Effect of different hydrogen Fugacities on the microstructure of additively manufactured Ti-6Al-4V

In this study we compared the microstructural changes in the presence of hydrogen of Ti-6Al-4V alloy prepared by two different additive manufacturing (AM) methods: electron beam melting (EBM) and selective laser melting (SLM). Cathodic hydrogenation of AM Ti-6Al-4V resulted in significant expansion of & beta; Ti phases due to the solute hydrogen, increasing of micro-strains, and & alpha; & RARR; titanium hydride/& beta;H transformation. It was shown that SLM Ti6Al-4V with acicular martensitic structure is more prone to hydrogen-induced phase transformation and hydrogen-assisted damage, compared to EBM. Moreover, it was revealed that different hydrogen charging environments cause different microstructural changes in EBM Ti-6Al-4V. Unlike cathodic hydrogen charging, gaseous hydrogen charging at elevated temperature reduced the process-induced micro-strains and promoted Al redistribution within the EBM Ti-6Al-4V. This resulted in massive precipitation of brittle & alpha;2-Ti3Al intermetallic compound, which act as a strong hydrogen trapping site aside from the Ti hydride phase.