Effect of oxygen content in local shielding atmosphere on cladding quality of Ti6Al4V during annular laser metal deposition

In annular laser metal deposition (ALMD), Ti6Al4V alloy is sensitive to oxygen. In this research, a coaxial double-layer shielding gas nozzle was used to provide a local inert gas atmosphere. The influence of the oxygen concentration in local shield on the cladding quality was evaluated regarding the appearance, chemical composition, microhardness, microstructure and phase constituent. The results show that the surface discoloration of cladding layer is related to oxygen contamination level. When the oxygen level in the local atmosphere is below about 1000 ppm, a shiny silver surface of the clad can be achieved, and the concentration of oxygen and nitrogen in the deposited material is below those allowable for ASTM Grade 5. In addition, as the oxygen concentration decreases, the microhardness of the samples decreases gradually, and microstructural characteristics experiences a change from the blocky plate α' martensite within the coarse prior β-grain boundary to the acicular α' martensite within the fine β-grain boundary.