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Application of surface technology in aircraft structure repair

in the service process of aircraft, due to the effect of environment and various stresses, its structure and accessories will be corroded and damaged. These corrosion or damage often start from the local surface of the structure, and then further expand or extend to the interior of the component. For example, corrosion starts from the structural surface, wear occurs on the structural surface, and fatigue cracks extend from the surface to the interior of the structure [1]. The wear and corrosion of aircraft structure or accessories shall be repaired in time, otherwise it will not only bring serious economic losses, but also endanger the Continuous Airworthiness of the aircraft, and may seriously endanger the safety of aircraft operation. How to repair the damaged structure surface efficiently, quickly and economically poses a severe challenge to the aircraft structure maintenance technology

surface technology refers to the technology that changes the morphology, chemical composition, microstructure and stress state of solid metal material surface or non-metal material surface through surface coating, surface modification, surface treatment and composite technology after surface pretreatment to obtain the required surface properties [2]. In the process of aircraft structure maintenance, the reasonable use of surface technology to repair the aircraft structure surface can not only restore the original functional characteristics of the aircraft structure, but also make the sensor of the aircraft tensile testing machine a class 1 Structure of electronic components with better performance than the base material, such as higher wear resistance, corrosion resistance and high temperature resistance. The research and promotion of surface technology in aircraft structure repair can not only effectively repair the damaged surface of aircraft structure, but also play a great role in energy saving and material saving, and effectively promote the development of aircraft maintenance technology

surface technology commonly used in aircraft structure repair

surface technology usually includes surface coating, surface modification and surface treatment. Surface coating is to prepare a coating layer on the surface of the substrate material. The material composition, structure and stress of the coating layer are prepared as required to improve the performance, including electrochemical deposition (electroplating and brush plating), chemical liquid deposition (electroless plating), vapor deposition (physical vapor deposition - vacuum evaporation, sputtering, ion plating, chemical vapor deposition including plasma enhanced chemical vapor deposition) Thermal spraying (flame spraying, arc spraying, plasma spraying, etc.), surfacing, hot dip coating, coating, molecular self-assembly, etc. Surface modification is to improve the performance by changing the chemical composition of the matrix material without adding a film layer, including diffusion infiltration (chemical heat treatment), ion implantation, conversion film, etc. Surface treatment does not change the chemical composition of the surface material, but only changes the structure and stress of the matrix material, so as to improve the performance. It does not attach a film layer, including surface quenching heat treatment, surface deformation treatment (including shot peening) and surface nano processing technology [3]. Because surface technology contains many contents, this paper simply introduces four surface technologies commonly used in aircraft maintenance

(1) brush plating technology

brush plating is to connect the surface treated workpiece with the negative pole of the special DC power supply as the cathode of brush plating; The plating pen is connected with the positive pole of the power supply as the anode of brush plating. During brush plating, make the plating pen filled with plating solution in the sheath move on the surface of the plated part at a certain relative speed, and maintain an appropriate pressure. In this way, in those parts where the plating pen contacts the plated part, the metal ions in the plating solution diffuse to the surface of the part under the action of the electric field force, and the electrons obtained on the surface are reduced to metal atoms. These metal atoms deposit and crystallize to form a coating. With the extension of brush plating time, the coating gradually thickens until it reaches the required thickness. Therefore, for worn parts, brush plating repair technology is more viable. The technology has the characteristics of simple process, many kinds of coatings, fast deposition and excellent performance

(2) electroless plating technology

electroless plating is a method that does not need to be powered on. According to the principle of oxidation-reduction reaction, metal ions are reduced to metal by using a strong reducing agent in a solution containing metal ions and deposited on the surface of various materials to form a dense coating. According to different plating solutions, electroless plating is often divided into electroless silver plating, nickel plating, copper plating, cobalt plating, nickel phosphorus plating, nickel phosphorus boron plating, etc. Electroless plating technology has attracted more and more attention because of its simplicity, energy saving and environmental protection. Electroless plating has a wide range of application, uniform coating and good decoration. In terms of protective performance, it can improve the corrosion resistance and service life of products; In terms of functionality, it can improve the wear resistance, conductivity, lubrication and other special functions of machined parts, so it has become a new development milestone of surface treatment technology all over the world

(3) anodizing technology

the metal material is immersed in an appropriate electrolyte solution as an anode, and the surface is energized to form an oxide film. This process is called anodization. After anodizing, the corrosion resistance, hardness, wear resistance and heat resistance of metal materials are greatly improved. The electrolytes for anodic oxidation can be divided into acidic, alkaline and non-aqueous solutions. At present, the industry is mainly acidic. Acid electrolyte includes sulfuric acid, chromic acid, oxalic acid, phosphoric acid, etc

(4) thermal spraying technology

thermal spraying technology is a method that uses a heat source to heat the sprayed materials to a molten or semi molten state, and spray and deposit them on the pretreated substrate surface at a certain speed to form a coating. Figure 1 shows the schematic diagram of the thermal spraying process. There are many methods of thermal spraying, such as plasma spraying, arc spraying, flame spraying and explosive spraying. Thermal spraying technology has the characteristics of flexible process, wide application range, wide base and spraying materials, less heating of processed workpiece, small stress and deformation, and high production efficiency

thermal spraying technology is widely used. By selecting coating materials with different properties and different process methods, functional coatings such as thermal barrier, wearable sealing, wearable sealing, high temperature oxidation resistance, conductive insulation and far-infrared radiation can be prepared. Coating materials involve almost all solid engineering materials, including metals, metal alloys, ceramics, cermets, plastics and other composite materials. Thermal spraying technology is widely used in aerospace, metallurgy, energy, petrochemical, machinery manufacturing, transportation, light industrial machinery, bioengineering and other fields of national economy

application of surface technology in aircraft structural repair

1 application in aircraft magnesium alloy structural maintenance

the surface protection of magnesium alloys used in the aviation industry mostly adopts the method of painting after chemical oxidation. This kind of facial mask is thin and soft, and it is easy to be scratched, scratched or worn in the process of use, resulting in local damage to the surface or having to be scrapped or replaced due to out of tolerance. According to the characteristics of magnesium alloy, some researchers have summarized a set of brush plating process on the surface of magnesium alloy, which is only electrically clean but not activated, and applied this process to repair and protect the surface of magnesium alloy parts of an aircraft maintenance and production unit. As long as the brush plating process is strictly controlled, the adhesion between the coating and the magnesium alloy substrate can be guaranteed; At the same time, in a humid environment, the magnesium alloy matrix is easy to form a galvanic cell with the coating metal, resulting in electrochemical corrosion and obvious selective corrosion. Therefore, when brush plating is used to repair surface defects, the coating should reach a sufficient thickness, and the surface is brushed with paint after plating. The quality of the brush coating after repair can fully meet the production and use requirements [4]. For example, if the chemical oxidation layer, primer and one side paint three-layer anti-corrosion repair process is adopted to repair the surface corrosion of aircraft magnesium alloy parts in situ, it can effectively prevent the further development of aircraft corrosion, prevent the failure of parts caused by corrosion, and increase the time interval of aircraft return to factory for overhaul. This process is simple, easy to operate, low cost, high benefit and reliable repair quality. It is not only applicable to the repair of corrosion of magnesium alloy parts of aircraft, but also has reference value for the repair of corrosion of magnesium alloy parts of other equipment. It has strong practicability and universality [5]

2 application in aircraft aluminum alloy structure maintenance

when aluminum and aluminum alloy are immersed in acid or alkaline cleaning solution, the oxide film is easy to be dissolved, and the exposed metal matrix is quickly oxidized, resulting in difficulties in electroplating on the surface of aluminum alloy, poor adhesion of aluminum and aluminum alloy coatings or difficult to be plated. In order to solve the problem of repairing the hard aluminum parts on the aircraft after local damage, the researchers used brush plating technology to repair the local scratches on the surface of aircraft aluminum alloy parts [6], discussed the process flow and process specification of rapid repair, and measured the hardness, adhesion and wear resistance of the repaired parts. After surface treatment, the hardness of the coating is 625hv, the wear of brush coating is 18.5mg/h, and the wear of new parts is 23.8 mg/h. The coating has good adhesion without peeling and falling off. Brush plating for repairing scratches on the surface of hard aluminum components of aircraft has the advantages of simple process, convenient operation, low cost, high bonding strength between the coating and the base metal, and the coating quality meets the performance requirements, so it is worth popularizing. In order to improve the performance of aluminum alloy, the author used electroless plating to repair the surface of aluminum alloy. The treated samples have higher hardness and stronger corrosion resistance [7]. Fig. 2 is an atomic microscope photograph of the treated sample

in order to improve the maintenance quality of aircraft skin, someone has carried out surface modification technology of cerium conversion film on the surface of aircraft skin [8]. By testing the film weight and corrosion resistance of the oxide film, they preliminarily determined the main components and process parameters of the formula, and optimized the formula of cerium salt modified solution by orthogonal test. Under the optimum process conditions, the corrosion resistance of the conversion coating prepared with cerium salt oxidation solution can meet the requirements of aircraft skin maintenance

The tail rotor blade of

Zhi 9 is made of full composite material, but the blade root sleeve is made of aluminum alloy. The blade root sleeve is a very easily worn part due to the frequent axial reciprocating motion of the blade during operation. The friction part of the blade root sleeve is hard anodized, but there are many micro cracks in the hard anodized layer, resulting in poor fatigue performance and easy peeling off. During the repair, it is necessary to change the hard anodizing layer to plasma spray coating. Parts shall also be sand blasted between spraying [2]

3 application in aircraft steel structure maintenance

the material of many parts such as aircraft engine frame and actuator is 30CrMnSiA, namely aircraft steel. During the use of the aircraft, the surface may be scratched, worn, corroded, and dented, which must be repaired in time. For this reason, someone studied the process specification for brush plating repair of 30CrMnSiA steel parts of aircraft, and solved the problem that hydrogen embrittlement is easy to occur in the electroplating repair of 30CrMnSiA steel [9]. There are many parts made of the same or similar materials on the aircraft, which are repaired by brush plating technology. It is not only simple, convenient and rapid, low cost, but also the quality can fully meet the requirements. 30CrMnSiA aircraft steel can also be effectively repaired by comprehensive use of friction electro spraying technology and n-a12o3/Ni-W nano composite brush plating technology [10]. With this repair method, the surface repair layer fully meets the repair requirements, improves the wear resistance and coating bonding strength of the repaired surfaces of aircraft actuators and other components, and has good corrosion resistance. At the same time, it also solves the problem of hydrogen embrittlement. N-a12o3/Ni Cr coating is also used to repair the surface of aircraft steel

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