Surface Nitriding Treatment and Tribocorrosion Properties of L-PBF Ti6Al4V

Titanium alloys are widely used in biomedicine because of their excellent corrosion resistance, biocompatibility, and other properties. Among them, Ti6Al4V is often used as orthodontic materials and bone application implants. In biomedicine, the alloy is always processed into various complex shapes to meet the needs of different fields, but the current traditional processing methods are difficult to meet the needs of mass production. Laser powder-laying additive manufacturing (L-PBF) is a new additive manufacturing technology, which has the unique advantages of fast, near-shape machining and a high degree of design freedom. This technology determines the shape data of the sample by software modeling, then melts the metal powder with high-energy laser beams, and accumulates the shape layer by layer from bottom to top to obtain the desired shape, which solves the problems of long processing cycle and low material utilization rate in traditional forging and other manufacturing methods. However, the tribological properties of titanium alloy are poor, and there are serious adhesive wear and fretting wear in the application process, which greatly limits the application of titanium alloy in many industrial fields such as drive transport parts. On the basis of the good corrosion resistance of titanium alloy, surface modification to improve the wear resistance and hardness of titanium alloy surface have become a common method. The nitriding treatment of titanium alloy can form a hard compound layer of TiN and Ti2N on the surface of the alloy to enhance the properties of titanium. The work aims to investigate the effect of nitriding temperature and original microstructure differences on nitriding results and wear and corrosion resistance of L-PBF Ti6Al4V and rolled Ti6Al4V alloys by heat treatment under high purity nitrogen environment. According to the experimental results, the microstructure evolution of Ti6Al4V alloy under different nitriding processes and the relationship between microstructure and tribocorrosion resistance were discussed. It was found that the nitride layers of L-PBF Ti6Al4V and rolled Ti6Al4V alloys were 10.2 μm and 8.23 μm, respectively. The more β phases in the rolled Ti6Al4V promoted the diffusion of N and thus obtained a wider solution zone in the sample; and the existence of high α' phase in L-PBF Ti6Al4V promoted the formation of nitride layer. The solution strengthening effect caused by the solid solution of N element in α-Ti also played a significant role in the improvement of the hardness, and the microhardness reached 1 251HV0.2 and 1 290HV0.2, respectively. After nitriding, the wear properties of L-PBF Ti6Al4V and roll Ti6Al4V alloys were greatly improved. After nitriding at 920 ºC, the wear rate of roll Ti6Al4V was the lowest under tribocorrosion and pure mechanical wear, which was (0.055±0.003)mm/a and (0.041±0.000 4)mm/a, respectively. The electrochemical test results show that the oxide layer and nitride layer produced by the nitriding process provided better corrosion resistance of the alloy. The corrosion current densities of the two alloys under tribocorrosion conditions were only 6.47×10−9 A/cm2 and 0.08×10−9 A/cm2. The wear mechanism of untreated Ti6Al4V alloy is mainly abrasive wear, while the wear mechanism of nitriding alloy is transformed into the combination of abrasive wear and adhesion wear. © 2023 Chongqing Wujiu Periodicals Press. All rights reserved.