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

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SUMMARY

TIG welding of aluminum requires alternating current (AC) due to the presence of a protective aluminum oxide layer that has a melting point exceeding 2000°C, while aluminum itself melts at 660°C. When using direct current (DC), the oxide layer does not melt, leading to poor weld quality characterized by overheating and deformation. In contrast, steels can be effectively welded with DC because their oxides are less problematic and do not form a similar barrier.

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  • Understanding of TIG welding processes
  • Knowledge of aluminum and its properties
  • Familiarity with electrical current types (AC vs. DC)
  • Basic principles of metallurgy and oxidation
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  • Research the effects of aluminum oxide on welding
  • Learn about the advantages of AC in TIG welding
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Welders, materials scientists, and engineers interested in the specifics of welding aluminum and the underlying chemical and physical principles involved.

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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