Laser Gas Nitriding Of Titanium Alloy And Analysis Of Mechanical Properties

In view of excellent strength and lightweight properties, Ti6A14V alloy is an attractive candidate material to guarantee the comprehensive performances for metal components in many innovative and high technological fields, ranging from aeronautic industry to biomedicine. However, hardness and tribological properties are too low to satisfy the extensive application. For this reason, surface strengthening of titanium alloys has attracted much attention. Titanium nitride with high hardness and wear resistance is an excellent material known suitable for surface strengthening. Laser gas nitriding of titanium alloy in nitrogen environment is developed as a promising technology recently to form titanium nitride layer with metallurgical bonding to the titanium substrate for modification of the traditional technology in mechanics, tribological properties. The quality of nitriding layer for industrial demands is the key to performance by adjusting the process parameters. Nevertheless, due to the influence of many process parameters, there are still some confronting problems for the actual production for extensive application. These problems are further studied and discussed in this work by experimental and theoretical analysis on the basis of previous studies. The main contents are as follows:(1) A self-consistent diffusion model is employed to simulate the atomic nitrogen transport in titanium alloy with the coupling effect of both initial activated nitrogen concentration and high temperature gradient. It is the first attempt at investigation of the effect of the maximum concentration limit of nitrogen in titanium nitride layer on nitrogen transport. It is worth noting that high temperature and concentration gradients lead to inhomogeneous distribution of nitrogen with the local maximum close to the surface which is confirmed by both the experimental and simulated results. Meanwhile, transient theoretical models are employed to investigate the effect of process parameters on the distribution characteristics of temperature and concentration field. It is used as the applicable reference for process optimization by trial-and-error.(2) The densely packed dendrites layers with high hardness and strength are prepared successfully under trial optimized condition by continuous CO2 laser and pulsed Nd:YAG laser. Meanwhile, considering the influence of substrate constraint and phase transformation during laser gas nitriding process, the distribution of residual stresses and the crack defects formed in nitriding layers are analyzed and summarized by experiments.(3) For the technical bottleneck problem of laser nitriding layer cracking, we report a laser surface nitriding process coupled with an applied stress field for the first attempt. A surprising finding is that the continuous and defect-free nitrided layer on the surface of Ti-6A1-4V alloy is achieved using a millisecond Nd:YAG laser coupled with an optimum applied stress field. The distribution rules of nitrides and stress states are not influenced by the applied stress. Meanwhile, the decreased residual stress, nitrogen concentration near the surface, and surface micro-hardness of the nirided layer is associated with increased friction coefficient and the increasing applied stress levels.(4) The elastic-plastic mechanical properties induced by the inhomogeneous composition in laser nitriding layer are quantitatively evaluated by nanoindentaion and nanoscratch experiments. A composition-dependent predicted model is developed to characterize the interplay between the inhomogeneous composition and mechanical performance which resembles the concentration profile of nitride generated from diffusion by laser irradiation. It is a new feasible method to confirm the mechanical properties by extracting the depth loading curvature profile involving the composition dependence.This study is the foundation of titanium alloy for extensive applications. Moreover, this gives us confidence in the industrial application of laser surface nitriding technology for rapidly developing and new green industrial revolution.