TIG welding produces clean, precise welds on aluminum, stainless steel, and other metals. But when defects appear, they look different from MIG or stick welding problems. Porosity in TIG often comes from contaminated filler metal or gas lens issues. Discoloration tells a different story on stainless than it does on titanium. These are TIG-specific problems that need TIG-specific solutions.
This guide covers 10+ common TIG welding defects. For each defect you will learn how to identify it by appearance, what causes it, and how to fix it. Material-specific notes for aluminum, steel, and stainless steel are included where they matter. AC and DC welding differences are noted throughout.
Common TIG Defects Quick Reference Table
| Defect | Appearance | Most Likely Cause | Fix Priority |
|---|---|---|---|
| Porosity | Tiny holes or pinholes in weld bead | Contaminated filler, gas coverage issue, draft | Check gas flow and filler cleanliness first |
| Discoloration | Blue, grey, or rainbow oxides on weld or HAZ | Inadequate shielding gas coverage | Adjust gas flow, check torch setup |
| Hot Cracks | Cracks along weld centerline during cooling | High travel speed, improper filler selection, high restraint | Reduce travel speed, verify filler metal |
| Crater Cracks | Small cracks at weld end crater | Arc stopped too abruptly, crater not filled | Use crater fill feature or taper current |
| Tungsten Inclusion | Bright particles in weld metal | Tungsten touched puddle, wrong polarity, high amperage | Regrind tungsten, correct polarity settings |
| Arc Wander | Unstable arc that moves erratically | AC balance settings, contaminated tungsten, gas flow issue | Check AC balance per manual, clean tungsten |
| Burn-Through | Hole blown through thin material | Excessive amperage, slow travel speed, poor fit-up | Reduce amperage, increase travel speed |
| Lack of Fusion | Gap between weld and base metal not bonded | Low amperage, improper torch angle, fast travel speed | Increase amperage within range, adjust technique |
| Undercut | Groove along weld toe below base metal surface | Excessive amperage, fast travel, wrong torch angle | Reduce amperage, slow down, adjust torch angle |
| Contamination | Dark, rough, or pitted weld surface | Base metal not cleaned, dirty filler, contaminated gas | Clean base metal and filler, purge gas lines |
Porosity in TIG Welding
Porosity appears as small holes or pinholes in the weld bead. In TIG welding, porosity usually comes from three sources: contaminated filler metal, gas coverage issues, or air reaching the weld pool.
Symptom: Small round holes on the weld surface or inside the weld, visible when grinding.
Likely causes:
- Contaminated filler rod with oil, grease, moisture, or oxides
- Gas lens blockage or damaged gas cup
- Draft or breeze blowing shielding gas away
- Leak in gas line or loose fitting
- Base metal contamination such as paint, oil, rust, or mill scale
How to fix: Stop welding. Clean the filler rod with a clean stainless steel brush or replace it. Check the gas lens and cup for spatter or damage. Verify gas flow is within the range recommended by your machine manual. Eliminate drafts near the welding area.
Prevention: Store filler metal in a clean, dry area. Clean base metal thoroughly before welding. Use a gas lens for better gas coverage. Check gas lines and fittings before each session. Keep fans off and close doors or windows to reduce drafts.
AC vs DC notes: On aluminum with AC TIG, porosity is often caused by inadequate oxide cleaning. On DC TIG, porosity is more commonly linked to gas coverage or filler contamination.
Porosity Visual Guide:
| Porosity Type | Looks Like | Common Cause | Check First |
|---|---|---|---|
| Wormhole porosity | Elongated, tubular holes | Gas flow too high or too low | Gas flow regulator |
| Scattered porosity | Random tiny pinholes across weld | Contaminated filler or base metal | Filler rod and base metal cleanliness |
| Surface porosity | Small craters on weld surface | Draft or gas coverage loss | Wind shields, gas lens condition |
| Cluster porosity | Tight group of holes in one area | Moisture or contamination in gas line | Gas line for leaks, gas purity |
TIG Weld Discoloration (Oxidation / Sugar)
Discoloration on a TIG weld indicates that the shielding gas did not fully protect the hot metal. The color of the oxide layer depends on the material being welded, the shielding gas used, the service requirements of the part, and the acceptance criteria for the job. No single color is universally acceptable or unacceptable.
Symptom: Colored oxide layer on the weld bead or heat-affected zone. Colors range from light straw through blue, grey, and dark grey.
Likely causes:
- Low gas flow rate
- Gas cup too small for the joint
- Torch held too far from the work
- Excessive heat input
- Lack of post-flow after the arc stops
How to fix: Verify gas flow rate against values in your machine manual. Use a larger gas cup or a gas lens. Reduce torch-to-work distance. Increase post-flow time to protect the cooling weld.
Prevention: Match gas cup size to the joint. Keep torch angle and distance consistent. Use trailing shields for reactive metals like titanium. Check gas purity from your supplier.
Discoloration Color Reference Chart:
| Color | Common Interpretation | Material Note | Caution |
|---|---|---|---|
| Silver / bright metallic | Good gas coverage | Stainless steel, steel, titanium | Target appearance for most welds |
| Light straw to gold | Slight surface oxidation | Stainless steel | Often acceptable per applicable codes |
| Blue to purple | Moderate oxidation | Stainless steel, aluminum | May be acceptable for non-critical service |
| Dark grey to black | Heavy oxidation | Any material | Typically indicates inadequate coverage |
| White or chalky | Severe oxidation | Aluminum | Weld quality is likely compromised |
Critical note: The interpretation of weld color depends on the material, the shielding gas composition, the service requirements of the part, and the acceptance criteria specified for the job. For structural or service-critical welds, consult the applicable code or specification and a certified welding professional.
Cracking
Cracks in TIG welds fall into three main types: hot cracks, crater cracks, and stress cracks. Each has a different cause and a different fix.
Hot Cracks: Hot cracks form along the weld centerline as the weld solidifies. They happen when the weld pool shrinks and the metal cannot stretch enough to accommodate the stress. High travel speeds, insufficient filler metal, and high joint restraint all contribute.
Symptom: A crack running down the center of the weld bead.
Fix: Reduce travel speed. Verify filler metal selection is correct for the base material. Preheat thick sections to reduce thermal stress.
Prevention: Match filler metal to base material. Use proper joint design. Keep travel speed within recommended ranges.
Crater Cracks: Crater cracks appear at the end of the weld bead where the arc stops. They form because the crater cools faster than the surrounding weld metal.
Symptom: Small cracks in the depression at the weld end.
Fix: Use the crater fill feature on your TIG machine. If no crater fill is available, taper the current manually over 2 to 3 seconds. Add a small dab of filler at the end before stopping the arc.
Prevention: Always fill the crater before stopping the arc. Use machine settings for crater fill if available.
Stress Cracks: Stress cracks occur after the weld has cooled, sometimes hours or days later. They come from high residual stress in the joint.
Symptom: Cracks in the weld or heat-affected zone that appear after cooling.
Fix: Grind out and reweld using a qualified procedure. Preheat and control interpass temperature.
Prevention: Use proper joint design to minimize restraint. Reduce gap or use a different joint configuration to reduce stress.
Crack Identification Guide:
| Crack Type | When It Forms | Location | Common Materials |
|---|---|---|---|
| Hot crack | During solidification | Weld centerline | Aluminum, some stainless steels |
| Crater crack | At arc termination | Weld end crater | All TIG weldable materials |
| Stress crack | Hours or days after cooling | Weld or heat-affected zone | High-strength steels, thick sections |
AC vs DC notes: Hot cracking is more common in aluminum with AC TIG due to thermal expansion. Stress cracking is more common in thicker DC TIG welds on steel.
Tungsten Inclusion
Tungsten inclusion happens when a piece of the tungsten electrode breaks off and gets trapped in the weld metal. These particles appear as bright, irregular spots on a radiograph or as small shiny specks on a ground weld surface.
Symptom: Small bright particles in the weld metal. Visible on x-ray or after grinding.
Likely causes:
- Tungsten touched the molten puddle
- Current setting too high for the tungsten diameter
- Wrong polarity (DC electrode positive instead of DC electrode negative)
- Contaminated tungsten tip
- Gas coverage failure that caused arc instability
How to fix: Stop welding. Grind out the tungsten inclusion and reweld. Inspect the tungsten. Regrind it to a clean point. Check that the tungsten size matches the amperage range in your manual.
Prevention: Use the correct tungsten size for your amperage range. Grind tungsten longitudinally along the length, not across it. Keep the tungsten tip clean. Do not dip the tungsten in the puddle.
Tungsten Inclusion Diagnostic Flow:
1. Did the tungsten touch the puddle? Yes: Regrind tungsten. Adjust technique to avoid dipping.
2. Is amperage too high for the tungsten diameter? Yes: Switch to a larger tungsten or reduce amperage per machine manual.
3. Is polarity correct? DC TIG uses DC electrode negative (DCEN). No: Switch to DCEN.
4. Is tungsten contaminated? Yes: Regrind or replace. Store tungsten clean.
5. Grind out inclusion. Reweld. Verify arc is stable before continuing.
Note: Regrinding a contaminated tungsten can improve arc stability, but check manufacturer guidelines for tungsten preparation and handling. Thoriated tungsten grinding dust is radioactive and requires local exhaust ventilation. Wash hands after handling.
Arc Wander / Arc Instability (AC TIG)
Arc wander is most common in AC TIG welding on aluminum. The arc moves erratically instead of staying focused on the weld joint. This makes it hard to control the weld puddle and produces inconsistent welds.
Symptom: Arc moves side to side or circles instead of staying in one place. Hissing or sputtering sound from the torch.
Likely causes:
- AC balance setting too far into cleaning action
- Tungsten contaminated or wrong type for AC welding
- Gas flow too high or too low
- Ground clamp connection is poor
- Magnetic field interference from nearby equipment
How to fix: Adjust AC balance per your machine manual. A typical starting range puts more time on the electrode negative side to stabilize the arc. Verify gas flow is within the range in your manual. Check the ground connection for solid contact on clean metal. Clean the tungsten or switch to a pure or zirconiated tungsten for AC welding.
Prevention: Start with AC balance settings from your machine manual for the material thickness. Check the machine manual for the recommended tungsten type. Modern inverter AC machines can work with lanthanated, ceriated, or zirconiated electrodes depending on manufacturer guidance. Avoid outdated universal recommendations about tungsten type for AC TIG. Make sure the ground clamp has solid contact on clean base metal. Keep filler rods and torch cable away from each other to reduce magnetic arc blow.
Burn-Through on Thin Materials
Burn-through happens when the weld pool melts through the base metal completely, leaving a hole. This is common when TIG welding thin sheet metal, exhaust tubing, or lightweight aluminum.
Symptom: A hole in the weld joint where the metal melted away.
Likely causes:
- Amperage too high for the material thickness
- Travel speed too slow
- Too much heat concentration in one spot
- Poor fit-up with excessive gap between pieces
How to fix: Stop welding. Fill the hole with additional filler metal by reducing amperage and dabbing carefully. If the hole is large, cut out the damaged section and start with a fresh joint and new filler.
Prevention: Use a foot pedal or fingertip amperage control to vary heat. Start with lower amperage and add heat as needed. Increase travel speed. Use a copper backup bar behind the joint to absorb heat. Ensure tight fit-up with minimal gap.
Lack of Fusion / Cold Lap
Lack of fusion, also called cold lap, happens when the weld metal does not bond properly to the base metal or between passes. The weld looks like it sits on top of the base metal rather than fusing into it.
Symptom: A visible line between the weld and base metal. The weld bead has a rounded, cold appearance with poor wetting at the edges.
Likely causes:
- Amperage too low for the material thickness
- Travel speed too fast
- Torch angle wrong, with too much push angle
- Filler metal added before the base metal reached melting temperature
How to fix: Grind out the unfused section. Increase amperage within your machine’s recommended range for the material thickness. Slow down travel speed. Hold the torch at a 70 to 80 degree angle from the base metal surface.
Prevention: Watch the puddle carefully. The base metal should be fluid before you add filler. Keep torch angle consistent. Clean the base metal to remove oxides before welding.
Undercut
Undercut is a groove that forms along the toe of the weld, below the surface of the base metal. It weakens the joint by reducing the cross-section of the base material.
Symptom: A groove or ditch along one or both edges of the weld bead.
Undercut Cause-Fix Table:
| Cause | How It Creates Undercut | Fix |
|---|---|---|
| Amperage too high | Melts base metal faster than filler can fill | Reduce amperage. Verify in machine manual. |
| Travel speed too fast | Weld pool washes up the sides without filling | Slow down. Let filler flow into edges. |
| Wrong torch angle | Arc pushes metal away from joint edge | Keep torch at 70 to 80 degrees. Point slightly into joint. |
| Insufficient filler addition | Not enough metal to fill the groove | Add more filler. Match filler rate to travel speed. |
Prevention: Use amperage settings from your machine manual for the material. Watch the weld edges. The filler metal should wet into both edges evenly. Adjust torch angle if one side shows undercut.
Contamination
Contamination in TIG welding shows up as a rough, dark, or pitted weld surface. The weld may look dirty, have poor wetting, or show grey smoke marks around the joint.
Symptom: Dark, rough, or dirty-looking weld. Poor wetting. Grey residue around the weld bead.
Likely causes:
- Base metal not cleaned of oil, grease, paint, or oxide
- Dirty filler rod from hand oils or storage contaminants
- Contaminated shielding gas from a low-purity tank or leak
- Dirty gas cup or gas lens with spatter buildup
- Improper gas purge for tubing or pipe
How to fix: Stop and clean everything. Remove all contamination from base metal using solvent cleaning and a stainless steel brush dedicated to each material type. Replace filler rod if contaminated. Purge gas lines. Clean or replace gas cup and lens.
Prevention: Clean base metal to bright metal before welding. Use a dedicated stainless steel brush for each material type, one for aluminum and one for steel. Store filler rods clean and dry. Check gas purity when ordering. Purge gas lines after changing tanks.
AC vs DC: Defect Differences
Some defects appear more often on AC TIG than DC TIG. Understanding these differences helps you diagnose faster.
AC TIG (typically aluminum and magnesium):
- Porosity is often oxide-related. The AC cleaning action removes aluminum oxide, but poor cleaning action settings can leave oxide behind.
- Arc wander is an AC-specific problem. DC arcs are naturally more stable.
- Tungsten inclusion risk is higher with pure tungsten. Alloyed tungsten electrodes such as lanthanated or ceriated hold a point better on AC.
- Hot cracking is more common. Aluminum contracts more during cooling.
DC TIG (typically steel, stainless steel, copper, titanium):
- Porosity is usually gas coverage or filler related. Base metal oxides are less of a factor.
- Discoloration on stainless steel is a DC TIG concern. Color is easier to control with good gas coverage.
- Undercut is more common on thicker DC welds if amperage is too high.
- Burn-through happens less often because heat input is easier to control on DC.
Material-specific notes:
- Aluminum: Focus on oxide removal and AC balance settings. Use a gas lens for consistent coverage.
- Stainless steel: Focus on gas coverage and heat input to avoid discoloration and carbide precipitation.
- Steel: Focus on cleanliness and proper filler selection. Undercut is a common defect to watch for.
Defect Prevention Checklist
Use this checklist before starting any TIG weld. Print it and keep it in your shop for reference.
Before You Strike the Arc:
- Base metal is clean with no oil, grease, paint, rust, or oxide
- Filler rod is clean and stored properly in dry conditions
- Tungsten is sharpened correctly for the job and material
- Gas cup and lens are clean with no spatter buildup
- Gas flow is set per machine manual, not from memory
- Gas line has been purged with no air in the line
- Ground clamp is secure on clean base metal
- Amperage settings match material type and thickness
- AC balance and frequency are set per manual for aluminum
- Draft-free welding area with fans off and doors closed
During Welding:
- Watch the puddle. Do not rush the weld.
- Maintain consistent torch angle at 70 to 80 degrees
- Keep torch-to-work distance steady at the recommended gap
- Fill the crater before stopping the arc at the end of the weld
- Use post-flow to protect the cooling weld
- Do not dip the tungsten in the puddle
After Welding:
- Inspect weld visually for any defects
- Check for discoloration that may indicate poor coverage
- Clean and store tungsten for the next use
- Replace gas cup or lens if damaged
When to Seek Professional Guidance
This article covers common defects for typical TIG welding jobs. Some applications require a certified welding professional or engineer to be involved.
You should consult a qualified professional for:
- Structural welds on buildings, bridges, or heavy equipment
- Pressure vessels or piping that carry gas or liquid under pressure
- Code-required welds that must pass formal inspection
- Safety-critical parts such as vehicle chassis, lifting equipment, or medical devices
- Welds on exotic materials including titanium, Inconel, and zirconium
For these applications, a qualified welding procedure specification (WPS) and a certified welder are required. Do not rely on general troubleshooting advice for critical welds.
Safety reminders:
- Tungsten grinding dust is hazardous. Thoriated tungsten contains radioactive material. Use local exhaust ventilation and wash hands thoroughly after handling.
- The TIG arc is brighter than MIG or stick arcs. UV exposure can cause arc eye and skin burns. Wear the proper shade lens and full protective clothing.
- Shielding gas such as argon and helium can displace oxygen in confined spaces. Ensure adequate ventilation when welding in tanks or small rooms.
- Hot metal and torch parts cause burns. Let parts cool before handling or use heat-resistant gloves.
Frequently Asked Questions
How do I know if my TIG weld has good gas coverage? Good gas coverage produces a clean, bright weld with minimal discoloration. On stainless steel, a silver or light straw color can indicate adequate coverage, but acceptance depends on the service requirements and applicable code. On aluminum, a clean bright silver appearance is typical. On titanium, even light discoloration can be unacceptable for critical service. Check your gas flow against your machine manual and verify gas purity with your supplier.
Why does my TIG arc wander on aluminum? Arc wander on AC TIG is usually caused by incorrect AC balance settings. If the balance is set too far into cleaning action with more electrode positive time, the arc becomes unstable. Other causes include contaminated tungsten, poor ground connection, and gas flow set too high. Adjust AC balance per your manual. Clean the tungsten. Verify the ground connection. Check gas flow.
Can I fix a TIG weld defect without grinding it out? Some surface defects can be repaired by welding over them with additional filler and good technique. But most defects including porosity, cracks, tungsten inclusions, and lack of fusion require grinding out the defective area before rewelding. A visual inspection or dye penetrant test can help determine if the defect is only on the surface or deeper inside the weld. For structural welds, consult a certified welding inspector.
Conclusion
TIG welding defects can be frustrating, but every defect has a cause and a fix. Look at the weld. Identify the symptom. Check the likely causes. Apply the correct fix. Then take steps to prevent it from happening again.
A systematic approach to diagnosis saves time and material. Start with the quick reference table at the beginning of this guide. Read the detailed section for your specific defect. Use the prevention checklist before your next weld.
Remember that every machine and material combination is different. Your machine manual is the final authority on amperage ranges, gas flow settings, AC balance, and frequency settings. When in doubt, consult the manual and contact the manufacturer for guidance.
For more TIG welding information, see related articles on what TIG welding is and how to fix a bad weld. For MIG or stick welding defects, different identification and repair methods apply, so refer to the MIG defects visual identification guide for those processes.
