[Opper77]

I think the popular misconception is that because aluminum has a lower melting temperature it "welds" at a lower temp also (read as aperage). The reality is that aluminum takes much more current to weld than the same thickness steel. Most 135 amp welders will JUST be able to weld aluminum properly with .035" wire on a maximum thickness of 1/8" material. If you must weld alum. with a smaller machine, my suggestion would be to use the smallest diameter wire possible and get a helium/argon mix (at least 80% helium) as it helps to input more heat into the material than just a straight argon shielding gas.

I used to TIG weld 600lb. aluminum castings at around 400A day in and day out; and even at that amperage, if I used a stright argon (as sometimes happened when we ran out of the proper helium mix) I could barely get a puddle started.

It all goes back to the high heat conductance of aluminum. If you take a 2 foot long rod and put heat to one end, the other end will get hot too. This is much different than steel of course (stainless steel in particular has a very low heat conductance, and takes LESS amperage that mild steel to weld)

One other nugget: The wire speed on a MIG welder is actually your AMPERAGE control. The thicker the material, the more amperage you need to properly fuse it; and thus the faster wire speed required. Of course your voltage must be set properly to help "wet" the puddle and "push" the weld puddle properly.