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Aluminum Workshop: Defining arc rectification
- By Frank Armao
- January 2, 2015
- Article
- Aluminum Welding
Q: Sometimes when I’m AC gas tungsten arc welding (GTAW) aluminum, the arc will start to flutter for a while. I can see it and hear it. Usually it goes away after a few seconds. What’s going on?
A: First of all, the good news is that you’re not imagining it. This phenomenon is fairly common in AC GTAW. It’s called arc rectification. While it won’t necessarily result in weld defects, it is distracting to the welder, and if it gets back enough, it will be difficult to add filler wire to the weld, which can cause real problems.
All GTAW power supplies exhibit arc rectification on AC to some degree. Newer square-wave power supplies generally rectify less than older, sine-wave power supplies. The new inverter GTAW power supplies generally are even less prone to rectify.
So what’s going on? As you say, the arc looks and sounds fluttery. What’s really happening is that the arc is acting like a rectifier, partially eliminating some of the electrode-positive cycles of the AC. This occurs because when the electrode is negative, electrons are emitted by the tungsten electrode. But when the electrode is positive, electrons must be emitted by the aluminum. However, the tungsten emits electrons more easily than the aluminum does, so some of the reverse-polarity AC cycles are small or missing.
Arc rectification was a relatively common problem 30 years ago when most GTAW power supplies supplied only sine-wave AC. In fact, when I first started in the aluminum welding industry, it wasn’t uncommon to see a 6- or 12-volt automobile storage battery wired in series with the welding circuit. Its purpose was to add a bias voltage to the circuit to minimize arc rectification. It looked strange, but it was effective.
I haven’t seen this arrangement in quite a few years, and I don’t miss it. The advent of square-wave power supplies reduced the incidence and severity of arc rectification.
Despite that, virtually all GTAW power supplies will rectify somewhat under the right circumstances. Some power supplies will show arc rectification only over a narrow range of welding current. Others will show it more generally. Almost all GTAW power supplies can be forced to rectify if you know how.
If you want to force your power supply to rectify so you will recognize it in the future, try this experiment (don’t worry, it won’t hurt your power supply).
Get a piece of aluminum about ½ to 1 in. thick. This is so you can put out a lot of current without melting it. Set your power supply on AC between 120 and 150 amps. Strike an arc and slowly move it along the surface of the plate for 3 or 4 in. Don’t worry about adding filler wire or even forming a weld puddle. All you really want to do it use the arc to clean the aluminum. Now, without stopping, back up and slowly draw the arc back over the cleaned area. Most of the time you will begin to see some amount of arc fluttering when you do this.
Why should this sequence cause arc rectification? Believe it or not, it’s because the surface of the aluminum is too clean. When we look at an arc, it looks to us like a uniform, bright light ½ in. or so in diameter. However, if you look at it on a much finer scale, it’s far from uniform. It’s going out and reigniting 120 times per second. When it reignites, the high frequency actually initiates the reignition. When trying to reignite in an electrode-positive cycle (that’s where we get arc cleaning), the HF actually will initiate from an impurity or oxide because it’s easier to initiate there—sort of like lightning hitting a church steeple rather than the ground. By welding over an area once, we’ve used the arc to strip off most of the impurities and oxides. When we weld over it immediately again, fewer oxides and impurities are available for the arc to reignite, so it’s harder to initiate the electrode-positive cycles.
About the Author
Frank Armao
Aluminum Consulting Inc.
440-479-0239
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