Evaluation of the Corrosion Behavior of the Laser Gas Nitrided Ti-6Al-4V

Abstract

Laser gas nitriding of Ti-6Al-4V alloy was carried out using a Nd:YAG pulsed laser under pure nitrogen environment at a flow rate of 30 l/min. The microstructure and corrosion behavior of the nitrided samples were examined using scanning electron microscopy, XRD, XPS, and anodic polarization tests in 2M HCl solution. For comparison, untreated samples were tested under the same conditions. After laser treatment, samples showed a relative flat surface with no problems of cracks or delamination of the alloyed tracks. Laser nitriding produced dendritic structures. The microstructure of the laser melted zone consisted of a thin continuous layer followed by a nearly perpendicular growth of dendrites. Below this a mixture of small dendrites and large needles with random orientation was produced. X-ray spectrum and XPS analyses from the surface of the laser nitrided specimen at different depth, confirmed that the thin top layer and large dendrites close to the surface corresponded to TiN. It can be also seen that the strong TiN peaks on the top surface gradually decrease with depth which suggests that the structure beneath the top surface is likely TiN0.3 and ά-Ti mixtures. In general, the corrosion potential of laser gas nitrided specimens was relatively nobler than the untreated sample. Furthermore, the proper laser nitrided specimen exhibited less corrosion current density, passivated more readily and also maintained a lower current density over the duration of the experiment. This was correlated with the formation of very thin, continuous TiNxOy film in an oxidation state that was confirmed by XPS analyses of the passive layers.