Which condition is most likely to cause porosity in a GMAW weld?

• A. Excessively slow travel speed
• B. Contaminants on the base metal or filler wire
• C. Using a clean, dry base metal
• D. Over-welding with too much heat

Answer: (B)

Porosity in a GMAW weld happens when gas gets trapped in the molten weld metal, forming tiny holes that weaken the joint. The most likely cause is contaminants on the base metal or filler wire, such as oil, grease, dirt, rust, or moisture. These substances release gases when heated or interfere with the shielding gas that protects the weld pool, allowing air to mix in or gases to form inside the weld. Keeping the metal and filler wire clean and dry minimizes these gas sources and helps the shielding gas do its job.

Other conditions don’t directly introduce gas pockets the same way. If the base metal and filler wire are clean and dry, porosity is much less likely. Excessive heat or overly slow travel can affect bead shape and fusion, but they aren’t the primary cause of porosity in the way surface contaminants are.