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A TIG-welded fillet joint fails by undercut when the arc melts into the edge of the base metal faster than filler can replace it. The result is a groove beside the weld toe, reducing the joint’s effective thickness and creating a stress riser. Avoiding it is less about turning up the machine and more about controlling arc length, torch angle, travel speed, and filler placement.

Prepare the Joint Before Striking an Arc

Clean both pieces to bright metal. Remove mill scale, paint, oil, rust, and cutting residue with a flap disc or stainless-steel wire brush reserved for the material. TIG is especially sensitive to contamination; even a small amount of oil can cause a wandering arc and make the puddle difficult to control.

Fit-up matters just as much. For a mild-steel inside fillet, aim for a tight, even joint with no more than about 1/32 inch of gap unless the design calls for one. A large gap forces you to add more filler, increases heat input, and makes it easier to wash the toes away. Tack the joint at intervals of roughly 1 to 2 inches, alternating sides when practical, so the parts do not pull out of alignment.

For thin sheet, use a backing bar or a temporary copper heat sink if the joint allows it. A backing bar can pull heat away from the root, but it may also make the puddle freeze too quickly. Test on scrap before welding the actual part.

Choose Settings and Consumables

Use the lowest amperage that produces a stable puddle and allows the filler to flow into both legs of the fillet. As a starting point for mild steel, estimate about 1 amp per 0.001 inch of material thickness, then adjust for joint design, fit-up, and heat sinking. A 1/8-inch joint might begin around 110 to 140 amps, while 16-gauge sheet may need only 45 to 75 amps. These are starting ranges, not fixed rules.

A gas lens and a larger ceramic cup can improve coverage, especially in corners. A standard collet body is cheaper and works fine for ordinary steel work, but a gas lens is worth considering when drafts, stainless steel, or awkward access make shielding difficult. You can compare TIG gas lens kits by cup size and tungsten compatibility.

Use filler that matches the base metal. For many mild-steel fillets, ER70S-2 or ER70S-6 in 1/16-inch diameter is practical. Use 3/32-inch filler for thicker joints or larger welds. Oversized filler chills the puddle and encourages you to linger with the arc, while undersized filler may not fill the joint quickly enough.

Control the Puddle at the Toes

Set the torch so the tungsten points into the corner, with the electrode about 1/16 to 1/8 inch above the work. A long arc spreads heat across the surface and makes the weld toe easy to undercut. Touching the tungsten contaminates it and can make the arc unstable, so stop and regrind if that happens.

Keep the torch roughly 45 degrees between the two workpieces, then tilt it 10 to 15 degrees in the direction of travel. This angle lets the arc reach the root without driving the puddle ahead of the tungsten. Too much work angle points the arc at one wall and can melt that edge away before filler reaches it.

Establish a small, controlled puddle at the root. Add filler at the leading edge of the puddle, not directly under the tungsten. Dip the rod every time the puddle reaches the size you want, withdrawing it just far enough to stay inside the shielding gas. Do not dab from outside the gas coverage; the end of the rod can oxidize and introduce contamination.

Watch both toes. A healthy fillet has smooth transitions into the base metal, with neither edge appearing to melt backward into a sharp groove. If one toe starts to recede, move the arc slightly toward the center, add a small amount of filler, and continue moving. Do not pause at the toe to “fix” it unless you are deliberately adding a short corrective pass.

Use Travel Speed to Prevent Undercut

Travel speed is the most common cause of undercut on a TIG fillet. Moving too quickly leaves the edges melted but the center underfilled. Moving too slowly can produce excessive convexity, overheating, distortion, and burn-through. For a 1/8-inch mild-steel fillet, a starting travel speed around 3 to 6 inches per minute is reasonable, but puddle size is a better guide than a stopwatch.

Use a steady speed and a small torch movement. A slight side-to-side weave can wash the puddle into both toes, but keep the weave narrow—usually no more than 1/8 inch. Wide weaving increases heat input and makes it harder to keep the arc centered. For thin material, a straight travel with regular filler dips is usually easier to control.

If the joint overheats, use pulsed TIG if your machine has it. A starting setup might be 1 to 2 pulses per second, with peak current near your normal welding amperage and background current around 30 to 50 percent. Pulse is not a substitute for technique, but it can give the puddle time to stiffen and reduce heat buildup.

Check the Weld and Correct Problems

After the weld cools, clean it and inspect both toes under good light. Undercut appears as a continuous or intermittent groove alongside the weld. Porosity looks like small holes or pinpricks; it commonly comes from inadequate gas coverage, leaks, drafts, dirty material, or an excessive arc length.

If the weld is undercut, do not simply turn up the amperage. Lower the current slightly, shorten the arc, slow down enough to keep the puddle full, and feed filler more consistently. Check for drafts around the work area and set gas flow in the recommended range. Excessive flow can create turbulence and be just as harmful as too little.

For a shallow, localized groove, clean the area and make a small repair pass using the same short arc and modest filler additions. A deep or widespread undercut may require grinding back to sound metal before rewelding. When buying equipment for this work, a TIG welder with a foot pedal gives better amperage control than a basic fixed-current machine, but a cheaper unit is perfectly adequate for occasional mild-steel fillets if its arc is stable and its duty cycle suits the job.

Finally, protect yourself with a properly rated helmet, TIG gloves, fire-resistant clothing, and ventilation. TIG produces less visible smoke than many processes, but stainless steel coatings, solvents, and contaminated metal can still create hazardous fumes.