Shielding Gases in MIG/MAG: Argon, CO₂ and Mixtures — How to Choose
Why Does Shielding Gas Matter?
Shielding gas in MIG/MAG serves two purposes:
- Protect the weld pool from atmospheric contamination (O₂, N₂, H₂O)
- Influence the arc, transfer mode and bead properties
Wrong gas selection can cause porosity, excessive spatter, insufficient penetration or overheating.
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The Base Gases
Argon (Ar)
- Inert gas — does not react chemically with the weld pool
- Soft arc, low central penetration, good stability
- Ideal for aluminium and stainless steels (MIG process)
- Pure: not suitable for carbon steel (unstable arc, "finger" penetration profile)
CO₂ (Carbon Dioxide)
- Active gas — reacts with the weld pool and forms CO
- High penetration, good side-wall fusion
- High spatter (short-circuit or globular transfer)
- Economical — cheapest of the MAG gases
- Suitable for carbon steel in rough work or robotics with subsequent cleaning
Oxygen (O₂)
- Used only in small percentages (1–5%) as an additive
- Improves arc stability and weld pool wetting
- Never use above 5% on steel or stainless
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Most Common Mixtures (EN ISO 14175)
| Mixture | Composition | Typical application |
|---|---|---|
| M21 | Ar + 15–25% CO₂ | Carbon and low-alloy steel — general use |
| M20 | Ar + 8% CO₂ | Carbon steel — fine bead, low spatter |
| M13 | Ar + 2% O₂ | Stainless steel — good wetting |
| M12 | Ar + 1% O₂ + 3% CO₂ | Stainless — stability/oxidation compromise |
| C1 | 100% CO₂ | Carbon steel — maximum penetration, high spatter |
| I1 | 100% Ar | Aluminium, copper, magnesium |
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Effect on Transfer Mode
| Gas | Short-circuit | Spray | Pulsed |
|---|---|---|---|
| 100% CO₂ | Yes (unstable) | No | No |
| Ar + 25% CO₂ | Yes (stable) | Limited | Yes |
| Ar + 8% CO₂ | Yes (very stable) | Yes | Yes |
| 100% Ar | Less stable | Yes | Yes |
Spray transfer requires a minimum of 80% Ar. Higher CO₂ eliminates spray and shifts the arc to globular or short-circuit.
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Flow Rate
| Welding position | Recommended flow |
|---|---|
| Flat / horizontal | 12–16 l/min |
| Vertical / overhead | 14–18 l/min |
| Outdoors / draught | 18–25 l/min |
Insufficient flow → porosity. Excessive flow → turbulence → porosity.
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Quick Selection Guide
- Carbon steel, general use: M21 (Ar + 18% CO₂)
- Austenitic stainless: M13 (Ar + 2% O₂)
- Aluminium: I1 (100% Ar)
- Maximum penetration, low cost: C1 (100% CO₂)
- Quality bead, low spatter: M20 (Ar + 8% CO₂)
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