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Welcome to our guide to specifying aerospace fasteners; 2020 edition.

The aerospace industry has been around longer than what we now call the aeroplane.

Throughout its history the aerospace industry has continued to develop and introduce new, ever more powerful products.

The jet engine, space-bound rockets, and ballistic missiles to name a few.

Aerospace fasteners have necessarily evolved alongside to meet the rising quality and performance standards required by ever more high-performance machines.

Achieving heavier than air travel requires a combination of engineering precision, physics and a strict adherence to the highest quality and manufacturing standards.

It’s as a result of these standards that aerospace fasteners are not solely limited to use in the aerospace industry.

The designation then of a fastener as an “aerospace fastener” can be used as a quality assessment for the whole of the fastener industry.

In this guide we explore some of the key specifications for aerospace fasteners and the performance characteristics that these specifications and manufacturing materials and techniques result in.

A Quick Overview of the Fastener Industry

There are various standard fasteners employed across the aerospace industry. These include:

• Screws;
• Rivets;
• Nuts;
• Bolts;
• Collars.

Aerospace fasteners are designed to withstand the extreme environments that they often experience.

For example, they need to be durable and resistant to corrosion. They also need to withstand extreme pressure and dramatic temperature changes.

As a result of these exacting needs, there are various different designs for high-quality fasteners which have been developed.

When it comes to aerospace fasteners it’s not a case of one size fits all. That being said, aerospace fasteners are always made to demanding specifications.

Aerospace Fasteners and Performance

Some of the key characteristics that aerospace fasteners, as a rule, are designed for include the following:

• Resistant to corrosion and oxidation (even in extreme conditions);
• Durable and strong with high shear and fatigue strength;
• Lightweight (often there is a careful balance to be found between lightweight for optimization of aircraft performance, loss of strength that comes with some lightweight materials, and the cost);
• Temperature resistant (they need to function optimally in a large temperature range).

Typical Aerospace Fastener Materials

As we’ve already mentioned there are various designs for different fasteners which are dependent on their desired characteristics of the fastener and the type of extreme conditions it will experience.

An instrumental part of a fastener’s characteristic are the materials that the fastener is made of.

Some of the more commonly used materials in aerospace fasteners include:

• Steel
• Aluminium
• Titanium
• Super-alloys

Steel

Steel and steel-based alloys typically feature greater strength than other materials. However, this extra-strength comes at a cost.

Steel is heavier, which can cause issues, especially when designing and constructing aircraft.

Serious careful consideration needs to be taken when using steel as a part in aerospace design.

Normally when steel is used for aerospace applications, they mean stainless steel or particular steel alloys.

Certain steel alloys are susceptible to heat damage, however, which makes it doubly important that the proper series of stainless steel is chosen for the specific requirements of its aerospace application.

Three examples of this are:

Series C300: This series is corrosion resistant (CRES) stainless steel. It doesn’t, however, have the same heat resistance as other types of stainless steel but is often used for covers.
CRES series 400: This series features much greater heat resistance than the series C300 but as a trade-off of sorts it’s more susceptible to corrosion.
Precipitation-hardened (PH) stainless steels of various grades are also used for some fastener applications.

There are also surface treatments that can be used on steels to improve their performance.

Aluminium

Aluminium has obvious benefits for the aerospace industry. It being much lighter than metals like steel.

However, it isn’t as strong and to attain industry acceptable performance characteristics it must be cold-heat formed and then undergo additional surface treatments.

Unfortunately, whilst these processes increase its strength and corrosion resistance, aluminium remains highly sensitive to dramatic temperature changes – and doesn’t operate well above around 125 degrees Celsius.

Titanium

Some of the advantages of using titanium are that it is lightweight (compared to steel), has strength that is comparable to steel, and it is resistant to heat and cold.

Specified operating temperatures range between -200C – 430C. This makes it a good material choice under certain circumstances.

Superalloys

High-performance alloys, as they are also known, are frequently used in the aerospace industry due to their broad range of specifications and their high-performance qualities.

They can withstand the many different types of stress that fasteners experience in aerospace equipment, whilst maintaining their structure and surface integrity.

Common super-alloys include:

A286: an iron-nickel-chromium alloy which can withstand temperatures ranging between minus 420 and 1200 degrees Fahrenheit. It exhibits high strength and corrosion and oxidation resistance. Suitable for use in engines, superchargers, and turbines.
H-11: a 5% chromium tool steel alloy which exhibits high impact resistance and surface hardness. Suitable for use in structural and highly stressed components, such as landing gears.
Hastelloy® (a registered trademark of Haynes International, Inc.): a nickel-molybdenum-chromium super-alloy which exhibits high corrosion resistance. Suitable for use in combustion and exhaust components.
Inconel 718® (a registered trademark of Special Metals Corporation): a nickel-based super-alloy, retains a 220ksi (kilopound per square inch) tensile strength up to 900 degrees Fahrenheit.
Monel® (a registered trademark of Special Metals Corporation): a nickel-copper alloy which exhibits high tensile strength and corrosion resistance. Suitable for use in structural components, as well as combustion and exhaust equipment.
Waspaloy® (a registered trademark of United Technologies Corp): a nickel-based super-alloy capable of withstanding temperatures up to 1600 degrees Fahrenheit, as well as exhibiting high corrosion and oxidation resistance.
MP35N® (a registered trademark of SPS Technologies, Inc.): a nickel-cobalt based alloy which exhibits high tensile strength, surface hardness, and corrosion resistance. Suitable for use in structural components.

Fastener Covers

An aspect of the aerospace fastener industry that ironically often gets overlooked is fastener covers.

These are required for the purpose of protecting fasteners from additional stress. Fastener covers are made from a broad array of different materials, which like the fasteners themselves are used for their specific characteristics.

Examples of materials used for fasteners covers include, but are not limited to:

• Phosphate;
• Zinc;
• Nickel;
• Silver;
• Black Oxide;
• Cadmium.

Conclusion

This guide provides a basic understanding of fasteners, their component materials and characteristics.

Aerospace fasteners are designed for purpose to the highest and most exacting quality standards. Having the right fasteners for your specific needs is vital for the optimal performance of any aircraft.