Using the laser gas nitriding method, the surface nitriding treatment of the titanium alloy is realized. The results show that when the power density is greater than 6.5 ×, the main product is TiN. Since the saturated vapor pressure and evaporation rate of Al are higher than that of Ti, an aluminum-lean layer is formed on the surface of the molten pool, which promotes the nitridation of the surface layer Ti. Thermodynamic analysis shows that the reaction to generate TiN in the laser melt pool has more thermodynamic advantages than the reaction to generate AlN.
Titanium and its titanium alloys have been rapidly developed and applied in the aerospace field since the 1950s because of their advantages such as high specific strength, good corrosion resistance, and high temperature resistance, and have become the main structure of contemporary aircraft and engines. One of the materials. Replacing stainless steel with titanium alloy can not only reduce the weight of the aircraft, but also improve the structural efficiency. At present, titanium and its titanium alloys account for an increasing proportion in aircraft materials. According to reports, the proportion of titanium alloys varies among different models. In passenger aircraft Boeing 777, it is 7%, transport aircraft C-17 is 10.3%, fighter F-4 is 8%, F-15 is 25.8%, F-22 is 39% . However, due to the high price of titanium alloy and poor wear resistance, its application field is limited. Therefore, it is very important to properly modify the surface of titanium and titanium alloy to improve its surface wear resistance and surface hardness.
Titanium nitride has the advantages of high hardness, wear resistance, corrosion resistance and so on. Using it as a protective layer can greatly increase the surface hardness, improve the wear resistance, and effectively extend the service life. A. Walker  et al. Studied laser nitriding on the surface of titanium and its alloys. In recent years, at home and abroad, ion beam enhanced deposition (IED) , plasma nitridation (PN) and other methods have been used to form titanium nitride films on the surface of titanium alloys. However, these methods have the disadvantages of thin coating thickness, low bonding degree, and easy peeling.

To this end, this article uses laser gas nitridation method to nitridize the surface of titanium alloy. This method improves the nitriding effect and simplifies the process. This paper mainly studies the effect of laser power density on nitriding, and analyzes the mechanism of laser nitriding.

  • 1. Experimental materials and methods
    Experimental material is TC4 alloy. After polishing and cleaning, blow dry to use. 500WCO2 continuous laser is used for laser nitriding. Preheat N2 gas to 200 ° C. The hot nitrogen beam is coaxial with the laser beam and reaches the sample surface at the same time. The laser scanning speed is 100 mm / min, and the laser power density ranges from 0.85 × 105 to 8.5 × 105 The phase analysis was carried out using Japanese Rigaku D / MAX-RB X-ray diffractometer.
  • 2. Experimental results and discussion
    The XRD results of the sample surface after nitriding treatment are shown in FIG. 1. As can be seen from Figure 1, compared with the original sample (Figure 1a), when the power density reached 3.7 ×, titanium nitride was formed on the surface. With the increase of laser power density, the phenomenon of nitriding is gradually obvious. When the power density is greater than 7.8 ×, it can be seen from Figure 1 (c), (d) that the main phase of the nitride layer is TiN at this time, and a small amount of Ti2N. This is because when the power density is low, on the one hand, a laser melting pool cannot be formed on the surface of the titanium alloy, and also nitrogen gas cannot be sufficiently activated, so it is difficult to form nitriding.
  • 3. Nitriding mechanism analysis
    After pre-heat treatment, the nitrogen molecules absorb part of the heat energy, so that the intramolecular energy increases, and then it is easy to become nitrogen ions when it is irradiated by laser. In addition, the hot nitrogen beam can promote convective movement in the molten pool, so that Ti can fully react with N atoms, and promote the progress of the nitridation reaction.
    It is worth mentioning that, in the Ti-6Al-4V alloy used in the experiment, theoretically, AlN should be generated in the nitride, but it has not been detected in practice. The reasons can be summarized as follows:
    First, on the surface of the laser melt pool, in addition to melting and nitridation processes, there is also an evaporation process of metal elements. Which kind of element evaporates quickly, correspondingly, there is less nitride generated on the surface by metallurgical reaction between this element and nitrogen. In the course of this study, the laser nitridation process was carried out under an industrial nitrogen atmosphere. Under this condition, the surface titanium and aluminum have obvious evaporation phenomena. At different temperatures, the results of the saturated vapor pressures PTi, PAl (in Pa) and evaporation rates vTi and vAl (in of Ti and Al are shown in Table 1.
    According to Table 1, in the liquid laser molten pool, the saturation vapor pressure and evaporation rate of Al are much higher than that of Ti. Therefore, the atomic percentage of Al in the molten pool is much lower than that of Ti, thereby forming an aluminum-depleted layer on the surface of the molten pool. It promotes the nitriding process of Ti in the surface layer.
    In addition, from a thermodynamic point of view, the results of Gibbs free energy calculation for the reaction of nitrogen and titanium to form TiN are shown in Table 2. It can be seen that the △ G of the formation reaction of TiN and AlN is negative. At the same temperature, TiN reacts? G is smaller than AlN. Therefore, the reaction to generate TiN is more advantageous.

4 Conclusion
Laser power density has a great influence on the nitriding effect. When the power density is greater than 6.5 ×, a nitride layer mainly composed of TiN is generated. At the same temperature, the saturation vapor pressure and evaporation rate of Al are much higher than that of Ti. The formation of an aluminum-lean layer on the surface of the molten pool promotes the nitriding process of Ti. From a thermodynamic point of view, the reaction to generate TiN is more advantageous.