Shielding gases for GMAW
November 5, 2012
Shielding gases do more than just protect the weld from atmospheric contamination. The gas and gas blends you use also influence the weld profile and the weld bead shape. Also, for an arc to occur in gas metal arc welding (GMAW), an ionized gas must be present to carry the charge.
Shielding gases probably are best-known for their ability to protect the molten weld metal from atmospheric contamination. While this is true and their presence is necessary, shielding gases and blends do so much more: They influence the final weld profile and the weld bead shape, and in gas metal arc welding (GMAW), an ionized gas must be present to carry the charge from the electrode to the base material. They also can influence how the molten metal is transferred across the arc.
The better you understand how shielding gases affect welds in carbon steel, stainless steel, and aluminum, the easier it will be to make adjustments to your blend if and when that time comes.
In certain materials, adding active elements provides the necessary stability to the arc, which presents itself in the way the molten metal pinches off at the end of the electrode as it melts. For other materials, however, adding active elements can affect the material properties adversely when all is said and done. It’s important to know which materials call for active elements and which don’t.
Carbon Steel. When using GMAW on carbon steels, it’s important to add oxygen—whether it’s in its pure form or in the form of CO2—to the equation. The welding electrode typically contains more manganese and silicon than the base metal. Those two elements serve as deoxidizers and react with the oxygen to form a solid to prevent porosity from forming.
Combining argon with CO2 is the most commonly used shielding gas blend for carbon steel GMAW. Another option is combining argon with oxygen. Carbon steel must have an active gas component present, and since argon is inert, you don’t want to weld carbon steel with 100 percent argon because it will be difficult to maintain a stable arc. Adding a little bit of CO2 or oxygen will help stabilize the arc. With that said, you can weld with 100 percent CO2 on carbon steel, but that will influence the metal transfer across the arc. For automated welding, 90 percent argon/10 percent CO2 is probably the most common mix.
Stainless Steel. A certain percentage of an active element must be present in your gas blend when welding stainless steel, but at the same time, you want to be able to maintain the stainless steel’s properties. Too much CO2 in your blend could cause chromium carbides to form in the weld, which might reduce the material’s corrosion resistance. The most common gas blend for welding stainless steel is 98 percent argon/2 percent oxygen. In recent years 98 percent argon/2 percent CO2 has become a popular blend. The CO2 provides a bead shape that differs slightly from the one produced with oxygen, and it provides more of a perceived arc stability.
The third type of gas that is commonly used is a helium-based trimix. For short-circuit transfer modes, generally it’s 90 percent helium/7.5 percent argon/2.5 percent CO2. The helium aids weld metal wetting out on the base metal, providing a smoother bead profile.
The problem welders who use this gas blend now face is the limited availability of helium and the corresponding high price. You can use an argon/oxygen blend or an argon/CO2 blend, but you might have to change your parameters a little bit—maybe voltage—but most folks find they can make an acceptable weld with a two-component gas blend.
Aluminum. Argon is the most commonly used gas for aluminum GMAW. It’s imperative to steer clear of using active elements on nonferrous materials as their presence will result in dirty, porous welds. Because aluminum has a higher thermal conductivity than other materials, you can use 100 percent helium because it allows for a hotter arc and a higher voltage for a given arc length than argon does. Doing so, however, results in its own share of trade-offs, which include a weld pool that is larger and more difficult to control and lower arc stability. Sometimes an argon/helium blend—possibly a 75/25 or 50/50—is perfectly OK. But, again, because of the recent shortage of helium and the associated cost, most shops will try and stick with 100 percent argon if they can.
Understand the Trade-offs. The more CO2 you use in your gas blends when welding steels, the deeper the weld penetration will be. However, there’s a trade-off to that. The more CO2, the more spatter you tend to get. There’s a fine line between getting the penetration you want and avoiding spatter. That comes through trial and error and finding the mix levels that work best for the result you’re hoping for.
Know Your Transfer Mode. Spray-transfer mode tends to be the smoothest, hottest, and most penetrating transfer mode in GMAW. But spray-transfer mode can’t exist if the blend that you’re using has anything less than 80 percent argon. If you use more than 20 percent CO2 with argon, the transfer mode changes from short-circuit at low current and voltage to globular at a higher current and voltage. In globular transfer, big globs of metal disperse across the arc, which creates spatter.
Short-circuit transfer mode is useful for applications that require very low heat input. A low-heat-input transfer mode is good for welding thin sheets where you’re likely to burn through with a hotter transfer mode, or for welding out of position, say, for instance, vertical or overhead. In those cases, you don’t want to try and fight gravity with a big weld pool. When welding carbon steel in short-circuit transfer mode, 100 percent CO2 will suffice.
Consider Your Material Thickness. With thick aluminum, for example, you’d be more likely to add helium because the thicker the metal is, the more it will carry the heat away, and for that reason you need a hotter arc.