Welding Thin Sheet Metal

Thin sheet metal punishes slow travel speed and excess heat. The difference between a solid weld and a hole is about half a second of dwell time on 20-gauge material. Success requires lower amperage, faster movement, and techniques that spread heat across the joint instead of dumping it all in one spot.

TIG (GTAW) is the precision process for sheet metal. Use the lowest amperage that maintains the puddle, typically 30-60 amps for 20-gauge mild steel. A foot pedal is essential for tapering heat at the end of beads and around corners where heat builds up. Run 1/16 inch filler rod or thinner, adding just enough to fill without building up a tall bead. On material under 24 gauge, autogenous (no filler) fusion welds work if the edges are fitted tight.

Pulse MIG handles sheet metal faster than TIG in production settings. The pulse cycle alternates between a high peak current (that transfers the droplet) and a low background current (that keeps the arc alive without adding heat). This drops average heat input by 20-30% compared to constant-voltage MIG. Set the peak and background parameters per the machine’s synergic programs, or start with a peak 30-40% above your normal voltage and a background 30-40% below.

Skip welding (stitch welding) is the most effective burn-through prevention technique on any process. Weld a 1-inch bead, skip 2-3 inches, weld another bead, and fill the gaps after the piece cools. This distributes heat evenly and prevents the cumulative buildup that blows holes.

Copper backing bars absorb heat from the root side and support the puddle on butt joints. Clamp a flat copper bar behind the joint, and the copper pulls heat away fast enough to prevent melt-through on material as thin as 26 gauge. The weld won’t stick to copper.

Fit-up on sheet metal is critical. Gaps that are acceptable on plate work will blow right through on 20-gauge. Tack every 1-2 inches with minimal heat, then weld between the tacks.