Which term describes melting completely through the base metal during MIG welding?

• A. Porosity
• B. Spatter
• C. Burn-through
• D. Cracking

Answer: (C)

Burn-through happens when the heat from MIG welding is so high or the welding is done in a way that the molten weld metal penetrates completely through the base metal, creating a hole or completely thinning to the back side. This is what the term describes—a weld that has melted all the way through the metal instead of just fusing the surfaces together. It’s most likely to occur on thin parts, near edges, or when heat input is excessive, travel speed is too slow, or the joint prep isn’t appropriate. To prevent burn-through, you’d lower heat input (adjust voltage and wire feed), increase travel speed, adjust the gun angle and distance, and consider using backing material or a stitch-welding approach for thin sections.

Porosity refers to gas bubbles trapped in the weld, spatter are ejecta on the surface from droplets, and cracking is a fracture in the weld or heat-affected zone.