The design and construction of aircraft usually use mechanical connections as a preferred solution, so there are many issues to consider in combination: application environment, temperature, load, material compatibility, etc. The fastener used in aircraft structure is a very broad concept. In terms of form, there are threaded fasteners and riveted fasteners.
From the way of installation, there are two-piece products, there are single-side installation products; In terms of function, Generalized fasteners also include screw sleeves, bearings, pipe joints, and lock structures.
In addition to load force and size, the correct selection of aerospace fasteners also needs to consider many factors comprehensively to ensure the effectiveness and stability of the mechanical connection.
In this article, we separate the aerospace fastener materials, aerospace fastener coating treatment, aerospace fastener lubrication, aerospace fastener corrosion, and other aspects of the expansion.
Aerospace Fasteners Materials
Most aerospace bolt materials are various alloy steel or stainless steel, as well as titanium alloy materials.
Alloy steel is a kind of high-strength carbon steel, so it will be used in places where the load force is relatively large, but it is not corrosion-resistant, because aerospace fasteners of this material must be surface-treated to prevent corrosion that may occur during application.
Stainless steel is a type of alloy steel, such as A286, which is an aged hardened austenitic iron-based material, ideal for prolonged exposure to a variety of temperatures, has extremely high oxidation resistance even at high temperatures, and is widely used in aerospace manufacturing due to its excellent creep strength.
The biggest advantage of titanium alloy is that it helps reduce weight. In some places where high-strength joints are needed, steel is still preferred, but titanium has gradually replaced it to reduce structural weight. In addition, titanium alloy has a similar potential to carbon fiber composites, so titanium alloy is more widely used with carbon fiber composites.
Analysis of Titanium Alloy Materials for Aerospace Fasteners
Since the 1950s, all countries in the world have chosen titanium alloy as the main material of fasteners used in aerospace. This is because titanium alloy has ideal high-temperature resistance and corrosion resistance, as well as good strength and elasticity in line with industry requirements, which can effectively solve the problem of bolt loosening and reduce the interference brought by a magnetic field.
Advantages of Titanium Alloy Materials
The difference between the characteristics of steel and titanium alloy is shown in Table 1. The research shows that the advantages of titanium alloy fasteners are mainly reflected in the following aspects:
Nonmagnetic
Titanium alloy only has a very small magnetic permeability, aerospace fasteners made from titanium alloy are non-magnetic aerospace fasteners, which can minimize the interference caused by a magnetic field. Austenitic stainless steel, which is also a non-magnetic material, usually produces a certain magnetic property after cold processing, while cold and hot processing will not change the magnetic properties of titanium alloy. Therefore, it can be seen that titanium alloy has a very prominent advantage in the manufacture of avionics equipment.
Low Density
The smaller density determines that titanium aerospace fasteners are lighter than those made from other materials aerospace fasteners.
High Melting Point
Tests have confirmed that titanium alloy has a higher melting point than steel material, so its heat resistance is often more consistent with aerospace fastener requirements.
High Yield Strength Ratio
The critical strength of aerospace fasteners under the influence of tensile load is the yield strength, followed by the tensile strength because the emergence of yield deformation will lead to aerospace fasteners can no longer play their proper fastening role.
High Strength-Weight Ratio
As a common metal with high specific strength, titanium alloy can be used as a substitute for light aluminum alloy. If the applied load is unchanged, the size of parts made of titanium alloy is usually smaller than that of aluminum alloy, which can make full use of the space. It is of great significance to the aviation industry.
Good Heat Resistance
The new titanium alloy can be used for a long time at temperatures of 600℃ or higher.
Highly Matched Composite Material
Another reason why titanium alloy is frequently used in aerospace fastener manufacturing is that its electrode potential is highly matched with carbon fiber material, which can avoid galvanic corrosion and lead to fastener failure.
The Elastic Modulus And Thermal Expansion Coefficient Are Small
The formula for calculating thermal stress is: Δσ=EαΔT
Therein, E represents elastic modulus, α represents thermal expansion coefficient and ΔT represents temperature change. It can be seen from the above formula that both steel and nickel alloys are larger than titanium alloys in terms of elastic modulus and thermal expansion coefficient. Therefore, under the same temperature, titanium alloys with small thermal stress are naturally superior to other materials in thermal fatigue performance.
Rivet Fastener Material
The fasteners used in the aviation industry can usually be divided into rivets, bolts, and special fasteners. Among them, the material requirements of rivets are ideal in cold plastic states, because only rivets with a good cold plastic state can meet the conditions of cold riveting installation. At present, titanium alloy rivets are mainly used in parts that require ideal corrosion resistance and low strength requirements. The β grains with outstanding cold working performance have naturally become the first choice for the manufacture of rivets. The titanium alloys introduced below are all β titanium alloys.
TB2 is a typical metastable titanium alloy, its nominal component is Ti-3Al-8Cr-5Mo-5V.
TB2 in a solid solution state has excellent welding performance and cold-forming performance. At present, it is mainly used to manufacture bolts, cold headings, rivets, satellite-arrow connecting belts, and satellite corrugated shells. In the future, TB2 will have greater application space, and it is necessary to carry out in-depth research based on it.
TB5, as the nominal component of metastable titanium alloy, is mainly Ti-15V-3Cr-3Sn-3Al.
In terms of cold-forming performance, the performance of TB5 is roughly the same as that of pure titanium. Relevant personnel can first do solution treatment for TB5, and then through cold forming, different types of fasteners can be obtained. In addition, the tensile strength of TB5 after aging can usually reach about 1000MPa. TB5 has been used to build Boeing planes for decades.
Ti-45nb is a stable titanium alloy for rivets. In the early stage, rivets were mostly made of pure titanium, but pure titanium fasteners with low strength could not be used in high-bearing parts. The aerospace industry was in urgent need of materials with similar plasticity and higher strength than pure titanium, and TI-45NB emerged at the right moment. In the development of Ti-45Nb, the researchers chose to use a metastable titanium alloy with greater deformation resistance and room-temperature plasticity than pure titanium to meet the requirements of tight firmware, for the synthesis of a new material, namely Ti45Nb. The TI-45NB proved to have a number of advantages.
Bolt Fasteners Material
Titanium alloy bolt is the most used aviation solid part, based on their use, titanium alloy bolt can be divided into three categories, are ordinary bolt, interference bolt, and high locking bolt. In general, titanium alloys used to make bolts should have ideal shear strength and tensile strength. One is the TB3. TB3 can be strengthened by heat treatment, and its nominal composition is Ti-10Mo-8V-1Fe-3.5Al.
The research shows that the advantages of TB3 are mainly reflected in the aspect of good cold-forming performance. Under general conditions, the cold heading ratio of TB3 can reach about 2.8, and the solid solution processing of TB3 can significantly improve the overall strength, which has been used for the manufacture of high-strength fasteners. The second is TB8. As a metastable titanium alloy, the nominal composition of TB8 is Ti-3Al-2.7NB-15Mo.
The advantages of TB8 are mainly reflected in its excellent cold and hot processing ability, good hardenability, outstanding creep resistance, and good corrosion resistance. In addition, with the addition of stable elements Nb and Mo, its self-diffusion coefficient and melting point give TB8 a more ideal oxidation resistance than other titanium alloys. At present, TB8 bolts have been widely used in the design and production of key products in the aviation industry.
Third, the TC4. TC4 fastener types can be subdivided into bolts, screws, pull nails, and rivet rings, its application fields are mainly satellite, aircraft, and aircraft equipment. The results show that TC4 is a two-phase titanium alloy with moderate strength, and most of the existing fasteners are made of TC4. It should be noted that the manufacturing process of TC4 tight firmware can only be used for hot upsetting, and the corresponding heating and hot upsetting equipment is required. Both the material utilization rate and the production efficiency are difficult to reach the ideal level. In addition, the strength of TC4 fasteners can not meet the requirements of high-strength fasteners. Poor hardenability determines that the section size of TC4 fasteners usually does not exceed 19 mm during solution aging. Four is TC6. TC6, a two-phase titanium alloy with good comprehensive properties, has the advantages of α titanium alloy and β titanium alloy. Its nominal composition is Ti-6Al-2.5M0-1.5Cr-0.5Fe-0.3Si.
Except for special circumstances, TC6 should be used after annealing. Of course, the relevant personnel can also use heat treatment to strengthen its strength.
Research shows that TC6 has the advantage of excellent oxidation resistance, which is one of the reasons why it is widely used in the aerospace fastener manufacturing field. As a two-phase titanium alloy, TC16 is characterized by solution aging strengthening, which is also the difference between TC16 and other titanium alloys. Its nominal composition is Ti-3Al-5M0-4.5V.
After solution treatment, TC16 can show good plasticity at room temperature, which is also the main reason for its excellent cold-heading performance. Generally speaking, the pier forging ratio of TC16 can reach about 1∶4. Based on TC16 carrying out the fastener manufacturing, there are two kinds of available technologies, namely hot heading forming and cold heading forming, which can be selected by relevant personnel at their discretion to ensure that the manufacturing efficiency and fastener quality can reach the ideal level. At present, TC16 is mainly used for the manufacture of screws, bolts, and nuts, but is not extended to other fields, which need special attention.
Aerospace Fastener Surface Plating and Coating
The application of plating or coating to the surface of a material is a common surface treatment. Most plating processes are performed by electrolysis, which produces hydrogen, so the plating process requires baking to prevent hydrogen embrittlement. The following are some of the more common coatings or coating options used in aerospace fasteners:
Cadmium plating processing
Cadmium plating is accomplished by means of electronic deposition. Because the melting temperature of cadmium is 600 ° F, the limit application temperature for cadmium plating is 450 ° F.
IVD aluminizing
Ion vapor deposition aluminum plating, a process originally designed for McDonnell Douglas, does not produce hydrogen arbitrability compared to cadmium plating, and most importantly, it is not as harmful as cadmium plating. The IVD aluminum plating process does not produce any hazardous waste. But its processing requires a vacuum environment, so the cost is higher.
Coating/Solid powder coating
Hi-kote and Kal-Gard are the most representative. It is a phenolic resin-based aluminum coating, usually with a certain lubricant added to the coating, which is very conducive to the installation of external thread fasteners under a close tolerance fit.
Anodizing
Anodizing is to making metal fasteners, in electrolyte solution through an applied anodic current, so that the surface of the formation of a layer of oxide film, so commonly known as “surface anodization”. Anodized surfaces block the passage of electrical current to prevent electrochemical corrosion.
Silver plating
Silver plating is extremely expensive. In the aviation industry, stainless steel bolts with silver-plated nuts are the most common. Silver is used both as a preservative and as a dry lubricant. Its application temperature can reach 1600 degrees Fahrenheit. Silver nuts need to be coated with transparent wax to prevent tarnishing because silver will fade when exposed to air.
Matters needing attention on surface treatment of Aerospace fasteners
The effect of plating or coating thickness on tolerance fit. For example, on the surface of a cylinder, the diameter of the part can be doubled by the coating or coating thickness; On the threaded surface, the pitch circle diameter increases fourfold.
Environmental and safety considerations. For example, how to handle electroplating water containing heavy metals, oils, grease, and suspended solids.
In the process of surface treatment, aerospace fasteners can be colored to provide different colors for parts, and thus provide the convenience of accurate identification and differentiation for the whole manufacturing field.
In the process of surface treatment, aerospace fasteners can be colored to provide different colors for parts, and thus provide the convenience of accurate identification and differentiation for the whole manufacturing field.
Aerospace Fastener lubrication
In aerospace threaded fasteners in the installation process, thread abrasion is a very troublesome problem, and the use of necessary lubricants on the fastener surface can effectively reduce the occurrence of wear. There are many choices of thread lubricants, the most common being grease or crystalline wax, graphite, and molybdenum disulfide.
Oils and fats, though the most common, are limited in their application temperature to 250 ° F, above which they will melt or boil. In addition, grease lubrication cannot be used in a vacuum.
Crystal wax lubrication is the most commonly used hexadecanol, it can adhere to the aerospace fastener in the form of dry film, has excellent lubrication performance, is different from black molybdenum disulfide, its dry film layer is transparent, will not affect the appearance of aerospace fasteners and aerospace fasteners special coloring requirements. Cetyl alcohol is therefore commonly used for the lubrication of ring-grooved nails, pull mandrel rivets, and high-lock bolts.
As a lubricant, graphite requires a finely powdered toner to be dissolved into oil or water to lubricate it. Therefore, it is similar to grease lubrication in that there is a limit to the maximum operating temperature, which is the boiling point of water or oil, and it cannot be used in a vacuum.
It is the most commonly used lubricating material in the aviation industry. Usually, molybdenum disulfide is mixed with epoxy resin, curing agent, etc., and it is attached to the surface of fasteners by spraying or soaking. After heating and curing, a film is formed to firmly adhere to the surface of fasteners, and this film is often referred to as the “molybdenum disulfide dry film lubrication layer”. It is important to note that molybdenum disulfide is oxidized to molybdenum trisulfide at about 750 ° F and failure occurs. Molybdenum disulfide is commonly used for the lubrication of self-locking nuts.
Corrosion of aerospace fasteners
Electrochemical corrosion
For all metals, including those used in aerospace fasteners, there is an electric potential. There are different electric potentials because of different metals. Therefore, when two different metals interact with water as an electrolyte, a kind of galvanic cell is formed, and currently begins to flow between the two metals. This low-energy electrochemical current begins to degrade one of the metals. More specifically, it is a metal with a higher potential that is degraded. The metal with the higher potential is called the “anode”, and the atoms of the anode metal lose electrons to the cathode, which is a metal with a lower potential.
Stress Corrosion
In aviation applications, especially in engine applications, there are many high-temperature environments. A sharp increase in ambient temperature will accelerate the rate of chemical reactions and the moisture content in the saturated air to accelerate corrosion. In addition, some corrosion starts during machining, and processes such as machining, molding, and heat treatment can leave stress on aircraft parts. When the stress corrosion threshold is exceeded, this residual stress can cause part failure in a corrosive environment.
When stress and corrosion work together, it is what we often call stress corrosion cracking. If there is uneven deformation of the metal during cold working or uneven cooling due to high temperature, this will cause the internal structure of the metal to be rearranged, and therefore internal stress will occur. This internal stress may also occur during the installation of aerospace fasteners, such as pressing deformation of bushings, installation of bolts under interference fit, etc.
Hydrogen Embrittlement
Due to its small size and lightweight, atomic hydrogen is highly diffused in most metals and easily penetrates through most clean metal surfaces. It migrates rapidly to a favorable position where it may remain in solution and precipitates in the form of molecular hydrogen, forming small pressurized cavities, cracks, or large bubbles. Hydrides may also be formed with base metals or alloying elements.
YIJIN Hardware Aerospace Fasteners Samples
Bolt Series
By providing a high-strength controlled preload system, this series of bolts can provide a universal solution for most structural installations. The bolt is easy to install smoothly by a single person on one side, even in the most restricted areas.
YIJIN bolts are the original solution for solidity and reliable assembly.
Main product features
Controllable preload;
A wide range of materials and coatings is suitable for installation on all structures;
It can be used for tensile and shear purposes, mounted on a plane or inclined plane;
Can be installed in areas with limited space with conventional installation tools;
Suitable for clearance, transition, and low interference with the installation.
Nuts, High Lock Nuts
Nuts and high lock nut products are designed to match associated custom bolts manufactured by YIJIN. The wrench structure of the high lock nut is designed to separate under a predetermined preload.
Main product features
Controllable preload of high lock nut;
A wide range of materials, and coatings, suitable for installation on all structures;
For shear or tensile stress environment, with fixed washer, automatic adjustment, or automatic sealing series;
It can be installed in a restricted area with traditional tools.
Blind Fasteners
Blind fasteners are a new solution that saves time by allowing fully automatic structural assembly, including automatic installation inspections. The fastener is designed to provide the same performance in terms of preload, shear force, and dimensions as fasteners currently used in shear environments.
Main product features
Designed for clearance and interference fit;
Can be used in metal, composite materials, and mixed structures;
Compatible with traditional aperture sizes;
Direct replacement for permanent structural fasteners such as self-locking bolts;
Optimized clearance, no scratches;
Can be automated or manual one-sided installation.
Screw
YIJIN series products are designed for metal and composite structures and are lightweight substitutes for cadmium-plated alloy steel or dry film-lubricated corrosion-resistant steel nuts on new-generation aircraft.
Main features
Compared with the traditional nut, it is lightweight, with the same mechanical properties;
The optimized contact area has no damage to the composite;
YIJIN coating is used for low friction and is environmental-friendly.
Aerospace Fasteners
Fasteners are used to increase the fatigue life of shear components and structures. These fasteners are installed through a specific pull-out process to solve the problems of push-in installation systems, such as bolt head damage, composite structure damage due to impact, and installation noise.
Main product features
Fast and quiet installation process;
Pull-stem has up to 1.5% interference, and Pull-in has up to 0.9% interference;
Suitable for metal construction and mixed (metal/composite) components;
YIJIN’s proprietary coating stripe technology can be provided, which is used for lightning protection.
Bushing Bolt
Bushing bolts have been developed to provide lightning protection for a new generation of aircraft structures. STL systems provide a high level of mechanical properties that allow interference fit without causing damage to composite structures.
Product Features
Suitable for limit area installation;
The tightly controlled radial expansion provides lightning protection, provides better conductivity, and avoids compound delamination；
Enhance mental structure’s life fatigue resistance；
The installation process is simple and reliable.
Thermal Insulation Blanket Fastener
This is a kind of fastener widely used in the aviation industry to fix the insulation blanket on the longitudinal beam, frame, or fuselage of the aircraft.
Main product features
Lightweight and Fire resistant;
Can be used for non-metallic and metal materials；
Non-penetrating fasteners can be installed without piercing the insulation blanket；
Mounting methods include clamping, riveting, or bonding.
Limited by space, the basic knowledge of aerospace fasteners is introduced here. The following are some customized aerospace fasteners produced by our company for customers.
