Blog

As an Amazon Associate I earn from qualifying purchases. This post may contain affiliate links at no extra cost to you.

A TIG-welded lap joint joins two overlapping pieces of metal by melting the edge of the top piece and a small amount of the lower piece, then adding filler metal as needed. It is useful for sheet-metal repairs, brackets, boxes, tanks, and thin tubing. The joint is straightforward, but poor fit-up can trap contaminants and make it easy to burn through the top sheet.

What You Need

You need a TIG welder with high-frequency start or lift-arc start, a torch with a suitable tungsten, shielding gas, filler rod, and clean work clamp. For most mild-steel lap joints, use 100% argon at roughly 15 to 20 cubic feet per hour (CFH). A gas lens can improve coverage, especially when welding at low amperage or in a drafty shop.

Wear a properly rated auto-darkening TIG welding helmet, fire-resistant clothing, leather gloves, and closed-toe boots. TIG produces less spatter than MIG or stick welding, but it still creates ultraviolet radiation, hot metal, fumes, and fire hazards. Remove coatings before welding and provide ventilation.

A basic inverter TIG machine is fine for steel and stainless work. AC capability is required for aluminum because its oxide layer is difficult to remove with DC. If you only weld steel, paying extra for AC/DC may not be worthwhile.

Prepare the Lap Joint

Overlap the pieces by about 1/2 to 1 inch for general sheet-metal work. Larger overlap increases strength only if both pieces are properly fused; it does not compensate for a dirty or badly fitted joint. Keep the sheets flat and in close contact. A gap makes the upper edge easier to melt and increases the chance of burn-through.

Clean both sides of the joint with a degreaser, then remove mill scale, paint, rust, and oxide with a dedicated stainless brush or abrasive pad. Do not use a contaminated brush that has previously been used on steel when preparing stainless or aluminum. Wipe away abrasive dust before welding.

Clamp the pieces so they cannot lift from heat. Tack weld every 1 to 2 inches on long sheet joints, alternating sides or working in short sections to control distortion. For thin material, a copper or aluminum backing bar can absorb heat and support the molten puddle, but it must be tight against the underside.

Choose Tungsten, Polarity, and Settings

Use DC electrode negative (DCEN) for mild steel and stainless steel. A 1.5% or 2% lanthanated tungsten is a practical general-purpose choice. For sheet metal, a 1/16-inch tungsten works well at low amperage; use 3/32 inch when the work is thicker or the machine will run above roughly 120 amps. Grind the tungsten lengthwise to a sharp point, with a taper about two to three times its diameter.

These are starting ranges, not fixed rules. Joint design, fit-up, torch angle, and machine control change the required heat. Begin near the low end and increase gradually. A foot pedal or torch amperage control is especially useful because you can reduce heat at the end of a weld or when the metal starts to glow.

If your machine does not include a gas regulator and flowmeter, choose a suitable TIG argon regulator and flowmeter. Excessive gas flow does not improve shielding. Above about 25 CFH, turbulence can draw air into the shield and cause porosity.

Weld the Lap Joint

Fit a sharp or slightly truncated tungsten in the torch and set the stick-out to about 1/8 inch with a standard cup. Use a larger gas cup and longer stick-out only when access requires it. Set post-flow for approximately 5 to 10 seconds so the tungsten and finished crater remain shielded as they cool.

Hold the torch about 70 to 80 degrees from the work, pointing in the direction of travel. Keep the tungsten 1/16 to 1/8 inch above the surface. Start the arc on the upper sheet near its edge, then move slightly toward the lower sheet until both surfaces form one small molten puddle. Add filler at the front edge of the puddle, not directly onto the tungsten.

Travel steadily along the overlap. A slight side-to-side movement can wash the puddle across both pieces, but avoid wide weaving on thin sheet. The goal is a narrow bead with visible fusion at both edges. Dip filler once per puddle or as needed; keep the rod end inside the argon shield between dips. For a clean appearance, feed short, consistent additions rather than long bursts.

At the end, reduce amperage if using a pedal or downslope control, add a final small amount of filler, and hold the torch over the crater during post-flow. Do not simply snap off the arc. That leaves a crater crack, which can become a leak or fatigue point.

Diagnose Common Problems

Burn-through: Amperage is too high, travel is too slow, the gap is too large, or the sheet has overheated. Lower the current by 5 to 10 amps, move faster, use shorter welds, and let the joint cool. A copper backing bar helps, but it cannot fix a large gap.

No fusion on the lower sheet: You are concentrating the arc on the top sheet or moving too quickly. Direct the puddle toward the lower piece briefly and verify that the sheets are tight together. A bead sitting on top of the joint is not a sound lap weld.

Porosity or a gray, dirty bead: Check for oil, moisture, drafts, a leaking hose, or an empty cylinder. Clean the filler rod and increase coverage only slightly. If the tungsten is contaminated, stop, grind past the damaged section, and restart.

Warping: Use smaller tacks, skip around the joint, reduce amperage, and clamp the work more effectively. For noncritical sheet-metal work, a series of short stitch welds may create less distortion than one continuous bead. The cheaper option—basic DC TIG with a hand torch control—is sufficient for this kind of steel work; spend more on AC/DC, pulse, or advanced controls only when your material and workload justify it.

Inspect the Finished Weld

Clean away discoloration and inspect both ends of the bead. Look for an even profile, smooth transitions, and fusion along both sheet edges. Reject the weld if it contains visible pinholes, cracks, severe undercut, or sections that can be lifted with a pick. For a leak-sensitive joint, test with the appropriate low-pressure method after the weld has cooled completely.