Oxidation induced mechanisms during directed energy deposition additive manufactured titanium alloy builds

Abstract

• Oxidation during out of chamber DED Ti6242 occurs concurrently in the liquid and solid state. • Oxygen species reverse the Marangoni force, causing centripetal flow, deepening the molten pool and assisting pore escape. • Oxygen can promote the formation of tetra modal α phase microstructure in Ti6242. • The α-phase Ti6242 alloy is more resistant to oxygen diffusion than Ti-6Al-4 V as SiO 2 inhibits TiO 2 formation. • New DED-AM industrial practices can be adopted to tailor ti alloys microstructure (texture, phases, and porosity). To prevent oxygen contamination, additive manufacturing (AM) techniques normally operate in an inert gas chamber (GC). An alternative method, useful for large builds and components repair, is the application of localised shielding gas (LSG). The effect of oxygen contamination on Ti6242 during directed energy deposition (DED) AM using an inert GC compared to LSG was investigated by in situ synchrotron x-ray experiments. When processing in LSG mode, the amount of oxygen absorbed from the atmosphere was sufficient to reverse the Marangoni flow leading to an alteration of the molten pool geometry and strongly influencing defect formation. Microstructural analysis reveals that, at high oxygen levels, the commonly developed α' martensitic microstructure was completely suppressed, forming precipitation of a tetra modal microstructure of α phase consisting of globular, primary and secondary lamellae (in colonies) and basketweave structure. These results help elucidate the influence of oxygen contamination in additively manufactured Ti alloys, potentially enabling improved industrial practices for AM of titanium alloy.