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Aluminium Alloys

METALS
In the bicycle industry aluminium alloys are often simply referred to as 'alloy'. Pure aluminium is quite soft and ductile. It is alloyed with small amounts of copper, manganese, silicon, magnesium and zinc to improve hardness and durability. It has good strength to weight; the same strength can be achieved with roughly half the weight of aluminium as of steel.

Properties
Aluminium alloys are designated with a four number code to indicate the type and amount of alloying elements: 1XXX is almost pure aluminium; 2XXX contains copper; 3XXX contains manganese; 4XXX contains silicon; 5XXX contains magnesium; 6XXX contains magnesium and silicon (these medium strength alloys are often the material of choice for bicycle frames); and 7XXX contains zinc. Each of these alloying elements produces different levels of strength, corrosion resistance, conductivity and workability.

In the presence of oxygen the surface reacts to form a protective oxide layer, which makes it almost maintenance free, although it can affect the appearance of brightly polished metal. The protective layer is enhanced and can be coloured with the anodising process. The surface of aluminium alloy can be polished to produce a very bright and highly reflective finish.

Extracting aluminium from bauxite is an energy intensive process. It takes roughly 3 kg of bauxite to produce 0.5 kg of aluminium. This is why it is so efficient to recycle aluminium rather than extract it from new bauxite.

Processing
Aluminium alloys are shaped by panel beating, metal spinning, metal stamping, deep drawing, superforming, forging, tube and section bending, casting, press braking and extrusion.

Aluminium alloys can be joined by certain arc welding processes (MIG and TIG), power beam welding (laser and electron beam), mechanical fixings (threads or rivets) and adhesive bonding. TIG welding is suitable for very thin materials and precise and intricate work. Whereas MIG welding is limited to materials between 1mm and 5mm.

Suitable finishing processes include anodising, spray painting, grinding, sanding, polishing and printing. Certain aluminium alloys are suitable for heat treatment. This is the process of controlled heating and cooling, and is used to produce either hard and brittle, or ductile and tough metals. A suffix – F, O, H or T – is added to the end of the four digit material identification code to indicate the type of heat treatment.

Relevant Applications
Bike Frames
Aluminium alloys are popular materials used in the construction of mass produced bike frames. Bicycle frames have to cope with high flexural load. High strength aluminium alloys tend to have low fatigue strength. As a result, aluminium alloy frames are made from thicker section tube (2–3mm) than steel equivalents to reduce deflection and fatigue and so are not much lighter. Increased stiffness means that aluminium frame bikes tend to feel stiffer to ride.

Bike Locks
Aluminium alloys have low shear strength and so are not usually suitable for bike lock applications that have to withstand cutting, breaking and impact.

Street Furniture
The application of aluminium alloys is limited in this area, mainly because steel equivalents tend to be less expensive.