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Aluminum Workshop: Electrical current in TIG and MIG

Q: When I gas tungsten arc weld (GTAW) aluminum, I always use alternating current (AC), but when I gas metal arc weld (GMAW) aluminum, I always use direct current electrode positive (DCEP). Why the difference? Does anyone GTAW aluminum using DC? Does anyone GMAW using AC?

A: In both GTAW and GMAW, if you use EP, you put about 70 percent of the arc energy into the tungsten electrode or GMAW filler wire, and if you use electrode negative, you put about 70 percent of the arc energy into the work.

In GTAW, you want to minimize the energy going into the tungsten electrode so it doesn’t melt. That’s why you use DCEN to GTAW most materials.

Additionally, in GTAW, DECEN maximizes weld penetration. However, when welding aluminum, you find that DCEP actually removes the oxide from the aluminum surface, making it easier to weld. Many years ago someone figured that if you use AC to weld aluminum, you get the arc cleaning you need from the EP part of the AC cycle, and you get the penetration you want from the EN part of the AC cycle.

Can you GTAW aluminum using DCEN? Yes. However, you don’t get any oxide removal from the arc cleaning, so preweld cleaning requirements are much more stringent. For this reason, DCEN GTAW is somewhat of a specialty process and isn’t common.

The situation is different with GMAW, in which you get both oxide removal and penetration from the DCEP current. If you used DCEN to GMAW aluminum, you would get no oxide removal and very little penetration. So really no good reason exists to use DCEN for GMAW. (Yes, I know that some FCAW electrodes are welded using DCEN, but we’re talking aluminum here.)

Actually, a few specialty AC GMAW power sources for aluminum can be found on the market. They are used primarily for welding very thin aluminum. In this case, adding a bit of EN current to the EP will actually cut down the weld penetration, which can be very helpful in the thin material. However, these power supplies tend to be complex and very expensive, and as a result they are not commonly seen.

About the Author
Aluminum Consulting Inc.

Frank Armao

President

Aluminum Consulting Inc.

440-479-0239

Frank Armao was an active member of the AWS D1 Committee, chairman of the AWS D1 Aluminum Subcommittee, and member of the Aluminum Association Committee on Welding and Joining. He also was the author of The WELDER's "Aluminum Workshop" column from 2001 to 2020.