New guy....came here for the fabrication more than the cars. You guys have some awesome fab threads going.
I've got a Millermatic 185(175A?) and a 200A inverter style TIG. I've done aluminum with both(I'm a beginner on both) and they both have they're upsides.
I cut/modified a full size Dodge pickup bed hardcover to fit a 521 for a friend. I welded the extruded aluminum channel back together with the mig, since that was all I had at the time. I bought a new liner, but used the same rollers. I lightened up the pressure on the rollers a bit. I was also running staight argon through it. The higher I turned the settings up, the better it welded. I also found that it seemed to work better with more stick out (how much wire is hanging out of the tip). The AL wire(.030) melts so fast that to push as much current through as it seemed to take, it wanted to melt in the tip. I think I had nearly an inch of stick out. I had the gas up pretty good, but I don't remember the setting. I also had no air flow in the room at the time. I moved the bed cover far enough from the welder that I could keep the cord almost completely straight. I've only done AL this way a few times and I did ball it up once. It did make some nice, flat weld beads with good penetration.
From talking with a couple of other welders, I got this..... You run a brand new liner because of all the roughness and wear inside one that has been used for steel wire. The rollers put a serrated texture on the wire when they squeeze it and that saws away at the liner....in miniscule amounts. It also causes the wire to leave behind fine steel particles...which you don't want coming out on the AL wire and into the weld. As mentioned, run one size bigger tip so that the AL doesn't drag or bind as the wire swells from the heat. When you switch back to steel wire....switch out the liner and keep that one just for AL. It was only about $14 if I remember right...so well worth the $'s to have a dedicated liner.
Had I had the tig at the time, I think I would have gone that route. The mig was simple and worked great at the time. But on nice clean, fairly new AL...the tig gives a great deal more control. This thread is more about welding AL with the mig, so I'll save the tig stuff
As for the valve cover...I still have a project that I haven't quite finished becuase it was such a nightmare trying to weld it!!! I was doing it with the tig, but I think I'm going to try the mig next time!! The AL of the valve cover is quite porous. So...getting it clean seems to be the biggest problem. I've used AL cleaners and SS wire brushes....about the only recommendation that I haven't tried yet is to bake it at 400-450 for about 45 min to burn out any contaminates.
Sorry...I can get a bit long winded