# How to Choose Shielding Gas in MIG/MAG and TIG: A Guide to Ar, CO₂, and He Mixtures

Shielding gas is not a mere accessory: it determines arc stability, penetration, spatter, bead appearance, and even the mechanical properties of the joint. Choosing the wrong gas can ruin an otherwise flawless weld. In this draft we review the most common mixtures of argon (Ar), carbon dioxide (CO₂), and helium (He), and how they perform depending on the base material and welding position.

What each gas does (and why they are blended)

• Argon (Ar): inert gas. Provides a stable, smooth arc, good control, and low penetration. It is the base of most mixtures and the standard gas for TIG welding.
• CO₂: active gas. Increases penetration and reduces mixture cost, but generates more spatter and a more aggressive arc. Used only in MAG welding (not in TIG).
• Helium (He): inert gas. Delivers more energy to the arc (higher voltage and heat input), improves penetration on highly conductive materials, and increases travel speed. It is expensive and lighter than air, requiring higher flow rates.
• Oxygen (O₂): in small proportions it improves weld pool fluidity and wetting on steels; included here for reference, though the focus is on Ar, CO₂, and He.

The common mnemonic: MIG = inert gas (Ar/He), MAG = active gas (mixtures with CO₂ and/or O₂), TIG = always inert.

MIG/MAG: mixtures by base material

The table below summarizes reference combinations. All percentages and flow rates must be confirmed against the wire manufacturer’s data sheet and the applicable WPS.

TIG: virtually all argon

In TIG welding the shielding gas must be inert to avoid attacking the tungsten electrode. The most common options are:

• 100% Ar: default choice for carbon steels, stainless steels, and aluminum. Stable arc and good control.
• Ar + He: for heavy sections or highly conductive materials (aluminum, copper). Increases heat input and travel speed, at the cost of a less smooth arc and higher gas consumption.
• Ar + H₂ (austenitic stainless steels only): improves penetration and bead finish, but MUST NOT be used on carbon steels or aluminum due to the risk of hydrogen cracking [VERIFY proportion].

The backing gas in TIG welding of stainless steel is typically argon to prevent root oxidation.

Influence of welding position

Welding position affects weld pool behavior, and the shielding gas can either help or hinder control:

• Flat (PA) and horizontal: accommodate mixtures with higher CO₂ content and more fluid weld pools without sagging issues.
• Vertical (PF/PG) and overhead (PE): a more controllable weld pool is advisable. Higher-Ar mixtures produce less spatter and a more manageable arc; the transfer mode (short-circuit, pulsed) is usually as critical as the gas selection.
• Aluminum with He: because helium is lighter than air, overhead positions or drafty environments may require a higher flow rate to maintain adequate shielding coverage [VERIFY].

Summary of effects: penetration and bead appearance

• More CO₂ → deeper penetration, more spatter, poorer surface finish.
• More Ar → more stable arc, less spatter, better finish, lower penetration.
• Adding He → higher heat input, greater penetration and travel speed, increased gas consumption, and a “hotter” arc.

Final recommendations before making a selection

• Always consult the wire or filler rod data sheet and the job’s WPS.
• Check which standards apply to your industry (e.g., the shielding gas classification under the applicable European reference standard).
• Run test coupons before production whenever you change gas or material thickness.

Technical draft pending expert review. Verify all parameters, percentages, flow rates, and normative references before publishing.

Safety: Shielding gases displace oxygen: work in well-ventilated areas to prevent asphyxiation, especially in confined spaces. Ar+H₂ mixtures are flammable and must be used on austenitic stainless steels only, in accordance with a qualified procedure; never on carbon steels or aluminum. Helium, being lighter than air, can accumulate in overhead areas. Ensure cylinders are properly secured and that appropriate pressure regulators are used. [VERIFY safety requirements and flow rates per manufacturer guidelines and local regulations].