Shielding gases fall into two categoriesnert or semi-inert. Only two of the noble gases, helium and argon, are cost effective enough to be used in welding. These inert gases are used in gas tungsten arc welding, and also in gas metal arc welding for the welding of non-ferrous materials. Pure argon and helium are used only for some nonferrous metals. Semi-inert shielding gases, or active shield gases, include carbon dioxide, oxygen, nitrogen, and hydrogen. Most of these gases, in large quantities, would damage the weld, but when used in small, controlled quantities, can improve weld characteristics.

Properties of shielding gases

The important properties of shielding gases are their thermal conductivity and heat transfer properties, their density relative to air, and how easy they undergo ionization. Gases heavier than air (e.g. argon) blanket the weld and require lower flow rates than gases lighter than air (e.g. helium). Heat transfer is important for heating the weld around the arc. Ionizability influences how easy the arc starts, and how high voltage is required. Shielding gases can be used pure, or as a blend of two or three gases. In laser welding, the shielding gas has an additional role, preventing formation of a cloud of plasma above the weld, absorbing significant fraction of the laser energy. This is important for CO2 lasers; Nd:YAG lasers show lower tendency to form such plasma. Helium plays this role best due to its high ionization potential; the gas can absorb high amount of energy before becoming ionized.

Helium is lighter than air; larger flow rates are required. It is an inert gas, not reacting with the molten metals. Its thermal conductivity is high. It is not easy to ionize, requiring higher voltage to start the arc. Due to higher ionization potential it produces hotter arc at higher voltage, provides wide deep bead; this is an advantage for aluminium, magnesium, and copper alloys. Other gases are often added. Blends of helium with addition of 5-10% of argon and 2-5% of carbon dioxide ("tri-mix") can be used for welding of stainless steel. Used also for aluminium and other non-ferrous metals, especially for thicker welds. In comparison with argon, helium provides more energy-rich but less stable arc. Helium and carbon dioxide were the first shielding gases used, since the beginning of World War 2. Helium is used as a shield gas in laser welding for carbon dioxide lasers. Helium is more expensive than argon and requires higher flow rates, so despite its advantages it may not be a cost-effective choice for higher-volume production. Pure helium is not used for steel, as it then provides erratic arc and encourages spatter.

Argon is heavier than air; lower flow rates are needed to blanket the weld. It is an inert gas, not reacting with the molten metals. It has low thermal conductivity. It ionizes easily, providing a stable arc with an excellent current path and high current density. It produces a very narrow arc cone and narrow penetration profile. It is often used as pure when welding aluminium and other nonferrous metals, though other gases can be added; pure argon does not provide sufficient penetration for welding steel. A blend of argon with 25-50% of helium is used for some nonferrous metals, as helium improves heat transfer into the base material and makes the molten metal more fluid. An oxidizing component (oxygen, carbon dioxide) is usually added to stabilize the arc for welding of steels; without it the arc control can be difficult as the arc tends to stray. In industrial gas business it is known as "the big A". Argon is used as a shield gas in laser welding for Nd:YAG lasers.

Carbon dioxide has good heat transfer properties; it dissociates in the weld and recombines in contact with the colder metal. Produces very deep weld but somewhat unstable arc and, due to its reactivity, intense spatter. Due to the presence of dissociated oxygen, the weld zone has oxidizing properties, producing more slag. Carbon dioxide can be used as pure (only for short-circuiting), or in a mixture with 5 to 25% argon, sometimes up to 50% (also for spray transfer); the argon addition inhibits sputtering. Increasing percentage of carbon dioxide increases the width and depth of the weld penetration. For welding of stainless steels where carbon content control is required, an argon-helium blend with 1-2% of carbon dioxide can be used. "Trimix" blends of argon-oxygen-carbon dioxide are more common in United Kingdom, while argon-carbon dioxide blends are more common in the USA. In comparison with argon-carbon dioxide mixture, for steel welding, pure carbon dioxide increases spatter and the arc is less stable. Pure carbon dioxide provides deep weld penetration and is very cheap. Pure carbon dioxide usage is limited to short circuit and globular transfer welding. It has high spatter and deep penetration. Provides good mechanical properties. Can be used for carbon steel. Has high production of smoke and fumes. It is very cheap.

Oxygen is used in small amounts as an addition to other gases; typically as 2-5% addition to argon. It enhances arc stability and reduces the surface tension of the molten metal, increasing wetting of the solid metal. It is used for spray transfer welding of mild carbon steels, low alloy and stainless steels. Its presence increases the amount of slag. Argon-oxygen (Ar-O2) blends are often being replaced with argon-carbon dioxide ones. Argon-carbon dioxide-oxygen blends are also used. Oxygen causes oxidation of the weld, so it is not suitable for welding aluminium, magnesium, copper, and some exotic metals. Increased oxygen makes the shielding gas oxidize the electrode, which can lead to porosity in the deposit if the electrode does not contain sufficient deoxidizers. Excessive oxygen, especially when used in application for which it is not prescribed, can lead to brittleness in the heat affected zone. Argon-oxygen blends with 1-2% oxygen are used for austenitic stainless steel where argon-CO2 can not be used due to required low content of carbon in the weld; the weld has a tough oxide coating and may require cleaning.

Nitrogen is used for welding of some stainless steels. It increases the weld penetration and enhances arc stability. It however can cause porosity in carbon steels. Argon-carbon dioxide-nitrogen blends can be used. Pure nitrogen is also used, or can be blended with 10% of hydrogen, depending on application. Blends with nitrogen content are used to weld nitrogen-containing alloys (up to 0.5% nitrogen increases mechanical properties and resistance to pitting corrosion) to prevent loss of nitrogen from the metal. Nitrogen can be used in some cases of laser welding; it is almost as good as helium for plasma formation suppression but can cause embrittlement of some steels.

Hydrogen is used for welding of nickel and some stainless steels, especially thicker pieces. It improves the molten metal fluidity, and enhances cleanness of the surface. It can however cause hydrogen embrittlement of many alloys and especially carbon steel, so its application is usually limited only to some stainless steels. It is added to argon in amounts typically under 10%. It can be added to argon-carbon dioxide blends to counteract the oxidizing effects of carbon dioxide. Its addition narrows the arc and increases the arc temperature, leading to better weld penetration. In higher concentrations (up to 25% hydrogen), it may be used for welding conductive materials such as copper. However, it should not be used on steel, aluminum or magnesium because it can cause porosity and hydrogen embrittlement.

Nitric oxide addition serves to reduce production of ozone. It can also stabilize the arc when welding aluminium and high-alloyed stainless steel.

Other gases can be used for special applications, pure or as blend additives; e.g. sulfur hexafluoride or dichlorodifluoromethane.

Sulfur hexafluoride can be added to shield gas for aluminium welding to bind hydrogen in the weld area to reduce weld porosity.

Dichlorodifluoromethane with argon can be used for protective atmosphere for melting of aluminium-lithium alloys. It reduces the content of hydrogen in the aluminium weld, preventing the associated porosity.

Common mixes

C-50 (50% argon/50% CO2) is used for short arc welding of pipes,

C-40 (60% argon/40% CO2) is used for some flux-cored arc welding cases. Better weld penetration than C-25.

C-25 (75% argon/25% CO2) is commonly used by hobbyists and in small-scale production. Limited to short circuit and globular transfer welding. Common for short-circuit gas metal arc welding of low carbon steel.

C-20 (80% argon/20% CO2) is used for short-circuiting and spray transfer of carbon steel.

C-15 (85% argon/15% CO2) is common in production environment for carbon and low alloy steels. Has lower spatter and good weld penetration, suitable for thicker plates and steel significantly covered with mill scale. Suitable for short circuit, globular, pulse and spray transfer welding. Maximum productivity for thin metals in short-circuiting mode; has lower tendency to burn through than higher-CO2 mixes and has suitably high deposition rates.

C-10 (90% argon/10% CO2) is common in production environment. Has low spatter and good weld penetration, though lower than C-15 one; suitable for many steels. Same applications as 85/15 mix. Sufficient for ferritic stainless steels.

C-5 (95% argon/5% CO2) is used for pulse spray transfer and short-circuiting of low alloy steel. Has better tolerance for mill scale and better puddle control than argon-oxygen, though less than C-10. Less heat than C-10. Sufficient for ferritic stainless steels. Similar performance to argon with 1% oxygen.

O-5 (95% argon/5% oxygen) is the most common gas for general carbon steel welding. Higher oxygen content allows higher speed of welding.

O-2 (98% argon/2% oxygen) is used for spray arc on stainless steel, carbon steels, and low alloy steels. Better wetting than O-1. Weld is darker and more oxidized than with O-1.

O-1 (99% argon/1% oxygen) is used for stainless steels. Oxygen stabilizes the arc

A-25 (75% argon/25% helium) is used for nonferrous base when higher heat input and good weld appearance are needed.

A-50 (50% argon/50% helium) is used for nonferrous metals thinner than 0.75 inch for high-speed mechanized welding.

A-75 (25% argon/75% helium) is used for mechanized welding of thick aluminium. Reduces weld porosity in copper.

H-2 (98% argon/2% hydrogen)

H-5 (95% argon/5% hydrogen)

H-10 (80% argon/20% hydrogen)

H-35 (65% argon/35% hydrogen)

Argon with 25-35% helium and 1-2% CO2 provides high productivity and good welds on austenitic stainless steels. Can be used for joining stainless steel to carbon steel.

Argon-CO2 with 1-2% hydrogen provides a reducing atmosphere that lowers amount of oxide on the weld surface, improves wetting and penetration. Good for austenitic stainless steels.

Argon with 2-5% nitrogen and 2-5% CO2 in short-circuiting yields good weld shape and color and increases welding speed. For spray and pulsed spray transfer it is nearly equivalent to other trimixes. When joining stainless to carbon steels in presence of nitrogen, care has to be taken to ensure the proper weld microstructure. Nitrogen increases arc stability and penetration and reduces distortion of the welded part. In duplex stainless steels assists in maintaining proper nitrogen content.

85-95% helium with 5-10% argon and 2-5% CO2 is an industry standard for short-circuit welding of carbon steel.

Argon-carbon dioxide-oxygen

Argon-helium-hydrogen

Argon-helium-hydrogen-carbon dioxide

Applications

The applications of shielding gases are limited primarily by the cost of the gas, the cost of the equipment, and by the location of the welding. Some shielding gases, like argon, are expensive, limiting its use. The equipment used for the delivery of the gas is also an added cost, and as a result, processes like shielded metal arc welding, which require less expensive equipment, might be preferred in certain situations. Finally, because atmospheric movements can cause the dispersion of the shielding gas around the weld, welding processes that require shielding gases are often only done indoors, where the environment is stable and atmospheric gases can be effectively prevented from entering the weld area.

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, in general, mean that more gas needs to 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 requirementsor the small weld pools of the short circuiting and pulsed spray modes, about 10 L/min (20 ft3/h) is generally suitable, while for globular transfer, around 15 L/min (30 ft3/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 2025 L/min (4050 ft3/h).

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