1. レーザー 表面 硬化
Laser surface hardening mainly uses a high-energy laser beam to irradiate the surface of a metal or alloy, and the thermal effect generated makes the surface of the substrate form a solid heating process that does not exceed the melting point. At the same time, the phase transformation is strengthened on the metal surface by using the presence of isomeric transformation in the matrix material combined with laser heating and material self-cooling effect. For titanium alloys, there have been studies in this area since the beginning of the 20th century. Dai Zhendong et al. significantly improved the hardness of TC11 titanium alloy by means of surface scanning laser quenching, and its friction coefficient could be reduced to the original 0.2 ~ 0.3, and the fretting wear resistance was increased by 123 times, which greatly improved the alloy surface properties. The surface structure and properties of TC11 titanium alloy were optimized by Zhang Hong et al. The results show that laser quenching can refine the surface structure obviously, and improve the hardness and wear resistance. Zhang Qi et al., through the study of various titanium alloys treated by laser self-quenching and rapid solidification, confirmed that the self-quenching treatment not only refines the and grain structure of the alloy, but also makes the surface chemical composition more homogeneous, and the segregation ratio after quenching can be reduced from the original state of 1.28 to 1.04, and no holes, cracks and other defects are found in the quenching layer. A smooth and uniform alloy surface can be obtained.
2. レーザー 表面 再溶解
Laser surface remelting is a method to melt and solidify the surface of the substrate rapidly by radiating the surface of the material under the protection of argon atmosphere, so as to refine the structure and improve the performance of the material. Guo Chun et al. carried out laser remelting treatment on the surface of TC4 alloy by laser beam. After microscopic observation, the surface structure of the matrix was refined, and the surface properties such as hardness and wear resistance were also significantly improved. In addition, some researchers used Nd∶YAG laser to remelt the surface of TiNi alloy, and the cladding layer and the matrix metallurgical combination are good, can form a continuous and dense passivation film, and the corrosion resistance is significantly enhanced. By laser surface remelting treatment of TA2 industrial pure titanium, Dai Jingjie believes that the improvement of surface wear resistance is due to the lattice distortion, fine crystal strengthening and dislocation strengthening caused by the melting process. However, surface remelting does not improve the performance of all titanium alloy materials, and its performance is also possible to deteriorate. The results show that the grains formed by laser surface melting of TA15 titanium alloy are abnormally coarsened. After laser surface melting treatment of TiZr alloy, Xu Bo found that the microstructure of the hot remelting zone was coarse flake grains, the microhardness of the modified zone was lower than that of the matrix, and the wear resistance was not significantly improved.
3. レーザー サーフェス 修復
Laser surface repair can be classified as a branch of laser forming repair technology, and it is also a synthesis of laser forming technology and laser cladding technology, which is a further application and development in the field of metal parts repair. The surface defects of titanium and titanium alloys can be eliminated by using laser surface repair technology. Deng Dewei et al. verified that laser repair can heal cracks on the surface of titanium alloy. After laser repair treatment, the matrix hardness value around the modified zone increased, and the hardness change curve between the modified zone and the heat-affected zone was relatively flat. Gong Xinyong et al. used laser continuous scanning to repair the TC11 titanium alloy impeller by laser melting and precipitation, and the experiment verified that the performance of the impeller was intact and the perfect repair of the parts was realized. Cui Aiyong et al. obtained Cr2O3/Ti cladding layer without internal cracks on the surface of TC4 alloy through laser cladding repair technology. The surface of the modified layer is smooth and has good metallurgical bonding with the substrate, thus achieving laser repair of the damaged parts of compressor blades.
4. レーザー 表面 合金化
レーザー 表面 合金化 は a method that uses 高エネルギー レーザー ビーム に 急速に heat and melt the surface of the material to promote the surface alloying reaction, so as to improve the surface properties of the alloy, which can be divided into surface gas alloying and surface powder alloying。
ガス 導入 によって ガス 合金 は 主に N2 または its mixture, also known as laser gas nitriding. It is in a nitrogen atmosphere, the use of high energy laser beam to activate nitrogen atoms, high-temperature action to melt the surface of the material, the active N atom and the liquid phase of the metal molten pool Ti alloying reaction, forming a hard phase TiN. after laser gas nitriding treatment of TA2 pure titanium with Nd∶YAG laser, Wang Pei et al. found that the friction coefficient on the surface of the material was about 0.22, which was about 1/4 of the matrix, and the wear resistance was greatly improved. At the same time, the denser the distribution of nitriding strengthened area, the smaller the friction coefficient and the better the wear resistance. The microhardness distribution curve extending from the nitriding surface to the matrix shows that laser gas nitriding can improve the hardness of the titanium alloy surface。
Laser surface powder alloying is the use of laser high energy and rapid heating characteristics, so that the substrate surface and the added alloy powder melting reaction solidification, forming a substrate material based on the surface alloy layer. Ge Xiaolan et al. used a high-energy laser beam to alloy the surface of TC4 alloy with Ti, Al and Nb mixed powder to produce TiAlNb alloy coating. The hardness of the alloy coating showed a gentle transition along the coating depth direction, and gradually increased from inside to outside. The average hardness of the coating was significantly higher than that of the TC4 matrix, and the friction coefficient was reduced, and the wear resistance was nearly 3 times higher than that of the matrix. Liu Qinghui et al. coated Ti/Si/C elemental element mixed powder on the surface of TC4 alloy, and used laser alloying technology to generate alloy coating on the surface of the matrix. It is found that the alloy coating is composed of Ti, Si and C compounds. The average microhardness of the coating is 80% higher than that of the substrate, and the average friction coefficient of the coating is about 0.38, which is about 16% lower than that of the substrate. The surface hardness and wear resistance are significantly improved.
5. レーザー 表面 クラッディング
レーザー 表面 クラッディング 缶 また be 分類された として a surface modification technology, is the basis of laser surface repair. It is the use of high energy density laser beam to add the cladd the cladd material to the substrate surface , to form the cladding material and the substrate good metallurgical combination of cladding layer on the surface of the substrate. The cladding process diagram is shown in the following figure.
プロセス of laser cladding is accompanied by laser alloying, but compared with simple laser alloying, the cladding layer material is not fully mixed with the matrix to alloying reaction, which can better reflect the special properties of the cladding material. At present, there are many material systems used for laser cladding of titanium and titanium alloys, including C, B, N, Si and Ni. に応じて に the composition and properties of the cladding layer, the prepared coatings can be divided into wear-resistant coatings, high-temperature oxidation-resistant coatings, biological coatings and thermal barrier coatings.
5.1 耐摩耗性 コーティング
The wear resistance of titanium alloy is poor compared with other properties, so the laser surface modification focuses more on improving the wear resistance of the matrix. In general, the higher the hard phase content in the wear-resistant coating, the higher the hardness and the better the wear resistance. There are many cladding materials that can improve the wear resistance of titanium alloys, including B, C, Ni, Si, B4C, Cr2C3, TiC, BN, SiC, TiB, TiB2 and Al2O3. Using NiCr/Cr3C2 and WS2 composite powders as raw materials, Wu Shaohua et al studied the improvement of the wear resistance of NiCr/Cr3C2 composite coatings by comparing the laser cladding with different amounts of WS2. The results showed that the composite coating had the best wear resistance when 20% of WS2 was added to the cladding material. The results show that the appropriate amount of WS2 can form a self-lubricating phase, and then show good wear and anti-friction properties. Sun Ronglu et al. used lasers to perform laser cladding experiments on the surface of TC4 samples. Ni and MoS2 mixed powders were used as coating materials. During the cladding process, metallurgical reactions occurred between the surface layer of the sample, the mixed powders and the mixed powders, and spherical CrxSy particles were uniformly mixed in the dendritic dendrites of the cladding layer. The friction coefficient is reduced while the wear resistance is improved. Liu et al. selected Co base alloy powder as cladding material and obtained cobalt base composite coating with fine dendritic and granular reinforced phases by laser cladding technology. The wear resistance of the matrix and cladding layer was measured by the wear testing machine, and it was found that the wear rate of the cladding layer was only 1/12 of the matrix, and the wear resistance was significantly improved. Weng et al. laser cladding TiN and Co-based mixed powder on the surface of TC4 alloy, the results show that Co/Ti intermetallic compounds with excellent wear resistance and dispersion strengthening phase can be formed on the cladding layer, and the wear resistance of the composite coating will be further improved with the increase of TiN powder addition.
5.2 耐酸化 コーティング
Structural parts for engineering applications are often in long-term service under high-temperature conditions. In order to reduce or avoid the chemical or electrochemical reaction between O, S, N and other elements in the high-temperature working atmosphere and the matrix, a dense high-temperature protective layer is generally constructed on the surface to protect the matrix from being destroyed. Yu Pengcheng et al. used laser cladding technology to prepare composite coatings on the surface of TC4 alloy with NiCr-Al-Si alloy powder as cladding material. The cladding coatings with continuous dense structure were uniformly mixed with Al2O3, NiO, TiO2, NiCr2O4 and other compounds by this method. The experimental results show that the high-temperature oxidation resistance of the coating is 7 ~ 9 times higher than that of the substrate. Liu et al. laser cladding TiN+Ti3Al mixed powder on the surface of TC4 alloy, and prepared a composite coating composed of -Ti, TiN, Al2O3 and TiO2 mixed phases. They carried out isothermal oxidation tests at 600 and 800 degree respectively. The experimental results show that the composite coating has better high-temperature oxidation resistance than the titanium alloy matrix.
5.3 サーマル バリア コーティング
The operating temperature in aerospace, gas turbine engines and other environments has reached the limit temperature of superalloy materials. The thermal barrier coating of alloy materials combines the performance of metal materials with the advantages of high-temperature resistance of ceramic materials to play the role of thermal insulation of ceramic materials, so that parts can work normally under high-temperature conditions. Shan Xiaohao et al. used laser cladding technology to prepare a high-temperature thermal barrier coating by mixing Nb, Al and Ti powders, and found that the high-temperature oxidation resistance of the coating was related to the degree of segregation of Ti and the content of Nb2Al compounds in the alloy. When the mass fraction of Ti is 15.18wt%, the segregation degree of Ti in the alloy is the lowest, and the oxidation resistance of the coating is the best. In view of the fact that loose porous TiO2 is the main reason for the poor oxidation resistance of TC4 alloy, Xu Jiangning et al. laser cladding NiCrNiSi mixed powder on its surface. The experimental results show that the oxide film coating composed of continuous dense Al2O3 and NiO can effectively prevent the erosion of oxygen atoms on the matrix, and the oxidation resistance can be significantly enhanced.
5.4 バイオコーティング
A bioactive coating is deposited on the surface of titanium alloy by laser cladding technology, which makes the titanium alloy implantation show better biocompatibility. Li Fuquan et al. coated the hydroxyapatite biological coating on the surface of TC4. The phase analysis of the coating mainly consisted of hard ceramic phases such as -Ti, Ti3P, TiO and CaTiO3, and the ceramic combination was good, the surface was uniform and smooth, and the wear resistance and biological wettability were good. In addition, in order to ensure that the titanium alloy matrix formed a uniform, flat, with strong metallurgical bonding biological coating, by cladding gradient coating and multiple cladding, to prevent the coating structure, stress mutation to reduce the internal stress. Shi Lei et al. coated pure HA, mixed HA and HA gradient coatings on the surface of TC4, and found in comparative experiments that the HA gradient coating showed more excellent performance, with high binding strength to the substrate, Ga/P ratio closer to natural bone, corresponding to higher HA content and superior biological activity.
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