Feb,08 2022
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Pure aluminum and pure magnesium are completely unsuitable as structural materials for the fuselage because of their very low strength. However, when alloyed (chemically mixed) with each other or with other metals, their strength is greatly increased, and they form the most widely used group of airframe materials. Alloying metals include zinc, copper, manganese, silicon and lithium, either alone or in combination.
There are many different variants, each with different characteristics and therefore suitable for different uses. Magnesium alloys are easily attacked by seawater, and their use in carrier-based aircraft is generally avoided. Aluminum alloys, while denser than magnesium alloys, are less susceptible to chemical attack, and are less expensive, making them more widely used. Known as duralumin, the 2024 alloy is composed of 93.5% aluminum, 4.4% copper, 1.5% manganese and 0.6% magnesium, and is the most widely used of all materials in aircraft construction. Aluminum alloys corrode more easily than pure aluminum, so pure aluminum is often rolled to form a protective layer on its alloy surface. This process is called cladding, and alloy sheets treated like this are called clad sheets or aluminum cladding. Another common method of protecting aluminum alloys is anodizing - an electrochemical process that converts the surface layer into a more corrosion-resistant form. Aluminum-lithium alloys are superior to aluminum-zinc and aluminum-copper alloys in strength and stiffness, so they can be used to reduce weight. Their use is limited because they cost about three times as much.
An interesting property that some aluminum alloys share with titanium is that they can be superplastically formed (SPF). When a material is heated to a temperature, well below its melting point, it can be stretched several times its own length without tearing or localized thinning. It can then be deformed using an inert gas such as argon to fill the mold and form accurately, without spring back when the pressure is released. There are various techniques based on this property for extremely complex shapes with precision and minimal weight. The high initial cost of tooling means that SPF is limited to certain high-cost items and is not yet suitable for mass production. Items such as pressure vessels, small tanks and reservoirs can be made using this technique.
The advantages of aluminum magnesium alloy
1. High strength to weight ratio
2. Various alloys to suit various applications
3. Low density, and therefore greater volume for the same weight, means they can use greater thicknesses than denser materials, and are therefore less prone to local buckling; this applies to magnesium alloys, even more than aluminum alloys
4. Available in a variety of standard forms - sheet, plate, tube, bar, profile
5. Aluminum alloy is easy to process after simple heat treatment
6. Can be superplastically formed (some aluminum alloys only)
shortcoming
1. Easily corroded, so protective finishes are required, especially magnesium alloys
2. Many alloys have limited strength, especially at high temperatures
3. Magnesium alloys have low strength (but high strength-to-weight ratio)
4. No fatigue limit (see section on fatigue later in this chapter)
There are many different variants, each with different characteristics and therefore suitable for different uses. Magnesium alloys are easily attacked by seawater, and their use in carrier-based aircraft is generally avoided. Aluminum alloys, while denser than magnesium alloys, are less susceptible to chemical attack, and are less expensive, making them more widely used. Known as duralumin, the 2024 alloy is composed of 93.5% aluminum, 4.4% copper, 1.5% manganese and 0.6% magnesium, and is the most widely used of all materials in aircraft construction. Aluminum alloys corrode more easily than pure aluminum, so pure aluminum is often rolled to form a protective layer on its alloy surface. This process is called cladding, and alloy sheets treated like this are called clad sheets or aluminum cladding. Another common method of protecting aluminum alloys is anodizing - an electrochemical process that converts the surface layer into a more corrosion-resistant form. Aluminum-lithium alloys are superior to aluminum-zinc and aluminum-copper alloys in strength and stiffness, so they can be used to reduce weight. Their use is limited because they cost about three times as much.
An interesting property that some aluminum alloys share with titanium is that they can be superplastically formed (SPF). When a material is heated to a temperature, well below its melting point, it can be stretched several times its own length without tearing or localized thinning. It can then be deformed using an inert gas such as argon to fill the mold and form accurately, without spring back when the pressure is released. There are various techniques based on this property for extremely complex shapes with precision and minimal weight. The high initial cost of tooling means that SPF is limited to certain high-cost items and is not yet suitable for mass production. Items such as pressure vessels, small tanks and reservoirs can be made using this technique.
The advantages of aluminum magnesium alloy
1. High strength to weight ratio
2. Various alloys to suit various applications
3. Low density, and therefore greater volume for the same weight, means they can use greater thicknesses than denser materials, and are therefore less prone to local buckling; this applies to magnesium alloys, even more than aluminum alloys
4. Available in a variety of standard forms - sheet, plate, tube, bar, profile
5. Aluminum alloy is easy to process after simple heat treatment
6. Can be superplastically formed (some aluminum alloys only)
shortcoming
1. Easily corroded, so protective finishes are required, especially magnesium alloys
2. Many alloys have limited strength, especially at high temperatures
3. Magnesium alloys have low strength (but high strength-to-weight ratio)
4. No fatigue limit (see section on fatigue later in this chapter)