The importance of preheat before laser nitriding a Ti–6Al–4V alloy

Laser nitriding of Ti–6Al–4V alloys has been developed to improve the surface properties. It is very important to control the processing conditions to produce a uniformly thick and crack-free surface layer. It has been found that the tendency to cracking is related to the volume fraction of titanium nitrides formed in the melt, and that preheating the material could avoid cracking. The present work, on a Ti–6Al–4V alloy, used a continuous 5 kW CO2 laser with a spinning beam to investigate the effect of preheating by laser glazing with overlapping tracks under argon prior to nitriding. The change in the specimen temperature was estimated using the temperature recorded at the bottom of the specimen and the tendency to cracking examined for each individual track to determine the number of preheated tracks necessary to give a crack-free surface. When preheating by laser glazing under argon was conducted prior to nitriding, not only was the tendency to cracking reduced, but also a more uniform depth of the nitrided layer was produced.