MIG Welding Definition: What It Is, Parts, and How It Works

A shop foreman points at a row of machines in the corner and says, “If you only learn one of these, learn that one.” The badge reads MIG. Most beginners can rig a MIG welder in a single afternoon, which is exactly why so many people assume they already understand it — and why so many of their first welds come out as ugly, porous little caterpillars. Here is the thing: once you know what MIG actually is, the rest of the process stops being mysterious.

MIG Welding Definition: What the Acronym Actually Means

Here is the short version. MIG welding stands for metal inert gas welding, and the American Welding Society formally designates the process as GMAW, gas metal arc welding. “MIG” is the trade and industry shorthand that nearly every shop, machine badge, and wire spool uses in everyday conversation. “GMAW” is the technical name that shows up in welding standards, qualification procedures, and code paperwork. Both describe the same process, and the terminology itself is governed by AWS A3.0M/A3.0:2025, Standard Welding Terms and Definitions. AWS has authored more than 350 standards covering welding practices and procedures, so when the society settles on a name, that name sticks.

Why the Name Exists

The acronym was coined for a practical reason: to tell you where the protective atmosphere comes from. In stick welding, shielding is generated by the flux coating on a consumable electrode, and the process leaves slag behind. In MIG, the shielding arrives as a separate stream of gas delivered through the torch. Calling it “metal inert gas” tells a welder that the metal electrode is covered by an inert gas. When that gas is chemically active rather than inert, the identical process gets called MAG, metal active gas.

This is why European shops say MAG and North American shops say MIG for what is, mechanically, the same machine. The distinction is about the gas, not the equipment.

Feature	MIG	MAG
Full name	Metal Inert Gas	Metal Active Gas
Formal AWS designation	GMAW	GMAW
Shielding gas	Argon, helium, or argon/helium mixes	Pure CO2 or argon/CO2 blends
Machine, torch, and wire	Identical for both
Typical behavior	Clean bead, minimal spatter, suited to thin and non-ferrous work	Deep penetration, heavier spatter, suited to carbon steel production work

The Two Ingredients That Define the Process

1. A continuously fed consumable wire electrode. The wire is both the electrode and the filler metal. It unwinds from a spool, passes through drive rolls and a liner, and arrives at the workpiece as the live end of the arc. Because it feeds continuously, the welder never stops to change an electrode.
2. An externally supplied shielding gas. A cylinder, regulator, and hose deliver a controlled stream of gas through the nozzle, where it blankets the arc and the molten pool beneath it.

Everything else in the system exists to serve those two elements. Change the gas and you change how the process behaves. Change nothing else and it is still the same MIG machine you bought.

No Flux Means No Slag

Because MIG wire is solid and carries no flux core, nothing burns off and nothing needs to be chipped away between passes. There is no de-slagging step, no grinder cleanup, and no chipping hammer in your hand. That single fact drives much of MIG’s appeal in production work, and it is the sharpest contrast with why SMAW still wins on dirty, thick, or outdoor jobs, where the slag is an acceptable trade for the extra versatility.

Where MIG Sits Among the Common Arc Processes

The four arc processes a fabricator is most likely to choose between are GMAW (MIG), FCAW (flux core, self-shielded or gas-shielded), SMAW (stick), and GTAW (TIG). MIG won the general-purpose slot because it combines continuous wire feed, no slag, high travel speed, easy mechanization, and a very forgiving arc. Lincoln Electric’s comparison of GMAW and FCAW lays out the technical trade between them in detail, and ESAB’s MIG welding guide covers the parameters that make the process repeatable.

MIG is also not limited to carbon steel. The same core process handles stainless, aluminum, and other non-ferrous metals with the right wire and gas, which is why our Aluminum MIG Welding: The Complete Practical Guide covers the hardware changes aluminum demands.

So the shorthand answer is this: MIG welding is metal inert gas welding, formally called GMAW by the American Welding Society, and it is defined by a continuously fed consumable wire electrode plus an externally supplied shielding gas. Everything else about the process, from the transfer modes that shape the bead to the choice between argon and carbon dioxide, follows from those two facts.

How MIG Welding Works: What Happens Between the Trigger and the Bead

People often describe MIG welding as “an arc between a wire and the metal.” True, but that skips the part that explains why the machine has a knob for everything. Here is the real sequence.

Pressing the Trigger Starts Three Things at Once

The trigger is a single command. Squeeze it and three events fire at once: the solenoid opens the shielding gas valve, the drive rollers pull wire off the spool and push it down the liner, and the power source energizes to strike the arc. That overlap keeps the process stable. If gas lagged half a second behind the arc, the pool would already be absorbing air before the shield arrived. Miller’s basics overview walks through the same sequence and is worth reading before your first power-on.

The Arc, the Molten Pool, and the Bead

The arc forms between the live end of the consumable wire and the workpiece. Its heat does two jobs at once: it melts the wire tip so fresh filler metal arrives at the joint, and it melts the base metal so the two can fuse. Both end up in a small molten pool in the groove or on the surface of the work. As the torch travels, the pool trails behind, cools, and solidifies into the bead. Every defect MIG is famous for, from porosity to undercut, starts in that pool while it is still liquid.

Why the Machine Runs Constant Voltage

MIG needs a constant-voltage (CV) power source to hold the arc steady. In CV mode the machine holds voltage fixed and lets current respond on its own. Voltage governs arc length: too low gives a short, harsh arc, too high stretches it long so it wanders and throws spatter. Wire feed speed sets amperage. Push more wire into the arc and the machine raises current to keep up; back off and current drops. Travel speed, voltage, and wire feed speed together decide bead width, penetration, and profile, so you tune voltage first, then wire feed speed, and recheck voltage when material thickness changes. ESAB’s process parameter guide has useful charts for reference.

The Shielding Gas Is Doing Real Work

Arc temperatures are hot enough to make surrounding air react with the metal. Without a shield, the molten pool absorbs atmospheric nitrogen, oxygen, and hydrogen. Nitrogen and oxygen drive porosity, the gas pockets that make a bead look like a sponge. Hydrogen promotes cracking, particularly the delayed kind that shows up hours after the weld cools. The shielding gas blankets the pool and keeps air out, which is the whole reason MIG and MAG are gas-shielded processes, and the reason a drafty bay or windy site can ruin a weld that ran perfectly indoors.

Inductance and Short-Circuit Spatter

When the wire tip touches the pool, the arc shorts out. That is short-circuit transfer, and it happens many times per second in a normal weld. The instant the wire touches down, current wants to spike hard, so inductance in the power source slows that spike down. More inductance means a gentler rise and less spatter, but the arc can feel sluggish and flutter at low settings. Less inductance means a faster spike, more spatter, and a more responsive arc. On modern machines you find this as an inductance setting, a spatter-control knob, or folded into the synergic program.

The Wire Feed System and the Copper Coating

Two knurled drive rolls grip the wire and push it forward at the speed you set, while the spool pays out wire against a drag brake. Wire travels through the liner, over the drive rolls, through the contact tip, and into the arc. Lincoln Electric notes that steel MIG wire is copper-plated to improve electrical conductivity, resist rusting in storage, and extend contact tip life. That coating is not decorative: it helps current pass cleanly from tip to wire and keeps the tip from eating through too fast.

Stick-Out (CTWD) and Polarity

CTWD means contact-tip-to-work distance, the length of wire sticking out past the tip. Fronius gives typical stick-out of roughly 10 to 15 mm for carbon steel solid wire and 12 to 20 mm for aluminum, which pulls heat away from the tip faster. Too short and the wire stubs into the work, causing erratic arc, harsh sound, and constant spatter. Too long and the arc goes unfocused, spatter climbs, and the wire can whip or burn back into the liner. Our aluminum MIG guide covers the aluminum-specific hardware and stick-out in more detail.

MIG also runs on DC positive polarity (DCEP) as the standard setup. With the electrode positive, more arc heat goes into the workpiece, giving deeper penetration, a faster travel rate, and a flatter bead. Wire it backwards and the workpiece becomes the negative terminal: penetration drops, the bead piles into a lumpy mound, and the liner cooks off quickly. It is a classic beginner error and one of the first things to check when a weld looks nothing like the manual.

The MIG Welding Machine, Torch and Consumables

A MIG welder looks like one box with a hose attached, and that is what trips people up. The box on your bench is only the power supply. The wire feed unit, torch, gas, work cable and the metal itself each have their own failure modes, and every one of them has to be right.

The Six Parts of a MIG Rig

1. Power source – the constant-voltage (CV) machine supplying arc current and the voltage that holds the arc.
2. Wire feed unit – motor, drive rolls and spool holder that push wire at a controlled speed.
3. Torch (gun) – handle, nozzle, contact tip and liner that guide wire and deliver current.
4. Shielding gas – cylinder, regulator, flowmeter and hose blanketing the arc in argon or a blend.
5. Work cable and clamp – the return path for welding current back to the machine.
6. The workpiece – the metal being joined, which is also part of the electrical circuit.

The wire and the workpiece are the two beginners forget, and the two that cause the most frustration.

Spool Gun, Push-Pull and Separate Feeder Systems

• Spool gun – the spool mounts on the torch, so the wire feeds a short distance. Standard for aluminum, since that wire is soft, expands when heated and is easily damaged on long or sharply bent feed paths.
• Push-pull – a remote feeder pushes wire while a small motor in the gun handle pulls it along, so the wire stays straight and untangled. The premium answer for aluminum and stainless in production, and the Aluminum MIG Welding practical guide covers the setup in depth.
• Separate feeder, standard gun – a floor-standing or bench-mounted feeder pushes wire down a long liner to a conventional torch. Cheapest and most common for steel, and it works when the liner is straight, the drive rolls are clean and the wire does not rub the liner wall.

Inside the Gun: The Consumable Stack

Four parts wear out on a schedule, and knowing them in order makes troubleshooting easier.

1. Drive rolls – in the feeder, not the torch. Knurled rolls grip and push the wire, but the knurl digs into soft aluminum, so V-shaped or U-shaped rolls cradle it without marring.
2. Liner – a coiled tube, steel for carbon steel and PTFE (Teflon) for aluminum or stainless, guiding wire to the contact tip. A kinked or burred liner is a classic cause of erratic feed and stubbing.
3. Contact tip – a small copper or copper-alloy nozzle the wire runs through. Current passes from here to the wire, so this is where the arc is struck. Worn or undersized tips cause stubbing and erratic bead shape.
4. Nozzle (gas cup) – the outer cup that shapes and directs shielding gas over the pool. Spatter builds up inside fast and, once it touches the liner or tip, it wrecks both.

Tension: Drive Rolls and Spool Hub

Drive roll pressure must grip the wire firmly enough to push it, but not so hard that it flattens or scores it. Too little and the wire stalls or slips; too much and it gets crushed, especially soft aluminum, and feeds unpredictably.

Spool hub tension is the other half. A spool that unwinds too freely tangles and surges, while one that drags stretches the wire until it slips. Both are set to the manufacturer’s spec and re-checked whenever you change wire diameter, material or spool size. The ESAB MIG welding guide on process parameters is a solid reference for feed speed and setup relationships.

Wire Diameter and Specifications

Most MIG wire falls between .023 and .035 inch (.6 mm to roughly .9 mm) for thin material, with heavier sizes for plate. ESAB lists the standard MIG wire diameter range as 0.6 mm to 2.4 mm. For home shop and motorsports work, .030 inch is the sweet spot: it handles thin automotive steel, tube and sheet without the hot, splashy behavior of .035, and it still has the bulk to lay down a decent fill bead.

Wire is classified by standard, and matching it to the base metal matters: carbon steel is AWS A5.18 or EN ISO 14341 (ER70S-6, ER70S-3), stainless is AWS A5.9 or EN ISO 14343 (ER308L for 304/304L, ER316L for 316/316L), and aluminum is EN ISO 18273 (ER4043, ER5356, ER5183).

Mild steel wire is copper-coated, and that coating is not cosmetic. It improves conductivity, extends contact tip life and gives better arc performance than bare wire. The Miller guide to MIG basics for mild steel ties wire, gas and machine settings together.

Aluminum Needs Its Own Hardware

Aluminum wire is roughly a third as stiff as steel and expands noticeably as it heats. Steel liners and steel drive rolls will make it buckle, flatten and tangle, so aluminum setups use PTFE guides and liners, U-shaped drive rolls and a spool gun or push-pull that keeps the feed path short and straight. The guide to prepping aluminum for welding covers the other half of the job, because clean metal matters even more with aluminum.

Air-Cooled and Water-Cooled Torches

Air-cooled guns are cheaper, lighter and fine for short-run work, home shops and repair jobs where you lay down a few beads and stop. Water-cooled torches circulate coolant through the handle and neck so the gun holds high current for long stretches without overheating, which is why they are standard in production, shipbuilding and robotic cells. The extra weight means they are not automatically the better choice for a beginner.

The Work Clamp Connection

The ground clamp is not optional, and it is often the weakest link in the chain. Welding current returns to the machine through the workpiece, and if that path is dirty the circuit fights itself. Paint, rust, oil and dirt are insulators, so a clamped-on lead does not work as intended. Clean the workpiece to bare metal and attach the clamp to clean metal as well. On cylindrical parts, use a dedicated clean lug or a scrubbed area away from the joint, never a dirty pipe wall. A poor clamp shows up as an arc that wanders, a bad bead pattern, or a shock through the work.

Wire Diameter and Material Thickness

Wire diameter	Typical material thickness	Notes
.023 in (.6 mm)	Up to about 1/16 in	Thin sheet, panels, body filler work
.025 in	1/16 to 1/8 in	Light automotive and repair work
.030 in	1/8 to 1/4 in	The all-round choice for home and motorsports
.035 in	1/4 in and heavier	Plate, structural and production work
.045 in (1.2 mm)	3/8 in and up	Heavy plate; needs a capable feeder and machine

With the hardware, wire and gas set correctly, the next question is how metal actually moves from the wire into the pool. That is where transfer modes come in, and they are the fastest way to diagnose most weld problems.

Shielding Gases: The Real Difference Between MIG and MAG

Here is where the MIG versus MAG argument actually lives: in the cylinder next to the machine. Same wire, same torch, same power source. The only meaningful difference is what comes out of the bottle.

MIG (metal inert gas) runs on inert gases such as argon, helium, or an argon/helium mix. MAG (metal active gas) runs on active gases such as pure CO₂ or argon/CO₂ blends. Inert gases are chemically noble and refuse to join the weld. Active gases take part in the arc itself, and that single fact changes the arc, the bead, and the spatter.

Why the Weld Needs a Blanket

A MIG arc melts steel, and the pool is hotter still. At those temperatures, nitrogen, oxygen, and hydrogen in the air react with the metal and the arc column, leaving porosity, brittle oxides, and hydrogen-assisted cracking in the bead. A stream of shielding gas wraps the arc and pool in clean atmosphere so the weld metal stays chemically pure as it solidifies. The gas adds no strength; it only protects the joint.

What “Active” Actually Means

Active gases improve the electrical characteristics of the arc, which is why CO₂-rich mixes feel so driven and focused. The trade-off is a rougher bead and more spatter. Inert gases give a quieter arc and a smoother bead, but penetrate less and cost more per cylinder.

Which Gas Should You Buy?

For mild steel, Miller’s guide to MIG basics is worth reading twice. Its pick for a shop that wants one gas to do everything is 75 percent argon and 25 percent CO₂, sold as 75/25 or “C-25.” That blend gives the least spatter and the best bead appearance of the common mixes, and it will not promote burn-through on thin metal. For structural work with an inspected surface, ESAB explains that 80/20 blends improve arc stability, reduce spatter, and create a smoother bead than straight CO₂.

Application	Recommended Gas	What You Get
Mild steel, budget work	100% CO2	Cheapest, deepest penetration, more spatter
Mild steel, general purpose	75/25 argon/CO2 (“C-25”)	Least spatter, best bead, less burn-through on thin metal
Structural steel, finish matters	80/20 argon/CO2	Better stability, less spatter, smoother bead
Stainless steel	90/10 or 98/2 argon-based blends	Stabilizes the puddle, keeps the bead bright
Aluminum, copper, non-ferrous	Pure argon or argon/helium	Clean arc, no reactive oxygen

Non-ferrous work has its own gas demands; our Aluminum MIG Welding practical guide covers that setup.

Flow Rate: The Number Most People Get Wrong

For mild steel, 20 to 25 cubic feet per hour is the typical working range. Too little flow lets air leak in at the nozzle and shield edges, and you get porosity. Too much flow creates turbulence that drags air right back in through the same path. Both ends of the dial cause the same symptom: a porous weld. Check flow first.

Leak Testing the Hose

Before you blame the gas, check the hose. Miller recommends a soapy water test: mix detergent and water, apply it along the full length of the hose including the connections, and watch for bubbles. If bubbles appear, replace the hose. A pinhole leak starves the nozzle and quietly fills your shop with nitrogen and carbon dioxide.

Cylinders: Cost, Handling and the OSHA Rules

Argon and argon blends cost more than CO₂, and larger cylinders cost more per cubic foot but less per unit of gas overall. Most shops buy the mix they use daily and swap the rest. The Air Gas welding supply guide compares suppliers and cylinder sizes, and many regions offer delivery and swaps for a flat monthly fee.

Cylinder handling is covered by OSHA 29 CFR 1910.253, and the rules are specific:

• Store cylinders away from heat and out of the way of traffic or damage.
• Close the valve on any cylinder that is empty or not in use.
• Keep oxygen cylinders at least 20 feet (6.1 m) from fuel gas cylinders or combustibles, or separate them with a noncombustible barrier.
• Never use a cylinder as a roller or support, strike an arc on one, or place one where it could join an electric circuit.

Argon gives the cleanest bead, the least cleanup, and the most forgiving arc: that is a MIG setup. CO₂ or a lean blend gives deeper penetration at the lowest cost per cubic foot, with spatter and a rougher bead: that is MAG. The machine is the same either way; the gas is your call at the bench.

Metal Transfer Modes: Short-Circuit, Globular, Spray and Pulsed

Ask a shop lead why one MIG bead looks like a tight stack of dimes and the next looks like burnt popcorn, and the answer is almost never the welder. It is almost always a different transfer mode, the physical way molten metal leaves the wire and lands in the puddle. Get that straight and troubleshooting gets far easier, because the mode tells you what the arc is doing at any given second.

Short-Circuit (Dip) Transfer

In short-circuit mode, the wire tip repeatedly touches the molten pool. The contact shorts the circuit, arc voltage collapses to nearly nothing, and the arc momentarily extinguishes. A few milliseconds later the pool surface cools and contracts, the gap reopens, and the arc strikes again. This cycle repeats up to about 200 times per second, the signature sound of a MIG welder running quietly at the bench.

Because the arc burns only part of the time, heat input stays low and the puddle stays small. That makes dip transfer the right choice for thin material, out-of-position work, and root passes where you want control over where metal lands. It is the easiest mode to learn, since the wire feeds itself in. The trade-off is penetration: it does not drive deep penetration the way spray does, so thicker sections need more passes or a different mode.

Globular Transfer

Globular is the awkward middle child. Droplets form at the wire tip, grow larger than the wire, then pinch off into the pool. They transfer irregularly, often sideways, producing heavy spatter and a rough bead compared with spray.

Globular shows up in the current range between dip and spray, and it is most strongly associated with pure CO2, which does not support the fine droplet formation argon blends produce. Gravity limits it too: large falling droplets behave badly in anything other than flat or horizontal work, so globular is effectively a flat-and-horizontal-only mode. Shops accept it where it is economical rather than pretty, since rough CO2 on a hidden bracket costs less than better gas.

Spray Transfer

Spray transfer is the clean, high-production mode. The arc is long, current and voltage are high, and fine droplets detach from the wire tip and are thrown axially across the arc by electromagnetic forces. The tip is pushed hard enough that droplets let go while still small, which is why the bead comes out smooth and free of spatter. Spray gives excellent penetration and high deposition rates on thicker material in flat and horizontal positions, but it needs the top of the machine’s current range. It is far too hot and fast for thin sheet or for vertical and overhead work.

Pulsed (Pulsed Spray) Transfer

Pulsed MIG is the modern answer to the old trade-off. The machine alternates rapidly between a short, very high current pulse that shoves a single droplet off the wire tip and a low background current that keeps the arc stable between pulses. Detachment happens at roughly 50 to 100 Hz, so the operator hears a steady buzz rather than the random crackle of globular or the hiss of short-circuiting.

Because the pulse does the detaching, the average current can stay lower than full spray requires. That means spray-type bead quality and penetration at a heat input you can live with, which makes pulsed MIG the practical answer for all-position welding on thin stainless steel, aluminum, and heat-sensitive alloys. Robots love it too, because every pulse is identical. If aluminum is part of your work, study this mode first, alongside the dedicated feeding hardware in our aluminum MIG welding guide.

How the Mode Is Actually Selected

There is no “mode switch” on a basic machine. Transfer mode is an emergent result, the product of several settings interacting:

• Voltage sets arc length. Longer arc, more heat spread, generally higher current.
• Wire feed speed drives amperage on a
• Wire feed speed drives amperage on a constant-voltage machine. Feed faster, draw more current.
• Travel speed changes how long the puddle has to solidify.
• Wire diameter sets the current range where each mode stays stable.
• Shielding gas changes droplet formation physics, which is why argon blends spray cleanly and CO2 trends globular.

Raise wire feed speed on a short-circuiting setup and you drift up through globular into spray, and the bead changes character as you go. Read the welding chart on the machine before you guess.

Transfer Mode	Position	Best For	Watch Out
Short-Circuit (Dip)	All positions	Thin material, root passes	Lower penetration and deposition
Globular	Flat and horizontal only	Economical CO2 work	Heavy spatter, rough bead
Spray	Flat and horizontal	Thicker material, production work	Needs high current
Pulsed	All positions	Thin stainless, aluminum, robotics	Needs a pulsed-capable machine

For a deeper look at the parameters behind these modes, ESAB’s MIG process parameter guide breaks down how wire size, current, voltage and gas interact, while Fronius’s MIG/MAG overview explains the same physics from the perspective of modern synergic power sources.

Why People Choose MIG: The Benefits and the Honest Limits

Every welding process has a résumé, and MIG’s is unusually easy to read. It is fast, it is clean, it never stops to cool down, and a beginner can put down a serviceable bead within a week of practice. That is why it swept fabrication shops and became the default arc process for good reason. But a résumé is not a guarantee, and the honest version of the MIG story includes a short list of places where a different process is simply the better tool.

What you gain by choosing MIG

• Continuous operation with very little downtime. There is no electrode to change every few minutes, so you are not constantly stopping to swap sticks and clear a stub. A spool of wire and a bottle of gas let a machine run through a shift, and one wire size often covers multiple positions.
• No slag and almost no cleanup. Solid wire carries its own deoxidizers, so the finished bead needs only a knock-off of loose spatter rather than chipping a slag layer.
• High deposition rate and travel speed. Of the common arc processes, MIG delivers the greatest volume of finished weld metal per hour, which is why it dominates production work where the clock is the expensive part.
• Low hydrogen input. ESAB describes MIG as a low-hydrogen process with minimal hydrogen content, which reduces cold-cracking risk and makes the process suitable for critical applications when the right shielding gas and wire are used.
• Mechanisation and automation are straightforward. Once the parameters are dialled in, the process transfers easily to robotic cells and automated fixtures. ESAB points to automotive body shells and chassis, shipbuilding, pipelines, pressure vessels, and bridges as representative work, which is a fair summary of where MIG earns its keep.
• A forgiving arc that is quick to learn. Shielded metal arc welding asks you to manage a consumable electrode and a flux bubble at the same time. TIG asks you to manage amperage, filler addition, and torch movement by hand. MIG mostly asks you to manage travel speed and stick-out, so the skill curve is short.

Where MIG falls short

The metal has to be clean. Solid wire has no flux to burn off or neutralise contaminants, so rust, dirt, oil, and paint on the joint will end up in your puddle. Miller is blunt about this: clean down to bare metal before striking an arc, because the process has no flux additives to compensate for contamination. Grinding a joint takes five minutes. Chasing porosity for an hour takes much longer.

You are married to the gas bottle. An external shielding supply limits portability and makes outdoor field work awkward, because wind and moving air strip the cover off the arc. Fronius lists sensitivity to rust and moisture along with difficulty maintaining shielding gas outdoors or in drafty halls among the standard disadvantages of MIG and MAG. Red-D Arc makes the same point from the other direction: because shielding gas is required, MIG is not commonly used for field fabrication and repair, where self-shielded flux-cored and stick welding remain more popular. That is the trade-off to understand, and it is exactly why stick welding still wins outdoors.

Gas choice changes the bead. Pure CO2 is cheap and penetrates well, but it throws noticeably more spatter and leaves a rougher surface than an argon/CO2 blend. Budget gas can cost you in grinding time and in appearance.

Heat control and distortion are harder. MIG runs hot and fast. Controlling heat input and managing warping on thin or mismatched sections takes more practice than the same job in TIG, where you add material by hand and can watch the pool moment to moment.

Thick material out of position is not its strength. Red-D Arc recommends gas-shielded flux-cored welding when thick metals must be welded out of position frequently, because the slag from those consumables assists fusion at productive amperages. Lincoln Electric’s GMAW versus FCAW-S comparison walks through the trade-offs in detail, and it is worth reading before you commit a thick structural joint to solid wire.

Special materials need special hardware and matching filler. Aluminum requires dedicated feeding equipment because the soft wire and its oxide demand specific drive rolls, liners, and spool guns, which our aluminum MIG welding guide covers in detail. Stainless needs matching wire, such as 308L for 304 and 304L base metal, and heavy equipment and structural members often call for tubular metal-cored consumables instead of solid wire.

So is MIG right for your project?

Choose MIG when you are welding carbon or stainless steel in a shop, when volume matters, when you want clean repeatable beads with minimal cleanup, or when you are just learning and want fast visible progress. Look at flux core for thick, out-of-position structural work, at stick for field repairs in wind and on dirty metal, and at TIG for thin, heat-sensitive, or cosmetic work. Pick the process the joint demands, not the one the badge is most familiar.

MIG vs TIG vs Stick vs Flux Core: Choosing the Right Process

Every shop owns at least three of these processes, and every beginner eventually argues about which one is “best.” There is no single best process, only the best one for a given metal, thickness, joint position, and set of expectations about finish, time, and cost. Pick wrong and you will spend a weekend fighting porosity on a job another process would have finished in an afternoon.

Get the official names straight first, since these designations show up on drawings, inspection reports, and code paperwork. The AWS A3.0M standard defines them: GMAW is gas metal arc welding (the technical name for MIG), GTAW is gas tungsten arc welding (TIG), SMAW is shielded metal arc welding (stick), FCAW is flux cored arc welding, and MCAW is metal cored arc welding, the metal-cored cousin of FCAW. A job traveler marked GMAW means MIG; FCAW-S means self-shielded flux core.

The Comparison Table

Process	AWS	Electrode / Filler	Shielding	Learning Curve	Speed	Bead Finish	Best Materials	Typical Uses
MIG (GMAW)	GMAW	Continuous solid wire electrode	External gas (argon/CO2, pure CO2, pure argon)	Easiest; fastest to proficiency	High deposition, travel speed	Good, slightly convex; spatter with CO2	Carbon steel, stainless, aluminum, magnesium	Production welding, fabrication, automotive, structural, ductwork, repair
TIG (GTAW)	GTAW	Non-consumable tungsten plus separate filler rod	External inert gas (argon, argon/helium)	Hardest; demands steady hand control	Slowest; manual filler addition	Excellent; no spatter, narrow bead	Stainless, aluminum, exotic alloys, thin sections	Aerospace, surgical, art, precision, pipe, thin-gauge fabrication
Stick (SMAW)	SMAW	Self-shielded flux-coated consumable electrode	None needed; flux generates its own shielding	Moderate; forgiving but less precise	Moderate; electrode changes interrupt work	Rough; slag must be chipped	Heavy carbon steel, pipe, structural, dirty or rusty metal	Field repair, shipbuilding, mining, outdoor work, structural erection
Flux Core (FCAW)	FCAW-S or FCAW-G	Flux-cored wire (or metal-cored, MCAW)	FCAW-S: self-shielded, no bottle. FCAW-G: external gas added	Moderate; similar handling to MIG	High; high deposition rates	Good, slightly flatter than MIG	Heavy carbon steel, shipbuilding, structural, wear plate	Heavy out-of-position work, thick plate, long production runs

What Actually Makes TIG Different

TIG uses a tungsten electrode that is not consumed; filler comes from a separate rod you feed by hand into the puddle, so you manage a torch, a foot pedal, and a rod, not a wire feed system.

As Miller frames it, the MIG/TIG difference is less about difficulty than control. In MIG you set feed speed and voltage, and the power source manages amperage and arc length. In TIG, amperage, filler addition, and torch movement all sit in your hands. One welder thinks in amps, the other in minutes per foot.

The payoff is a smaller heat-affected zone, less distortion, zero spatter, and a machined-looking bead, which is why an aerospace TIG welding standard is often a contract requirement and why TIG handles thin stainless and aluminum. The cost is speed and consumables: more filler rod, more argon, steeper learning curve.

Why Stick Still Wins on the Job Site

SMAW carries its own shielding, so no gas cylinder, no flowmeter, no hoses to trip over. It runs in wind, in rain, on a pipeline, and on steel never cleaned since the mill, and it handles thick sections that would make a MIG gun stutter. Stick welding’s advantages come with tradeoffs: electrode changes every few minutes, slag to chip between passes, and a rough bead. Codes still accept it for structural fillet welds because it is easy to inspect and qualify.

Flux Core: Two Different Animals

FCAW-S uses a flux-cored wire that produces its own shielding gas and fluxing agents, so it needs no external bottle, making it the site-friendly cousin of stick with MIG-like deposition rates. FCAW-G adds external shielding gas on top of the core, giving a cleaner bead and better control at the price of hauling a bottle. For heavy out-of-position work, FCAW-G is typically preferred because high deposition plus out-of-position control is what structural work needs.

Flux core also covers options MIG lacks, including self-shielded wire for aluminum, detailed in our guide to aluminum flux core welding.

The Strength Question, Answered Honestly

Ask ten welders which process makes the strongest weld and you get ten answers, none useful. The honest answer is that no welding type is strongest across all applications. Strength depends on the metal, the thickness, and the expected loading. A proper MIG fillet in 1/4-inch steel can outperform a sloppy SMAW bead in plate. What settles it legally is qualification: structural work under AWS D1.1, aluminum under AWS D1.2, and welder and procedure qualification under ASME Section IX.

Machine Sizing for the Process You Chose

Process choice constrains the power source. Lincoln Electric recommends compact 115V MIG wire feeder welders for indoor work on clean new steel, roughly 24 gauge to 12 gauge. Heavier work needs a 230V machine, covering about 1/8-inch to 1/4-inch material and beyond. A 115V unit cannot push enough amperage into thick plate, producing cold, poorly fused beads that look like failure but are really a power supply problem.

For thin metals, MIG’s small-diameter wires and pulsed waveforms hold a stable arc at low amperage without burning through, where TIG is the traditional answer. For thick sections out of position, go FCAW-G. For aluminum, plan gas and hardware up front, as covered in our aluminum MIG welding guide. Learn two processes well, not four poorly: MIG for general fabrication, TIG for thin or high-quality work, and a stick machine for field repairs and thick sections you meet outside the shop.

Common MIG Welding Mistakes and How to Fix Them

Most MIG defects don’t stem from the machine itself, but from preparation, settings, or technique. The most common thread between nearly all of them is dirty metal. By troubleshooting one variable at a time, you’ll spend far less time grinding and far more time laying down sound welds.

Porosity: Tiny Holes With Big Consequences

Porosity appears as pinholes or tiny bubbles throughout your bead and is almost always caused by atmospheric contamination. According to ESAB’s MIG welding defect guidance, porosity is caused by surface contamination and hydrogen from moisture.

Fix it: Clean the workpiece down to bare metal, keep your wire dry and sealed when not in use, use moisture-free hoses, and verify your gas flow is stable and leak-free. For aluminum especially, this level of prep is non-negotiable, as covered in Aluminum Cleaner for Welding: How to Prep Aluminum Right.

Spatter: The Pesky Metal BBs

A small amount of spatter is expected with MIG, but excessive amounts point to incorrect parameters. ESAB states that spatter is caused by excess current or incorrect gas.

Fix it: Drop your current slightly, add a touch of inductance if your machine allows, and switch from pure CO2 to an argon/CO2 blend such as 75/25 for a cleaner bead.

Lack of Fusion: The Hidden Weak Spot

Lack of fusion occurs when the weld metal never bonds properly to the base metal. It can look acceptable on the surface while being critically weak. ESAB identifies this defect as being caused by low current or improper torch angle.

Fix it: Increase your current to a level appropriate for your material thickness, adjust your inductance if available, and correct your torch weave to drive the arc into the joint rather than solely into the puddle.

Burn-Through, Undercut, and Poor Bead Appearance

Thin sheet metal is the most prone to burn-through, which happens when excess heat punches a hole through the workpiece. Combat this by reducing heat input, using pulsed transfer if available, opting for a smaller wire diameter, or adding a backing bar for support.

Undercut is a gouge along the toe of the weld, often from dragging too steeply or traveling erratically. Hold a shallow push angle between 5 to 15 degrees, maintain a consistent stick-out, and keep a steady travel speed. For carbon steel solid wire, ESAB recommends a 10–15 mm CTWD to help prevent it.

For poor bead appearance, the culprit is often low heat input, cold starts, or a lack of cleanup between passes. Dial in your parameters, refine your torch manipulation, and make inter-pass cleaning a required step in your workflow.

Feeding Issues: Bird-Nesting, Stubbing, and Burn-Back

Feed problems can bring a job to a halt. Wire bird-nesting and erratic feeding are typically caused by incorrect drive roll tension, incorrect spool tension, a kinked liner, or a mismatched contact tip. Work through these in order, from the spool down to the tip.

Stubbing and burn-back are often caused by the contact tip being too close to the work, the wrong tip size for your wire diameter, or reversed polarity. For MIG, DCEP (Direct Current Electrode Positive) is required. Running the opposite polarity will lead to an erratic arc, stubbing, and poor penetration. Always confirm your tip matches your wire diameter, as an incorrect fit can create its own feeding issues.

Quick Troubleshooting Reference

Symptom	Likely Cause	Fix
Porosity (pinholes)	Contamination, moisture, poor gas coverage	Clean to bare metal, dry wire, use moisture-free hoses, check gas flow
Excess Spatter	Excess current, pure CO2	Reduce current, add inductance, switch to argon/CO2 blend
Lack of Fusion	Low current, improper torch angle	Increase current, adjust inductance, correct torch weave
Burn-Through	Heat input too high on thin material	Reduce heat, use pulsed transfer or smaller wire, add a backing bar
Undercut (toe gouge)	Travel angle too steep, dragging, erratic speed	Hold a 5–15 degree push angle, steady travel speed, consistent stick-out
Bird-nesting, erratic feed	Drive roll or spool tension off, kinked liner, wrong contact tip	Set roll and spool tension, replace liner, fit the correct contact tip
Stubbing, burn-back	Tip too close to work, wrong tip size, reversed polarity	Extend stick-out, match tip to wire diameter, confirm DCEP
Poor bead appearance	Low heat input, cold starts, no inter-pass cleanup	Adjust parameters, refine torch manipulation, clean between passes

Most defects announce themselves before you lay a full bead. If wire feed and gas flow are correct, focus on the joint, settings, or torch angle. Change one variable at a time and keep notes on your machine. That habit, more than any single setting, is what makes good beads repeatable.

For aluminum, that logic holds true with less margin for error. The oxide layer reforms instantly, so contamination drives porosity faster and consumes filler metal at a higher rate. For this reason, the cleaning and feeding steps in Aluminum MIG Welding: The Complete Practical Guide are especially important. Polarity and contact tip sizing remain the two quickest checks for erratic arcs, and Miller’s walkthrough of MIG basics for mild steel is a solid reference for dialing in a stable setup. Fix the root cause, and the symptom will disappear with it.

Safety First: PPE, Welding Fumes, and the OSHA Rules That Apply

Welding looks like a bright, noisy, spark-throwing operation, and it is tempting to assume the hot metal is the dangerous part. It is not. The serious long-term hazard is the air coming off the arc, and the rules governing it live in the Code of Federal Regulations under 29 CFR 1910.252. Take one thing away: the arc is the source of the fumes, the UV, the spatter, and the fire risk, and every control you need points back to the arc and the air around it.

Protecting Your Face and Eyes

OSHA 1910.252 requires helmets or hand shields for all arc welding, and its filter-shade guide points to shade 12 for gas-shielded arc welding on ferrous materials and shade 11 for nonferrous work. A MIG arc emits infrared and ultraviolet radiation intense enough to burn your retina in a fraction of a second and to trigger arc eye, the painful inflammation that shows up hours after the fact. Use a properly rated auto-darkening helmet and confirm its shade range covers the light you are working with. Shade 10 can still work for low-current TIG, but a solid 12 is the sensible default for MIG on steel.

Screens are not optional. OSHA requires nearby employees to be protected, so hang curtains or a screen that blocks line of sight to the arc. In a small garage, the person grinding steel across the room is exposed to your arc and your fumes unless something physical stands between you.

Clothing, Gloves, and Skin

OSHA 1910.252(b)(3) requires employees exposed to welding, cutting, and brazing hazards to wear protective clothing in accordance with 1910.132. Practically that means flame-resistant cotton or leather, sleeves down, and nothing synthetic. Polyester and nylon melt and stick to skin instantly, turning a small spatter burn into a serious one. Wear leather gauntlet gloves, keep your cuffs tucked, and use a leather apron or bib for overhead work. MIG spatter is hot and fast and hides in the creases of jeans cuffs and boot tops, so closed leather footwear beats canvas sneakers.

Ventilation: The Rule That Saves Your Lungs

OSHA 1910.252(c)(1)(iii) requires local exhaust or general ventilating systems arranged to keep toxic fumes, gases, and dusts below the maximum allowable concentration in 1910.1000. The wording matters. The obligation is not “open a window” but a system designed and arranged to hold concentrations under the limit. A garage door open across the driveway is general ventilation in the loosest sense. A fume extractor with a hood that captures at the source is local exhaust, and that is what actually works.

The second half of the rule, 1910.252(c)(1)(iv), requires that potentially hazardous materials in fluxes, coatings, coverings, and filler metals be covered by a Hazard Communication program with labels and safety data sheets, and it carries the standard notice that adequate ventilation must be used. The SDS for your wire and the certification sheet for your gas belong in your safety file, not in a drawer. OSHA’s main welding, cutting, and brazing page collects these requirements in one place.

What Welding Fumes Actually Do to You

NIOSH describes welding fumes as composed of metals, most containing a small percentage of manganese. Prolonged exposure above 1 mg/m3 may cause manganism, a Parkinsonian syndrome whose symptoms resemble Parkinson’s but do not respond to the same treatment. NIOSH also notes that confined space welding significantly increases manganese exposure, because the fume has nowhere to go but into the welder’s breathing zone.

The exposure limits are worth knowing by heart:

• NIOSH REL: 1 mg/m3 (8-hour TWA) and 3 mg/m3 (short-term exposure limit)
• OSHA PEL: 5 mg/m3 ceiling concentration
• ACGIH TLV: 0.02 mg/m3 TWA for the respirable fraction

Note how far apart those numbers are. A respirable TLV of 0.02 mg/m3 is a demanding target, which is exactly why NIOSH’s welding fume research pushes capture at the source rather than relying on masks alone. The same resource documents welder’s anthrax, a rare but documented occupational disease among metalworkers involving dermal infection from contaminated fume, one more reason to wear long sleeves and gloves and to wash your hands before eating lunch at the bench.

Hexavalent Chromium

Welding stainless steel is not a neutral act for your lungs. OSHA states that hexavalent chromium is known to cause cancer and that a major source of worker exposure is hot work such as welding stainless steel and other alloy steels containing chromium. It targets the respiratory system, kidneys, liver, skin, and eyes. The OSHA hexavalent chromium page is worth reading before a stainless production run in an enclosed shop. If you TIG or MIG stainless regularly, ask about local exhaust filtration rated for chromium instead of settling for a fan pointed at a window. For aluminum work in a closed space, our guide to aluminum MIG welding covers filler and surface prep in detail.

Fire Prevention and the 35-Foot Rule

Hot work and flammable materials do not mix, and OSHA spells out the distances. Combustibles must be relocated at least 35 feet (10.7 m) where practicable, and floors must be swept clean for a 35-foot radius before you start. Anything that cannot be moved needs fire blankets or noncombustible covers, and a working extinguisher has to be within reach. A fire watch must be maintained for at least half an hour after welding finishes, which in practice means most shop work and all evening work when you are tired and ready to bolt.

A related rule catches people: OSHA 1910.252(a)(3) prohibits hot work on used drums, tanks, and containers until they are thoroughly cleaned, vented, and purged with inert gas. An “empty” drum that once held solvents can hold a flammable vapor pocket, and welding into it without purging is how workshops burn down. Purge and vent, and if you cannot, do not weld it.

Cylinder Safety

Shielding gas cylinders fall under 29 CFR 1910.253. Secure every cylinder upright with a chain or strap and keep cylinders away from heat, sparks, and open flames. Never use a cylinder as an electrical ground, and never let the regulator take a side load from a dropped cylinder. Separate oxygen from fuel gas by 20 feet or a barrier, close the valve when you finish, and cap the cylinder when it is empty. If you are buying gas for the first time, a supplier guide to air gas welding supply and cylinder safety is worth a read before the truck arrives.

Your First MIG Project: A Step-by-Step Plan and What Comes Next

Everything so far has been theory. Now comes the part where understanding meets hot metal. Your first MIG project should be small, expendable and chosen so a bad weld costs you a coupon instead of a bracket. A simple shelf bracket, a small square tube frame or a scrap weldment you can cut apart all work. The goal is not the object itself. The goal is to learn how a machine responds to your hand.

Match the Machine to the Metal

Lincoln Electric’s practical rule of thumb is a good place to start: a compact 115V MIG wire feeder welder suits clean, new steel indoors in roughly the 24 gauge to 12 gauge range, while material from about 1/8 inch up to 1/4 inch needs a higher-capacity 230V machine. That step up is not about mystique. It is about how much power is available to melt the joint and how fast the gun can feed wire. A 115V unit asked to weld 1/4-inch plate will spend the afternoon fighting burn-through and cold laps instead of teaching you anything.

Buy the wire alongside the machine, because drive rolls, contact tip and liner are matched to wire diameter and material. Set aside the right cylinder of shielding gas too. A welder that arrives without gas and consumables is not ready to weld.

Choose Your Wire and Gas

Start with .030-inch wire. It feeds smoothly, works from light-gauge material up through 1/4-inch plate and does not demand a large machine. On classification, Miller recommends ER70S-3 as an all-purpose mild steel classification, with ER70S-6 chosen when the extra deoxidizers in the wire help on dirty or rusty steel. Either will work on a clean first project, and ER70S-6 is the more forgiving of the two.

For gas, 75/25 argon and CO2 is the best all-round pick for carbon steel because it gives the cleanest bead and the least spatter. Pure CO2 is workable if cost is the deciding factor and you can tolerate more spatter and a rougher bead. Aluminum needs pure argon plus dedicated feeding hardware, because aluminum wire is soft and expands as it heats; our complete aluminum MIG guide walks through that setup in detail.

The Setup Sequence

1. Clean the metal back to bare metal. Grind, wire-wheel or sand every surface that will become part of the weld. Mill scale, paint, oil and rust all cause porosity, and no amount of extra voltage fixes contaminated steel.
2. Bevel the joint on thicker material. A 30 to 45 degree bevel lets the arc reach the bottom of the groove for full penetration instead of melting a shallow skin across a wide gap.
3. Clamp to clean metal. Attach the work clamp to bare, unpainted steel close to the joint. A clamp on mill scale is a ground connection to nothing, and you will chase phantom problems all afternoon.
4. Set and leak-test the gas. Set the regulator to 20 to 25 cubic feet per hour for mild steel, then check the hose and fittings with soapy water. Bubbles mean a leak, and a leak means porous welds.
5. Set drive roll and spool tension. The drive rolls should squeeze the wire firmly enough to feed without slipping, and the spool should turn freely with just enough drag to prevent over-running. Throw away any wire with rust on it.
6. Set wire feed speed first, then voltage. Start from the machine’s own chart if it has one. If it does not, run a trial on scrap of the same thickness and joint, adjust one parameter at a time and write down what worked.

Four Technique Details That Decide the Weld

Stick-out. ESAB gives the CTWD range as 10 to 15 mm for carbon steel solid wire and 12 to 20 mm for aluminum. Many shops run closer to 1/4 inch, roughly 6 mm, on thin material for finer control. Too long and you get spatter and a wandering arc; too short and the tip touches the work.

Travel angle. Hold a shallow 5 to 15 degree push angle and keep it consistent. Dragging the torch backward through the puddle opens a groove and invites undercut.

Travel speed. Steady and moderate. The goal is a bead width that matches the thickness of the joint, and only practice teaches your hand that rhythm.

Sound. Listen as you weld. Crackling short-circuit transfer sounds like frying bacon, while spray transfer hisses like steady rain. That shift in sound is one of the most reliable ways to know your parameters have moved into a different transfer mode.

Prove the Weld Before You Trust It

Run a test coupon on scrap of the same thickness and joint configuration, let it cool, cut across the weld, then break it. Inspect penetration and fusion instead of trusting appearance alone. A bead that looks like a textbook can still be cold at the root. A coupon you deliberately destroyed teaches you more in ten minutes than a finished frame that only looks right.

A Three-Week Practice Ladder

• Week 1: consistent flat beads. Run short passes on mild steel until width, ripple and standoff stop changing from one end of the coupon to the other.
• Week 2: stringing beads and weave control. Work on keeping the weave even without letting the edges melt into a wide puddle.
• Week 3: joints in position. Butt joints, lap joints and T-joints, flat and horizontal first, then vertical. Add fillets and a short fillet cap you have to grind smooth.

Budget for safety gear, ventilation and plenty of practice pieces. A helmet with the correct filter shade, leather gloves, a jacket and local exhaust or general ventilation are not optional. OSHA’s welding guidance covers what the rules require and how fume control is meant to work.

So what is MIG welding, once you have actually done it? MIG welding is metal inert gas welding, the trade name for GMAW, the gas metal arc welding process in which a continuously fed consumable wire electrode and an externally supplied shielding gas work together to lay down a molten pool that cools into a solid bead. That is the whole idea, and it is why the process earned the place it holds in nearly every shop, garage and fabrication line on earth. A machine you can learn in an afternoon will still surprise you in a year. Keep welding, keep coupons, and the surprises will start working for you instead of against you.

Frequently Asked Questions

What does MIG welding stand for?

MIG stands for Metal Inert Gas welding. It is a form of arc welding in which a continuously fed solid wire electrode both carries the current and melts into the weld pool, while a shielding gas protects the arc and molten metal from the air. The American Welding Society calls the same process GMAW, or Gas Metal Arc Welding, in its codes and standards.

Is MIG welding the same thing as GMAW?

Yes. GMAW is the formal technical name for the process and MIG is the trade name most people use at the bench. The only technical wrinkle is that when chemically active shielding gases are used, the process is properly called MAG, Metal Active Gas welding, though the equipment and the mechanics are identical.

What is the difference between MIG and MAG welding?

It comes down to the shielding gas. MIG uses inert gases such as pure argon or argon/helium mixtures and is the choice for aluminum, copper, and other non-ferrous metals. MAG uses active gases such as pure CO2 or argon/CO2 blends and is the workhorse for carbon and low-alloy steels.

Is MIG welding harder to learn than TIG or stick welding?

MIG is generally the easiest of the three to pick up because the wire feeds automatically and the arc is forgiving. Stick is a close second and is faster to set up. TIG has the steepest learning curve because the welder must manage amperage, filler addition, and arc length simultaneously rather than relying on preset parameters.

What shielding gas should I use for mild steel?

A 75% argon / 25% CO2 blend, often called 75/25 or C-25, is the best all-purpose choice because it produces the least spatter and the best bead appearance while resisting burn-through on thin metal. Pure CO2 is cheaper and penetrates more deeply but throws noticeably more spatter and leaves a rougher bead, which costs cleanup time.

Why is my MIG weld full of holes?

That is porosity, and the usual culprits are dirty or oily metal, moisture on the wire or in the gas line, or a leak in the shielding gas hose. Clean the workpiece to bare metal, keep the wire dry, replace moisture-contaminated hose, and verify the flow rate — typically 20 to 25 cubic feet per hour for mild steel.

Can you MIG weld aluminum and stainless steel?

Yes, with the right consumables. Aluminum needs pure argon, a spool gun or push-pull feeder, and hardware designed for soft wire, while stainless steel uses argon-rich blends and matching filler metal such as 308L for 304/304L base material. Neither is a difficult upgrade, but neither should be attempted with a setup intended for mild steel.

Can MIG welding be done outdoors?

It can, but wind is the enemy because the shielding gas is what protects the weld. A light breeze will pull air into the arc and cause porosity, so outdoor and field work is often better served by self-shielded flux-cored or stick welding. A windshield, a fabric tent, or a gas-flow adjustment helps, but shielding gas coverage is a genuine limitation of MIG.

Do MIG welds need slag removal or grinding?

No slag is produced at all, because solid MIG wire carries no flux — that is one of the process’s biggest practical advantages over stick welding. You will still want to chip or brush off spatter, especially between passes, so that it does not end up trapped inside the finished weld.

What thickness of metal can a MIG welder handle?

A compact 115V machine is generally comfortable on thin material from about 24 gauge up to about 12 gauge, while 1/8-inch to 1/4-inch steel calls for a 230V higher-capacity unit. Beyond that, or for thick sections welded out of position, flux-cored wire is usually the more productive and more forgiving choice.