Grant Erwin writes:

I'm wondering if a small CO2 bottle, the 20 pound kind, can be used
for MIG welding aluminum.

No, gotta be argon. When I tried it, you get something like a thermite
reaction where the molten aluminum takes the O2 off the CO2 and there's all
this amorphous C spraying around as soot.

Pretty strange to think that (oxidized) soot is what makes beverages fizz.