Understanding the Significance of Microstructure Heterogeneity in Wire-Arc Based High Deposition Rate Additive Manufacturing of Ti6Al4V Alloy

Abstract

Additive Manufacturing (AM) is a manufacturing process which produces near net-shape components by progressively building them layer by layer. There has been considerable interest in using additive manufacturing of titanium alloys for aerospace, due to advantages such as part cost reduction and increased flexibility in both manufacturing and product design. However, using Ti6Al4V in AM processes can lead to microstructural heterogeneity and, in particular, a coarse columnar beta grain structure, which results in mechanical anisotropy. In this research, the titanium alloy Ti6Al4V walls produced by the Wire Arc-Based Additive Manufacturing (WAAM) process was investigated, which has a high deposition rate suitable for building large-scale aerospace components. In the first part of this study, an as-deposited, single bead wide parallel Ti6Al4V wall built with the WAAM process was characterised with a multi-scale approach using various microscopy techniques, in order to understand and investigate the mechanisms involved that relate to the formation of their heterogeneous microstructure. This work was underpinned by using thermal simulations to study the effect of rapid heating on the transus temperature and to replicate the thermal cycle experienced in HAZ bands. The nature of the finer, weak contrast fusion boundary segregation bands was also examined, and microsegregation of Al, V and Fe was found, with the use of EPMA compositional analysis. The transformed microstructures of the as-deposited (unrolled) samples were then compared with an inter-pass rolled material, which incorporated an additional process step that lightly deformed the deposited material between added layers and results in a recrystallised finer beta grain structure. This resulted in an improvement in the mechanical properties, such as the tensile strength and overall anisotropic behaviour, which was found to be due to the refinement of the alpha-lath microstructure and the formation of a more uniform prior beta grain size and texture randomisation. Suggestions for the overall reduction in alpha lamellar spacing seen during the inter-pass rolling build process were also explored. The effect of the subsequent related transformation microstructure heterogeneity on the mechanical behaviour was investigated, in both standard Ti6Al4V WAAM materials and samples subjected to inter-pass rolling. Full-field strain maps were produced by Digital Image Correlation (DIC), using tensile samples loaded in different orientations. In the unrolled specimens, the results showed that the strain localisation behaviour was highly dependent on the loading orientation. This was then followed by electron backscatter diffraction (EBSD) texture analysis, to explain the effect of local texture on the strain localisation behaviour. Overall, the beta grain refinement, through the application of inter-pass rolling was found to be very effective at homogenising the strain distribution. However, an abnormally rolled condition was found to produced locally coarse grains, which was shown to be mechanically far more detrimental than the original coarse beta columnar grain structure.

Date of Award	31 Dec 2019
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Philip Prangnell (Supervisor) & Joao Quinta Da Fonseca (Supervisor)