Gas Metal Arc Welding Procedure: The Complete Guide

A fabrication shop in Ohio ran three MIG guns on the same aluminum trailer frame, and the frame that came back from the customer was rejected for porosity — the inspector asked for the procedure that produced it, and there wasn’t one. The fix was not a better machine; it was a written GMAW procedure telling every welder which wire, which gas, which settings, and which cleaning step to use. That gap between ‘the guy who knows how’ and ‘the procedure we all follow’ is exactly what this guide closes.

What GMAW Really Is, and Why a Written Procedure Changes Everything

Gas metal arc welding, or GMAW, is simpler than most people expect. A continuous wire fed from a spool acts as both the electrode and the filler metal. The arc is struck between the tip of that wire and the workpiece. A shielding gas, usually argon, carbon dioxide or a blend of the two, floods the area and keeps air out of the molten pool. That last detail carries more weight than beginners assume. Oxygen and nitrogen pulled into a hot weld pool react with the metal, and the result is oxides, porosity and a deposit that can look fine, pass visual inspection, and still be weak. Keeping the air out is the whole reason the gas exists, and it is the reason gas flow rate belongs on the procedure sheet.

One Arc, Three Names

The trade calls this process MIG in the United States and MAG in Europe. Both refer to the same arc. The regional names are simply shorthand for the shielding gas in use: inert (argon) in one case, active (CO2) in the other. AWS uses the neutral term GMAW because the gas can be inert or active depending on the base metal, and because the trade names invite confusion rather than clarity. If you have picked up a shop job using the phrase “MIG wire” or “MAG welding,” you now know what is actually happening. Our guide to aluminum MIG welding walks through the same process on the metal where gas purity matters most.

The broader family has three members, and AWS treats them as distinct processes with their own qualification rules. Solid-wire GMAW uses a bare wire and external gas. Gas-shielded flux-cored wire (FCAW-G) carries flux inside a tubular wire and adds a shielding gas on top. Self-shielded flux-cored wire (FCAW-S) draws its protection from the flux core itself and needs no cylinder at all. The arc looks the same. The rules, the filler classifications and the fume profile do not.

Why GMAW Out-Produces Stick

The reason GMAW dominates production and fabrication work is a single mechanical fact: the wire feeds continuously. A stick welder stops every couple of minutes to throw out a spent electrode, sweep the slag, and start again. A GMAW welder keeps feeding filler into the arc as fast as the machine will melt it. That is why deposition rate, the pounds or kilos of weld metal laid down per hour, is the primary reason shops choose it over stick. It is not about which weld is stronger. It is about how much steel you can put down before lunch, and how much of it goes into the part instead of into slag.

There is a second payoff. AIHA notes that SMAW (stick) generally carries the highest fume, hexavalent chromium and manganese emission rates of the common arc processes, while GMAW is far lower-emitting. Cleaner air and higher deposition in the same process is why stick welding still wins in the field but rarely wins inside a production shop.

The Recipe Problem

Now the part that matters. A GMAW procedure is not paperwork, and it is not a form you fill out to satisfy an auditor. It is a recipe. It says which wire, which gas, which flow rate, which amperage, which voltage, which wire feed speed, how fast to travel, how to hold the torch, and how to clean the joint beforehand. Its purpose is to make the result repeatable by someone who was not there the day the original weld was made.

Without one, every welder invents their own settings, and small differences compound fast. Picture the same steel bracket going out to three welders on one shift:

Welder	Wire	Shielding gas	What came off the bench
A	.035 solid	100% CO2	Deep, narrow, spattery bead; slight burn-through on the corner
B	.035 solid	75/25 argon/CO2	Wider, flatter bead, clean face, similar penetration
C	.045 solid	75/25 argon/CO2	Higher amperage, softer penetration, more heat into the part

Same drawing, same material, three different weldments. The inspector cannot call any of them wrong, because nothing in the paperwork says what the weld was supposed to look like. The code only cares whether the joint meets the strength requirement, and suddenly the shop is grinding and re-welding brackets to prove it. A written procedure would have collapsed all three into one answer before the arc was struck.

That is the entire argument of this guide. Read on and you will be able to read a GMAW procedure, write one from scratch for your own work, and audit one that someone else wrote.

I’ll rewrite this section, tightening the prose and adding useful detail while keeping every fact and link intact.

The Documents Behind the Procedure: WPS, PQR, and Essential Variables

A procedure is only credible if somebody can prove it works. That proof lives in two documents, and confusing them is a common mistake in small fabrication shops. The Welding Procedure Specification (WPS) is the recipe: wire, gas, settings, positions, cleaning steps. The Procedure Qualification Record (PQR) is the evidence: test results showing a welder, using those parameters on that joint, produced welds that met the code. The WPS tells you what to do. The PQR tells an inspector you may do it.

Without a PQR, a WPS is a wish. Without a WPS, a PQR proves something nobody can repeat.

Prequalified vs. Qualified: What AWS D1.1 Actually Requires

AWS D1.1, the Structural Welding Code for steel, splits the world into two camps in Clause 5. Procedures listed as prequalified are those AWS has already blessed, provided you stay inside the stated ranges for base metal, thickness, joint design, filler metal, position, and preheat. Inside those limits no qualification testing is required. Outside them, the procedure must be qualified by testing: build test coupons, have a qualified welder make them, and the results become the PQR.

The distinction matters commercially. Qualification testing costs money in coupons, destructive testing, and lab fees. Prequalification costs nothing but discipline. Which side of the line your job sits on is an estimator’s first question.

The GMAW-S Wrinkle: Short-Circuit Is Not Prequalified

Here is the wrinkle that catches shops out. In D1.1, GMAW is prequalified largely through spray-type transfer with the specified argon blends. GMAW-S, the short-circuit transfer designation used in Annex K, is not a prequalified process under D1.1. The reason is worth remembering: short-circuit mode can generate lack of fusion, and lack of fusion is a structural defect that cannot be tolerated in a prequalified procedure without qualification testing to contain it. See welding industry discussion of short-circuit MIG prequalification in AWS welding codes.

Sheet shops have a route. AWS D1.3, the aluminum building and bridge code, provides in Clause 3 a path to prequalified short-circuit procedures for sheet aluminum, with its own essential variable ranges. For aluminum sheet, aluminum MIG welding follows different rules than steel plate, and the prequalified route is more forgiving. Everything else needs a qualified PQR.

Preheat and Interpass: The Variable That Trips People Up

Preheat and interpass temperature are essential variables, and they are where hand-waving gets punished. In D1.1, Table 5.8, titled “Prequalified Minimum Preheat and Interpass Temperature,” is cited from Clause 5.7 and sets minimums based on the carbon equivalent of the base metal and the thickness range being welded.

But Table 5.8 is not the only way. Annex B, “Guidelines on Alternative Methods for Determining Preheat,” is a normative annex with two methods that calculate preheat from steel composition, thickness, filler metal diffusible hydrogen content, and joint restraint. Clause 6.8 (Essential Variables), specifically Subclause 6.8.4, guides when Annex B or other alternative methods may replace Table 5.8.

For higher-strength or higher-restraint joints, AWS D14.8M (ISO/TR 17844:2004 IDT) is the newer reference. It gives four methods to determine the preheat needed to avoid cold cracking, and it applies to SMAW, self-shielded FCAW, gas-shielded FCAW, GMAW, and SAW. It works off filler metal diffusible hydrogen levels, classified H4, H8, and H16 (3, 8, and 16 mL of hydrogen per 100 grams of deposited weld metal), running from 3 up through over 15 mL per 100 g for the highest hydrogen consumables.

One more rule catches experienced welders out. Preheat lookups are driven by the combined thickness of all parts joined by the weld, not the thickest member alone. A 1-inch plate welded to a 1/4-inch flange is a combined 1-1/4 inch joint for preheat purposes. Get this wrong on a restrained assembly and you may hydrogen-crack a joint that looked easy.

What Belongs on a GMAW WPS Sheet

Your WPS should be readable by someone who has never welded the job. Check that it carries:

• Base metal specification, grade, and thickness range
• Joint design: groove type, angle, root face, root opening, backing
• Filler metal classification and diameter, plus shielding gas type and flow rate
• Transfer mode and qualified positions
• Amperage, voltage, wire feed speed, and travel speed
• Electrode extension (stickout) and torch angle or work angle
• Preheat and interpass temperature limits
• Cleaning method, including chemical or mechanical prep
• Technique notes: stringer or weave, push or pull, weave width and frequency
• Inspection and acceptance criteria

If a line on that list is missing, it is a variable someone will invent on the shop floor. That is how three welders end up with three different settings on the same bracket, and how a frame comes back rejected for porosity. For comparison, see why SMAW still wins in certain applications.

Choosing Filler Metal and Shielding Gas: The Chemistry Half of the Procedure

Two decisions drive almost everything after you strike an arc: what you feed into the puddle and what you surround it with. Get both right and the machine settings in your procedure have an easy job. Get either wrong and you can lose a shift to spatter, porosity, or a bead that fails inspection.

The Standards That Govern Your Consumables

Every filler metal carries a classification number on the spool, and each family of base metals has its own AWS specification. For carbon steel, AWS A5.18 covers GMAW electrodes and rods, and the workhorse sitting in every wire stand is ER70S-6. Stainless and nickel alloys fall under AWS A5.9, where ER308L is standard for 304 and 316L work. Aluminum is covered by AWS A5.28, with ER4043 and ER5356 appearing on nearly every aluminum job.

Those four characters are worth decoding. In ER70S-6, ER identifies a filler metal rather than a consumable electrode. The 70 is minimum tensile strength in ksi, so 70 ksi is roughly 70,000 psi. The S means solid wire, as opposed to T for tubular flux-cored. The trailing 6 identifies deposit chemistry and the level of deoxidizers, which is why ER70S-6 is a good general-purpose wire while ER70S-3, with less silicon, behaves differently in the puddle.

Matching Filler Strength to the Base Metal

The general rule is simple: pick filler at least as strong as the base metal. A 60 ksi base welded with 70 ksi filler gives extra margin, while the reverse can leave you under the code minimum. Codes formalize this through matching filler tables, so check what your governing specification allows for the joint and the loading before choosing anything. Where the code permits two or three classifications, the decision usually comes down to ductility, corrosion behavior, or how the deposit machines.

Aluminum shows why matching is not only a strength question. ER5356 is a silicon-magnesium alloy, stronger than ER4043, and it gives up strength faster at elevated temperature, which is why it dominates structural work. ER4043 flows better, has a wider operating window, and is the filler most often used on thin sheet and castings. Our Aluminum MIG Welding: The Complete Practical Guide breaks down both in detail.

Shielding Gas: What It Does to the Weld

Gas is not a comfort feature. It shields the molten pool from nitrogen, oxygen, and hydrogen in the air, and it dictates transfer mode, bead shape, spatter level, and fume generation. For carbon steel, 75/25 and 90/10 argon/CO2 blends are the standard general-purpose choices, giving a stable arc, a good bead, and manageable spatter. 100% CO2 digs deepest and is cheap, but it throws noticeably more spatter and runs a less stable arc at low amperage, so it belongs on heavier fabrication where penetration matters more than finish.

Stainless steel prefers 85/15 argon/CO2 for general work, with argon/helium blends and tri-mix (a small hydrogen addition) for higher deposition rates and control. Aluminum runs 100% argon or an argon/helium blend for a chemical reason, not a cosmetic one: the gas cannot oxidize molten aluminum, and pure argon keeps the arc stable on a metal that conducts heat far better than steel. Adding a heavier gas such as helium changes the arc characteristics, one of the tools used for alloy wheel repair.

Gas choice also shifts your fume profile. Shielding aluminum with pure argon can raise fume levels compared with a CO2 blend, so ventilation planning has to follow the gas you actually run, not a generic assumption.

Polarity, Stickout, and Wire Size

Polarity is a fixed decision for most shops. Carbon and stainless steel GMAW run DCEP (direct current electrode positive). Aluminum GMAW runs DCEN (direct current electrode negative), where cathodic cleaning helps break up the stubborn oxide layer on aluminum.

Electrode extension, the stickout beyond the contact tip, runs about 3/8 to 1/2 inch (10 to 13 mm) for steel. Aluminum uses a longer stickout to help control the arc and cut spatter, and this is a setting worth stating on the WPS rather than leaving to habit.

Wire diameter has to fit the amperage you intend to run. A wire too heavy for the machine causes stubbing and excess spatter.

Wire diameter	Typical amperage range
.023 in	30 to 130 A
.030 in	40 to 145 A
.035 in	50 to 180 A
.045 in	75 to 250 A

Gas Flow and Cylinder Handling

Set flow at the torch, not at the regulator, and measure it with a flowmeter or flow gauge. For GMAW, roughly 20 to 30 cfm at the torch is the normal band, and flux-cored work wants more because it has to displace more air. Too little shows up as porosity; too much creates turbulence around the arc that can disturb the pool and increase fume. Our AllGas Welding Supply guide covers cylinder options and supplier comparisons.

Common cylinder sizes are 40, 250, and 300 cubic feet. Keep cylinders upright, chained, and secured so a passing forklift cannot topple them. An unsecured cylinder is both a dropped-object hazard and a code violation under the welding rules in 29 CFR 1910.254.

Setting the Machine: Amperage, Voltage, Wire Feed Speed, and Heat Input

Everything upstream of the arc can be perfect and still produce a bad weld if the machine settings are off. Settings are also the easiest part of a gas metal arc welding procedure to write down and the easiest to quietly change on the fly. Get them onto paper, recorded as the welder actually ran them.

The Three Controls and What They Physically Do

Amperage controls how fast the wire melts and how hard the arc pushes into the puddle. More amperage means more filler per unit of travel, a wider bead and deeper penetration. Too little and the wire barely melts, the puddle cools between passes, and the bead comes out cold and sluggish.

Voltage on a constant-voltage machine is essentially a command for arc length. Push it up and the arc stretches longer, spreading heat across a wider, flatter bead. Wire feed speed then drives amperage: crank the feed and the power source pushes more current to keep melting wire at that rate, raising amperage and penetration. That chain reaction is why a slipping feeder drive suddenly gives you a cold weld.

Modern machines reverse the logic. On synergic and pulsed setups the operator picks material, wire, gas and thickness, and the machine manages amperage and voltage to hold a constant arc length. Miller’s parameter-setting walkthrough covers how those controls interact.

Starting Points: Thickness and Wire Feed

A common shop starting rule is roughly 1 amp of output per 0.001 inch of material thickness. That puts 0.125 inch (1/8 inch) steel at about 125 amps and 0.250 inch steel at about 250 amps. Treat it as a starting point, not a law: joint design, welding position, wire diameter and travel speed all move it.

Wire diameter	Starting multiplier	Example at 125 A
.023 in	3.5 in per amp	about 440 ipm
.030 in	2 in per amp	about 250 ipm
.035 in	1.6 in per amp	about 200 ipm
.045 in	1 in per amp	about 125 ipm

Reading the Bead to Tune Voltage

• Voltage too high: a wide, flat bead, often with undercut along the toes, excess spatter and more fume. The arc is long and the shield is stretched thin.
• Voltage too low: a stubbing, popping, erratic arc, a narrow convex bead sitting on the surface, and poor fusion into the joint.

Convex and humped means raise voltage slightly before touching anything else. Undercut and flying spatter mean back the voltage down.

Stickout, Work Distance, and Push Versus Pull

Electrode extension, the wire sticking out past the nozzle, is usually 3/8 to 1/2 inch for steel and longer for aluminum. More extension adds resistance in the wire, so the arc runs cooler and softer with a broader bead and slightly less penetration. Less extension gives a tighter, more focused arc that digs deeper. That is why stickout belongs on the WPS rather than left to habit.

Nozzle-to-work distance stays around 1/2 inch. Pushing the gun (travel, or “gun leading”) gives better penetration and a wider bead but puts the arc where you cannot see it. Pulling the gun (drag) gives a narrower, more controllable bead with better visibility, at the cost of some penetration. ESAB’s machine settings guide walks through the tradeoffs.

Polarity

Steel, stainless and most flux-cored work run DCEP (direct current electrode positive). Aluminum and magnesium run DCEN, where the torch is negative and the work is positive. Polarity shifts heat distribution at the work and the wire, so it is never a “try it and see” adjustment. More on aluminum setups is in our aluminum MIG welding guide.

Heat Input and Why It Belongs on the WPS

Heat input is the single number that ties machine settings to weld quality, distortion and preheat requirements. The common shop calculation is:

kJ/mm = (voltage × amperage × 6) ÷ travel speed in mm/min
The inch-based equivalent is roughly (V × A × 14.3) ÷ ipm.

High heat input means more distortion, coarser heat-affected zone grain growth, more hydrogen pickup and a greater chance of lamellar tearing in thick or restrained joints. Low heat input means risk of lack of fusion and cold cracking in hardenable steels. Since the value shifts with material thickness, heat input belongs in the written gas metal arc welding procedure as a calculated range. For code acceptance, use the arc energy calculation in AWS A3.1 with the applicable arc efficiency rather than a shop shortcut.

Modern Machine Features and Why You Log the Real Numbers

Synergic and programmable parameter sets, pulse GMAW and CMT-style controlled metal transfer have moved the skill from knob-turning to mode selection. Pulse gives spray-like bead quality at lower amperage, so it works out of position and on thinner material. Controlled metal transfer manages the drop transfer cycle to keep heat low while deposition stays high. They only work if the actual parameters get recorded. Have the welder log amperage, voltage, wire feed speed, travel speed and stickout on a traveler for each joint. A procedure verified against remembered settings is not verified at all.

Preliminary Note: Preheat and Interpass Temperature

Preheat and interpass temperature are machine-setting companions, not paperwork leftovers. AWS D1.1 Table 5.8, cited in Clause 5.7, sets prequalified minimum preheat and interpass temperatures, and Annex B offers alternative methods based on steel composition, thickness, diffusible hydrogen and joint restraint. Write the required minimum preheat and maximum interpass temperature into the WPS as explicit numbers with a thermometer reference point. Running cooler than Table 5.8 puts you back into qualification testing territory.

Transfer Modes: Short-Circuit, Globular, Spray, and Pulse

Before any parameters are tuned, it’s important to understand one of the most influential parts of GMAW: the transfer mode. Put simply, a transfer mode is how the molten filler metal crosses the arc from the consumable wire to the weld puddle. The way that metal is transferred dictates heat input, bead shape, spatter, deposition rate, and even the risk of defects. For this reason, mode selection isn’t a matter of preference, it’s something that needs to be documented within a procedure.

Short-Circuit Transfer

Short-circuit transfer, often referred to as GMAW-S, is the most common mode for thin material. In this cycle, the wire physically dips into the molten puddle, extinguishing the arc. The puddle briefly cools, the wire continues to feed forward, and the arc re-strikes, repeating this process hundreds of times per second.

This rapid cycling is what gives short-circuit transfer its biggest strength: heat control. It produces minimal warping, allows for easy puddle control, and can be used with smaller wire diameters on light gauge steel. However, this low heat input comes at a cost. Due to its tendency to not fully fuse into the side walls of a joint, short-circuit transfer carries the highest risk for lack of fusion out of all of the modes.

Because of this exact risk, GMAW-S is excluded from prequalification under AWS D1.1. If your procedure plans to utilize short-circuit transfer for code work, it cannot be assumed prequalified and must be accounted for through qualification.

Globular Transfer

As amperage increases past the short-circuit range, the mode transitions into globular transfer. Instead of dipping into the puddle, the wire forms much larger droplets of molten metal that fall across the arc under gravity. These droplets are often larger than the diameter of the wire itself.

Globular transfer is most often seen in the mid-thickness range, however, it’s rarely the “go to” choice for a clean procedure. The arc is less stable, spatter levels rise drastically, and bead control can be inconsistent compared to its counterparts. With that being said, some older machines without the capability for spray or pulse may run in this range, making awareness of its drawbacks vital for troubleshooting.

Spray Transfer

Spray transfer is the mode of choice when speed, penetration, and a clean appearance are the priority. With the aid of an argon-rich shielding gas, typically 85–90% argon blends, the magnetic pinch force of the arc propels tiny droplets across the arc. These droplets are smaller than the wire diameter, creating a smooth, axial stream of metal.

This results in a wide, flat bead, minimal spatter, the highest deposition rate out of the non-pulsed modes, and essentially no burn-through risk on appropriately sized joints. The tradeoff is heat. Spray transfer requires higher amperage and voltage to achieve, making it best suited for thicker material in the flat and horizontal positions. For applications involving lighter, non-ferrous materials, processes such as those outlined within the Aluminum MIG Welding: The Complete Practical Guide often utilize spray-like transfer characteristics with the proper gas and equipment.

Pulsed GMAW: The Best of Both Worlds

Pulsed GMAW was developed to solve the classic tradeoff between spray quality and low-heat control. Rather than a constant stream of amperage, a pulsed power source alternates between a high “peak” current to detach a single, spray-sized droplet, and a low “background” current to maintain the arc without overheating the puddle.

This allows welders to obtain spray-like bead characteristics at a much lower overall amperage. The result is far less heat input, allowing for out-of-position welding in vertical and overhead, better control on thinner sections, reduced distortion, and a drastic drop in spatter. For shops looking to maximize versatility without sacrificing appearance, pulse is often the ideal balance.

Choosing the Right Mode By Thickness

Transfer Mode	Ideal Thickness Range (Mild Steel)	Key Traits
Short-Circuit (GMAW-S)	24ga – 3/16″ (0.7mm – 4.8mm)	Great heat control, out of position capable, highest lack of fusion risk.
Globular	3/16″ – 1/4″ (4.8mm – 6.4mm)	High spatter, unstable arc, generally avoided in modern procedures.
Spray	1/4″ and up (6.4mm+)	Cleanest non-pulsed bead, highest deposition, flat/horizontal only.
Pulsed GMAW	Thin gauge – heavy plate (versatile)	Spray-like quality at low amperage, all-position capable with minimal spatter.

At the end of the day, the transfer mode is the bridge between your machine settings and the quality of your puddle. Picking the correct one for your thickness, position, and code requirements is one of the simplest ways to prevent the exact defects that would otherwise have an inspector asking for a procedure to begin with.

Joint Preparation, Fit-Up, Cleanliness, and Welding Position

Most GMAW defects are decided before the arc is ever struck. Joint geometry, root opening, tack quality, surface condition, and welding position set the conditions for penetration, fusion, and distortion. When a weld looks like a machine problem, it is often a fit-up problem.

Joint Design and Edge Preparation

Most GMAW joints are butts, either square or V-groove. A square butt works only on thin material, where the plates are close enough in thickness and alignment for a short root pass to tie them together without much filler. As material gets heavier, edge preparation becomes necessary so the root pass has somewhere to go.

Three variables define a prepared groove: groove angle, root face (or land), and root opening. A wider angle cuts the metal that must be melted, lowering weld volume and heat input. A root face supports the molten pool at the bottom of the joint and controls penetration and melt-through. Root opening lets the root pass reach the bottom of the joint, and its size is a procedure variable, not a shop-floor guess.

This is where the governing code draws a useful line. AWS D1.1 applies to material 1/8 inch (3 mm) and thicker, so structural joint design usually starts with a V-groove rather than a square edge. Thinner material belongs under AWS D1.3, with different joint details and prequalified short-circuit procedures.

Fit-Up and Tacks

Fit-up is the physical positioning of parts before welding, and it controls how much heat the root pass sees. Poor fit-up, meaning a wide root gap, high or low mismatch, and out-of-square plates, forces the root pass to stretch to bridge uneven gaps. The result is burn-through on the tight side, lack of fusion on the open side, and inconsistent penetration. Clamp and stitch the parts, then check the root opening with a gauge rather than by eye.

Tack welds get treated carelessly, which is why they cause so much rework. AWS D1.1 requires tacks to use the same welding process and consumables as the production weld, and requires tacks left in the finished weld to be ground flush or removed. A carbon steel tack in a stainless weld, or a flux-cored tack in a GMAW joint, leaves contamination or an unfused lump behind. Position tacks where the production pass will remelt them, then grind them flush before you continue.

Cleanliness and Cross-Contamination

Grind or wire-brush every joint face back to bright metal, then degrease. Mill scale, rust, paint, and oil all prevent fusion, and a dirty joint is the single most common root cause of a lack-of-fusion callout.

The subtler hazard is cross-contamination. A carbon steel wire brush, or a grinding wheel that has touched carbon steel, used on stainless reintroduces iron particles that rust and streak the finished weld. On aluminum, the same contamination is a leading cause of porosity. Keep dedicated brushes and dedicated grinding tools for stainless and aluminum work. Our guide to prepping aluminum for welding covers the specific cleaning products and methods, and the aluminum MIG welding guide goes deeper on joint detail.

Aluminum deserves special attention because its oxide layer melts at a far higher temperature than the metal itself. The oxide does not melt away into the bead; it has to be removed mechanically. A cleaned surface must be welded promptly, because the oxide re-forms. OSHA’s welding safety fact sheet is worth a read as well, since grinding and cleaning stainless generate hazardous dust in addition to welding fume.

Welding Position, Bead Sequence, and Distortion

GMAW is strongest flat and horizontal, where the arc is stable, the puddle is easy to see, and gravity works with you. Vertical-up is normally run in short-circuit transfer with a forehand (push) technique, dragging the torch up the joint at a work angle of roughly 60 to 75 degrees. Short-circuit gives precise heat control, but it is also the mode most associated with lack of fusion, the concern that keeps GMAW-S out of D1.1 prequalification, as The Fabricator explains.

Overhead is the most restrictive position. Many shops redesign the joint or switch to FCAW rather than fight it, and for aluminum, aluminum flux core welding is a common answer.

Finally, think about bead sequence. Stringer beads, one pass at a time, are easier to control than weave beads, which can undercut or entrap oxide if the weave is too wide. Where the first pass sits changes how the joint pulls. A procedure that specifies the sequence, the number of passes, and which side you weld first controls distortion better than any amount of post-weld straightening.

Safety: Fume Control, PPE, and the OSHA Rules That Apply

Safety should never be an afterthought in a GMAW procedure. The gap between a safe shop and a preventable incident usually comes down to whether the required controls are written into the procedure and followed on the floor. For gas metal arc welding, the hazards are fumes, fire, and electrical shock, and all of them sit under enforceable standards.

Ventilation: The First Line of Defense

Fumes are the most consistent hazard for any GMAW operator. Under 29 CFR 1910.252, general ventilation must be provided at a minimum of 2,000 cubic feet per minute per welder unless local exhaust hoods and booths under paragraph (c)(3), or airline respirators, are provided. That is a baseline requirement, not a recommendation.

For indoor or confined-space work involving lead, cadmium, beryllium, or mercury-bearing materials, local exhaust ventilation or airline respirators are required with no general-ventilation exception. Exposure sampling must be taken inside the welding helmet within 50 mm of the breathing-zone centerline; lapel sampling does not reliably predict a welder’s actual exposure.

Source reduction is worth weighing too. Per the American Industrial Hygiene Association, some GMAW choices have hexavalent chromium emission rates as little as 1/250th of SMAW. Process selection is a control measure in its own right, especially with stainless steel filler metals.

Hexavalent Chromium: Know the Numbers

Cr(VI) is one of the most tightly regulated welding fume constituents. Per OSHA 1910.1026, the permissible exposure limit for chromium (VI) is 5 µg/m³ as an 8-hour time-weighted average, with an action level of 2.5 µg/m³. Employers must apply engineering and work practice controls first, supplement them with respiratory protection meeting 1910.134 when controls fall short, and offer medical surveillance to employees exposed at or above the action level for 30 or more days a year.

Chromium (VI) Exposure Limit	Value
OSHA PEL (8-hour TWA)	5 µg/m³
OSHA action level	2.5 µg/m³
NIOSH recommended exposure limit	0.2 µg/m³

The NIOSH recommended exposure limit for hexavalent chromium is 0.2 µg/m³, and the AIHA notes that even the OSHA PEL is very difficult to achieve. That gap is exactly why a procedure must lean on engineering controls before treating respirators as the default.

Fire Prevention

Welding sparks travel much farther than the operator expects, so fire prevention belongs in the procedure. Where appreciable combustible material is within 35 feet of the work, a fire watch is required, equipped with proper extinguishing equipment and trained to use it, and maintained for at least half an hour after welding stops. Smoldering debris, rags, and dust can ignite long after the arc goes out, so that half-hour requirement should never be trimmed.

Personal Protective Equipment (PPE)

Engineering controls come first, but PPE is mandatory for every GMAW operation. Per 29 CFR 1910.252(b)(2)(i)(A), helmets or hand shields must be used during all arc welding and arc cutting operations. A procedural PPE list should include:

• Eye and face protection: a properly rated welding helmet with respirable safety glasses worn underneath for impact protection.
• Flame-resistant clothing: long-sleeved FR jackets and pants; avoid synthetics that melt and stick to skin.
• Gloves: dry, flame-resistant welding gloves suited to GMAW.
• Hearing protection: easy to overlook on MIG, but necessary in loud shops or when grinding nearby.
• Foot protection: closed-toe leather boots with no open laces or mesh exposed to sparks.

Welding screens must also shield workers adjacent to the area. Use booths painted with a low-reflectivity finish such as zinc oxide or lamp black, with screens mounted about 2 feet above the floor to contain spatter without creating glare.

Electrical Safety and Cylinder Storage

Electric shock is an ever-present risk, especially on damp floors or in tight, confined work areas. 29 CFR 1910.254 requires that the frame or case of the welding machine be grounded and that all ground connections be checked for mechanical strength and electrical adequacy. Inspect electrode holders, work clamps, and cables daily for cuts, exposed conductors, or loose connections before energizing the machine.

Cylinder safety for shielding gas is just as important. Cylinders must stay upright, chained or otherwise secured to a stable structure, away from direct heat, and in a well-ventilated area. Check hoses and regulators for leaks, and segregate incompatible gases such as oxygen and fuel gases by the required clearance. Quick checks, yet among the most commonly missed on a shop-floor inspection.

Make Safety a Part of the Procedure

Handing a welder PPE and wishing them luck is not a control. Ventilation type, the need for local exhaust, Cr(VI) precautions, fire watch duration, required PPE, machine grounding checks, and cylinder storage should all be listed where they apply. Written down, safety becomes auditable, repeatable, and enforceable. A solid GMAW procedure does not just produce consistent welds; it sends people home the same way they came in.

Common Defects and How to Diagnose Them at the Bench

With GMAW, your bead will tell you what’s wrong before any inspection tool will. Instead of guessing, read its profile, toes, and arc behavior. More often than not, defects can be traced back to gas coverage, heat input, prep, or fit-up and corrected on the spot.

Porosity: Trapped Gas

Porosity is made up of tiny gas pockets throughout the weld. While it can be missed on the surface, it’s almost always a shielding gas issue. Low flow, leaking fittings, moisture, shop drafts, contaminated wire, or mill scale can all allow atmospheric gases into the puddle. For aluminum, the naturally occurring aluminum oxide is an especially common culprit.

Fix: Verify your gas flow and check for leaks, purge your gas lines, use fresh wire, and thoroughly clean the joint. For oxidized aluminum stock, using a proper aluminum cleaner for welding can go a long way in preventing it.

Undercut and Lack of Fusion: Structural Concerns

Undercut is a groove melted into the base metal at the toe of the weld, leaving a stress riser. It’s most often caused by excessive voltage, too much travel speed, too little filler metal, or an incorrect torch angle.

Lack of fusion is far more severe. This occurs when the weld metal fails to bond to the base metal or a previous pass. Low heat input, poor fit-up, surface contamination, or using GMAW-S (short-circuit transfer) on material that’s too thick are all common causes.

Fix: For undercut, lower your voltage, slow your travel speed, aim your torch into the joint, and use a slight forehand weave to deposit more filler into the toes. For lack of fusion, increase heat input, correct your fit-up and cleaning, and reassess if short-circuit transfer is appropriate for the thickness. For heat sensitive alloys, considerations from our guide on aluminum MIG welding can help narrow down the right approach.

Spatter, Burn-Through, and Bead Shape Issues

Excessive Spatter: BB sized droplets are usually tied to an unstable arc. High voltage, 100% CO2, an oversized wire for your amperage, wrong polarity, excessive electrode extension, or a dirty gas nozzle are all worth checking first.

Burn-Through: On thin gauge material, too much heat will blow straight through the root. Combat this by dropping your amperage, utilizing pulsed or short-circuit transfer, or adding a copper backing bar or heat sink to draw heat away.

Convex Beads and Humping: A rope-like, overly convex bead is usually caused by too little travel speed paired with excessive wire feed speed. Humping, on the other hand, creates ridges down the bead when the puddle solidifies faster than filler can be worked into it. The fix for both is to increase travel speed while keeping your wire feed and voltage proportionate to one another.

Cold Lap and Excessive Reinforcement: Cold lap occurs when filler rolls over unfused base metal, typically from low heat input or an improper work angle. Excessive reinforcement is almost always a fit-up issue, caused by joint mismatch, poor tacking, or over-depositing filler with little regard for the required profile. Both are best corrected by improving joint alignment and driving the arc into the base metal with adequate heat input.

Bench-Side Diagnosis at a Glance

Before chasing numbers, build a checklist. Clean your joint, confirm your wire diameter for your amperage, check gas coverage for drafts, and keep your wire feed and voltage in balance. On a constant voltage machine, wire feed drives amperage, while voltage dictates arc length. Small changes to either will shift the puddle more than you may expect.

Symptom	Likely Cause	First Adjustment To Try
Porosity (pinholes)	Low gas flow, leaks, moisture, drafts, rusty wire, or aluminum oxide	Verify gas flow and fittings, purge lines, clean the joint, and use fresh wire
Undercut at toes	Excessive voltage, too fast travel, too little filler, or improper torch angle	Lower voltage, slow travel, aim into the joint, and add a slight forehand weave
Lack of fusion	Low heat input, poor fit-up, contamination, or GMAW-S on thick material	Increase heat input, correct prep and fit-up, and reassess transfer mode
Excessive spatter	High voltage, 100% CO2, oversized wire, wrong polarity, or excessive stickout	Lower voltage, shorten stickout, verify polarity, and reassess shielding gas
Burn-through	Excessive heat on thin gauge material	Drop amperage, use pulsed or short-circuit transfer, or add a copper heat sink
Humping or convex bead	Too slow of travel with excessive wire feed speed	Increase travel speed while proportionally matching wire feed and voltage
Cold lap	Low heat input or improper work angle	Increase heat input and direct the arc into the base metal
Excessive reinforcement/mismatch	Poor fit-up or over-deposition	Improve joint alignment and control filler with consistent travel speed

The key isn’t memorizing every number, it’s building the habit of reading your puddle, your bead, andyour surroundings. If you do that consistently, your procedure won’t just be on paper anymore, it’ll be etched into every single pass you lay down.

GMAW Compared With SMAW, FCAW, and GTAW

There is no best welding process, only the best fit for the joint in front of you, the shop you work in, and the code you must satisfy. Reaching for MIG on everything gets a fabricator burned on dirty repair work. The useful skill is knowing which process earns its place on a job, and why.

SMAW (Stick Welding): Survives the Worst Conditions

Shielded metal arc welding needs no external shielding gas. The flux on the electrode does the protecting, which is why stick is the default for field erection, structural repair, and any job where a gas cylinder is impractical. It also tolerates rust, scale, mill slag, and paint better than any other arc process, because the flux cleans the puddle as it works.

The trade-offs are speed and fume. Stick runs slower than GMAW, and the electrode changes at roughly 15 to 20 minutes of arc time, turning a production weld into a series of restarts. Fumes are the bigger issue: AIHA notes that SMAW generally produces the highest emission rates of total fume, hexavalent chromium, and manganese among common arc welding processes. That is why stick wins in the field and loses indoors under a hood. Advantages of stick welding makes the full case for the process.

FCAW: Deposition Rate and Dirt Tolerance

Flux-cored wire comes in two families that behave very differently at the bench.

FCAW-S, self-shielded wire requires no external shielding gas, so it works anywhere a stick electrode does, but faster and at a far higher deposition rate. It tolerates poor surface preparation, which makes it standard for out-of-position structural work where cylinders and grinding are impractical. The cost is slag, more fume, and a rougher bead that needs cleanup. On aluminum the question changes again: flux core is a genuinely different material problem. Our guide to aluminum flux core welding covers where it works and where it fights you.

FCAW-G, gas-shielded wire pairs the flux chemistry of the core with an external shielding gas. That combination delivers deep penetration and high deposition rates, and it is frequently specified for fillet welds that must meet structural code requirements. It is the workhorse for out-of-position work where you want FCAW deposit rates but the slag-free handling of GMAW.

GTAW (TIG): The Cleanest Weld You Can Make

GTAW gives you direct control over amperage, travel speed, and filler addition, which makes it the cleanest and most controllable weld in the group. With no filler metal needed for thin sections, it produces the best cosmetic results in the industry, and it is the process of choice for critical stainless and aluminum work.

It is also the slowest and the most skill-intensive. GTAW is very sensitive to surface cleanliness and amperage control, so prep time eats into any speed advantage. It earns its keep on thin material, root passes, and stainless or aluminum work where appearance and corrosion performance matter more than tonnage. Aerospace TIG welding shows the discipline the process demands.

Side-by-Side Comparison

Criterion	GMAW (MIG/MAG)	SMAW (Stick)	FCAW	GTAW (TIG)
Deposition rate	High, best of the clean processes	Low	Highest	Lowest
Fume level	Low to moderate	Highest	Moderate to high, more slag	Low
Positioning ability	Good with pulse and short-circuit	Good	Excellent, especially FCAW-S	Fair, best flat and horizontal
Surface tolerance	Poor to fair, needs cleaning	Excellent	FCAW-S excellent, FCAW-G fair	Poor, must be clean
Equipment cost	Moderate, includes gas and wire	Low, no gas system	Moderate, FCAW-S needs no gas	Highest, plus torch and rod
Typical applications	Production sheet metal, general fabrication	Field repair, structural, dirty base metal	Structural fillets, field erection, heavy steel	Thin stainless and aluminum, root passes, critical work

Which Process Should You Reach For?

• Field erection with no gas: FCAW-S or SMAW. Self-shielded flux core gives you the speed advantage without a shielding gas supply.
• Stainless food-grade or sanitary work: GTAW for the root and any cosmetic pass, GMAW with a tri-mix or 85/15 blend for fill passes. Backing gas and full cleaning discipline are non-negotiable.
• Thick structural steel: GMAW or FCAW-G for the bulk deposit, SMAW for out-of-position or confined work. Never hand a structural fillet to a process your code does not permit.
• Thin aluminum: GMAW with 100% argon and DCEN. Read our aluminum MIG welding guide before you dial in settings, and clean the metal properly first.
• Repair on dirty or unknown base metal: SMAW or FCAW-S. Both tolerate scale and rust, but expect stick-level fume rates, so ventilation and PPE must match. Spark test unknown base metal first.

On fume: if you are switching a job away from stick to reduce emissions, check local exhaust and respiratory protection requirements against OSHA’s hexavalent chromium standard. Changing the process is the best control, but the controls still have to be written into the procedure.

Building, Rolling Out, and Auditing Your Own GMAW Procedure

Going back to the Ohio shop from the beginning, the fix was never a new welder or a new machine. It was a written plan anyone could follow without guessing. That is the point of building your own gas metal arc welding procedure: take everything in this guide and turn it into a repeatable system. Here’s a practical workflow for doing it right.

1. Define the Job Envelope

Before touching a coupon, lock down the scope of work: base metal grade and specification, thickness range, joint design and preparation, welding position, applicable code, and required inspection and acceptance criteria. These set the boundaries for everything that follows and tell you whether you are working inside a prequalified scope or need qualification testing.

2. Select Consumables and Gas

With the envelope set, match your filler metal and shielding gas to the base metal and the code. For carbon steel, AWS A5.18 ER70S-6 is the workhorse, while stainless and aluminum call for their own classifications. Then match gas to goal: a 75/25 argon/CO₂ blend is a solid all-rounder for mild steel, 100% CO₂ gives deeper penetration at the cost of more spatter, and aluminum needs 100% argon or an argon-helium blend. For shops working lighter metals, the Aluminum MIG Welding: The Complete Practical Guide is a useful reference when narrowing these choices down.

3. Establish and Record Starting Parameters

Now prove your setup on test coupons. Use the rule of thumb of roughly 1 amp per 0.001″ of steel and the wire-feed multipliers for your wire diameter to land on a safe starting point, then tune voltage off bead appearance. The most important step, as The Fabricator’s GMAW 101 stresses, is to record the actual parameters that produce a sound, clean bead. Do not rely on memory. Amperage, voltage, wire feed speed, travel speed, stickout, and gas flow all need to be written down.

4. Check Prequalification vs. Qualification

Not every procedure can be written from a table. Under AWS D1.1, procedures may fall under prequalified Clause 5 provisions, or they must be qualified and supported by a Procedure Qualification Record (PQR). Be aware of the major exception: GMAW-S (short-circuit transfer) is not prequalified under AWS D1.1. However, AWS D1.3 Clause 3 does provide a route for prequalified short-circuit procedures in sheet metal building systems. If you intend to run lower preheat than D1.1 Table 5.8 lists, the AWS Welding Digest states that qualification testing is still required. For complex, crack-sensitive joints, AWS D14.8M can determine required preheat through its four cold-crack avoidance methods.

5. Write a Followable WPS

With qualified parameters, compile them into a Welding Procedure Specification (WPS) that someone who has never welded this joint before can follow. Be explicit about base metal spec and thickness range, joint details (groove angle, root face, root opening, land), filler classification and diameter, gas type and flow rate, qualified positions, amperage, voltage, wire feed speed, travel speed, electrode extension, preheat and interpass temperatures, cleaning method, technique (push or pull, stringer or weave), and inspection criteria. Name your controls too: fume extraction, PPE, and inspection method, consistent with the expectations of OSHA 29 CFR 1910.252.

6. Train, Qualify, and Control

A perfect WPS means nothing if the welder can’t reproduce it. Train welders to the procedure as written, qualify them under your governing code, and monitor continuity from there. Reinforce proper joint preparation, since poor cleanliness causes countless defects. For aluminum in particular, that means removing oxide promptly and using dedicated equipment, a process covered in the Aluminum Cleaner for Welding: How to Prep Aluminum Right guide.

7. Audit, Track, and Revise

The final and most overlooked step is upkeep. Implement a simple audit routine: periodic welder continuity checks, spot checks of machine parameters against the WPS, and a review of rework and nonconformance data. Treat every recurring defect as feedback. When a fix proves permanent and valid, revise the WPS through the correct qualification path and retrain affected welders.

Procedure Rollout Checklist

• Envelope: Base metal, thickness range, joint, position, code, and acceptance criteria confirmed.
• Consumables: Filler, gas, and flow rates selected and verified.
• Proven Parameters: Tested on coupons with all values recorded.
• Qualification: Prequalified status verified, or a PQR completed.
• WPS: Written to be followable, with all required elements and safety controls.
• People: Welders trained, tested, and continuity tracked.
• Audit Trail: Spot checks, rework data, and a revision process in place.

This checklist bridges the gap that cost the Ohio shop so much. A good weld is impressive; a repeatable weld, made by anyone following the procedure, is profitable. The procedure turns knowledge into a standard, but the welder brings it to life. Your gas metal arc welding process is only as strong as the discipline to follow, verify, and audit it for the life of the job.

Frequently Asked Questions

What is the difference between GMAW and MIG welding?

There is no difference in the process — GMAW is the formal AWS term, and MIG is the common trade name in North America, with MAG used in much of Europe. In practice, ‘MIG welding’ usually means solid-wire GMAW with a shielding gas, while FCAW (flux-cored wire) is a different process even though it is sometimes sold on the same machine.

What voltage and wire feed speed should I start with for 1/8-inch mild steel using .035 wire?

A good starting point is roughly 125 amps, which with .035 wire and a multiplier of about 1.6 inches per amp gives a wire feed speed near 200 ipm, with voltage set in the high teens to low twenties and trimmed by watching the bead. Treat these as starting points only — always run a test coupon on the actual material and joint before committing to production.

Is short-circuit MIG welding prequalified under AWS D1.1?

No. AWS D1.1 defines GMAW-S as short-circuit transfer mode in Annex K and excludes it from prequalified procedures because the mode carries a risk of lack of fusion. AWS D1.3, for sheet metal building systems, provides a route to prequalified short-circuit procedures, and any other use of GMAW-S requires qualification testing and a supporting PQR.

Do I need a written WPS for shop work that never leaves the building?

Commercially, yes — most quality systems, customer specifications, and insurance programs expect a documented procedure even for internal work, and code work requires one. Practically, even for hobby work a one-page written recipe prevents the most common failure: three welders running three different versions of ‘the same’ bracket.

How much shielding gas flow does GMAW need?

Most GMAW work runs around 20–30 cubic feet per minute at the torch, with flux-cored typically needing more, and the exact figure belongs in your procedure. Too little flow lets air in and causes porosity; too much wastes gas and can create turbulence that disturbs the puddle.

Why does my MIG weld come out full of porosity?

Porosity is almost always a gas, wire, or cleanliness problem rather than a settings problem. Check shielding gas flow first, then purge the hose and regulator for moisture, inspect the wire for rust or oil, confirm the nozzle is not splattered, and for aluminum make sure the oxide was mechanically removed immediately before welding.

What shielding gas should I use for stainless steel and for aluminum?

For stainless, a tri-mix of argon with a few percent CO2 or oxygen, or an argon/helium blend, gives penetration plus a clean, wide bead — and always use a dedicated stainless brush. For aluminum, use 100% argon or an argon/helium blend, run DCEN, and expect higher voltage and much higher wire feed speed than steel of the same thickness.

What preheat do I need for thick carbon steel?

Start with AWS D1.1 Table 5.8, ‘Prequalified Minimum Preheat and Interpass Temperature,’ which is cited from Clause 5.7, and use the combined thickness of all parts joined by the weld. Annex B and AWS D14.8M offer lower, calculation-based requirements — but if you use preheat below Table 5.8, qualification testing is still required.

Do I have to wear a respirator if I have a good fume extractor?

It depends on what you are welding and what your exposure measurements show. OSHA requires engineering and work practice controls first, with respiratory protection under 29 CFR 1910.134 added where those controls cannot hold exposure below the limit — and for indoor or confined-space welding of lead, cadmium, beryllium, or mercury-bearing materials, local exhaust or airline respirators are required by rule.

Can I weld aluminum on my existing MIG machine?

Almost certainly yes, if it is a constant-voltage wire-feed machine with a polarity switch and you can run 100% argon. You will need aluminum wire (ER4043 or ER5356), a spatter-resistant liner and contact tip, dedicated aluminum grinding tools, and you should expect to re-learn your settings because aluminum needs higher voltage and much faster wire feed speed than steel.