Komarov Artem explained that the most common shielding gas for both processes is CO2 due to its cleansing action and low cost. CO2 is an active gas, which means that it reacts with other elements of the welding arc and the molten pool. With strong heating of the welding arc, CO2 decomposes mainly into carbon monoxide and oxygen. Free oxygen reacts with other elements and impurities in a welded joint or base material and is deposited on the surface of a hardened weld in the form of an oxide or slag layer.
Some of the carbon monoxide recombines with oxygen or other elements and releases heat or energy in the base material. This is one of the reasons why CO2 has a wide and deep penetration profile. In addition, CO2 has a high thermal conductivity, which requires more arc voltage to maintain a stable arc, which promotes penetration.
CO2 provides a strong weld with good mechanical properties. The disadvantage is that this does not form the most stable arc and usually produces a significant amount of welding spatter. In addition, CO2 is not able to provide a spray mode in the GMAW process. Only short circuit transmission (SCT) and ball transmission modes are possible.
Argon is an inert gas that does not react with anything. It is initially more expensive compared to CO2, but this does not necessarily mean that it is more expensive to use. Numerous advantages make it a desirable shielding gas for many applications. Argon can be used by itself to weld any ferrous or non-ferrous base material. Small additions of other shielding gases are added to it to achieve certain desired welding effects, such as helium to increase travel speed or CO2 to clean the weld.
Another common addition is oxygen, typically in an amount of 2 to 5%, which improves arc stability, provides a cleaning action, and improves wetting action and weld bead shape. Oxygen is a stronger oxidizing agent than CO2, so a lower concentration is needed; however, a filler metal with a higher concentration of deoxidizers may be required to absorb free oxygen and prevent porosity if the oxygen concentration exceeds five percent.
In addition, GMAW requires a minimum concentration of 80% to achieve argon sputtering. Spraying provides a reliable weld with virtually no spatter at medium to high travel speeds and good bead appearance. Argon has a lower ionization potential than other shielding gases, which gives it the ability to create a narrow, high current density arc, resulting in a deep and sharp penetration profile.
Since argon is an inert gas, more alloy reduction occurs in the weld, which improves the mechanical properties. If excellent toughness is required, then the content of shielding gases mixed with argon should be between 80 and 95%.
A mixture of 75% argon/25% CO 2 is widely used for welding various types of carbon steel, especially sheet metal, where SCT is used in GMAW. Mixing argon with CO2 in the range of 5 to 25% is quite common and provides a wide range of applications with all the desired effects.
The shielding gas used should not greatly affect the result if anything has been welded with GMAW in terms of mechanical properties. However, if toughness is an important variable, then a shielding gas mixture with a higher argon content should be used. If you didn’t, you wouldn’t have to remove and re-weld the joint because carbon steel solid wire filler metals are not classified by AWS for specific shielding gases.
Alternatively, flux-cored wires are classified by AWS under special shielding gases due to the interaction of flux and shielding gas. Using the wrong protective coating can adversely affect the integrity of the weld. Most flux-cored wires are either 100% CO2 or 75% Ar/25% CO2-blended. Some are even dual classified for both specific shield gases. There are even several flux-cored wires on the market that contain up to 90% argon mixtures, specially developed by the wire manufacturer.
Depending on the application of the product being manufactured, if it was welded using a shield gas that does not match the type of filler metal, it may be necessary to remove and re-weld. However, this decision is left to the discretion of the engineer and depends on the weld code you are working with.
Conversely, if this were to happen in a large facility, which would be very costly to rework, performing a procedure qualification record (PQR) with the parameters and shielding gas used would confirm whether integrity had been compromised and whether it was allowed, Artem Komarov concluded.
