Application Of Titanium Alloy Fasteners In Aerospace Field

Application of titanium alloy fasteners in aerospace field

One C919 requires 200,000 pieces of titanium alloy fasteners, and the first batch of 100 orders directly generates a demand for 20 million pieces. Behind this number, there is a key fact hidden in the aerospace manufacturing industry, that is, without titanium alloy fasteners, the aircraft cannot fly.

Why are titanium alloys irreplaceable in the aviation field?

The strength of titanium alloy is comparable to steel, and its weight is only about 60% of steel. For aircraft, every kilogram of weight reduction means reduced fuel consumption, increased range or increased load capacity. For advanced passenger aircraft such as the Boeing 787 and Airbus A350, the amount of titanium alloy accounts for about 15% of the structural weight. More importantly, the corrosion resistance of titanium alloys far exceeds that of aluminum alloys, and it can maintain stability in environments with high altitudes, high humidity, and severe temperature differences. These properties superimpose each other, making titanium alloy the most reliable material for connecting aircraft frames.

In the segmented field of fasteners, the advantages of titanium alloys are becoming more and more obvious. The weight of an ordinary steel bolt is more than twice that of titanium alloy. On an aircraft, tens of thousands or even hundreds of thousands of fasteners are accumulated, and the weight reduction effect is extremely impressive. Moreover, the thermal expansion coefficient of titanium alloy is similar to that of carbon fiber composite materials . Titanium alloy processing is used in aerospace applications . It is used in composite material structures that are widely used in modern aircraft. It will not cause loosening or stress concentration due to temperature changes. This compatibility makes titanium alloy fasteners an obvious choice for composite fuselages.

How difficult is it to make a titanium alloy fastener?

For the production of titanium alloy fasteners, strict thresholds are set at the initial stage of raw materials. Wire materials and rods must meet AMS or GB/T The chemical composition and tolerance range stipulated by the standards, etc. After the raw materials enter the factory, they must undergo ultrasonic flaw detection to remove materials with internal cracks or inclusions. The subsequent upsetting and forming process is a more prominent test of the technological level. Titanium alloys have poor plasticity at room temperature and must be formed in a specific temperature range. Materials will crack at low temperatures, and grains will become coarse at high temperatures, which affects performance.

Heat treatmen There are extremely high requirements for temperature control accuracy. Once the furnace temperature deviation exceeds plus or minus 5 degrees, it is very likely that the entire batch of products will be scrapped. After heat treatment, the fasteners also need to undergo mechanical performance tests such as tensile strength, shear strength, fatigue life, etc. Each data must be within the narrow window specified by the standard.

How to solve difficult problems in processing

The thermal conductivity of titanium alloy is only one-sixth that of steel. During processing, a large amount of heat generated is concentrated in the cutting area. When cutting at high speed, the tool tip temperature can instantly reach more than 1,000 degrees, and ordinary carbide inserts will be worn out and scrapped in just a few minutes. A common practice in the industry is to use positive-angle blades. The cutting edges of this blade are very sharp and can reduce cutting force and heat generation. At the same time, the tool coating is also required to have high temperature resistance. If it is a TiAlN or AlCrN coating, it can form an aluminum oxide protective layer in a high temperature environment to delay tool wear.

The application method of cutting fluid has a direct impact on the processing quality. It is difficult for traditional pouring cooling to send the cutting fluid to the real cutting area. Titanium alloy fasteners are used in the aerospace field. Therefore, high-pressure and large-flow internal cooling must be used. The pressure of the cutting fluid usually needs to reach 70 to 100 bar, and it is sprayed directly from near the blade edge. This high-pressure cooling method can not only take away heat, but also wash away chips, preventing chips from wrapping around the workpiece and causing scratches on the surface. For difficult processes such as deep hole machining, it is also necessary to use a peck drilling process to make the tool withdraw periodically to achieve chip removal and prevent chip clogging and tool breakage.

How to prevent hidden dangers during installation

There is a problem that is easily overlooked during the installation process of titanium alloy fasteners, and that is gap corrosion. When there is a small gap between the fastener and the connection hole, moisture and salt in the environment will penetrate and cause electrochemical corrosion. Titanium alloy itself is corrosion-resistant, but the aluminum alloy structure in contact with it will accelerate corrosion. To solve this problem, the hole diameter tolerance must be strictly controlled during installation. Generally, the interference fit process is used to make the fastener diameter slightly larger than the hole diameter, and an interference fit is formed by press-fitting, thereby completely eliminating the gap.

Another installation difficulty is the protection of the surface coating. Aircraft structural parts are coated with primer and topcoat before assembly. However, the coating is easily scratched when fasteners are installed. Once the coating is damaged, corrosive media will invade from the damaged area. The current advanced technology is to re-apply sealant to the head and surrounding areas after the fastener is installed. This sealant must be oil-resistant and media-resistant, and must be able to adapt to vibration and temperature changes during aircraft service. Both Airbus and Boeing have sealing process specifications specifically for the installation of titanium alloy fasteners.

Looking at the reality and challenges of localization from C919

The C919 single aircraft uses 200,000 pieces of titanium alloy fasteners. This number has surprised many people. In fact, these fasteners are not all standard parts. They cover dozens of varieties such as bolts, screws, rivets, studs, etc., with specifications ranging from 2 mm to more than 20 mm in diameter. Among them, the most technically demanding ones are high-strength titanium alloy bolts used for wing docking and engine suspension. Such products have long been monopolized by a few companies such as Alcoa of America and LISI of France.

In recent years, domestic companies have made significant breakthroughs in the field of titanium alloy fasteners. Many companies such as Aerospace Seiko and aviation industry standard parts have built a number of production lines with a high degree of automation, achieving independent control of the entire process from the beginning of raw materials to the end of finished products. Driven by the C919 project, domestic titanium alloy fastener suppliers successfully passed NADCAP certification and entered the list of qualified suppliers of COMAC. However, for some high-end products, such as large-sized titanium alloy bolts with a diameter of more than half an inch, the localization rate is still low, and there is room for further improvement in the stability of materials and processes.

Differences between civil aviation and aerospace requirements

Many people think that aerospace standards are definitely higher than aviation standards. However, in the field of fasteners, the situation is exactly the opposite. Once a spacecraft is launched, the working time is limited. However, commercial aircraft have to fly for more than ten hours a day, more than 300 days a year, and have a continuous service life of 20 to 30 years. Such long-term and high-intensity use conditions put forward more stringent requirements for the fatigue resistance and stress corrosion resistance of fasteners.

Aircraft fasteners must undergo a complete set of airworthiness certifications before they can be used for installation. Take the anti-fatigue performance as an example of titanium alloy processing for aerospace applications . Fasteners need to undergo vibration tests and fatigue life tests to simulate the stress conditions of an aircraft in hundreds of thousands of take-off and landing cycles. The surface treatment process also needs to undergo strict weather resistance tests, including salt spray test, damp heat test and medium immersion resistance test. These test cycles last for several months or even a year. As long as any link fails to meet the standards, it will not be able to obtain airworthiness approval. It is this stringent certification system that ensures the safe flight of tens of thousands of flights every day.

A seemingly inconspicuous small part, that is, a titanium alloy fastener, is actually a key yardstick used to measure the level of a country's aviation manufacturing. In the field of high-end manufacturing, what other "small but critical" components like titanium alloy fasteners do you think deserve our attention? Feel free to share your thoughts in the comment area.