Most welding defects trace back to a small number of root causes: contaminated or poorly prepared material, incorrect machine settings, inadequate shielding, worn consumables or the wrong filler metal. Identify which fault you have, correct the cause rather than welding over the symptom, and the majority of defects disappear.
This hub is the starting point for diagnosing any weld fault. It explains what each defect looks like, why it happens and how to prevent it — then links to in-depth sub-topic guides for porosity, cracking, fusion and penetration problems, undercut, spatter, wire feeding faults, burn back and arc instability. Use the answer-first sections below to find your fault quickly, then follow the link to the detailed page.
What are the most common welding defects?
The most common welding defects are porosity, cracking, lack of fusion, lack of penetration, undercut, overlap and excessive spatter, together with process faults such as poor wire feeding, burn back and an unstable arc. Each has its own causes, but clean material, correct parameters and quality consumables prevent the majority of them.
Defects fall into a few broad families:
- Gas-related — porosity, caused by gas trapped in the solidifying weld.
- Cracking — hot, cold and hydrogen cracking, driven by stress, heat input and consumable choice.
- Fusion and penetration — lack of fusion and lack of penetration, where the weld fails to bond or fails to reach through the joint.
- Weld profile — undercut and overlap, which create weak points and stress concentrations.
- Process faults — spatter, bird nesting, poor wire feeding, burn back and arc instability, mostly tied to settings, consumables and torch condition.
What causes porosity in welding?
Porosity is caused when gas becomes trapped in the weld as it solidifies, leaving small holes or cavities. Common causes include contaminated parent material, moisture, incorrect shielding gas flow, damp consumables or poor welding technique.
To prevent it, ensure the material is clean and dry, use the correct shielding gas and flow rate, store consumables correctly and protect the weld area from wind or draughts. Consistent consumables and correct preparation are essential for sound, porosity-free welds. For a full diagnosis — including why MIG and TIG welds go porous and how to repair them — see our guide to weld porosity causes and prevention.
Why is my weld cracking?
Weld cracking may occur during cooling or after welding due to excessive restraint, hydrogen, incorrect filler metal or unsuitable welding procedures. The type of crack — hot, cold or hydrogen — determines the likely cause, so identifying it is essential before any repair.
Using the correct consumables, controlling heat input and following approved welding procedures significantly reduces the risk. For thicker steels, preheating and low-hydrogen consumables are often required. Read more in our guide to weld cracking causes and prevention.
What is lack of fusion and lack of penetration?
Lack of fusion occurs when the weld metal fails to bond completely with the parent material or previous passes; lack of penetration occurs when the weld fails to extend fully through the joint thickness. Both are serious because they reduce joint strength and may not be visible from the surface.
They are commonly caused by insufficient current, excessive travel speed, poor torch angle or incorrect joint preparation. Sufficient heat input, correct travel speed and proper joint design help achieve complete fusion and full penetration. See lack of fusion and lack of penetration.
What is undercut and overlap?
Undercut is a groove along the edge of the weld where the parent metal has melted but not been filled with weld metal, creating a weak point. Overlap occurs when weld metal flows beyond the weld toe without fusing to the parent material, creating stress concentrations. Both are caused by incorrect current, travel speed or torch angle.
Correct welding parameters and technique produce a smooth weld profile that avoids both. See undercut, overlap and weld profile defects.
How do I reduce weld spatter?
Reducing spatter starts with correct machine setup — matched voltage and wire feed speed, correct stick-out and suitable shielding gas. Argon/CO₂ mixtures generally produce less spatter than pure CO₂, and premium-quality welding wire produces a smoother, more stable arc.
Clean material preparation and a well-maintained contact tip and nozzle complete the picture. For the full breakdown, see weld spatter causes and prevention.
Why won’t my MIG wire feed properly?
Poor wire feeding usually results from worn drive rolls, damaged liners, incorrect roller tension, blocked contact tips or poor spool braking. Bird nesting — where the wire tangles between the drive rolls and the liner — is a related fault caused by worn liners, incorrect tension or excessive resistance in the torch.
Systematically checking each part of the wire feed system, replacing worn consumables and using quality wire restores reliable feeding. See MIG wire feeding problems.
What causes burn back?
Burn back occurs when the welding wire melts back into the contact tip after the arc stops. It is commonly caused by incorrect wire feed speed, excessive voltage, worn or oversized contact tips or poor electrical contact.
Correct settings, proper stick-out and regular contact-tip replacement prevent it. See burn back and contact tip problems.
Why is my arc unstable or my welds inconsistent?
An unstable arc may be caused by incorrect voltage, inconsistent wire feed, contaminated material, worn torch consumables or poor electrical connections. Inconsistent welds usually stem from variations in technique, settings, consumable quality or material preparation.
Checking machine settings, the work return clamp and consumable condition — and following approved procedures — restores a stable, repeatable arc. See arc, gas and weld consistency problems.
Can the right consumables prevent defects?
Yes. A large share of defects — porosity, spatter, inconsistent feeding, burn back and cracking — share a root cause in worn, contaminated or low-quality consumables. Premium consumables are manufactured to tighter tolerances, offering more consistent wire feed, improved arc stability and reliable mechanical properties, which reduces downtime, spatter and rework.
When the fix for a recurring defect is consumable quality or storage, starting with the right wire, electrode or filler rod is the cheapest way to avoid rework. Explore our welding consumables guide, or contact the Weldability SIF technical team for advice on the right consumables for your application.
Choosing the right consumable for the job
Many defect repairs also come down to selecting the correct filler metal, electrode or shielding gas. Our defect cluster includes detailed selection comparisons drawn from real questions welders ask:
- MIG and TIG filler wire comparisons — ER70S-6 vs ER70S-3, 308L vs 316L, 4043 vs 5356 and more.
- MMA electrode comparisons — 6013 vs 7018, rutile vs basic, cellulosic vs low-hydrogen.
- Shielding gas comparisons — Argon vs CO₂, gas mixtures and TIG gases.
- Stainless steel grade comparisons — 304 vs 316, 304L vs 304, duplex.
- Process and equipment comparisons — MIG vs TIG, laser welding, torches and machines.
Sub-topic guides in this category
- Weld porosity causes and prevention
- Weld cracking causes and prevention
- Lack of fusion and lack of penetration
- Undercut, overlap and weld profile defects
- Weld spatter causes and prevention
- MIG wire feeding problems
- Burn back and contact tip problems
- Arc, gas and weld consistency problems
- MIG and TIG filler wire comparisons
- MMA electrode comparisons
- Shielding gas comparisons
- Stainless steel grade comparisons
- Process and equipment comparisons
Related categories: MIG Welding · TIG Welding · MMA Welding · Welding Consumables · Welding Standards & Compliance