Howdy Everyone,
This is my first post here, although I've been a daily lurker for quite a long time. I've been enjoying the friendly environment, and having a ball soaking up all the great knifemaking knowledge flying around everywhere. That is, as much knowledge as my itty bitty pea-brain can absorb.....
I thought I'd throw my 2 cents (if it's actually worth that) into the hat here, since welding is one of the only subjects that I actually know more about than just enough to get me into trouble. I'm a welding school honor graduate and class valedictorian, with an Associate's Degree In Applied Science In Welding Technology, plus additional training in mechanical engineering and advanced metallurgy. In my 25 years of experience, I've worked in nearly every scope of welding you can imagine except for underwater welding, and have held 28 different welding certifications. As an aside, I have 6 years full-time experience as a machinist, with 5 of those in tool and die.
I'm afraid I must echo the advice given by the other posters to this thread. The Longevity MIGWELD 140 is rated at only 20% duty cycle at 90 amps, which means it can run at 90A for only 2 minutes out of every 10. This limits it to about 16 gauge aluminum
at best, and even then only in short bursts of a minute or so because you'll need some serious heat for aluminum. I'll try to explain this in the best manner my po' ol' feeble mind will allow:
In welding, heat energy is spent bringing the added filler metal up to the temperature of the molten puddle. Unlike TIG, with MIG the filler metal is continously being added automatically, which means a high, fixed rate of heat-sink effect. To make matters worse, even though aluminum melts at a lower temperature than steel it's (A) either completely liquid/molten or completely solid and lacks the fluid, in-between plastic state of ferrous metals, and (B) has a very high degree of thermal conductivity, which means heat is constantly being conducted away from the weld zone at lightning speed. Thus, a LOT of heat is required to maintain adequate wetting action and prevent a cold-lap situation where the filler wire doesn't fuse together with the base metal, but merely lays on the surface. Reducing the wire feed speed in order to curb the filler's heat sink effect and therefore increase the heat, fluidity and wetting action of the weld pool results in greatly increased spatter and poor weld bead appearance. It also increases heat at the contact tip and causes more wire burnbacks, a sure-fire recipe for going through expensive contact tips like water. Not a good option.
The other fact is that in most aluminum welding situations, "pushing" the MIG gun is preferred to a "pulling" angle, because it better helps to break up the invisible oxide film on the surface of the aluminum (called the "cleaning action"), allowing better wetting action and fusion with the base metal. The problem here is that when pushing you're laying down filler metal partially in front of the arc, so that the arc doesn't strike the base metal as directly as when you're "pulling" the gun. This translates to lower penetration, which of course means (you guessed it!) more heat is required when pushing than when pulling.
All this means you'll probably not be satisfied with a 120V MIG for aluminum, even on very thin stuff. Also, at the least you'll have to put it on a dedicated 20A circuit by itself to have a fighting chance of welding without tripping the breaker every couple of minutes. In my professional opinion, a 230V machine is an absolute necessity for aluminum. Sorry to be the bearer of bad news.....
J.E. Vaughn