Browsing by Author "Caballero, Armando E."

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    Microstructure tailoring of a wire-arc DED processed Ti6242 alloy for high damage tolerance performance
    (Elsevier, 2025-05-05) Zakir, Farhana; Syed, Abdul Khadar; Zhang, Xiang; Davis, Alec E.; Sahu, Vivek K.; Caballero, Armando E.; Biswal, Romali; Prangnell, Philip B.; Williams, Stewart W.
    This paper examines the effects of interpass hammer peening and post-process β annealing on the tensile properties, high-cycle fatigue, and fatigue crack growth behaviour of the titanium alloy Ti-6Al-2Sn-4Zr-2Mo-0.1Si (Ti6242), processed via wire-arc directed energy deposition (w-DED, also known as WAAM). A major challenge in additive manufacturing of titanium alloys is the development of a coarse columnar grain structure under standard build conditions, leading to significant anisotropy and variability in mechanical properties. This study demonstrates that interpass peening effectively refines the grain structure by inducing recrystallization, resulting in isotropic properties and increased strength without compromising fatigue crack growth resistance. Additionally, post-deposition annealing above the β-transus temperature (β annealing) significantly reduces the fatigue crack growth rate by an order of magnitude through microstructural refinement. The formation of coarse single-variant lamellar colonies promotes crack path branching and deviation, enhancing fatigue crack growth performance. Combining in-process grain refinement via peening with post-process β annealing further increases the threshold stress intensity factor by 2.5 times. These improvements provide substantial benefits for damage-tolerant design principles.
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    β Grain refinement by yttrium addition in Ti-6Al-4V Wire-Arc Additive Manufacturing
    (Elsevier, 2021-11-14) Kennedy, Jacob R.; Davis, Alec E.; Caballero, Armando E.; Byres, Nicholas E.; Williams, Stewart; Pickering, Ed J.; Prangnell, Philip B.
    Wire-Arc Additive Manufacturing (WAAM) of large near-net-shape titanium parts has the potential to reduce costs in aerospace applications. However, with titanium alloys, such as Ti-6Al-4V, conventional WAAM processing conditions generally result in epitaxial solidification from the melt pool fusion boundary, which over many layers can generate coarse cm-scale,<001>//ND fibre textured, columnar β grain structures within the deposited metal. The mechanical anisotropy caused by this coarse primary grain structure cannot be eliminated by subsequent solid-state phase transformations. In order to attempt to refine the size of the solidified β-grains and reduce their strong texture, the growth restriction efficiency of low addition levels of the strongly partitioning element (k = 0.1) yttrium (Y) has been investigated. Less than 0.8 wt.% Y was sufficient to reduce the widths of the solidified columnar β grains from 1 to 2 mm to 100–300 µm. Y was also found to induce a columnar-to-equiaxed transition (CET) in the latter stages of melt pool solidification, which benefits from a lower liquid thermal gradient and higher solidification velocity. Inter-dendritic segregation of Y was also found to be significant and oxygen scavenging led to the formation of Y2O3 particles in the inter-dendritic liquid, with a previously unreported irregular eutectic morphology. High-resolution EBSD analysis showed these particles exhibited specific orientation relationships with the solidified β grains, which were confirmed experimentally.