Arc welding (MIG/MAG, TIG, stick)

Fusion: an electric arc melts the edges and usually a filler

Arc welding joins metal parts by melting their edges with an electric arc, so they flow together and freeze as one piece. It is the most common way to join steel, from bike frames and trailers to bridges and ships. The other fusion welding methods in this chapter refine the same idea: melt, mix, refreeze.

Photo: William M. Plate Jr., Public domain, via Wikimedia Commons

Arc welding · MIG/MAG side view with TIG and stick insetsCircled numbers match the steps below. The two boxes on the right show how TIG and stick welding change steps 4 and 5.

Arc welding (MIG/MAG, TIG, stick) in a minute, by MecMade

How it works

Close the circuit. A power source sends current through a cable to the torch, and a ground clamp on the workpiece carries it back. Typical settings are tens to a few hundred amps at only about 10–35 volts.
Strike the arc. When the wire touches the metal and pulls back, an electric arc (a continuous spark through hot, electrically conducting gas) forms. It reaches several thousand degrees Celsius.
Melt a pool. The arc melts the edges of both parts into one small puddle of liquid metal, the weld pool. For comparison, steel melts at about 1,500 °C.
Feed the filler. In MIG/MAG a motor pushes a thin wire through the torch nonstop. Its tip melts in the arc and drips into the pool, adding metal to fill the joint.
Shield the pool. Gas flowing from the nozzle pushes the air away. Without it, oxygen and nitrogen would cause porosity (trapped gas bubbles) and brittle welds.
Travel and freeze. The welder or a robot moves the torch along the joint. Behind it, the pool freezes into a bead (the raised ridge of weld metal) that is one piece with both parts.

Three variants, one principle. MIG/MAG (metal inert gas / metal active gas) feeds wire automatically under argon or an argon–CO2 mix. TIG (tungsten inert gas) uses a tungsten electrode that does not melt, while the welder dips a separate filler rod into the pool. Stick welding uses a rod coated with flux (a chemical powder) that burns to make its own shielding gas and a crust called slag.

Key facts

Materials: Carbon and stainless steels (all three variants); aluminum, titanium, copper and nickel alloys mostly with TIG or MIG
Typical parts: Steel frames, car chassis, pipes, tanks, ship hulls, bridges, bike frames
Part size: From sheet about 0.5–1 mm thick (TIG) to plates over 100 mm (many passes); assemblies of any size
Precision: About ±1 mm on a welded assembly; heat distortion is the main limit, so precise faces are machined afterward
Surface finish: Raised, rippled bead; spatter (MIG) or slag (stick) to clean off; ground flush when looks matter
Batch size it suits: One-offs and repairs up to mass production with robots
Tooling and machine cost: A few hundred to a few thousand dollars for a hand set; roughly $100,000 or more for a robot cell
Speed: Typically 0.1–0.5 m of weld per minute by hand; TIG is the slowest, robots the fastest

Strengths and limits

Strengths

Joins almost any steel, and most other metals with TIG
Cheap, portable equipment; stick works outdoors and in wind
A good weld can be as strong as the parts it joins
Works at any scale, from bike frames to ships

Limits

Heat warps thin parts and changes the metal next to the weld (the heat-affected zone)
Quality depends on the welder's skill or on a well-tuned robot
Fumes, intense UV light and spatter demand ventilation and protective gear
Hidden flaws (porosity, cracks, lack of fusion) need inspection on critical parts

Where it's used

ConstructionSteel beams, columns, bridge girders
AutomotiveChassis, suspension arms, exhaust systems (robotic MIG)
EnergyPipelines, pressure vessels, wind-tower sections
ShipbuildingHull plates, decks and frames
Aerospace and food equipmentThin stainless and aluminum parts (TIG)
Repair and farmingField repairs of machinery with stick welding

How to recognize it

A raised bead along the joint with fine half-moon ripples; neat TIG beads look like a row of stacked coins.
Tiny metal balls (spatter) stuck next to MIG welds.
Straw, blue or rainbow heat tint on stainless steel beside the weld.
Rougher beads with leftover glassy slag on stick welds.

Compare with

Resistance spot welding7.2clamps overlapping sheets and welds them at spots with current and pressure; no arc, no filler, a fraction of a second per spot.Laser welding7.4a focused beam makes narrow, deep welds with much less heat and distortion, but the equipment costs far more.Brazing and soldering7.3the parts never melt; a lower-melting filler flows into the gap, so joints are weaker but heat damage is small.

Watch it

TimWelds · How the variants differ, side by side

Miller Welders · The MIG torch, wire feed and weld pool up close

DIY PRO · Tungsten, filler rod and arc in close-up

FANUC America · Robots welding a large part in a factory cell

See real photos

Wikipedia: Arc welding — overview with photos of welders, torches and beads
Wikipedia: Gas metal arc welding — MIG/MAG torches, setups and weld close-ups
Wikipedia: Gas tungsten arc welding — TIG torches, tungsten electrodes and arc photos
Wikimedia Commons photo search

Remember

An electric arc melts both edges, usually with a filler, into one pool that freezes into the joint.
Shielding gas or flux keeps air away from the hot metal; without it, welds turn porous and brittle.
MIG/MAG is fast and easy, TIG is slow and precise, stick is rugged and portable.