MIG Welding Defects: Causes, Fixes, and Prevention

You finish a 6 mm fillet joint on mild steel, grind the bead flat, and the fabricator signs off the frame. Two days later a customer taps it with a hammer and it breaks at the toe. The weld looked fine — but a cold, dull bead with a fine crack running along the root was hiding inside it. Almost every MIG welding defect announces itself early, if you know the fingerprints it leaves behind.

What Counts as a MIG Welding Defect — and Why the Difference Matters

Before any repair starts, the question isn’t how to grind something out, it’s whether what you’re looking at is actually rejectable. For MIG welding, that judgement comes down to one key distinction: the difference between a discontinuity and a defect.

According to AWS A3.0M, standard welding terms, a discontinuity is an interruption in the typical structure of a material that doesn’t necessarily affect its usefulness. A defect is a discontinuity that exceeds the acceptance criteria of the governing specification. Put simply, the feature is the imperfection, and the verdict comes from the code.

This is mirrored by the harmonised terminology used alongside ISO 6520-1, which classifies weld imperfections and defines a welding defect as a discontinuity or a deviation from the assumed weld geometry. The same bead of MIG weld metal can be perfectly acceptable under a general fabrication drawing, but fail outright under AWS D1.1 for structural steel or ASME Section IX for pressure work. For that reason, “acceptable” is never a universal word in this trade.

The Six Root-Cause Families

GMAW, known as MAG across much of Europe, the UK and Australia, is widely used for good reason. However, with that volume comes a predictable set of failures. Nearly every MIG welding defect can be traced back to one of six root-cause families:

1. Contamination and chemistry – Rust, mill scale, oil, paint, moisture, zinc or oxide films that poison the weld pool.
2. Shielding gas problems – Wrong gas mix, incorrect flow, drafts, leaks, a blocked diffuser or a fouled nozzle.
3. Parameter mismatch – Amperage, voltage, wire feed speed and travel speed falling out of sync with one another or with material thickness.
4. Technique and geometry errors – Incorrect work or torch angle, poor root gap, bad bead placement or improper filler addition.
5. Heat and restraint problems – Excess or insufficient heat input, high restraint, and the cracking or distortion that can follow.
6. Equipment faults – Worn contact tips, a loose earth clamp, an erratic power source or a spatter-prone gun.

Knowing which family you’re working from is far more productive than guessing at a fix. Many of the issues that appear complex are, more often than not, caused by one of these six.

Surface Irregularities vs Weld Discontinuities

To speak the language of inspectors, it helps to use the split laid out in Kobelco’s reference booklet on welding defects. For MIG work, imperfections generally fall into two groups:

• Surface irregularities: Undercut, overlap, excess reinforcement, uneven bead, herringbone (fish-scale) pattern and underfill.
• Weld discontinuities: Porosity, inclusions, incomplete fusion, incomplete penetration, melt-through, hot cracks and cold cracks.

Spatter deserves its own note. It is strictly an event, not an inherent defect. Only when it lands in an undesirable location, heavily pockmarks the plate, or gets trapped and becomes an inclusion does it cross the line into a rejectable issue.

Why Getting This Right Pays Off

The cost of mislabelling isn’t just cosmetic. Rework, scrapped material, NDT fees, and code non-conformance all add up, and on pressure or structural jobs, a single defect can require a formal repair procedure that isn’t always available on the floor. More quietly, consistent, code-compliant welds build a reputation that keeps a shop on bid lists, whilst repeated rejections tend to remove it from them.

With setup playing such a large role, it’s worth brushing up on the specifics for trickier materials with our aluminum MIG welding guide, and paying extra attention to gas selection by reading a solid buyers guide to welding gas supplies and equipment. Both are areas where the line between a harmless discontinuity and a costly defect is often decided before the arc is struck.

Porosity: The Most Common MIG Welding Defect

Ask any CWI and porosity is the defect they name most often on a MIG weld. It is also the most misunderstood, because a pore can look like a pinprick while hiding a cavity several millimetres deep.

What porosity actually is

Molten steel holds dissolved gases far more willingly than solid steel. The pool soaks up nitrogen, oxygen and hydrogen from the arc atmosphere, the shielding gas and any contamination on the joint. As the puddle freezes those gases want out: some escape, the rest are trapped as cavities. The AWS definition of porosity calls them voids formed by gas during solidification. Find the source and you have found the fault.

The five forms and what they tell you

Not all porosity looks the same. The Australian Welding Institute groups porosity into forms that map straight back to causes:

Form	Where it appears	What it tells you
Scattered	Evenly through the bead	Shielding or surface contamination
Cluster	Grouped in one area of the bead	Local gas source, dirty spot or leak
Linear	Along the centreline or root	Moisture or gas on a dirty root face
Elongated	Long voids at the toe or root	Fast travel, or gas over a dirty surface
Piping / wormhole	One large channel at the root	Gas ingress via a root gap or bad fit-up

Wormhole porosity is not a cluster of bubbles but one open channel, so gas is usually being drawn in through the joint rather than through the arc.

The three gas sources

1. Nitrogen and oxygen from poor shielding. The gas has to arrive as a laminar column that sweeps air away from the arc. Break that and nitrogen and oxygen dissolve straight into the pool. Culprits: drafts and open doors, a leaking hose, flow set so high the gas churns into turbulence, a spatter-packed diffuser, and the venturi effect, where a shallow torch angle pulls the column in along the plate instead of up around the arc.

On air entrainment the Australian Welding Institute states that as little as 1% air entrainment causes distributed porosity and that more than 1.5% produces gross surface-breaking pores. That narrow window is why an enclosed booth matters in winter.

A partially blocked diffuser fails counter-intuitively: blocked holes force all the flow through the remaining openings, giving a strong jet on one side and no coverage on the other. Turbulent flow then aspirates air in rather than pushing it away.

2. Hydrogen from moisture and organics. Hydrogen is the most damaging weld gas: it makes weld metal hard and brittle and drives cold cracking. It comes from damp wire, moisture on the joint, oil, grease, rust, paint and primer. The commonest root cause is a joint prepped in the morning and welded after primer went on, or wire stored in a humid room.

3. Oxide and sulphide from dirty joint faces. Mill scale, rust and slag dragged along the joint face melt into the pool and float as oxides or sulphides that fail to rise out, which is why root faces are ground back to bright metal in double-sided work.

The diagnostic order that actually works

Work this sequence before you touch any machine settings:

1. Compare torch flowmeter to station flowmeter. A torch reading well below the station reading points to a leak between them. The University of Alberta GMAW reference flags a calibration trap: a meter calibrated at 20 psig on a 50 psig line reading 40 scfh is really delivering about 54.6 scfh.
2. Leak test the hose and regulator with a liquid leak detector, not soapy water, then apply the calibration correction for your line pressure and confirm it sits inside the flow range for your nozzle size.
3. Check cylinder condition and gas mix. A nearly empty cylinder cannot hold pressure, and slumping pressure means the flow you set is not the flow you get. Confirm the mix matches the procedure.

Prevention checklist

• Clean the joint back to bright metal before welding.
• Keep nozzle and gas diffuser clean; replace the tip before it traps spatter.
• Use a wind screen. A PVC curtain around the workpiece removes more porosity than most people expect.
• Set flow to the procedure range and verify it at the torch.
• Store wire dry; keep spools sealed until use.
• Hold the correct stickout and torch angle.

One cross-check is the material. Hydrogen porosity behaves very differently on aluminium than on steel, so an aluminium pore hunt shifts the whole diagnostic. See our aluminum MIG welding guide for the oxide and hydrogen problems specific to that metal.

Repair and inspection

Do not reweld a pored joint until the cause is fixed, or you reproduce the same pores. Gouge or grind back to sound metal and re-weld with a qualified procedure. Pores run deeper than they look: a small surface opening can be the mouth of a cavity extending several millimetres into the section, so grind until you reach solid parent metal.

On detection, keep expectations straight. Small surface pores may not register on a radiograph at all, lying in the plane of the film or too small to resolve, and radiography remains a long-standing tool for internal porosity and inclusions. That is why visual inspection for surface porosity stays routine in any MIG welding parameter review.

Undercut: The Groove That Quietly Weakens Your Weld

Of all the MIG welding defects, undercut is the one most likely to walk straight past you. There is no hole to feel, no crack to see, no colour change in the bead. The weld comes off the gun with a smooth, even profile, and yet along the toe or sidewall the base metal has been melted away and never replaced with filler.

That is the definition: a groove melted into the base metal at the weld toe or along the fusion boundary of a sidewall, left open because filler metal never filled it. Kemppi’s breakdown of common MIG/MAG defects groups undercut with lack of fusion and flooding, because all three trace back to the same trio: too much heat, too little filler, or both.

Why Undercut Is a Mechanical Problem, Not a Cosmetic One

An undercut is a notch, and notches behave badly under load. It acts as a stress raiser, concentrating force at the exact point where a crack would prefer to start, and it reduces the effective cross-sectional area of the weld available to resist applied loads. On a fillet weld it cuts into the effective throat, so the joint is weaker than the bead’s tidy appearance suggests. This is why structural codes limit the depth and accumulated length of undercut rather than banning it outright, and why undercut is a classic fatigue crack initiation site in cyclically loaded structures such as crane rails, trailer chassis and pressure-vessel skirts.

Two welds can look identical and behave completely differently. One has full-throat fillets with clean toes. The other has a 2 mm groove running the length of the leg, and it may fail under a loaded impact test that the first passes with room to spare.

What Causes Undercut

Undercut is almost always a balance problem: more base metal melted than filler metal deposited. The usual suspects:

• Excessive welding voltage. Weman’s MIG/MAG guide states plainly that undercut is a sign of too high a voltage, and Kemppi lists it among the root causes of lack of fusion and flooding. A long, soft, wandering arc spreads the pool and eats the toe.
• Excessive current, especially with too low a travel speed. Too much amperage melts the sidewalls ahead of the arc.
• Travel speed too fast for the amperage. The pool is dragged along rather than filling the joint.
• Insufficient filler metal addition. The wire is not being fed fast enough to replace what the arc melts.
• Wrong work angle. Held parallel to the plate instead of angled about 45° into the joint, the arc reaches one sidewall and starves the other.
• Wrong torch angle. A torch that is not square to the direction of travel favours one toe over the other.
• Too long an arc. Beyond the correct arc length you lose penetration at the toe and pick up spatter and oxidation.
• Arc blow. The arc is deflected into the sidewall, melting it where the torch is not pointing.
• Excessive weave width without adequate filler. A wide weave that outruns the wire feed rate leaves the extreme edges of each pass starved.

The High-Current, Fast-Travel Trap

Here is the counter-intuitive part. A welder who sees shallow penetration raises the amperage, and the heat input on paper looks generous. But too much current plus fast travel means the pool is wide and thin, it does not have time to wet the sidewalls, and the toes are melted and left unfilled. The bead still looks good.

Raising amperage to get penetration is a very common shop habit, and on fillet welds it is exactly what produces undercut and melt-through. The correct fix is usually more filler metal and a slower, more deliberate travel, not more heat. The parameter relationships in the ESAB MIG process parameters guide and the Miller weld defect troubleshooting reference show how the numbers interact.

Preventing Undercut Before It Starts

1. Hold the correct work angle, about 45° into the joint for a fillet, and keep the travel angle square to the bead.
2. Keep a constant arc length and a steady wire stickout. Set the machine by the sound of the arc, not by the dial alone.
3. Add filler at the toe. If you weave, dwell slightly at the extreme edges of each pass so filler reaches the corners.
4. Slow the travel until the pool has time to wet both sidewalls, and favour a small stringer over a wide weave on fillet work.
5. Match filler addition to consumption. Keep the wire in the leading edge of the puddle and raise wire feed speed before you raise voltage.
6. Re-check amperage and voltage against the WPS rather than trusting a remembered setting, and clean rust and mill scale from the toe, since a dirty toe is already a notch waiting to happen.

Repairing an Existing Undercut

1. Remove the defective metal down to sound base material. Grinding works for shallow grooves; for deeper ones, air arc welding and carbon arc gouging opens the toe quickly without heat distortion.
2. Restore the joint geometry and toe profile, removing all hardened, remelted and gouged surface before depositing new metal.
3. Reweld with a smaller stringer or a second pass, using the corrected work angle, a shorter arc and more deliberate filler addition.
4. Re-inspect to the same criteria you would have applied to the original weld, and confirm the accumulated undercut length against the code limit before signing off.

Industry guidance on fusion and porosity fabrication defects from the Australian Welding Institute follows the same principle: remove the defective metal to sound material, restore the joint, then reweld and re-inspect. And always ask why the undercut happened in the first place. Grind it out, weld it up the same way, and it will be back by the next shift.

Lack of Fusion and Incomplete Penetration

A weld can look picture perfect from the top, only to fail the moment it’s put to load. With MIG, the two defects that get rejected the most are lack of fusion and incomplete penetration, and both hide beneath a clean cap. Knowing the difference between them is often the first step to fixing them.

Know The Difference

Although they’re often grouped together, these are two separate failures.

• Incomplete fusion occurs when the weld metal fails to bond with the joint face or a previous bead. Instead of melting into the parent metal, it sits against it, creating a thin, unfused boundary that can run along the sidewall or toe.
• Incomplete penetration occurs when fusion never reaches the full thickness of the joint. The top may appear closed, but an unmelted ligament of base metal remains at the root, leaving the load path compromised.

According to the TWI guidance on incomplete root fusion and root penetration, these imperfections are more common with consumable electrode processes such as MIG, MAG, FCAW, MMA and SAW. With a non-consumable process like TIG, heat input and filler can be controlled separately, making these defects far easier to avoid.

The MIG Catch-22

For MIG in particular, penetration comes down to a delicate balance between current, voltage and travel speed. As noted by troubleshooting guides and MIG and MAG fundamentals, all three must be matched to the joint condition.

• Too little heat: A current that’s too low won’t melt through the root face. The puddle will bridge the gap or sit on top of it, leading to lack of fusion at the root.
• Too much heat and too much speed: Cranking up the current without adjusting for travel speed can backfire. A fast, overpowered arc can bridge a gap before it has time to melt through, giving a deceptively deep-looking bead with no true root penetration.

Joint Preparation and Position Traps

Many of these defects are engineered in before the trigger is ever pulled. TWI highlights several common triggers: an excessively thick root face, a root gap that’s too small, misplacement of the weld, failing to cut back in double-sided joints, too little arc energy, too small a bevel angle, or an electrode diameter too large for the gap.

These factors become especially punishing in tight joints. The narrow-groove trap leaves little room for the wire or nozzle to reach the bottom of the bevel, making access to the root near impossible. This is just as critical for thin or conductive metals, which is why a good aluminum MIG welding guide puts such emphasis on bevel angle and root gap, as aluminum’s conductivity punishes cramped setups even harder than steel.

Even perfect geometry can be ruined by contamination. Rust, mill scale, oil or grinding debris at the root will prevent bonding, and in a narrow groove, there simply isn’t room to clean it after tacking. As the Australian Welding Institute’s guidance on fusion defects states, sound, clean joint preparation is the first and most important line of defence.

Position can be just as restrictive. Running a thicker joint vertical-down gives the puddle no time to dwell at the root, pushing molten metal ahead of the fusion zone. For root passes, vertical-up is almost always the safer and more forgiving choice. Technique and filler considerations for these positions are expanded on in the all-position rod and technique guide.

Prevention and Diagnosis

The most reliable way to avoid these defects is to treat the root as its own operation, not an extension of the cap. Keep these practical checks in mind:

• Follow the procedure: Set current and voltage against the WPS for your wire size and thickness, not by feel.
• Prioritise access: Trim your root face and open an adequate root gap with a bevel wide enough for the torch to reach the bottom.
• Control the arc: Keep a short arc and weave just wide enough to fuse the full sidewall, with a slight dwell at the toes.
• Tack with intent: Be aware that tacking can close the root gap. Check it after tacking before running your root pass.
• Go up, not down: Use vertical-up for root passes on thicker vertical joints.

Should suspicion arise after welding, don’t rely on looks alone. A weld defect troubleshooting approach may start with dye penetrant for surface-breaking lack of fusion at the toe, but full confirmation often requires ultrasonic testing or radiography. For a definitive answer, a macro cross-section is unmatched. Both defects are code controlled, and under AWS D1.1, their acceptance is strictly defined.

Spatter, Poor Bead Profile and Arc Blow

Spatter is not a defect category. It is a physical event: a droplet of molten metal thrown out of the arc that lands on the plate or your clothes and freezes as a hard lump. A few flecks beside a fillet are cosmetic. Spatter driven into the root of a joint and welded over is an inclusion, and inclusions are among the worst things an inspector can find in a weld. So treat spatter as a symptom: the pattern tells you which parameter is wrong before you run a test coupon.

Spatter that matters, spatter that does not

• Pockmarks are shallow craters in the bead face where a spatter droplet bounced off the pool. Mostly cosmetic, but they act as stress raisers on a bead under-matched to the joint.
• Mouse-foot marks are the shallow dimples left when spatter lands on the parent metal beside the weld and is later knocked or ground flat. Also mostly cosmetic.
• Excessive reinforcement happens when spatter adds metal to a bead that already carries too much. That is a dimensional issue, and dimensional issues get rejected.
• Spatter inclusions are the dangerous ones. Spatter landing in the joint, root face or inside a groove and then covered by a later pass becomes trapped contamination in the weld metal.

Why the arc throws metal

Start with the transfer mode. Short-circuit transfer inherently spatters because the wire touches the pool repeatedly and each contact is a small explosion. The Welders and Welding Technology MIG guide notes that short-circuit frequency in short arc welding normally runs between 50 and 200 Hz, with the arc short-circuited roughly 25% of the time. Some spatter in dip mode is normal; a lot of it is not.

The most common cause is wrong voltage for the wire feed speed. Hobart Brothers calls a voltage too low in relation to that speed the biggest contributor to excessive spatter, whatever the transfer mode. Too little arc voltage leaves the arc too short to properly heat and shield the wire tip, so the droplet melts unevenly and detaches badly.

Second is inductance. Too little lets current rise sharply on short circuit and melt the wire almost explosively. Raising inductance lengthens arc time, reduces dip frequency and cuts spatter. Synergic machines set it for you; on a constant-current set you control it with the DIP setting.

Third, voltage that is too high. An over-long arc is unstable, and an unstable arc wanders and spits. Long wire stickout does the same by letting the wire tip oxidise before it reaches the pool, as does base metal covered in rust, scale or oil.

Finally, gas and wire. A 75% argon/25% CO2 mix typically gives less consistent transfer and more spatter than 90% argon/10% CO2, and metal-cored wire usually spatters less than solid wire. Get the gas right before blaming the machine: the welding supply buyer guide covers cylinder and gas choices, and the aluminium MIG welding guide covers the pure argon and argon/helium mixes needed for non-ferrous work.

Reading the bead profile

Shape is a free diagnostic: every profile fault points back to a parameter or technique problem.

Profile problem	Usually means
Too flat, barely crowned	Too much heat input, slow travel, or a pool pushed flat.
Too convex, humped	Travel too fast for the amperage, or voltage too low on a narrow pool.
Too narrow and thin	Wire or feed speed too small, or travel too quick.
Too wide, spread out	Heat too high for the wire size, high voltage, or a wide weave.
Rippled or fish-scaled	Unstable arc: turbulent gas flow, worn liner, poor gas mixing, voltage hunting.
Overlap, a cold lap	Leading edge fuses, trailing edge does not. Too much heat up front or slow travel.

Arc blow: when the arc refuses to stay put

Arc blow is magnetic deflection of the welding arc. The current running from torch to workpiece creates a magnetic field that pushes the arc sideways, usually toward the work clamp. The signatures are unmistakable: erratic spatter at one repeating point along the bead, undercut on one side only, and a bead that wanders. Remedies, roughly in order of how often they work:

• Move the earth clamp to another point on the workpiece, ideally opposite the joint.
• Hold the shortest workable arc length you can keep consistent.
• Reduce current slightly; the deflecting force rises with current.
• Angle the torch against the blow, and travel toward the clamp rather than away.
• In severe cases, ground the clamp directly to the joint being welded.

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Cracks: Hot Cracking, Cold Cracking and the Crater Crack

Cracking is one of the few MIG defects that can be genuinely unpredictable. The key isn’t what it looks like, but when it forms. A crack while the puddle is still liquid calls for a completely different fix than one that appears hours after cooling. For this reason, most investigations rely on timing alone, a classification that is backed by sources such as the KOBELCO weld defect guide.

Hot cracking: tearing while the weld is still molten

Hot cracking forms along the centreline or root as the weld solidifies. Towards the end of freezing, the last remaining liquid gets pushed between growing grains. If that liquid contains low-melting constituents, it forms an extremely weak film, and any shrinkage strain at that moment will tear it open.

The main culprits are sulphur and phosphorus. To prevent them from forming low-melting iron sulphides at grain boundaries, manganese is added to the filler. As noted in the University of Alberta’s GMAW reference, manganese will combine with sulphur first, stopping it from segregating. This is why a sufficient manganese-to-sulphur ratio is so vital, especially for higher-sulphur base metals.

The fix is usually straightforward. Opt for a narrower bead, moderate your heat input, and relieve restraint wherever possible. Above all, check the base metal’s sulphur content before changing technique. On some high-sulphur plates, chemistry will be the limiting factor no matter how precise your travel speed is.

Crater cracks: the risk at the end of the weld

Not every crack comes from the chemistry of a joint. Some form at the very last drop of molten metal, known as a crater crack. When the arc is broken abruptly, two things happen at once: the heat source is removed from the largest, most liquid pocket of the puddle, and that pocket is left to freeze with nowhere for its contraction to go. As the Weman MIG/MAG guide explains, interrupting welding directly allows a crater to form as the final metal solidifies and shrinks, whereas controlled crater filling allows it to solidify from the top down and avoid it entirely.

Most modern MIG machines have crater-fill, burn-back or similar functions to do this for you. The Kemppi guide to common MIG/MAG defects recommends using them on any weld that ends at a critical location.

Cold cracking: hidden until hours later

Cold cracking is the most deceptive of the three. It forms after the weld has fully cooled, often hours later, and can pass a visual inspection whilst already present. This type of cracking is the result of three factors converging: diffusible hydrogen, a hard and brittle microstructure, and tensile stress from restraint.

Hobart Brothers’ MIG troubleshooting guide for metal-cored and solid wire points out that whilst solid and metal-cored wires are low hydrogen and resist moisture pickup, the joint, procedure and fit-up are far more common instigators. Its checklist of causes includes insufficient weld size, excessive joint restraint, poor joint design, rapid cooling, incompatible filler chemistry, and mismatched tensile strength and ductility.

Prevention revolves around that list. Match your weld size to the plate thickness, use preheat where required to slow cooling, and reduce restraint through better sequencing or fit-up. Avoid narrow, deep joints on thick plate, as these stack restraint onto a fast-cooling, harder deposit. Use a hydrogen-controlled filler, store it correctly, and be aware of gas choice. Some argon-rich mixes increase tensile strength at the cost of ductility, so when standardising a mix for structural or pressure work, it can be worth consulting welding gas suppliers and cylinder services to weigh that trade-off.

Crack type	Forms	Main driver	First fix
Hot / centreline	While solidifying	Sulphide segregation plus restraint	Narrower bead, lower heat input, less restraint
Crater	As the arc stops	Uncontrolled final shrinkage	Crater fill or burn-back function
Cold	Hours later, after cooling	Hydrogen, hardness, restraint, stress raisers	Preheat, hydrogen control, correct weld size

Regardless of the type, document the location, conditions and time of appearance before grinding. That small step is often the difference between a repair that holds, and one that fails the exact same way again.

Inclusions, Oxide and the Surface Irregularities Inspectors Actually Reject

Ask an inspector what they rejected last week and you will rarely hear the word “porosity.” More likely: “ugly bead,” “cold lap,” “slag in the root,” “heel is hollow,” or “a welding blob.” That is the working rejection list, and much of it lands in this section.

Inclusions: the defect you cannot see

Start with a misconception: MIG welding has no flux, so there is no slag to trap in the puddle. Flux inclusions are therefore rare, but rare is not never, and inclusions stay among the most dangerous defects because they hide inside sound-looking metal.

Where do they come from in GMAW?

• Oxide and mill scale from the base metal. Scale flakes that survive preparation detach as the puddle forms and freeze into the weld.
• Aluminium oxide, the classic failure in aluminium welding. The film reforms within seconds of cleaning, and disturbing it releases it into the pool.
• Copper contamination from the contact tip, or a copper-coated wire dropped into the joint.
• Back-gouging residue. A root not cleaned and ground before rewelding carries carbonised scale into the new bead.
• Spatter welded over. A bead ground only superficially leaves spatter pockets the next pass traps.
• Zinc from galvanised or zinc-coated steel, which boils out of the coating and enters the pool.

The Australian Welding Institute notes that slag normally appears as elongated lines along the weld, is readily identified in a radiograph, and while usually associated with flux processes it can also occur in MIG welding. Prevention guidance: a smooth bead profile lets slag be removed between runs, while a poor, convex profile traps pockets of slag. Add adequate inter-run overlap, remove all slag between runs, and grind narrow V-grooves back before the next pass.

Kobelco’s defect guide lists the main causes of slag inclusions as too low welding amperage, too much arc length, too much weaving width, too narrow a groove, and slag left on the preceding layer. Low amperage and a long arc let the pool cool too fast to flush lighter contamination out. The fix is to gouge or grind back to clean metal and reweld properly, not to run a hotter bead over the top.

The surface irregularities that fail visual inspection

Most code rejections happen before an instrument is switched on, because the eye is fast and the standard is written in observable terms. AWS D1.1 sets explicit limits on profile and surface condition, and these features are on the list:

• Undercut, the groove at the toe, both a profile and a strength defect.
• Overlap, a cold lap where the leading edge fuses but the trailing edge does not, leaving the bead unmerged to the plate.
• Excess reinforcement, a bead too proud of the surface.
• Underfill, a bead below the plate or below the specified fillet leg.
• Uneven bead ripples and irregular narrow-and-wide patterns, usually an unstable arc.
• Herringbone or fish-scale pattern, a stacking-fault-like texture from an unstable arc and low heat input.
• Pockmarks and mouse-foot marks, craters and splayed marks left by spatter and excess leading-edge heat.
• Excessive convexity, a bead that humps rather than flattens.

The cause of overlap is counter-intuitive. Most welders assume a cold lap means too little heat. It is usually the opposite: too much heat at the leading edge, or travel speed too slow for the amperage. The molten leading edge runs ahead and pools over the plate, the trailing edge cools and rolls onto itself without fusing, the weld dips, the metal beneath oxidises, and you have a lap.

So is the fix. Do not simply add heat. Slow the travel slightly, reduce amperage, check the gas mix, and push the bead rather than dragging it. On aluminium the oxide film is a constant companion, so preparation matters more than parameters: our guide to prepping aluminium for welding covers that step, and the complete guide to aluminium MIG welding covers the rest.

One last piece of advice from the defect literature: never weld over anything questionable. Grinding over spatter, scale, or a cold lap is how hidden inclusions get created. The Miller weld defect troubleshooting reference makes a good one-page checklist by the machine.

Material-Specific Traps: Stainless Steel, Aluminium, Galvanised Steel and Thin Gauge

While most MIG welding defects stem from the same core issues, the way they present can change drastically between materials. Applying generic carbon steel advice to stainless, aluminium, galvanised, or thin gauge sheet will often create the very defect you’re trying to prevent.

Stainless Steel: Heat Retention and Oxidation

Stainless steel holds onto heat far more than carbon steel. With roughly a third of its thermal conductivity and a much higher rate of expansion, heat builds up within the weld pool. This leads to excessive distortion, wider heat-affected zones, and grain growth.

The classic MIG failure for stainless, however, is porosity. Both the base metal and filler contain chromium, which oxidises incredibly readily. Any disturbance to its protective oxide film will form a fresh, tenacious oxide layer that traps shielding gas as it solidifies. Due to this, stainless is extremely sensitive to gas flow and wind. A clean gas diffuser is non-negotiable, and any drafts must be blocked with a wind screen.

On top of this, welding stainless can convert chromium to its hexavalent state. According to OSHA’s hexavalent chromium fact sheet, Cr(VI) fumes can damage the eyes, skin, nose, throat and lungs and cause cancer, with a permissible exposure limit of just 5 micrograms per cubic metre as an 8-hour TWA.

Aluminium: Oxide, Hydrogen and Tight Heat Control

Aluminium brings its own set of obstacles. Its oxide film reforms within seconds of exposure to air, so the joint must be cleaned immediately before welding. For reliable results, preparation is just as important as your parameters, which is covered in detail within Aluminum Cleaner for Welding: How to Prep Aluminum Right.

With hydrogen being the main culprit, porosity is by far the biggest threat. For aluminium, molten metal takes up far more hydrogen than solid aluminium can hold, so the gas is forced out as the weld freezes. This makes joint cleanliness and keeping the filler dry far more critical than it is on steel. As explained within Aluminum MIG Welding: The Complete Practical Guide, even the smallest amount of moisture or contamination can ruin an otherwise perfect pass.

Thermally, aluminium sinks heat at an alarming rate. Too little heat will cause lack of fusion, whilst too much will cause burn-through. Additionally, a wide, flat puddle combined with high restraint is a prime recipe for hot cracking. It’s a delicate balancing act that leaves little room for error.

Galvanised and Zinc-Coated Steel: Volatilised Zinc

Welding galvanised or zinc-coated steel creates a unique form of porosity. The zinc coating volatilises at a much lower temperature than the melting point of steel, rapidly turning to gas within the weld pool and becoming trapped as it solidifies. Alongside this, dense zinc oxide fumes are produced at a hazardous rate.

These fumes must be controlled. OSHA 29 CFR 1910.253 requires local exhaust ventilation or airline respirators for welding zinc-bearing base or filler metals indoors and in confined spaces. Natural airflow will never be sufficient for this work.

Thin Gauge Sheet: Precision Over Power

With thin gauge sheet, the window between a sound weld and burn-through is incredibly small. Excess heat input can result in burn-through, underfill, distortion, and overlap at the root. To combat this, short-circuit transfer should be used deliberately to limit heat input. This must be paired with lower wire feed speeds, a tight arc, minimal weave, and plenty of tacks to combat restraint. Even a moment of dwell can punch a hole straight through the material.

Each of these materials fights back in its own way. By respecting their thermal and chemical quirks, you’ll be able to diagnose the defect for what it truly is, rather than chasing a fix that was never meant for it.

Diagnosing, Repairing and Knowing When Not to Repair

Prevention only works if you can diagnose what went wrong. The moment a defect lands on an inspector’s screen or your own grinder, you are running a root-cause investigation with a deadline attached.

A repeatable diagnostic sequence

1. Name the defect in code terms. Use ISO 6520-1 vocabulary: porosity, undercut, lack of fusion, incomplete penetration, overlap, crack, inclusion. Welder shorthand rarely matches inspector language, and the mismatch turns a repair into an argument.
2. Locate it in the weld. Start, middle or end? Toe, root or face? Undercut at the start of a bead points to travel speed or filler addition; undercut down the whole length points to amperage.
3. Place it in the pass sequence. Root, fill or cap? A root-pass defect usually means the current was wrong for the joint thickness; a cap-pass defect points to wire feed speed, technique or cleanliness on that pass alone.
4. Map it to a cause family. Parameter, gas, cleanliness, geometry, metallurgy or equipment. Test one family at a time.

Defect to cause: the quick lookup

Defect	Where to look	Check first
Clustered porosity	Mid-bead, root passes	Gas flow, leaks, nozzle
Scattered porosity	Whole bead	Moisture, oil, rust, wet wire
Undercut	Toe or sidewall	Amperage, travel speed, filler
Lack of fusion	Toe, sidewall, prior bead	Heat input, prep, bead position
Incomplete penetration	Root	Current vs thickness, gap, root face
Centreline crack	Weld centreline	Sulphur, restraint, puddle shape
Crater crack	Weld end	Crater fill, burn-back time
Spatter, pockmarks	Along bead	Inductance, voltage vs WFS, dirt
Inclusions	Root, between runs	Mill scale, spatter, gouge residue

What your eye catches and what it cannot

Most visual rejections are surface features: undercut, overlap, underfill, excess reinforcement, irregular bead width, pockmarks, herringbone and visible cracks, judged by eye against the acceptance criteria.

What the eye cannot reach is anything buried: internal porosity, trapped inclusions, lack of fusion, root penetration that never fused and subsurface cracks. TWI’s guidance on incomplete root fusion and penetration explains why those need testing, not opinion.

The NDT menu

• Dye penetrant. Finds surface-breaking cracks only, never a crack buried under a weld, and needs a clean, uncoated, scale-free surface.
• Magnetic particle. Works on ferromagnetic steels, finding surface and near-surface defects penetrant can miss; requires magnetization of the part.
• Radiography. Shows internal porosity and inclusions as density differences on film or digital. Ideal for pipe and closed sections, but needs access both sides.
• Ultrasonic testing. The standard answer for lack of fusion and root penetration, effective for toe cracks. Needs a skilled technician and sound access surfaces.

What “rejectable” actually means

AWS D1.1:2025, the 25th edition approved by ANSI on 19 March 2025, draws a hard line on piping porosity. Transverse tension CJP groove welds have zero tolerance for visible piping porosity. Other static groove and fillet welds limit visible piping porosity of 1/32 in. or larger to 3/8 in. in any 1 in. of weld and 3/4 in. in any 12 in. of weld. Interpretation ID1-91-010-00 adds nuance: the code sets no surface porosity limit against visual inspection, so where surface porosity is a concern, the limit shall be agreed between owner and contractor. See the AWS D1.1:2025 preview, AWS porosity guidance and the Welding Inspector’s Handbook.

Repair rules, and when not to repair

KOBELCO’s welding defect handbook and the Australian Welding Institute’s notes on fusion and porosity defects agree on the order of operations: gouge out to sound metal, and re-weld only once the root cause is resolved. Slag and inclusion defects must be gouged out and re-welded with proper preparation and technique.

1. Remove the defect back to clean sound metal, leaving a surface fit for re-welding. Air arc gouging is standard on carbon steel; on stainless and aluminium, mechanical removal protects the parent metal.
2. Restore the joint geometry and the required weld size. A grind-and-re-weld that leaves a thinner section is not a repair.
3. Re-weld to a qualified procedure, not a “same as before but slower” guess.
4. Re-inspect the repair to the same criteria as the original weld.

Knowing when not to repair matters too. Some code-controlled items need the Engineer’s approval, and members with reduced section thickness after gouging may fall outside the qualified procedure. If the root cause is unknown, the base metal is suspect, or the joint is in a fitness-for-service assessment, escalate rather than grind and re-weld. Rule out the supply side too: a poor-condition cylinder, an uncalibrated flowmeter or a failed leak check will spoil every repair. Our guides to welding equipment checks and leak detection and gas cylinder and supply service cover that.

The exposure you create while fixing it

Repair work doubles your exposure: grinding out a defect throws metal dust and coating particles into the air, then you re-weld in the same space. OSHA 29 CFR 1910.253 requires general mechanical ventilation of at least 2,000 cubic feet per minute per welder when welding in a space under 10,000 cubic feet per welder, in rooms with ceilings under 16 feet, in confined spaces, or where partitions block cross-ventilation. Local exhaust hoods must hold 100 linear feet per minute at the welding zone. See eCFR 1910.253 and OSHA Fact Sheet 3647 on welding.

Grinding hot-rolled steel, coated stock or stainless adds a particulate load a welding hood will not capture. Size extraction for the grinder, not just the arc, and treat the repair as a second job with its own controls.

The Ten-Step MIG Welding Defects Prevention Checklist

Nearly every MIG welding defect can be traced back to one of five causes: gas, cleanliness, heat, geometry or restraint. With that in mind, the following ten steps can be worked through in order at the bay to prevent most issues before they start. As TWI’s guidance on weld defects and the Australian Welding Institute’s fabrication defect advice both state, the majority of defects come down to process control, not luck.

1. Prepare the Joint Properly

Before striking an arc, the joint geometry must be correct. A root face that is too thick can prevent penetration, a bevel that is too narrow can restrict sidewall fusion, and a root gap that is too tight can lead to lack of fusion or burn-through. Tacks can also be deceptive, as overly heavy tacks can close a gap that the root pass needs to melt through. From there, all joint faces, edges and tack points must be free of rust, mill scale, paint, oil, grease or primer, and cleaned immediately before welding to prevent contamination.

2. Keep Filler Clean and Wire Dry

Moisture is the direct cause of hydrogen porosity. Spools left open to shop air will pick up moisture over time, so they should be stored off of the floor and kept sealed when not in use. For aluminium specifically, the oxide layer reforms extremely quickly after cleaning, so joint prep and welding must be done back to back. For more on this specific process, refer to our guide to prepping aluminium for welding.

3. Match Shielding Gas to the Material

Gas selection is just as important as wire selection. For carbon steel, ER70S-6 is the most versatile due to its deoxidisers, whilst ER70S-5 is better suited for dirtier or higher-sulphur steels. For shielding, 90/10 argon-CO2 tends to provide a stable arc with less spatter for many fabricators, whilst 75/25 can be more forgiving on less pristine steel at the cost of increased spatter. Stainless steel and aluminium require argon, or an argon-helium mix, with CO2 never being used for either.

4. Set Flow Rate and Seal the System

Leaks are one of the most overlooked sources of porosity. Check cylinder valves, hose fittings and connections before welding, and verify the flow rate at both the regulator and at the torch with a flowmeter. Any large discrepancy between the two readings will point towards a leak. Avoid cranking the flow to excessive amounts, as turbulent shielding gas will pull in atmosphere far easier than a controlled, laminar flow. In draughty areas, a windscreen is a necessity.

5. Maintain the Torch

A clean torch is a consistent torch. Nozzles, gas diffusers and contact tips should be kept free of heavy spatter, with the contact tip recess set to the manufacturer’s recommendation. According to Miller’s troubleshooting guidance, the nozzle must be large enough to fully shield the weld pool, and stickout should be kept to around half an inch past the nozzle. Going much longer will destabilise the arc and drastically increase atmospheric contamination.

6. Hold a Stable Current and Voltage Relationship

A stable transfer mode is audible. A smooth, consistent crackle is the goal for short circuit MIG, whereas harsh popping indicates that voltage is too low for the set wire feed speed. If your machine has inductance control, utilise it to soften the short circuit spikes that create excess spatter. These adjustments directly combat spatter whilst keeping the arc controlled for better fusion.

7. Get the Angles Right

There are two angles to watch for. The torch angle is the gun’s angle to the plate, typically around 45 degrees into the joint, whilst the work angle is the angle of the torch relative to the travel direction. Holding the gun perfectly perpendicular will often undercut the toes of the weld, whilst an inconsistent arc length will destabilise the pool. Keeping the wire slightly leading into the puddle with constant travel is the most reliable way to maintain fusion.

8. Protect Against Wind and Manage Flow

This discipline ties directly back to step four. Never increase gas flow as a reaction to suspected wind contamination. This will create turbulence and draw in more air, not less. Instead, eliminate draughts by repositioning the work, closing doors or using wind protection. If the environment cannot be controlled, the shielding gas cannot be reliably controlled either.

9. Tack Well and Finish Properly

Tacks should hold the fit-up without collapsing the root gap. Fit-up bars can be used to achieve this, preventing the root pass from being starved of space to fuse. Equally as important, use the crater-fill or burn-back function if your machine is equipped with it at the end of a weld. Simply releasing the trigger will allow the molten crater to shrink rapidly, leaving it prone to crater cracking.

10. Inspect and Clean Up Before the Next Pass

Between passes, all spatter, slag and loose debris must be removed. Spatter left on the previous bead can become trapped under the next, forming inclusions that are almost impossible to detect once buried. Take a moment to inspect the toe, face and edges of each run before continuing. Small corrections now prevent defects that would require extensive rework later.

These ten steps may seem simple, but their consistency is what separates rework from first pass acceptance. By controlling gas, cleanliness, heat, geometry and restraint at each stage, the vast majority of MIG welding defects become entirely preventable.

Frequently Asked Questions

What is the most common MIG welding defect?

Porosity is the defect MIG welders encounter most often, and it is also the most often misdiagnosed. The usual root causes are poor shielding (drafts, leaking hoses, too high or too low gas flow, a dirty gas diffuser, excessive stickout), contamination or moisture on the joint, and hydrogen from damp wire or coated surfaces. Because almost every porosity case traces back to gas, cleanliness or heat input, a systematic check of those three areas resolves most cases.

What is the difference between a weld discontinuity and a weld defect?

A discontinuity is any interruption of the normal structure of a weld, such as a small pore or a minor undercut. A defect is a discontinuity that falls outside the acceptance criteria of the governing specification. The distinction decides whether a weld passes or fails, so always work from the acceptance criteria in the code or contract that governs the job rather than from general appearance.

Can a MIG weld with porosity be ground out and rewelded?

Often yes, provided the rules of the governing code allow it. The defect must be removed by grinding or gouging to sound metal, the joint restored to full size and geometry, the area rewelded with a qualified procedure, and the repair re-inspected to the same acceptance criteria. If the cause is not corrected first — a leaking gas line, for example — the repair will simply repeat the defect.

Why am I getting excessive spatter from my MIG welder?

In most cases the voltage is too low for the wire feed speed you are running, which is the single most common contributor to spatter regardless of transfer mode. Other culprits include inductance set too low, a dirty nozzle or gas diffuser, rust or mill scale on the plate, a 75/25 argon-CO2 mix where a 90/10 would suit the job, and arc blow. Raise the voltage a touch, check the inductance setting, and clean the gun.

How do I tell lack of fusion apart from incomplete penetration?

Lack of fusion means the weld metal failed to bond to the joint face or to the preceding bead, and it usually shows as a dark line along the sidewall or between passes. Incomplete penetration means the root of the joint was not fused through its full thickness, and it is only really visible in a cross-section or confirmed by radiography or ultrasonic testing. Both usually trace back to insufficient heat input, too narrow a groove, or a root gap that closed up.

Does shielding gas choice really change defect rates?

Yes, and in several ways. A higher argon content gives smoother, more consistent transfer with less spatter, which also helps when welding stainless and aluminium; pure argon is required for aluminium and for most MIG welding of non-ferrous metals. Adding CO2 increases penetration and weld metal strength but reduces ductility and increases spatter. Gas purity and cylinder condition matter just as much: moisture or a depleted cylinder feeds hydrogen straight into the puddle and causes porosity.

What is the safest way to test a shielding gas line for leaks?

Use a liquid leak detector solution on the threaded joints, the hose connections and the fittings, and verify actual delivery with a torch-mounted flowmeter rather than trusting the regulator. If the torch flowmeter reads much lower than the station flowmeter, the fault is between them. Remember that flowmeters are calibrated at a specific inlet pressure, so a reading on a 50 psig line with a 20 psig-calibrated meter understates the true flow by roughly a third.

How do I stop my weld cracking at the end of the bead?

That is almost always a crater crack caused by breaking the arc before the pool has filled and solidified. Use the crater-fill function on your power source so the arc continues at reduced heat input while the end of the pool solidifies from the top down, and allow the correct burn-back time so the wire does not freeze into the pool. Keep the bead size matched to the material thickness, because an undersized bead on restrained joints is a common underlying cause.