What's inside
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A corner joint on aluminum sheet looks simple, but it exposes nearly every weakness in a TIG setup: poor cleaning, unstable heat control, contaminated tungsten, and excessive filler. Aluminum also conducts heat away quickly, then suddenly becomes fluid as the workpiece gets hot. The result can be a cold-looking joint at the start, followed by burn-through or a collapsed corner near the end.
For most thin sheet, use an AC TIG welder with a foot pedal or fingertip amperage control. A machine with adjustable AC frequency and balance is useful, but it is not essential for occasional repairs. Your AC TIG welder with foot pedal should have enough output for the material without running continuously at its limit.
Joint Design and Material
A corner joint joins two sheets at roughly 90 degrees. You can weld the outside corner, the inside corner, or both. An outside corner is usually easier to see and access, while an inside corner traps oxide and makes torch positioning more difficult.
For sheet from about 1/16 to 1/8 inch thick, leave a small, consistent fit-up rather than forcing the edges together. A gap of approximately 1/32 inch can help establish a sound root, but a large gap demands more filler and increases burn-through risk. Clamp the parts securely, then place several small tack welds before running the joint.
| Aluminum sheet thickness | Typical starting amperage | Common tungsten | Filler starting point |
|---|---|---|---|
| 0.040 inch | 35–55 A | 1/16 inch | 0.035–1/16 inch |
| 0.063 inch | 55–85 A | 1/16 inch | 1/16 inch |
| 0.090 inch | 80–120 A | 3/32 inch | 1/16–3/32 inch |
| 0.125 inch | 110–160 A | 3/32 inch | 3/32 inch |
These are starting ranges, not fixed settings. Joint geometry, heat sinking, alloy, and machine characteristics can move the correct amperage substantially. A copper or aluminum backing bar will pull heat away and may require more current.
Prepare the Aluminum
Cleanliness is more important on aluminum than on ordinary mild steel. Remove paint, anodizing, oil, and adhesive at least 1 inch on both sides of the joint. Use a dedicated stainless-steel brush reserved for aluminum, and brush immediately before welding. Do not use a brush that has previously touched steel; embedded iron can contribute to contamination and rust staining.
Wipe the cleaned area with acetone or another suitable degreaser using a clean, lint-free cloth. Let the solvent evaporate completely. Do not weld over wet solvent or use chlorinated cleaners, which can create dangerous fumes when heated.
Deburr the edges, check the angle with a square, and make sure the sheets sit flat against the clamps. Aluminum expands considerably during welding, so a joint that starts square can pull open. Tack from the center outward, alternating sides where possible. If the tacks crack, contain black residue, or leave obvious pinholes, stop and correct the fit-up before welding the full seam.
Torch and Machine Settings
Use AC with high-frequency start. For thin sheet, a 1/16-inch or 3/32-inch lanthanated tungsten works well. Grind it to a clean, tapered point; although AC can ball the end slightly, a large ball makes the arc wander. A gas lens and a #7 or #8 cup can improve shielding, especially at the outside corner.
Set argon flow around 15–20 cubic feet per hour indoors with a normal cup. Excessive flow can create turbulence and draw air into the shielding gas. Keep the torch angle close to 70–80 degrees to the surface and use a short arc, roughly 1/16 inch or less. Shield the hot weld pool until it loses its bright appearance.
Start with AC balance near the machine’s midrange. More electrode-positive cleaning action helps remove oxide but also heats the tungsten and reduces penetration. More electrode-negative time improves penetration and electrode cooling, provided the aluminum is genuinely clean. AC frequency around 80–120 Hz is a practical starting range: higher frequency tightens the arc but can make the arc harsher and increase tungsten heating.
A sharp, consistent arc is easier to control with a TIG gas lens kit. It is not mandatory for a small corner joint, and the cheaper standard collet body is fine when access is open and your gas coverage is already stable.
Welding the Corner
Position the sheet so you can see the leading edge of the puddle, not just the arc. Begin on a tack and slowly add current with the pedal. Aluminum often takes a moment to heat, then the puddle forms suddenly. Do not chase the puddle with the torch. Hold the torch steady and watch both edges melt into a small, even pool.
Once the puddle wets both sides, add filler at the front edge of the pool. Dip the rod decisively, then withdraw it without dragging the rod through the arc. A 1/16-inch filler rod is easier to control than a thick rod on most sheet. Keep the filler inside the argon shield; pulling it far away can leave an oxidized tip that contaminates the weld.
Move steadily and use little or no torch weaving. A corner weld that is too wide adds heat without adding useful strength. For 0.063-inch sheet, a travel speed of roughly 4–8 inches per minute may be a reasonable starting point, but the correct speed is whatever maintains fusion without letting the edges sag.
As the joint gets hotter, reduce pedal pressure or move faster. This is the main advantage of foot control. If the edge begins to round over, stop adding filler, back off current, and let the puddle catch up. End by tapering current down while maintaining shielding gas for several seconds.
Common Failures and Fixes
Black soot or pepper-like deposits: Usually contamination, poor gas coverage, or an unstable arc. Stop, clean the area, trim the tungsten, check for drafts and leaks, and restart.
Pinholes: Look for oil, moisture, porous cast material, excessive gas flow, or a long arc. Grind out the defect before rewelding; simply covering it can trap contamination.
Burn-through: Current is too high, travel is too slow, the gap is too large, or the joint has become heat-soaked. Use a heat sink, reduce amperage, shorten the arc, and avoid repeatedly reheating one spot.
Lack of fusion: The puddle is not reaching both sheet edges. Increase current briefly, improve fit-up, and aim the arc at the joint rather than depositing filler on top of cold metal.
Safety and Useful Gear
Wear a properly rated welding helmet, flame-resistant clothing, gloves, and closed footwear. Aluminum reflects arc radiation strongly, and TIG welding produces intense ultraviolet exposure even at modest amperage. An auto-darkening TIG welding helmet with a clear lens and dependable low-amp performance is worth buying; a basic fixed-shade helmet is cheaper but awkward for precise starts.
Use ventilation that removes fumes without blowing directly across the argon shield. Secure the gas cylinder upright, inspect the regulator and hoses, and keep flammable materials away from the work area. Practice on scrap cut from the same alloy and thickness before welding the finished part. A sound practice corner should have consistent width, smooth tie-in at the tacks, and no visible soot, craters, or unmelted edges.