The Influence Of Nitrogen Content On Layer Crack Of Ti-6Al-4V Alloy By Laser Nitriding

In order to improve the surface hardness and wear resistance of titanium alloy and control the nitriding cracks, the laser nitriding process has been carried out on Ti-6A1-4V substrate. The influences of processing parameters on laser niriding are analyzed and the characteristics of cracks are studyed. Base on the results above and the mechanism why cracks formed, several solutions, including adjusting the content of N2in mixed gas and changing the processing parameters, are used for surface cracking control. The results show that the surface cracks can be controlled effectively by using these solutions above.This research shows that, by increasing laser power or reducing the scanning speed, the thickness of nitride layer increases. The gauss distribution of laser beam energy, the different cooling rate and the gas effect cause the convection in melting pool. By reducing the laser scanning speed, the convection time will increase. As a result, the N will diffuse sufficiently and get the uniform distribution. Ti-6A1-4V laser nitriding, substrate has a chilling effect, hindered the nitrogen deep diffusion, so the nitride layer hardness shows trapezoidal distribution. As a result, the surface hardness is the highest.The crack of nitride layer begins from the substrate and stops at the surface. There are more lattice defects and severe compositional inhomogeneity at the beginning of metal solidifying. In the condition of certain temperature and stress, the lattice defects migrate and accumulate in the nitride layer. Because of the tensile stress and the poor plasticity at a high temperature, the cracks appear in the lattice defect gathering place. The phase of the nitride layer was analyzed by XRD. It shows that while the nitrogen content of the nitriding gasis more than60%, there will be a large number of TiN in the nitride layer. The results of EDS scanning of the crack show that the segregation is higher in N element. The method of adjusting the nitriding gas content of nitrogen to control the nitride layer crack was used in this paper. The effect of crack in the nitride layer with different nitrogen is studied at the parameter of laser power2000W, scanning speed5mm/s and nitrogen gas flow rate20L/min. The results show that when the nitrogen content is40%, a nitrogen uniform distribution layer with no cracks on the surface is achieved.The hardness and wear resistance of titanium alloy are improved obviously after laser nitriding. When the nitriding gas nitrogen content is40%, the wear resistance of nitriding layer is5times of the substrate, and the hardness of surface, which can be achieved778HV0.2, is nearly2.5times of the substrate.