TIG Welding Problems: Diagnosis and Fixes

Q: "Why does my tungsten keep turning black?"

"Black tungsten means contamination, almost always from dipping the electrode into the molten puddle or from filler rod touching the tungsten. Break off the contaminated section, re-grind to a clean point, and focus on maintaining a consistent 1/8-inch gap between the tungsten and the puddle. If the blackening happens without contact, check your post-flow gas time. Insufficient post-flow lets atmospheric oxygen attack the hot tungsten."

Q: "What is the correct argon flow rate for TIG welding?"

"15-25 CFH covers most TIG welding applications. For a standard size 7 cup, 15-20 CFH works well. Larger cups (size 10-12) or gas lens setups can use 12-18 CFH because they produce smoother laminar flow. Too much gas creates turbulence that pulls air into the shielding column, which is worse than slightly too little."

Q: "How do I prevent color on the back of stainless steel TIG welds?"

"You need a back purge. Flow argon behind the joint to displace atmospheric oxygen before and during welding. Use purge dams (aluminum tape or silicone plugs) to contain the gas in pipe joints. For open joints, use a backing plate with a gas channel. A light straw color is acceptable on the back side. Blue, purple, or black means the purge failed and the chromium oxide layer is compromised."

Published June 15, 2025 · Updated March 17, 2026 · 7 min read

🛑 Safety Warning

TIG welding produces intense UV radiation. Always use shade 10-13 lens. Ensure adequate ventilation, especially when welding stainless steel or chromoly.

Quick Diagnosis

Symptoms, causes, and fixes at a glance
Symptom	Likely Cause	Likelihood	Fix
Tungsten contamination (grey/black deposits on tungsten)	Dipping tungsten into the weld puddle	high	Maintain 1/8" gap between tungsten and puddle. Rest your hand or use a finger guide for stability. Practice on scrap to build muscle memory.
	Filler rod touching the tungsten	high	Keep filler rod at a low angle (15-20 degrees) and dab into the leading edge of the puddle, not the arc column. Pull the rod back out of the heat zone between dabs.
	Tungsten too small for the amperage	medium	Use the correct tungsten diameter for your amp range. 3/32" handles 60-120A, 1/8" handles 100-200A. Undersized tungsten overheats and balls, then transfers material to the weld.
Porosity	Insufficient argon coverage	high	Set flow to 15-25 CFH. Keep the torch cup within 1/4" to 3/8" of the workpiece. Hold the torch in position for 3-5 seconds after releasing the pedal to post-flow shield the cooling puddle.
	Contaminated base metal	high	Clean with stainless steel brush and acetone. Remove all oxide, oil, and mill scale. For aluminum, use a dedicated stainless brush (never one used on steel).
	Contaminated filler rod	medium	Wipe filler rods with acetone before use. Store rods in a sealed tube. Fingerprint oils are enough to cause porosity in TIG welds.
	Draft disrupting gas shield	medium	Block air movement with screens. Even HVAC airflow can disturb TIG gas coverage. A gas lens cup significantly improves shielding in light drafts.
Lack of fusion / cold lap	Amperage too low	high	Increase amperage until the puddle forms within 1-2 seconds. If it takes longer, you don't have enough heat to fuse both pieces.
	Travel speed too fast	high	Slow down and watch the puddle. You should see the leading edge melting into the base metal. If the puddle is just sitting on top, you're moving too fast or running too cold.
	Wrong torch angle	medium	Keep torch at 15-20 degrees from vertical, tilted in the direction of travel. Too steep an angle concentrates heat on the puddle surface instead of the base metal.
	Adding too much filler too fast	medium	Dab smaller amounts of filler, less often. Flooding the puddle with filler cools it and prevents proper fusion to the base metal edges.
Excessive oxidation (rainbow/purple/black discoloration)	Insufficient post-flow time	high	Set post-flow to 1 second per 10 amps (minimum 5 seconds). The weld must be below 400F before the gas stops, or atmospheric oxygen attacks the hot metal.
	No back purge on stainless steel or titanium	high	Purge the backside of stainless and titanium joints with argon. Use solar flux paste, purge dams, or a sealed backing system. A light straw color on the back is acceptable. Purple or black means failed purge.
	Gas flow too low or torch held too far from work	medium	Increase flow to 20-25 CFH. Bring the cup closer to the workpiece. Consider using a larger cup (size 8 or 10) for better coverage area.
	Contaminated argon (moisture in cylinder or line)	low	Crack the cylinder valve briefly before connecting. If you get consistent oxidation with proper flow, swap to a fresh cylinder.
Tungsten spitting / popping	Tungsten contaminated (must be re-ground)	high	Break off the contaminated end and re-grind to a clean point. Grind lengthwise along the tungsten, not across it. If the contamination has traveled up the rod, cut back farther.
	Wrong polarity for tungsten type	medium	Use DCEN (DC electrode negative) for steel, stainless, and chromoly. Use AC for aluminum. Running pure tungsten on DCEN causes spitting. Use 2% thoriated or ceriated tungsten on DC.
	Amperage too high for tungsten diameter	medium	Reduce amperage or switch to a larger tungsten. A 3/32" 2% thoriated tungsten maxes out around 120-130A. Beyond that, it overheats and ejects material.
Wandering arc	Contaminated tungsten tip	high	Re-grind the tungsten to a clean, sharp point. Even small amounts of filler metal or base metal bonded to the tip will deflect the arc.
	Magnetic arc blow	medium	Reposition the work clamp closer to the joint. On thick or magnetized steel, demagnetize with an AC arc pass or change clamp position to shift the magnetic field.
	Incorrect tungsten grind (off-center point)	medium	Re-grind with the tungsten running parallel to the grinding wheel face. An off-center point creates a lopsided arc cone that wanders to one side.
Difficulty starting arc	High frequency start not functioning	high	Check that HF start is enabled in machine settings. Listen for the high-frequency snap when you press the pedal. If absent, inspect the HF circuit board and spark gap points.
	Work clamp not making good contact	high	Clamp directly to clean, bare metal on the workpiece. A poor ground connection makes arc initiation difficult and inconsistent.
	Tungsten ground to a blunt tip	medium	Sharpen the tungsten to a point. A blunt or balled tip on DC requires more voltage to initiate the arc. Grind to a taper 2 to 2.5 times the tungsten diameter.
	Oxidized or dirty tungsten surface	low	Re-grind the tungsten to expose clean material. Surface oxides from previous overheating increase arc start voltage requirements.
Crater cracking	Arc terminated too abruptly	high	Use the foot pedal or torch switch to gradually reduce amperage at the end of the weld. A smooth taper over 2-3 seconds fills the crater and prevents shrinkage cracking.
	No crater fill programmed	high	Enable crater fill (downslope) on your machine. Set it to reduce amperage over 3-5 seconds at the end of the weld. Add a small amount of filler as amperage decreases.
	Hot-crack-susceptible alloy (certain stainless and nickel alloys)	medium	For crack-sensitive alloys, use a longer downslope and slightly overfill the crater. On critical joints, back-step the arc over the filled crater before breaking.
Undercutting	Amperage too high	high	Reduce amperage until the puddle is manageable. If you need higher amps for penetration, increase travel speed to prevent the edges from melting away.
	Travel speed too fast with insufficient filler	high	Add more filler metal at the toes of the weld to fill the undercut zones. Slow down or dab filler more frequently.
	Torch angle directing arc at one toe	medium	Keep the torch centered on the joint. Fillet welds are especially prone to undercut on the vertical member. Adjust angle to distribute heat evenly.
Dirty/sooty weld appearance	Contaminated base metal (oil, marker, oxide)	high	Clean all contaminants before welding. Even Sharpie marks leave carbon deposits. Use acetone and a stainless brush.
	Filler rod oxidizing in the heat zone	medium	Keep the hot end of the filler rod within the gas shield after dabbing. Don't pull it so far away that it exits the argon envelope and oxidizes.
	Wrong filler alloy for base metal	low	Verify filler matches base metal. ER70S-2 for mild steel, ER308L for 304 stainless, ER4043 or ER5356 for aluminum. Mismatched filler creates discolored, weak joints.
Inconsistent bead width	Unsteady hand or body position	high	Brace your wrist or forearm on the workpiece or table. Find a comfortable position before striking the arc. Awkward positions kill consistency.
	Inconsistent travel speed	high	Focus on moving at a constant pace. Count in your head or use a metronome rhythm. The puddle width tells you your speed: constant puddle width means constant travel speed.
	Fluctuating amperage (foot pedal issues)	medium	Check the foot pedal potentiometer for wear. Clean the pedal pivot and slider contacts. If the pedal is intermittent, replace it. Alternatively, switch to torch-mounted amperage control.
Filler metal contamination (black specks or inclusions)	Dirty filler rod	high	Wipe every rod with acetone and a clean cloth before use. Filler rods stored loose in a shop collect oil, dust, and grinding particles.
	Wrong filler alloy creating intermetallic compounds	medium	Double-check filler compatibility. Using carbon steel filler on stainless creates chrome-depleted zones that appear as dark inclusions and fail in corrosive service.
	Tungsten inclusions from arc contact	medium	If tungsten dips into the puddle, stop. Grind out the contaminated weld area completely and re-weld. Tungsten inclusions are hard, brittle defects that act as crack initiators.

TIG welding demands more from you than any other arc process. The tradeoff is full control over the weld pool and the cleanest results possible. When problems show up, they’re almost always technique-related, not equipment failures.

The symptom table above gives you fast answers. This section covers the deeper causes and the habits that prevent these problems from happening in the first place.

Gas Coverage: The Foundation of Every Good TIG Weld

Gas problems hide behind many TIG defects. Porosity, oxidation, contamination, and sooty welds all trace back to shielding failures. Get your gas coverage right and half the problems on this page disappear.

Argon Flow and Cup Selection

The standard recommendation is 15-25 CFH. The exact number depends on your cup size, whether you’re using a gas lens, and the ambient conditions.

Cup Type	Cup Size	Recommended Flow (CFH)	Best For
Standard alumina	#5 (5/16")	10-15	Tight access, thin material
Standard alumina	#7 (7/16")	15-20	General purpose
Standard alumina	#10 (5/8")	20-25	Wide coverage, thick material
Gas lens	#7-#8	12-18	General purpose, improved shielding
Gas lens	#12-#14	15-22	Maximum coverage, visible puddle
Jumbo gas lens (Furick style)	#16-#20	20-35	Stainless, titanium, long runs

Gas lens vs. standard collet body: A gas lens replaces your standard collet body with a stacked-screen diffuser that turns the argon flow laminar. The result is a wider, more stable gas shield at the same or lower flow rates. If you’re fighting oxidation or porosity, a gas lens is the single best upgrade you can make to your TIG torch.

Post-Flow Settings

Post-flow keeps argon flowing after the arc stops, shielding the cooling tungsten and weld crater from oxidation. The rule of thumb is 1 second per 10 amps, with a minimum of 5 seconds.

At 150 amps, set post-flow to 15 seconds. That sounds like a lot, but stainless steel and titanium oxidize rapidly above 400F. The tungsten itself needs protection too. Skimping on post-flow turns your expensive tungsten purple and degrades its emission properties.

Hold the torch in position during post-flow. Moving the torch away from the weld while gas is still flowing wastes it. Keep the cup over the crater until post-flow completes.

Back Purging Stainless Steel

Any stainless steel joint that’s open on the back side needs a back purge. Without it, the backside of the weld oxidizes heavily. That oxidation destroys the chromium-rich passive layer that gives stainless steel its corrosion resistance.

Pipe and tube purging: Seal both ends of the pipe with purge dams (aluminum tape, silicone discs, or inflatable bladders). Flow argon through a fitting at one end. Vent from the other end through a small hole. Wait until an oxygen analyzer reads below 50 PPM (parts per million) before striking the arc. On critical work, below 20 PPM.

Open joints and flat plate: Use a backing bar with a gas channel, or tape off the backside with aluminum tape leaving just the root gap open, and flood the cavity with argon from a trailing port.

Reading the colors: Straw/light gold on the back is acceptable for most applications. Blue means marginal protection. Purple, dark blue, or black means the purge failed. Grind out and re-weld.

Tungsten Preparation Guide

Tungsten preparation directly controls arc stability, arc start reliability, and bead consistency. A badly ground tungsten causes more problems than most welders realize.

Grinding Technique

Grind the tungsten lengthwise (parallel to the rod axis), not perpendicular. Crosswise grind marks act like tiny channels that steer the arc unpredictably. Lengthwise marks align the arc path along the point.

Use a dedicated grinding wheel for tungsten only. Contamination from grinding other metals transfers to the tungsten and then into your weld. A small diamond wheel or belt grinder kept exclusively for tungsten is worth the investment.

Taper and Tip Geometry

The grind angle controls arc width and penetration:

Taper Length	Arc Shape	Best For
1x diameter (blunt)	Wide, soft	AC aluminum, low amperage
2x diameter (standard)	Focused, moderate	General purpose DC welding
3x diameter (sharp)	Tight, concentrated	Thin material, precision work, tight joints

For DC welding on steel and stainless, a taper of 2 to 2.5 times the tungsten diameter works for most situations. Leave a tiny flat (about 0.010-0.020") on the tip rather than grinding to a needle point. The flat stabilizes the arc and the tip lasts longer.

For AC aluminum welding, the tungsten balls naturally during use. Start with a slight taper and let the arc form a small ball on the end. If the ball grows larger than 1.5 times the tungsten diameter, you’re running too much amperage or your AC balance is shifted too far toward electrode positive (cleaning action).

Tungsten Selection

2% Thoriated (red band): Best all-around DC tungsten. Excellent arc starts, stable arc, long life. Contains low-level radioactive thorium oxide. Grind in ventilated area.
2% Ceriated (grey band): Non-radioactive alternative to thoriated. Good for low-amperage DC work. Starts easily at low amps.
2% Lanthanated (blue band): Versatile. Works on AC and DC. Good replacement for thoriated without the radioactivity concern.
Pure tungsten (green band): AC aluminum only. Balls up cleanly on AC. Poor choice for DC because it’s harder to start and less stable.

Common Torch Angle Problems

Torch angle affects penetration, gas coverage, and bead profile. Small deviations from optimal cause visible defects.

Torch too vertical (less than 5 degrees from perpendicular): The arc blows straight down into the puddle. Gas coverage is good but you lose visibility of the leading puddle edge. Penetration is deep but the bead tends to be narrow.

Torch too angled (more than 30 degrees from perpendicular): Gas coverage deteriorates because the cup tilts away from the trailing weld. The arc spreads, reducing penetration. Undercut becomes likely on the far side of the joint.

Optimal range: 15-20 degrees from perpendicular, tilted in the direction of travel. This gives you clear visibility of the puddle, good gas coverage over both the puddle and the solidifying bead behind it, and balanced penetration.

Filler Rod Angle

Keep the filler rod at a low angle, 15-20 degrees from the workpiece surface. Dab into the leading edge of the puddle, not into the arc itself. Pull the rod back after each dab, but keep the hot end inside the gas envelope. If the rod exits the gas shield and oxidizes, that oxide transfers into the next dab.

On horizontal and flat joints, feed filler from the side opposite your torch hand. On vertical-up, feed from above. The goal is always to add filler to the front edge of the puddle without interfering with the arc column.

Dealing with Specific Materials

Aluminum on AC

Aluminum TIG on AC introduces its own set of problems. The AC cleaning action strips the aluminum oxide layer during the electrode-positive half-cycle, but it also heats the tungsten more than DC does.

Common aluminum-specific issues:

Tungsten balling too large: Reduce AC balance toward electrode negative (more penetration, less cleaning)
Black smut on the weld: Increase cleaning action (more electrode positive) or clean the base metal more aggressively
Puddle hard to control: Aluminum’s high thermal conductivity pulls heat away fast. Preheat thick sections to 200-300F. Use a bigger cup and higher flow for better coverage on the wide heat-affected zone.

Stainless Steel

Stainless is unforgiving about heat input. Too much heat causes carbide precipitation in the 800-1500F range (sensitization), which destroys corrosion resistance. Too little heat causes cold lap.

Best practices for stainless TIG:

Use the lowest amperage that gives good fusion
Travel fast enough to keep the heat-affected zone narrow
Back purge every joint (see above)
Use ER308L filler for 304, ER316L for 316, ER309L for dissimilar joints to mild steel

Chromoly (4130)

Chromoly is air-hardenable. The heat-affected zone gets hard and brittle after welding if the part cools too fast. On tube thicknesses above 0.060", preheat to 300-400F and wrap the part in welding blankets to slow the cooling rate. Use ER70S-2 filler for non-critical applications or ER80S-D2 for strength-matched joints.

Diagnostic Flowchart

When a TIG weld doesn’t look right, work through this sequence:

Is the weld discolored? Gas coverage problem. Check flow rate, post-flow, back purge, and cup size.
Is the arc unstable? Tungsten problem. Re-grind, check diameter vs. amperage, verify polarity.
Is penetration inconsistent? Technique problem. Check amperage, travel speed, torch angle, and joint cleanliness.
Are there pinholes? Contamination. Clean base metal, clean filler rod, check gas purity.
Is the bead uneven? Body position. Brace yourself, get comfortable, practice on scrap.

Start at the top and work down. Fix the most likely cause first before chasing rare problems.

Frequently Asked Questions

Why does my tungsten keep turning black?

Black tungsten means contamination, almost always from dipping the electrode into the molten puddle or from filler rod touching the tungsten. Break off the contaminated section, re-grind to a clean point, and focus on maintaining a consistent 1/8-inch gap between the tungsten and the puddle. If the blackening happens without contact, check your post-flow gas time. Insufficient post-flow lets atmospheric oxygen attack the hot tungsten.

What is the correct argon flow rate for TIG welding?

15-25 CFH covers most TIG welding applications. For a standard size 7 cup, 15-20 CFH works well. Larger cups (size 10-12) or gas lens setups can use 12-18 CFH because they produce smoother laminar flow. Too much gas creates turbulence that pulls air into the shielding column, which is worse than slightly too little.

How do I prevent color on the back of stainless steel TIG welds?

You need a back purge. Flow argon behind the joint to displace atmospheric oxygen before and during welding. Use purge dams (aluminum tape or silicone plugs) to contain the gas in pipe joints. For open joints, use a backing plate with a gas channel. A light straw color is acceptable on the back side. Blue, purple, or black means the purge failed and the chromium oxide layer is compromised.

Quick Diagnosis

Gas Coverage: The Foundation of Every Good TIG Weld

Argon Flow and Cup Selection

Post-Flow Settings

Back Purging Stainless Steel

Tungsten Preparation Guide

Grinding Technique

Taper and Tip Geometry

Tungsten Selection

Common Torch Angle Problems

Filler Rod Angle

Dealing with Specific Materials

Aluminum on AC

Stainless Steel

Chromoly (4130)

Diagnostic Flowchart

Frequently Asked Questions

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