Gas Metal Arc Welding feeds a continuously supplied solid wire through a gun while shielding gas protects the puddle. It is the most productive process for carbon steel fabrication and the first process most shops adopt.
MIG (Metal Inert Gas, formally GMAW) uses a solid wire electrode fed at a set speed through a gun. The wire carries current from a constant-voltage power source and melts into the joint as the arc travels. Shielding gas — typically 75/25 argon/CO₂ for steel — flows around the arc to protect the puddle from oxygen and nitrogen in the air.
The process has four transfer modes. Short-circuit dips the wire into the puddle dozens of times per second for thin metal and all-position work. Globular transfer is a middle mode best avoided. Spray transfer atomizes the wire into a fine droplet stream — high deposition, flat position only. Pulsed MIG (GMAW-P) delivers spray-like transfer out of position by pulsing the current. Mode choice decides wire size, voltage, gas, and what the weld will look like.
| Thickness | Wire | Volts | Wire Feed (IPM) | Gas (CFH) |
|---|---|---|---|---|
| 24–22 gauge | 0.023" | 15–17 V | 120–160 | 15–20 |
| 1/16" | 0.030" | 16–18 V | 170–220 | 20–25 |
| 1/8" | 0.035" | 18–20 V | 220–280 | 25–30 |
| 3/16" | 0.035" | 19–21 V | 280–340 | 30–35 |
| 1/4" | 0.045" | 21–23 V | 300–380 | 35–40 |
| 3/8" | 0.045" | 23–25 V | 360–420 | 35–45 |
Starting points only — dial in on scrap before production welds. Travel angle 10–15° push or pull depending on material and transfer mode.
A constant-voltage power source drives the arc while a wire feeder pushes solid wire through the gun. The wire is the electrode, the filler, and the heat conductor — all in one continuous consumable.
Constant-voltage power source holds voltage steady; wire feed speed controls the current and deposition rate.
Wire feeder pushes the solid wire through the liner at a set IPM, through drive rolls matched to the wire diameter.
Contact tip transfers welding current to the wire, which extends past the nozzle as stick-out.
Gas flow — the solenoid opens with the trigger and shielding gas floods the arc zone before and after the arc starts.
Arc transfer — the wire touches the work and shorts; in short-circuit mode it melts into the puddle dozens of times per second.
Puddle formation — the wire and base metal melt together; voltage shapes the bead while wire feed fills the joint.
Solidification — the puddle freezes under the gas stream, forming the weld with no slag layer to remove.
Post-flow — gas continues for a moment after the trigger releases so the hot weld end does not oxidize into a crater.
MIG shares its rig with FCAW, so the same machine often runs both. Here is everything needed for a solid-wire setup.
Constant-voltage DC machine — 140 A for hobby and light sheet, 250 A for shop production, 350 A+ for heavy wire.
Smooth constant-speed feed with knurled V-groove drive rolls sized to the wire. Pressure set light enough not to crush or deform the wire.
Air-cooled for light duty, water-cooled above ~300 A. Match the liner to wire type and length — steel liner for steel wire, Teflon for aluminum.
A poor ground causes arc wander and heat loss. Use a clean, tight clamp on bare steel near the joint.
75/25 argon/CO₂ for steel, 100% CO₂ for budget work, pure argon for aluminum. Flowmeter set per the wire and joint.
Work lead and whip sized for the output; oversize for long runs to prevent voltage drop at the arc.
V-groove rolls matched to the wire diameter; liner matched to wire and gun length. Wrong sizes cause bird's nests and erratic feed.
One size per wire diameter. A worn tip (oversized bore) gives an erratic arc; a spatter-clogged nozzle blocks gas flow.
ER70S-6 for general steel, ER70S-3 for clean shop work, ER308L for stainless, ER4043/5356 for aluminum.
Wire brush, anti-spatter spray, tip cleaner, and pliers for trimming the wire and clearing spatter.
Auto-darkening helmet, welding gloves, long-sleeve FR jacket, and approved footwear — MIG throws UV and spatter.
On-gun or local exhaust extraction for regular production; welding fume is a known hazard — ventilate.
The wire class must match the base metal and the service. The diameter sets the amperage range and the transfer mode you can run — and the gas finishes the recipe.
| Wire | Base Metal | Best For |
|---|---|---|
| ER70S-6 | Carbon steel | General purpose, mill scale |
| ER70S-3 | Carbon steel | Clean shop work |
| ER308L | 304/316 stainless | Stainless fabrication |
| ER4043 | Aluminum | All-purpose aluminum |
| ER5356 | Aluminum | 5xxx alloy, stronger welds |
Typical flow: 15–45 CFH depending on wire and position. Wind over ~5 mph breaks coverage.
The tip bore must match the wire exactly — oversize bores give a wandering, erratic arc and burnback. Keep nozzles clean with anti-spatter compound, and replace tips the moment the arc starts hunting. A spare tip and nozzle on the bench saves an hour of frustration.
Anti-spatter spray or gel on the nozzle and work area keeps spatter from bonding. A wire brush and grinder clean the toes between passes — spatter is cosmetic, but it hides defects and ruins paint adhesion.
Solid wire is less moisture-sensitive than flux-cored, but rust on the surface still becomes porosity. Keep spools covered, avoid condensation, and never use wire that has rusted on the outer wraps.
The same machine can weld sheet metal and heavy plate — the transfer mode is the difference. It is set by wire size, voltage, and gas, and it decides what positions you can run.
| Mode | Gas | Positions | Thickness | Deposition |
|---|---|---|---|---|
| Short-Circuit | 75/25 or CO₂ | All | 24 ga – 1/8" | Low–moderate |
| Globular | CO₂ rich | Flat | Avoid | Moderate, spattery |
| Spray | Argon ≥80% | Flat / horizontal | 1/8" and up | High |
| Pulsed (GMAW-P) | Argon rich | All | 1/16" and up | High, out of position |
Spray and pulsed modes need at least 80% argon — CO₂-rich gas will not sustain a spray stream. Pulsed machines cost more and are the standard for aluminum and out-of-position production.
Short-circuit transfer runs every position. Spray transfer is flat-position only. If the job is vertical or overhead, the mode and wire decide whether it is possible at all.
Maximum speed and deposition. Any transfer mode works — spray shines here. Push angle 10–15° for a flat, clean bead.
Keep the work angle tilted slightly up into the top member of a fillet. Short-circuit and pulsed handle it well; spray can too.
Short-circuit or pulsed with 0.035" or smaller wire. Small U or triangle weave, pausing at the edges so each step freezes. No spray.
Short-circuit or pulsed only, low heat, stringers, tight arc. The puddle must stay small — watch for spatter and falling metal.
Rule of thumb: below 1/8" material, short-circuit is your all-position mode. Above it, consider pulsed for out-of-position work or reposition the part to flat and run spray.
MIG is less forgiving of dirty material than stick or FCAW. Porosity and lack of fusion are almost always prepped into the joint before the trigger is pulled.
Mill scale is a poor conductor and destabilizes the arc. Grind or brush to bright metal in the joint and 1" past each edge.
Rust carries moisture, oil boils into the puddle, and paint burns into porosity. Degrease, then grind — in that order. On galvanized steel, remove the zinc first; the fume is toxic.
Typical fillet fit-up is tight; grooves need a root opening of 1/16–1/8" per the WPS. Too tight causes lack of penetration; too wide burns through.
60° included angle is the standard for full-penetration grooves on plate. Thinner material can run square-edge butts with a small gap.
Mismatch over 1/16" makes the arc wander and the weld cross-section uneven. Check alignment at both ends and the middle before tacking.
Tack at both ends and intermittently along long seams — enough to hold, not so many they slow you down. Grind tacks smooth when they will be buried by a groove weld.
Six techniques cover nearly every MIG job. Push, pull, stringer, weave — each has a purpose, and the bead profile tells you if you used the right one.
Straight, no oscillation — preferred for roots, thin material, and overhead. Narrow heat zone, less distortion, fewer trapped defects.
Oscillate the gun to widen fill passes — keep the weave to about 2–3× the wire diameter and pause at each edge to fill the toes.
Gun tilted 10–15° forward in the direction of travel. Gives a flatter, wider bead with less penetration — used for thin metal and spray transfer.
Gun tilted back 10–15°. Deeper penetration, narrower bead — the default for short-circuit on thicker steel and dirty material.
Small triangle or U steps, pausing at the edges so each step freezes. Short-circuit or pulsed only — the puddle must solidify before it runs.
Root, hot pass, fill, cap — wire-brush each pass and watch interpass temperature on thick plate. MIG leaves no slag, but scale and spatter still hide defects.
The table above is a starting point. These are the controls behind every number — and what happens when you move them.
Sets arc length and bead width. Too high — flat, spattery bead; too low — tall, ropey bead with poor wetting. Adjust in 0.5 V steps.
Wire feed sets the amperage — more wire per minute, more current and fill. Too slow burns back; too fast ropes and sputters.
Contact tip to work distance: typically 10–19 mm. Too long — low heat and poor gas coverage; too short — tip contact, burnback, and blocked gas.
Controls the pinch-off of the short circuit. More inductance gives a smoother, wetter puddle; less gives a crisper, more dig-in arc. The second most-used dial on a MIG machine.
Controls deposit per inch and heat per inch. Too fast — lack of fusion and undercut; too slow — burn-through and excess reinforcement.
15–45 CFH depending on wire and position. Too low — porosity; too high — turbulent gas that sucks air into the puddle.
MIG is the default process for sheet and light structural work across almost every industry.
Light frames, handrails, stairs, and moment connections in the shop — ER70S-6 with 75/25.
Ductwork, enclosures, and panels — short-circuit with 0.023–0.030" wire, minimal distortion.
Body panels, floor pans, and brackets on thin steel — the standard process for collision repair.
Tube frames, railings, and ornamental work — clean beads that need little grinding before paint.
Fuel tanks, equipment frames, and agricultural gear in 1/8–3/8" steel, pulsed for out-of-position seams.
Internal or mechanized roots on mainline pipe — pulsed MIG roots with proper back purge.
Trailers, truck bodies, and marine work with spool gun or push-pull feeders on pure argon.
Shop repairs on brackets, guards, and light frames — fast setup, immediate results.
Eight defects cover nearly every MIG rejection. For each: what it looks like, what caused it, and how to stop it.
Appearance
Pin holes or cavities on the bead surface, sometimes a spongy centerline.
Causes
Low gas flow, wind, dirty or wet base metal, clogged nozzle, wrong stick-out, leaking hose.
Prevention
Clean the joint, set gas 15–45 CFH, block drafts, check the nozzle and hose, hold spec stick-out.
Appearance
The bead looks laid on top of the base metal instead of melted in — a cold edge at the toes or root.
Causes
Low wire feed/heat, travel too fast, wrong gun angle, mill scale, cold thick material.
Prevention
Raise wire feed and voltage, slow down, correct the angle, grind the joint, preheat when specified.
Appearance
A groove melted into the base metal along the weld toes, weakening the cross-section.
Causes
Voltage or travel speed too high, wrong work angle, weave too wide, arc length too long.
Prevention
Drop voltage slightly, fix the work angle, narrow the weave, slow down, cap the toe with a stringer.
Appearance
Holes or sagging drops blown through thin material, usually on the back side of the joint.
Causes
Too much heat, wire too large, travel too slow, excessive root gap on thin metal.
Prevention
Smaller wire, lower feed, faster travel, tighter fit-up — or switch to short-circuit transfer.
Appearance
Weld metal rolls over the base at the toe without fusing — a rounded lip hiding a cold joint.
Causes
Low current, excessive wire, wrong angle, puddle too big for the position.
Prevention
More heat, less fill, correct the angle, keep the arc on the leading toe until it melts in.
Appearance
Metal globules welded to the plate around the joint — cosmetic damage and extra cleanup.
Causes
Voltage off for the feed speed, wrong polarity, worn tip, contaminated surface, wrong gas mix.
Prevention
Tune voltage and inductance, verify polarity, replace worn tips, use anti-spatter, check the gas.
Appearance
Wire tangles and jams between the drive rolls and the gun — the arc dies mid-weld.
Causes
Wrong drive-roll size, too much roll pressure, kinked liner, worn liner, damaged spool.
Prevention
Match rolls and liner to the wire, set pressure just enough to feed, replace the liner annually.
Appearance
The wire fuses to the contact tip, and the arc stops feeding — tip destroyed.
Causes
Wire feed too slow for the voltage, worn tip, feeding problems, contact tip too small.
Prevention
Raise feed speed, replace the tip, fix the feed system, and trim wire with a proper cut — not the grinder.
MIG throws UV, spatter, and fume. Treat it the same as any arc process: protect eyes and skin, ventilate, and clear the work area.
Welding fume is a known hazard. Zinc, chrome, and manganese in coated metals raise the risk — ventilate always.
Local exhaust or outdoor work. Confined spaces need forced ventilation and a trained watchman.
Spatter travels meters. Clear flammables, cover openings, and check for smoldering after grinding too.
The arc emits intense UV — arc eye and skin burn within seconds. Full shade-rated lenses, covered skin, screens for bystanders.
Auto-darkening helmet, welding gloves, long sleeves, no synthetic fabric. Spatter sticks — cover skin.
Rated respirators when ventilation cannot do the job — galvanized, stainless, and confined work.
Galvanized and painted steel produce toxic fume — strip the coating first and wear respiratory protection.
Fresh beads and the base metal around them stay hot for minutes. Mark hot work and watch where you put hands and knees.
These documents govern MIG wire, procedure, and welder qualification. If you are quoting work or signing off welds, you will meet them.
| Standard | Scope | Why It Matters |
|---|---|---|
| AWS A5.18 | Carbon steel MIG wire | Classifies ER70S-3 and ER70S-6 — chemistry, mechanicals, usability |
| AWS A5.9 | Stainless steel wire | ER308L, ER309L, ER316L classes for stainless MIG |
| AWS A5.10 | Aluminum wire | ER4043 and ER5356 — the aluminum MIG wire spec |
| AWS D1.1 | Structural steel welding code | Procedure and welder qualification plus acceptance criteria for MIG welds |
| ASME Section IX | Welding & brazing qualifications | WPS/PQR and performance qualification for pressure equipment |
| ISO 14341 | Solid wire for non-alloy steel | International classification equivalent to the AWS series |
The questions welders actually ask about MIG — answered straight.
MIG (GMAW) feeds a continuously supplied solid wire through a gun while shielding gas protects the puddle. It is fast, easy to learn, and the default process for carbon steel fabrication from 24 gauge sheet to 1" plate.
75/25 argon/CO₂ is the standard — smooth bead and low spatter. 100% CO₂ is cheaper with deeper penetration but more spatter. Aluminum runs on pure argon, stainless on tri-mix or 98/2 Ar/O₂.
Yes — with pure argon, an aluminum wire like ER4043 or ER5356, and a spool gun or push-pull feeder. Soft aluminum wire will not feed through a standard 3 m lead without tangling.
Almost always voltage off for the wire feed speed, a worn contact tip, contaminated base metal, wrong polarity, or a gas problem. Set voltage first, then check the tip and gas.
DCEP (electrode positive) for solid wire — the work is negative, giving deep penetration. DCEN is rare and only for specific thin-sheet or specialty applications.
Short-circuit dips the wire into the puddle rapidly — low heat, all positions, thin metal. Spray atomizes the wire into a fine droplet stream — high deposition but flat position only and it needs argon-rich gas.
Yes — short-circuit transfer with 0.023" or 0.030" wire on 24 gauge and up. Keep heat low, travel steady, and use stitch welding on long seams to control distortion.
Wrong drive-roll size or pressure, a kinked or worn liner, a bird's nest at the feeder, a worn contact tip, or a damaged spool. Check the feed path before blaming the machine.
All-in-one machine settings helper for a fast MIG starting point.
Set your 75/25 or CO₂ mix and read the flow and bottle life you need.
Match wire class to base metal and service — from ER70S-6 to ER5356.
Volts × amps ÷ travel speed — the number that controls distortion and cracking.
What wire and process fit 24 gauge vs 3/8" plate — before you strike the arc.
See wire feed and voltage balance, transfer modes, and technique in motion.
Watch the technique sections — push vs pull, transfer modes, and vertical up — demonstrated on real steel.
Watch VideosA shop-floor cheat sheet with the parameter table, transfer mode chart, and gas quick reference.
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