Browsing by Author "Garner, Alistair"

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    Microstructural characterisation and mechanical properties of Ti-5Al-5V-5Mo-3Cr built by wire and arc additive manufacture
    (Taylor and Francis, 2022-08-18) Caballero, Armando; Davis, Alec E.; Kennedy, Jacob R.; Fellowes, Jonathan; Garner, Alistair; Williams, Stewart; Prangnell, Philip
    The as-deposited microstructure and mechanical properties of the near-β titanium alloy Ti-5Al-5V-5Mo-3Cr (Ti-5553) produced by wire-arc additive manufacture (WAAM) were investigated, to understand its microstructural evolution under WAAM deposition conditions and to establish correlations between the microstructure features formed and the thermal cycles experienced during deposition. The ‘as-deposited’ Ti-5553 WAAM material exhibited higher tensile strengths than other as-deposited additively manufactured Ti-5553 deposits previously reported in the literature, but had significant anisotropy in elongation, as a consequence of the coarse and columnar β-grain structure that formed on solidification, which exhibited a strong {001}β⟨001⟩β cube texture. The multiple reheating cycles, inherent to the WAAM process, were recorded using a novel ‘harpoon’ thermocouple technique, and the α precipitation evolution was related to the thermal history. Electron probe microanalysis chemical maps revealed significant solute microsegregation during solidification, which influenced the subsequent precipitation due to its effect on the local β-phase stability. As each layer experienced more reheating cycles, the microstructure evolution could be ‘time resolved’ and the α laths were found to precipitate in a specific sequence of nucleation sites, starting at the β-grain boundaries and then inter-dendritically, where there was lower matrix β stability. However, after the reheating peak temperature was insufficiently high to have any further effect, the microstructure consisted of a relatively uniform distribution of α laths.
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    On the observation of annealing twins during simulating β-grain refinement in Ti–6Al–4V high deposition rate AM with in-process deformation
    (Elsevier, 2020-01-08) Donoghue, Jack; Davis, Alec E.; Daniel, Christopher S.; Garner, Alistair; Martina, Filomeno; da Fonseca, João Quinta; Prangnell, Phil B.
    Additive Manufacture (AM) of Ti–6Al–4V frequently leads to undesirable, coarse, columnar β-grain structures with a strong <100> fibre texture. In Wire-Arc AM (WAAM), it has been found that the application of a low plastic strain, by methods such as inter-pass rolling, can disrupt β columnar growth and produce a refined, equiaxed grain structure that is more randomly orientated. The origin of this desirable effect has been investigated by thermo-mechanical simulation, direct in-situ EBSD observation, as well as by real-time synchrotron X-ray diffraction (SXRD) during rapid heating. These complementary approaches have shown that, when starting with a WAAM microstructure, the grain refinement process produces a unique micro-texture represented by a four-pole motif symmetrically centred on the parent grain {100} orientations. These new β-grain orientations can be reproduced by a double {112}<111> twinning operation, which produces 12 new, unique, β-orientation variants. High-resolution orientation-mapping techniques and in-situ SXRD heating simulations suggest that the prior β does not twin during deformation, but rather the grain refinement and related texture may be caused by annealing twinning during β re-growth on rapid re-heating of the deformed AM microstructure. Although this is the first time such a unique texture has been observed in a deformed and β annealed Ti–6Al–4V material, it was only found to dominate under the unusual conditions that occur in AM of rapid heating – a fine, lightly deformed α transformation microstructure, with a very coarse starting β-grain structure