Yesterday a buddy brought over a new exhaust he bought from autozone for me to weld up. it consisted of a "Thrush welded" POS aluminized muffler and some 2" aluminized pipe. Everything welded nice as it should until i got to the muffler then i dont know what kept happening but it kept violently popping the weld pool out of the weld. At one point it popped up so bad that the weld went verticvally up and landed rigth in my hair giving me a nice bald spot to deal with. looking at the weld afterward i saw a major crater in the weld where the material had popped out. Logically to me it seemed like some sort of gasses were getting trapped behind the weld and expanding violently but im not understanding how when the pipe was open on one side and the muffler was open on the other. It was definely scarry and i'd like to figure out what happened to avoid it in the future.

Machine was a sync 250
2% thoriated
er70s filler
90 amps

Anyone know what was happening. The only pics i have is when it was completed. None of the carnage.

Flange came out pretty good