Why can't you TIG weld Aluminium with DC current?

AI Thread Summary
TIG welding aluminum requires AC current due to the presence of a tough oxide layer on its surface. While aluminum has a relatively low melting point of 660°C, its oxide layer, aluminum oxide, has a melting point exceeding 2000°C. When using AC, the thermal cycling effectively breaks down this oxide layer, allowing for a clean weld pool. In contrast, DC current does not provide the necessary thermal cycling to melt the oxide, resulting in poor weld quality, overheating, and the formation of globules that do not bond with the base metal. This issue is specific to aluminum, as other metals like stainless steel do not have such problematic oxide layers and can be welded effectively with DC.
Adrian Baker
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Why can't you TIG weld Aluminium with DC current?

You can TIG weld it beautifully with AC, but if you turn the TIG welder to DC it doesn't work... You don't get a nice little weld pool, the sample seems to overheat and deform and it goes all black on the outside. The welding rod just leaves circular globules on the top that don't melt to the main body. You can happily do stainless and other steels on DC, but not Aluminium.

I assume that this is a Chemistry question, as I can't think of a Physics reason...

Anyone? Thanks for listening.
 
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Because of the oxide layer on aluminum. The melting point of Aluminum is just 660C while that of aluminum oxide is over 2000C. When you use AC, you actually are constantly breaking off the oxide coating by thermal cycling. Remember, Aluminum Oxide (or alumina is a ceramic, and is very brittle, so it breaks off easily. With DC, you get nowhere because you're heating the oxide but not enough to melt it.

Steels don't oxidize nearly as easily as Aluminum and their oxides are not nearly as nasty. So they can be welded happily using DC.

PS : Not the best place for this post. I'd have put it in General Engineering or Materials Science. If you're not happy with this answer, you might think about moving it. Also, I consider myself more a physics person than a chemistry person.
 
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I'm very happy with your answer - thank you.
 

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