The effect of processing parameters on rapid-heating β recrystallization in inter-pass deformed Ti-6Al-4V wire-arc additive manufacturing

Relatively low levels of inter-pass deformation have been found to be very effective in refining the coarse columnar grain structures normally seen in Ti-6Al-4V components, built using wire-fed high-deposition-rate additive manufacturing processes. The most important process parameters that control the level of beta recrystallization - the final grain size and micro-texture - were systematically investigated by simulating the deformation and high heating rate conditions in controlled samples, to develop the process knowledge required to optimise inter-pass deformation and obtain predictable grain sizes. Overall, it was found that the level of beta-grain refinement achieved by inter-pass deformation was surprisingly insensitive to the ranges of deformation temperatures, deformation speeds, and changes to the as-deposited alpha + beta microstructure, expected within the WAAM process window, provided a minimum plastic strain of only 14% was achieved in each added layer. Conversely, the final component grain size was shown to be strongly affected by rapid grain growth on reheating above the beta transus. The texture results obtained were consistent with previous work which suggested that, with fine AM transformation microstructures, new beta-grain orientations may be produced during the alpha -> beta transformation from the development of twinning faults, induced by the prior deformation and rapid heating. In contrast, greatly increasing the starting alpha lamellar spacing - to be more similar to that found in a wrought material - greatly reduced the level of recrystallization and also appeared to change the recrystallization mechanism to favour new beta orientations produced largely by local lattice rotation.