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.