In situ alloying of directed energy deposition fabricated Ti6Al4V with nitrogen

Abstract

The effect of increasing N₂ content in the shielding gas on the microstructure and mechanical properties of directed energy deposition (DED) fabricated Ti6Al4V is studied. In the presence of 10 % N₂ in the Ar-N₂ gas mixture, the melt-pool dimensions are higher than that fabricated in pure Ar at all powers. Irrespective of the N₂ content in the gas mixture, all fabricated builds predominantly consist of ɑ-Ti laths and traces of β-Ti. The presence of N₂ coarsens the microstructure significantly and randomizes the texture. For upto 8 % N₂ injected in the gas mixture, the absorbed nitrogen in the build is only 0.2–0.4 %, but at higher N₂ concentrations, TiN phase forms. The compressive strength of the builds fabricated in 2 % N₂ and 10 % N₂ are 25 % and 100 % higher, respectively than that of the build fabricated in pure Ar, although the presence of N₂ completely compromises plasticity. Similarly, there is a 66 % increase in the hardness of the builds fabricated in the presence of N₂. The observed improvement in the strength of the builds is discussed in the context of the microstructural changes owing to the absorption of N₂ in the alloy during the DED process.