Choosing the shielding gas for a gas metal arc-welding job will directly affect productivity and costs. The right gas mixture, engineered to compliment the electrode that's being used, can help to improve the mechanical properties and strength of the weld, keep spatter down, and allow the welder to increase his the speed of welding.

Shielding gases initially were used to prevent contamination of the weld pool. Oxygen, nitrogen and ambient humidity from the air all can lead to poor welds, so inert gases such as argon and helium were put into service to eliminate contamination. Later, it was found that shielding gases also helped to improve arc stability, welding speed and other variables in the welding process, while improving the quality and the look of the weld and reducing spatter.

There are many shielding gases and shielding gas mixtures in common use with today's welding processes. Some have a broad range of applications while others serve specialized applications. According to gas suppliers, straight CO₂ is the gas most often used for welding carbon and mild steels; however, CO₂ tends to cause spatter, poor surface finishes on the weld bead, and could lead to burn-through on thin materials. An argon-CO₂ mixture, or an argon-CO₂ mixture with an added oxygen component, can produce welds in mild steel with higher strength, impact resistance and better surface finishes. Pure argon, pure helium and argon-helium mixtures typically are called for to weld nonferrous metals, such as aluminum, copper, magnesium and their alloys.

Gas suppliers recommend welders first need to identify the material used for the parent metal and the weld metal, along with their thickness. Next, the metal transfer mode has to be determined. This depends on the voltage used during welding: a low current operation results in short-circuit and pulsed metal transfer, while higher currents, which require a different shielding gas, produces a spray metal transfer mode.

Then, gas suppliers say the desired quality for the welded assembly has to be taken into consideration. This includes the mechanical and chemical requirements that the part will need in its specific application, and the surface finish desired by the customer.

Finally, the cost of the weld has to be considered. This includes the acceptable level of spatter and any post-welding clean up that would have to be done; the speed at which the weld would be laid down; and the prices and volumes of gases that would be needed to complete the weld. While premium mixtures of shielding gases cost more than standard argon or helium — and argon and helium are more costly than CO₂ — gases typically contribute only 12 percent to the overall cost of a finished product, so paying more for a gas mixture may be offset by the improvements in weld quality and production speed.

It is possible to find several gas mixtures that would produce a good weld for a specific application, but it isn't possible to develop an all-purpose mixture that will give good welding characteristics under all conditions. Each of the primary gas suppliers — Praxair; Air Liquide; Linde AG; BOC Group; Air Products & Chemicals Inc.; AirGas Inc.; and Matheson Tri Gas — provide recommended mixtures of gases to match or go with a variety of base metals and electrodes. Their recommendations are available in charts on line at the following sites:
www.praxair.com
www.airliquide.com
www.us.linde-gas.com
www.boc-gases.com
www.airproducts.com
www.airgas.com
www.mathesontrigas.com

The gases normally used for commonly welded materials are:
steels

CO₂
Argon +2 to 5% oxygen
Argon +5 to 25% CO₂

non-ferrous

Argon
Argon / helium