PROPERTIES OF SHIELDING GASES USED FOR WELDING

 Shielding gases are inert or semi-inert gases that are commonly used in several welding processes, gas metal arc welding and gas tungsten arc welding (GMAW and GTAW, popularly known as MIG (Metal Inert Gas) and TIG (Tungsten Inert Gas), respectively). Their purpose is to protect the weld area from oxygen and water vapor. Depending on the materials being welded, these atmospheric gases can reduce the quality of the weld or make welding more difficult. Other arc welding processes use alternative methods of protecting the weld from the atmosphere as well. Shielded metal arc welding, for example, uses an electrode covered in a flux that produces carbon dioxide when consumed, a semi-inert gas that is an acceptable shielding gas for welding steel.

The important properties of shielding gases are;

• Thermal conductivity and heat transfer properties.
• Its density relative to air.
• The ease with which 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 the formation of a cloud of plasma above the weld, absorbing a significant fraction of the laser energy. Helium plays this role best due to its high ionization potential; the gas can absorb a high amount of energy before becoming ionized.

Argon is the common shielding gas, widely used as the base for the more specialized gas mixes.

Carbon dioxide is the least expensive shielding gas, providing deep penetration. However, it negatively affects the stability of the arc and enhances the molten metal's tendency to create droplets (spatter). Carbon dioxide in a concentration of 1-2% is commonly used in the mix with argon to reduce the surface tension of the molten metal. Another common blend is 25% carbon dioxide and 75% argon for GMAW.

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 a 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 aluminum, magnesium and copper alloys. Other gases are often added. Blends of helium with the 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 aluminum 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 shielding 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.

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 the wetting of the solid metal. It is used for spray transfer welding of mild carbon steels, low alloy, and stainless steel. 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 aluminum, magnesium, copper, and some exotic metals. Improper choice of a welding gas can lead to a porous and weak weld, or excessive spatter; the latter, while not affecting the weld itself, causes loss of productivity due to the labor needed to remove the scattered drops. Excessive oxygen, especially when used in an application for which it is not prescribed, can lead to brittleness in the heat-affected zone. Argon-oxygen blends with 1–2% oxygen is used for austenitic stainless steel where argon-CO2 cannot be used due to required low content of carbon in the weld; the weld has a tough oxide coating and may require cleaning.

Hydrogen is used for welding of nickel and some stainless steels, especially thicker pieces. It improves the molten metal fluidity and enhances the cleanness of the surface. 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; its application is usually limited only to some stainless steel.

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

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

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

Improper choice of a welding gas can lead to a porous and weak weld, or excessive spatter; the latter, while not affecting the weld itself, causes loss of productivity due to the labor needed to remove the scattered drops.

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