Nitride formation in titanium based substrates during laser surface melting in nitrogen-argon atmospheres

Laser nitriding of a commercial purity (CP) Ti and a near gamma TiAl based alloy in Ar-N-2 gas mixtures has been investigated both experimentally and theoretically. Observations indicate that the nitride TiN forms in CP Ti, while TiN and Ti2AlN form in the near gamma alloy. Melt dimensions and temperature distributions, that correlate well with the phases observed, have been theoretically simulated using a three-dimensional analytical model of the heat transfer problem. A marked increase in melt volume is observed when the nitride phases are formed, and this has been attributed to the increase in the rate of laser power absorption induced by nitride formation rather than the exothermic heats of formation. Thermocapillary driven fluid hows, which increased with increases in the N-2 concentration of nitriding atmosphere and the laser-material interaction time play a major role in determining the shape and surface contours of the melt pool. (C) 1999 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.