Abstract
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 β 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 β-grain refinement achieved by inter-pass deformation was surprisingly insensitive to the ranges of deformation temperatures, deformation speeds, and changes to the as-deposited α + β 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 re-heating above the β transus. The texture results obtained were consistent with previous work which suggested that, with fine AM transformation microstructures, new β-grain orientations may be produced during the α → β transformation from the development of twinning faults, induced by the prior deformation and rapid heating. In contrast, greatly increasing the starting α 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 β orientations produced largely by local lattice rotation.
Highlights
The α + β Ti alloy Ti-6Al-4V (Ti64) is widely used in the aerospace industry due to its excellent mechanical properties
To obtain a broad overview of the effect of the process variables on the refined β-grain size seen that can be obtained with inter-pass deformation in the Wire-Arc AM (WAAM) process, plane-strain compression (PSC) deformation and rapid heating simulations were first conducted on a large matrix of samples to systematically vary the strain, strain rate, deformation temperature, and the starting microstructure prior to deformation
The current work sought to determine the influence of the most important process parameters that affect the level of β recrystallization and the final grain size, when inter-pass deformation is employed to refine the coarse β-grain size normally seen in Ti64 components built using the WAAM process
Summary
The α + β Ti alloy Ti-6Al-4V (Ti64) is widely used in the aerospace industry due to its excellent mechanical properties. Wire-fed, high power (> 2 kW) directed energy deposition (DED) AM processes are capable of efficiently producing parts with kilogram-per-hour deposition rates and build envelopes of several metres, which makes these processes attractive for the manufacture of larger-scale aerospace components [3]. Such processes employ lasers [4], electron beams [5,6], or arc-plasma welding torches [7,8,9,10,11,12], as directed energy heat sources to deposit layers 1–2 mm thick. Because of the larger melt pool dimensions, the solidification and cooling rates are considerably lower (< 102 °C s−1) than experienced in powder bed technologies (104–5 °C s−1) [13,14,15,16,17] and as a consequence, components produced by high deposition rate DED can suffer from more severe microstructural heterogeneity and mechanical anisotropy [7,14,18]–[27]
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