How to choose shielding gas in MIG welding

MIG (Metal Inert Gas) welding is a popular welding process that uses a shielding gas to protect the weld from contaminants.  There are a variety of shielding gases to choose from, which can be confusing for anyone new in welding. This article is aimed to increase understanding of shielding gas, their drawbacks and benefits as well as help in selecting the best one.

Working principle of shielding gas

One of the main functions of all protective gases is to isolate oxygen, nitrogen, and water vapor in the air and protect the welding pool and electrodes. The shielding gas enters through the welding gun, ejects from the welding tip, surrounds the electrode, replaces the air around the electrode, and forms a temporary protective gas shield around the molten pool and arc. 

These protective gases promote the formation of the arc plasma zone, which is the current channel for the welding arc. The type of shielding gas also affects the conduction of arc heat and the amount of arc force exerted on the molten pool.

Why is shielding gas important?

Shielding gas is an essential component of any arc welding process as well as MIG welding process. MIG welding with shielding gases produces cleaner and faster welds and removes the need to frequently stop to replace electrodes like you would in stick welding. Reduced clean-up and increased efficiency also come with using shielding gases. 

Shielding, whether in the form of flux or gas, prevents harmful elements such as oxygen and hydrogen from getting into the weld pool. Without shielding, the welding arc becomes completely erratic, leading to a globby mess of metal riddled with holes and contaminants. 

The most commonly used shielding gases for MIG welding are Carbon Dioxide (CO2),  Argon (Ar),  Helium(He) and mixtures of these gases. Each type of gas has its own unique properties and is suitable for different welding applications.

The insert gases in MIG welding

Insert gases, such as argon and helium, are inert or non-reactive gases that do not undergo chemical reactions with external elements like oxygen and nitrogen under normal conditions. They are chemically stable and provide excellent shielding properties in welding. Argon is the most commonly used inert gas in welding applications due to its affordability and effectiveness.

Argon

Argon is a noble gas and one of the most widely used shielding gases due to its ability to provide a stable arc and excellent shielding of the weld with high-quality and clean welds.

When using pure argon for MIG welding steel, it is generally not recommended. Pure argon lacks the necessary active components to stiffen the arc and provide satisfactory results when welding mild steel. The low thermal conductivity of argon can lead to a narrow, reduced penetration with less fusion, resulting in a tall, narrow bead and potential issues such as undercutting and reduced ductility in the weld. 

However, some sources suggest that using pure argon can be advantageous when welding thin steel sheets, as it reduces the likelihood of melting through the base metal.

Helium

Like argon, helium is another inert gas that does not react chemically with other elements, including MIG wire, weld metals, or the atmosphere. Helium also provides good arc characteristics, but welders usually choose it due to its high heat due to greater ionization potential. The high heat of Helium produces deep weld penetration and creates a broader bead profile, which works excellent with thicker pieces, which is where pure argon comes short.

The active gases in MIG welding

Active gases are reactive gases that participate in chemical reactions. Active gases can react with other substances, leading to changes in composition, structure, or physical properties. MIG welding utilizes carbon dioxide and oxygen reactive gases, of which CO2 plays a significantly more crucial role in MIG welding

Carbon Dioxide (CO2)

The most common of the reactive gases used in MIG welding is Carbon Dioxide (CO2). It is the only one that can be used in its pure form without the addition of an inert gas. CO2 is also the least expensive of the common shielding gases, making it an attractive choice when material costs are the main priority.

Pure CO2 provides very deep weld penetration. The CO₂ makes the gas slightly electrically conductive, this in turn raises the arc voltage, which increases penetration. However, it also produces a less stable arc and more spatter than when it is mixed with other gases.

Next it allows for greater welding speed setting or gun travel. Finally, it offers far greater cleaning action and can even cut through rust and mill scale.

Due to unfavorable weld appearance and high penetration properties, CO2 is often mixed with argon gas. By mixing these two, you get the best out of each, and maybe the most popular shielding gas mixture, C25.

Oxygen

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

Shielding Gas Mixtures

Shielding gas mixtures are used in MIG welding to provide the necessary protection for the weld pool and to enhance the welding process. Different shielding gas mixtures have varying properties, such as stability, reactivity, and thermal conductivity, which can significantly impact the welding performance and the quality of the weld.

25% CO2, 75% Argon (C25)

75% or argon with 25% carbon dioxide, which is often referred to as the C25 mix, is one of the best and most popular shielding gas mixtures for mild steel due to its versatility, arc stability, and weld penetration. It is also known for producing minimal spatter, making it a cost-effective option for welding mild steel. 

The 25% carbon dioxide content in the C25 mix enhances penetration and bead characteristics, while the high argon content contributes to clean welds with great arc stability and minimal spatter. 

Argon/Helium Mixtures

Argon/helium mixtures are blends of two inert gases, argon and helium, which are commonly used as shielding gases in MIG welding. These mixtures are typically composed of 75-90% argon and 10-25% helium.

Mixing 25-75% of argon with 25-75% of helium increases the arc performance and penetration while retaining the excellent cleanliness of the weld. 

So basically, this mixture shows a similar result to the C25 mix. However, since helium is more expensive than carbon dioxide, this mixture is not so cost-efficient when looking to GMAW weld mild steel.

Tri-Mix (Mixture of helium, argon, and CO2)

Helium makes up a majority of the composition of trimix gases, followed by argon, CO2, or sometimes O2 (oxygen). Helium is inert, just like argon, which helps in preventing contamination. Helium is considered a “hotter” gas, leading to increased weld pool fluidity and the ability to travel faster. It’s also easier to make starts with helium. Stainless steel, like aluminum, is very sensitive, which limits CO2 or O2 additions to a small percentage (around 2%). \

Choosing The Right Gas: What Gas Is Used For MIG Welding?

Different gases play different roles in the welding process, from weld penetration to arc stability to the finished weld itself. Choosing the consumables that provide continuous and even gas delivery is also a very important aspect to consider in your MIG welds.

Be sure to evaluate your project goals in order to select the right gas for the weld at hand. Things to keep in mind when selecting include:

• Cost
• What preparation entails
• The base material you’ll be welding
• The finished weld properties
• What needs to be done for post-weld clean-up