An alloy is metal comprised of two or more elements (usually other metals). The value of alloys has been known for thousands of years with the copper alloys of bronze (copper and tin) and brass (copper and zinc) being discovered in ancient times.
Alloys will have more desirable properties than the pure metal such as greater corrosion resistance, hardness, toughness, ductility etc depending on the components and their proportions.
The most used metal in the world is steel, an alloy of iron with a small amount of carbon.
In most aluminium applications, alloys are used. Pure aluminium is rarely employed.
Elements such as copper, magnesium, manganese, tin, and zinc will make up to 15% of the weight of an aluminium alloy.
These alloys can be placed into three categories: commercially pure, heat treatable and non-heat treatable.
Commercially pure alloys (1xxx series) are 99% or more aluminium by weight and used for products such as foil and power lines due to their corrosion resistance, workability, and electrical conductivity.
Heat treatable alloys (2xxx, 6xxx & 7xxx series) are extruded/worked at high temperatures and then quenched/ cooled rapidly. This is known as solution heat treatment which strengthens the alloy greatly. Followed by natural ageing (room temperature) or artificial ageing in an oven.
2xxx series mainly employ copper as an additive and are used in aerospace due to their high strength and toughness.
6xxx series are the most common in extrusion due to their versatility, strength, formability, and great corrosion resistance. Silicon and magnesium are the main alloying elements.
7xxx series are commonly used in aircraft due to their high strength. Zinc is the main alloying element.
Non-heat treatable alloys (3xxx, 4xxx & 5xxx series) are cold worked through rolling or forging to increase strength. These alloys cannot be strengthened significantly by heat treatment. Along with heat treatable alloys these are between 90 and 99% aluminium by weight depending on the alloy.
3xxx series contain manganese and usually some magnesium giving them moderate strength and good workability. Because of this they serve as general purpose alloys for products such as aluminium cans.
4xxx series contain silicon which lowers the melting point making them great for use as welding wire.
5xxx series containing magnesium are used widely in construction and marine applications due to their high strength, weldability, and corrosion resistance.
Temper designations refer to how the alloy has been mechanically or thermally treated to reach the desired properties. There are four categories as follows:
F – as fabricated
0 – annealed
H – Strain hardened
T – thermally treated
Regarding aluminium extrusion the temper of significance for heat treatable alloys is T – thermally treated
which is further broken down, the most common designations are as follows:
T4 – solution heat treated and naturally aged
T6 – solution heat treated and artificially aged