Titanium alloy has many excellent properties, high specific strength, low density, biocompatibility and other characteristics, therefore, it is widely used in the military industry, people's livelihood supplies and aerospace industry, however, due to the poor wear resistance of titanium alloy, the hardness is also lower, the contact with other metals corrosion hazards are relatively large, which have affected the scope of application of titanium alloy. In order to improve the physical defects of titanium alloys, we have treated the surface of titanium alloys to maximise their excellent performance and effectively expand the application range of titanium alloys. Micro-arc oxidation technology greatly improves the wear and corrosion resistance of titanium alloys, by reducing the coefficient of friction, the formation of a layer of ceramic film on the surface, preventing contact corrosion, this paper briefly describes the micro-arc oxidation technology and the advantages of the micro-arc oxidation and analyses the progress of the research and the prospects for application.
1 Micro-arc oxidation technology and advantages
Micro-arc oxidation belongs to the application of titanium alloy new technology, it introduces the oxidation of Faraday work area to the high-pressure discharge area, resulting in the surface of its alloys as well as non-ferrous metals such as A,l T,i Mg placed in the treatment solution, spark discharge, micro-arc discharge, corona and other phenomena in the electrochemical, thermo-chemical, plasma chemistry, a layer of oxidised ceramic film on the surface of the material in-situ growth, which in turn makes the material surface properties are strengthened. In general, the MAO process can be divided into four stages, before the spark stage for the first stage, the power supply is turned on, there are a large number of oxygen bubbles precipitation, the formation of an air seal, the air seal to oxygen-based, generating a layer of insulating passivation film on the surface of the specimen, the current density from zero very quickly rose to the peak, and then fell. Spark stage for the second stage, when the passivation film by the voltage continues to rise and breakdown, the current density stops falling and began to rise, the strong electric field between the electrodes to form a plasma and make the gas seal gas discharge, a large number of travelling small sparks on the surface of the specimen, this is because it is always in the region of the film layer is thin to break through the passivation film, and constantly changing the breakdown site. Micro-arc stage for the third stage, with the continuation of MAO, dispersed micro-arc will gradually appear on the surface of the sample, and fast swimming, micro-arc intensity increased, the density gradually decreased, with the extension of time the current density tends to stabilise, the resistance and the thickness of the film layer gradually increased, the local arc stage for the fourth stage, the later stages of the MAO, the number of arc spots on the surface of the sample is gradually reduced, the arc spot obviously slows down the mobile speed, and the current becomes lower density. Through the above four stages, the anode electrochemical reaction, plasma bombardment, melting, diffusion, sintering phase transition, solidification and other processes can be formed with the substrate combined with a strong and relatively thick ceramic film layer. Micro-arc oxidation technology has the following advantages: ① Because the ceramic film is in situ growth, so the combination is relatively high. ② Adjust the composition of the electrolyte and process conditions, so that the composition and performance of the film layer changes, the functional design of the film layer is effectively improved. ③ Instantaneous discharge temperature is relatively high, the temperature Krys-mann calculates that it can reach 8000K, Van believes that the temperature exceeds 2000℃, the oxide will melt in this region, the substrate temperature does not exceed 300℃, the performance of the substrate will not deteriorate. ④Even the complex shape of the workpiece can be formed on the inner and outer surfaces of the film; ⑤Simple operation, cost-effective, less pre-treatment process, do not need high temperature or vacuum conditions. ⑥ Combine the advantages of ceramics and metal, effectively improve the corrosion resistance and wear resistance of the metal surface.MAO film layer comprehensive mechanical and physicochemical properties are good, so it effectively improves the scope of its application, with the continuous improvement of the technology as well as the continuous deepening of the research, the MAO technology will surely bring us better economic benefits.
2 Titanium alloy MAO wear and corrosion resistance treatment
Due to the control of many factors, titanium alloy MAO has an extremely complex process, whether it is the frequency, current and voltage in the electrical parameters, or the temperature, concentration and composition in the electrolyte, all of which will bring certain effects on the characteristics and organisation of the titanium alloy MAO film layer. Therefore, if the electrolyte parameters and electrical parameters are selected appropriately, it can make the surface of titanium alloy grow a layer of MAO film layer in situ, which has good corrosion resistance. There are many factors affecting the abrasion resistance of titanium alloy MAO film layer, including the shape characteristics of the surface, the organisational structure of the film layer, temperature, hardness, etc. In general, the hardness of the film layer is very low. In general, the hardness and wear rate of the film layer are inversely proportional to each other, and with the reduction of the coefficient of friction and the increase of hardness, the wear resistance of the film layer also increases. Therefore, we should reduce the friction coefficient and increase the hardness to improve the wear resistance of the film layer. The corrosion resistance and densification of the film layer are inextricably linked. Under normal circumstances, the wear resistance of the film layer is increased with the increase in densification.
If the surface layer of titanium alloy is prepared with dense structure, suitable thickness, low coefficient of friction and high hardness, and firmly combined with the body of the MAO film layer, it will play a significant role in improving the function of corrosion resistance and abrasion resistance of the surface of titanium alloy, and the prospect of the application of MAO film layer is the corrosion resistance and abrasion resistance.
3 Titanium alloy MAO film layer research progress
From the 1980s, Russia began the study of titanium alloy MAO film layer, compared with other countries, not only started early, but also more thorough and in-depth research. The protection performance of the film layer, analysis of the chemical composition of the film layer and optimisation of the electrolyte formulation are the important contents of its research. After Russia, the research work on titanium alloy MAO began to be carried out comprehensively. However, there are not many relevant reports on the treatment of corrosion and wear resistance of titanium alloy MAO at home and abroad.Xue et al. studied the organisation and structure of the membrane layer of MAO made in sodium meta-aluminate system and silicate system, and also analysed some mechanical properties including elastic abrasion, the hardness of the membrane layer, and the general distribution of the membrane layer. The use of bipolar pulsed power supply affects the electrical parameters in the MAO process of titanium alloys, which was also studied in detail and depth by Wu et al. As for the electrical parameters and the composition of the treatment solution on the wear resistance, like composition and growth rate of titanium alloy MAO film layer, Wang et al. also made a relevant study. Some related literature points out that in the phosphorus or calcium components of the treatment solution, will generate titanium alloy MAO membrane layer, not only corrosion resistance, wear resistance, but also has the characteristics of biocompatibility, in the medical bone transplantation, the application of this membrane layer is very promising.
4 Application prospects of titanium alloy MAO film layer
Analysing its physical and chemical properties, the application prospect of MAO film layer is very wide, especially its corrosion and wear resistance. In the modern hull structure field of application, in the size difference, complex shape of the parts, the formation of hard, dense, uniform MAO film layer, in steel and other piping components with contact and Cu alloys, alloys to form a TiO2 film layer, able to seawater corrosion performance is further improved. MAO film on titanium alloys used in other industrial areas, with strong corrosion resistance and heat barrier properties, especially for some key parts, such as the engine cover of the car, its protection is more significant, can be its heat insulation film layer is easy to fall off the shortcomings of the effective way to overcome. Also due to its good wear resistance, in the textile industry also has a wide range of applications, especially for yarn cups and other key parts. In addition, under the strict conditions of high speed and heavy load, high pressure and high temperature, MAO film layer also has good magnetic shielding ability, irradiation ability and resistance to high-energy rays, which is widely used in electronic shielding board.
5 Conclusion
Titanium alloy has many excellent properties, high specific strength, low density, biocompatibility and other characteristics, therefore, it is widely used in the military, people's livelihood supplies and aerospace industry, however, due to the poor abrasion resistance of titanium alloys, the hardness is also lower, the corrosion hazards of contact with other metals is relatively large, which affect the scope of application of titanium alloys. MAO technology has an economic, environmentally friendly, simple and at the same time, in the processing of the The process will not damage the body and many other advantages. At present, the research work on titanium alloy MAO is also being gradually launched and carried out, and has made progress in stages. Domestic research is still in its infancy. So we must expand the field of application, in-depth understanding and mastery of the technology. This paper briefly describes the micro-arc oxidation technology and advantages, and analyses the corrosion-resistant film layer as well as the research progress of micro-arc oxidation wear-resistant in-situ generation on the surface of titanium alloys, and looks forward to the prospects for its application and development direction.
