CRACK-FREE HARD SURFACES PRODUCED BY LASER NITRIDING OF COMMERCIAL PURITY TITANIUM

The results of laser nitriding of commercial purity titanium (CPTi) in diluted and undiluted nitrogen environments are described in this paper in terms of surface cracking, surface hardness, hardness profile and microstructure. Surface cracking can now be totally eliminated either by nitriding in a dilute nitrogen atmosphere or by controlling the laser parameters and gas now rate in a pure nitrogen environment. The hardness developed in a crack-free trail surface produced in a dilute nitrogen environment was less than half of the maximum hardness that can be attained using pure nitrogen. The surface hardness after glazing in a dilute environment was dependent on the nitrogen concentration in the mixture, and increased with increasing nitrogen content. The nitrided layer generated in a dilute environment possessed a fine needlelike structure, the needle size increasing with increasing nitrogen concentration in the gas mixture. In a pure nitrogen environment the nitrided layer possessed a dendritic structure of titanium nitride. The trail surfaces were found to be relatively smooth when nitrided at 50 mm s(-1) using 2.8 kW laser power with a 5 mm defocused distance.