Common welding related questions we hear. Part 7
More so now than in the past, there is renewed interest in DC TIG welding of aluminum. Without a doubt, a quick online search will yield dozens of forum threads, and videos documenting this practice. You’ll see both DC negative and DC positive used, and with highly variable results. Is it possible to weld with DC? Yes, of course. Early forms of TIG welding before HF stabilized AC current was used, employed DC only welding.
Airplanes in World War II were often pieced together with DC TIG. In these days Helium was the preferred and readily available gas. These days, Helium has fallen out of favor, making Argon king. However, Argon does not have the same properties as Helium, which helped in penetration and cleaning of the aluminum while welding. Helium is also much more expensive, which has curtailed its use. But you can buy some Argon/Helium gas blends which restore much of what was lost with pure Argon.
DC positive polarity works well, for cleaning and welding on oxidized aluminum material, reducing the need to clean, but it requires huge diameter tungsten electrodes along with high amperage to weld even the thinnest material because the direction of the electron flow concentrates the heat on the Tungsten rather than the work. In DC negative, you have to have perfectly clean metal, free from oxidation and impurities. Coupled with a high percentage of Helium, you can find that that a passable job can be done. However, it does take lots of patience and skill to get this done right.
If you are not careful, you’ll have a thick crusty scum on top of the aluminum while welding that does not seem to want to flow or even weld and eventually you’ll have a huge molten lake of aluminum that forms right under the surface of the scum…and a huge blowout. In either case whether you use DC+ or DC-, you are going to have to be on top of your game and pull together your best skills to be able to competently complete a weld. Arc stability suffers somewhat as well with these processes as the arc will want to wander around.