I will need to weld some aluminium in the next month or so and have been doing a little research. AC TIG welders can weld aluminuim, but DC TIG welders can't. The AC TIG welders cost a fortune.
MIG welders are also supposed to be OK for aluminium so long as you have the aluminium wire and pure Argon gas. Both sides of the aluminium need to be completely cleaned to a very shiny surface immediately before you weld.
There are two things that make Aluminium tricky with MIG - the melting point is lower than steel, and aluminium doesn't conduct heat away from the weld as quickly as steel. This means there is a fine line between getting the aluminium to melt and blowing holes. I think the reason that TIG is preferred for aluminium is it's improved controllability.
I'm planning to buy a sheet of aluminium, the wire and gas and do some practicing with MIG.
I've heard that cleaning the ali surface with white vinegar works. Though acetone is probably best.
For a home MIG be sure to change the liner for a plastic/teflon one if yours is steel. The wire picks up minute amounts of steel on the way thru and it will make a difficult job even harder. The whips tell me that a slightly oversize tip also helps. ie: 0.9 for 0.8 wire etc.
My comments today look like I'm a Halfords salesman, but here goes anyway...
Halfords sell the small 60L disposable bottles of gas (Pure CO2, Pure Argon, and the usual 80-ish% mix, about £8 each) as well as the Al wire, and I believe stainless wire too (wire is the usual stupid £11 for half a kilo price, don't know if this is normal for Al/Stainless).