INTRODUCTION

• SHIELDING GAS used in a Welding process has a significant influence on the overall performance of the welding system.
• While shielding gas protects the welding from the effects of oxygen and nitrogen in the atmosphere there are other uses also for stabilizing the welding arc and ensuring a uniform metal transfer.
• In MIG / MAG and FCAW the shielding gas used has a substantial influence on the method of molten metal transfer to the base metal during welding. This could be in the form of spray, globular, short circuiting or pulsed spray transfer.
• Choosing the proper gas is essential for successful welding operation.
• The shielding gas protects and sometimes interacts with the base material and with the filler material, if any to produce a weld conforming to the mechanical and metallurgical properties expected as per the procedure established.
• Shielding gas, welding current, voltage and speed are essential parameters which determine the weld bead and penetration profile.
• Proprietary gas mixtures with different combinations and percentages are available which will   improve the weld bead profile and increase productivity for the given processes.
• The profile means weld bead shape, penetration and the UTS expected as per the WPS and PQR specified by the Customer / Designer.
• The applications of shielding gases are limited by
• Welding process chosen
• Gas cost
• Equipment cost
• Its effects on the Weld bead
• Care should be taken to ensure the welding area is free from drafts of air / wind which will blow away the shielding gas and expose the molten weld bead to oxygen and nitrogen in the air.
• Gas type and flow required is based on the process of welding. Other factors to be considered are 
• Weld geometry
• Speed
• Current
• Type of gas
• Metal transfer mode being utilized

SHIELDING GASES USED

• The different Shielding gases which are used in welding process are

• Argon is an inert gas with a low ionization potential; argon prevents oxidation of the Tungsten rod ensures easy arc starting and smooth arc operation. It’s the primary gas used in TIG welding and Plasma welding.
• It promotes axial "spray" transfer in certain forms of MIG welding. This is because it is a cooler gas with low thermal conductivity.

• Helium is again an inert gas lighter than air and has a low density; therefore larger flow rates are required, as compared to argon gas to perform the same job.
• Its thermal conductivity is much higher compared with argon.
• It has high ionization potential.
• It provides clean weld bead with deep penetration.
• In comparison with argon, helium has difficulty in arc starting and stability but provides a higher temperature   arc at the same current level.
• Since helium is costly it is combined with argon or argon mixture to enhance the overall performance of the blend.
• It is used in TIG welding for special applications

• Oxygen is added as 2–5% addition to argon in MIG welding. To improve the stability of the welding arc developed as well as to increase the fluidity of the weld puddle.
• It reduces the surface tension of the molten metal ensuring better gap bridging. It also increases the arc stability.
• In the spray-transfer mode, small additions of oxygen enhance the range of welding where the spatter is reduced.
• Oxygen is not suitable for welding nonferrous and exotic metals.

• Carbon dioxide is used in certain types of MIG welding. Carbon dioxide added to argon improves arc stability, better penetration and fluid weld puddle flow characteristics. Carbon dioxide being a diatomic gas transfers more heat to the base material than does argon alone.
• The higher heat levels increases penetration pattern as compared to argon. The higher temperature may increase base material distortion .Gap-bridging ability in the weld zone may also be a problem.

• A small percentage of Hydrogen mixed with Argon is used for welding Stainless steels and nickel alloys especially the one with thicker dimensions.
• Hydrogen is the lightest known element and is a flammable gas.
• It enhances the fluidity of molten metal and the cleanliness of the surface by removing oxygen from the weld atmosphere. This prevents oxidation in the weld bead.
• Its addition narrows the arc ensures a higher arc temperature. This increases weld bead penetration 
• Hydrogen and Argon mixtures have a high heat potential and is used in operations involving cutting and gouging.
• It is used in limited way because some materials are sensitive to hydrogen related contamination.

• Nitrogen is not used as a primary shielding gas. It can be used combined with other gases for some welding applications (e.g., copper) Nitrogen and hydrogen / argon mixtures are used in plasma cutting.

SHIELDING GASES FOR WELDING PROCESSES

• Generally the shielding gases in welding process are decided according to the methodology which is adopted to weld the material. Different processes uses different Shielding gases. Some of the welding processes are listed below,

• Argon
• Helium
• Argon-Hydrogen mixture Hydrogen mixtures containing between 2 to  8 % Hydrogen balance Argon are  used for sheilding of stainless steel and nickel-base alloy welds . Hydrogen reacts with atmospheric oxygen preventing it from coming in contact with the weld pool ensuring cleaner welds. The higher arc temperature gives deeper penetration as well as higher speed of welding. Hydrogen also increases the fluidity of the weld pool increasing wettability and gap filling ability.   
• Helium - Argon mixtures is a high arc voltage arc releasing higher levels of energy with   high heat input which ensures narrow beam weld arc giving   deep penetration with faster speeds. Arc striking with helium is a issue as well as stability of the arc compared with Argon.

• The type of shielding gas used in MIG Welding has a major influence on the weld quality. The gases used are,
• Reactive shielding gases:
• oxygen argon mixtures
• nitrogen argon mixtures or pure nitrogen
• carbon dioxide 
• hydrogen mixed with inert gases 
• Inert shielding gases:
• Argon
• Helium

• The shielding gases used for FCAW- Flux Cored Arc Welding process are,
• Carbon Dioxide
• Argon
• Argon/Carbon Dioxide

• A plasma welding torch uses separate  gases for the plasma arc and for shielding the weld pool
• The plasma gases used in this process are:

• Argon or Argon + 2% to 8% Hydrogen mixture as the shielding gas for Stainless Steel.
• Pure Argon for arc starting and shielding on other metals.
• Pure helium is normally used for aluminum alloys

SHIELDING GASES USED FOR DIFFERENT BASE METALS

• Argon
• Helium
• Argon – Helium

• Helium
• Argon

• Argon
• Argon – Hydrogen

• Argon
• Argon – Helium
• Argon – Hydrogen
• Helium

• Argon
• Argon – Helium
• Helium

• Argon
• Helium