Influence of Directed Energy Deposition Build Orientation on the Final Mechanical Properties of Large Ti-6Al-4V Components

Directed Energy Deposition (DED) is an additive manufacturing technology rapidly consolidating for the fabrication and repair of complex aerospace Ti-6Al-4V components, ensuring high deposition rates and large build volumes. However, in high build-rate regimes, process-induced phenomena, such as oxygen pick-up, overheating, and residual porosity, introduce material anisotropy. While residual porosity can largely be mitigated by Hot Isostatic Pressing (HIP), dissolved oxygen cannot be removed by any post-processing method, raising uncertainties regarding the final mechanical properties of large DED parts and limiting the predictive performance of design simulation tools. To fully exploit the advantage of design for additive manufacturing methods, it is fundamental to assess how the DED process influences the material performance of large Ti-6Al-4V builds. This work investigates the effect of build orientation and process parameter set on the final mechanical behaviour of DED Ti-6Al-4V samples in high-build-rate conditions. Compared to cast and annealed Ti-6Al-4V, all the realized samples exhibit an average 14% and 22.5% increase in yield strength and ultimate tensile strength, respectively, but 38% lower Elongation at Break. Microstructural analysis reveals that both the process parameter set and the build orientation influence DED material anisotropy, particularly ductility, with vertically built specimens showing 68% and 33% higher elongation at break than longitudinal and lateral specimens, respectively, highlighting a direction-dependent mechanical response consistent with the columnar prior β-grain morphology and extension. LECO analysis confirms that oxygen pick-up also occurs in an enclosed deposition environment and strongly increases material strength at the expense of ductility. Oxygen content varies with build orientation, from 0.15 ± 0.02 wt.% in vertical to 0.19 ± 0.01 wt.% in longitudinal specimens. Finally, fracture surface analysis indicates that lack-of-fusion defects act as preferential crack initiation sites.