Gases for Gas Tungsten Arc Welding (FCAW)

• Argon.  Argon, an inert gas, is the most widely used (in its pure form) as a shielding gas for Gas Tungsten Arc Welding (GTAW). Its mild thermal conductivity produces a narrow, constricted arc column which allows greater variations in arc length with minimal influence on arc power and weld bead shape. This characteristic makes it the preferred choice for manual welding. In addition, argon provides good arc starting due to its low ionization potential. This property allows argon to carry electric current well when compared to other shielding gases.
  
  For AC welding applications, argon is preferred over helium because of its superior cleaning action, arc stability, and weld appearance. When welding thicker aluminum alloys (> 1/4"), argon is mixed with helium to enhance the thermal conductivity of the shielding gas.
  
  While pure argon may be used for mechanized applications, depending on the base material, thickness and composition, argon-helium or argon-hydrogen blends promote higher welding travel speeds. The hotter arc characteristics of argon-helium blends also make them more suitable for welding metals with high thermal conductivity, such as copper.
    

• Helium.  Helium, also an inert gas, has high thermal conductivity and high ionization potential, which produces higher arc voltages when compared to argon for a given current setting and arc length. This produces a "hotter" arc. The increased heat input affects depth of penetration and its wider, less constricted arc column increases weld bead width.
  
  The use of helium is generally favored over argon at the higher current levels which are used for the welding of the thicker materials, especially those having high thermal conductivity or relatively high melting temperatures. It is often used for high-speed mechanized applications.
  
  Although argon is widely used for AC welding of aluminum, pure helium has been successfully used for DCEN mechanized welding of this material. It produces greater penetration at higher travel speeds. However, surface oxides must be cleaned from the weld joint to obtain acceptable results, since the cleaning action of the AC arc is not present. Argon-helium mixtures are widely used with AC current when welding with aluminum alloys.
  
  The physical properties of helium definitely offer advantages in some applications. However, due to it high ionization potential, it also produces a less stable arc and a less desirable arc starting characteristic than argon. Its higher cost and higher flow rates are also factors to be considered. In some cases, an argon mixture is used for igniting the arc and pure helium is used for welding. This technique is used for DC GTAW welding of heavy aluminum.
   

• Argon-Helium Mixtures.  Each of these gases (argon and helium), as explained above, has specific advantages. USW's blends (argon-helium blends) are basically used to increase the heat input to the base metal while maintaining the favorable characteristics of argon, such as arc stability and superior arc starting.
  
  A-75 Gas Blend.  This blend is sometimes used for DC welding when it is desirable to obtain higher heat input while maintaining the good arc starting behavior of argon.
  
  A-50 Gas Blend.  This blend is used primarily for high-speed mechanized and manual welding of nonferrous material (aluminum and copper) under 3/4 inch thick.
  
  A-25 Gas Blend.  The speed and quality of AC welding on aluminum can be improved with this blend. It is sometimes used for manual welding of aluminum pipe and mechanized welding of butt joints in aluminum sheet and plate. The A-25 gas blend is also used for many of the GTAW hot wire applications to increase the energy input while accommodating the high filler metal deposition rates of the process.