The Influence of Microstructure Heterogeneity on the Tensile Deformation Behaviour of Cold Metal Transfer Additively Manufactured Ti6Al4V

Cold metal transfer (CMT)-based wire-arc directed energy deposition is an effective and novel high-speed additive manufacturing method for making titanium alloy engineering parts at low costs. However, the large-scale industrial implementation continues to lag due to concerns related to the potential formation of heterogeneous microstructures and anisotropic properties in the as-deposited state. This research aims to address these concerns by advancing the understanding of the deformation behaviour of the CMT fabricated Ti6Al4V alloy through systematic correlations between microstructure characterisation and tensile tests. We show that the CMT process can effectively reduce columnar grain formation and promote isotropic mechanical properties. However, the resulting microstructure is dominated by grain boundary α (GBα) variant selection, which leads to the presence of coarse, similarly oriented α-colonies. It is shown that, irrespective of the loading direction, a diffuse and heterogeneous deformation distribution is developed during tensile loading. This is further related to the presence of heat affected zone (HAZ) banding, GBα and adjacent regions consisting of soft and hard α-laths within distinct parent β-grains. The soft regions are oriented favourably for basal or prismatic slip. Our findings indicate that the stress concentration along the GBα/soft α-colony and hard basketweave α-laths leads to premature fracture and reduced ductility in both horizontal and vertical sections of the CMT deposit. The results underscore the importance of understanding and avoiding GBα variant selection and macrozone formation during CMT. Based on these findings, we make recommendations for microstructure tailoring and mechanical performance manipulation during post-CMT heat treatments.