In MIG/MAG welding, the shielding gas does more than protect the molten metal from the atmosphere: it directly affects arc stability, penetration profile, spatter level, bead appearance and even the mechanical properties of the weld metal. The wrong gas choice comes back as porosity, lack of fusion and unnecessary grinding costs.
Gases are classified according to EN ISO 14175, and this classification is stated explicitly on the WPS. Below we summarise the most common options and the applications where each is preferred.
Active vs. Inert Gas: MIG or MAG?
- MAG (Metal Active Gas, process 135): Uses active gases containing CO₂ or O₂. Standard for carbon and low-alloy steels.
- MIG (Metal Inert Gas, process 131): Uses inert gases such as argon, helium or their mixtures. Preferred for aluminium, copper and other non-ferrous metals.
For the parameter side, see our article on MIG/MAG welding parameters.
The Most Common Shielding Gases
| ISO 14175 | Composition | Typical Use | Key Characteristic |
|---|---|---|---|
| C1 | 100% CO₂ | Carbon steel, heavy sections, flux-cored wire | Deep penetration, low cost; high spatter |
| M21 | Ar + 15–25% CO₂ (18% most common) | Structural steel S235–S355, general fabrication | Stable arc, low spatter, spray transfer possible |
| M20 | Ar + 5–15% CO₂ | Thin sheet, pulsed welding | Low spatter, smooth bead appearance |
| M12 | Ar + 0.5–5% CO₂ | Stainless steel (304L, 316L) | Low carbon pick-up, good wetting |
| I1 | 100% Argon | Aluminium MIG, TIG welding | Clean arc, oxide cleaning action |
| I3 | Ar + He | Thick aluminium, copper | Higher heat, broader penetration |
Gas Selection by Material
Carbon and Structural Steels
For series production, the most balanced option is an M21 (Ar/18% CO₂) mixture. 100% CO₂ is cheaper and gives deep penetration, but spatter and grinding time can raise the total cost. On visible surfaces and painted parts, a mixed gas usually works out more economical.
High-Strength Steels
For S690QL, S960QL and wear plate, M21 or M20 class mixtures are preferred together with low-hydrogen consumables. Here, heat input and preheating matter more than the gas; for details see our articles on S960QL welding parameters and welding Hardox.
Stainless Steel
Gases with a high CO₂ content carry carbon into the weld metal and can reduce corrosion resistance. For this reason, low-CO₂ M12 class mixtures or Ar/O₂ mixtures are used. To prevent oxidation on the root side, root shielding (backing gas) is applied using forming gas or argon.
Aluminium
Pure argon (I1) is the standard. For thicknesses above 10 mm, helium-containing mixtures (I3) increase heat transfer and reduce the need for preheating. See aluminium welding.
Gas Flow Rate: More Isn't Better
As a general rule, the flow rate in MIG/MAG welding is set in the range of 12–18 l/min depending on wire diameter and nozzle diameter. Too low a flow rate causes porosity; too high a flow rate creates turbulence that draws air into the arc zone and again causes porosity. Outdoors or in draughty areas, wind screens should be used.
Quality note: The gas class is one of the essential variables of a WPS. A procedure qualified with M21 in the WPQR cannot be used with a gas outside its range of qualification. A gas change may require a WPS revision.
Common Gas-Related Defects
- Porosity: Insufficient flow, a leaking hose, a dirty nozzle or draughts.
- Excessive spatter: Parameter mismatch with a high CO₂ content and short-arc setting.
- Discolouration on stainless: Inadequate root shielding or a high CO₂/O₂ content.
For defect types and repair approaches, read our article on welding defects.
Frequently Asked Questions
Which gas is most commonly used for MAG welding?
For structural steels, the most common gas is an argon mixture with 15–25% CO₂, classified as M21 under EN ISO 14175; in practice, 82% Ar / 18% CO₂ is typically used.
Is CO₂ used for MIG welding stainless steel?
Only in small amounts. For stainless steel, M12 class mixtures containing 0.5–5% CO₂ or Ar/O₂ mixtures are generally used; high CO₂ can reduce corrosion resistance.
What should the shielding gas flow rate be?
In MIG/MAG welding, the typical flow rate is 12–18 l/min depending on wire and nozzle diameter. The value should be specified in the WPS and checked in production.
Let's Choose the Right Process for Your Material
Share the material, thickness and quantities, and we'll prepare a quote with a suitable process and WPS proposal.
Request a Quote →