Aluminium alloys – Alloys for extrusion

Aluminium for extrusion is mostly alloys with the following serial numbers:
1000 series – Al
6000 series – Al + Mg + Si
7000 series – Al + Zn + Mg

The 1000 series is non heat treatable. These alloys are often chosen in products where high thermal and electrical conductivity are desired. They have low strength.

The 6000 and 7000 series are heat treatable. They are the most commonly used extrusion alloys and have a wide range of applications.

The 6000 series has good extrudability and can be solution heat treated at the extrusion temperature. Furthermore, these alloys have medium to high strength, are easy to weld and offer good resistance to corrosion, even in marine environments. The bulk of the extruded material for load bearing constructions is made from these qualities. They are used for load bearing constructions both on land and at sea.

The 6060 alloy offers medium strength and is easy to extrude even for complicated cross-sections. This alloy is the most used extrusion alloy. It has good formability during bending in the T4 condition. Typical applications are extrusions for windows and doors, lighting, awnings, handrails and furniture. This material is highly suitable for anodising, both for decorative and protective reasons.

The 6101 alloy offers virtually the same production possibilities as 6060. It is especially suitable for electrical applications where fairly high strength is required.

The 6063 alloy has slightly higher strength than 6060, but is also marginally more difficult to extrude, especially if the cross-section is complicated. Applications are for the most part the same as for 6060. This material is well suited for anodising, both for decorative and protective purposes.

The 6005A alloy has higher strength than 6063 but is slightly harder to extrude. It is suitable for anodising for protective purposes but the quality of the surface makes decorative finishing more difficult.

The 6082 alloy has high strength and is suitable for extrusion of cross-sections that are not too complicated. Typical applications are load carrying structures in the ship, offshore, transport, and building industries such as platforms, bridges, stairs, scaffolds and handrails. The material is suitable for anodising for protective purposes.

The 7000 series has the highest strength of the most used construction alloys. They have good weldability and obtain lower reduction of strength in heat affected zones than the 6000 series. Their corrosion resistance and formability are, however, not as good as those in the 6000 series. But by adding small amounts of ZrCr or Mn this can be improved. Typical applications are automotive parts, aircraft containers, bicycle frames and high speed boats.

The 7108 alloy has high strength and good fatigue strength, but has a limited extrudability and formability. The alloy is susceptible to stress corrosion in areas with high stresses. The resistance to stress corrosion is slightly increased with overageing. Welding should only be carried out in areas where the loading is lower. Typical applications are structures for building and transport applications where high strength is required. The material is suitable for anodising for protective purposes.

The magnesium and silicon contents of various 6000 alloys can overlap in certain cases (see diagram above). Alloys can therefore be made up with the focus on optimal function adaptation and the ability to produce them. This process takes place continuously and today there are a number of variants of the alloy types above that have been adapted to suit specific conditions.

Temperature – Mechanical Properties
Care should be taken when using aluminium at high temperatures. Mechanical properties could significantly be reduced at temperatures above 100°C, especially if the material has been thermally hardened or cold worked.

In general the 606060636005A and 6082 alloys should not be used in structures at temperatures above 100°C. The tensile strength decreases as the temperature increases while elongation on fracturing usually increase. It should be noted that the characteristics are dependent of alloy, temperature and time. If the designer is unfamiliar with the exact temperature characteristics for a given alloy, it can be assumed as a starting point that properties such as tension, shear and fatigue strength vary in proportion to the tensile strength.

Low Temperature Properties
In contrast to steel, aluminium alloys do not become brittle at low temperatures. In fact aluminium alloys increase in strength and ductility whilst impact strength remain unchanged. As the temperature decreases, below 0°C, the yield strength and tensile strength of aluminium alloys increase.

Alloy Summary
Special material properties can be achieved with special thermal hardening.
With, for example, extended heating, ie thermal hardening longer than the optimal hardening time, lower strength is achieved but in return the alloy becomes tougher and in certain cases has greater resistance to corrosion.

F – Extruded and air cooled
O – Softened, annealed at 350-500°C, for 1-5 hours
T4 – Solution heat treated and naturally aged at 20°C, for 5-10 days
T6 – Solution heat treated, artificially aged