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

The surface modification of a Ti-6Al-4V alloy using a gas tungsten are, 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, alpha-titanium, and al pha "-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 con stitution in the resolidified surfaces were determined and analyzed, and th e mechanism of the formation of titanium nitrides is discussed. The results show that the nitriding kinetics obey parabolic laws and are, therefore, c ontrolled by nitrogen diffusion. The nitrogen-concentration depth profiles, calculated using Fick's second law of diffusion, are compared to experimen tal nitrogen depth profiles, showing satisfactory agreement.