What MIG/MAG welding is
MIG welding is an arc welding process in which a continuous solid wire electrode is fed from a welding gun into the joint. An arc struck between the tip of the wire and the workpiece melts both, forming a weld pool; the same wire is therefore the heat source and the filler metal at once. A shielding gas, fed through the gun alongside the wire, protects the molten pool from the surrounding air[S1].
The process was first patented in the USA in 1949 for welding aluminium, with helium shielding the arc. From about 1952 it took hold in the UK — argon for aluminium, CO₂ for carbon steels — and a naming split arrived with it: strictly, MIG (metal inert gas) covers the inert shielding gases such as argon, while CO₂ and argon–CO₂ mixtures are MAG (metal active gas)[S1]. Workshops say “MIG” for all of it, and this sheet does too, but the distinction matters the moment you buy gas.
Why it became the trade’s workhorse is no mystery: against MMA stick welding it offers high deposition rates and high productivity, because the wire feeds continuously instead of stopping at every stub[S1]. TWI puts MIG/MAG at more than half of all weld metal deposited[S1].
The equipment, part by part
FIG. 1 · The MIG/MAG process at the arc
Behind that torch sit four pieces of equipment worth understanding before you touch any of them:
- Power source. MIG usually runs with the wire positively charged from a constant-voltage power source[S1]. Constant voltage is what makes the process self-regulating: the arc length settles where the wire burn-off matches the feed speed.
- Wire feed unit. A motor drive pulls the wire from a reel and pushes it up the torch liner[S1]. Wires may be solid, or cored — a metal sheath around a powdered flux or metal filling[S1].
- Torch. The wire passes through a copper contact tube — the contact tip — which conducts the welding current into it; the gas nozzle around it delivers the shield[S1].
- Gas supply. Cylinder, regulator and flowmeter. Which gas, and at what flow, is a materials decision — see shielding gas below.
If you need equipment for a short-term job, compare welder hire options from National Tool Hire or welding equipment hire options from Mammoth Hire only after confirming the process, current range, supply, duty cycle, torch, gas and lead requirements. That comparison does not replace machine settings or suitability checks.
Manual MIG is often called semi-automatic: the power source controls the wire feed rate and the arc length, while travel speed and wire position stay in the welder’s hands[S1]. Mechanised and fully automatic variants exist where progressively less is under manual control[S1].
The variables you actually set
A MIG set has fewer meaningful knobs than its front panel suggests, and they interlock:
- Wire diameter and wire feed speed set the current. Wire selection usually falls between 0.6 and 1.6 mm, and because burn-off must balance feed, the feed speed you dial in effectively chooses the welding current[S1]. There is no independent “amps” knob in the MMA sense.
- Voltage sets the arc — its length and, with it, the transfer mode you get (next section).
- Inductance tames dip transfer. When the wire short-circuits into the pool the current surges; inductance controls that surge, and setting it well against the feed speed is what keeps spatter down[S1].
- Travel speed, stick-out and torch angle are yours alone — the workmanship variables no machine setting can replace.
The boundary matters more than the list: the numbers themselves — feed speed, voltage, inductance for a given wire, gas and thickness — belong to your machine manufacturer’s settings data and, on qualified work, to the WPS. Modern synergic sets exist precisely to hold those relationships for you[S1]. This desk publishes no parameter tables, and any figure below is TWI’s published illustration, not a setting to copy.
Metal transfer modes: dip, spray, pulsed
How molten metal crosses from the wire tip to the pool largely determines how the process behaves — its sound, its spatter, its positional ability, the thickness it suits. There are three principal modes[S1].
FIG. 2 · Three transfer modes
- Dip (short-circuiting) transfer. At low voltage the molten tip of the wire transfers by dipping into the pool — a rapid make-and-break of short circuits. TWI’s illustration for a 1.2 mm wire runs from roughly 17 V at 100 A to 22 V at 200 A; voltage and inductance must be set carefully against feed speed to keep spatter down[S1]. It is the low-current, thin-sheet and positional mode most beginners meet first.
- Spray transfer. At much higher voltage and current the wire never touches the pool: metal crosses the open arc as a spray of droplets around the wire diameter and smaller — roughly 27 V at 250 A up to 35 V at 400 A on the same 1.2 mm illustration. Below a threshold current the arc forces cannot project droplets, which is what confines true spray to high currents[S1].
- Pulsed transfer. Developed to stabilise an open arc below the spray threshold: current pulses, each with enough force to detach one droplet, avoid both short-circuiting and spatter. Synergic pulsed sets tune the pulse parameters to the wire composition and diameter, and the pulse frequency to the feed speed[S1].
Shielding gas: more than a curtain
The gas does more than fence off the air. It forms the arc plasma itself, stabilises the arc roots on the material surface, and governs how smoothly droplets transfer from wire to pool — which is why changing gas changes penetration and pool behaviour, not just spatter[S1].
The customary pairings TWI lists[S1]:
| Material | Typical gases |
|---|---|
| Steels | CO₂ · argon + 2–5% O₂ · argon + 5–25% CO₂ |
| Non-ferrous (aluminium, copper, nickel alloys) | argon · argon/helium mixtures |
Two behavioural notes carry most of the practical weight. Argon-based mixtures are more tolerant of parameter setting and give less spatter in dip transfer than CO₂ — but they run colder, which raises the risk of lack-of-fusion defects. And CO₂ cannot support the open-arc modes at all: for spray or pulsed transfer you need an argon-based mixture[S1]. How long a cylinder actually lasts at your flow rate is arithmetic, not folklore — the gas cylinder duration calculator walks it with your figures.
A first-welds sequence that respects the boundaries
A beginner’s guide owes you an order of operations, not a parameter sheet. This one plans a first session the way the rest of this desk plans everything — decisions first, arc last:
- Sort the fume route before anything else. All welding fume can cause lung cancer, and the law requires exposure to be controlled no matter how small the job[S2]. Indoors, that usually means extraction at source — HSE notes on-torch extraction can be very effective for MIG[S3]. Work the options through the fume extraction planner and let a competent person make the assessment.
- Clean and prepare the metal. Mill scale, rust, paint and oil ruin fusion and add to the fume; proper cleaning and preparation is on HSE’s own exposure-reduction list[S3].
- Fit up and clamp so the joint cannot move while you learn to watch the pool.
- Choose wire and gas from the manufacturer’s data for your material and thickness — and on qualified work, from the WPS.
- Set the machine from its own tables. Use the synergic program or the manufacturer’s settings chart for that wire–gas–thickness combination. Remember what the knobs mean: feed speed is current, voltage is arc length, inductance is dip smoothness[S1].
- Check the duty-cycle rating for the current you will draw, so the set is not asked to run beyond its nameplate — the duty-cycle planner turns the rating into minutes.
- Practise on scrap of the same material and thickness. Consistent stick-out, steady travel, watch the pool rather than the arc. In dip transfer, a steady crackle is the classic sign the voltage and inductance are working together.
- Keep notes. Wire, gas, program, plate thickness, what the bead looked like. A planning record beats a memory — and the wire runtime estimator will tell you what is left on the spool for next time.
What this sheet will not tell you
No universal parameter recipes: the workable settings for your machine, wire and gas live in your manufacturer’s data and, on coded work, in a qualified WPS — the voltage and current figures quoted above are TWI’s published illustrations for one wire size, cited to show scale, not settings to copy[S1]. No fume verdicts: this page and the planner can order the control questions, but assessment belongs to a competent person under COSHH[S2]. And no illusions about ease: MIG is quick to start and genuinely hard to do well — TWI’s dry phrasing is that a high degree of manipulative skill is demanded of the MIG welder[S1]. The wider hazard picture — eyes, hearing and lungs together — has its own sheet: welding safety fundamentals.