Floating Cast Aluminum

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I've searched through all the other cast aluminum posts and I feel like mine a little unique. I'm using a Syncrowave 250DX with a watercooled torch. The tungsten is 2% thoriated, 3/32" in diameter. Gas is pure argon and i'm using about 60 amps on max cleaning to just build up material on a, roughly, 1/2" x 1/2" area chunk of cast aluminum. I'm not preheating the work piece either. To prep the metal, i hit it with some phosphoric acid to get rid of any surface oxides, and then after it's dry and rinsed off, i'll brush it with an aluminum dedicated stainless steel brush. Since the surface area of the piece is so small, i can't even really get a bead going. I just need to build up more material. The thing is, after I add the first bit of filler, i'll stop and brush the piece then while i'm adding the second layer of filler, it's as if the oxide forms between the base material and the first layer of aluminum filler, and then it just floats off to the side. Also, it's not as if the first layer of filler didn't fuse with the base material. I broke off a piece of the new aluminum to see if the oxide layer formed between the base material and filler, but it took the base material with it. So the question is: what's causing it to float? lack of preheat? dirty aluminum? bad casting? The metal looks clean, it does have the exterior signs of a dirty metal. I don't really have anything to preheat the piece with, so i can't test that. I hope what i wrote isn't too disorganized so you can picture it.Last edited by Chucez; 11-30-2010 at 02:26 PM.Reason: forgot to throw in the question
Reply:i'd like to also add that in the bits that broke of there is a lot of porosity.
Reply:I'm so stupid the darn thing is made of magnesium.
Reply:Well, -that- would explain why the aluminum filler didn't really weld to the magnesium base material.    The best laid schemes ... Gang oft agley ...
Reply:ruined another aluminum only brush I see....
Reply:Originally Posted by slopecarverruined another aluminum only brush I see....
Reply:When tiging magnesium, you see a greenish glow or aura when you establish an arc.You can tig it with the right rod.You now have three mag brushes.Just my  opinion, not from a book, just from the road.Howes Welding Inc.www.howesweldinginc.com