ANALYSIS OF THE PHASES DEVELOPED BY LASER NITRIDING TI-6AL-4V ALLOYS

Laser nitriding Ti-6Al-4V (Ti64) alloys produces complex microstructur es in the solidified melt pool developed during the processing. The mi crostructures contain mainly titanium nitrides and the alpha (or alpha ') titanium phase in the matrix. Difficulties have been encountered in identifying the phases and examining the level of residual stress ind uced by the processing, because of the rapid cooling rate and the nitr ogen concentration introduced during the laser gas alloying which have a combined influence in determining the lattice parameters of the pha ses. The present work has investigated the microstructures formed main ly in a Ti64 alloy under pure argon and various nitrogen concentration environments during the processing, and also to a lesser extent comme rcial purity titanium (under 20 vol.% N only) studied for comparison p urposes, using metallography, X-ray diffractometry (XRD) and X-ray pho toelectron spectroscopy (XPS). A 5 kW continuous CO2 laser with a spin ning beam was used for the processing. As a result of this analysis, t he identification of the TiN0.3 phase formed in a Ti64 alloy was confi rmed, and the influence of the nitrogen concentration was elucidated. This is considered to be helpful for a better understanding of the las er processing process. (C) 1997 Acta Metallurgica Inc.