In situ alloying of directed energy deposition fabricated Ti6Al4V with nitrogen

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 alpha-Ti laths and traces of beta-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.