Corrosion Resistance and Wear Resistance of Titanium Alloy

1 Surface modification technology of titanium alloy
Titanium alloy has low density, high specific strength, good corrosion resistance and strong fatigue resistance. It has been widely used in aerospace, enclosure, automobile, medical and other fields. However, titanium alloy has high friction coefficient, is very sensitive to adhesive wear and micro-wear, has poor wear resistance, is easy to ignite at high temperature and high speed friction, and has relatively poor resistance to high temperature oxidation, which seriously affects the safety and reliability of its structure and greatly limits its application.

Therefore, further improving the surface properties of titanium alloy such as wear resistance, high temperature oxidation resistance and corrosion resistance has become an urgent problem to be solved. In addition to improving the composition and preparation process of the alloy, surface modification of titanium alloy is currently the most effective method.

In traditional surface modification technology, ion implantation is restricted by ion implantation energy, and the strengthening layer is very shallow; ion carburizing, boronizing and nitriding have the disadvantages of long processing cycle and easy deformation of workpieces at high temperature; the thermal spraying modified layer has a loose structure and relatively low bonding strength with the substrate, and it is not easy to form a metallurgical bond with high bonding strength. Laser surface modification technology is the product of the combination of laser technology and metal heat treatment. It is to apply extremely high energy to the surface of the material to cause physical and chemical changes, thereby significantly changing the surface hardness, wear resistance, corrosion resistance and high temperature performance of the material. Due to the extremely fast heating speed produced by high energy density, precise controllable power output and selectivity of modified local surface, laser surface modification technology has attracted widespread attention and attention.

2 tic/ti composite material cladding layer
Laser cladding, also known as laser coating or laser melting, is a new type of material processing and surface modification technology. Its essence is to spray powder with special properties (such as wear resistance, corrosion resistance, oxidation resistance, etc.) on the metal surface or feed powder synchronously with the laser beam, and then make it melt, expand and solidify rapidly under the action of the laser beam, forming a metallurgical bonding layer without cracks and pores on the surface of the substrate. A surface modification technology. Laser cladding technology has the following advantages: 1) The laser beam has high energy density, fast cooling speed during solidification, and fine solidification structure of laser cladding layer; 2) Different cladding can be performed on different parts of the same part according to needs: 3) The combination of substrate and cladding layer is metallurgical bonding, and the cladding layer structure has obvious gradient characteristics, so that the cladding layer and the substrate have good bonding.

Laser cladding is the most commonly used titanium alloy laser surface modification technology. According to the different components and properties obtained by the laser cladding layer, it is mainly divided into wear-resistant, high-temperature oxidation-resistant, biological and thermal barrier cladding layers. The wear resistance of titanium alloy is relatively poor, so the research on laser cladding of titanium alloy surface is mainly focused on improving wear resistance. The materials for laser cladding wear-resistant modified layer on the surface of titanium alloy are mainly B, C, Ni, Si, B4C, Cr2C3, TiC, BN, Si C, Ti B, TiB2, A1203, etc.

Ceramic materials have excellent wear resistance, corrosion resistance, heat resistance and high temperature oxidation resistance, but their brittleness, poor fatigue resistance, sensitivity to stress and cracks, and difficulty in processing limit their application. The research on laser cladding ceramic coating technology on titanium alloy surface has broadened the application scope of ceramic materials, organically combining the excellent performance of ceramic materials with the toughness and good processability of titanium alloy materials, giving full play to the comprehensive advantages of the two types of materials, and meeting the needs of structural performance (strength, toughness, etc.) and environmental performance (wear resistance, corrosion resistance, high temperature resistance, etc.), and obtaining a very ideal composite material structure.

3 Laser alloying of titanium alloy surface
Laser alloying is to rapidly melt one or more alloying elements with the surface of the substrate under the action of a high-energy laser beam, that is, to use laser to change the chemical composition of the metal and alloy surface. This method has the following advantages:

1) The performance of advanced alloys can be obtained after local surface treatment of metal parts;

2) The depth and width of the modified layer are precisely controlled;

3) Due to the large temperature gradient of the laser heating layer, the bonding layer is narrow, the bonding quality is good, and the adverse effect on the performance of the base metal is minimal. The difference between laser alloying and laser cladding is that laser alloying is to fully mix the added alloying elements and the surface layer of the substrate in a liquid state to form an alloying layer; while laser cladding is to melt the pre-coating layer completely and slightly melt the surface layer of the substrate, so that the cladding layer and the substrate material form a metallurgical bond while keeping the composition of the cladding layer basically unchanged.

4 Preparation of laser melted titanium plate

Laser melting refers to the use of a high-energy-density laser beam to scan the surface of the workpiece, melt a thin layer of the surface and solidify it at an extremely fast cooling rate. Laser melting can obtain a modified layer with high hardness, high resistance and high temperature performance on the surface of the metal material, while keeping the core of the part still in good toughness, so that the part has the characteristics of good corrosion resistance, high impact toughness and high fatigue strength. Laser melting is a local rapid solidification non-equilibrium physical metallurgical process. This method has the characteristics of high power density, small workpiece deformation, and simple process. In particular, the temperature gradient of the solidification interface of laser melting can be as high as 103K/mm, and the solidification speed can be as high as several meters per second, which can obtain ultra-fine organization and structure. Therefore, in recent years, laser melting technology has been widely used in metal surface modification technology, and has also been applied to titanium alloy surfaces.

5 Solving the problem of laser surface cracks
Although titanium alloy laser surface modification technology has been widely used at this stage, there are still many problems to be solved in terms of process. The first is the problem of laser surface modification cracks. In addition, the repeatability of laser processing results, especially the poor repeatability of laser alloying and laser cladding results using ceramic powder, is also a major problem.

5.1 Laser surface modification laser beam process parameters Laser surface modification process parameters mainly include laser power, spot size, scanning speed, overlap rate, laser beam energy distribution, etc., which have an impact on the quality of the modified layer. The energy distribution of laser beam is described by the model of laser beam, which mainly includes four models: Gaussian model, multi-model, rectangular model and concave top model. Gaussian beam is most suitable for cutting and welding applications, but not suitable for laser surface modification. It tends to cause gasification and melting deep in the matrix. When multiple lasers are modified, serious defects such as collapse and pores are easily generated at the overlap of the modified layer. To obtain a flat and high-performance modified layer, these parameters must be comprehensively optimized to achieve the best combination.

5.2 Nano-density and crack formation mechanism during modification
Laser surface modification of titanium alloy is a metallurgical process of rapid melting and solidification. There are complex physical and chemical phenomena such as heat transfer, mass transfer, convection, diffusion, and phase change during laser processing. Cracks are very easy to occur on the surface of the modified layer and in the transition zone between the modified layer and the matrix. Internal defects such as cracks seriously affect the quality of the modified layer. Therefore, the mechanism of crack generation during laser modification has become a hot topic for researchers in various countries. Many scholars have proposed crack control methods during laser modification, mainly including optimizing laser modification process parameters, adjusting modified layer materials, modifying laser modified layers with additives, preheating and slow cooling of titanium alloy substrates during laser modification, and introducing ultrasonic vibration during laser modification. Studies have shown that the introduction of ultrasonic vibration during laser cladding can improve the fluidity of the liquid molten pool, allowing bubbles to escape quickly and making the tissue distribution more uniform; applying ultrasonic vibration during the solidification process can break up the growing dendrites and disperse them to various parts of the melt to form uniformly distributed small nuclei. Ultrasonic vibration can also make it easier for liquid to replenish the pores between dendrites, which is beneficial to reducing shrinkage cavities and partially eliminating the source of tensile stress, thereby reducing cracks in the remelting layer. The cavitation and stirring effects of ultrasonic waves in the molten pool can make the temperature of the molten pool uniform, improve the solidification state of the molten pool, and reduce residual thermal stress and cracking sensitivity.

5.3 Composite gradient coating
While the single laser surface modification technology is developing, the composite laser surface modification technology that integrates two or more surface modification technologies has developed rapidly. Composite laser surface modification technology can not only improve the surface properties of titanium alloys, but also improve the surface morphology of the modified layer and reduce the generation of cracks. Golbeeiwski M et al. prepared a composite gradient coating containing TiN and Ti2N on a titanium alloy substrate by glow plasma nitriding and laser remelting, which greatly improved the surface properties of the titanium alloy. Yilbas B.S, et al. deposited TiN on laser nitrided titanium alloy using PVD technology and found that the bonding strength and shear strength of the formed TiN film layer with the titanium substrate were significantly improved compared with the film layer prepared by PVD technology alone.

5.4 Structural characteristics of plasma sprayed nanocomposite ceramic coatings
Due to the special structure of nanomaterials, they have excellent properties that are difficult to obtain in general materials, which provides favorable conditions for improving the performance of titanium alloy laser surface modification layers. Combining nanopowders with laser surface modification technology to prepare a surface composite modification layer containing nanostructures on the surface of titanium alloys can change the mechanical, physical and chemical properties of the titanium alloy surface, give the titanium alloy surface new functions, and achieve the purpose of combining material surface modification with functionalization.

Laser surface modification is an ideal technology for titanium alloy surface treatment, which has attracted great attention from all countries, especially the wide use of titanium alloys in military, aerospace, automobile, medical and other fields, which has made the research and development work in this field more concerned. Compared with the research on titanium alloy laser surface modification technology abroad, the relevant theoretical and experimental research in China started late, and there are still few practical applications. There is a big gap in equipment, process, materials and basic research. In order to further expand the development of titanium alloy applications, it is urgent to carry out research on titanium alloy laser surface modification technology.