Choosing the Right Gas for Welding: A Comprehensive Guide

In welding processes such as CO2 gas shielded welding, inert gas shielded welding, mixed gas shielded welding, plasma arc welding, brazing in a protective atmosphere, and oxy-acetylene gas welding and cutting, the use of corresponding gases is essential. The selection of welding gases primarily depends on the welding and cutting methods.

Additionally, it is related to factors such as the properties of the base metal, the quality requirements of the weld joint, the thickness of the workpiece, the welding position, and the process method.

Selection of Gases According to Welding Methods

The choice of gases for welding, cutting, or gas shielded welding varies according to the welding method employed during the welding process, as shown below:

Gas welding

  • C2H2+O2
  • H2

Gas cutting

  • C2H2+O2
  • Liquefied petroleum gas+O2
  • Coal gas+O2
  • Natural gas+O2

Plasma arc cutting

  • Air
  • N2
  • Ar+H2
  • Ar+N2
  • N2+H2

TIG welding (manual or automatic)

  • Ar
  • H3
  • Ar+He

GMAW (semi-automatic or automatic)

Solid welding wire

  • MIG welding:Ar, He, Ar+He
  • MAG welding:Ar+O2, Ar+CO2, Ar+CO2+O2
  • CO2 welding: CO2, CO2+O2

Flux cored welding wire

  • CO2
  • Ar+O2
  • Ar+CO2

Gas shielded brazing

  • Ar
  • H2
  • N2
  • Decomposition of nitrogen: 75% Hydrogen and 25% Nitrogen; 7% to 20% Hydrogen (by volume), the remainder is Nitrogen.

Selection of Gases According to the Base Metal

In gas shielded welding, apart from self-shielded wires, both solid wires and flux-cored wires pose the issue of appropriate gas (medium) combination for protection. The impact of this combination is relatively clear and not as complex as the wire-flux combination, as protective gases are only of two types: inert gases and active gases.

For easily oxidizable metals such as aluminum, titanium, copper, zirconium, and their alloys, inert gases should be used for protection, with higher purity requirements for inert gases used on more easily oxidizable metals. When welding carbon steel, low alloy steel, stainless steel, and other metals using consumable electrode gas shielded welding, it is not advisable to use pure inert gases.

It is recommended to use oxidizing protective gases such as CO2, Ar+O2, or Ar+CO2. This can improve welding process performance, reduce spatter, stabilize droplet transition, and achieve good weld formation. Table 6-21 provides the protective gases suitable for gas shielded welding of different metal materials, while Table 6-22 lists the protective gases suitable for consumable electrode inert gas shielded welding of different base materials.

Table 6-21 Selection of Base Materials and Shielding Gases

Base MaterialsShielding GasesMixture Ratio and Chemical Composition (Volume Fraction)Chemical PropertiesWelding Methodsbrief explanation
Aluminum and Aluminum AlloysAr InertTIG, MIGTIG welding uses alternating current, while MIG welding uses direct current reverse polarity. The unique advantage of argon is its stable arc burning, and during melting pole welding, the wire metal easily transitions into a stable axial jet flow, with minimal spattering. For easily oxidizable metals such as Al, Ti, Cu, Zr and their alloys, as well as nickel-based alloys, inert gases should be used for protection.
Ar+HeUsually adding 10% He for MIG welding, 10% to 90% He for TIG welding, various proportions up to 75% He + 25% ArInertTIG, MIGHelium has a high heat transfer coefficient. Under the same arc length, the arc voltage is higher when using helium compared to argon, resulting in a higher arc temperature and greater heat input to the base material, leading to a higher melting rate. A mixture of Ar and He can combine the advantages of both gases. When welding thick aluminum plates and their alloys, it can increase penetration depth, reduce porosity, and improve production efficiency. The amount of helium added depends on the thickness of the plate; for thicker plates, more helium is added, typically around 10% helium. However, if the proportion of helium added is too high, spattering increases. For welding thick aluminum plates, such as 20mm, helium is sometimes added in proportions exceeding 50%.
Titanium, Zirconium, and Their AlloysAr InertTIG, MIGThe arc burns steadily, providing excellent protection.
Ar+He Ar75%, He 25%InertTIG, MIGIt can increase the heat input, suitable for jet arc, pulse arc, and short-circuit arc (with a 75:25 mixture ratio), and can improve the penetration and wettability of the weld metal.
Titanium, Zirconium, and Their AlloysAr InertTIG, MIGA stable jet arc is generated during the melting electrode welding, but preheating is required when the plate thickness exceeds 5mm.
Ar+HeAr 50%, He 50%; Ar 30%, He 70%InertTIG, MIGThe greatest advantage of using Ar+He mixed gas is the improvement of the wettability of the weld metal, thus enhancing the welding quality. Because He transfers more heat to the base metal than Ar, it can therefore reduce the preheating temperature.
Copper and Copper AlloysN2melting electrode gas shielded weldingIncreasing the heat input can reduce or eliminate preheating, but it can lead to increased spattering and smoke. Nitrogen arc welding is generally only used for deoxidized copper welding. Nitrogen is readily available and inexpensive.
Ar+N2Ar 80%, N2 20%melting electrode gas shielded weldingThe arc temperature of pure Ar is higher than that of Ar+He. Compared to Ar+He, Ar+N2 is cheaper and readily available, but it produces more spatter and smoke, and the bead formation is poorer.
Carbon Steel and Low Alloy SteelAr inertTIGsuitable for welding thin plates
Ar+N2add 1% to 4% N2inertTIGWhen welding austenitic stainless steel, it can increase arc stiffness and improve weld bead formation.
Ar+O2add 1% to 2% O2oxidizingmelting electrode gas shielded welding (MAG)When using pure Ar to shield melting electrode welding for stainless steel, low carbon steel, and low-alloy steel, the main drawback is the instability of the arc cathode spots, which can lead to irregular weld penetration and formation. The high viscosity and surface tension of the liquid metal make it prone to defects such as porosity and undercut. Therefore, using pure Ar for shielding in melting electrode welding is not suitable. However, adding a small amount of O2 can improve and overcome these issues by refining the droplet and reducing the critical current for jet transition. When welding stainless steel, the oxygen content should not exceed 2% by volume, as excessive oxygen can lead to severe oxidation of the weld surface, thereby reducing joint quality. This method is suitable for jet arc and pulse arc welding.
Ar+O2+CO2add 2% O2, add 5% CO2oxidizingMAGFor jet arc, pulse arc, and short-circuit arc
Ar+CO2add 2.5% CO2oxidizingMAGFor short-circuit arc. When welding stainless steel, the addition of CO2 should be less than 5%, otherwise, it will result in severe carbon infiltration.
Nickel-based AlloysAr+O2add 20% O2oxidizingMAGAdding 20% O2 is primarily used for welding low carbon steel and low alloy steel. Ar+20% O2 offers higher productivity and better resistance to porosity compared to Ar+CO2 and pure CO2 shielding gases. It also enhances the weld joint toughness. When used for narrow gap vertical welding of high-strength steel with jet arc and in cases where high weld joint requirements are needed, Ar+20% O2 can reduce the tendency for interlayer cracking. Due to its strong oxidizing nature, Ar+20% O2 should be used with welding wire containing higher levels of Mn and Si.
carbon steel and low-alloy steelAr+CO2Ar70%~80%, CO220%~30%oxidizingMAGAr+CO2 is widely used for welding carbon steel and low-alloy steel. It combines the advantages of Ar, such as stable arc, minimal spattering, and ease of obtaining axial jet transition, with the oxidizing properties that can overcome issues like cathode drift and poor weld bead formation when using pure Ar for welding. It exhibits good process performance, minimal spattering, and good impact toughness of the weld metal. Although the cost is higher than CO2 welding, it is still widely adopted. The typical Ar:CO2 ratio is (70~80):(30~20), which can be used for spray, short-circuit, and pulse transition arcs. However, for vertical and overhead welding using short-circuit transition arc, the optimal Ar:CO2 ratio is 50:50, which is beneficial for controlling the weld pool. As CO2 increases, the impact toughness of the joint decreases.
Ar+O2+CO2Ar 80%,O225%, CO215%oxidizingMetal Active Gas (MAG)The 80% Ar, 15% CO2, and 5% O2 mixture is the optimal blend for welding low carbon steel and low alloy steel, providing satisfactory weld bead formation, joint quality, and good process performance, with improved penetration depth. It can be used for both spray transfer and short-circuiting arcs.
CO2oxidizingMetal Inert Gas (CO2)Suitable for short-circuiting arcs. It exhibits some spattering, and the impact toughness of the weld metal is lower compared to Ar+CO2 welding.
CO2+O2CO275%~80%, O220%~25%oxidizingMetal Active Gas (MAG)By adding a certain amount of O2 to CO2, the oxidation reaction in the arc zone intensifies, releasing heat to accelerate the melting of the wire, raising the temperature of the weld pool, increasing the penetration depth, and allowing for a higher deposition rate. This method is highly efficient. The addition of O2 reduces the concentration of free hydrogen in the arc column and in the molten metal, resulting in a lower hydrogen content in the weld, thus enhancing its resistance to hydrogen porosity. However, it is essential to control the O2 content within a certain range and to use wire with strong deoxidation capabilities (with higher levels of Si, Mn, Al, Ti, etc.) to control the oxygen content in the weld metal.
nickel-based alloysAr+He He20%~25%inertTIG MIGThe heat input is greater compared to pure Ar.
Ar+H2H2<6%reducingNon-consumable electrodeIt can suppress and eliminate CO gas pores in the weld, increase the arc temperature, and enhance heat input.

Table 6-22: Protective Gases Suitable for Inert Gas Shielded Arc Welding of Different Base Materials

Shielding gasBase metals
Ar All metals except for steel
Ar+He All metals, particularly suitable for welding copper and aluminum alloys
He All metals except for steel
Ar+O21%High-alloy steel
Ar+O21%~3%Alloy steel
Ar+O21%~5%Non-alloy steel and low-alloy steel
Ar+CO225%Non-alloy steel
Ar+CO21%~3%Aluminum alloy
Ar+N20.2%Aluminum alloy
Ar+H26%Nickel and nickel alloys
CO2Non-alloy steel
CO2+O215%~20%Non-alloy steel
Water vaporNon-alloy steel
Ar+O23%~7%+CO213%~17%Non-alloy steel and low-alloy steel

In recent years, there has been a growing promotion and application of a coarse Ar mixture, consisting of Ar=96%, O2≤4%, H2O≤0.0057%, and N2≤0.1%.

This coarse Ar mixed gas not only improves weld seam formation and reduces spatter, enhancing welding efficiency, but also, when used for welding low-alloy high-strength steel with a tensile strength of 500~800MPa, the mechanical properties of the weld metal are comparable to those achieved when using high-purity Ar. The coarse Ar mixed gas is cost-effective and offers good economic benefits.

When welding ultra-thin materials using manual TIG welding, it is advisable to use Ar for gas protection. For welding thick parts, high thermal conductivity or refractory metals, or when conducting high-speed automatic welding, it is recommended to use He or Ar+He for gas protection. When performing manual TIG welding on aluminum using alternating current, Ar should be chosen for gas protection.

This is because compared to He, Ar exhibits better arc ignition performance and cathode cleansing effect, resulting in excellent weld seam quality. The selection of shielding gas for gas metal arc welding depends not only on the base metal but also on the form of droplet transfer and the current conditions.

Tables 6-23 to 6-26 respectively list the selection of shielding gases for jet transition, short-circuit transition, high current plasma arc welding, and low current plasma arc welding.

Table 6-23 Selection of gases for gas tungsten arc welding with jet transition melting

MetalsShielding Gases (Volume Percent)Advantages
AluminumArgonThickness 0.25mm: Best for metal transfer and arc stability, minimal spattering
75% Helium + 25% ArgonThickness 25~76mm: Greater heat input compared to using argon
90% Argon + 10% NitrogenThickness greater than 76mm: Maximum heat input, minimal porosity
MagnesiumArgonExcellent cathodic cleaning action
Carbon steelArgon + 3%~5% OxygenGood arc stability, produces a well-controllable welding pool with good fluidity and weld joint shape, minimal undercut, allowing for higher speeds compared to argon
Carbon DioxideHigh-speed mechanized welding, low cost
Low-alloy steelArgon + 2% OxygenMinimal undercut, provides good toughness
Stainless steelArgon + 1% OxygenGood arc stability, produces a well-controllable welding pool with good fluidity and weld joint shape, minimal undercut when welding thicker stainless steel
Argon + 2% OxygenGood arc stability when welding thinner stainless steel, better joint shape compared to using argon + 1% oxygen mixture, and higher welding speeds
Copper, nickel, and their alloysArgonGood wetting for materials with a thickness of 3.2mm, excellent control of the welding pool
Helium + ArgonThe higher heat input of 50% helium + argon and 75% argon mixtures can offset the high thermal conductivity of thicker plates
Reactive metals (Ti, Zr, Ta)HydrogenGood arc stability, minimal welding contamination. To prevent air contamination on the backside of the welding area, inert gas protection is required.

Table 6-24 Selection of Gases for Short-Circuiting Transfer Metal Inert Gas (MIG) Welding

Metals:Protective Gases (Volume Fraction)Advantages:
Carbon SteelArgon + 20%~25% CO2For thicknesses less than 3.2mm, high welding speed, minimal burn-through, minimal deformation, and minimal spatter, with good penetration.
Argon + 50% CO2For thicknesses greater than 3.2mm, minimal spatter, clean weld appearance, and good weld pool control in vertical and overhead positions.
CO2Greater penetration, faster welding speed, and lowest cost.
Stainless Steel90% Helium + 7.5% Argon + 2.5% CO2No impact on corrosion resistance, small heat-affected zone, minimal undercut, minimal deformation, and good arc stability.
Low Alloy Steel60%~70% Krypton + 25%~35% Argon + 4%~5% CO2  Argon + 20%~25% CO2Minimal reactivity, good toughness, best arc stability, wettability, and weld bead shape, with minimal spatter. Moderate toughness, excellent arc stability, wettability, and weld bead shape, with minimal spatter.
Aluminum, Copper, Magnesium, Nickel, and their AlloysArgon and Argon + KryptonFor welding thin metal sheets, argon gas is satisfactory; for thicker metals, argon mixed with other gases is preferable.

Table 6-25 Selection of Shielding Gases for High-Current Plasma Arc Welding

Base MaterialsPlate Thickness/mmProtective Gases
Gas ShieldingPenetration Welding
Carbon Steel<3.2Ar Ar 
>3.2Ar He 75% + Ar 25%
Low Alloy Steel<3.2Ar Ar 
>3.2Ar He 75% + Ar 25%
Stainless Steel<3.2Ar or Ar 92.5% + He 7.5%Ar 
>3.2Ar or Ar 95% + He 5%He 75% + Ar 25%
Copper<2.4Ar He or He 75% + Ar 25%
Nickel Alloys<3.2Ar or Ar 92.5% + He 7.5%Ar 
>3.2Ar or Ar 95% + He 5%He 75% + Ar 25%
Reactive Metals<6.4Ar Ar 
>6.4Ar + He (50-75)%He 75% + Ar 25%

Table 6-26 Selection of Protective Gases for Low Current Plasma Arc Welding

Materials to be WeldedPlate Thickness (mm)Protective Gas
Aperture MethodFusion Penetration Method
Aluminum<1.6Ar, He 
>1.6He He 
Carbon Steel<1.6Ar,He25%+Ar75%
>1.6Ar, He 75% + Ar 25%Ar,He75%+Ar25%
Low Alloy Steel<1.6Ar,He,Ar+H2(1~5)%
>1.6He 75% + Ar 25%, Ar + H2 (1-5)%Ar, He,Ar+H2(1~5)%
Stainless SteelAll ThicknessesAr, He 75% + Ar 25%, Ar + H2 (1-5)%Ar,He,Ar+H2(1~5)%
>1.6He 75% + Ar 25%, HeHe,He75%+Ar25%
Nickel AlloysAll ThicknessesAr, He 75% + Ar 25%, Ar + H2 (1-5)%Ar, He,Ar+H2(1~5)%
Reactive Metals<1.6Ar, He 75% + Ar 25%, He + Ar Ar 
>1.6He 75% + Ar 25%, HeAr, He75%+Ar25%

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