Aluminum makes for a bad 3D printer material because it oxidizes when getting warm. This is the same reason you need tons of flux and gas when trying to TIG or MIG weld aluminum. Also, aluminum can actually burn -- there were some explosions in Chinese factories where aluminum dust from polishing cell phone or laptop cases ignited.
You can 3D print with iron/steel, although it's typically eaiser to do it with something that melts easier, like bronze. I think this is what Shapeways does with their "steel" option, because it's bronze colored, and then filled in with steel dust.
Note that 3D metal printers use selective laser sintering -- this means that a thin layer of powder is laid down, then a laser melts/fuses this powder as desired; lay down another layer; ... This is, as has been said, pretty expensive equipment, although I *believe* you can get a SLS system in the mid 5 figures if you're willing to compromise a little bit.
If you just have a few things you want to get 3D printed, sending your STL files to Shapeways and having them print it in your choice of material is a lot easier and cheaper than buying your own printer, except for the cheap-and-easy print-at-home materials like PLA or ABS. (Note: Shapeways "white strong flexible" is Nylon, which is higher strength than ABS.)