MIG Welding Defects – Types, Causes and How to Fix Them

I remember the first time my weld got rejected on a job site. I was on a structural fabrication project, second week of my first proper contract. The QC inspector walked over, looked at my fillet weld on a bracket, and circled two spots with a marker. He said one word — "undercut." I had no idea what that meant at that point. I thought the weld looked fine visually.

My foreman explained it to me after. And then he showed me three more defects on my other joints that I had completely missed. That was the day I started taking MIG welding defects seriously.

Every welder faces defects at some point. Beginners face them often. Even experienced welders hit problems when material condition, machine settings, or site environment changes. The difference between a good welder and an average one is knowing exactly what went wrong and how to fix it properly. This guide covers every major MIG welding defect, what causes it, and how to repair it.

Why MIG Welding Defects Happen

Welding defects in MIG welding occur when the weld metal, heat input, shielding gas coverage, or base metal preparation does not meet the required standard. Some defects are visible on the surface. Others are internal and only detected through radiographic or ultrasonic testing.

Most MIG welding problems come from three areas — wrong machine settings, poor base metal preparation, and incorrect welding technique. Fix those three areas and you eliminate 90% of all defects.

1. Porosity in Weld

Porosity is the most common MIG welding defect I have encountered in fabrication work. It appears as small holes or pits on the surface of the weld or inside the bead. It looks like the weld has small craters scattered across it. Under bend test or X-ray, internal porosity shows up as cavities that should not be there.

What causes porosity in MIG welding:

Contamination on the base metal is the most frequent cause. Oil, grease, rust, mill scale, paint, and moisture all release gases when the heat hits them. Those gases get trapped in the molten pool as it solidifies and porosity forms.

The second major cause is gas coverage failure. Too low gas flow means the weld pool is exposed to atmosphere. A damaged gas line, worn-out O-rings, a broken gas solenoid, a cracked MIG liner, or loose fittings will all disrupt gas flow and allow air into the weld.

I once had a full shift of porosity problems and could not find the cause. Finally found a small crack in the gas hose near the regulator. That tiny leak was enough to pull atmosphere into the weld zone throughout the entire shift.

How to fix porosity:

The only way to properly repair weld porosity is to remove the porous section of the weld down to the base material and then clean it again before welding that section again. Attempting to weld over the affected bead will only end in the same result.

Grind out the affected area completely, clean the base metal, check gas flow, and re-weld.

2. Undercut in Weld

Undercut is a groove or channel that forms along the edge of the weld bead where it meets the base metal. Instead of the bead tying smoothly into the base material, it cuts into it and leaves a notch. This reduces the effective thickness of the base metal at that point and creates a stress concentration area.

My QC inspector used to check for undercut with a weld gauge. Anything more than 0.5mm to 1mm deep depending on the WPS requirement meant automatic rejection on the projects I worked on.

What causes undercut:

Undercut can be created when the welder focuses the arc too much on the bottom plate and not enough on the top plate of the weld joint. Other common causes are excessive voltage and travel speeds that are too fast without using enough filler metal.

Incorrect torch angle is usually the main problem for beginners. I had undercut problems specifically on horizontal fillet welds until my supervisor showed me to direct the arc more toward the vertical plate instead of the flat plate.

How to fix undercut:

Reduce voltage slightly and slow down your travel speed. Correct your torch angle. For existing undercut on a completed joint, grind the affected area smooth and apply a repair weld pass to fill the groove. Then blend and grind to profile.

3. To Much Spatter

Spatter means small droplets of molten metal flying out from the arc and sticking to the base metal surface around the weld. It does not affect the weld strength directly but it creates extra cleaning work, can cause surface contamination, and looks unprofessional on finished fabrications.

On every job I have worked, excessive spatter was treated as a sign of poor technique or wrong settings.

What causes excessive spatter in MIG welding:

Voltage too low for the wire speed is the most common cause. When voltage is insufficient, the arc becomes unstable and short circuits repeatedly in an erratic way, throwing spatter in all directions. Using pure CO2 gas also produces significantly more spatter than C25 Argon mix.

Contaminated base metal and incorrect torch angle also contribute to spatter levels.

How to reduce spatter:

Adjust voltage and wire speed to achieve a smooth, consistent arc sound. Switch from pure CO2 to a C25 mix if spatter remains high. Clean base metal before welding. Apply anti-spatter spray to the nozzle and surrounding base metal surface before starting. For existing spatter, use a spatter chisel or grinder to remove it from the base metal.

4. Lack of Fusion

Lack of fusion is a serious defect where the weld metal does not properly fuse with the base metal or with the previous weld pass. The bead sits on top of or alongside the base metal without actually bonding to it. Lack of fusion, sometimes referred to as "cold lapping," occurs when there is little or no union between the weld metal and the base metal, normally caused by lack of heat input at the start of the weld.

This is one of the defects that gets caught in bend tests and radiographic inspection. Visually the weld can look acceptable but under testing it fails.

What causes lack of fusion:

Voltage too low is the main cause. Travel speed too fast also reduces heat input below what is needed for proper fusion. Incorrect torch angle on groove welds means the arc is directed away from the fusion face instead of into it. Contamination or tight joint fit-up also prevents proper fusion on the root pass.

How to fix lack of fusion:

This defect requires complete removal of the affected weld pass. Grind back to clean base metal, correct your voltage and travel speed, verify torch angle, and re-weld. Never attempt to weld over a cold lap — it will not fix the fusion problem underneath.

5. Burn-Through

Burn-through happens when the heat input is too high and the arc melts completely through the base metal, creating a hole. This is most common on thin material and on root pass welding where the gap is too large.

In my early days of MIG welding thin sheet metal, burn-through was my biggest problem. I was running settings that worked fine on 6mm plate but were completely wrong for 1.5mm sheet.

What causes burn-through:

Voltage too high and wire speed too low for the material thickness. Travel speed too slow allows heat to build up. On thin material, even a brief pause in travel movement can cause instant burn-through. Root gap too wide on groove welds also increases burn-through risk.

How to fix burn-through:

For thin material, reduce voltage, increase travel speed, and use a smaller wire diameter. For detailed settings matched to specific material thickness, our guide on MIG Welding Settings for 1mm, 2mm, 3mm Steel covers exact parameters.

For existing burn-through holes, fit a backing plate or backing tape, fill the hole with a proper weld pass at correct lower settings, then grind and profile to finish.

6. Overlap Weld

Overlap is when the weld metal rolls over onto the base metal surface without fusing into it. It looks like the bead has flowed over the edge of the joint but the bond is not there. The weld pool was too fluid and gravity pulled it over before it fused.

This defect is common in overhead and vertical down welding when settings are not adjusted correctly for the position.

What causes overlap:

Voltage too high makes the weld pool too fluid. Travel speed too slow allows excess weld metal to pile up and roll over. Incorrect torch angle directing the puddle away from the joint also contributes.

How to fix overlap:

Grind off the overlapping weld metal and re-weld with corrected settings — reduce voltage, increase travel speed, correct torch angle. On positional welding, always adjust settings downward from flat position values.

MIG Welding Defects Reference Table

Correct MIG welding voltage and wire speed settings are the foundation for preventing most of these defects. Our complete settings reference is at MIG Welding Settings – Voltage and Wire Speed.

What My QC Inspector Taught Me About Defect Prevention

My QC inspector on a Gulf refinery project had a habit of watching the welder set up before any welding started. He used to say — "A welder who checks his gas flow, cleans his base metal, and reads the WPS before striking the arc will give me almost zero defect work. A welder who just picks up the torch and starts immediately gives me problems every day."

That observation changed how I start every welding job. Before striking the arc I always check gas flow, inspect the nozzle for spatter buildup, verify my settings against the WPS, and confirm the base metal is clean and properly tacked. Those five minutes of preparation eliminate most defect causes before they happen.

FAQ

Q: What is the most common MIG welding defect? Porosity is the most frequently encountered MIG welding defect. It is caused by gas coverage failure, base metal contamination, or moisture on the wire or base material.

Q: Can you weld over a defect to fix it? No. Most weld defects including porosity, lack of fusion, and cracks must be completely removed by grinding back to clean base metal before repair welding. Welding over a defect traps it inside and creates a worse condition.

Q: How do I know if my MIG weld has internal defects? Surface defects are visible on inspection. Internal defects like subsurface porosity and lack of fusion are detected through radiographic testing (RT), ultrasonic testing (UT), or destructive bend testing as specified in the WPS or project inspection plan.

Q: What causes undercut on MIG fillet welds? Excessive voltage, travel speed too fast, and incorrect torch angle directed toward the flat plate instead of the vertical member are the main causes of undercut on fillet welds.

Q: How do I prevent burn-through on thin MIG welding? Use smaller wire diameter, reduce voltage, increase travel speed, and maintain continuous movement without pausing. Using a backing plate or copper backing bar also helps control heat on very thin material.

Conclusion

MIG welding defects are not random. Every defect has a cause and every cause has a fix. Porosity comes from gas or contamination issues. Undercut comes from voltage and angle problems. Burn-through comes from wrong settings on thin material. Lack of fusion comes from insufficient heat input.

In my years of fabrication work, the welders who produced the least rework were not always the most naturally talented. They were the ones who understood their defects, knew what caused them, and corrected the problem at the root before continuing. That is the mindset that keeps a QC inspector satisfied and keeps you employed on quality projects.