ti alloys were also identified. To further explore the most promising of the novel Ti alloys, full PBF-LB builds of Titan 23 were fabricated and investigated. In the as-built condition, Titan 23 initially maintained a single metastable β-phase, typical of previously studied commercial metastable β-Ti alloys. Subsequent heat treatments significantly altered the microstructure and resulting mechanical properties by the formation of α-phase precipitates. The location and size of the precipitates can be easily controlled and allowed for a wide range of mechanical properties to be developed in PBF-LB fabricated Titan 23.
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