this metal's hard, thin oxide layer gives it natural corrosion resistance.
The weld did not break through the aluminum oxide layer. This created a weld with the filler metal mixed in with the partially melted oxide to create the contaminated bead seen here.
The electrode is connected to the negative terminal of the power supply so the current flows from the tungsten electrode to the work surface. 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.
Direct current electrode posititive (DCEP) 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. The positive ions of the shielding gas bombard the workpiece, knocking off oxide ﬁlm and producing a clean weld surface. * solves the oxide problem because the current flows from the workpiece to the tungsten, lifting the oxide off the material in the arc zone. very little penetration because the heat is concentrated on the tungsten instead of the workpiece so a shallow weld is produced requires a large-diameter, water-cooled electrode in order to prevent the electrode tip from melting but causes the tungsten to ball up at the end. 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. AC allows the electrode positive (EP) portion of the cycle to blast away the aluminum oxide while the electrode negative (EN) portion melts the base metal Do not start welding until the puddle has the appearance of a shiny dot. This indicates that the oxide has been removed and it is safe to add filler and move forward. Adding filler to the weld zone before the oxide layer is adequately removed will result in contamination.
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 goes out and affects the quality of the welding arc – and the quality of the weld. To correct this unstable arc, welding manufacturers superimposed a low-current, high-voltage radio frequency ( NOT shown here) on top of the welding current. The high frequency is present continuously and acts like a pilot arc.
In 1974: the square wave increased the amount of time the arc spends at full-current flow in both DCEN and DCEP and eliminated the tendency for the arc to extinguish 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.
Development of the advanced square wave further decreases the time it takes for the current to reverse directions, increasing arc stability even more and eliminating the need for continuous high frequency
four different waveforms currently available different types of welds because the wave shape affects the arc and puddle characteristics and the penetration profile. An advanced square wave (A) has very fast transitions between EN and EP and is good for fast travel speed - responsive, dynamic, and focused arc with better directional control forms a fast-freezing puddle with deep penetration A soft square wave (B) provides maximum puddle control. provides a smooth, soft, "buttery" arc with a fluid puddle and good wetting action 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. a soft arc with the feel of a conventional power source. good wetting action and actually sounds quieter than other waves fast transition through the zero amperage point eliminates the need for continuous high frequency The triangular wave (D) reduces heat input peak amperage while reducing overall heat input into the weld. quick puddle formation, low weld distortion, and fast travel speeds especially good for welding thin aluminum.
A feature called AC balance control allows you to tailor the EP-to-EN ratio. This wave is balanced with both cleaning and penetration. If you notice a brownish oxidation or flakes that look like black pepper in your weld puddle, simply increase the cleaning action. However, note that too much EP causes the tungsten to ball excessively and provides too much etching
Here, you can see the difference in welds between maximum cleaning and maximum penetration. Greater amounts of EN create a deeper, narrower weld bead, better joint penetration and a smaller etched zone. This helps when welding on thick material or when appearance (i.e. a minimal etched zone) is important. Setting an inverter's EN duration to the maximum level creates the potential to deliver more heat into the work, permitting faster travel. Lesser amounts of EN (e.g., more EP) remove more oxide and create a shallower, wider bead. Miller's Syncrowave® 250 and Syncrowave® 350 LX let you adjust EN values from 45 to 68% (32 to 45% EP).
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.
Some TIG inverters, provide extended balance control . Operators can fine tune the duration of the EN portion of the cycle from 30 to 99 percent. AC Balance should be fine tuned according to how heavy or thick the oxides are. Some companies have even experimented with welding ferrous metals using AC TIG with a high percentage of EN. These metals tend to be contaminated, such as saturated with an oil, and benefit from just a few percentage points cleaning action.
The AC frequency controls the width of the arc cone. 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 The result is significantly improved arc stability, ideal for fillet welds and other fit-ups requiring precise penetration. 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.
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. 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. This is an ADVANCED feature – it is very similar to balance control but offers more fine-tuning.
Slide show finished 03042013
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 it only used on Aluminum and magnesium?• And will it clean grease and paint?
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 it 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.
• 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