Back purging is always a good idea. I don't think it's required for aluminum though. Stainless is one metal where no BP is a really bad thing. But most all aluminum is welded without BP. Most aluminum frames are done in production settings and I can't see them BPing it, but maybe they do. The main issue with Aluminum is that it oxydizes instantly but only superficially, so you need an AC welder properly set up to blow the crud off the weld while you run, but unlike steel it isn't going to be burning in from the back since it oxydizes so fast you can't purge it fast enough to do any good, but once it skins over it's about as bad as it gets. It oxydizes so fast the show is over the moment you stopped shaping the joint.