Question

what happens to the duty cycle for a gmaw gun when \\(75\text{ar}/25\text{co}_2\\) gas is used instead of a \\(100\\%\\) \\(\text{co}_2\\) shielding gas?

the duty cycle will be reduced by 15 to 20%.
the duty cycle will be reduced by 40 to 50%.
the duty cycle will be increased by 15 to 20%.
the duty cycle will be increased by 40 to 50%.

Explanation:

Analyze shielding gas thermal properties

Using the Duty Cycle Definition knowledge point
Carbon dioxide (\(\text{CO}_2\)) has a higher thermal conductivity and provides better cooling to the GMAW gun nozzle and neck compared to argon (\(\text{Ar}\)). When switching from \(100\%\) \(\text{CO}_2\) to an argon-rich mixture like \(75\%\) \(\text{Ar} / 25\%\) \(\text{CO}_2\), the gun receives less cooling from the gas flow.

Determine the effect on duty cycle

Because argon-rich mixtures provide less cooling than pure \(\text{CO}_2\), the GMAW gun will heat up faster at a given current. Consequently, the rated duty cycle of the welding gun must be derated (reduced) when using argon-rich shielding gases. Standard industry specifications typically reduce the gun's duty cycle rating by \(40\%\) to \(50\%\) when switching from \(100\%\) \(\text{CO}_2\) to argon/carbon dioxide mixtures.

Answer:

• (A) The duty cycle will be reduced by 15 to 20%.
• (B) The duty cycle will be reduced by 40 to 50%. (Correct answer)
• (C) The duty cycle will be increased by 15 to 20%.
• (D) The duty cycle will be increased by 40 to 50%.