How to Tell If a MIG Weld Is Good Quality

Written By: Liam Bryant

Reviewed By: Russell Egan

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A good quality MIG weld exhibits even bead width, consistent ripple pattern, minimal spatter, proper penetration, and smooth transitions at the start and finish. Inspect for a uniform appearance, lack of visible defects (cracks, porosity, undercutting), and proper fusion between the weld and base metal.

Welding is a very versatile technique that you’ll find wherever you find different metals. However, not all welds are created equal. Some welds are not the best quality, whether due to a mistake in the welding set-up or mistakes on the part of the welder. Whether you are examining the quality of someone else’s weld or checking to see if your own mastery of MIG welding is complete, you should know how to tell if a MIG weld is good.

A few factors can help you determine if a MIG weld is good based on sight alone. These include the consistency of the weld pattern, coloration, bead profile, and more. These same factors help you identify if a MIG weld is bad.

Here is your detailed guide to identifying if a MIG weld is good or bad and what to look for.

prevent welding undercut

Signs of Good Weld Quality

MIG welding is fairly easy for beginners once you master the set-up. One of the advantages of MIG welding is that it creates good quality welds, as long as you know what you are doing. Examining your work after you’re done can help you tell if you created a good quality weld.

Here are some of the main signs of a quality MIG weld to look out for.

1. Consistent Welding, No Weld Pattern

Many beginners use weld patterns to master MIG welding. They use patterns such as whipping, weaving, and circle welding to achieve consistency throughout the joint. However, the true mark of a master welder is when there is no visible pattern. Top-notch welders don’t need a pattern to achieve consistency across their weld. Instead, they rely on their own control over the MIG welding machine

When you look at a weld joint, it should be smooth and consistent, without any visible weld patterns. If you or the welder did use a weld pattern, it should be consistent throughout, without major differences in size, spacing, and other factors.

2. No Cracks

Another sign of a quality MIG weld is a completely smooth weld. The joint shouldn’t have any cracks. Cracks are not only ugly to look at, they are also a sign that the structural integrity of the weld is compromised. 

3. No Slag

Slag is a byproduct produced in welding. It looks like ripples or deposits of molten metal that forms around the weld. A good MIG weld will not have any slag or minimal slag. The joint is smooth and uniform throughout.

4. Coloration

The color of the weld joint is a surprising tool that can help you tell if the MIG weld was a quality job or not. The color of the joint should be the same color as the base metal or a very similar shade. This means that no oxidation occurred and that the weld is of a sound quality.

5. Bead Profile

Finally, the bead profile is another thing to look for that can help you tell if a MIG weld is quality or not. The bead profile should be completely flat along the joint. If it is concave, or sunken, or convex, which means raised, then the quality of the weld is not as high as it should be. 

The bead should also be a completely even thickness throughout the joint and evenly fill the joint. The bead can tell you if the weld has enough structural integrity or not.

How to Identify a Bad Weld

Just as you should know how to identify a good weld, you should also know how to identify a bad weld. Whether you’re looking at a weld you made yourself or one performed by someone else, there are a few reasons why you should know how to identify a bad weld.

  • You want to know if your project is structurally sound.
  • You want to know where you need to improve on welding.
  • You want to give feedback to someone you are working with or supervising.

Here are the main signs of a bad MIG weld to look out for.

1. Cracks

The biggest sign of a bad weld is cracks, slag formation, or holes. These indicate that there was a lack of consistency in the welding. They are also very visible signs of a bad weld. 

If you are welding a structurally important joint and notice cracks or other gaps in the joint, try to repair it immediately. Cracks also affect the strength of the weld, not just the appearance.

2. Discoloration

Another very visible sign of a bad weld is too much discoloration or not enough discoloration. MIG weld joints change color a little bit because the heat changes the properties of the metal, so you should notice a slight difference in shade between the base metal and the joint. 

However, the joint shouldn’t be a completely different color. If it is very discolored, that means the joint oxidized as you were MIG welding. Oxidation is common when gas interacts with the weld, which also affects the strength of the weld.

3. Thin Bead

Another thing to look for is the quality of the bead. If the bead is very thin, that is a sign of a bad weld. The bead should have the same thickness across the joint and fill out the gap between the two metals nicely. A thin bead is lacking in strength, which could cause the joint to collapse.

4. The Weld Is Not Straight

Finally, a major sign of a bad MIG weld is any lack of consistency, including the direction. If the weld is not straight, that is a sign of a bad weld. Other common problems include dips, bumps, and visible patterns.

Final Thoughts

Once you know how to weld yourself, you will always be examining the quality of the welds around you. You may be disappointed to see how many welds have thin beads, crooked lines, discoloration, and cracks, which are common signs of bad MIG welds.