| Titanium and its alloys have some good characters such as great intensity,low density, and high corrosion resistance, which make the more attractive candidate materials for structural application in the field of national defense and civilian applications. Along with the development to the direction of high parameter, large capacity, the demand for Titanium and its alloys has increased rapidly in recent years. However,their high friction coefficient, high sensitivity to adhesive wear and fretting wear, as well as their weak resistance to wear and bad resistance to high temperature oxidation restricts the application of titanium alloys. Surface modification technique can change these surface defects of titanium alloys under the condition of keeping the virtues of titanium alloy substrate. For the reason, surface modification is one of effective methods.The aim of the present investigation is to mainly make a systematic introduction the failure modes and failure mechanism of titanium-based alloys in the power plants under equilibrated conditions in a simulated power industry environment. Based on mechanism analysis for water droplet erosion of titanium-based alloys, the relative incident velocity of water droplets of the last stage turbine blades of 600MW ultra supercritical steam turbine is 381.7 m/s by calculating. The results showed that the maximum influence depth of stress waves by waterdrop impact in titanium-based alloys is 76.45μm at this speed, so the thickness of surface modification is above and beyond the distance; the density and elastic modulus the material have little effect on impact pressure operation on the material and could be ignored.To improve water droplet erosion resistance and corrosion-resistance of titanium-based alloys (TC4 and TA2) surface remelting by micro-plasma arc (MPA) was designed. The suitable technical parameters of micro-plasma arc surface modification for each type of coating were established. Influences of different technical parameters to the micro-plasma arc remelting coating had been investigated emphatically and the most suitable technical parameters had been obtained. By means of SEM and micro- hardness tester, microsurtcutre of the coatings was investigated, and micro-hardness was measured. The results showed that micro-hardness of TC4 reaches 304HV0.3 by using air-cooled MPA remelting method and 456HV0.3 by using water-cooled MPA remelting method; micro-hardness of TA2 reaches 420HV0.3 by using air-cooled MPA remelting method 450HV0.3 by using water-cooled MPA remelting one time and 490HV0.3 by using water-cooled MPA remelting two times. It’s obviously that corrosion-resistance of the coating prepared by water-cooled MPA remelting method is much better than that of the coating prepared by air-cooled MPA remelting method:the corrosion rate of TC4 by water-cooled MPA remelting method in 3.5% NaCl solution (0.0002g/m△-h) is less than that of TC4 by air-cooled MPA remelting method (0.005g/m1·h). Both of corrosion rate of treated TC4 are below untreated TC4 (0.01g/m--h). the corrosion rate of TA2 by water-cooled MPA remelting 2 times in 3.5%NaCl solution is 0.0006g/m2·h; the corrosion rate of TA2 by water-cooled MPA remelting 1 times is 0.0003g/m2·h; the corrosion rate of TA2 by air-cooled MPA remelting is 0.001g/m2·h; all three are below untreated TA2 (0.03g/m2·h).Moreover, based on the failure modes and failure mechanism of used titanium-based alloys in the power plants and characteristics of the low-power laser surface modification, three types of laser surface alloying in-situ formed coating reinforced by the ceramic particles were designed:laser surface in-situ formed TiC/Ti、B4C/Ti and SiC/Ti high strength wear-resistant coating system. The research result shows that reinforcement coatings by the ceramic particles can be synthesized through low-power laser. The TiC/Ti composite coating had the best grain size(2μm) and hardness (about 2900 HV0.5); then the B4C/Ti composite coating is composed of the TiB2 and TiC with a hardness of about 1600 HV0.5; and finally SiC/Ti composite coating is composed of the Ti5Si3 and TiC with a hardness of about 1100 HV0.5. Because the coating show a more uniform microstructure and higher hardness, the abrasion resistance of the coating has been greatly improved in comparison to the substrate by wet sand abrasive wear testing research. The abrasion resistance of TiC/Ti composite coating is 12 and 5 times higher than that of the untreated TC4, respectively. The abrasion resistance of B4C/Ti composite coating is close to that of SiC/Ti composite coating.Due to high temperature wear failure、1fretting wear failure and corrosion failure, three corresponding types of coating were designed:NiCr-75%Cr2C3 composite coating、MoS2/Ti coating and Ta/Ti coating. By the test of performances and microstructure of NiCr-75% Cr2C3 composite coating, the metallurgical bonding exists between the coating and the substrate, and the bonding strength is high. The microhardness of coatings higher than that of the original coating after 300 thermal shock experimentations at 1000℃,which appears to signal that microhardness of coatings can enhance by post-weld heat treatment due to grain refinement in the coating after thermal shock experimentations. The composite coating also shows better abrasive wear, high temperature erosion and thermal shock resistance than those of the substrate. The MoS2/Ti composite coating with sub-micron grade structure has been prepared on Ti6A14V by laser method under argon protection. The morphology, microstructure, microhardness and friction coefficient of the coating were examined. The results indicated that the friction coefficient (0.261) of the MoS2/Ti coating were lower than those (0.623) of Ti6A14V. The composite coating exhibits excellent adhesion to the substrates, better wear resistance, fretting wear resistance and harder surface (about 1200HVo.3). The microstructure, hardness and corrosion resistance of SO42" of single-layer Ta/Ti composite coating are higher than those of multi-layers coating due to the dilution of the crack. The corrosion rate of single-layer Ta/Ti composite coating in 0.5M H2SO4 solution (0.0001g/m2·h) is less than that of multi-layers Ta/Ti coating (0.001g/m2·h). Both of corrosion rates of treated TC4 are below untreated TC4 (0.01g/m2·h). |
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