Laser gas nitriding of pure titanium using CW and pulsed Nd:YAG lasers

The influence of parameters of continuous wave (CW) and pulsed Nd:YAG lasers on the laser gas nitriding (LGN) process of pure titanium models made by selective laser melting is presented. The morphology, microstructure, hardness and chemical composition of the nitrided layers are analysed. The layers formed by the Nd:YAG laser in CW and Q-sw modes show high roughness and porosity, respectively. The morphology of the layers formed by ms-pulsed laser varies from rough to smooth by increasing the peak power from 1 to 3 kW but cracking may occur. The thickness of the layers by using CW and pulsed lasers varies from 10 to 60 μm and the thickness of the layers in Q-sw mode varies from 1 to 2 μm. For pulsed laser experiments, cracking is avoided by using a stepwise pulse shape to decrease the cooling rate. X-ray photoelectron spectroscopy shows that the layers have high nitrogen concentration at the top surface and by grazing incidence X-ray diffraction (GIXD) it is possible to realize that the layer has a gradient TiN/TiN0.3 composition. The smooth and defect free TiN layers formed by ms-pulsed laser do not have any detrimental effect on the fatigue strength of the specimens.