February 27, 2003
Q: I've heard different opinions about which size and type of tungsten electrode I should use to gas tungsten arc weld (GTAW) aluminum. Could you clear up this subject for me?
A: As you know, we use direct current electrode negative (DCEN), or straight polarity, to weld steels and stainless steels. For this type of welding, 2 percent thoriated tungsten electrodes, ground to a conical point, are recommended almost universally. To make this type of electrode, we disperse small particles of thorium oxide, or thoria, in the tungsten so it makes up 2 percent by volume of the electrode.
Thoria is used for two reasons. First, it makes the tungsten electrode better able to resist sagging or deforming at the high temperatures it reaches during welding. Second, it makes it easier for the tungsten electrode to emit electrons. Together, these mean that a thoriated tungsten electrode can carry more current than a pure tungsten electrode of the same diameter.
In DCEN welding, about 80 percent of the arc energy goes into the work, and the tungsten electrode has to dissipate only about 20 percent. Because we don't have to dissipate much heat, we can weld with relatively small–diameter electrodes. A 3/32–inch–diameter electrode can carry up to 250 amps. It also means we can grind the electrode tip to a sharp, conical point to concentrate the arc, and it won't deteriorate quickly.
Aluminum is different. Although it's possible to weld aluminum using DCEN and helium shielding gas, it's more difficult and requires stringent preweld cleaning. It's more common to use AC GTAW on aluminum alloys.
When we use AC, the electrode negative (EN) part of the AC wave gives good penetration, which we want, while the electrode positive (EP) part of the AC wave strips any remaining oxides off the surface of the aluminum, which we need.
This cleaning action makes it easier to make a good weld. In fact, you can see it. If you look at a good GTAW, you will see a bright, frosty stripe about 1/16 to 1/8 in. wide right next to the weld bead. This is the area where the oxides have been removed by the AC arc.
Early AC GTAW power supplies used a simple 60–cycle sine wave AC that gave equal amounts of EN and EP. However, this isn't optimum. Newer power supplies use square wave AC that allow you to vary the balance between EP and EN. We don't need 50 percent EP to get good cleaning, and EP puts more heat into the tungsten electrode.
Also, we want to maximize the amount of EN to get the most weld penetration. It's common to run conventional AC GTAW power supplies at 65 percent EN and 35 percent EP for best results.
What does all this have to do with your choice?
More heat goes into the tungsten electrode in AC GTAW than in DCEN GTAW. This means two things. First, you need a larger–diameter tungsten electrode to carry, for example, 200 amps AC than you would to carry 200 amps DC (see Figure 1).
Second, if you grind the tungsten to a point and use it in AC welding, the tip rapidly deteriorates. The traditional solution to this is not to grind a point on the tungsten. Most AC GTAW is performed with a blunt–tip electrode. This tip rapidly forms a round ball while welding.
If you get a 2 percent thoriated electrode ball, you will find that as it rounds over, it grows small irregularities on the surface. The arc then wanders from one irregularity to another on the tip and becomes somewhat unstable. For this reason, 2 percent thoriated electrodes usually aren't recommended for AC welding.
Instead, use either pure tungsten or zirconiated tungsten electrodes. More recently rare–earth electrodes—ceriated and lanthanated—have become available. These electrodes substitute cerium oxide or lanthanum oxide for the thorium oxide in the tungsten and can work well on either DC or AC. They have the added advantage of not being radioactive, but they are more expensive than the other electrode types.
In summary, follow these three recommendations:
All of these recommendations are valid for traditional GTAW power supplies. In the last few years, however, most manufacturers have introduced GTAW power supplies based on inverter technology, which allows you to vary the AC frequency over a range of about 20 to 150 Hz. This means that less heat goes into the tungsten than when welding with conventional power supplies. Also, these power supplies can produce acceptable welds on AC using 10 to 15 percent reverse polarity.
With inverter power supplies, you can use a smaller–diameter tungsten and can grind it to a point. If your welding amperage is low, the point will last a long time. If the current is higher, it will deteriorate more quickly.
So what electrode should you use for AC welding aluminum? It comes down to your power supply. If you're using a conventional power source, use pure tungsten or zirconiated tungsten and let the end form a ball. If you're using an inverter–based machine, use 2 percent thoriated tungsten ground to a point.
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