Don't know if this will be of much help, but I had to fabricate a
rectangular box out of 3/8 aluminum that had to be welded "in place" on
the rear bumper of my wife's motor home. I have a Miller dial-arc 2000
tig welder and I couldn't get it hot enough to take a weld and no way
to pre-heat. Finally had to drive the motor home over to a friend that
had a mig set up for aluminum. That did the trick.
Bill