Consumables Corner: Why a shop using GMAW with C25 shielding gas experienced porosity

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Q: Our shop does a lot of welding-related repair on construction and farm equipment, as well as large steel castings and forgings primarily using flux-cored arc welding (FCAW) with CO₂ shielding gas. However, these processes generate smoke and welding spatter. We tried using a gas metal arc welding (GMAW) filler metal with C25 shielding gas but experienced a lot of porosity. Any ideas as to why?

A: Based on the information you provided, we see several possible causes for the porosity you are experiencing.

Porosity occurs when nitrogen, oxygen, or hydrogen become trapped in the weld pool during solidification. Oxygen and nitrogen typically come from atmospheric contamination, while hydrogen comes from grease, oil paint, dirt, or water contamination.

Ineffective shielding gas coverage caused by flow rates that are too high or too low, hose leaks, air drafts, or holding the welding gun too far away from the weld joint can lead to porosity as well. Even a small amount of atmosphere mixed with the shielding gas in any of these scenarios can trigger it.

Porosity caused by hydrogen typically comes from welding on joints that were cleaned and prepared incorrectly. You should remove any surface contamination and then preheat to eliminate any moisture that may be present.

Since you mentioned repair work, we suspect your GMAW porosity issues stem from a surface area that is not cleaned properly. Use soap and water or acetone to remove any visible contaminants, dry the surface, and grind or use a wire wheel on the surface.

Since you did not mention any porosity issues with your current process, it is possible the FCAW wire you use has a greater cleaning capability than the GMAW wire.

Depending upon their respective classifications, GMAW and FCAW may have similar or very different levels of manganese and silicon, common deoxidizing elements.

Deoxidizers in solid GMAW wires come solely from the steel used to make the wire. With FCAW wires, the deoxidizing elements come primarily from the wire’s core.

FCAW wires may be designed with core ingredients (both slag and deoxidation ingredients) specifically tailored to provide better porosity resistance. Slag cover resulting from FCAW contributes to porosity resistance by preventing absorption of atmospheric nitrogen through the molten weld metal surface.

As an active gas, 100% CO₂ improves porosity resistance with FCAW because the high arc temperature causes the gas molecules to disassociate—or separate—allowing the controlled amount of oxygen to react with other elements. This traps impurities in the slag layer that are removed after welding.

A C25 shielding gas—which is 25% CO₂ and 75% argon—does not provide as much cleaning action. Argon is an inert gas that provides no cleaning action of the weld puddle, although it displaces air from the weld joint.

A 100% CO₂ shielding gas does tend to produce more smoke and spatter, but it also reduces the risk of porosity. You could incorporate vigorous preweld cleaning with the GMAW process, or you could invest in localized fume extraction and stick with FCAW.

A closer look at the costs (including rework) of mitigating spatter, porosity, and fume should help you to determine what is best for your shop.