The role of surface roughness on hydrogen embrittlement of electron beam powder bed fusion Ti−6Al−4V

Hydrogen−assisted cracking remains a critical challenge for titanium alloys exposed to hydrogen environments. In this study, the hydrogen−assisted cracking of electron beam powder bed fusion (PBF−EB) Ti−6Al−4V was investigated, focusing on the effect of surface condition. Slow strain rate tests (at 10 −6 s −1 ) were carried out under in−situ electrochemical charging at −5 mA cm −2 in a 3.5 wt.% NaCl solution on specimens with machined (Ra of 3 μm) and as−built (Ra of 27 μm) surface conditions. Fracture behavior and hydrogen−affected microstructural features were characterized using scanning electron microscopy and scanning transmission electron microscopy. Hydrogen−assisted degradation was observed for both surface conditions. However, the depth of the hydrogen−affected fracture zone decreased from approximately 2 mm in machined specimens to approximately 0.5 mm in as−built specimens. Correspondingly, the reduction in ductility was lower in as−built specimens (20 ± 13%) compared to machined specimens (35 ± 25%). Fracture morphology exhibited a transition with increasing distance from the surface. Within 100 to 500 μm from the surface, regions of higher hydrogen content showed hydride−assisted quasi−cleavage fracture. In contrast, regions at greater depths dominated by solute hydrogen showed mainly transgranular cracking controlled by phase−dependent plasticity within the lamellar α/β microstructure. The continuous β−phase network characteristic of the PBF–EB microstructure appeared to promote hydrogen transport, leading to a relatively high apparent effective hydrogen diffusivity of approximately 1.6 × 10 −11 m 2 s −1 . Under the investigated conditions, the results indicate that the as−built surface condition reduces the extent of hydrogen−assisted cracking in PBF−EB Ti−6Al−4V. • As-built surfaces reduced H-affected fracture depth (2 mm to 0.5 mm). • As-built specimens showed lower HE index than machined (20% vs 35%). • Fracture changed from hydride-assisted quasi-cleavage to transgranular as H decreased. • The β-phase network enhanced H transport (D eff of 10 -11 m 2 s -1 ).