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Aluminum is one of the most common materials used in aircraft construction and has been a staple of the aviation industry

for years. This versatile material offers strength, light weight, corrosion resistance and malleability.  It is used to fabricate

many aircraft parts and is the primary component of most certified, commercial and many homebuilt aircraft.  Not just any

aluminum is suitable, but there is a group of aluminum alloys that does possess the required structural properties for

aircraft construction.  

Pure aluminum is “alloyed“ to improve many of its properties, including strength.  Alloying is achieved by adding other elements to the aluminum.  The typical alloying elements are copper, magnesium, manganese, silicon, tin and zinc. There are two principal classifications of alloys: casting alloys and wrought alloys.  It is the wrought alloys that are suitable for aviation applications, and are discussed here.  

The manipulation of aluminum by alloying creates many different classes of aluminum.  These aluminums can be identified by a series of numbers and letters such as 6061-T4.  Each digit designates a property of the aluminum.  The first number indicates the alloy elements used. 

1 is essentially pure aluminum with a minimum of 99% purity.
2 is alloyed with copper 
3 is alloyed with manganese.
4 is alloyed with silicon.
5 is alloyed with magnesium.
6 is alloyed with magnesium and silicon.
7 is alloyed with zinc.

The second number indicates modifications in impurity limits. Zero means there are no controls of impurities but 1 through 9 indicate control of one or more impurities. Since the types of aluminum covered here all have 0 as the second number, we won’t go into this.

The last 2 numbers indicate the aluminum content.  The aluminum yet to be alloyed has a minimum content of 99% pure aluminum.  So the last two numbers indicate the aluminum content above that 99%.  Thus, 6061 would be made up of 99.61% aluminum.

After the first four numbers, the next numbers or letters are temper designations.  Non heat treatable alloys are designated with the letter “H” followed by one or two numbers.  The first number indicates the method the temper was achieved (H1 means strain hardened. H2 means strain hardened and partially annealed, H3 means strain hardened and then stabilized).  The next number indicates the temper (2 is ¼ hard, 4 is ½ hard, 6 is ¾ hard, 8 is full hard, 9 is extra hard).  An example may be 3003-H12.

Heat-treatable alloys are followed by a letter and by one or more numbers.  These numbers are used to designate the method used to temper the alloy.  The letter “F” means as fabricated, “O” is full soft annealed and “T” is heat treated.  The most common aviation heat treated designations are T3, T4 and T6.  Below are the heat-treated designations and explanations.

T1  Cooled from hot working and naturally aged (at room temperature).
T2  Cooled from hot working, cold-worked, and naturally aged.
T3  Solution heat treated and cold worked.
T4  Solution heat treated and naturally aged.
T5  Cooled from hot working and artificially aged (at elevated temperature). 
T51  Stress relieved by stretching.. 
T510  No further straightening after stretching.
T511  Minor straightening after stretching.
T52  Stress relieved by thermal treatment.
T6 Solution heat treated and artificially aged.
T7  Solution heat treated and stabilized.
T8  Solution heat treated, cold worked, and artificially aged.
T9  Solution heat treated, artificially aged, and cold worked.
T10  Cooled from hot working, cold-worked, and artificially aged.
W  Solution heat treated only. 

While there are many alloys out there that the builder can find, the list below compiles the most common you will find for aviation applications. This is not meant to be a complete list for your aircraft project and heat-treated designations are not added.  

2024 - This well-known alloy uses copper as the primary alloying element and has a high strength to weight ratio.  With its high strength and excellent fatigue resistance, it is good for creating structural parts.  In annealed form it can be formed then subsequently heat treated.  It can be machined to a high finish.  Its corrosion resistance is relatively low and welding is not generally recommended.  Suitable for universal applications such as aerospace, aircraft structural components and fittings.

3003 – Is a manganese alloy with moderate strength, good workability, good corrosion resistance and is widely used.  It may be deep drawn or spun, welded or brazed. It is non heat treatable. Suitable for decorative trim, tanks.

5052 – is a magnesium alloy and the highest strength of the common non heat-treatable aluminums.  It has high fatigue strength and has excellent resistance to corrosion. It has excellent workability and can be drawn or formed into complex shapes.  It is used in a wide variety of aircraft applications.  Suitable for universal applications such as aerospace and marine.

6061- is a precipitation-hardened aluminum alloy containing magnesium and silicon as its major alloying elements. It is one of the most common alloys of aluminum for general-purpose use. It is commonly available in pre-tempered grades such as 6061-O and tempered grades such as 6061-T4 and 6061-T6.  It has good mechanical properties and good corrosion resistance.  It has good workability with good forming properties with most common techniques.  It can be welded using all methods.  Suitable for universal applications, structural parts and aerospace.

6063 – is a magnesium and silicon alloy that has good mechanical properties and allows for complex shapes to be formed with very smooth surfaces.  It was developed as an extrusion alloy with relatively high tensile properties, excellent finishing characteristics and a high degree of resistance to corrosion. Suitable for universal applications such as marine and aerospace.

7075 – is a zinc alloy and has excellent mechanical properties with high strength and good resistance to fatigue.  It is one of the most commonly used aluminum alloys for highly stressed structural applications and has been used extensively in aircraft structural parts.  It is one of the highest strength aluminum alloys available with excellent strength-to-weight ratio.  It can be formed when annealed and subsequently heat treated. Arc and gas welding are not recommended.  It is available in the Alclad form to improve the corrosion resistance with its high strength only being moderately affected. Used where highest strength is needed. Suitable for universal applications, aerospace and forging.

If in doubt about the proper alloy for your needs, be sure to ask your supplier or another builder for their input and advice.  Then buy and build with confidence.

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