Gases for Gas Plasma Arc Welding (FCAW)

The physical configuration of PAW requires the use of two gases, a "plasma" or orifice gas and a shielding gas. The primary role of the plasma gas, which exits the torch through the center orifice, is to control arc characteristics and shield the electrode. The shielding gas, introduced around the periphery of the arc, shields or protects the weld. In many applications, the shielding gas is also partially ionized to enhance the plasma gas performance.

Low current (< 100 amps).  Argon is the preferred plasma gas because its low ionization potential ensures easy and reliable starting. Argon-helium mixtures are also used for applications requiring higher heat input.

The choice of shielding gas is dependent on the type and thickness of the base material. When welding aluminum, carbon steel, and copper, the gases commonly used are argon, helium, and argon-helium mixtures. It is generally recommended that the percentage of helium be increased as the base-plate thickness increases. When welding low alloy steels, stainless steels, and nickel alloys, the aforementioned gases in addition to argon-hydrogen mixtures are used.

High Current (> 100 amps).  The choice of gas used for high current plasma arc welding also depends on the material to be welded. In all but a few cases, the shielding gas is the same as the orifice gas.

Shielding Gases

• Argon.  Argon is suitable as the orifice and shielding gas for welding all metals, but it does not necessarily produce optimum welding results. In the Melt-In mode, additions of hydrogen to argon produce a hotter arc and more efficient heat transfer to the work. Limits on the percentage of hydrogen are related to its potential to cause cracking and porosity. However, when using the Keyhole technique, a given material thickness can be welded with higher percentages of hydrogen. This may be associated with the Keyhole effect and the different solidification pattern it produces.
  
  Argon is used for welding carbon steel, high strength steel, and reactive metals such as titanium and zirconium alloy. Even minute quantities of hydrogen in the gas used to weld these materials may result in porosity, cracking, or reduced mechanical properties.
    

• Argon-Helium Mixtures.  Helium additions to argon produce a hotter arc for a given arc current. Argon-helium mixtures containing between 50% and 75% helium are generally used to make keyhole welds in heavier titanium sections and for fill and capping passes on all materials when the additional heat and wider heat pattern of these mixtures prove desirable.
    

• Argon-Hydrogen Mixtures.  Argon-hydrogen mixtures are used as the plasma and shielding gases for making keyhole welds in stainless steel, Inconel, nickel, and copper-nickel alloys.  Permissible hydrogen percentages vary from 5% to 15%, used for highest welding speeds in stainless steel in tube mills. See Table 5 for high-current gas selection.