One of aluminum's weaknesses is it's lack of strength is its pure form. To get around this and preserve aluminum's low density and lightweight other elements are added to the metal to "pin" dislocations reducing ductility but increasing strength. By this method some aluminum alloys can be as strong as steel. Adding different elements achieves slightly different effect but almost all alloys are stronger than the original aluminum metal.

Copper, an important part of some aluminum alloys
Adding copper to aluminum increases aluminum's strength, and hardness and also makes it heat treatable. Under a classification system that is currently in place all aluminum alloys are give a four digit number. Those with copper come in the form 2XXX. Alternatively adding magnesium causes increased tensile strength, resistance to marine corrosion and ease at which welding can occur. The code for these alloys begins with a 5.

Manganese also forms important aluminum alloys.
There are three further common elements which can feature in aluminum alloys. Manganese is often added to give increased strength and resistance to corrosion. The addition of silicon lowers the melting point and improves castability, and alloys with zinc have increased strength and hardness. What makes these alloys really special is they retain the lightweight property of aluminum whilst gaining the extra properties that aluminum itself lacks.

A dislocation is pinned by a different element in an alloy, increasing strength
But how do the addition of these materials strengthen aluminum? It is all to do with aluminum's structure which has dislocations which make it ductile, and malleable. Even though these properties are sometimes very useful often strength is more important and alloys use other elements (usually of different sizes) to stop the dislocations from moving. This is done by disrupting the regular crystal structure and therefore making it harder for atoms to slip past each other.

Most of the uses that aluminum has are in fact as part of an alloy rather than as a pure metal. Pure aluminum simply isn't strong enough for usage in aeroplanes, cars, trains and buildings.

Summary: Alloys of aluminum are more useful that aluminum itself, as they give real strength to the material, by pinning dislocations whilst maintaining aluminum's other properties. Using different materials in alloys gives slightly different effects, so alloys can be hand picked for their specific job.

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