Selecting shielding gas for GMAW and FCAW

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Q: In my welding shop, we use a variety of GMAW and FCAW wires to make fillet and butt welds in all positions. Can you help us select the appropriate shielding gas for these different wires and applications?

A: Selecting an appropriate shielding gas is critical for obtaining sound welds, but it can be a challenge. Always remember that some basic principles can make gas selection a little easier.

The first thing to consider in shielding gas selection is the material being welded. When welding aluminum, copper, and nickel-based alloys, use a fully inert gas such as argon (Ar), helium (He), or an argon/helium blend. When welding ferrous alloys, the gas will, in most cases, have an oxidizing component (oxygen or CO₂), although straight argon is sometimes used for welding stainless steel.

With GMAW, the transfer mode used must be considered when welding ferrous alloys. To obtain spray transfer, the gas must contain less than 18% CO₂ or 8% oxygen. However, combining the high arc energy of spray transfer with a highly oxidizing gas blend, such as 82% Ar/18% CO₂ and 92% Ar/8% oxygen, causes a thick oxide layer to form on the weld bead. You can achieve better results with less oxidation by using gas blends that contain no more than 10% CO₂ or 5% oxygen. For stainless steel, use a gas blend with minimal oxidizing components—no more than 3% oxygen or 3% CO₂.

Pulsed welding is normally done with a gas that is used for spray welding.

Welding carbon and low-alloy steel using GMAW with short-circuiting transfer is normally done with either 100% CO₂ or 75% Ar/25% CO₂. You also can use other Ar/CO₂ blends that contain more than 18% CO₂ in some applications. When welding low-alloy steels that require oxidization to be kept to a minimum, use a He/Ar/CO₂ tri-mix that contains less than 5% CO₂. This type of tri-mix also is used when welding stainless steel with short-circuiting transfer.

Another thing to consider is the cleanliness of the material. If there is heavy mill scale or rust, utilize a gas blend with more CO₂ to help clean the weld metal and minimize porosity. For materials that are relatively clean, a less-oxidizing gas can reduce spatter and fumes.

When welding with flux-cored wires, you should only use the shielding gas or gases recommended by the wire manufacturer. Using a flux-cored wire with a gas that is not recommended by the manufacturer can result in weld defects, including porosity and cracking, and also can result in a weld with inadequate mechanical properties.