Why You Get Porosity in MIG Welding and How to Fix It

Table of Contents > 1. What is Porosity in Welding?     1.1 Is Porosity Bad for Your MIG Welds?     1.2 Types of Porosity in MIG Welding 2. What Causes Porosity in MIG Welding and How to Solve It?     2.1 Inadequate shielding gas flow and porosity         2.1.1 How to solve porosity caused by poor gas flow?     2.2 Wrong shielding gas type for application         2.2.1 How to Choose the Right Gas to Avoid Porosity?     2.3 MIG welding in draft or wind         2.3.1 How to Deal With Wind or Draft?     2.4 Dirty metal surfaces and poor preparation         2.4.1 Clean the Surfaces to Reduce the Risk of Porosity     2.5 Moist or dirty MIG wire and porosity         2.5.1 How to Handle the Wire to Reduce the Risk of Porosity?     2.6 Contaminated cylinder or hoses         2.6.1 Check the Cylinder and Hoses     2.7 Clogged nozzle or contact tip         2.7.1 Replace Worn-out or Damaged Nozzle and Contact Tip to Avoid Porosity     2.8 Too-long wire stickout         2.8.1 What is the Recommended MIG Stickout?     2.9 Using the wrong torch angle         2.9.1 What is the Right Technique to Avoid Porosity?     2.10 Moving too fast and taking the torch out of joint         2.10.1 Consistency is Crucial in Avoiding Porosity 3. How To Fix Porosity? 4. Final Thoughts 5. 🧐 Why You Get Porosity in MIG Welding and How to Fix It FAQ

Welding defects are common and can occur even among experienced welders. Among them, porosity is one of the most common.

Although porosity is common, many welders, especially less experienced ones, wonder why it occurs in MIG welding and how to fix it. The key to addressing the issue lies in understanding why it happens and how you can avoid it.

Our article discusses MIG welding porosity, including its causes, solutions, and possible fixes. Read more to learn how to deal with “swiss cheese” in your welds.

What is Porosity in Welding?

Porosity is one of the most common welding defects in MIG welding and other welding processes. It occurs when atmospheric gases or contaminants enter a molten weld pool.

Gases and contaminants become trapped in the molten puddle. As it solidifies, contaminants evaporate, leaving holes, pores, and small cavities. Whether you are a professional or a new welder, you've likely seen tiny holes on top of your weld; that's porosity.

Is Porosity Bad for Your MIG Welds?

The severity of porosity in MIG welding depends on the size, amount, and location of porosity. Tiny surface porosity is, in most cases, mostly an aesthetic defect. Surface pinholes affect weld appearance, making it difficult to paint, polish, or machine.

However, each hole indicates a lack of weld metal within the joint. Severe, internal (subsurface) porosity affects the structural integrity of the weld. Voids and wormholes reduce the load-bearing capacity of the weld.

As a result, undetected internal porosity increases the risk of cracking or joint failure under stress. It can cause failures or leaks in critical components in welding applications.

Types of Porosity in MIG Welding

Welding porosity is often grouped by location and appearance. These are the most common types of porosity in MIG welding:

• Surface porosity: If gases and contaminants are trapped near the surface, they appear as small holes on the surface of the welds. Surface porosity is the most common and easiest to detect, as it is visible to the naked eye. Some welders call it "Swiss cheese" because of the distinctive holes on the weld surface.
• Subsurface porosity (internal porosity): Once gases become trapped within the welds, they can create internal porosity. This is a much more severe type because it cannot be detected without NDT testing.
• Distributed porosity: Pores spread uniformly or randomly throughout the entire weld metal.
• Clustered porosity: A group or localized pores concentrated in one specific section.
• Linear porosity: Individual pores arranged in a distinct line, commonly along the fusion boundary.
• Wormhole porosity: Elongated pores resembling a worm crawling through the weld. They look like a herringbone in an X-ray.
• Crater pipes: Craters form at the end of the weld due to insufficient metal and contraction.

What Causes Porosity in MIG Welding and How to Solve It?

Understanding the leading causes of porosity is crucial in addressing the defects. The leading causes of porosity in MIG welding are grouped as follows:

• Poor shielding gas coverage (inadequate shielding gas flow, wrong shielding gas type, wind or draft)
• Dirt and contamination (dirty or rusty surface metal, moist or contaminated filler wire, dirty or contaminated gas cylinder, clogged nozzle or tip)
• Poor welding technique (incorrect wire stickout, wrong torch angle, excessive travel speed)

Before troubleshooting each issue in detail, use this quick table to match the most common porosity causes with the right fix.

Let's explore each cause in detail and explain how to fix it.

1. Inadequate shielding gas flow and porosity

Shielding gas coverage is a leading cause of porosity because it allows contaminants to reach the molten weld puddle. Among the most common reasons is inadequate shielding gas flow; either too low or too high.

Low gas flow creates poor shielding, allowing atmospheric gases such as nitrogen, hydrogen, or oxygen to reach the molten weld pool. Keep in mind that too high flow can also cause issues. Unnecessarily high gas flow (over 50-60 CFH) creates turbulence that sucks in contamination, rather than creating shielding.

Porosity Caused by 50CFH Gas Flow
Source: https://www.youtube.com/watch?v=aQsDHMIJ6H0

How to Solve Porosity Caused by Poor Gas Flow?

To avoid porosity caused by poor shielding, adjust the flow rate based on current conditions. If you are MIG welding indoors, with little to no draft, use 15-25 CFH.

You can use a lower end in well-protected areas and on thinner metals (15-20 CFH). Increase the flow to 20-25 CFH if a draft is present in the workshop.

2. Wrong shielding gas type for application

MIG welding uses several shielding gas types, typically Argon, CO2, or a combination of both. Choosing the wrong shielding gas for the given metal type or thickness can cause porosity.

There is no one-size-fits-all approach when choosing MIG welding shielding gas. Whenever you change the metal type and thickness, make sure you use the correct gas.

Choosing the Right MIG Gas
Source: https://weldguru.com/best-gas-for-mig-welding/

How to Choose the Right Gas to Avoid Porosity?

When MIG welding mild steel, most welders use C25 shielding gas (75% argon, 25% CO2). This mixture provides the best balance between argon protection and CO2 penetration.

Welders often use pure (100%) CO2 shielding to reduce costs. While economic, CO2 is a semi-inert gas, so it can cause increased spatter and rough welds. In some applications, it can even cause porosity if used incorrectly.

MIG Welding with 100% CO2
Source: https://www.youtube.com/watch?v=pTsithkxj8U&t

MIG welding aluminum, due to its reactivity, requires a nonreactive (inert) shielding gas. Therefore, always use 100% argon, or add some helium on thicker pieces.

MIG welding stainless steel usually requires a two-gas mix of 98% argon and 2% oxygen, or a tri-gas mix of helium, argon, and CO2. These increase fluidity in the pool, but the amounts are low to reduce the risk of oxidation and porosity.

3. MIG welding in draft or wind

Even a slight breeze, wind, or draft can blow away the shielding gas. Although the nozzle directs the shielding gas into the joint, the wind can still make the molten pool vulnerable.

The lack of shielding gas allows atmospheric gases to enter the joint. As a result, you can see porosity, which can eventually cause cracks or failure.

How to Deal With Wind or Draft?

If you notice a draft in a workshop or increased air movement, increase the gas flow or use a larger nozzle for better coverage. However, as previously noted, do not overdo it, as you might create turbulence.

Avoid MIG welding outdoors. Gas welding processes, including MIG, are not suitable for outdoor welding due to the risk of porosity. Instead, use self-shielded, flux-cored wire, or stick welding.

4. Dirty metal surfaces and poor preparation

Contamination, such as dirt, dust, rust, oil, grease, paint, mill scale, and coatings, can become trapped in welds. As you burn contaminants, they turn into gases and fumes.

Gases and fumes trapped in welds can cause porosity; they can boil, causing spatter, or form toxic fumes that can be harmful. The issues are even more significant on non-ferrous or reactive metals.

Poor joint fit-up and preparation can suck air in from the back side of the joint. You may get porosity, even if you did everything else right.

Clean the Surfaces to Reduce the Risk of Porosity

One precaution to prevent porosity is to clean the metal surfaces before welding. While Stick welding can safely burn through some surface contamination, MIG is not forgiving.

Always clean the surface with a wire brush, clean clothing, or solutions if there is stubborn contamination. Spending some time cleaning before MIG welding will save you time on rework and porosity.

In addition, don't forget about joint fit-up. Follow the procedure specifications and ensure a tight fit-up to reduce the risk of porosity.

5. Moist or dirty MIG wire and porosity

Contaminated, moist, or dirty MIG wire can also cause porosity. Moist or contaminated wire can introduce hydrogen or oxidation into welds. Hydrogen and oxidation create void zones, pores, and cracks you want to avoid.

This issue may happen with any filler metal, such as TIG rod, Stick electrode, or MIG wire. Even dirty or greasy gloves can contaminate your wire if you touch it directly.

Damaged or contaminated liner can also cause porosity. Wire will pick up contamination as it runs to the joint, eventually causing porosity in the welds.

Contaminated MIG Wire Caused Porosity
Photo by @DanWelds (TikTok)

How to Handle the Wire to Reduce the Risk of Porosity?

To reduce the risk of MIG wire contamination, handle and store it properly. Store your MIG wire in a dry, moisture-free environment (e.g., a dry cabinet or an airtight bag).

Store MIG Wire in an Airtight Bag
Source: https://www.youtube.com/watch?v=vkf0HHS9L9E

Before welding, check your wire for signs of damage, rust, or oxidation. These can introduce porosity into welds, resulting in inconsistent welds. In addition, avoid touching the wire with bare hands or dirty gloves, as it can grease it.

Although MIG is not as clean as TIG, it still needs contamination-free surfaces and consumables to avoid porosity. If greasy, wipe the wire with steel wool or wire wipes.

Don't forget to check the liner for signs of damage or contamination. If in poor condition, replace it before welding.

6. Contaminated cylinder or hoses

Even though everything is clean and set right, contamination can come right from the source - the cylinder. The cylinder contaminates the shielding gas, which sprays contaminants directly into welds, causing porosity and other defects.

In addition, your gas hoses may be damaged or contaminated. A typical example is a gas hose you used for other applications that may contain oil or dirt. Switching it to a welding gas hose may introduce contamination into the joint.

Also, check hoses and connections for leaks or signs of damage. Damaged hoses supply less shielding gas than required, eventually causing porosity in the joint.

Hose Leak Causing Weld Porosity

Check the Cylinder and Hoses 

Contact the gas and cylinder suppliers to ensure you receive clean shielding gas. Check the hoses and connections for signs of damage, and replace them if needed.

7. Clogged nozzle or contact tip

MIG torches use a nozzle that directs shielding gas into the joint, protecting the internal components from damage. But the nozzle is first in the line of "fire," so it picks up a lot of spatter and molten metal.

Once the nozzle gets clogged, it affects wire feedability and shielding gas coverage; poor coverage leads to porosity. The same goes for the contact tip, which guides the wire and conducts the welding current.

Clogged Contact Nozzle
Photo by @juraganlas_id (TikTok)

Replace Worn-out or Damaged Nozzle and Contact Tip to Avoid Porosity

Nozzles and contact tips are consumables that wear out during use. Check your nozzle and contact tip before welding to ensure they can provide sufficient shielding and good wire feedability.

If necessary, clean the nozzle with pliers to reduce the risk of porosity. If there is a lot of hard spatter, it is better to replace the nozzle rather than clean it completely. The same goes for the contact tip.

Also, be aware of excessive use of anti-spatter gel. Anti-spatter gel is useful, but only when used correctly. Excessive gel can become a contaminant and burn off in the gas stream, leading to porosity.

8. Too-long wire stickout

In MIG welding, the stickout is the length of the wire extending from the contact tip. In general, MIG welding uses a consistent stickout of 3/8 to 1/2 inch.

If your stickout is too long, it increases resistance and lowers your actual welding amperage. However, it also means your nozzle is far from the joint, meaning a limited amount of gas reaches the weld. As previously noted, a limited amount of gas means a risk of porosity.

Long Stickout Causes Porosity
Source: https://www.youtube.com/watch?v=hEB_aUqYC8Y

What is the Recommended MIG Stickout?

To reduce the risk of porosity and achieve consistent welds, keep the stickout between 3/8 and 1/2 inch. That way, you weld near the joint, meaning your nozzle is close enough to supply sufficient shielding.

But be careful not to overdo it. Welding too close to the joint increases the current, which can lead to burn-through. In addition, it can melt the contact tip and clog the nozzle, leading to porosity once again.

9. Using the wrong MIG torch angle

The tilt of the MIG welding torch, or the angle along the direction of travel, is called the travel angle. Using the right angle is crucial in producing consistent, spatter-free welds.

However, angles over 20 degrees can cause porosity. The nozzle is tilted far away from the plate, so the gas does not completely cover the weld pool. As a result, the molten puddle reacts with atmospheric contaminants, leading to porosity.

Wrong Torch Angle in MIG Welding
Source: https://www.youtube.com/shorts/NS48tFW_thU

What is the Right Technique to Avoid Porosity?

Using an appropriate MIG travel angle of 5-15 degrees can help eliminate various defects, including porosity. At push angles up to 15 degrees, with consistent speed and a distance of 3/8-5/8", you will get even gas coverage.

In addition to gas, the correct technique ensures stable wire feedability, consistency, and proper fusion. Assuming everything else is right, good technique will yield stable results and high-quality welds.

10. Moving too fast and taking the torch out of joint

If you move too fast across the joint, you risk porosity, among other defects. Too fast a travel speed means you move the shielding before the puddle has cooled and solidified.

As noted, a hot, molten weld puddle readily picks up contamination. Therefore, you shouldn't fly across the joint.

Also, don't remove your torch from the joint before filling the joint. Finishing the weld prematurely before allowing the wire to fill the end may create a crater. Craters can affect structural integrity and cause cracks.

Crater Defect in MIG Welding

Consistency is Crucial in Avoiding Porosity

Keep the travel speed moderate and consistent. Going slowly isn’t a solution, as you can burn through the metal. However, spend some time at the end of the weld to make sure you fill the crater.

How To Fix Porosity?

Porosity can occur even if you do everything right, for professionals and beginners alike. In case of light surface porosity, you can easily fix it, and here is how:

• Grind the affected area: Remove the porosity from the weld by grinding to the base metal. Do it until the holes are gone.
• Clean the area to remove contaminants: Clean the ground area again with a wire brush and solvent.
• Check your gas, parameters, and technique: Make sure you are using the right, clean shielding gas, proper parameters, and the right technique for the job.
• Move back and re-weld: Start a new weld slightly ahead of the existing weld, then back into it before continuing.
• Inspect the new welds: Check for signs of porosity. If there are signs, check your setup.
• Completely redo welds if needed: If there is deep porosity across the welds, you will have to remove the welds completely and redo them.
• Don't weld over porosity: Welding over porosity only covers the defects; it doesn't address them. The structural defects can still develop beneath the surface.

Final Thoughts

Porosity is one of the most common welding defects, evident in many processes, including MIG welding. Depending on the severity and location, it can be an aesthetic defect or cause severe weld failure.

To address porosity in MIG welding, you must understand its causes. The leading causes are poor shielding, contamination, and poor technique. Adjusting these will help you get the highest-quality welds, ideal for critical applications.

🧐 Why You Get Porosity in MIG Welding and How to Fix It FAQ

1. What is porosity in MIG welding?

Porosity is a welding defect that appears as small holes, pores, or cavities in the weld. It happens when gas becomes trapped in the molten weld pool before it solidifies.

2. What causes porosity in MIG welding?

The most common causes are poor shielding gas coverage, dirty base metal, contaminated MIG wire, clogged nozzles, and incorrect torch technique. Any air, moisture, oil, rust, or dirt entering the weld pool can create porosity.

3. Is porosity bad for a MIG weld?

Yes, especially when it is deep, widespread, or inside the weld. Small surface pinholes may mainly affect appearance, but severe porosity can weaken the joint and increase the risk of cracks or failure.

4. Can MIG wire cause porosity?

Yes. Wet, rusty, oily, or oxidized MIG wire can carry contamination into the weld. Store wire in a dry place and avoid touching it with dirty gloves or bare hands.

5. Why does dirty metal cause porosity?

Rust, oil, paint, grease, moisture, and mill scale can burn during welding and release gas. When that gas gets trapped in the weld, it leaves holes after the bead cools.

6. How do you fix porosity in a MIG weld?

Grind out the porous area until the holes are removed, then clean the metal and fix the cause. After checking gas flow, wire condition, and technique, re-weld the area.

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• Most Common MIG Welding Defects
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