I have seen a webpage where a bloke made a furnace to melt aluminium out of a what looks like a stainless steel beer keg so theres not much reason that a forge couldn't be made from one but that had refractory of some sort in it to protect the keg from the heat.
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Don't waste it on a furnace, I've built numerous furnaces, all using plastic buckets as forms, and then just sling the set furnace out and reuse the bucket. Mind you, the furnace is so cold on the outside, you can touch it on the outside when on on the inside, and it's walls are only about 40mm thick, so you'd have no warping problems. And my furnace runs fairly hot, I have managed to melt cast iron, but not melt enough to pour, and that needs bags and bags of charcoal and hours of heating. Aluminium is easy to melt, even brass (about 900C) is not hard with a furnace. Plus, you can use it as a forge easily enough.
You could line it with conctrete and use it as a nice stove though, be pretty warm and would hold it's heat well after the fire went out.
