Tre’Heflin-KingClick image to enlargeby Tre’Heflin-King

Shielding gas can impact performance

The Impact of Shielding Gas on Filler Metal Performance, 
Along with the formulation of the filler metal being used, shielding gas directly impacts welding performance—for example, how smoothly the wire welds, its spatter levels and its arc qualities.

Shielding gas can also affect the amount of weld fume generated and can alter the mechanical properties of the completed weld. Knowing how to select the appropriate shielding gas for an application helps achieve the desired welding performance and also minimize rework caused by poor weld quality.

Carbon dioxide, argon, helium and oxygen are the most common shielding gases used in the welding process. Take into account the desired welding properties, and consider the cost and available labor for weld preparation and other non-welding activities when making the selection.

What to know about carbon dioxide and argon
Carbon dioxide (CO2) gas is the least expensive reactive gas, the most common and also the only shielding gas usable without adding an inert gas. Since using 100 percent CO2 shielding gas provides broad, deep joint penetration, it is ideal for welding thick material. The downside of straight CO2 is that it tends to create a less stable arc and more spatter than when mixed with other gases such as argon. Additional spatter requires post-weld cleaning, leading to unwanted downtime.

Using 100 per cent argon is common when welding aluminum, magnesium or titanium because it creates a stable arc, provides consistent weld quality and appearance, and offers good weld pool control. It also produces a narrow penetration profile, which makes it ideal for fillet and butt welds. Since argon produces less spatter, it also minimizes downtime for post-weld cleanup.

Knowing how to select the appropriate shielding gas for an application helps welding operators achieve the desired welding performance and also minimize rework caused by poor weld quality. Click image to enlargeMany operators add argon to a CO2 mixture when welding materials like carbon steel because it generates high productivity rates and helps create visually appealing welds. These mixtures typically consist of 75 to 95 percent argon and 25 to 5 percent CO2.

Shielding gas with higher argon content produces welds with higher tensile and yield strengths but lower ductility, while higher CO2 content in a shielding gas mixture improves ductility and crack resistance but lowers the tensile and yield strengths.

Some applications may not require higher argon content because the higher strength it delivers is unnecessary and would needlessly make the weld more susceptible to cracking. However, other welds may call for this higher content because they might otherwise fail to meet minimum strength standards.

Additional shielding gases
Helium is typically used for welding non-ferrous metals or when welding stainless steels with a tri-mix formula of argon and CO2. Helium creates a hot arc, which helps increase travel speeds and productivity rates, and a wide, deep penetration profile for welding thick materials. Helium is often used in ratios of 25 to 75 percent helium with an appropriate balance of argon. Adjusting these ratios can change weld penetration, bead profile and travel speeds. Helium is more expensive than other gases and requires a higher flow rate than argon because it is so light; weigh the value of the increased productivity against this higher cost.

Oxygen is a reactive gas that is usually used in ratios of 9 percent or less. Adding it to an argon mixture helps improve weld pool fluidity, weld penetration and arc stability — especially when welding carbon, low alloy and stainless steels. Since oxygen causes oxidation of the weld metal, in most cases it should not be used to weld aluminum, magnesium, copper or other exotic metals.

Always follow the filler metal manufacturer’s recommendations for the type and amounts of shielding gas that should be used for a given wire. It is vital to gaining good weld quality and minimizing unnecessary downtime. SMT

Tre’ Heflin-King is a welding engineer-CWI, applications engineering, Hobart Brothers

 

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