Laser Surface Nitriding of Ti–6Al–4V Alloy in Nitrogen–Argon Atmospheres

Surface-nitrided layers of Ti–6Al–4V alloy were fabricated using a diode laser in pure and mixed gas atmospheres of nitrogen and argon. The surface morphology, microstructure, hardness, and cracks of the nitrided layers were investigated. In all gas atmospheres, the layers showed smooth and humped regions, and consisted of planar nitrogen titanium (TiN), dendrites, and acicular martensite. The surface roughness was improved dramatically as the nitrogen concentration of the atmosphere was diluted with argon. Overall, the hardness of the nitrided layer was greatest for pure nitrogen and it tended to decrease as the concentration of argon in the atmosphere increased. However, the hardness of the layer for pure nitrogen also decreased rapidly, from the surface to matrix, in comparison to the diluted nitrogen atmospheres. It was shown that the number and size of dendrites, which determine hardness, are controlled by the nitrogen concentration. The dendrites of the nitrided layer were denser and smaller in a pure nitrogen atmosphere, than in diluted nitrogen atmospheres. Longitudinal and transverse cracks were observed in the nitrided layers. These two types of cracks were decreased or even eliminated as the argon concentration of the nitrogen–argon atmosphere was increased. Therefore, by diluting the nitrogen atmosphere with argon, the nitrided layer properties, in terms of surface roughness and cracks, can be improved, but this may also cause a reduction in the layer hardness.