Titanium alloy fasteners are widely used in modern advanced aircraft structures due to their excellent overall performance such as light weight, high specific strength, fatigue resistance and corrosion resistance. In the aircraft connection assembly, there is a high potential difference between the titanium alloy fastener and the aluminum alloy material. If the joint is improperly treated, the potential corrosion is easily generated, and the corrosion will seriously cause the structure to fail. Therefore, solving the potential corrosion problem when the titanium alloy fasteners are connected with the aluminum alloy structure is an important task to ensure the long-life and high-reliability connection of modern aircraft [1]. There are many ways to prevent potential corrosion of aluminum alloy structure assembled titanium alloy fasteners. The best solution is to apply a layer of anti-corrosion aluminum coating on the surface of titanium alloy fasteners. The manufacturing industry is heavily adopted. This paper describes the protection mechanism of titanium alloy fasteners and aluminum alloy structure connection, titanium alloy fastener coating technology and aluminum coating effect, and provides efficient and reliable aluminum coating for domestic titanium alloy fasteners used in aluminum alloy structures. Technical method. 1 aluminum coating protection mechanism and effect When titanium alloy is assembled in aluminum alloy structure, electrochemical corrosion will occur under the action of electrolyte due to the high potential difference. There are two basic ways to protect against electrochemical corrosion: (1) Insulate the anode or cathode; 2) Eliminate the ionic conductivity of the anode or cathode region. Based on the above two approaches, there are mainly four protection methods: (1) fasteners using cadmium-plated or aluminized coatings; (2) fasteners coated with organic or inorganic coatings; and (3) installation A wet primer or elastomeric sealant is used in the process; (4) the fastener or structural component is coated with a zinc-chromium coating. However, none of the above methods can fully protect the surface, and the surface layer produced by some of the methods does not have sufficient toughness or viscosity, and there are problems such as excessive manufacturing cost, which prompts people to conduct research to obtain more. Good method, such as coating a layer of aluminum, fluorine and chromate containing or molybdenum disulfide, fluorine and chromate type coatings on the outer surface of fasteners or structures, resulting in aluminum coating technology The birth of. Titanium alloy fastener anti-corrosion aluminum coating developed by foreign companies [2] can coat coatings with uniform thickness, smooth and compact, and good protection performance, not only can meet the high temperature use conditions of 235 ° C, but also need not be prevented Electrochemical corrosion provides additional sealing during assembly and has excellent bonding strength with the matrix material without adversely affecting the matrix material. In addition, aluminum coating is used for the protection of various titanium alloys and high-strength corrosion-resistant steel fasteners. It has excellent corrosion inhibition effect, and has good stress and embrittlement resistance under high temperature and high stress. It can resist various hydraulic liquids, fuel oils, paint removers and cleaning agents without deterioration, heat resistance and good adhesion. 2 aluminum coating technology Titanium alloy fasteners used in China mainly use bare state or blue anodic polarization state. With the development of various domestic models, there is a strong demand for aluminum alloy fastener aluminum coating technology. Titanium alloy fastener surface coating aluminum technology mainly includes aluminum coating, coating method and coating process. Aluminum coatings are the basis of aluminum coating technology, and their quality is related to whether the performance requirements such as anti-corrosion can be achieved after coating the surface of the fastener and applying it to the surface of the fastener. The coating method is related to the quality consistency of the product and the ability to achieve mass production, and the coating process is the key to the bonding ability and quality stability of the aluminum coating and the titanium alloy fastener. 2.1 aluminum coating The specific composition, size, content and other factors contained in the anti-corrosion aluminum coating have a major impact on performance. The ingredients mainly include aluminum powder, binder, preservative, lubricant, thinner, etc., all of which have certain fineness requirements. The resin used is benzaldehyde resin, the preservative is a chromium salt compound, and the lubricant is PTFE. The powder is adjusted by spraying with a thinner. The coating has strong bonding strength, the surface color is consistent with foreign products, and the tissue state is also consistent. The specific situation is shown in Figure 1. Fig.1 Structures of domestic (a) and exotic (b) aluminum coating 2.2 Coating process The coating process must ensure that the appearance after coating is free of defects and the coating thickness is uniform. The aluminum coating for the aluminum coating of the fastener is in a liquid state, and the coating method can be spray coating and centrifugal dip coating. For the surface coating of civil fasteners, most of the industry uses centrifugal dip coating and spraying. The centrifugal dip coating process is to soak the workpiece in the paint. After the paint completely covers all the workpieces, the workpiece is taken out and placed in a centrifuge, and the paint attached to the surface of the workpiece is homogenized by the action of centrifugal force, and the excess paint is thrown. Out, thereby achieving high-volume coating of the workpiece. The process has high production efficiency, low labor intensity and easy automatic control, and is a commonly used method in the industrial production of steel fasteners. However, for the surface of titanium alloy fasteners coated with aluminum, this process has the following problems: the surface of the workpiece is rough after centrifugal drying. During the centrifugation process, due to the mutual contact between the workpieces, the coating is likely to be deposited on the lap joint. Or the phenomenon of sticking out and not conforming to the requirements of the standard. The spraying process is divided into manual spraying and equipment spraying. Manual spraying can obtain a relatively uniform surface after coating. The thickness of the coating can be controlled within a certain range to meet the standard requirements, but its efficiency is low, generally relying on the skill of the operator. Degree guarantee. Taking into account the working principle of the centrifugal dip coating process and the spraying process, a process method using an aluminum coating machine for spraying is developed. The aluminum coating machine has the characteristics of high production efficiency, high utilization rate of paint, simple operation, data acquisition and printing, and can meet the mass production requirements of titanium alloy fastener coated aluminum coating, and meet the coating performance and quality. Uniformity requires uniform and smooth surface. The metallographic examination of the test piece coated with the aluminum coating machine is carried out to observe and measure the coating thickness of the bolt portion and the threaded portion, and the obtained coating thickness is between 7 and 10 μm (the standard prescribed coating thickness is 5 to 12.5 μm). ), fully in line with the standard requirements (Figure 2). Fig.2 Shaft (a) and thread (b) Profile of the area of ​​the fastener 3 performance test of aluminum coated parts After the aluminum alloy is coated with aluminum, its performance is qualified. Physical, chemical and metallurgical inspections are required, and the test parts are subjected to installation test, coating performance test and mechanical property test to verify the conformity of the coating and coating process. Through a series of tests, various performance indexes of products coated with domestic aluminum coating and coating process have met the requirements of the standard. The specific test results are as follows: (1) Installation test: The test piece was interferingly mounted into the aluminum plate, and the rod portion was observed after the installation, and no peeling of the coating was observed. (2) Appearance: The surface of the coating is smooth and flat, with golden yellow luster, and there are no uneven defects such as pores, bubbles and pits. (3) Coating thickness: The coating thickness of all working faces of the tested fasteners is between 5 and 12.5 μm, and the coating of the sharp edges of the parts is completely covered. (4) Bonding test: The results of the drop weight test showed that the coating of the fastener did not show obvious peeling of the coating. (5) Paint adhesion test: Test according to the standard, the test pieces meet the requirements of the index. (6) Fluid resistance: The workpiece is immersed in a certain type of fluid and immersed for 30 days at a certain temperature. The hardness of the coating and the bonding strength of the coating are all required. (7) Paint-resistant paint: The fastener is immersed in a certain type of paint remover for 24 hours at room temperature, and its bonding force and coating hardness meet the requirements. (8) Brittleness: 4 countersunk titanium alloy fasteners are assembled in high-strength aluminum alloy blocks as required, and loaded according to the standard. At least 3 days after exposure at a certain temperature, no rounding of the fastener head is found. There are cracks in the area and the threaded area, so the brittleness test meets the requirements. (9) Anti-corrosion: Install 5 fastener gaps on the aluminum test piece and perform acetic acid-salt spray test. The test results show that the corrosion surface of the aluminum alloy surface with the aluminum-coated titanium alloy fastener is smaller than the uncoated aluminum. Fastener aluminum alloy to meet design requirements. (10) Heat resistance: The test piece is heated to a certain temperature and kept warm, cooled in air, and subjected to visual inspection and bonding test of the heated fastener sample, and the coating is free from bubbles or cracks. The coating did not show significant peeling of the coating after the hammer test. (11) Assembly pressure: The assembly pressure is installed according to the requirements, and the test results are all satisfactory. (12) Mechanical properties test: For the tested parts and uncoated coatings, the test pieces are based on GJB715, and the shear test, tensile test, long-term stress and high cycle fatigue are performed according to HB6568. The test results are all in accordance with the requirements. . 4 Conclusion For the installation of titanium alloy fasteners for aluminum alloy structures, since the two have a high potential difference, if the isolation measures are not used, the aluminum alloy structure is severely peeled off due to the action of the electrolyte, causing the structure to fail. The potential corrosion problem can be well solved by spraying aluminum coating technology on the surface of the fastener. Through the research and development in recent years, the company has already had the aluminum coating technology and the automatic coating technology that meet the performance requirements. The coated workpieces have passed the identification to meet the requirements of foreign BMS10-85 and other standards. Through serialized testing and production, it has been able to carry out For mass production, the next step is to test on other types of titanium alloy fasteners, alloy steel fasteners and other material fasteners to meet aircraft assembly requirements. references [1] Niu Chunyun. Design and manufacture of aircraft composite structure [M]. Xi'an: Northwestern Polytechnical University Press, 1995: 231 [2]Hi-Shear Engineering Report 4-01020 [R]. Hi-Shear Corporation, 1972 6 Inch Floor Trap,6 Inches Floor Drain,6 Inches Square Shower Floor Drain,6 Inch Stainless Steel Shower Drain Kaiping City Jinqiang Hardware Products Co.,Ltd , https://www.jmpowerdrain.com
