this metal's hard, thin oxide layer gives it natural corrosion resistance.
The current flows from the tungsten electrode to the work surface, and the positively charged argon gas ions flow from the work surface to the tungsten which puts too much heat into the metal and causes the base metal underneath the oxide layer to liquefy while the surface remains hard and impenetrable. The electrode is connected to the negative terminal of the power supply. Electrons are emitted from the tungsten electrode and accelerated while traveling through the arc. A significant amount of energy, called the work function, is required for an electron to be emitted from the electrode. When the electron enters the workpiece, an amount of energy equivalent to the work function is released. This is why in GTAW with DCEN more power (about two-thirds) is located at the work end of the arc and less (about one-third) at the electrode end. Consequently, a relatively narrow and deep weld is produced.
Direct current electrode posititive (DCEP) solves the oxide problem because the current flows from the workpiece to the tungsten, lifting the oxide off the material in the arc zone. DCEP alone provides the oxide cleaning action and very little penetration. Because the heat is concentrated on the tungsten instead of the workpiece, DCEP also causes the tungsten to ball up at the end. Direct-Current Electrode Positive (DCEP) This is also called the reverse polarity. The electrode is connected to the positive terminal of the power source. The heating effect of electrons is now at the tungsten electrode rather than at the workpiece. Consequently, a shallow weld is produced. Furthermore, a large-diameter, water-cooled electrodes must be used in order to prevent the electrode tip from melting. The positive ions of the shielding gas bombard the workpiece, knocking off oxide ﬁlm and producing a clean weld surface. Therefore, DCEP can be used for welding thin sheets of strong oxide-forming materials such as aluminum and magnesium, where deep penetration is not required.
AC, then, combines DCEN and DCEP to provide good heat penetration with cleaning action. The oxide is sandblasted away – as you can see in the following pictures.
Historically, though, AC has posed an obstacle to GTAW because the arc frequently extinguishes itself as the current reaches a zero point before reversing directions. Without any current passing between the tungsten and the base metal, the arc simply goes out and affects the quality of the welding arc – and the quality of the weld.
The introduction of Squarewave technology brings a new level of TIG-welding performance to the market. Improvements in transformer-based GTAW machines created the square wave, which increased the amount of time the arc spends at full-current flow in both DCEN and DCEP. Square-wave technology eliminated the tendency for the arc to extinguish when the current came to a halt as it reversed directions by making the transition very quickly. This greatly improved the stability of the arc and made square-wave technology the preferred method for GTAW of aluminum and other materials that form an oxide layer, such as magnesium.
This wave is balanced with both cleaning and penetration. 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.
Here, you can see the difference in welds between maximum cleaning and maximum penetration. 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
This controls the amount of arc-cleaning action and width of the arc-etching zone around the weld.
The second major revolution in frequency technology came with the invention of the inverter, which created the ability to increase or decrease output frequency beyond the standard 60 Hz, which is the standard frequency delivered to every outlet in the U.S. (other countries, such as Germany, England, and France, deliver AC power at 50 Hz). The inverter also allowed for the development of the advanced square wave, which decreases the time it takes for the current to reverse directions, increasing arc stability even more and eliminating the need for continuous high frequency. Increasing frequency above 60 Hz causes the current to change direction more often, which means that it spends less time per cycle in both DCEN and DCEP mode. By spending less time at each polarity, the arc cone has less time to expand. An arc cone at 400 Hz is much tighter and more focused at the exact spot the electrode is pointing than an arc cone operating at 60 Hz (see Figure 1). The result is significantly improved arc stability, ideal for fillet welds and other fit-ups requiring precise penetration. Combined with adjustable balance control to increase the electrode negative polarity—resulting in deeper penetration and tungsten that doesn't ball up—high AC frequency can weld very tight joints with good penetration and without the risk of laying down too much filler metal. Workpieces with wide gaps to fill or that require buildup will benefit from the softer, wider arc cone that results from lower frequencies Unlike other types of waveform control, such as balance and amplitude, frequency control provides good penetration at both low and high frequencies. The primary difference between the two is the width of the arc cone and resulting weld bead.
Controls the width of the arc cone. Increasing the AC Frequency provides a more focused arc with increased directional control. Decreasing the AC Frequency softens the arc and broadens the weld puddle for a wider weld bead. A Good starting point for working with adjustable frequency for the first time is between 80 and 120 Hz.
At 60 Hz, you can see the bead doesn’t quite penetrate the thick aluminum.
At 200 Hz, the bead is much tighter and penetrated the thicker metal.
The AC frequency controls the width of the arc cone.
Arc shaping capabilities are enhanced by improved balance control. On the left is the tungsten with a balled end, due to more time spent in the electrode positive (EP) part of the cycle, which creates shallower penetration. On the right is the tungsten with a sharp end, due to more time spent in the electrode negative (EN) part of the cycle, which creates deeper penetration and allows faster travel speeds.
The four different waveforms affect the arc and puddle characteristics and the penetration profile. An advanced square wave (A) allows for fast travel speed. 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. A soft square wave (B) provides maximum puddle control. 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. The sine wave (C) permits welding with traditional characteristics. 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. The triangular wave (D) reduces heat input. 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 AC Amperage Control allows the EN and EP amperage values to be set independently. Adjust the ratio of EN to EP amperage to precisely control heat input to the work and the electrode. EN amperage controls the level of penetration, while EP amperage dramatically effects the arc cleaning action along with the AC Balance control. 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.
Cleaning Action Bill Sommer • In the Beginning… Cori Zandstra • Cleaning Action in Action Anna Tivol • Cleaning in the Big Picture
Questions about Cleaning Action• Can it be used on mild steel?• Can it be used in the D.C. mode?• Is it A.C. ONLY?• Is only used on Aluminum and magnesium?• And will it clean grease and paint?
No no yes
Once More but Slower…• Can it be used on mild steel?.............No• Can it be used in the D.C. mode? …..No• Is it A.C. ONLY? ………………………Yes• Is only used on Aluminum and magnesium? …………………………..Yes• And will it clean grease and paint?.....No
Mechanical Means of Removing Oxides …and rust …and dirt …and corrosion.
How is welding aluminum like eating an Oreo?
• Can it be used on mild steel?..............No• Can it be used in the D.C. mode?.......No• Is it A.C. ONLY? ………………………Yes• Is only used on Aluminum and magnesium? ………………………..…Yes• And will it clean grease and paint?......No
TIG Weld – Magnification
Etch Zone Close-up
Zero Crossover Area
Balanced AC Wave
Cleaning vs. Penetration
Extended Balance Control
AC Frequency Control
AC Frequency Control
Frequency Adjustment at 60 Hz
Frequency Adjustment at 200 Hz
AC Frequency Control
Frequency: Bead and Cleaning
Independent Amperage Control
AC Waveform Selection
Independent EN and EP current control may optimize the weld bead profile and penetration.