Effect of laser gas nitriding on the microstructure and corrosion properties of Ti-6Al-4V alloy

Laser surface melting of Ti-6Al-4V alloy under a pure nitrogen environment of 30 L/min gas flow rate, was carried out with 200-600 mJ laser beams, produced by a Nd-YAG pulsed laser at 5 mm defocused distance, and 0.5-3 mm/s sample traverse velocities. The microstructure, hardness and corrosion behaviour of the nitrided samples were examined, using scanning electron microscopy. X-ray diffractometry (XRD), microhardness measurements across the workpiece cross-section and anodic polarization tests in 2 M HCl solution. The microstructures consisted mainly of a thin continuous layer of titanium nitride followed by nearly perpendicular dendrites, and below this, a mixture of small dendrites and large needles, which had a random orientation. The denderitic structure was the TiN phase, and the needle phase and the phase of the matrix between the dendrites were nitrogen-enriched α′-Ti. The density of TiN dendrites decreased gradually towards the interface between the nitrided layer and the substrate. The melted zone showed a range of hardness of between 400-1300 Hv, the hardness being found to be related to the dendrite populations. An improvement in corrosion behaviour, associated with the presence of a good TiN coating, was observed. Also, the improved pitting corrosion resistance is obtained due to the microstructural changes after laser treatment.