What effect does shielding gas purity have on weld quality in GMAW?

• A. It doesn't affect weld quality.
• B. It only affects the color of the weld.
• C. Impure gas can cause porosity, oxidation, and poorer bead stability.
• D. It only affects the shielding gas flow rate.

Answer: (C)

Shielding gas purity is crucial because the gas creates the protective envelope around the arc and molten weld metal. If the gas contains moisture, oxygen, or other contaminants, those impurities can enter the molten pool and cause defects. Porosity forms when gas bubbles get trapped in the solidifying metal, oxidation occurs when oxygen reacts with the hot metal surface, and bead stability suffers when the shielding is inconsistent due to contaminants. In short, impure shielding gas disrupts the protective environment, leading to porosity, oxidation, and a less uniform weld bead.