1. GMAW SHIELDING GASSES

2. 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.

3. 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.

4. 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

5. 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

6. Helium
An inert gas with a specific gravity of 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

7. 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)

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