Contrary to iron, silver or gold, aluminium has to be chemically extracted. The mineral you extract aluminium from is bauxite. This chemical process was mastered so recently that the Washington Monument is tipped with it just to show off. Even then the tip is so small that you can barely see it with your own eyes, but you can see it in Spiderman :)
I'm not sure on all the particulars, but metals are not the same: some require more energy to work with or to refine than others.
Steel (IIRC) needs a lot more energy to melt than aluminum, however I think aluminum requires more energy to extract from ore. Also, IIRC, aluminum needs more energy to weld just because it conducts electricity so much better, even though it has a lower melting point.