I'm pretty sure I have the issue knocked down to cheap Chinese alloy with impurities but thought I would ask before I scrapped the parts. I went to weld up some 3" aluminum intercooler piping and was having a heck of a time getting it to actually weld. It was customer supplied (issue #1), polished and fairly thin walled. The fitment of the butt joints was excellent, I cleaned the area and hit it with a stainless bristled brush I use only for aluminum TIG work. Pure Argon, pure tungsten rod with 4043 filler (tried 1/6" and 3/32" filler with no difference). Issues is 1 it takes forever to get through the cleaning phase to the point where it has a shiney puddle (this is away from the edge of the joint) just as you start to add filler and move along the area suddenly starts getting this black crud in it and the arc jumps around. Pretty soon it all turns to crap. If you try to join the 2 edges it starts the black crud producing properties almost immediately. Same settings, same filler, used on a known good piece of 6061 tubing and it welds beautifully. Am I correct in thinking it is impurities coming out of the alloy? Thanks in advance. [P