Most MIG welding defects come back to the same handful of causes — incorrect machine settings, contaminated or poorly prepared material, worn consumables or the wrong wire and gas. Using quality consumables, clean materials and correct welding parameters prevents the majority of faults.
This guide is a fault-by-fault reference for the most common MIG problems, with the causes and fixes for each.
What is bird nesting in MIG welding?
Bird nesting occurs when welding wire tangles inside the wire feeder, forming a knot that prevents smooth wire feeding. It is usually caused by excessive drive roller tension, worn liners, blocked contact tips or incorrect wire feeding components.
Regular maintenance and correctly adjusted drive rollers help prevent bird nesting and reduce downtime.
What causes porosity in MIG welding?
Porosity is caused by gas becoming trapped inside the weld as it solidifies. Common causes include inadequate shielding gas, contaminated material, excessive drafts, incorrect gas flow or moisture.
Keeping materials clean, checking gas equipment and using quality welding consumables helps minimise porosity and improve weld integrity.
What causes excessive spatter when MIG welding?
Excessive spatter is often caused by incorrect voltage, unsuitable wire feed speed, poor shielding gas selection or incorrect polarity. Low-quality wire and worn consumables can also contribute.
Correct machine setup and high-quality welding wire improve arc stability and significantly reduce spatter. For polarity guidance, see how MIG welding works.
What is burn back in MIG welding?
Burn back occurs when the welding wire melts back into the contact tip, causing the wire to stick. It is commonly caused by low wire feed speed, excessive voltage or worn contact tips.
Replacing worn consumables and correctly setting welding parameters helps prevent burn back and extends torch life.
What is lack of fusion?
Lack of fusion occurs when the weld metal fails to bond properly with the parent material. It is usually caused by insufficient heat input, incorrect torch angle, poor joint preparation or excessive travel speed.
Proper preparation, suitable welding parameters and correct technique help ensure complete fusion throughout the joint.
Why is my MIG wire not feeding properly?
Poor wire feeding can result from worn liners, incorrect drive roller tension, damaged contact tips or contaminated wire. Incorrect spool brake settings may also restrict wire movement.
Routine maintenance and replacing worn consumables improve wire feed consistency and reduce production downtime.
What causes an unstable arc during MIG welding?
An unstable arc may be caused by incorrect voltage, inconsistent wire feed, worn contact tips, poor earth connections or contaminated materials. Shielding gas problems can also affect arc stability.
Checking equipment condition and using high-quality consumables helps maintain a smooth, consistent welding arc.
What is undercut in welding?
Undercut is a groove that forms along the edge of the weld, reducing joint strength. It is often caused by excessive heat input, high travel speed or poor torch angle.
Correct welding parameters and technique help eliminate undercut and improve weld quality.
How can I reduce welding distortion?
Welding distortion occurs because heated metal expands and contracts as it cools. Distortion can be reduced by minimising heat input, using balanced weld sequences, applying tack welds and securing components with clamps or fixtures.
Planning the welding sequence before starting helps maintain dimensional accuracy.
What causes weld cracking?
Weld cracking may occur due to excessive restraint, poor filler metal selection, rapid cooling or hydrogen contamination. Cracks can develop during or after welding depending on the material and application.
Using the correct consumables, proper joint preparation and suitable welding procedures significantly reduces the risk of cracking.
Prevention starts with the right consumables
Most of these defects share a root cause — worn consumables, contamination or the wrong wire. Starting with quality wire and correct setup is the cheapest way to avoid rework. See our MIG wire selection guide and how to set up a MIG welder.