TIG welding inside corner joints feels like welding blindfolded. The puddle hides in the joint, the arc glare bounces off both plates, and your torch cup blocks whatever's left. It's one of the steepest learning curves in welding, and it has almost nothing to do with amperage or filler technique.
Here's the thing: inside corners are just fillet welds in a tight spot. Per AWS D1.1 structural welding standards, a fillet weld needs complete fusion at the root. That means reading the puddle correctly, even when you can barely see it.
We'll break down the exact torch angle, settings, and visual cues that make TIG welding inside corner work reliable.
Why the Inside Corner Is a Visual Problem
An inside corner is a 90-degree groove between two plates. When you strike the arc, the plates trap the light. Instead of a clear puddle on flat steel, you get a bright glare bouncing off both walls.
Your helmet does its job. The problem is geometry. The puddle sits at the bottom of a V, below your line of sight.
The torch cup blocks the top of your view. And the arc naturally wants to climb up the vertical plate, pulling the weld away from the root.
That's what makes the inside corner a visual problem first, technique second. You're not fighting the weld. You're fighting your eyes.
You have to learn to read the glow, the ripple, and the "wash" of the puddle against the vertical plate. Once you can see those cues, the weld starts to make sense.
Quick Answer: The Recipe for a Clean Inside Corner
Here's the formula that works for most TIG welding inside corner joints:
- Torch angle: 45 degrees or slightly less
- Arc length: tight, about the diameter of your tungsten
- Filler rod: 1/16-inch or 3/32-inch, dabbed at the leading edge
- Travel speed: steady enough to let the puddle wet both plates
Watch the puddle wash evenly into both plates. If it climbs one side, tilt your torch toward the other plate. If it bulges, you're adding filler too fast.
If it looks flat and dull, drop your amperage by 10 to 15 amps.
Getting to Know the Joint: Inside Corner vs. T-Joint vs. Fillet
An inside corner is a fillet weld. Same geometry as a T-joint, but the orientation changes everything. In a T-joint, the vertical plate stands upright, and you can watch the puddle from the side.
In an inside corner, the plates form an L-shape or a box section, and you're reaching into the corner.
The weld itself has two legs and a throat. The legs are the fusion length on each plate. The throat is the deepest point of fusion in the joint.
A good fillet looks slightly concave, like it's settling into the corner. A convex bead means too much filler or a cold puddle. A flat bead that doesn't wet into the vertical plate means cold lap, which is a fusion failure.
| Joint Type | Torch Angle | Visibility | Difficulty |
|---|---|---|---|
| T-joint | 45 degrees | Clear from the side | Easier |
| Inside corner | 45 degrees or less | Blocked by cup and walls | Harder |
| Outside corner | 45 degrees | Clear | Needs less filler |
The key difference: inside corners trap heat. Both plates absorb energy, so the joint heats up faster than flat steel. That changes your amperage and travel speed.
If you're new to the process, it helps to understand the TIG process itself and why it behaves differently from other methods in tight spots.
The Setup That Makes or Breaks the Weld
Before you strike the arc, the joint has to be right. Inside corners hide defects. A gap that looks tiny from the side turns into a blowout when you hit it with the arc.
Clean both plates. Mill scale, oil, and oxide layers pull contaminants straight into the puddle. Use a dedicated stainless steel wire brush for steel, a separate brush for aluminum.
Wipe with acetone if the material is greasy.
Fit-up matters more here than on flat plate. Clamp the pieces tight so heat doesn't pull the corner open. Tack weld both ends of the joint first.
If the plates shift, the root gap changes, and you'll get inconsistent fusion along the weld.
For the torch setup, use a small cup, size 5 or 6, so you can actually see the corner. A gas lens improves shielding gas coverage and lets you bring the cup closer without snagging. Tungsten sharpened to a taper, with a short stick-out of about 1/4 inch.
Long stick-out in a corner means you're fighting the cup for space.
Checklist before you weld:
- Cup size 5 or 6 for visibility
- Gas lens for better coverage in tight corners
- Tungsten: 1/16-inch or 3/32-inch, sharpened to a taper
- Stick-out: 1/4 inch or less
- Dedicated wire brushes for steel vs. aluminum
Choosing the Right Settings for Inside Corners
Inside corners trap heat. Two plates feed energy into the joint, so it heats up faster than flat steel. Manufacturer specifications indicate you'll need slightly less amperage than the same material on a flat plate.
For mild steel:
- 1/16 inch: 50-80 amps
- 1/8 inch: 80-120 amps
- 1/4 inch: 120-180 amps
For stainless steel:
- 1/16 inch: 40-70 amps, keep heat input low
- 1/8 inch: 70-110 amps
For aluminum (AC):
- 1/8 inch: 110-150 amps at 120-150 Hz
- Balance around 65-70 percent EN for cleaning action
The rule of thumb: inside corners need about 10-15 percent less amperage than a flat weld on the same material. If the puddle runs or you get burn-through, step down. If the weld looks cold, step up.
Gas flow: 15-20 CFH of pure argon. Inside corners can deflect the gas, so aim for the middle of that range. As of 2026, most inverter TIG machines have digital gas timers built in, which helps consistency.
| Material | Thickness | Amperage | Gas Flow |
|---|---|---|---|
| Mild steel | 1/16 in | 50-80 A | 15-18 CFH |
| Mild steel | 1/8 in | 80-120 A | 15-18 CFH |
| Stainless | 1/16 in | 40-70 A | 15 CFH |
| Aluminum | 1/8 in | 110-150 A (AC) | 18-20 CFH |
These are starting points, not absolutes. Your machine, your tungsten, and your technique all shift the numbers. Use them as a baseline, then adjust for what the puddle tells you.
The same logic applies whether you're deciding on the right approach for steel or dialing in your aluminum settings. The joint geometry matters as much as the material.
