Shielding Gases Used in Welding
The inert or semi-inert gases used in gas metal arc welding and gas tungsten arc welding (GMAW/GTAW or MIG/TIG) protect the weld area from atmospheric gases. Gases in the atmosphere include oxygen, nitrogen, carbon dioxide, and water vapor. They may impair weld quality. Improper choice of shielding gas may lead to a porous and weak weld, to excessive spatter, and reduced productivity.
Shielded metal arc welding uses an electrode covered in flux that produces carbon dioxide when heated. This semi-inert gas provides an appropriate shield when welding steel. In laser welding the shielding gas prevents a plasma cloud from forming above the weld.
The six noble gases (odorless, colorless, monatomic, low chemical reactivity) are helium, neon, argon, krypton, xenon, and radon. Of those only helium and argon are cost effective enough to be used in welding. These inert gases are used in GTAW and GMAW for non-ferrous materials.
Semi-inert gases include carbon dioxide, oxygen, nitrogen, and hydrogen. These semi-inert gases, when in controlled quantities, can improve welds. Most, when over applied, would damage the weld. Gases may be used pure or as a blend of two or three gases.
The first essential properties are thermal conductivity and heat transfer. Relative density and ease of undergoing ionization are also crucial. Heat transfer is necessary to heat the weld around the arc. The ability for ionization affects the arc’s start and voltage requirements.
Helium, a lighter than air gas, requires larger flow rates. The higher flow rate and its expense cause Helium to lose its place as first choice for high volume welding. Thermal conductivity is high. Helium requires a higher voltage to start the arc because it is not easy to ionize. Ideal for aluminum, magnesium, and copper; helium provides a deep wide bead. Helium blends may be used for stainless steel or aluminum welding. Pure helium provides an erratic arc and encourages spatter when working with steel.
Argon, heavier than air gas, needs a lower flow rate. The inert gas does not react with molten metals, has low thermal conductivity and ionizes easily. The stable arc with excellent current path and high current density produces a very narrow arc cone and narrow penetration profile. Pure argon is used often with aluminum and nonferrous metal welding. Pure argon is not used for welding steel. Adding Helium will improve heat transfer. Oxygen or carbon dioxide will stabilize the arc.
Carbon dioxide has good heat transfer and produces a very deep weld, but the arc is somewhat unstable and spatter is increased. Argon-Carbon dioxide blends are common as the argon inhibits spattering. Carbon dioxide is cheap, but has a high production of smoke fumes. It can be used for carbon steel.
Oxygen is used as an additive. Two-five percent added to argon will enhance arc stability, reduce surface tension, and increase wetting of the solid metal. Due to oxidative properties it cannot be used for welding aluminum, magnesium, or copper. Oxidation of the electrode leads to a porous deposit (without sufficient deoxidizers). Too much oxygen can lead to brittleness.
Nitrogen increases weld penetration and enhances arc stability. It is used for some stainless steels, but will cause porosity in carbon steels. Pure nitrogen, hydrogen-nitrogen, or argon-carbon dioxide-nitrogen may be used. When using an alloy containing nitrogen, nitrogen gas blends will increase mechanical properties, resist pitting corrosion, and prevent loss of nitrogen from the metal. It can be used in some laser welding.
Hydrogen improves metal fluidity, enhances surface cleanliness, and can be used with nickel and some stainless steels. Many alloys and carbon steel may be made more brittle. When added to argon-carbon dioxide it counteracts oxidation, narrows the arc, increases arc temperature, and improves weld penetration. It may be used with copper.
There are many combinations and special gas additives. Nitric oxide reduces ozone. Sulfur hexafluoride shields aluminum welding. Dichlorodifluoromethane is added for aluminum-lithium alloys. Watch for a future post on common gas mixes and applications.
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