Heat treatment effects on the hydrogen embrittlement of Ti6Al4V fabricated by laser beam powder bed fusion

Abstract

Hydrogen embrittlement (HE) behavior of Ti6Al4V produced by laser beam powder bed fusion (PBF-LB) was investigated via tensile tests with/without electrochemical H-charging, and the heat treatment (HT, 950 °C for 2 h) effects on HE mechanisms were also explored. Decreased resistance against HE after long-term H-charging is verified for the HT PBF-LB Ti6Al4V and this phenomenon is not obvious for the as-built (AS) PBF-LB counterparts. To understand this observation, we carried out microstructural characterization experiments including backscattered electron (BSE) image, electron backscatter diffraction (EBSD), transmission electron microscope (TEM), and hydrogen analyses based on X-ray diffraction (XRD) and thermal desorption spectrometry (TDS). These results and others reveal that the β phase introduced by the HT process attracts more hydrogen ingress compared with the full acicular α΄ martensitic phase in the AS PBF-LB parts. The lattice expansion of the hydrogenated β phase compels a localized stress field near the phase boundary, leading to an increased work-hardening rate under H-charging for the HT PBF-LB Ti6Al4V. Consequently, brittle titanium hydrides occur along with the α/β interfaces after H-charging, and the micro-voids preferentially initiate therein and coalesce to micro-cracks under plastic deformation. Therefore, a larger ductility loss is verified for the HT PBF-LB Ti6Al4V compared with the AS PBF-LB counterparts under the same H-charging condition.