In the welding process, using mixed gas instead of a single gas as the shielding gas can effectively refine the droplet, reduce the splash, improve the forming, control the penetration, prevent defects, and reduce the porosity productivity, thus significantly improving the welding quality.
Commonly used welding shielding gas mixtures include binary gas mixture, ternary gas mixture and quaternary gas mixture.
Binary gas mixtures include Ar He, Ar-N2, Ar-H2, Ar-O2, Ar-CO2, CO2-O2, N2-H2, etc;
Ternary gas mixtures include Ar-He-CO2, Ar-He-N2, Ar-HeO2, Ar-O2-CO2, etc;
The quaternion gas mixture is used less and is mainly composed of Ar, He, N2, O2, H2, CO2, etc.
The proportion of each component in various mixed gases can vary in a large range, which is mainly determined by welding process, welding material, welding wire model and many other factors.
In general, the higher the requirements for weld quality, the higher the requirements for the purity of the single gas used to prepare the mixture.
1. Binary gas mixture
Adding a small amount of oxygen to argon for MIG welding can improve the stability of the arc, improve the refinement rate of molten droplets, reduce the spray transfer current, improve the wettability and weld bead formation.
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For example, Ar+(1% – 2%) O2 is commonly used for spray arc welding of carbon steel, low alloy steel and stainless steel.
Properly increase the oxidizability of the arc atmosphere to raise the temperature of the molten metal in the molten pool and improve the fluidity.
The molten metal can flow fully to the weld toe, reduce the undercut tendency, and make the weld bead flat.
For example, Ar+(5% – 10%) O2 can be used for the welding of carbon steel to improve the welding speed.
Sometimes a small amount of oxygen is added for welding nonferrous metals.
For example, when welding very clean aluminum plates, adding 1% oxygen in volume fraction can make the arc stable.
(2) Argon carbon dioxide
This kind of mixed gas is mainly used for carbon steel and low alloy welding, and has limited application for stainless steel welding.
Ar-CO2 has less spatter than pure CO2, and reduces the burning loss of alloy elements, which is helpful to improve the strength and impact toughness of the weld.
A small amount of CO2 is added to Ar as a small amount of O2 is added to produce a jet arc.
The biggest difference is that the critical current of Ar-CO2 mixture is higher than that of Ar-O2 mixture.
Ar-CO2 is the most widely used binary welding gas mixture in China.
In order to meet the market demand and standardize the quality requirements, the chemical industry standard HG/T3728-2004 Welding Gas Mixture Argon Carbon Dioxide has been formulated, which specifies the purity of the feed gas used to prepare Ar-CO2 gas mixture, the technical requirements, test methods, inspection rules, etc. for the mixed gas products.
The ratio of Ar-CO2 mixed gas can be almost any ratio.
For example, it is common to add 5% CO2 to the mixture gas for all position pulsed MAG welding of low alloy steel thick plates.
Generally, compared with adding 2% O2, the weld oxidation is less, the penetration is improved, and the porosity is less;
Ar+(10% – 20%) CO2 is used for narrow gap welding of carbon steel and low alloy steel, all position welding of thin plates and high-speed MAG welding;
Ar+(21% – 25%) CO2 is commonly used for short circuit transfer welding of low carbon steel;
Ar+50% CO2 is used for high heat input deep penetration welding;
Ar+70% CO2 is used for welding thick wall pipes.
(3) Argon helium
The Ar He mixture is used for welding non-ferrous metals, such as aluminum, copper, nickel alloys and active metals, regardless of its proportion.
These gases use different combinations to improve the arc voltage and heat of TIG welding and MIG welding, while maintaining the favorable characteristics of argon, which is particularly suitable for occasions where the weld quality is highly required.
The helium content shall be at least 20% to produce and maintain the effect of stable spraying arc.
(4) Argon nitrogen
When welding duplex stainless steel, 2% – 3% N2 can be added into the mixed gas to improve the resistance to pitting corrosion and stress corrosion of the joint.
(5) Argon helium
H2 is a diatomic molecule with high thermal conductivity.
When Ar-H2 mixture is used, the temperature of the arc can be increased, the penetration ability can be increased, the welding speed can be improved, and undercut can be prevented.
In addition, hydrogen has a reducing effect, which can prevent the formation of CO pores.
Ar-H2 mixed gas is mainly used for welding nickel base alloys, nickel copper alloys, stainless steel, etc.
The hydrogen content should be controlled below 6%.
2. Ternary gas mixture
(1) Argon carbon dioxide oxygen
The mixed gas containing these three components generally has CO2 below 20% and O2 below 5%.
The main advantage is that it can weld carbon steel, low alloy steel and stainless steel of various thicknesses. No matter which transition form is suitable.
(2) Argon carbon dioxide hydrogen
A small amount of H2 (volume fraction of 1% – 2%) is added to the stainless steel during pulse MIG welding, which improves the weld wetting and makes the arc stable.
Therefore, CO2 should also be less (volume fraction is 1% – 3%) to reduce carburizing amount and maintain good arc stability.
This gas is not recommended for low alloy steel, because it causes high hydrogen content in weld metal, poor mechanical properties and cracks.
(3) Argon helium carbon dioxide
The addition of He and CO2 in Ar can increase the heat input of the weld and improve the stability of the arc, and the wetting and forming of the weld bead are better.
When welding carbon steel and low alloy steel, He is added to increase heat input and improve the fluidity of the molten pool.
He is also inert, which has no effect on the burning loss of the oxidation alloy of the weld metal.
For example, Ar+(10% – 30%) He+(5% – 15%) CO2 is used for pulsed jet arc welding of carbon steel and low alloy steel;
(60% – 70%) He+(20% – 35%) Ar+5% CO2 is used for high-strength steel, especially all position short-circuit transfer welding;
90% He+7.5% Ar+2.5% CO2 is widely used for all-position short circuit arc welding of stainless steel.