Microstructure evolution and mass transfer behavior during multi-pass laser surface nitriding process on titanium alloy

Laser surface nitriding technique is successfully applied to improve the surface modification efficiency of titanium alloys. Moreover, adjustment of nitride layer thickness can be achieved by applying multi-pass laser surface nitriding. Mass transfer behavior of laser surface nitriding could be totally different from the steady-state gas nitriding processes because it involves rapid heating and cooling procedures for many times, which is systematically analyzed in this research by conducting different number of laser surface nitriding passes and in-situ laser quenching after laser surface nitriding experiment on commercial pure titanium TA2 alloy. Results indicate that the thickness of nitridation layer increases with the increase of nitriding passes. After three passes laser surface nitriding, the depth of nitridation region can reach to about 56 μm. During multi-pass laser surface nitriding process, the pre-formed TiN layer will be partially decomposed under the thermal impact of subsequent laser processing. The decomposed nitrogen will further diffuse deeply into the substrate along the nitrogen concentration gradient and combine with the nitrogen from the environment to continually offer the nitrogen source during the entire laser nitriding process with high processing efficiency.