What's inside
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TIG welding an outside corner on sheet metal looks simple because the joint is easy to reach. The challenge is keeping the thin edges aligned while adding enough filler to bridge the corner without melting it away. A steady fit-up, short weld segments, and a small torch angle matter more than high amperage.
Prepare the joint
Cut the parts square and remove burrs. For a typical 18- to 22-gauge steel panel, aim for a tight, even seam; a gap wider than about 1/32 inch makes burn-through more likely. Degrease the edges, then remove paint, rust, mill scale, and coatings for at least 1/2 inch on either side. Use a dedicated stainless or carbon-steel brush as appropriate, and avoid contaminating stainless with tools used on mild steel.
Hold the pieces at 90 degrees with a square and clamps, leaving the outside corner accessible to the torch. Tack at both ends and near the middle, then check that the angle has not pulled out of square. Add more tacks for a long seam—roughly every 1 to 2 inches is a useful starting point. If the panels are very thin, use smaller, closely spaced tacks rather than a few large ones.
Equipment and starting settings
For mild steel, use DC electrode negative (DCEN) and clean filler rod matched to the base metal, often ER70S-2. A 1/16-inch rod suits many thin-sheet jobs; 3/32-inch filler can be easier to handle on thicker sheet but may add too much metal on very light stock. A small tungsten, commonly 1/16 inch, is easier to control at low current than a large one. Grind it to a tapered point with the grind marks running lengthwise.
Start around 30 to 50 amps for thin sheet and adjust for material thickness, joint fit, and machine control. That is only a starting range: foot-pedal control, travel speed, tungsten size, and heat sinking all change what works. Set argon flow around 15 to 20 cubic feet per hour indoors, with the nozzle close enough to shield the puddle. Excessive flow can create turbulence and pull air into the shield. Use a welding helmet with a suitable TIG-rated shade and gloves that let you feed filler accurately.
A basic AC/DC TIG machine is not necessary for mild-steel corners; a DC TIG unit is enough. If buying equipment for occasional sheet-metal repairs, prioritize stable low-amp control and a usable foot pedal over a high maximum amperage rating. A DC TIG welder with a foot pedal gives more control as the panel heats up. For aluminum, you need AC TIG capability, and the setup and cleaning requirements differ.
Weld the corner in short runs
Clean the tungsten if it touches the puddle, then hold the torch about 10 to 15 degrees off vertical, aimed into the corner. Keep the arc short—roughly the diameter of the tungsten tip or less. The corner concentrates heat, so do not linger while waiting for a large puddle to form.
Start at a tack and establish a small puddle on both edges. Add a small dab of filler at the leading edge, move forward, and repeat. Keep the rod tip inside the argon shield; pulling it out hot can oxidize it. On thin panels, use a series of short runs about 1/2 to 1 inch long, alternating locations along the seam and letting the work cool between passes. This reduces warping and keeps the far end from becoming hotter than the start.
Where access permits, copper backing can absorb heat and support the joint, especially at a gap. It is helpful, not mandatory, and it must fit tightly behind the seam. Do not try to compensate for a poor fit by turning up the current or flooding the corner with filler; both tend to produce a bulky, weak-looking bead or a hole.
Common problems and fixes
| Problem | Likely cause | What to change |
|---|---|---|
| Burn-through | Gap, excess heat, or stopping in one spot | Tighten fit-up, lower current, move sooner, or weld shorter sections |
| Bead sits on top | Edges did not fuse, often from low heat or a long arc | Shorten the arc and direct it at both edges before adding filler |
| Black or sooty weld | Contamination, poor gas coverage, or a draft | Clean again, check gas flow and connections, and shield the work from wind |
| Panel pulls out of square | Too much heat concentrated in one direction | Add more tacks, stitch in alternating sections, and clamp the assembly |
If you burn through, stop and let the area cool. Clean the damaged edge and bridge the opening with small, controlled tacks rather than trying to drag a long bead across it. A filler rod can help span a tiny gap, but it cannot restore an edge that has melted away substantially; trim and refit the part if needed.
Finish and inspect
Let the panel cool naturally; quenching can distort it. Check that the corner remains square and look for pinholes, undercut, cracks, and areas where the bead has not tied into both sides. A sound sheet-metal corner weld does not need to be tall. Avoid grinding it flush unless the part requires a smooth finish, because thin sheet is easy to grind through.
For a first practice session, use offcuts of the same thickness and make a few tack-and-stitch joints before welding the finished part. A 1/16-inch ER70S-2 TIG filler rod is a practical choice for many mild-steel sheet repairs. If the work is outdoors, wind can disrupt argon even at modest speeds; use a windbreak rather than simply increasing gas flow. A TIG welding wind screen can help when a sheltered workspace is not available.