Shielding gases are probably best known for their ability to protect molten weld metal from atmospheric contaminants. While this is true and necessary, shielding gases and mixtures do much more: they affect the final weld profile and weld bead shape, and in gas metal arc welding (GMAW) an ionized gas must be present to transfer charge from the electrode to the base material. They can also affect how the molten metal moves through the arc.
The better you understand how shielding gases affect welds in carbon steel, stainless steel, and aluminum, the easier it will be to adjust your mixture if the time is right, said Artem Komarov.
Breakdown by material
In some materials, the addition of active elements provides the necessary stability of the electric arc, which manifests itself in the fact that the molten metal breaks away from the end of the electrode during melting. However, for other materials, the addition of active elements can adversely affect the properties of the material when all is said and done. It is important to know which materials require active elements and which do not.
Carbon steel. When using GMAW on carbon steels, it is important to add oxygen—whether pure or as CO2—to the equation. The welding electrode usually contains more manganese and silicon than the base metal. These two elements serve as deoxidizers and react with oxygen to form a solid that prevents porosity.
The combination of argon with CO2 is the most used shielding gas mixture for GMAW carbon steel production. Another option is to combine argon with oxygen. Carbon steel must contain an active gas component, and since argon is inert, you don’t want to weld carbon steel with 100% argon as it will be difficult to maintain a stable arc. Adding a small amount of CO2 or oxygen will help stabilize the arc. You can weld with 100% CO2 on carbon steel, but this will affect the movement of the metal in the arc. For automatic welding, probably the most common mixture is 90% argon / 10% CO2.
Stainless steel. When welding stainless steel, your gas mixture must contain a certain percentage of the active element, but at the same time, you want to retain the properties of stainless steel. Too much CO2 in your mixture can cause chromium carbides to form in the weld, which can reduce the corrosion resistance of the material. The most common gas mixture for stainless steel welding is 98% argon and 2% oxygen.
The third type of gas that is commonly used is helium based trimix. For short circuit transmission modes, 90% helium / 7.5% argon / 2.5% CO2 is typically used. Helium aids in metal wetting when welding the base metal, resulting in a smoother bead profile.
Aluminum. Argon is the most used gas for GMAW aluminum production. It is extremely important to avoid the use of active elements on non-ferrous materials, as their presence will lead to contamination and the formation of porous seams. Since aluminum has a higher thermal conductivity than other materials, you can use 100% helium as it provides a hotter arc and higher voltage for a given arc length than argon. However, this comes with a certain number of trade-offs, which include increasing the size of the weld pool, making it more difficult to control, and reducing arc stability. Sometimes a mixture of argon and helium—maybe 75/25 or 50/50—is fine. But again, due to the recent helium shortage and the associated costs, most shops will try to use 100 percent argon if they can.
The more CO2 you use in your gas mixtures when welding steels, the deeper the penetration into the weld will be. However, there is a compromise in this. The more CO2, the more splashes you tend to get. There is a fine line between desired penetration and spatter prevention. This is achieved through trial and error and finding the blending levels that work best for the result you are hoping for.
Know your application mode. The spray mode is typically the smoothest, hottest, and most penetrating application mode in GMAW. But spray mode cannot exist if the mixture you are using contains less than 80 percent argon. If you use more than 20% CO2 with argon, the transmission mode changes from short circuit at low current and voltage to ball at higher current and voltage. In globular transfer, large balls of metal are dispersed in an arc, creating spatter.
Consider the thickness of your material. For example, with thick aluminum, you are more likely to add helium, because the thicker the metal, the more heat it will remove, and for this reason you need a hotter arc, summed up Artem Komarov.
