Argon

Slightly less than one percent of the earth's atmosphere is  composed of argon, which is colorless, odorless, tasteless, and nontoxic. As an inert gas, argon does not react with other compounds or elements. Argon is about 1.4 times heavier than air and cannot sustain life. The inert properties of argon make it ideal as a shield against atmospheric contamination, thus it is used in many welding processes. Argon promotes good arc starting characteristics and arc stability due to its low ionization potential.

Carbon Dioxide

Carbon Dioxide, a reactive gas, is about 1.5 times heavier than air. It is an odorless, colorless gas with a slightly pungent, acid taste and is slightly toxic. It will not sustain life or support combustion. Differing from other reactive gases such as oxygen, carbon dioxide can be used alone for GMAW shielding gas applications. Its relatively high oxidizing potential can be countered by the use of GMAC or FCAW wires higher in alloying elements, such as silicon and manganese. Carbon Dioxide is commonly mixed with argon to improve productivity and penetration in GMAW.

Helium

Helium is the second lightest element, after hydrogen, and is lighter than air. Like argon, it is chemically inert and will not sustain life. Due to its high thermal conductivity and high ionization potent ional, helium is used as a shielding gas for welding applications when increased heat input is desired, and low tolerance for oxidizing elements exist such as with aluminum and magnesium welding.

Hydrogen

Hydrogen, the lightest known element, is a flammable gas. Explosive mixtures can be formed when certain concentrations of hydrogen are mixed with oxygen, air, or other oxidizers. Hydrogen is not life sustaining. Small quantities are useful in gas blends for plasma cutting and some welding applications because of its high thermal conductivity and reactive nature. It is very useful for GMAW and GTAW of 300 series austenitic stainless steels. 

Nitrogen

Nitrogen is a colorless, odorless, and tasteless gas, which forms 78 percent of the earth's atmosphere (by volume). It is nonflammable, does not support combustion, and is slightly lighter than air. Nitrogen is inert except at arc welding temperatures, where it will react with some metals, such as aluminum, magnesium, and titanium. It is not recommended as a primary shielding gas with GMAW, but is commonly applied as an assist gas with laser cutting on stainless steels. It can be used in combination with other gases for some welding applications and is also widely used in plasma and laser cutting.

Oxygen

Fifty percent of the earth's crust and approximately 21 percent of the earth's atmosphere (by volume) is oxygen. Oxygen combines with almost all known elements except rare or inert gases, and it vigorously supports combustion. Because of its highly oxidizing and combustion-supporting properties, oxygen is an ideal gas for increasing flame temperatures and improving performance for oxy-fuel welding and cutting. Small amounts of oxygen may be added to argon for GMAW to increase arc stability and improve the wetting and shape of the weld bead when working with mild or stainless steels. It is also used to enhance cutting speeds with plasma and laser processes.