Microstructure characterization and evolution mechanism of titanium during laser surface nitriding

Laser surface nitriding technique has been successfully applied to greatly improve the nitriding efficiency of titanium alloy with marginally changing the surface roughness of substrate. However, due to the ultra-short action time, it is hard to understand the evolution mechanism of the microstructure during nitriding process, which is exactly the foundation to better control the quality of the nitride layer. In order to avoid the interference of alloying elements in titanium alloy on diffusivity of nitrogen, a commercial pure titanium TA2 alloy was used as the substrate in this research. Combined with finite element modeling (FEM) technology and microstructure characterizations, evolution mechanism of the microstructures during laser nitriding were systematically analyzed. Results indicate that microstructures of TiN -, α-Ti(N) - and (α’ + α) are formed successively from the surface to the center. In the laser heating stage, TiN phase and stabilized α-Ti(N) phase formed, while in the cooling stage, α’ martensite formed prior to the surrounding α phase, result in continuous orientation transition in α grains. Due to the gradual decrease of nitrogen contents from the surface to the center, α’ martensite laths coarsened with the directional growth characterizations.