The Arc Itself

Push electricity hard enough and air stops blocking it: the current flows across the gap as a glowing bridge hotter than lava. Hold that bridge over steel and the steel melts into a small shiny pool called the puddle.

Here is what makes welding different from glue. Melted metal from both pieces mixes together in the puddle. When it cools solid there is no seam left inside — it is one piece of metal, the way one ice cube frozen from two drops of water is one cube.

The machine's current tears electrons off atoms in the gap, turning the gas into plasma — gas so hot that its charged pieces conduct electricity. Once the plasma exists, the current flowing through it keeps it alive, and that flowing charge heats it the same way current heats a toaster wire — except concentrated into one spot, near 6,000 °C.

That heat melts a pool — the puddle — of base metal plus melted filler rod. As the arc moves on, the puddle freezes behind it into the bead. Because both pieces melted into the same pool, the finished joint is continuous metal: fusion. A bolt clamps two pieces; glue sticks to their surfaces; a weld makes them one object.

In numbers: a typical stick arc runs around 22 volts while carrying about 110 amps. Power is volts times amps — roughly 2,400 watts — delivered into a spot around a centimeter across. Concentration, not just quantity, is the trick: the same 2,400 watts spread through a room warms it gently; focused on a fingertip of steel, it drives the surface past the 1,510 °C melting point in under a second.

The plasma sustains itself only while current flows: collisions keep the gas ionized, and the ionized gas keeps conducting. Break the circuit — lift the rod too far — and the plasma cools, recombines into ordinary air, and the arc snaps out in a millisecond. Fusion's consequence is structural: metal grains solidify across the old boundary, so a sound weld is as strong as the plate around it — a claim no clamp or adhesive layer can make.

Electricity usually cannot jump through air. A welding machine pushes it so hard that it jumps a tiny gap and keeps glowing there. That steady glow is the arc.

The arc is hotter than anything in a kitchen. It melts a little pool in the metal. When the pool cools, the two pieces have become one piece.

The arc is a self-sustained gas discharge: electron emission at the cathode, impact ionization in the column, and a conducting channel in rough local thermal equilibrium at 6,000 to 20,000 K depending on gas and current. Arc voltage divides across the cathode fall, the column, and the anode fall; most workpiece heating arrives through the fall zones and charge condensation at the surfaces rather than from the luminous column itself.

Melting establishes a fusion zone whose solidification is epitaxial — new grains grow off the existing base-metal grains at the fusion boundary, which is why the joint has metallurgical continuity rather than an interface. Beside it lies the heat-affected zone: base metal that never melted but was thermally cycled, with microstructures of its own. Most engineering questions about weld strength are questions about these two regions.

Treat the column as a plasma in local thermodynamic equilibrium: ionization fraction from the Saha equation, electrical conductivity rising steeply with temperature, and a falling voltage-current characteristic at low current — the negative differential resistance that makes constant-current power sources the natural pairing (folio 5 returns to this). The energy balance partitions among cathode fall, anode fall, column radiation, and convection; workpiece heat input is customarily folded into an arc efficiency η near 0.7 to 0.9 for SMAW.

Downstream, heat input per unit length Q = ηVI/v governs the thermal cycle. Solidification proceeds epitaxially, with columnar grains growing along the maximum thermal gradient and a possible columnar-to-equiaxed transition at the centerline. The metallurgy of folios 11 and 16 — defect diagnosis and bend testing — is this thermal history read backward from the finished bead.