Browsing by Author "Prangnell, Philip B."

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    Confirmation of rapid-heating β recrystallization in wire-arc additively manufactured Ti-6Al-4V
    (Elsevier, 2020-08-13) Davis, Alec E.; Caballero, Armando; Prangnell, Philip B.
    The coarse columnar β grains in Ti-6Al-4V WAAM can be refined by relatively low strain inter-pass deformation. Simulation, with rapid heating, has shown this may partly occur by a novel recrystallization mechanism that involves twinning during β regrowth through the α-β transus, as a result of the prior deformation promoting faults in the α-β interface, which produces a unique micro-texture in each parent β grain. Here, this potential mechanism has been investigated further, using a different deformation mode – uniaxial tensile deformation rather than plane strain compression – to enable the texture contribution from conventional recrystallization to be more unambiguously discriminated. The tensile-deformed samples are shown to produce the same unusual unique micro-texture seen previously, at low strains, despite the different deformation mode, but this disappeared at higher strains, which provides more evidence in support of this new rapid heating β recrystallization mechanism
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    Effect of deposition strategies on fatigue crack growth behaviour of wire+ arc additive manufactured titanium alloy Ti-6Al-4V
    (Elsevier, 2021-04-03) Syed, Abdul Khadar; Zhang, Xiang; Davis, Alec E.; Kennedy, Jacob R.; Martina, Filomeno; Ding, Jialuo; Williams, Stewart; Prangnell, Philip B.
    The influence of three deposition strategies on the fatigue crack growth behaviour of Wire + Arc Additive Manufactured (WAAM) Ti–6Al–4V has been investigated in the as-built condition. Test samples were prepared using single pass, parallel pass, and oscillation deposition strategies and tested with cracks propagating parallel and normal to the plane of deposition. Due to the higher local heat input, the oscillation build exhibited a significantly coarser columnar β grain structure as well as a coarser transformation microstructure, compared to the single pass and parallel pass builds, which were very similar. Among the three build methods, the lowest crack growth rates were found with the oscillation build. The crack growth data was found to broadly fall between that of a recrystallized α (mill-annealed) and β annealed wrought material, with the oscillation strategy build behaving more similarly to a β annealed microstructure. The fatigue crack growth rate was lower when cracks were propagated perpendicular to the build layers. For each build strategy, a greater microstructural influence on crack growth rate was found at lower levels of stress intensity factor range (<25 MPa m1/2). However, the anisotropy and scatter in the data was much more significant in the case of the oscillation build. These differences have been attributed to the stronger α microtexture heterogeneity present in the oscillation build, which led to a greater crack deflection and bifurcation, giving rise to lower crack growth rates and a higher sensitivity to the anisotropy caused by the directional β grain structure.
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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.