Impact of local shielding on microstructural defects and fatigue behaviour in L-DED repaired Ti–6Al–4V

Abstract This study investigates the influence of deposition direction and shielding environment on the fatigue performance of laser-directed energy deposition (L-DED) repair of Ti–6Al–4V. Fatigue tests and microstructural analysis show insignificant differences due to deposition direction or environment. However, locally shielded repairs exhibited fatigue initiation from oxygen-rich defects, which were absent in coupons repaired under an inert gas environment. These defects were undetectable by bulk compositional analysis or X-ray micro computed tomography and were only identified through fractography. Microscopy revealed that the defects formed during solidification, with an equiaxed microstructure and enriched in oxygen and nitrogen. They were also significantly harder and stiffer than the surrounding matrix, promoting crack initiation. These features resemble low-density inclusions or hard alpha defects known from cast Ti–6Al–4V, and this study is the first to report such defects in laser additive manufacturing. Spatter analysis and infrared imaging suggest that high-temperature particles escaped local shielding and reacted with the interstitials available in the atmosphere. Subsequent deposition resulted in incomplete melting and limited diffusion, resulting in chemically distinct inclusions. These finding highlight the critical role of environment in L-DED of Ti–6Al–4V, particularly for repair applications with limited deposition height.