Gas Tungsten Arc Welding (GTAW), or TIG, is often specified to meet strict aesthetic, structural, or code/standard requirements. The TIG process is complex, and it is undisputedly the most difficult process to learn. This article contains photos and descriptions of common TIG mistakes and basic tips on how to prevent these errors from happening.
Figure 2A: Aluminum welded in DC with argon
Figure 1 – Poor Gas Coverage Leads to Contamination
The weld here shows contamination caused by lack of shielding gas, which can happen when the shielding gas is not turned on, there is either too little or too much gas shielding, or the gas shielding is blown away.
To troubleshoot gas contamination issues, first check the gas cylinder label to be sure you’re using the right type of gas for TIG welding, generally 100 percent argon (or perhaps an argon/helium blend for thick aluminum). Attempting to weld with an AR/CO2 mix (used for MIG welding) will cause immediate contamination.
Figure 2B: Ideal aluminum weld
Next, set the proper gas flow rate, which should be 15 to 20 cubic feet per hour (cfh). Welders commonly—and incorrectly—assume that a higher gas flow/pressure provides greater protection. In fact, excessive gas flow creates turbulence and swirling currents that pull in unwanted airborne contaminants (and it can cause arc wandering). Generally, err on the lower side of recommended shielding gas rates to ensure proper shielding coverage without turbulence.
Third, check all the fittings and hoses for leaks. Any breach may pull air into the shielding gas stream, which can cause the weld to be contaminated (and you’ll waste money if gas escapes). Rub soapy water over the hose and all fittings. If bubbles form, you have a leak and need to replace the defective components.
Figure 2C: AC balance set too high
Finally, assuming you have a full cylinder, the right type of gas and no leaks, consider that you may have a tank contaminated with moisture. Shielding gas cylinder contamination does not happen frequently, but it is possible. Check with your gas supplier to resolve this issue.
Figure 2A-2D – Welding Aluminum in the Wrong Polarity/Adjusting Balance
This TIG weld (Figure 2A) was created with the machine’s polarity set on direct current electrode negative (DCEN). As you can see, the weld did not break through the aluminum oxide layer. This created a weld where the filler metal mixed in with the partially melted oxide and created the contaminated bead seen here. To defeat this, always TIG weld aluminum with the polarity set to alternating current (AC).
Figure 2D: Balled tungsten
TIG welding in AC (Figure 2B) 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. A feature called AC balance control allows operators to tailor the EP to EN ratio. If you notice a brownish oxidation and or flakes that look like black pepper in your weld puddle (Figure 2C), increase the cleaning action. However, note that too much EP causes the tungsten to ball excessively (Figure 2D) and provides too much etching. Lastly, when TIG welding aluminum, 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.
Figure 3: Grainy aluminum weld
Figures 2B and 3 – Weld Graininess
Figure 2B shows the way an aluminum TIG bead should look. Figure 3 (see page 74) shows a bead with a grainy appearance, which is typically caused by filler metal problems. For instance, a 4043 aluminum filler rod from one manufacturer may have different properties than a 4043 rod from another manufacturer. The welder (if the application permits) may need to adjust filler brand accordingly. The rod may also be defective (too much of a certain ingredient). The welder may even have the wrong type of filler rod, such as 4043 filler instead of 5356 filler.
Figure 4: Lack of fusion in the root
Prior to welding, always check the filler metal type and remove all grease, oil and moisture from the surface to prevent contamination.
Figure 4 – Lack of Fusion in the Root
Lack of fusion at the root of a T-joint or a fillet weld can be caused by a number of factors: improper fit-up, holding the torch too far away from the joint (increasing arc length) and improperly feeding the filler rod, to name a few. This issue may be seen more often with a transformer-based machine, as the arc tends to wander between the two sides of the joint as it seeks the path of least resistance. In this case, reducing arc length will provide better directional control and help increase penetration. It is also important not to under-fill the joint or weld too quickly.
Note that inverter-based machines (especially those with an advanced output controls such as adjustable frequency and pulsing controls) offer more control over the arc. These controls create a narrower, more focused arc cone that provides better directional control over the weld puddle and deeper penetration (and often at increased travel speeds).
Figure 5A: Poorly filled weld craters
Figure 5A and 5B – Craters
Craters, such as the one shown in Figure 5A, typically occur at the end of the weld, and they often lead to cracking. Causes include instantly reducing the welding power (which causes the puddle to cool too quickly) and removing the filler rod too quickly at the end of the weld. You can easily fix crater cracking issues by continuing to feed filler rod while slowly reducing current at the end of a weld. Note that some TIG welders feature a “crater control” function that automatically reduces the current at the end of a weld. The result is a good-looking weld bead, as seen in Figure 5B.
Figure 5B: Weld crater filled
Figure 6A through 6D – Dirty Base and/or Filler Metal
On day one of welding school, your instructor should have taught you to clean materials prior to welding. Figure 6A (see page 76) shows what happens when you don’t clean the mill scale off of hot-rolled mild steel. All base and filler metals need to be cleaned, whether it’s mill scale, oxide on aluminum, or dirt and grease on filler metals. Grind, brush and wipe away all potential contaminants. For cleaning aluminum, dedicate a stainless steel brush to the task to prevent contamination from other metals.
Figure 6B shows what happens when a weld on mild
steel has been properly cleaned before welding. Figure 6C shows a weld made on chrome-moly tubing that has not been cleaned, hile 6D shows a weld made that has been cleaned prior to welding.
Figure 6A: Uncleaned steel weld
Figure 7A and 7B – Poor Colour on Stainless
Figure 7A shows discoloration on a stainless steel weld caused by overheating, which not only affects a material’s colour, but degrades its corrosion resistance and mechanical properties as well. Unfortunately, once this error is made, there is nothing that can be done to fix it except for scrapping the part and starting over. To prevent overheating, reduce amperage, slightly increase travel speed or shorten the arc length. If your welding equipment features pulsing capabilities, learn how to use them. Pulsing reduces heat input, and it offers excellent control of the weld puddle. Figure 7B shows proper coloration of stainless.
Figure 6B: Clean steel weld
Figure 6C: Uncleaned chrome-moly
Figure 6D: Cleaned chrome-moly
Figure 7B: Good colour on stainless steel
Figure 8: Sugaring on stainless steel
Figure 8 – Sugaring on Stainless
Figure 8 shows sugaring on the backside of a stainless steel weld. Sugaring (oxidation) occurs around the weld when it is exposed to oxygen in the air. The best way to prevent this is to back purge the weld with argon shielding gas or reduce welding amperage.
Figure 9: Excessive amperage/heat input
Figure 9 – Too Much Amperage on Aluminum
Figure 9A shows what a weld bead looks like on aluminum with the amperage set too high. This creates a wider profile, an ill-defined bead and can potentially lead to burn-through. To solve this problem, reduce amperage and/or increase travel speed. Reference back to Figure 2B to identify an ideal weld.
Figure 10: Change in arc length
Figure 10 – Proper Arc Length Control
The color change in the middle of this aluminum weld bead (Figure 10) resulted from an increase in arc length (arc length, the distance between the electrode and the base metal, determines TIG welding voltage). Holding too long of an arc increases overall heat input, increases the potential for distortion, widens the weld bead while decreasing penetration and affects weld bead appearance. Practice holding a consistent arc length to improve heat input control and improve weld bead quality.
Brad Hemmert, welding engineer, Miller Electric Mfg. Co.