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
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