Shielding gas

Shielding gas
Shielding gases are necessary for gas metal arc welding to protect the welding area from atmospheric gases such as nitrogen and oxygen, which can cause fusion defects, porosity, and weld metal embrittlement if they come in contact with the electrode, the arc, or the welding metal. This problem is common to all arc welding processes, but instead of a shielding gas, many arc welding methods utilize a flux material which disintegrates into a protective gas when heated to welding temperatures. In GMAW, however, the electrode wire does not have a flux coating, and a separate shielding gas is employed to protect the weld. This eliminates slag, the hard residue from the flux that builds up after welding and must be chipped off to reveal the completed weld.
The choice of a shielding gas depends on several factors, most importantly the type of material being welded and the process variation being used. Pure inert gases such as argon and helium are only used for nonferrous welding; with steel they cause an erratic arc and encourage spatter (with helium) or do not provide adequate weld penetration (argon). Pure carbon dioxide, on the other hand, allows for deep penetration welds but encourages oxide formation, which adversely affect the mechanical properties of the weld. Its low cost makes it an attractive choice, but because of the violence of the arc, spatter is unavoidable and welding thin materials is difficult. As a result, argon and carbon dioxide are frequently mixed in a 75%/25% or 80%/20% mixture, which reduces spatter and makes it possible to weld thin steel workpieces.
Argon is also commonly mixed with other gases, such as oxygen, helium, hydrogen, and nitrogen. The addition of up to 5% oxygen encourages spray transfer, which is critical for spray-arc and pulsed spray-arc GMAW. However, more oxygen makes the shielding gas oxidize the electrode, which can lead to porosity in the deposit if the electrode does not contain sufficient deoxidizers. An argon-helium mixture is completely inert, and is used on nonferrous materials. A helium concentration of 50%–75% raises the voltage and increases the heat in the arc, making it helpful for welding thicker workpieces. Higher percentages of helium also improve the weld quality and speed of using alternating current for the welding of aluminum. Hydrogen is added to argon in small concentrations (up to about 5%) for welding nickel and thick stainless steel workpieces. In higher concentrations (up to 25% hydrogen), it is useful for welding conductive materials such as copper. However, it should not be used on steel, aluminum or magnesium because of the risk of hydrogen porosity. Additionally, nitrogen is sometimes added to argon to a concentration of 25%–50% for welding copper, but the use of nitrogen, especially in North America, is limited. Mixtures of carbon dioxide and oxygen are similarly rarely used in North America, but are more common in Europe and Japan.
Recent advances in shielding gas mixtures use three or more gases to gain improved weld quality. A mixture of 70% argon, 28% carbon dioxide and 2% oxygen is gaining in popularity for welding steels, while other mixtures add a small amount of helium to the argon-oxygen combination, resulting in higher arc voltage and welding speed. Helium is also sometimes used as the base gas, to which smaller amounts of argon and carbon dioxide are added. Additionally, other specialized and often proprietary gas mixtures claim to offer even greater benefits for specific applications.
The desirable rate of gas flow depends primarily on weld geometry, speed, current, the type of gas, and the metal transfer mode being utilized. Welding flat surfaces requires higher flow than welding grooved materials, since the gas is dispersed more quickly. Faster welding speeds mean that more gas must be supplied to provide adequate coverage. Additionally, higher current requires greater flow, and generally, more helium is required to provide adequate coverage than argon. Perhaps most importantly, the four primary variations of GMAW have differing shielding gas flow requirements—for the small weld pools of the short circuiting and pulsed spray modes, about 10 L/min (20 ft?/h) is generally suitable, while for globular transfer, around 15 L/min (30 ft?/h) is preferred. The spray transfer variation normally requires more because of its higher heat input and thus larger weld pool; along the lines of 20–25 L/min (40–50 ft?/h).