Mechanism of surface modification of the Ti-6Al-4V alloy using a gas tungsten arc heat source

The surface modification of a Ti-6Al-4V alloy using a gas tungsten arc, as a heat source, was studied. The experimental results show that the titanium alloy surface can be melted and nitrided using pure nitrogen or a nitrogen/argon mixture shielding atmosphere. The resolidified surfaces are 0.9 to 1.2-mm thick and contain titanium nitride dendrites, α-titanium, and α″-titanium (martensite). The average dendrite arm spacing is influenced by the electrode speed. Small titanium nitride dendrites are homogeneously distributed in the resolidified surfaces. The microstructure and phase constitution in the resolidified surfaces were determined and analyzed, and the mechanism of the formation of titanium nitrides is discussed. The results show that the nitriding kinetics obey parabolic laws and are, therefore, controlled by nitrogen diffusion. The nitrogen-concentration depth profiles, calculated using Fick’s second law of diffusion, are compared to experimental nitrogen depth profiles, showing satisfactory agreement.