WHAT IS THE BEST SHIELDING GAS FOR MIG WELDING?

WHAT IS THE BEST SHIELDING GAS FOR MIG WELDING?

MIG welding with shielding gas and a solid wire electrode produces a clean, slag-free weld without the need to continually stop welding to replace the electrode, as in stick welding.

Increased productivity and reduced clean-up are just two of the benefits possible with this process. The primary purpose of shielding gas is to prevent exposure of the molten weld pool to oxygen, nitrogen, and hydrogen contained in the atmosphere. The reaction of these elements with the weld pool can create a variety of problems, including porosity (holes within the weld bead) and excessive spatter. Different shielding gases also play an important role in determining weld penetration profiles, arc stability, mechanical properties of the finished weld, the transfer process you use and more.

Choosing the Right Gas.

Many MIG welding applications lend themselves to a variety of shielding gas choices, and you need to evaluate your welding goals in order to choose the correct one for your specific application. The cost of the gas, finished weld properties, preparation, post-weld clean-up, the base material, weld transfer process and your productivity goals all need to be taken into account when selecting a shielding gas.

Argon, Helium, Carbon Dioxide and Oxygen are the four most common shielding gases used in MIG welding, with each providing unique benefits and drawbacks in any given application.

Shielding gases for MIG/GMAW welding

Metal Inert Gas welding is sometimes referred to as Gas Metal Arc welding .The basic gas for MIG/MAG welding is argon (Ar). Helium (He) can be added to increase penetration and fluidity of the weld pool. Argon or argon/helium mixtures can be used for welding all grades. However, small additions of oxygen (O2) or carbon dioxide (CO2) are usually needed to stabilize the arc, improve the fluidity and improve the quality of the weld deposit. For stainless steels there are also gases available containing small amounts of hydrogen (H2).

Shielding gases for TIG/GTAW welding

The normal gas for TIG welding is argon (Ar). Helium (He) can be added to increase penetration and fluidity of the weld pool. Argon or argon/helium mixtures can be used for welding all grades. In some cases, nitrogen (N2) and/or hydrogen (H2) can be added to achieve special properties. For instance, the addition of hydrogen gives a similar, but much stronger, effect as adding helium. However, hydrogen additions should not be used for welding martensitic, ferritic or duplex grades.

Alternatively, if nitrogen is added, the weld deposit properties of nitrogen alloyed grades can be improved. Oxidizing additions are not used because these destroy the tungsten electrode.

Recommendations for shielding gases used in TIG welding of different stainless steels are given in the table. For plasma-arc welding, the gas types with hydrogen additions in the table are mostly used as plasma gas, and pure argon as shielding gas.

Gases for welding

Carbon Dioxide (CO2): It is the most common of the reactive gases used in MIG welding especially in Nigeria and the only one that can be used in its pure form without the addition of inert gas. CO2 is also the least expensive of the common shielding gases, making an attractive choice when material costs are the main priority. Pure CO2 provides very deep weld penetration, which is useful for welding thick material; however, it also produces a less stable arc and more spatters than when it is mixed with other gases. It is also limited to only the short circuit process. For many companies, including those that place an emphasis on weld quality, appearance and reducing post-weld clean-up, a mixture of between 75% to 95% Argon and 5% to 25% CO2 will provide a more desirable combination of arc stability, puddle control and reduced spatter than pure CO2. This mixture also allows the use of a spray transfer process, which can produce higher productivity rates and more visually appealing welds.

Argon: Italso produces a narrower penetration profile, which is useful for fillet and butt welds. If you’re welding a non-ferrous metal; aluminum, magnesium or titanium, you will need to use 100% Argon.

Oxygen: It’s also a reactive gas. It is typically used in the ratio of nine percent or less to improve weld pool fluidity, penetration and arc stability in mild carbon, low alloy, and stainless steel. It does cause oxidation of the weld metal, however, so it is not recommended for use with aluminum, magnesium, copper or other exotic metals.

Helium: Unlike pure Argon, it is generally used with non-ferrous metals, but also with stainless steels. Because it produces a wide, deep penetration profile, Helium works well with thick materials and is usually used in ratios between 25% to 75% Helium and 75% to 25% Argon. Adjusting these ratios will change the penetration, bead profile, and travel speed. Helium creates a ‘hotter’ arc, which allows for faster travel speeds and higher productivity rates.

However, helium gas is more expensive and requires a higher flow rate than Argon, so you’ll need to calculate the value of the productivity increase against the increased cost of the gas. With stainless steel, helium is typically used in a tri-mix formula of Argon and CO2.

When Carbon Dioxide or Oxygen is added to the mixture, MIG welding is not technically MIG welding anymore. This is due to the fact that both Carbon Dioxide and Oxygen are not inert gasses. The process then becomes GMAW or Gas Metal Arc Welding.

Characteristics of MIG Welding Gasses

Different gasses produce different types of weld penetration and arc characteristics.

• Argon gas has shallow wide penetration and a very smooth fluid-like arc.
• Helium produces a very hot weld with good weld productivity and mild penetration with a fluid arc.
• Carbon dioxide produces a deep narrow penetration with a stiff harsh arc that works well in /out of position welding.
• Adding carbon dioxide to argon or helium deepens penetration and stiffens the arc improving out of position welding.
• Oxygen added to helium or argon in small amounts deepens the weld penetration and stiffens the arc characteristics.
• Adding helium to any mix makes the arc hotter.

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