Best way to clean old aluminum for TIG

[Jason]

All the places I have gone around me wont give anything away. I think they make up the prices for each purchase made. I got some round bar one day and ended up short so the very next day I went to get some more and the rate had gone up over night.

[Spike]

Rather than messing with acids and chemicals and all the mess that goes with safety, cleanup, and disposal of hazmat chemicals... i just mechanically clean aluminum. I invested in a big variable speed buffer and just polish large oxidized or pitted aluminum areas. Where i'm going to weld i use a scotch brite pad on my angle grinder.
And then, i turn the 'clean width' way up on my welder and make a couple of quick passes over the weld area (no filler, and not at full heat) to bring contaminants up to the surface. If there's a lot of contaminants i will knock them off with the scuff pad again, but usually i don't have to. The little contaminants will float on the surface of the weld puddle. After welding, hit it with a stainless wire brush.

If you're going to weld a plate with lots of porosity, or contamination thats been absorbed into a porous aluminum casting (like aluminum engine parts) a good trick is to either set your welder to 100% clean width, or switch over to DCEP and run over the area you're working on with light heat. It will bring porosity and the contamination to the surface and you can knock it off with the scuff pad or wire wheel. I use this when working on vintage aluminum pieces like aluminum cast engine and motorcycle parts (even from back as far as the 1900's - 1920's).

[jakeru]

Regarding using low-current TIG torch cleaning: lately I haven't been bothering to move the AC Balance knob. It seems to work just as well, even with 20% EP (minimum cleaning setting) on my machine, and even with a Helium mix (the Helium content further reduces the cleaning action compared to pure argon.) It seems like the real "cleaning" action happens from heating the surface (without melting it, at least not excessively deep puddle.)

It's even OK if the surface melts slightly during cleaning (as if often does for me with 20% EN and a Helium mix), just as long as the puddle doesn't melt very deep, that works well. I like to see a smooth-frozen surface ideally after it freezes.

If there are any hydrocarbon contaminants in or on the surface, you don't want to just form a deep puddle because then, that it what causes porosity to be frozen down underneath the surface and it's not easy to "cure" once it's there. Heating the surface sufficiently often causes the hydrocarbon contaminants (e.g., water, oils, grease, etc) to vaporize. When I weld two sides of something, even when I find cleaning the "front side" is very helpful and important, I almost always find cleaning the "back side" accomplishes little because the heat from welding on the first side vaporizes the contaminants off the back side.

A lot of the above relates to weld-repairing cracked aluminum automotive wheels. Usually, some nasty contaminants are embedded down into the crack. Even welding new materials, some etching is useful to "test" the cleanliness of the metals that you think are clean, (before you really weld them.) I like to see no blackish or discolored looking deposits around the etched area. Whitish to grey etched area color is perfect - it means it's perfectly clean, and going to weld up beautifully with a glassy puddle (and look really nice too.)