Shielding gas and its uses

Shielding gas is a crucial part of welding and its selection affects many factors. Proper gas selection is crucial in welding in the most cost-effective manner as shielding gas protects the molten pool of metal from atmospheric gases especially hydrogen which can lead to several discontinuities like porosity.

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But first here are the most common arc welding process that uses shielding gas:

• GMAW
• GTAW

SMAW though makes use of shielding gas through a flux coating which is generated during the welding process. The flux coating is also present with FCAW which has a tubular core. Note! SMAW and FCAW do not use an external shielding gas, GMAW and GTAW do.

Here are the factors that affect the selection of shielding gas from ASME Section II:

1. Type and thickness of base metal being welded
2. Arc characteristics
3. Metal transfer
4. Travel speed
5. Depth and width of fusion
6. Cost of welding
7. Mechanical properties
8. Root opening
9. Cleanliness of the base material
10. Spatter
11. Arc cleaning action
12. Gas purity
13. Joint configuration
14. Welding position
15. Fume generation

Intended use of Shielding Gases

1. Argon - chemically inert gas which can be used singularly or in combination with other gases. It can be used for both ferrous and nonferrous materials. For GMAW process, argon produces a constricted arc column at a high current density which causes the arc energy to be concentrated on the weld pool (produces a distinct finger-like shape). Can be used for single-side melt-through welding with or without consumable inserts
2. Carbon dioxide- active gas used primarily for GMAW and FCAW. Carbon dioxide dissociates and forms carbon monoxide and free oxygen. This free oxygen forms scale and slag. Since carbon dioxide is a reactive gas, the electrode needs to have deoxidizing elements to compensate. It is often mixed with argon to improve operating characteristics and to maximize mechanical properties. Economically, carbon dioxide has lower cost per unit volume compared with other gases.
3. Helium- chemically inert gas used for higher heat inputs. It can improve wetting action, depth of fusion and travel speeds (wetting action is the ability of the liquid filler metal when melted to adhere in a thin layer). It does not produce a stable arc compared with argon and produces a wider arc column. It is advantageous when welding magnesium-based and copper-based alloys. For GMAW, can only be applicable with globular transfer, and needs to be mixed with argon for a continuous spray transfer
4. Oxygen- never used as a base shielding gas, always a minor component
5. Hydrogen- used as a minor component because it is chemically active- most commonly used at smaller percentages (1-35%).

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