Mass transport in laser surface nitriding involving the effect of high temperature gradient: Simulation and experiment

A self-consistent diffusion model has been employed to simulate the atomic nitrogen transport in titanium alloy with the coupling effect of both initial activated nitrogen concentration and high temperature gradient. Transport equations are solved through coupling the transient heat transfer equation and incorporating the surface physical phenomena in the governing equations as boundary conditions. For comparisons, laser gas nitriding of Ti6Al4V alloy has been performed by means of a pulsed Nd:YAG laser irradiation in a controlled nitrogen chamber. Results indicate that the simulated nitriding molten pool geometry and nitrogen concentration profiles are broadly consistent with the experimental results for various laser intensities and nitrogen pressures. Due to the combined driving of both concentration and temperature gradient, a rapid increase of nitrogen concentration profile during the heating stage is observed. While in the cooling stage, the redistribution of nitrogen in substrate and the local maximum of nitrogen close to the surface are mainly caused by the transient temperature gradient which is confirmed by both the experimental and simulated results. In addition, the influence of laser intensity on nitrogen transport is strongly related to the initial nitrogen concentration at surface. Especially, when the initial nitrogen concentration at surface approaches to the maximum concentration limit of nitrogen in titanium nitrided layer, slight drops of nitrogen concentration in molten zone are observed with the increase of laser intensity. This indicates that the earlier formed diffusion barrier due to the maximum concentration limit will impede the further penetration of atomic nitrogen and thus lead to the relatively lower concentration.