____ is denser than air, so it effectively shields welds in deep grooves in the flat position.

• A. Argon
• B. Nitrogen
• C. Helium
• D. Hydrogen

Answer: (A)

The main idea is how shielding gas behavior affects protection of the weld pool. Gases that are heavier than air tend to settle around the joint and form a stable blanket, which is especially important for deep grooves in the flat position where the bottom of the groove is exposed to air. Argon is denser than air, so it blankets the weld effectively, keeps oxygen and nitrogen away from the molten metal, and stabilizes the GTAW arc. This makes it the best choice for shielding welds in deep grooves in the flat position.

Helium, by contrast, is lighter than air and tends to rise away from the weld, offering less shielding in that geometry. Nitrogen and hydrogen can introduce unwanted reactions or porosity in many metals, so they’re not preferred shielding gases for typical TIG welding.