High power diode laser nitriding of titanium in nitrogen gas filled simple acrylic box container: Microstructure, phase formation, hardness, dendrite and martensite solidification analyses

This work presents a detailed microstructure-property correlation of high power diode laser nitrided titanium based on its dendrite and martensite microstructures, structural phase and microhardness. For the laser nitrided titanium, cooling rate was estimated for the first time using empirical power law relationship by considering the martensite width and dendrite arm spacings and the results were validated by analytical thermal model. A processing window was formulated for laser gas nitriding of titanium by relating the experimental parameters such as laser processing parameters, nitrogen gas pressure and titanium nitride phases. The novelty of this work is demonstration of laser nitriding of titanium in laminar flow nitrogen gas environment using a simple acrylic container setup that avoids design and fabrication of a complex gas nozzle delivery system. In this setup, a thin transparent acrylic sheet was used as optical window instead of expensive quartz glass, to transmit high power diode laser beam at 980 nm wavelength to irradiate the substrate work piece in a closed container. This simple laser nitriding technique has created characteristic golden colored TiN surface with high surface hardness. Dendrite and martensite microstructures of the laser nitrided titanium were analyzed by optical and FESEM microscopy. EDAX analysis revealed the presence of nitrogen in laser nitrided titanium and X-ray diffraction confirmed the TiN phases. The microhardness of laser nitrided titanium is six times higher than its substrate.