Optimization of the mechanical properties of Ti-6Al-4V alloy fabricated by selective laser melting using thermohydrogen processes

2-step Thermo Hydrogen Process (THP) including hydrogenation and dehydrogenation steps was applied to Ti-6Al-4V alloy fabricated by selective laser melting (SLM) process to refine the microstructure and to increase the ductility of the alloy. It was observed that as-fabricated alloy's surface was composed of oxides of titanium and aluminum, which may alter the hydrogenation kinetics. The hydrogen treatment for 1 h at 650 °C, the maximum hydrogen solubility temperature of the alloy, transformed starting non-equilibrium α′-martensitic phase to β- and δ (TiH2) phases. On the other hand, very fine discontinuous β-phase was formed along with α-phase as a result of dehydrogenation at 700 °C for 18 h, which decreased the hydrogen level well below the starting value and caused transformation of δ-phase to fine α-phase. In contrast to commonly used THP treatments consisting of betatizing at high temperature, 2-step treatment prevented grain boundary α-phase formation as well as excessive grain growth. About 110% and 240% increments were detected in % Elongation (EL) and % Reduction in Area (RA) values, respectively, as a result of 2-step THP, while the strength decrease was limited to 10%. The change in ductility of the alloy was also verified by transformation of flat and shiny fracture surfaces seen in SLM fabricated alloy to a fracture surface containing equiaxed dimples after THP treatment. The hardness drop in the alloy after hydrogenation and dehydrogenation treatments was attributed to relief of residual stress and formation of equilibrium α and β-phases.