What is the purpose of purge gas in aluminum TIG welding and how is it applied?

• A. To prevent backside oxide formation; purge gas flows along the backside (through purge cap or dam) during welding.
• B. To cool the weld area rapidly.
• C. To increase filler deposition.
• D. To stabilize the arc.

Answer: (C)

Purging is used to keep the backside of the aluminum weld from reacting with air as it solidifies. Aluminum oxide forms quickly and can become trapped in the root if the backside isn’t protected, leading to porosity and a weak seal. By directing an inert gas along the backside—often through a purge cap or dam—the root area is shielded from the atmosphere during welding. The purge gas is maintained throughout the root pass and for a short time after to ensure the backside remains protected as it cools. This function is distinct from simply shielding the visible surface, cooling, increasing filler deposition, or stabilizing the arc.