Application of laser cladding technology in the aviation field
Laser cladding technology can significantly improve the wear resistance, corrosion resistance, heat resistance and oxidation resistance of metal surfaces. At present, the research on laser cladding mainly focuses on process development, cladding layer material system, rapid solidification structure of laser cladding, interface bonding with the substrate and performance testing.
The aviation field is an important field related to national security and a strategic industry that the country supports. How to better apply laser cladding technology to my country’s aviation manufacturing is of great strategic significance. Aviation materials are an important material basis and technological leader for the research and development and production of weapons and equipment. Strengthening the hardness and wear resistance of aviation material substrates is of great significance to the improvement of aviation materials. For example, the development and application of high-power lasers provide new means for the surface modification of aviation materials and open up a new path for the development of material surface strengthening technology. Ceramic materials have high hardness and high chemical stability that are incomparable to metal materials. Therefore, suitable ceramic materials can be selected according to different service conditions of parts. The high energy density laser heating temperature and fast heating speed can be used to melt ceramic coatings on the surface of metal materials (such as titanium alloys), thereby organically combining the excellent wear resistance and corrosion resistance of ceramic materials with the high plasticity and high toughness of metal materials, which can greatly improve the service life of aviation parts.
Application of laser cladding technology in aircraft parts manufacturing
In addition to bearing loads under working conditions, titanium alloy components of aircraft bodies and engines will also form thermal fatigue loads due to the start/stop cycle of the engine. Under the dual effects of alternating stress and thermal fatigue, cracks of varying degrees will be generated, which will seriously affect the service life of the body or engine and even endanger flight safety. Therefore, it is necessary to study the surface strengthening methods of aviation titanium alloy structures to give full play to their performance advantages and enable them to be more widely used.
Ceramics are divided into oxide ceramics and carbide ceramics. Aluminum oxide, titanium oxide, cobalt oxide, chromium oxide and their composite compounds are widely used oxide ceramics and are also the main materials for preparing ceramic coatings. Carbide ceramics are difficult to prepare coatings on their own. They are usually prepared with self-melting alloys with cobalt or nickel bases to form metal ceramics. Such metal ceramics have high hardness and excellent high-temperature performance and can be used as wear-resistant, abrasion-resistant and corrosion-resistant coatings. Commonly used ones include tungsten carbide, titanium carbide and chromium carbide . Laser cladding can be used to prepare ceramic coatings by first adding transition layer materials (such as NiCr, NAI, NiCrAl, Mb, etc.) to the surface of the material, and then using pulsed laser cladding to melt the Ni and Cr alloys in the transition layer and Al2O3, Zr02 and other materials in the ceramics onto the surface of the substrate to form porosity. The metal molecules in the substrate can also diffuse into the ceramic layer, thereby improving the structure and performance of the coating. Using laser cladding of ceramic coatings for aircraft engine turbine blades is a high-tech technology with great application value.
Titanium alloys are widely used in aircraft manufacturing, such as Ti-6AI-4V titanium alloy, which is used to manufacture key components with high strength/weight ratio, heat resistance, fatigue resistance and corrosion resistance. However, in the processing and manufacturing of these titanium alloys, traditional process methods have many weaknesses that are difficult to overcome. For example, the production of partitions is made of toothed alloy plates that are several inches thick and weigh tens of kilograms, and it takes more than a year to obtain these finished alloy plates. Because it is difficult to process, it takes hundreds of hours of work in the machining center to process such parts, and a lot of tool wear. Laser cladding technology has great advantages in this regard, which can strengthen the surface of titanium alloys and reduce manufacturing time.
Laser cladding is one of the surface modification technologies with the greatest potential for modern industrial applications and has significant economic value. In the early 1980s, Rolls-Royce of the United Kingdom used laser cladding technology to harden the joints of the RB211 turbine engine casing and achieved good results.
In recent years, the research and development of AeroMet in the United States has made substantial progress, and their multiple series of Ti-6Al-4V titanium alloy laser cladding parts have been approved for use in actual flights. Among them, the two full-size joints on the F-22 fighter meet the requirement of 2 times fatigue life, the wing root ring of the F/A-18E/F meets the requirement of 4 times fatigue life, and the connecting rod for lifting meets the flight requirements and the life exceeds the original technical requirements by 30%. Titanium alloy parts manufactured by surface strengthening using laser cladding technology not only outperform parts manufactured by traditional processes in performance, but also reduce production costs by 20%~40% and shorten production cycles by about 80% due to the advantages of materials and processing. The application of laser cladding in the repair of aviation parts has achieved ideal results.
Laser cladding technology has a direct impact on the repair of aircraft. Its advantages include automation of repair processes, low thermal stress and thermal deformation. As people expect the life of aircraft to continue to increase, more complex repair and overhaul processes are required. Parts such as turbine engine blades, impellers and rotating air seals can be repaired by surface laser cladding strengthening. For example, laser melting technology is used to repair cracks in aircraft parts. Some non-penetrating cracks usually occur in thick-walled parts. The crack depth cannot be directly measured, and other repair technologies cannot play a role. Laser cladding technology can be used. According to the crack situation, the cracks can be gradually removed by grinding and flaw detection for multiple times. The grooves after grinding can be filled with laser cladding and powder-added multi-layer cladding process to rebuild the damaged structure and restore its performance.
Please consult Huirui-Laser for the base material and alloy powder used for laser cladding of engine turbine blades. The powder particles used for cladding are spherical and less than 150μm in size. Different process parameters should be used for the cladding layer of different alloy powders to obtain the best cladding effect.
Repair the top of the damaged turbine blade to the original height. During the laser cladding process, the laser beam forms a very shallow melting depth at the top of the blade, and the metal powder is deposited on the top of the blade to form a weld. Under computer numerical control, the weld layer increases the cladding layer. Unlike the damaged blades clad by laser cladding, the blades of manual tungsten inert gas arc cladding must undergo additional post-processing after cladding. The top of the blade must be precisely processed to expose the gap formed during the cooling process, while laser cladding omits these processing processes and greatly reduces time and cost.
In the aviation field, the spare parts of aircraft engines are very expensive, so in many cases, spare parts repair is more cost-effective. However, the quality of the repaired parts must meet the flight safety requirements. For example, when the surface of the propeller blade of an aircraft engine is damaged, it must be repaired through some surface treatment technology. Laser cladding technology can be well used for laser three-dimensional surface cladding repair of aircraft propeller blades.
There are actual cases where the aircraft engine blades are repaired by laser. The cladding material (alloy powder) is Inconel625 (Cr-Ni-Fe 625 alloy powder), and the blade material is Inconel713. By detecting the cross-section of the cladding layer by metallographic method, it can be found that after laser cladding, a metallurgical cladding transition zone is formed between the blade base material and the cladding layer. The conclusion proves the high efficiency and reliability of laser cladding technology and the excellent performance.
Laser cladding can strengthen the alloy cladding layer on the surface of the material and improve the mechanical and chemical properties of the alloy surface. Overlay alloy powder is an ideal laser cladding material with high application value. Overlay alloy powder can be quickly melted under the irradiation of the laser beam and then clad on the surface of the aviation component. This process can adopt the pre-coating method. The pre-coated material can be wire, plate, powder, etc. The most commonly used material is alloy powder. Laser cladding first pre-places the cladding material on the surface of the substrate to be clad, and then uses the laser beam to scan the melted cladding material and the substrate surface to achieve surface strengthening.
The cladding area is formed under the action of the laser beam and the powder feeding system. The substrate material and alloy powder determine the properties of the surface cladding layer. The laser directly irradiates the substrate surface to form a molten pool, and the alloy powder is sent to the surface of the molten pool. Argon is also sent into the molten pool during the laser cladding process to prevent oxidation of the substrate surface. The molten pool is formed on the surface of the substrate. If the alloy powder and the substrate surface are both solid, the alloy powder particles will be ejected when they come into contact with the substrate surface, and will not adhere to the substrate surface for cladding. If the substrate surface is in a molten pool state, the alloy powder particles will be adhered when they come into contact with the substrate surface, and laser cladding will occur under the action of the laser beam to form a cladding belt.
The wear resistance of the laser cladding layer is proportional to the hardness. It is generally difficult to balance the hardness, wear resistance, corrosion resistance and fatigue resistance of the cladding layer. The microstructure and chemical composition of the substrate surface can be improved by the laser cladding process.
The laser cladding process has great advantages over the tungsten inert gas welding (TIG) cladding process. The properties of the laser cladding layer depend on the proportion of the cladding alloy elements. In order to achieve the best expected effect, the dilution effect of the substrate material must be avoided as much as possible, because the hardness of the cladding layer is inversely proportional to the dilution of the substrate material. On the surface of Inconel 792 alloy, laser cladding and tungsten inert gas welding were used to clad Rene142 alloy powder. The comparison of microhardness shows that the hardness of the strengthened surface layer produced by laser cladding is higher than that of the surface hardness of tungsten inert gas welding cladding. The reason is the high solidification rate of the laser cladding layer and the strong convection effect generated in the molten pool. Therefore, laser cladding technology is more valuable in the aviation field than tungsten inert gas welding cladding.
Relevant data show that the strength of aviation parts repaired by laser cladding technology can reach more than 90% of the original strength. More importantly, it shortens the repair time and solves the problem of rapid repair of rotating parts that must be solved for the continuous and reliable operation of important equipment.
Application of laser cladding in surface modification of aviation materials
Laser cladding of high hardness, wear-resistant and high-temperature resistant coatings
In order to prevent the scrapping of important parts working in high-speed, high-temperature, high-pressure and corrosive environments due to local surface damage and to increase the service life of parts, countries around the world are committed to developing various technologies to improve the surface performance of parts. Traditional surface modification technologies (such as spraying, plating, surfacing, etc.) are not ideal due to poor interlayer bonding and poor solid-state diffusion. The emergence of high-power lasers and broadband scanning devices provides a new and effective means for material surface modification. Laser cladding is a new type of surface modification technology with high economic benefits. It can prepare high-performance cladding layers on cheap, low-performance substrates, thereby reducing material costs, saving precious rare metals, and increasing the service life of metal parts.
Titanium alloys and aluminum alloys are widely used in modern aircraft manufacturing. For example, the use of titanium alloys in the fuselage of the fourth-generation fighter F-22 in the United States has reached 41%, and the use of titanium alloys in the advanced V2500 engine in the United States has also reached about 30%. Titanium and titanium alloys have high specific strength, excellent corrosion resistance, and good high temperature resistance, which can reduce the weight of the fuselage and improve the thrust-to-weight ratio. The disadvantages of titanium alloys are low hardness and poor wear resistance. The hardness of pure titanium is 150~200HV, and titanium alloys usually do not exceed 350HV. In many cases, a dense oxide film will be formed on the surface of titanium and titanium alloys to play a role in corrosion prevention. However, when the oxide film is broken, the environment is bad or crack corrosion occurs, the corrosion resistance of titanium alloy will be greatly reduced.
The total amount of aluminum alloy on the US F-35 fighter jet, which first flew in 2000, is more than 30%. However, the strength of aluminum alloy is not high enough, and it is easy to produce plastic deformation during use, especially the low surface hardness and poor wear resistance of aluminum alloy, which restricts its application to some extent.
The microhardness of the titanium alloy surface after laser cladding is 800-3000HV. Using laser cladding technology to strengthen the surface of aluminum alloy is an effective way to solve the problems of poor wear resistance and easy plastic deformation of aluminum alloy surface. Compared with other surface strengthening methods, this method has metallurgical bonding characteristics between the strengthening layer and the aluminum matrix, and has high bonding strength. The thickness of the cladding layer reaches 1~3mm, the structure is very fine, the cladding layer has high hardness, good wear resistance, and strong bearing capacity, thus avoiding the formation of cracks due to strain inconsistency between the soft matrix and the strengthening layer. In addition, by cladding high-performance ceramic coatings on the surface of titanium alloys and aluminum alloys, the wear resistance and high temperature resistance of the materials can be greatly improved.
Laser cladding to obtain thermal barrier coatings
In recent years, aviation engine gas turbines have developed in the direction of high flow ratio, high thrust-to-weight ratio, and high inlet temperature. The gas temperature and gas pressure in the combustion chamber have been continuously improved. For example, the temperature before the turbine of military aircraft engines has reached 1800℃, and the temperature in the combustion chamber has reached 2000℃~2200℃. Such high temperatures have exceeded the melting point of existing high-temperature alloys. In addition to improving cooling technology, preparing thermal barrier coatings (Thermal Bamer Coating, TBCs) on the surface of high-temperature alloy hot end components is also a very effective means. It can achieve a heat insulation effect of 1700℃ or higher to meet the requirements of high-performance aviation engines to reduce temperature gradients, thermally induced stresses and service stability of base materials. In the 1970s, ceramic thermal barrier coatings (TBCs) were successfully used for J-75 gas turbine blades, and countries around the world have invested heavily in in-depth research on them from materials to preparation processes.
Since the 1980s, the laser melting of ceramic layer on the surface of the material has obtained a dense columnar crystal structure, which improves the strain tolerance; the dense and uniform laser remelting structure and the low porosity can reduce the oxidation rate of the bonding layer and prevent the penetration of corrosive media. The high-power laser can be used to directly irradiate the ceramic or metal powder, melt it and form a metallurgical bond on the metal surface to obtain a columnar crystal structure perpendicular to the surface. Since the solidification order of the cladding layer is from the surface to the inside, the surface structure is relatively fine, and such a structure is conducive to relieving thermal stress. For example, 8% (mass fraction) yttria partially stabilized zirconia (YPSZ) thermal barrier coating was obtained by laser cladding. The uniformly mixed powder can also be placed on the substrate, and the mixed powder is irradiated by a high-power laser. The powder is melted well and a molten pool is formed by adjusting the laser power, spot size and scanning speed. On this basis, the alloy powder is continuously added to the molten pool by changing the composition, and the above process is repeated to obtain a gradient coating.
Laser cladding of super wear-resistant and corrosion-resistant alloys on the surface of key components can increase the service life of components and shorten the manufacturing cycle without deformation of the surface of components. The thermal barrier coating produced by laser cladding has a good thermal insulation effect and can meet the requirements of high-performance aircraft engines to reduce temperature gradients, thermally induced stresses and stable service of base materials.
Conclusion
Laser cladding technology plays a vital role in the development of the aviation industry. Laser cladding technology can improve the hardness, wear resistance, corrosion resistance and fatigue resistance of the surface of aircraft parts, increase the service life of materials, and can also be used for the repair of worn parts to save processing costs. Laser cladding technology is applied to the manufacture of aircraft parts, which can reduce the manufacturing process of workpieces and improve the quality of parts. With the advancement of today’s science and technology, the overall performance of aircraft will be further improved, and the requirements for materials will become higher and higher. The further improvement and development of laser cladding technology will play an important role in the technological progress of the aviation industry. Aviation materials will present a new look with the development of laser cladding technology.
