June 12, 2003
Q: I've read that constant-current (CC) power supplies are preferred for gas metal arc welding (GMAW) aluminum, but when I visit companies that sell welding equipment, they tell me I want a constant-voltage (CV) power supply. What's the real story? Can I use the more common CV power supplies or not?
A: This controversy has gone on for 40 years or so. At one time the statement that CC power supplies are preferred was true. However, welding power supplies have changed a lot, especially in the last 20 years.
It's helpful to know what CC and CV power supplies are and how they work.
Shielded metal arc welding (SMAW) and gas tungsten arc welding (GTAW) were developed before GMAW. Therefore, power supplies initially were developed for SMAW and, a bit later, GTAW. In both SMAW and GTAW, the operator holds a stinger, or torch, and welds manually.
During welding, it's inevitable that the welder's hand will move to change the arc length, which controls the arc voltage. This means that as the arc length changes, the arc current changes too.
|Figure 1In power supplies designed with steep volt-amp curves, even large changes in voltage caused only small fluctuations in welding current.|
Large current changes would have made it difficult to control the arc and could have caused welding defects, so manufacturers designed power supplies with steep volt-amp curves (see Figure 1).
In these power supplies, even large changes in voltage caused only small fluctuations in welding current. This situation applied to SMAW and GTAW. Welders still use these types of power supplies today. Strictly speaking, the term constant current is a misnomer because the current can vary a bit, but it's in common usage.
When GMAW was invented in the late 1940s, only CC power supplies were available, so they were used for GMAW as well. However, it quickly became apparent that they weren't user-friendly for GMAW.
The main reason for this is that no coupling took place between the wire feed set on the wire feeder and the welding current set on the power supply. The first important consideration in GMAW is that the filler wire is burned off at the same rate as it's being fed. If the current on a CC power supply is set too high, the wire will burn back to the tip almost immediately. If it's set too low, the wire will stub on the workpiece and the arc will be extinguished. The second concern is that a CC power supply gives no current spike on arc initiation, so it's often difficult to start large-diameter filler wires.
The CV power supply features a shallow volt-amp curve slope.
In response to the rising popularity of GMAW, manufacturers invented a new type of power supply—CV. In this type of power supply, the slope of the volt-amp curve is shallow (see Figure 2).
When welding with a CV power supply, even small changes in arc voltage can result in large changes in the welding current. In this way, the GMAW arc was made self-regulating.
If the welder pulled the torch away from the workpiece—raising the arc length and arc voltage—the power supply dropped the arc current to burn off wire at a slower rate until the preset arc voltage was re-established. If the welder pushed the torch toward the work—shortening the arc and reducing the arc voltage—the power supply quickly raised the welding current to burn off more wire until the preset arc voltage was re-established.
So the CV power supply made it easier for the welder to set up the power supply and wire feeder. The welder basically had to set a reasonable arc voltage on the power supply, set a reasonable wire feed speed (WFS) on the wire feeder, and weld.
At any reasonable combination of voltage and WFS settings, the welder got an arc. It could be trimmed by changing the voltage to get a longer or shorter arc or by changing the WFS to deposit more or less metal. In any case, though, the power supply reacted quickly to set the right welding current. In fact, wide current swings during welding were common and expected.
CV power supplies gained popularity and continue to be the most common type of power supply for GMAW today. However, when people started welding aluminum with the GMAW process in the 1950s, they had problems.
The difficulty arose because of the large, rapid current swings that the CV power supplies made during welding. Rapid current changes of as much as 200 amps were common. The high thermal conductivity of aluminum—five times that of steel—meant that if the current dropped too rapidly or too far, lack of fusion defects often resulted. This phenomenon never has been seen in steel welds, in which the thermal conductivity is lower.
For this reason, CC power supplies were recommended for aluminum GMAW. However, it's important to remember that the problem was seen on relatively simple transformer-controlled CV power supplies.
While such power supplies still are made, most modern CV power supplies are more sophisticated. In the older machines, which were made for steel welding, hardware components controlled the arc response. Current rise and fall rates were uncontrolled.
Today's CV power supplies can be programmed for welding materials with different characteristics, such as aluminum and steel. Current rise and fall rates are controlled rigidly, and the current fluctuation during welding is less than that encountered in 1950s CV power supplies.
So, recommending CC power supplies for aluminum GMAW made sense 30 years ago. And in some applications, it still makes sense today. For example, high-current (at least 600 amps) aluminum GMAW using large-diameter (at least 532-inch) wire still works better with CC than CV. However, most of us don't do this sort of welding. For most applications, modern CV power supplies are suitable for aluminum GMAW.
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