Great Question! Aluminum is refined in many ways, the companies that produce will tell you the way they do it best. Aluminum by nature is a very soft metal. So when you make it to use for cars and airplanes, you add other metals to aluminum. The common varieties often used are 6061, and 7 thousand series. Aluminum has a lot of odd properties to it. Used correctly it can be very strong, but make no mistake, aluminum is not steel. Bridges are not built with aluminum, and there is a reason for this. First let us think of a bicycle made of aluminum. You cut the tubes, then have someone weld the bike together. When you weld aluminum, you create stresses, caused by the heat. After welding you would send the bike frame to be heat treated, this reduces the stresses, and makes the metal stronger. The problem with aluminum, and there is a problem (look for articles on the airforce retiring jets) is fatigue. Fatigue is basically when something has a stress applied to it and is damaged by the stress. Steel alloy, used in bridges and building supports, has a high fatigue point, you can flex the metal a million times and as long as you dont bend it, it will be just as strong as the time before (flex a spoon a little, you could do this forever and it wont break). When you bend a metal you go past its fatigue point, basically, you are damaging the very structure of the metal, and it is not weaker for it. Aluminum essentially has no fatigue point. Any stress on aluminum will be there forever, even the smallest stress will weaken the aluminum a little bit (maybe only a teeny, tiny bit, but still a bit). The problem this creates is in load bearing structures. You design a bridge with a fatigue point of 30000 pounds. As long as nothing goes over the bridge is heavier than 30000 pounds, the steel bridge will last forever (in theory). Not so with aluminum. Every car, even ones weighing 2000 pounds will stress the bridge, and each time this happens it is weaker. Its ok on a bike frame, something that carries one person, or a car, which has a pretty determined life cycle and is very overbuilt. But on a bridge or building, its not acceptable. On bike, the frame cracks, and you buy a new one, maybe after a crash. On a car, they replace your worn, fatigued suspension part after 100000 miles or whatever the manufacturer has determined. On a building or bridge, the predicted stresses are hard to compute and failure could result in a very bad situation.

Smelting process to extract the pure ore with is blended with other additives to effect strength.