Understanding AC TIG Balance Control
|Fig. 1. AC current is used to weld aluminum because its positive half cycle provides a "cleaning" action and its negative half cycle provides penetration.|
In the mid-1970s, Miller Electric Mfg. Co. set the standard for AC TIG welding technology when it developed its first Syncrowave® AC/DC welder. One key to this welder’s success was its squarewave AC output with balance control. Today, all professional-grade TIG welders feature balance control.
To understand how balance control works, you first need to understand why aluminum and magnesium require an AC welding output. When exposed to air, these metals form an oxide layer that melts at a much higher temperature than the base metal. For example, aluminum oxide melts at about 3600 degree F, while aluminum melts at just 1200 degree F. If not removed, the oxide layer can inhibit proper weld fusion and puddle fluidity.
Fortunately, the squarewave alternating current (see Fig. 1) inherently provides a "cleaning" action (see Fig. 2). The electrode positive (EP) portion of the AC cycle—where current flows from the work to the electrode—actually "blasts" off surface oxides. This then allows the electrode negative (EN) portion of the cycle (where the current flows from the tungsten into the work) to melt the base metal and fuse the join).
|Fig. 2. Notice the lighter colored "etched zone" or "cleaning action" created by the AC arc.|
Until Miller invented balance control, the amount of AC “cleaning action” and “weld penetration action” remained balanced at 50 percent each. Unfortunately, this “one size fits all” approach didn’t provide the ideal arc for all applications. Today, however, welders can use balance control to tailor the TIG arc for the job at hand.
Syncrowave technology gives you the ability to set balance control on range from 0 to 10, or maximum cleaning to maximum penetration, respectively. Syncrowave technology allows fine tuning the duration of the EN portion of the AC cycle from 45 to 68 percent. In other words, you are spending 68 percent of the AC cycle (which is 1/60th of a second) putting energy into the work piece. Inverter-based TIG welders, such as the Dynasty™, extend balance control even further, allowing EN duration times from 30 to 99 percent.
|A comparison between greater (top) and lesser (bottom) amounts of EN.|
Balance Control Benefits
Making the EN portion of the cycle last longer:
• Achieves greater penetration, such as for thick sections.
• May permit increasing travel speeds.
• Narrows the weld bead.
• Increases tungsten electrode life and reduces balling action.
• May permit using a smaller diameter tungsten to more precisely direct the heat or make a narrower weld bead.
• Reduces the size of the etched zone for improved cosmetics (see Fig. 4).
Reducing the EN portion of the cycle:
• Produces greater cleaning action to remove heavier oxidation.
• Minimizes penetration, which may help prevent burn-through on thin materials.
• Widens the bead profile, such as for catching both sides of a joint.
• Decreases tungsten electrode life and increases balling action.
There are no hard rules about setting balance control, but the typical error involves too much cleaning. Too much cleaning action (electrode positive duration) causes excess heat build-up on the tungsten. This creates a large ball on the end of the tungsten. Subsequently, the arc loses stability and you lose the ability to control the direction of the arc and the weld puddle. Arc starts begin to degrade as well. Too much penetration (or, more precisely, insufficient cleaning action) results in a “scummy” weld puddle. If the puddle looks like it has black pepper flakes floating on it, add more cleaning action to “blast” away oxides and other impurities.
For a Syncrowave welder working on clean aluminum, a good starting point would be “7”. For a Dynasty, set the EN between 60 and 75 percent. In all cases, make practice welds on scrap prior to welding to determine the best balance control settings. Lastly, it may help to keep a journal of your favorite balance control and weld parameter settings. Note the material type, thickness, joint configuration, welding position, tungsten type and diameter and application.