About Aluminum:

Aluminum is the most abundant metallic element, making up 8% of the earth's crust by weight. It reacts with water and air to form powdery oxides and hydroxides so it is never found in nature in the metallic state. Many minerals (including feldspars) contain aluminum but extracting the metal from most minerals is very expensive.

The main ore of aluminum is bauxite (the source of over 99% of metallic aluminum).
Bauxite is a mixture of minerals that contain hydrated aluminum oxides and is therefore a rock and not a mineral. Soil-like in appearance, bauxite forms when silica in aluminum-bearing rocks (rocks with high feldspar content) is washed away (leached). This weathering process occurs in tropical and subtropical climates such as Africa, western India, South America and Australia.

Most bauxite is first processed to make alumina, or aluminum oxide. Alumina is lighter than bauxite because the water has been removed and it flows readily in processing plants, unlike bauxite, which has a sticky consistency. Aluminum metal is refined from alumina. In the refining process, called the Hall-Heroult process, alumina (aluminum oxide) is dissolved in molten cryolite. The alumina is then separated into its elements by electrolysis. Because this process requires so much energy, it is usually done in countries with large supplies of cheap hydroelectric power. 

Processing
Aluminum ingots can be processed in three ways:
Casting - pouring melted aluminum into steel molds of various designs.
Rolling - processing aluminum using large presses. The raw material is processed into flat aluminum plates of different thickness.
Extrusion - forcing aluminum that has been melted to flow through a shaped opening in a die.

Extrusion
Extruding aluminum has been compared to squeezing toothpaste from a tube where the paste takes the shape of the tube opening. Here's how it works: an aluminum billet, heated to approximately 470° C (900° F), is placed in the extrusion press and forced under pressure through a steel die. The die determines the desired finished cross section. Extruded material emerges as an elongated piece with the same profile as the die opening.

Press size determines how large of an extrusion can be produced. Extrusion size is measured by its longest cross-sectional dimension, i.e. its fit within a circumscribed circle. A circumscribed circle is the smallest circle that will completely enclose the cross section of an extruded shape.

The most important factor to remember in the extrusion process is temperature, which is most critical because it gives aluminum characteristics such as finish and hardness.

Steps in the Process:
1. Billets must be heated to approximately 425°-500°C (800-925° F).

2. After a billet reaches the desired temperature, it is transferred to the loader where a thin film of smut or lubricant is added to the billet and to the ram. The smut keeps the two parts from sticking together.

3. The billet is transferred to the cradle.

4. The ram applies pressure to the dummy block, which, in turn, pushes the billet until it is inside the container.

5. Under pressure, the billet is crushed against the die, becoming shorter and wider until it has full contact with the container walls. While the aluminum is pushed through the die, liquid nitrogen flows around some sections of the die to cool it.

6. As a result of the pressure added to the billet, the soft but solid metal begins to squeeze through the die opening.

7. As an extrusion exits the press, its temperature is taken. This is done to enable the maintenance of maximum press speeds. The target exit temperatures vary with the different alloys used.

8. Extrusions are pushed out of the die to the leadout table and the puller, which guides metal down the run-out table during extrusion. While being pulled, the extrusion is cooled either by using special water quenching equipment or by a series of fans along the entire length of the run-out and cooling table. 

9. The unusable part of the billet (butt), which contains oxides from the billet skin, is sheared off and discarded while another billet is loaded and welded to a previously loaded billet.

10. When the extrusion reaches a desired length, it is cut with a profile saw or a shear.

11. Metal is transferred from the run-out table to the cooling table.

12. After the aluminum has cooled, it is then moved to the stretcher. Stretching straightens the extrusions and performs "work hardening"(molecular re-alignment which gives aluminum increased hardness and improved strength).

13. After extrusions have been stretched they are transferred to a saw table and cut to specific lengths.

Relevant Terms
Temper -the combination of aluminum hardness and strength produced by mechanical and/or thermal treatments.
Tensile -an indication of the maximum pulling load that a material can stand without failure, usually measure in pounds per square inch of a cross-sectional area.
Yield - the stress at which a material first exhibits a specific permanent set
Elongation - maximum percentage of stretch a material will stand before breaking.

Advantages over different production methods
In machining, the cost of adding each piece is additive whereas in extrusion, once a shape is available, it can be reproduced indefinitely with no additional preparation costs.
Extruding is almost always less expensive than welding. The cost of an extrusion die can be significantly less than the cost of a welding jig for the same shape.
Extrusion dies are much less costly than roll-forming dies and allow a variance in wall thickness that roll-forming dies do not. Extrusion dies can be finished with less lead time than forming dies and casting molds.

Advantages of Aluminum
An amazing metal, aluminum is one-third the weight of steel, will not burn, is not magnetic, has a high inherent resistance to corrosion, and gets stronger as it gets colder. Of the common metals, on a weight basis, aluminum is the most efficient thermal conductor. (Strength and hardness are acquired by the addition of alloying elements like silicon, magnesium, manganese and copper.) Other advantages include high strength-to-weight ratio, low cost, excellent joining, forming and machining capabilities and high electrical conductivity.