Microstructural heterogeneity and anisotropic mechanical propertiers of titanium alloys manufactured by wire arc additive manufacturing: A review
Mohammad amin Zarei, Moein G. Shabestari, S.G. Shabestari, H.R. Abedi
Abstract
Wire Arc Additive Manufacturing (WAAM), as an advanced manufacturing method, enables the efficient production of large parts from engineering alloys such as Ti–6Al–4V. However, a review of various literature has shown that the nature of this process leads to successive thermal cycles, resulting in the formation of heterogeneous microstructures, strong columnar textures (coarse columnar β grain structure), and HAZ that contribute to the anisotropy of mechanical properties in the produced parts. This article reviews the literature related to anisotropy and heterogeneity in the microstructure and mechanical properties of metal AM parts, with a particular focus on WAAM. The main focus of the research is on the formation of columnar β grains, HAZ bands, and micro-segregations, which play a significant role in tensile deformation behavior. The results indicate that initial columnar β grains with preferential alignment in the build direction, along with the formation of α/β lamellar structures and heterogeneous distribution of alloying elements, lead to local variations in strength and ductility. Furthermore, micro-segregations resulting from non-equilibrium solidification and recrystallization phenomena in HAZ bands significantly impact the anisotropy of mechanical properties A comparison of WAAM samples with those processed by other manufacturing methods reveals that the non-equilibrium microstructure has a direct effect on reducing fatigue resistance and fracture toughness. Additionally, the mechanical properties of WAAM-Ti-6Al–4V alloys are contrasted with those of other AM methods, and solutions for reducing microstructural heterogeneity are proposed. Notably, refining the β grain structure through inter-pass rolling has proven effective in homogenizing strain distribution. In conclusion, the factors influencing anisotropy and heterogeneity in AM metal parts stem from unique microstructural characteristics or manufacturing deficiencies. The study highlights possible solutions to overcome these challenges, including grain morphology, crystallographic texture, phase transformation, heterogeneous recrystallization, layer banding, and grain coarsening.