May 19, 2009
Gas tungsten arc welding (GTAW), or tungsten inert gas (TIG) welding, often is thought of as the most difficult welding process to master. Fortunately for welders, several technological advances have occurred, forcing many to rethink their assumptions.
Alternating current (AC) GTAW is no longer just a way to blast through oxides to weld the aluminum underneath. AC GTAW inverters—through new arc controls and waveform options—have made it easier for operators to tailor arc characteristics, weld puddle behavior, weld bead profile, penetration, and appearance. Such controls include those for:
Today's AC GTAW inverters let the operator choose from four different waveforms: advanced square wave, soft square wave, sine wave, and triangular wave (see Figure 1). Each wave changes the arc and puddle characteristics as well as the penetration profile.
Advanced square wave. The advanced square wave waveform offers fast transitions between EN and EP for a responsive, dynamic, and focused arc with better directional control. It forms a fast-freezing puddle with deep penetration and fast travel speeds.
Soft square wave. Soft square wave provides a smooth, soft, "buttery" arc with a fluid puddle and good wetting action. The puddle is more fluid than with advanced square wave and more controllable than with sine wave.
Sine wave. The sine wave offers a soft arc with the feel of a conventional power source. It provides good wetting action and actually sounds quieter than other waves. Its fast transition through the zero amperage point also eliminates the need for continuous high frequency.
Triangular wave. The triangular wave offers peak amperage while reducing overall heat input into the weld. This leads to quick puddle formation, low weld distortion, and fast travel speeds. It is especially good for welding thin aluminum.
Independent amperage (or amplitude) control allows the EP and EN amperages to be set independently. This precisely controls heat input into the work and even takes heat off the electrode. The EN portion of the cycle controls the level of penetration, and the EP portion affects the arc cleaning action.
A current with greater EN than EP creates a narrow bead with deeper penetration and no visible cleaning action, ideal for fillet welds and automated applications. A current with greater EP than EN gives the operator a wider bead with less penetration and clearly visible cleaning action, ideal for buildup work (see Figure 2).
For example, when welding a thick piece of aluminum, the operator can pour 350 amps of EN into the weld and only 175 amps of EP into the tungsten. This allows faster travel speeds, faster filler metal deposition, deeper penetration, and the potential to eliminate preheating. Case studies about GTAW inverters with independent amperage control suggest that companies can cut production time by as much as two-thirds.
Increasing EN while maintaining or reducing EP also permits the use of a smaller-diameter tungsten. This takes heat off of the tungsten and more precisely directs it into the weld. Companies have reported that this has allowed them to purchase thinner-diameter electrodes, which are less expensive than the thicker variety.
Adjusting the AC frequency—the number of times per second that the direction of the electrical current completes a full cycle—gives welders excellent control over bead appearance and penetration profile (see Figure 3). While conventional GTAW technology limits AC frequency to 50 or 60 hertz (Hz), new GTAW inverters allow frequency to be adjusted anywhere from 20 to 400 Hz.
Frequencies between 80 and 120 Hz are comfortable to work with, increase control of the arc, and boost travel speeds. Setting the frequency from 120 to 200 Hz provides an ideal frequency for most aluminum welding. An arc cone at 400 Hz is even tighter and more focused; improves arc stability; and is ideal for fillet welds or other fit-ups requiring deep, precise penetration.
Figure 4 provides an example of a weld done at 150 Hz and 40 Hz.
In general, increasing AC frequency provides a more focused arc with increased directional control and a narrower bead and cleaning area. This improves performance when welding in corners, on root passes, and in fillet welds. A narrower bead also prevents overwelding, which is a significant waste of time and filler metal.
Turning up the frequency while turning down the balance (see next section) has allowed some manufacturers to reduce scrap and increase productivity by achieving deeper penetration without putting too much heat into the part. This achieves the desired bead profile and production speed without warping the part.
A lower frequency softens the arc and results in a wider weld puddle and bead. This removes impurities well and transfers the maximum amount of energy to the weld piece, which speeds up applications requiring heavy metal deposition, such as building up a worn part or making a fill pass. A good starting point for such applications is 60 Hz with adjustments made from there.
AC balance control allows the operator to adjust the balance between the penetration (EN) and cleaning action (EP) portions of the cycle. Some inverters have adjustable EN as great as 30 percent to 99 percent for control and fine-tuning of the cleaning action.
For instance, if the operator sets EN at 70 percent, it means that 70 percent of the AC cycle is putting energy into the work, while 30 percent of the cycle is cleaning.A good starting point on clean aluminum is between 60 percent and 75 percent. Some companies have even experimented with AC GTAW on ferrous metal, where a few extra percentage points of cleaning action proved beneficial.
Extending the EN portion of the cycle narrows the weld bead, achieves greater penetration (good for thick welds), and may permit increased travel speeds (see Figure 5). It also reduces the size of the etched zone for improved cosmetics. It reduces balling action, increases tungsten electrode life, and may permit the use of a smaller electrode to more precisely direct the heat into the weld.
Reducing the EN portion of the cycle widens the weld bead and decreases its penetration (see Figure 6), which may be beneficial in catching both sides of the joint.
It produces a greater cleaning action to remove heavy oxidation and minimizes penetration, which may help prevent burn-through on thin materials. Reducing the EN cycle, however, decreases tungsten electrode life and increases balling action because more heat is being directed into the electrode. This creates a large ball at the end of the tungsten and causes the arc to lose stability, making it hard to direct the arc weld puddle.
In terms of bead appearance, too much penetration (higher EN) can result in a scummy-looking weld puddle that may still contain oxides and inclusions. Increasing cleaning action will blast away those contaminants. Ultimately, operators should practice adjusting balance control on scrap material and find which settings work best for them.
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