If used carelessly, shielding gasses can displace oxygen, causing hypoxia and potentially death.
Ionizability influences how easy the arc starts, and how high voltage is required.
[5] Carbon dioxide is the least expensive shielding gas, providing deep penetration, however it negatively affects the stability of the arc and enhances the molten metal's tendency to create droplets (spatter).
[6] Carbon dioxide in concentration of 1-2% is commonly used in the mix with argon to reduce the surface tension of the molten metal.
Blends of helium with addition of 5–10% of argon and 2–5% of carbon dioxide ("tri-mix") can be used for welding of stainless steel.
Helium and carbon dioxide were the first shielding gases used, since the beginning of World War 2.
[8] Helium is more expensive than argon and requires higher flow rates, so despite its advantages it may not be a cost-effective choice for higher-volume production.
[9] Pure helium is not used for steel, as it causes an erratic arc and encourages spatter.
Excessive oxygen, especially when used in application for which it is not prescribed, can lead to brittleness in the heat affected zone.
Argon-oxygen blends with 1–2% oxygen are used for austenitic stainless steel where argon-CO2 can not be used due to required low content of carbon in the weld; the weld has a tough oxide coating and may require cleaning.
Other gases can be used for special applications, pure or as blend additives; e.g. sulfur hexafluoride or dichlorodifluoromethane.
Faster welding speeds, in general, mean that more gas needs to be supplied to provide adequate coverage.
Perhaps most importantly, the four primary variations of GMAW have differing shielding gas flow requirements—for the small weld pools of the short circuiting and pulsed spray modes, about 10 L/min (20 ft3/h) is generally suitable, while for globular transfer, around 15 L/min (30 ft3/h) is preferred.
The spray transfer variation normally requires more because of its higher heat input and thus larger weld pool; along the lines of 20–25 L/min (40–50 ft3/h).