Which defect in a MIG weld indicates potential failure and should be avoided?

• A. Porosity in the weld
• B. Uniform bead height
• C. No cracks
• D. Perfect throat thickness

Answer: (A)

Porosity in a MIG weld is a defect where tiny gas pockets get trapped in the solidifying weld metal. Those pores weaken the weld because they reduce the effective cross‑section and create stress concentrators, which can lead to premature failure under load or vibration. Porosity also undermines fatigue strength and corrosion resistance, and it often signals problems with shielding or cleanliness. Common causes include moisture on the base metal or filler wire, oil, grease, rust, or other contamination, and inadequate shielding gas coverage due to incorrect flow, leaks, long distance, or wind. To prevent it, clean and dry all surfaces, use dry filler wire, ensure good shielding gas coverage with the correct flow, keep a proper arc length, and adjust welding parameters so gas can escape as the weld cools.

Other characteristics described—uniform bead height, no cracks, and perfect throat thickness—point to a sound weld with good fusion and strength, not a defect.