Which of the following is a commonly used shielding gas in GMAW?

Carbon dioxide (CO2) is widely used as a shielding gas in Gas Metal Arc Welding (GMAW) due to its effectiveness and cost efficiency. It provides adequate protection to the weld pool against atmospheric contamination, preventing defects like porosity and ensuring good weld quality. CO2 also enhances arc stability and produces a higher deposition rate, which is beneficial for various welding applications, particularly in MIG welding of mild and low alloy steels.

Helium, while also used in certain applications, is less common due to its higher cost and lower effectiveness in specific contexts compared to CO2. Hydrogen can be used as a shielding gas but is generally more applicable to specialized welding processes, primarily for high-alloy steels and is less common in typical GMAW scenarios. Oxygen, while important in welding processes, is not used as a shielding gas in GMAW since it can lead to oxidation of the weld and adversely affect the weld quality. This contrast reinforces why carbon dioxide is the preferred choice among these options for general GMAW applications.