What are some Shielding Gases that are used for Welding?

Shielding gases are used primarily to protect the molten weld pool from contamination from oxygen, nitrogen, and other contaminants from the atmosphere. Shielding gases also determine weld transfer, weld bead penetration, arc stability, mechanical properties, and overall quality of the finished weld. To understand the role of gases used for welding we must look at the processes and types of metals that are welded. 

 

The Gas Tungsten Arc Welding (GTAW) process is used to weld all types of metals and the number one shielding gas used is Argon. Argon is an inert gas which does not react with any metals, so it won’t contaminate the non-consumable tungsten electrode which is used in GTAW welding to create the welding arc. Therefore, Argon can be used for shielding when welding ferritic metals, such as steel and for nonferrous metals such as aluminum.  When welding thicker aluminum, the addition of Helium, which is also an inert gas, to the argon may be used to create a mixture that will generate more heat and penetration. Argon-Hydrogen mixtures are used to weld austenitic stainless steels along with nickel-copper and nickel-based alloys.

 

The Gas Metal Arc Welding (GMAW) process is also used to weld all types of metals, both ferrous and nonferrous. The four most common gases used in GMAW welding are Argon, Helium, Carbon Dioxide (CO2) and Oxygen (O2). Both Argon and Helium are inert gases and can be used alone or mixed with each other, however, Argon alone is not recommended for welding steel and other ferrous metals. The gas does not produce enough thermal conductivity to create a fluid weld pool and the outer edges of the arc are cold. This results in a high peaked weld bead shape with undercut and, a narrow penetration profile which can be unacceptable to some standards, see Figure 1.

 Figure 1, GMAW welding with 100% Argon Shielding on Steel

Figure 1, GMAW welding with 100% Argon Shielding on Steel

 

Carbon Dioxide (CO2) is a reactive gas, and it is the only reactive gas that can be used in its pure form to weld ferrous metals such as steel. The CO2 shielding gas provides deeper and wider weld penetration especially on thicker plate. However, the arc is less stable and there is more spatter than when it is mixed with an inert gas such as Argon. See Figure 2.

 Figure 2, GMAW welding using 100% CO2

Figure 2, GMAW welding using 100% CO2

Today, most shielding gases for GMAW welding of steel are a mixture of Argon and CO2. For welding thin steel from gauge to 3mm thick a mixture of 25% CO2 and 75% Argon is commonly used, and the type of arc transfer is known as short circuit transfer. This type of transfer can be used to weld in all positions. Short circuit transfer occurs in the lower range of welding currents with smaller electrode diameters and produces a small, fast freezing weld suited for welding thin material and allows control of the arc in out of position welding and bridging larger root openings. 

 

When we reduce the amount of CO2, in the ranges of 8% to 15% CO2 in Argon, we can now increase the current and achieve spray transfer which is used for welding thicker material from 3mm and above. Spray transfer occurs in the higher ranges of current and it generates a larger weld pool that is best suited for flat and horizontal welding. The introduction of pulsed GMAW transfer, which switches the current from a high peak current to a lower background current up to hundreds or even thousands of times per second, allows the process to weld in a spray type transfer and achieve welding in all positions.  

 

We can also use Oxygen-Argon mixtures to offer better weld metal fluidity. Mixtures such as 2% Oxygen in Argon can achieve spray transfer and be used on welding steel and stainless steel. Figure 3 shows the different weld penetration and profiles using the above-mentioned gas mixtures.

 Figure 3, Images of weld bead shape and penetration with different gas mixtures

Figure 3, Images of weld bead shape and penetration with different gas mixtures

For GMAW welding of nonferrous metals such as Aluminum, the most common shielding gases are 100% Argon or a mixture of Argon-Helium

 

The Flux Cored Arc Welding (FCAW) process also uses either 100% CO2 for welding steel or austenitic stainless-steel alloys. The FCAW filler wires are developed to use 100 % CO2 and, with the correct consumable selection can be used in all positions. The type of arc transfer using 100% CO2 is a globular, and it offers deep weld bead penetration. To achieve a smoother and stabler arc the use of Argon-CO2 shielding gas mixtures are recommended. The most common mixture is 75% Argon – 25% CO2.

 

The Metal Cored Arc welding (MCAW) process also uses Argon-CO2 gas mixtures for shielding. The process is recommended to be used in the spray transfer mode and therefore uses a mixture of 8% to 15% CO2 in Argon to achieve spray transfer. It is used to weld in the flat and horizontal positions.

 

The above processes and gas mixtures are just the most common used in today’s fabrication. There are others that can be tailored for specific applications and where you can seek the advice of a welding specialist.

 

Bill Eccles, CSA W178.2 Level 2, IWS

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