Getting the Most From Your Weld-Shielding Gas

By: Andy Monk

Sunday, December 1, 2019

Shielding gas prevents exposure of the molten weld pool to the atmosphere, thus playing a vital role in process performance and weld quality.
Shielding gas plays a vital role in the performance and quality achieved in a gas metal arc welding (GMAW) operation, preventing exposure of the molten weld pool from the atmosphere, where it can react with elements and lead to issues such as porosity and excessive spatter. Different shielding gases offer different weld-penetration profiles and arc stability; they also affect the weld’s mechanical properties.

Gas Selection

Many GMAW applications lend themselves to a variety of shielding gas choices. Shops should carefully evaluate their welding goals to choose the best shielding gas for the job, accounting for price, desired weld properties, and the base material. Also, consider productivity goals and the amount of pre- and post-weld cleanup you are willing or able to accept.

Carbon dioxide (CO2) and argon find the most use in shielding gases, in addition to oxygen and helium. CO2 and oxygen are reactive gases—the electrons in these gases react with the weld pool, producing distinct characteristics. Argon and helium are inert gases; they do not react with the base material or the weld pool.

Shops can use CO2—less expensive than other shielding gases—in its pure form (100 percent) without adding an inert gas, making it an attractive choice for many welding operations. Using pure CO2 provides very deep weld penetration, useful for welding thick material, and it works well when welding mild or carbon steels. Limitations: Welders can only use pure CO2 in short-circuit welding processes; and the gas produces a less stable arc and more spatter than when mixed with other gases.

Inadequate shielding gas coverage can cause porosity, seen here on the face and interior of the weld bead.

Mixtures of argon (75 to 95 percent) and CO2 (5 to 25 percent) provide good arc stability and weld-pool control, and result in less spatter than 100-percent CO2, helping to minimize rework and post-weld cleanup. In addition, these mixtures allow the use of spray-transfer welding, which can improve productivity and result in more visually appealing welds. Argon also creates a narrower penetration profile, useful for fillet and butt welds. Use argon-CO2 mixtures for welding carbon, mild and stainless steels. For welding nonferrous alloys—aluminum, magnesium or titanium for example—100-percent argon gets the call.

To optimize weld-pool fluidity, weld penetration and arc stability on mild, low-alloy and stainless steels, consider adding oxygen to the shielding gas mix, typically in ratios of nine percent or less. Oxygen, however, will cause oxidation of the weld metal, so we do not recommend its use for welding aluminum, magnesium, copper or other exotic metals.

Lastly, helium-argon typically shielding-gas mixtures (25 to 75 percent helium) typically get the call for welding nonferrous alloys, as well as stainless steels. Because it produces a wide, deep penetration profile, helium works well for welding thick materials. It also creates a hotter arc, which allows for relatively fast travel speeds and high productivity rates. However, the more costly helium requires a higher flow rate than argon, so weld shops will need to calculate the value of the productivity increase against the increased cost of the gas. Another option for stainless steel welding: use helium in a tri-mix formula with argon and CO2.

Achieving Good Gas Flow

This graphic illustrates how welding-gun consumables can affect shielding gas coverage. Compare the image on the left showing good gas coverage to that on the right showing turbulent gas flow, less likely to protect the weld pool from atmospheric contamination.

GMAW gun consumables—the diffuser, contact tip and nozzle—play a crucial role in properly directing the shielding gas to the weld pool. Use of a nozzle too narrow for the application or allowing the diffuser to clog with spatter likely will result in insufficient shielding gas coverage. Likewise, a poorly designed diffuser might not channel the shielding gas properly, resulting in turbulent, unbalanced gas flow. Both scenarios can allow pockets of air into the shielding gas and lead to excessive spatter, porosity or weld contamination. Select consumables that resist spatter buildup and look for a wide enough nozzle bore to provide adequate shielding gas coverage.

Also important: Be sure to use a gas regulator designed for the shielding gas mixture in use, and the proper connectors. As part of a routine maintenance plan, check the regulators often to ensure proper functioning. In addition, take care to correctly set shielding gas flow rates (measured in ft.3/hr.) to ensure proper weld-pool coverage based on wire-feed speed and weld-travel speed. Increasing wire-feed speed can increase travel speed and the size of the weld profile. As a result, shielding gas flow rate may need to increase as well. Beware, though, of turbulent gas flow—it usually indicates excessive gas-flow rate.

Lastly, when welding a deep joint or bevel, consider using gas preflow—allowing the shielding gas to flow for a few seconds prior to starting the welding process. This helps ensure adequate gas coverage. MF


See also: Bernard Welding

Related Enterprise Zones: Welding

Reader Comments

There are no comments posted at this time.


Post a Comment

* Indicates field is required.

YOUR COMMENTS * (You may use html to format)




Visit Our Sponsors