| Due to the features of low density, high strength to weight ratio and good corrosion resistance, titanium alloy can replace steel in manufacturing the highlight functionality component for aviation transmission mechanism. But using titianium alloy for the application of gear and other transmission parts, its low hardness, poor wear resistance and prone to serious adhesive wear may limit its applications. For solving this problem, it is necessary to improve the surface hardness and wear resistance of titanium alloy. Surface hardening technology seems to be an effective way to improve the surface hardness and tribological properties of titanium alloy, which could lead the surface properties of titanium gear to meet the required operational performance. In this thesis, TC21-DT titanium alloy was taken as the research object. To obtain the manufacturing technology of titanium gear, the surface hardening technology and mechanism for improving the surface hardness of titanium alloy was investigated, and then the grinding process of the hardened titanium alloy were also studied. Based on the above research, the adhesive wear and contact fatigue performance of titanium gear were further discussed.The main research contents of the thesis are as follows:1. Based on the analyzing of the unsteady diffusion process of boron into TC21-DT titanium alloy by Rare Earth addition-Boriding(RE-B), the accelerated kinetics model of the boriding process by rare earth addition and the model of boride layer growth were established. The microstructural evolution and phase transformations and surface hardness of boride layers were also investigated. The results showed that the boride layers included TiB2 layer at the top and TiB sub-layer. The surface hardness of boride layer was about 3200HV0.01. Moreover, the hardness of boride layer emerged in a gradient distribution. Based on the above analysis, the surface boriding of TC21-DT titanium alloy used RE-B technology was finally achieved.2. By analyzing the characteristics of hard phase particles in the boride layer, the uniaxial compressive model and the surface wear modeling for boride layer were established. Furthermore, based on evaluating the size-effect of hard phase particles on the tribological properties of boride layer, the tribological properties were also obtained. The results showed that the surface plastic deformation and adhesion of asperities were reduced by the strengthening effect of hard phase particles, which consequently caused the severe adhesive wear of titanium alloy to be decreased.3. The strain gradient model of grinding contact zone during grinding of the boride layer was firstly established, and then the specific grinding energy model was also proposed. Moreover, the effects of grinding parameters and grinding wheel on grinding force, grinding temperature and specific grinding energy were discussed. The results showed that when the size of material remove was equivalent to the size of hard phase particles, the shear strength of grinding surface was increased, however, the elastic deformation was reduced, which caused the friction between abrasive grains and boride layer to be decreased.4. The surface characteristics of grinding boride layer, such as surface roughness, crack, plastic deformation, microhardness, burning surface and the surface residual stress were analyzed. Meantime, by evaluating the effects of grinding parameters and hard phase particles on the surface integrity, the grinding process for boride layer to achieve the desirable grinding surface integrity was obtained.5. The contact fatigue performance and the fatigue failure mechanism of RE-B TC21-DT titanium gear were analyzed through the contact fatigue test. The results showed that higher grinding surface integrity of titanium gear, produced by RE-B treatment and grinding process, led the adhesive wear resistance and fatigue performance of titanium gear to be improved. |
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