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Vacuum Heat Treatment of Aerospace Fasteners

Aviation fasteners have gone through 5 generations from the beginning to the present. The general trend is that the structure is developing towards light and small size and combination, and the performance is innovating towards high strength and high reliability. There are many types of fasteners used in aerospace structures, such as solid rivets, threaded pins, bolts, nuts, and one-sided fasteners, and of course other common fasteners, such as cotter pins, quick release locks, washers, etc. For these fasteners, there are only a few common materials. In addition to alloy steel and stainless steel, the common ones are aluminum alloys (including 2024, 6061, 7075) and titanium alloys (6-4Ti).

Aluminum alloy is probably the most widely used fastener material. In addition to its excellent strength-to-weight ratio, some aluminum alloys can also be vacuum heat treated to obtain relatively high strength. For example, aircraft manufacturers will use high-strength 7075 aluminum alloys for aircraft structures, because zinc and copper are added to this alloy to improve its own strength and processing performance. The advantages of titanium alloys are more, but the most important is that in composite material applications, it can meet the requirements for strength and corrosion characteristics at the same time.

All fasteners must undergo vacuum heat treatment during the manufacturing process, and many fasteners will undergo vacuum heat treatment multiple times. Aluminum alloy solid rivets, which are more common on aircraft, usually require solid solution strengthening and aging treatment to achieve the required mechanical properties. Solution treatment first heats the aluminum alloy to a range slightly below the eutectic melting temperature. At this temperature, the alloy components are dissolved before vacuum hardening. This rapid vacuum hardening retains the grain structure of the alloy, but softens the material itself, requiring subsequent age hardening at high temperatures. In the process of solution treatment, temperature control is very important, because the temperature of solution treatment is very close to the eutectic melting temperature.

This aging process increases the strength and hardness of the material, and the aging time and temperature control need to be adjusted according to the alloy used and the required mechanical properties. In general, the higher the aging temperature, the shorter the time required to achieve maximum performance.

Whether in fixed-wing, rotary-wing aircraft or in many spacecraft, the strength, corrosion and weight of structural components are important, and as long as there is a mechanical connection between structural components, the application of fasteners is inseparable. Therefore, it can be said that fasteners play a key role in defining the integrity and life of most aerospace structures.

With the development of aerospace technology, from the demand of fasteners, special aerospace fasteners, special-purpose fasteners and standard fasteners with strong technology and high performance will develop vigorously in the future. The application of the above-mentioned fasteners in emerging markets such as non-military and nano-aerospace will also increase. In addition, the rapid development of the aircraft manufacturing industry and the increase in military spending will further promote the development of the aerospace fastener market.