Weld Undercut in TIG Welding: Causes, Code Limits, and How to Fix It

Undercut is one of the most common defects you will encounter in TIG welding, and one of the most misunderstood. Welders argue about what causes it, inspectors flag joints that are within code, and fabricators grind welds that did not need it. This guide covers what undercut actually is, what the codes allow, how to diagnose it by pattern, and what to do when you already have it.

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What Is Weld Undercut?

Weld undercut is a groove melted into the base metal at the weld toe that the weld metal does not fill back in. You can see it as a notch running along the line where the bead meets the surrounding plate. Undercut reduces the base metal cross-section at the exact point where stress concentrates, making it a crack initiation site under fatigue loading. It is not cosmetic.

Undercut vs. Similar Defects

Welders regularly confuse undercut with three other defects. Getting the diagnosis right matters because each one has a different fix.

Underfill is a low bead face where the weld surface sits below the plate surface, but the base metal itself is intact. Nothing has been eroded from the parent material. Overlap is weld metal that rolls over the plate surface without fusing, creating a mechanical notch from excess material rather than missing material. Concavity is a fillet weld whose face curves inward, producing a throat smaller than specified.

Run your fingernail along the weld toe. If your nail drops into a groove cut below the original plate surface, that is undercut. If the plate surface is intact and the bead simply sits low, that is underfill. If your nail catches on weld metal rolled over the plate, that is overlap.

Toe Undercut vs. Root Undercut

Toe undercut forms at the visible weld toe and is detectable during visual inspection. Root undercut forms on the interior of a groove weld and requires radiography, ultrasonic testing, or back-gouging to find. Root undercut is particularly serious in pressure piping and structural members because it sits at the point of maximum stress in a tension-loaded joint.

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Code Acceptance Limits

Some undercut is permissible. The codes set dimensional limits based on loading type, material thickness, and service conditions. Knowing these numbers prevents you from scrapping acceptable work or passing work that should be rejected.

ASME Section IX covers procedure and performance qualification, not dimensional acceptance. The dimensional limits for pressure vessels appear in ASME Section VIII Division 1, and for process piping in ASME B31.3. Always reference the current edition of the governing document on your project. Code limits do get revised.

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What Causes Undercut in TIG Welding

Undercut always traces to the same root problem: the arc is removing base metal at the toe faster than filler metal replaces it. TIG welding gives you independent control over amperage, travel speed, arc length, and filler addition, which means more variables to cause undercut and more precise tools to eliminate it.

Amperage and Heat Input

Too much amperage drives aggressive melting at the toe while the puddle struggles to keep up. Heat input is a function of amperage, voltage, and travel speed together, so you can run higher amperage without undercut if you increase travel speed and keep arc length consistent. A tungsten that wanders or starts erratically creates local hot spots that preferentially erode the toe.

2% lanthanated tungsten or an E3 tungsten provides a stable, consistent arc across the amperage range. CK Worldwide's LaYZr tungsten delivers the same arc stability on both AC and DC applications, keeping heat distribution consistent at the toe and reducing the intermittent undercut that follows an erratic arc.

Filler Timing and Addition Rate

This is the most TIG-specific cause of undercut and the one that gets the least attention. If you dip too infrequently or fail to direct the rod toward the toe, the puddle fills at the center while the arc continues eroding the edges. The result is undercut along both toes even when amperage and travel speed look correct. Consciously feed filler toward the toe on each dip. Slow your travel until you can see the molten metal wash fully into both toes before you advance.

Travel Speed

Travel speed that is too high is among the most common causes. When you move faster than the puddle can follow and fill, the arc drags a depression behind it and the bead solidifies before molten metal wets into the toe. Slow down enough to watch the puddle trailing edge catch up with the arc before you move again. If you can see the toe is not filling behind the arc, reduce travel speed before touching your amperage.

Torch and Electrode Angle

The arc angle determines where heat concentrates. On a fillet weld, a work angle of 45 degrees bisecting the joint and a travel angle of 10 to 15 degrees from perpendicular gives balanced heat across both toes. Directing the arc too far toward one member focuses heat at that toe and creates one-sided undercut.

Holding a consistent angle across a full weld length is what prevents intermittent undercut that appears and disappears along the bead. The CK Flex-Loc torch lets you set the head angle to match the joint geometry rather than forcing an awkward wrist position you cannot hold steady. For high-amperage work, water-cooled torches like the CK 20 and CK 18 prevent heat fatigue in your torch hand, which directly affects angle consistency on long welds.

Arc Length and Gas Coverage

A long arc length spreads heat over a wider area at the surface, sending more of it toward the toe rather than the puddle center. Keep arc length roughly equal to your tungsten diameter. Erratic arc behavior from turbulent or insufficient shielding gas creates localized overheating at unpredictable points along the toe.

A gas lens collet body converts shielding gas flow from turbulent to laminar, keeping the gas column coherent at the extended stickout you need to watch the toe clearly on tight joints. CK Worldwide's Gas Saver Kit includes the gas lens collet body along with a valve that eliminates the initial gas surge at arc start. For most steel and stainless TIG work, 15 to 20 CFH of argon provides adequate coverage. Dropping below that in any air movement increases arc instability and undercut risk directly.

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Undercut on Aluminum

Aluminum TIG undercut deserves separate treatment because the material behaves differently and the margin for error is narrower.

Aluminum has approximately four times the thermal conductivity of steel, so heat dissipates rapidly and you need significantly higher amperage to maintain a puddle. That necessary high heat input also means the range between enough fusion and undercut is tighter than on steel. On steel, color cues tell you how hot the base metal is getting. Aluminum gives you no such warning. The base metal looks the same at room temperature and at 900 degrees Fahrenheit. The puddle appears suddenly and the transition to undercut or burn-through happens in seconds.

The AC cleaning action that breaks down the oxide layer also means you cannot simply reduce amperage to prevent undercut without risking oxide inclusions. Optimize travel speed and filler addition rate to maintain adequate puddle temperature without dwelling in one spot.

Filler selection affects toe fill on aluminum. 4043 flows more easily and fills toes with less mechanical pressure on each dip. 5356 is stiffer and requires a more deliberate dip angle to direct metal to the toes. If you are seeing consistent undercut with 5356 on compatible base alloys, 4043 often reduces it without any parameter changes.

Undercut on aluminum is harder to see than on steel. Use a bright light at a low angle to cast shadows into the toe area and confirm with a weld gauge. The Gas Saver Kit with a gas lens is particularly valuable on aluminum because AC arc instability and undercut risk reinforce each other: stabilizing the gas column improves arc consistency and cleaning action simultaneously.

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Undercut by Position

Flat and Horizontal

In the flat position, gravity works in your favor and undercut is almost always a parameter problem: amperage too high, travel speed too fast, filler added too infrequently. Horizontal fillet welds introduce the first complication. The puddle sags toward the lower toe, overfilling it while the upper toe goes unfilled. Direct your arc slightly toward the upper member and make sure each filler dip reaches the upper toe explicitly.

Vertical and Overhead

Position-specific undercut is common and gets almost no attention in standard guides.

In vertical-up welding, the puddle sags downward during a weave. Without a deliberate pause at each toe, the molten metal runs to the bead center before solidifying and leaves the toes unfilled. Reduce amperage 10 to 15 percent compared to your flat position setting and pause long enough at each toe to see the metal wet in before moving. A foot pedal lets you modulate heat in real time as the joint preheats and the puddle becomes increasingly fluid.

Vertical-down TIG is not recommended on material thicker than thin-gauge sheet. The arc leads the puddle and toes cannot fill at any meaningful travel speed. Use vertical-up on thicker material.

In overhead welding, the puddle sags away from the toes toward the bead center. Keep your arc length as short as possible and stay on the lower end of your amperage range. Torch access angle matters as much as parameters in the overhead position.

The CK 17 and CK 26 air-cooled torches are popular for positional work because their lighter weight reduces fatigue over long welds in awkward positions. For heavy-amperage overhead work, the water-cooled CK 18 keeps the torch body cool at 200 amps and above, where an air-cooled torch becomes uncomfortable to hold at the required angles.

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Diagnosing Undercut by Pattern

The location and distribution of undercut tells you more about the cause than anything else.

Undercut on one side only points to torch angle. The arc is directed toward one member. Check your work angle first, and if access is the problem rather than technique, the CK Flex-Loc torch resolves confined-access angle problems without compromising your grip.

Undercut on both sides symmetrically points to globally high heat input, travel speed too fast, or filler addition rate too low. Reduce travel speed first. If it persists, add filler more aggressively toward both toes. If it still persists, reduce amperage.

Undercut only in vertical, not in flat points to gravity effects. Your flat position parameters are fine. Reduce amperage 10 to 15 percent, slow travel, and add pause time at the toes during weave passes.

Intermittent undercut along the weld length points to arc instability. Check for gas leaks, verify flow rate is at 15 to 20 CFH, switch to a gas lens collet body, and inspect your tungsten for contamination or improper preparation.

Undercut that gets worse as the weld progresses points to heat buildup. Effective heat input is increasing as the joint preheats even though your settings have not changed. Pause between passes to let the joint cool, or use foot pedal control to reduce amperage as the joint warms up.

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How to Detect and Measure Undercut

Visual inspection is the first step. Use a strong light at a low angle to cast shadows into any groove at the toe. A weld fillet gauge or dedicated undercut gauge with a pointed depth probe lets you measure depth directly and record it against code limits. AWS D1.1 inspection requires measured depth, not an estimate. Bridge cam gauges measure fillet leg length, throat, and undercut depth in a single tool and are standard equipment for structural inspection.

For joints where visual access is limited or consequences of missed defects are severe, dye penetrant testing (PT) reveals surface-breaking undercut and any cracks initiating from it. Radiographic testing (RT) and ultrasonic testing (UT) are required to detect root undercut on groove welds where interior access is not possible.

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Repair Options When Undercut Exists

When undercut exceeds the applicable code limit, you have two options: blend or fill.

Blending by grinding is appropriate when the undercut is shallow and widening the groove into a smooth transition will bring the remaining depth within the code limit. Grind parallel to the weld axis, not across it, and verify with a gauge that the remaining profile meets the limit. Do not remove more base metal than necessary.

Filling by welding is required when the undercut is deep enough that grinding cannot bring it within limits without removing excessive base metal. Clean the area thoroughly, set your amperage on the lower end of your range for the material thickness, and run a tight repair bead directly into the undercut groove. On aluminum, re-clean the oxide from the groove with a stainless steel brush before welding. Inspect the repair visually and with a gauge before signing off.

Address the cause before repairing. If you fill undercut without fixing the parameter or technique issue that created it, you will recreate the same defect on the repair pass.

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For consistent arc stability, precise shielding gas coverage, and torch control across positions and materials, CK Worldwide's Gas Saver Kit, LaYZr tungsten, and Flex-Loc torch address the most common mechanical contributors to undercut directly. The right equipment does not replace technique, but it removes the variables that make good technique harder to execute consistently.