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Welding Gas Mixtures

Argon-Oxygen Shielding Gas Mixtures

 

The addition of small amounts of 02 to argon greatly stabilizes the weld arc, increases the filler metal droplet rate, lowers the spray arc transition current, and improves wetting and bead shape. The weld puddle is more fluid and stays molten longer allowing the metal to flow out towards the toe of the weld. This reduces undercutting and helps flatten the weld bead. Occasionally, small oxygen additions are used on non- ferrous applications. For example, it’s been reported by NASA that .1% oxygen has been useful for arc stabilization when welding very clean aluminum plate.

 

Argon-1% Oxygen

This mixture is primarily used for spray transfer on stainless steels. One percent oxygen is usually sufficient to stabilize the arc, improve the droplet rate, provide coalescence and improve appearance

 

Argon-2% Oxygen

This mixture is used for spray arc welding on carbon steels, low alloy steels and stainless steels. It provides additional wetting action over the 1% 02 mixture. Mechanical properties and corrosion resistance of welds made in the 1 and 2% 02 additions are equivalent.

 

Argon-5% Oxygen

This mixture provides a more fluid but controllable weld pool. It is the most commonly used argon-oxygen mixture for general carbon steel welding. The additional oxygen also permits higher travel speeds.

 

Argon-8-12% Oxygen

Originally popularized in Germany, this mixture has recently surfaced in the U.S. in both the 8% and 12% types. The main application is single pass welds but some multi-pass applications have been reported. The higher oxidizing potential of these gases must be taken into consideration with respect to the wire alloy chemistry. In some instances a higher alloyed wire will be necessary to compensate for there active nature of the shielding gas. The higher puddle fluidity and lower spray arc transition current of these mixtures could have some advantage on some weld applications.

 

Argon-12-25% Oxygen

Mixtures with very high 02 levels have been used on a limited basis but the benefits of 25% 02 versus 12% 02 are debatable. Extreme puddle fluidity is characteristic of this gas. A heavy slag/scale layer over the bead surface can be expected which is difficult to remove. Sound welds can be made at the 25% 02 level with little or no porosity. Removal of the slag/scale before subsequent weld passes is recommended to insure the best weld integrity.

 

 

Argon-Carbon Dioxide Shielding Gas Mixtures

 

The argon-carbon dioxide mixtures are mainly used on carbon and low alloy steels and limited application on stainless steels. The argon additions to CO2 decrease the spatter levels usually experienced with pureCO2 mixtures. Small CO2 additions to argon produce the same spray arc characteristics as small 02additions. The difference lies mostly in the higher spray arc transition currents of argon - CO2 mixtures. In GMAW welding with CO2 additions, a slightly higher current level must be reached in order to establish and maintain stable spray transfer of metal across the arc. Oxygen additions reduce the spray transfer transition current. Above approximately 20% CO2 spray transfer becomes unstable and random short circuiting and globular transfer occurs.

 

 Argon-3-10% CO2

These mixtures are used for spray arc and short circuiting transfer on a variety of carbon steel thicknesses. Because the mixtures can successfully utilize both arc modes, this gas has gained much popularity as a versatile mixture. A 5% mixture is very commonly used for pulsed GMAW of heavy section low alloy steels being welding out-of-position. The welds are generally less oxidizing than those with98 Ar-2% 02. Improved penetration is achieved with less porosity when using CO2 additions as opposed to 02additions. In the case of bead wetting, it requires about twice as much CO2 to achieve the same wetting action as identical amounts of 02. From 5 to 10% CO2 the arc column becomes very stiff and defined. The strong arc forces that develop give these mixtures more tolerance to mill scale and a very controllable puddle.

 

 

 Argon-11-20% CO2

This mixture range has been used for various narrow gap, out-of-position sheet metal and high speed GMAW applications. Most applications are on carbon and low alloy steels. By mixing theCO2 within this range, maximum productivity on thin gauge materials can be achieved. This is done by minimizing burn through potential while at the same time maximizing deposition rates and travel speeds. The lower CO2 percentages also improve deposition efficiency by lowering spatter loss.

 

 Argon-21-25% CO2

This range is universally known as the gas used for GMAW with short circuiting transfer on mild steel. It was originally formulated to maximize the short circuit frequency on .030 and .035-in. diameter solid wires but through the years has become the defacto- standard for most diameter solid wire welding and commonly used with flux cored wires.  This mixture also operates well in high current applications on heavy materials and can achieve good arc stability, puddle control and bead appearance as well as high productivity.

 

 Argon-50% CO2

This mixture is used where high heat input and deep penetration are needed.  Recommended material thicknesses are above 1/8 in. and welds can be made out-of-position. This mixture is very popular for pipe welding using the short circuiting transfer. Good wetting and bead shape without excessive puddle fluidity are the main advantages for the pipe welding application. Welding on thin gauge materials has more tendency to burn through which can limit the overall versatility of this gas. When welding at high current levels, the metal transfer is more like welding in pure CO2 than previous mixtures but some reduction in spatter loss can be realized due to the argon addition

 

 Argon-75% CO2

A 75% CO2 mixture is sometimes used on heavy wall pipe and is the optimum in good side-wall fusion and deep penetration. The argon constituent aids in arc stabilization and reduced spatter.

 

 

Argon-Helium Shielding Gas Mixtures

 

Regardless of the percentage, argon-helium mixtures are used for non-ferrous materials such as aluminum,copper, nickel alloys and reactive metals. These gases used in various combinations increase the voltage and heat of GTAW and GMAW arcs while maintaining the favorable characteristics of argon. Generally, the heavier the material the higher the percentage of helium. Small percentages of helium, as low as 10%, will affect the arc and the mechanical properties of the weld. As helium percentages increase, the arc voltage,spatter and penetration will increase while minimizing porosity. A pure helium gas will broaden the penetration and bead but depth of penetration could suffer. However arc stability also increases. The argon percentage must be at least 20% when mixed with helium to produce and maintain a stable spray arc.

 

 Argon-25% Helium

This little used mixture is sometimes recommended for welding aluminum where an increase in penetration is sought and bead appearance is of primary importance.

 

 Argon-75% Helium

This commonly used mixture is widely employed for mechanized welding of aluminum greater than one inch thick in the flat position. HE-75 also increases the heat input and reduces porosity of welds in 1/4 and 1/2 in. thick conductivity copper.

 

 Argon-90% Helium

This mixture is used for welding copper over 1/2 in. thick and aluminum over 3-in.thick. It has an increased heat input which improves weld coalescence and provides good X-ray quality. It is also used for short circuiting transfer with high nickel filler metals.

 

 

Argon-Nitrogen

 

Small amounts of nitrogen have been added to Ar-1% 02 to achieve a completely austenitic microstructure in welds made with type 347 stainless steel filler metal. Nitrogen concentrations in the range of 1.5 to 3% have been used. Quantities above 10% produced considerable fuming but welds are sound. Additions greater than 2% N2 produced porosity in single pass GMAW welds made in mild steel; additions less than 1/2%caused porosity in multipass GMAW welds in carbon steel. A few attempts have been made to utilize N2 rich argon mixtures for GMAW welding of copper and its alloys, but spatter percentage is high.

 

 

Argon-Oxygen-Carbon Dioxide

 

Mixtures containing these three components have been termed ”universal” mixtures due to their ability to operate using short circuiting, globular, spray, pulse and high density type transfer characteristics. Several triple-mixes are available and their application will depend on the desired metal transfer mechanism and optimization of the arc characteristics.

 

 Argon-5-10%, Carbon Dioxide 1-3%, Oxygen

This ternary mixture range has gained popularity in the U.S. over the last several years. The chief advantage is its versatility to weld carbon steel, low alloy steel and stainless steel of all thicknesses utilizing whatever metal transfer type applicable. Stainless steel welding should be limited to spray arc only due to the stiffness of the puddle at low current levels. Carbon pick-up on stainless steel should also be considered in some instances. On carbon and low alloy steels, this mixtures produces good welding characteristics and mechanical properties. On thin gauge materials, the 02 constituent assists the arc stability at very low current levels (30 to 60 amps) permitting the arc to be kept short and controllable.  This helps minimize burn through and distortion by lowering the total heat input into the weld zone.

 

 Argon-10-20%%, Carbon Dioxide 5%, Oxygen

This mixture is not common in the U.S. but has found applications in Europe.  The mix produces a hot short circuiting transfer and fluid puddle characteristics. Spray arc transfer is good and seems to have some benefit when welding with triple deoxidized wires since a sluggish puddle is characteristic of these wires.

 

 

Argon-Carbon Dioxide-Hydrogen

 

Small additions of hydrogen (1-2%) have been shown to improve bead wetting and arc stability when Pulse Mig welding stainless steel. The CO2 is also kept low (1-3%) to minimize carbon pick-up and maintain good arc stability. This mixture is not recommended on low alloy steels in the excessive weld metal hydrogen levels could develop causing weld cracking and poor mechanical properties.

 

 

Argon-Helium-Carbon Dioxide

 

Helium and CO2 addition to argon increase the heat input to the weld and improve arc stability. Better wetting and bead profile is achieved. When welding on carbon and low alloy steels, helium additions are used to increase the heat input and improve the puddle fluidity in much the same way that oxygen is used except that helium is inert and oxidation of the weld metal and alloy loss are not a problem. When welding low alloy material, mechanical properties can be achieved and maintained more easily.

 

 Argon-10-30%, Helium-5-15%, CO2

Mixtures in this range have been developed and marketed for pulse spray arc welding of both carbon and low alloy steel. Best performance is on heavy section, out-of-position applications where welding is desired at maximum deposition rates. Good mechanical properties and puddle control are characteristic of this mixture. Pulse spray arc welding with low average currents is acceptable but mixtures with low CO, and/or 0, percentages will improve arc stability.

 

 Helium-60-70%, Argon-20-35%, CO2

This mixture is used for short circuiting transfer welding of high strength steels, especially for out-of- position applications. The CO2 content is kept low to insure good weld metal toughness. The helium provides the heat necessary for puddle fluidity. High helium contents are not necessary, as the weld puddle may become too fluid for easy control.

 

 Helium-90%, Argon-7.5%, CO2-2.5%

This mixture is widely used for short arc welding of stainless steel in all positions. The CO2 content is kept low to minimize carbon pickup and assure good corrosion resistance,especially in multipass welds. The CO2 + argon addition provides good arc stability and penetration. The high helium content provides heat input to overcome the sluggish nature of the stainless steel weld puddle.

 

 

Argon-Helium-Oxygen

 

Just as a helium addition to argon increases the arc energy when welding non-ferrous materials, so does a helium addition to argon-oxygen affect the arc with the GMAW process on ferrous materials. Ar-He-O2mixtures have been used occasionally for spray arc welding and surfacing low alloy and stainless steels to improve puddle fluidity and bead shape and reduce porosity.

 

 

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