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

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.