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Aluminium and its alloys used in aircraft
This unit of work addresses aspects of the following syllabus outcome:
A student:
H 1.2. differentiates between the properties of materials and justifies the selection of materials, components and processes in engineering.
Extract from Engineering Studies Stage 6 Syllabus © Board of Studies NSW 1999
By studying the following pages and visiting the web links students will learn to analyse the structure, properties, uses and appropriateness of materials in aeronautical engineering applications. They will also learn about the effects of heat treatment.
Source: Corus Aluminium Walzprodukte web site 
.
Viewed on 9 July 2003.
Introduction
Any brief history of aircraft materials must mention timber as one of the first materials used to make a powered, manned flight. The Wright Flyer consisted mainly of Sitka spruce and bamboo glued and screwed together to form a canvas-covered frame. Wooden aircraft were very successful in the early years of flight but by the end of World War I their days were numbered. Today, timber is only suitable for comparatively small aircraft. As aircraft became larger, materials with better specific strength (strength to weight ratios) became necessary. Today aircraft consist largely of aluminium alloys with steel, titanium alloys and polymer composites forming the smaller proportion. The balance of materials does depend on the type of aircraft as military fighter planes have much higher proportions of composites and titanium alloys.
Aircraft need to be made of lightweight materials to increase payload and save in fuel consumption. The more passengers a plane can carry, the more profit an airline company can make.
Whilst pure aluminium has low specific gravity, good corrosion resistance and excellent thermal and
electrical conductivity it is too weak and ductile to be used on its own. In
1906 Dr Alfred Wilma, a German metallurgist, discovered that aluminium alloyed
with copper and heat treated correctly could be made far stronger. The alloy of
aluminium with 4% copper is called Duralumin and the heat treatment process is
called precipitation hardening. These alloys have typically low specific gravity
(around 2.7) and high strength (450 MPa). They are limited by a maximum service
temperature of about 660°C. Since then, other heat treatable aluminium
alloys have been developed for aircraft use. These include a range of complex
aluminium-zinc alloys which develop the highest strength of any aluminium alloy.
These alloys have led to modern aircraft design where the skin of the fuselage
and wings are stressed aluminium alloy members which reduces the overall weight.
The aluminium alloys mentioned above have the disadvantage of not being as corrosion resistant as pure aluminium so a thin layer of pure aluminium is often pressure welded to both sides of the alloy. This material is called Alclad.
Titanium, though very expensive, is used where high strength is needed in load bearing applications such as landing gear and engine mounting brackets.
Steel is used where strength is needed in restricted spaces, for example in the carriageways. Alloy steels can be heat treated to give very high mechanical properties and take up less volume, which is very important, as there is not very much “free space”. It is used sparingly though as it is heavy and suffers from increased brittleness (low energy to cause fracture) at the low temperatures found at very high altitudes.
Lithium is also used as an alloying element for improved properties.
One property that is appropriate for selecting materials for aircraft use is their specific strength. The following list indicates the approximate specific strength for some materials:
- Aluminium 30 Mpa
- Mild Steel 51 MPa
- Duralumin (heat treated) 150 MPa
- Al-Zn (heat treated) 220 Mpa
- Titanium alloy (heat treated) 270 MPa
Modern aircraft are extremely efficient and fast flying machines. The advances in materials technology accounts for the economy and reduction in weight while speed comes from development of the aircraft's shape and its engines.
Advances in polymer composites are making them increasingly more popular. They have very attractive low density and high mechanical properties. Composites consist of fibres of glass, carbon, Kevlar or boron reinforced in an epoxy resin matrix. They are replacing some of the aluminium alloys in commercial aircraft and find even greater applications in military aircraft such as the Eurofighter.
Activity 1
Discuss why there has been a change from one material to another in aircraft development?
Aluminium and its alloys
There is a broad range of aluminium alloys, and because of this a classification system has been developed for wrought and cast alloys.
Wrought alloys are registered by a four-digit number, which can have a further letter and number to indicate temper and condition. Various domestic nomenclature schemes exist for the casting alloys.
| Four-digit number | Alloying element(s) |
|---|---|
| 1XXX | aluminium of 99% minimum purity |
| 2XXX | aluminium-copper alloys |
| 3XXX | aluminium-manganese alloys |
| 4XXX | aluminium-silicon alloys |
| 5XXX | aluminium-magnesium alloys |
| 6XXX | aluminium-magnesium-silicon alloys |
| 7XXX | aluminium-zinc-magnesium alloys |
| 8XXX | miscellaneous, e.g. Aluminium-lithium alloys |
The following information has been provided by courtesy of Professor Alan Crosky of the University of New South Wales.
The most commonly used aluminium alloys for airframe construction are the precipitation hardening alloys in the 2XXX and 7XXX series, as shown below:
Aluminium alloys used in aircraft construction.
Boeing Aircraft Co. (n.d.).Seattle
2XXX series aluminium alloys
The 2XXX series aluminium alloys are alloyed with copper from 1.9-6.8% and often contain additions of manganese, magnesium and zinc. Their precipitation hardening has been widely studied. They are used for applications such as, forgings, extrusions and liquefied gas storage tanks in civil transport and supersonic aircraft. These alloys have lower crack growth rates and thus have better fatigue performance than the 7XXX series alloys. Therefore, these are used on the lower wings and body skin (see Figure above). The alloys used are 2224, 2324 and 2524 (both modified versions of 2224). These alloys are often clad with 99.34% pure aluminium for increased corrosion resistance. Compositions of these alloys are included in the table below, Aluminium airframe alloy compositions.
7XXX Series aluminium alloys
The Al-Zn-Mg system offers the greatest potential for precipitation hardening (out of the aluminium alloys) though copper is often added to improve stress corrosion cracking resistance (with the drawback of reducing weldability). Stress corrosion cracking resistance decreases with increasing Zn:Mg ratio. The stress corrosion cracking problems have been the biggest restriction upon the use of these alloys but they have still been used in lightweight military bridges, railway carriages, military and civil aircraft.
Table: Aluminium airframe alloy compositions
Source: Flower, H. M. (1995) High performance materials in aerospace. Chapman Hall: London
Activity 2
List the three main reasons why aluminium alloys are used instead of pure aluminium?
Heat treatment of precipitation hardening alloys
Aluminium alloys are strengthened in a number of ways including: solid solution hardening, cold working, dispersion hardening and precipitation hardening.
Precipitation hardening (otherwise known as age hardening) is a process whereby a fine precipitate structure is formed in the alloy matrix following a heat treatment process.
For an alloy to be precipitation hardened it requires:
- decreased solid solubility with decreasing temperature
- the ability to suppress the formation of precipitates by quenching from a solid solution
- the formation of metastable coherent precipitates
The precipitation hardening process follows three main steps:
- Solution treatment. The alloy is heated above the solvus temperature to dissolve any precipitates and ensure the alloying elements are in solid solution.
- Quench, The alloy is quenched. The alloying elements in solution do not have time to diffuse and form precipitates. Thus, the alloying elements remain in solution forming what is known as a supersaturated solid solution.
- Ageing. The alloy is heated to an intermediate temperature below the solvus temperature. The alloying elements are able to diffuse to form coherent precipitate clusters (known as GP zones).
Example of age hardening 2XXX series aluminium alloy system
Source: Gibson, J. (n.d.) Equilibrium diagrams and common metal alloy systems.
Sydney: Clarendon.
The coherent precipitates increase the strength of the alloy by distorting the crystal lattice and creating resistance to dislocation motion. The number of precipitates increases with increasing time thus increasing the strength of the alloy. However, with excessive time the precipitates become large and incoherent and their strengthening effect decreases. Thus, during precipitation hardening there are four main stages:
- solid solution strengthening in the supersaturated solid solution
- coherency stress hardening from the coherent precipitates
- precipitation hardening by resistance to dislocation cutting
- hardening through resistance to dislocation between precipitates.
Activity 3
Describe how an age hardening rivet could be inserted into the wing of an aeroplane during assembly?
Other Useful Resources
The Boeing 747 family
Studying about engineering materials
A site about how aeroplanes fly
A site about design choices for a small STOL aircraft
A manufacturer of small ultralight aircraft
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