Exactly where and why would you want to tig weld aluminum on DC?

Because helium carries so much more energy and heat than argon, aluminum can be puddled very quickly so a puddle can be made very quickly even on very thick aluminum.

That translates into less distortion because travel speed is quicker, the beads are smaller, and penetration is deeper.

So some applications for tig welding aluminum using DC are:  thick aluminum parts where distortion limitations are tight, large aluminum parts that would otherwise need to be preheated, tack welding thick aluminum to prevent cracking of tack welds.

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