Gases for Metal Arc Welding (GMAW)

Gas Metal Arc Welding (GMAW) is used to weld all commercially important metals, including steel, aluminum, copper, and stainless steel. The process can be used to weld in any position, including flat, vertical, horizontal, and overhead. It is usually connected to use direct current electrode positive (DCEP). It is an arc welding process that incorporates the automatic feeding of a continuous, consumable electrode that is shielded by an externally supplied gas.

• Argon. Argon (Ar) is used on nonferrous base metals such as aluminum, nickel, copper, magnesium alloys, and reactive metals, such as zirconium and titanium. Argon provides excellent arc welding stability, penetration, and bead profile on these base metals. When welding ferrous-based metals, argon is usually mixed with other gases, such as oxygen, helium, carbon dioxide, or hydrogen.
  
  The low ionization potential of argon helps create an excellent current path and superior arc stability. Argon produces a constricted arc column with high current density which causes the arc energy to be concentrated over a small surface area.
    

• Carbon Dioxide.  Carbon dioxide (CO₂ ), a reactive gas, dissociates into carbon monoxide and free oxygen in the heat of the arc. Oxygen then combines with elements transferring across the arc to form oxides from the weld pool in the form of slag and scale, generating a great deal of smoke and fumes. Although carbon dioxide is an active gas and produces an oxidizing effect, sound welds can be consistently achieved with pure CO₂ .
  
  Carbon dioxide is often used in its pure form with welding of carbon steel, because it is readily available and produces good welds at low cost. However, this may be a false economy as the low cost per unit of gas does not always translate to the lowest cost per foot of deposited weld. Other factors, such as lower deposition efficiency due to spatter loss, can influence the final weld cost and should be carefully considered.
  
  Carbon dioxide will not support spray transfer. Metal transfer is restricted to the short circuiting and globular modes. A major disadvantage of carbon dioxide is harsh globular transfer with its characteristic spatter.
  
  The weld surface resulting from carbon dioxide shielding is usually heavily oxidized. An electrode with higher amounts of deoxidizing elements is needed to compensate for the loss of alloying elements across the arc. This may cause problems when the completed part requires paint. The advantages of carbon dioxide are good width of fusion and the achievement of good mechanical properties.
   

• Helium.  Helium (He) is a chemically inert gas that is used for welding applications requiring higher heat inputs. It may improve wetting action, depth of fusion, and travel speeds. It does not produce the stable arc provided by argon. Helium has higher thermal conductivity than argon and produces a wider arc column. The higher voltage gradient provides a higher heat input than argon, promoting greater weld pool fluidity and better wetting action. This is an advantage when welding aluminum, magnesium, and copper alloys.
  

• Argon-Oxygen Mixtures.  The addition of small amounts of oxygen to argon greatly stabilizes the welding arc, increases the filler metal droplet rate, lowers the spray transition current, and influences bead shape. The weld pool is more fluid and stays molten longer, allowing the metal to flow out towards the weld toes.
  
  1%  Gas Blend (99/1).  This blend is primarily used for spray transfer on stainless steels. One percent oxygen is usually sufficient to stabilize the arc and improve the droplet rate and bead appearance.
  
  2% Gas Blend (98/2).  This blend is used for spray arc welding of carbon steels, low-alloy steels and stainless steels. It provides greater wetting action than the 1% oxygen mixture. Weld mechanical properties and corrosion resistance of welds made with 1% and 2% oxygen additions are similar. However, bead appearance will be darker and more oxidized for the 2% blends with stainless steels.
  
  5% Gas Blend (95/5).  This blend provides a more fluid but controllable weld pool. It is the most commonly used argon-oxygen mixture for general carbon steel welding. The additional oxygen also permits higher travel speeds.
    

• Argon-Carbon Dioxide Mixtures.  Argon-carbon dioxide blends are mainly used on carbon and low-alloy steels and have limited application on stainless steels. Carbon dioxide added to argon, at higher current levels, increases spatter.
  
  In GMAW, a slightly higher current level must be reached when using argon-carbon dioxide in order to establish and maintain stable spray transfer. Above approximately 20% carbon dioxide, spray transfer becomes unstable and periodic short-circuiting and globular transfer occurs.
  
  5% Gas Blend (C-5).  This blend is used for pulsed spray transfer and short-circuiting transfer on a variety of material thicknesses. A 5% mixture may be used for GMAW-P of low alloy steels for out-of-position welding. The arc forces that develop give this mixture more tolerance to mill scale and a more controllable puddle than an argon-oxygen blend.
  
  10% Gas Blend (C-10).  This blend performs similarly to the C-5, but with increased heat input providing a wider, more fluid weld puddle in either short-circuit or spray transfer.
  
  25% Gas Blend (C-25).  This blend is commonly used for GMAW with short-circuiting transfer on low carbon steel. It was formulated to provide optimum droplet frequency on short-circuiting transfer using .035 and .045 diameter wire. USW's C-25 operates well in high current applications on heavy base metal. It promotes good arc stability, weld pool control, and weld bead appearance. This blend will not support the spray type mode of metal transfer.
    

• Argon-Carbon Dioxide-Oxygen Mixtures.  Mixtures containing these three components are versatile, due to their ability to operate using short-circuiting, globular, spray, pulsed, and high-density transfer modes. Several ternary compositions are available and their application depends on the desired metal transfer mode.
  
  The advantage of this blend is its ability to shield carbon steel and low-alloy steel of all thicknesses using any metal transfer mode applicable.  The tri-mix gas produces good welding characteristics and mechanical properties on carbon low-alloy steels and some stainless steels. On thin gauge base metals, the oxygen constituent assists arc stability at very low current levels (30 to 60 amps) permitting the arc to be kept short and controllable. This helps minimize excessive melt-through and distortion by lowering the total heat input into the weld zone. Tri-mixtures are is generally used for spray arc welding, providing high deposition rates and often higher travel speeds than carbon dioxide.
   

• Argon-Helium Mixtures.  Helium is often mixed with argon to obtain the advantages of both gases. Argon provides good arc stability and cleaning action, while helium promotes wetting with a broad width of fusion.
  
  Argon-helium blends are used primarily for nonferrous base metals, such as aluminum, copper, nickel alloys, magnesium alloys, and reactive metals. Helium additions to an argon-base gas will increase the heat input. Generally, the thicker the base metal, the higher the percentage of helium. Small percentages of helium, as low as 20%, will affect the arc. As helium percentages increase, the arc voltage, spatter, and weld width to depth ratio increase, while porosity is minimized in aluminum. The argon percentage must be at least 20% when mixed with helium to produce and maintain a stable spray transfer.
  
  A-25 Gas Blend.  This blend is used for welding nonferrous base metals when an increase in heat input is needed and weld bead appearance is of primary importance.
  
  A-50 Gas Blend.  This blend is used primarily for high-speed mechanized welding of nonferrous materials under 3/4 inch thick.
  
  A-75 Gas Blend.  This blend is used for mechanized welding of aluminum greater than one inch thick in the flat position. It increases heat input and reduces porosity of welds in copper.
    

• Argon-Helium-Carbon Dioxide Mixtures.  Helium and carbon dioxide additions to argon increase the heat input to the weld, which improves wetting, fluidity, and weld bead profile. Patents and pending patents cover GMAW with such three-part blends.
  
  A-1025 Gas Blend.  This blend is widely used for short-circuiting transfer welding of stainless steel in all welding positions. The carbon dioxide content is kept low to minimize carbon absorption and assure good corrosion resistance, especially in multi-pass welds. The argon and carbon dioxide additions provide good arc stability and depth of fusion. The high helium content provides significant heat input to overcome the sluggish nature of the stainless steel weld pool.