Forming characteristics and analysis of nitride layer on laser surface of Titanium alloy

Titanium and titanium alloy with high specific strength, has a comparable to steel tensile strength, good corrosion resistance, low temperature performance of stability, in the aerospace, Marine, petrochemical, car is widely used in areas such as construction, and titanium alloy because of its good biological compatibility, has been widely used in biomedical aspects.However, the low wear resistance of titanium alloy under the condition of friction and contact load limits the application of titanium and titanium alloy in some fields. Therefore, the surface modification technology of titanium alloy is of great significance to expand the application field of titanium alloy. In the surface modification method of titanium alloy, laser gas nitriding is regarded as a potential new and effective surface nitriding method, which has the advantages of strong combination of nitriding layer and matrix, small workpiece deformation, short preparation time, deep nitriding layer and so on. Since K Atayamal6 [1983] used continuous wave CO2 laser for surface nitriding of titanium plates for the first time, in recent decades, the laser nitriding method of titanium and titanium alloy has been extensively studied by scholars at home and abroad, and great progress has been made.

Conclusion: 1) The surface nitriding of titanium alloy can be effectively realized by using semiconductor laser. The surface hardness of nitriding layer can reach more than 9000 MPa. Compared with the base material, the hardness of nitriding layer can be increased by nearly 3 times. The results show that it is feasible to use semiconductor laser to nitride Ti-6Al-4V alloy.

2) The surface color of the ideal nitriding layer is golden yellow, and the surface of the overheated nitriding layer will turn silver white, and there will be black granular material perpendicular to the scanning direction; The surface morphology of nitride layer can be divided into smooth surface and rough surface. With the increase of laser power at the same line energy, the depth and width of nitride layer increased, and the forming coefficient decreased. When the laser power is constant, the depth and width of nitride layer increase with the scanning speed decreasing, and the forming coefficient decreases. However, when the line energy is 600 J/mm and the laser power is 1400W, the depth of penetration decreases instead.

3) There is convection in the molten pool, and the form and strength of convection will lead to the formation of hemispheric fusion pool, finger pool and W shaped fusion pool. In addition, multiple deformations of fusion interface will occur. The weld pool with different hemispherical morphology above mostly appears under the condition of small forming coefficient. Firstly, the weld is wider than the laser beam diameter, and The outflow of Marangoni is now at the edge of the weld pool, which will obviously affect the shape of the trajectory by increasing the weld width or depth. Secondly, when the shape coefficient is very small, the corresponding laser power is large and the scanning speed is small, the temperature of the molten pool is high and the interaction time between the laser beam and the substrate is long, there are more nitrogen particles dissolved in the molten pool, and the nitride generation reaction is an exothermic reaction, which will also have an impact on the geometry size and even the morphology of the molten pool.

Track 4) due to the molten pool shape and structure has many unknown factors, the spatial distribution of the heat and quality is not very good characterization, laser nitriding reaction at different stages in the process of dynamics and the convection and diffusion of heat and mass transfer in the molten pool, the relative contribution of uncertainty so Ma Lange, flow's influence on the laser nitriding process remains to be further research.