Browsing by Author "Fellowes, Jonathan W."

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    Mechanical performance and microstructural characterisation of titanium alloy-alloy composites built by wire-arc additive manufacture
    (Elsevier, 2019-08-14) Davis, Alec E.; Breheny, Cameron I.; Fellowes, Jonathan W.; Nwankpa, U.; Martina, Filomeno; Ding, Jialuo; Prangnell, Phil
    A first stage study has been performed to investigate the potential for exploiting high deposition rate WAAM to print dual-alloy microstructures. Samples were built using alternating feed wires of commercially-pure Ti and Ti–6Al–4V. A high level of dilution occurred during deposition accompanied by effective liquid-phase mixing, producing a regular distribution of solidified melt tracks of approximate bimodal composition each less extreme than that of their respective constituent feed wires. The yield strength of the dual alloy composite material was approximately midway between that of the two alloys from which it was produced and exhibited a double inflection yield behaviour. Overall, because of the relatively coarse length scale there was not a significant property advantage in tensile loading above that of a chemically homogenous material, thus the main advantage of printing alternate alloys at this length scale is likely to reside more with increasing crack path tortuosity during fracture or fatigue loading. Importantly, the deposited material was found to have a refined β-grain structure suggesting that the composition gradients introduced by dual-alloy printing can disrupt the epitaxial columnar growth normally seen in WAAM deposits.
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    Microstructure transition gradients in titanium dissimilar alloy (Ti-5Al-5V-5Mo-3Cr/Ti-6Al-4V) tailored wire-arc additively manufactured components
    (Elsevier, 2021-11-03) Kennedy, Jacob R.; Davis, Alec E.; Caballero Ramos, Armando; White, M.; Fellowes, Jonathan W.; Pickering, E. J.; Prangnell, P. B.
    The nature of the chemical mixing and microstructure gradients that occur across the interface transition, when manufacturing tailored components with the two high-performance dissimilar titanium alloys (Ti-6Al-4V (Ti-64) and Ti-5Al-5V-5Mo-3Cr (Ti-5553)) by the wire-arc additive manufacturing (WAAM) process, are reported. It has been shown that a relatively long-range chemical gradient occurs during the transition between layers produced with the two different titanium alloys, due to convective mixing in the melt pool between the substrate layers and new alloy wire. This resulted in a stepwise exponential decay composition profile normal to the layers, the width of which can be described by a simple dilution law, with steep local composition gradients seen within the boundary layers at the fusion boundary of each individual layer. The alloy-alloy composition gradients had little effect on the β-grain structure. However, they strongly influenced the transformation microstructure, due to their effect on the parent β-phase stability and the β → α transformation kinetics and reaction sequence. The microstructure gradient seen on transitioning from Ti-64 → Ti-5553 was significantly more abrupt, compared to when depositing the two alloys in the reverse order. Under WAAM thermal conditions, Ti-64 appears to be more sensitive to the effect of adding β-stabilising elements than when Ti-5553 is diluted by Ti-64, because at high cooling rates, stabilisation of the β phase readily suppresses α nucleation when cooling through the β transus, and the normal Ti-64 lamellar transformation microstructure is abruptly replaced by finer scale α laths generated by precipitation during subsequent reheating cycles.
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    On the origin of microstructural banding in Ti-6Al4V wire-arc based high deposition rate additive manufacturing
    (Elsevier, 2019-01-04) Ho, Alistair; Zhao, Hao; Fellowes, Jonathan W.; Martina, Filomeno; Davis, Alec E.; Prangnell, Philip B.
    Directed energy high deposition-rate additive manufacturing processes involve a larger melt pool diameter (∼5–10 mm) and layer height (1–2 mm) than powder bed technologies, which generally leads to greater microstructural heterogeneity and more severe Heat Affected Zone (HAZ) banding. While HAZ banding has been widely reported in AM, in this study the banding features seen in samples produced by Wire-Arc Additive Manufacturing (WAAM) have been more rigorously quantified than previously possible, using statistically reliable compositional and, purpose developed, microstructure analysis mapping tools, which has provided new insight into their nature and mechanisms of formation. In addition to HAZ banding, a segregation layer has also been discovered at the fusion boundary from each melt track. This transient segregation layer and the weak coring seen, for the first time in the AM deposits, can be attributed to the lower partition coefficient of Fe in titanium, as well as limited V and Al segregation. The detailed microstructure evolution occurring in the HAZ bands has been revisited, based on new evidence, and is shown to involve both dark and white etching bands. The lower temperature dark etching region is caused not just by an increase in the α lamellar spacing due to coarsening, but also by greater chemical partitioning with temperature rise. In addition, it is shown by thermal simulation that the thin white band occurs on re-heating to just below the β transus temperature, which is shifted upwards owing to the high heating rate in AM. This white band is associated with a morphological change to a fine α lamellar colony morphology, which exhibits less solute partitioning. The mechanisms involved are discussed. The rapid coarsening that occurs in the range of the β approach curve is attributed to interface migration from β re-growth, rather than conventional surface tension driven effects, whereas the fine colony microstructure is proposed to be caused by colony nucleation, in subsequent cooling, on a low volume fraction of residual α.