On the observation of annealing twins during simulating β-grain refinement in Ti–6Al–4V high deposition rate AM with in-process deformation
Jack Donoghue, A. Davis, Christopher Stuart Daniel, Alistair Garner, Filomeno Martina, João Quinta da Fonseca, P.B. Prangnell
Abstract
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.