EFFECTS OF NITROGEN GAS-FLOW RATES ON THE MICROSTRUCTURE AND PROPERTIES OF LASER-NITRIDED IMI318 TITANIUM-ALLOY (TI-4V-6AL)

Laser surface melting of IMI318 titanium alloy under a pure nitrogen e nvironment of different gas flow rates, was carried out with a 1.4 kW laser beam, produced by a 5 kW CO2 continuous laser at 5 mm defocused distance, and 10 and 50 mm s(-1) sample traverse velocities. The nitri ded tracks were analysed in terms of microstructures, hardness, melt d epth and surface conditions, including surface cracking. The tracks co ntained surface cracks, and showed gold-coloured melt surfaces, with d ecreasing colour intensity at lower laser energy densities and reduced gas flow rates. The microstructures consisted mainly of dendrites, th e concentration of dendrites being found to increase with increasing g as flow rates and laser energy densities. A maximum surface hardness o f 1480 VHN was developed after glazing in a pure nitrogen environment, the hardness being found to be related to the dendrite populations. L ower hardnesses were recorded with decreasing gas flow rates and micro structures containing mixtures of dendrites and needles. (C) 1998 Else vier Science S.A. All rights reserved.