That aluminium contains zinc, i'm pretty sure thats the trouble. Any time i have been asked to tig a "mystery metal", that cannot be welded, usually turns out to be a metal containing zinc.
7075 is a sod because it’s aluminium, magnesium, zinc, copper and a little bit of iron.
When you weld it all the alloying elements go into solution and start forming weird phases and compounds together.
You get Micro cracking which starts in the partially melted area of the HAZ.
I don't know much about the aluminium alloys to be honest, any time i have worked with aluminium, i either knew the grade, and that it was weldable, or it was try it and see job.
