GMAW SHIELDING GASSES

Shielding gas is the component of the GMAW process that elevated the GMAW process to its current position of prominence in the welding industry. Before effective shielding of the molten weld puddle was attained, the GMAW process was little more than a novel idea. With all arc welding processes, protecting the molten weld puddle from atmospheric contamination is of utmost importance. With the GMAW process the puddle is protected by a stream of externally supplied shielding gas. For the well trained welder, knowing and understanding the nature and characteristics of the shielding gasses commonly used in conjunction with the GMAW process is essential.

Shielding Gas Delivery Systems Shielding gas is delivered to the welding equipment in one of two fashions; Stand Alone Cylinders Makes system more portable Manifold delivery system Allows for use of bulk liquid shielding gasses Ability to custom mix shielding gasses when a gas mixer is integrated into the manifold system Cheaper in cost (less handling and purchasing) GMAW shielding gasses Three pure gasses commonly used: Carbon Dioxide Argon Helium It is common practice to use a mixture of these gasses to obtain a particular characteristic or result.

Carbon Dioxide A non reactive gas Consists of one part carbon, two parts oxygen Specific gravity of 1.52, therefore heavier than air Several characteristics make it a good choice as a shielding gas when used with the GMAW process: Produces a wide weld pattern and deep penetration without the tendency of undercutting the base metal Produces a weld with good surface contour but has a tendency to create more weld spatter due to the unstable nature of the arc (To help combat the tendency of excessive weld spatter, the operator uses the shortest electrode extension possible) Cheaper than the pure inert shielding gasses Because of the oxygen present in carbon dioxide, an electrode with deoxidizing (ER70S-3 or ER70S-6) agents should be used to produced x-ray quality welds

Argon An inert gas with a specific gravity of 1.38 and is heavier than air Pure argon is not used as a shielding gas for welding carbon based metals due to its tendency to produce uneven penetration, undercutting and poor bead contour Pure argon use in GMAW is limited to welding of non –ferrous metals Use of argon as a base for a mixed shielding gas is GMAW is common Argon has many attributes that can be utilized by using a gas mixture: Provides a very stable arc with little weld spatter Argon has low thermal conductivity which produces high arc density Higher arc densities allow for more energy (heat) input into the base metal Argon requires slightly lower arc voltage for a given wire size than helium or carbon dioxide due to its ability to ionize easily and transmit some electrical energy Argon is more costly than carbon dioxide

Helium An inert gas with a specific gravity of 0.137 and is much lighter than air Produces higher arc heat than argon and broad but someone shallow penetration profile Pure helium used in GMAW is limited to welding non-ferrous metals The use of helium when welding ferrous metal is generally as a minor component in a mixed shielding gas Helium is more costly than carbon dioxide and argon Cannot be used in a pure state for spray arc transfer High ionization potential can create difficulty in arc starting unless using high-frequency or capacitive arc starting is used for GTAW Promotes Globular Transfer in GMAW

Mixed Shielding Gasses The use of mixed shielding gasses in GMAW is common By mixing gasses in varying amounts, specific characteristics of each gas can be utilized to produce better welds On of the most common gas mixtures is argon and carbon dioxide By mixing the two gasses, the characteristics of each gas are combined to produce welds of the highest quality. Carbon Dioxide advantages: Wide, deep penetration with good contour No undercutting Low cost Argon advantages: Stable arc Low spatter High arc density A wide variety of shielding mixtures are available for different applications It is not uncommon for large users of the GMAW process to mix their own blend of shielding gas to best fit their application (Manifold System)

AWS Shielding Gas Designation System The American Welding Society has the following shielding gas system to visually identify shielding gas cylinder contents