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MIG Welding Gas: A Comprehensive Guide

I typically stick weld, but recently I switched to MIG welding and it’s allowed me more control over my work. I can vary the blend of gases to find the combination that works best for different projects and materials. Seeing my results made me want to learn more about MIG welding gases.

MIG welding gas is an integral part of the welding process. Different gases can be used for MIG welding, each with its own benefits and drawbacks. The purpose of these gases is to shield the electrode and base metal from atmospheric oxygen and humidity.

This guide provides an overview of the common welding gases: argon, helium, carbon dioxide, and oxygen. By understanding the benefits and drawbacks of each gas, you can choose the right one for your project.

Picture of me using MIG welding gas.

Why is gas used for MIG welding?

Before we go ahead, let us clarify that there’s no such thing as “gasless” MIG welding (MIG stands for Metal Inert Gas). Below, we’ll be comparing MIG to flux-cored arc welding (FCAW), which isn’t technically gasless but does lack an external gas source.

The primary purpose of using gas when MIG welding is to protect the weld pool from oxidation, water vapor, and contaminants. Gases like hydrogen and nitrogen can disrupt the quality and texture of the work. They get absorbed by molten metal, and as the metal freezes, they become stuck as cavities.

Gas welding uses a solid wire and a “shielding gas” (Usually argon-CO2 mix though other gases are also used). A gas container supplies the shielding gas and releases it through a nozzle.

Without an external shielding gas, you need to replace your solid wire spool with a flux-cored wire spool. These wires produce their own shielding gases upon melting, which is why they are also called self-shielding (this process is called flux-cored arc wielding or FCAW).

MIG welding has less fumes.

When using a MIG welder, the welder releases shielding gases like argon and CO2 (note that CO2 is a semi-inert gas) to prevent the hot wire and base metal from coming in contact with atmospheric oxygen to prevent combustion and oxidation.

In contrast, gasless welding uses a flux-cored wire. That’s a wire with a cleaning agent called “flux” at its center. The flux gets melted during the welding process and releases gases and liquid slag that prevent oxidation of the base metal.

These gases, along with fume from the consumables and base metal, are made of metal particles and small amounts of carbon monoxide. They can cause short-term effects like nausea and light-headedness and severe long-term effects like lung damage and cancer if inhaled regularly.

Thus, it’s strongly advisable to wear a respirator and have good ventilation or a fan when welding gasless. These gases also interfere with the visibility of the weld pool.

The fumes might also taint the metal you are working with. However, you can easily clean the imperfections with a spray or piece of cloth.

Though MIG welding creates fewer fumes, it still produces certain harmful gases like nitrogen dioxide and ozone. Ozone is colorless and odorless and is not filtered by a respirator. Hence, a ventilation system is still necessary when MIG welding.

MIG doesn’t create (typical) slag and generates less spatter.

Slag is a coating that forms on top of your weld. It’s made up mostly of chemical compounds and other materials like oxides but can also include silica sand. The presence of slag in your weld can be identified as elongated crust either continuous or discontinuous along the length of the weld.

Generally, gas-shielded MIG welding does not involve a flux-cored wire. While during FCAW, this slag spews on the base metal every time you weld, solidifies, and has to be cleaned using a wire brush, chipping hammers, or grinder.

Spatter refers to molten metal droplets that spew in every direction and get solidified as you weld. Though it’s tough to avoid spatter altogether, gas-shielded welding generally creates less of it. In comparison, using a flux-cored wire will create more spatter due to the flux. The spatter can be cleaned using manual tools or an anti-spatter spray.

In this sense, using gas with your welding makes the job less messy, less time-consuming, and doesn’t demand extra clean-up work.

You can’t weld exotic metals without gas.

If you’ve been into welding long enough, every now and then you meet someone selling a flux-cored wire that allegedly welds aluminum or other exotic metals. While some laboratory fluxes can weld aluminum, they require extreme care to operate and store and are basically impractical for general use.

Aluminum is soft and highly sensitive, so it’s very reactive with atmospheric gases and impurities in its molten state. It also doesn’t noticeably glow when it gets hot, making it difficult to determine the state of its weld.

With flux-cored arc welding (FCAW), the gases released by the flux are less reactive than aluminum and do not provide an effective shield for your weld. Plus, molten aluminum absorbs hydrogen and moisture very fast. You’re likely to end up with an overly-porous weld that doesn’t hold together very firmly.

Welding aluminum with FCAW may lead to burn-through, porosity or cause the weld pool to fall through the workpiece – unless you pay incredible detail to the voltage, wire-speed, travel speed, and the kind of flux you use.

Hence, to safely fuse aluminum, MIG or TIG welding is the way to go. However, you can’t use CO2 as the shielding gas because molten aluminum is more reactive than CO2. You’d need to use 100% argon or, for aluminum thicker than 0.5 inches, an argon-helium mix.

Picture of industrial oxygen cylinders for welding.

What type of gas do you use for MIG welding?

There are several gases you can use with your MIG welder. However, carbon dioxide (CO2) and argon are the most popular gases. Small percentages of oxygen and helium are also used for certain welds.

For most commercial welding, you’d use a blend called C25 – which contains 25% carbon and 75% argon. Welding gases are stored in high-pressure cylinders. You can purchase C25 bottles from your local welding supply store and even order them online from Amazon.

Argon

Argon is usually mixed with CO2 or other gases, but it is often used independently. It is one of the chemically “noble” or inert gases. That is, it has a very low tendency to react with the atmosphere or the base metal.

Using argon gives a narrower penetration profile and helps in keeping a stable arc angle. Pure argon gas is used for welding non-ferrous metals like aluminum, titanium, or copper.

Carbon dioxide

Pure CO2 as the shielding gas is a popular choice for many MIG welders. It’s the only gas that works without adding an inert gas and is relatively inexpensive.

CO2 MIG welding offers a deep penetration profile, a hot arc and is suitable for thick materials. However, it produces more spatter, and the arc is not as stable as with an argon-carbon mix. 100% CO2 only allows a short circuit transfer.

Oxygen

Oxygen is often used at less than 1/10th to improve penetration, obtain good wetting (i.e., increase the molten weld pool fluidity), and stiffen the arc. It’s used in spray transfer welding of alloy and stainless steel.

However, it can create slag and should not be used with reactive alloys like copper and aluminum. Excessive oxygen use can cause the MIG electrode to oxidize, leading to more pores in the weld pool.

Even though we may consider all oxygen the same this isn’t necessarily true. The oxygen cylinders you see in hospitals contain the purest form of oxygen which isn’t the same grade as what’s used for welding.

Helium

Helium is another inert gas commonly mixed with 25 – 75% argon. It’s used to get a broader and deeper penetration and weld stainless steel or non-ferrous metals like aluminum. It has a high flow rate, meaning that the cylinder will run out more quickly.

Helium requires a higher voltage and causes a hot, energy-dense arc. It’s often used in small amounts in a “tri-mix” blend to adjust arc characteristics and penetration profile and prevent oxidation. Helium is added to pure argon when welding exotic metals thicker than 0.5 inches.

What is the best argon-carbon mix for MIG welding? (What mix of gas is used to weld sheet metal?)

The exact argon-carbon mix depends on what metal you work with and which welding transfer type you prefer. I usually work with C25, but I can also do most of my work with pure CO­when I run out of C25.

The most common mix for MIG welding is C25 (75% argon and 25% argon). C25 only allows short circuit and globular transfer of the metal. For an average hobbyist, this method has the best quality-to-price ratio.

  • More argon generally translates to narrower, reduced penetration. It can be preferable for fillet and butt welding. However, if you are welding stainless steel, for example, you’d want a lower argon-to-CO2 ratio.

Using argon helps stabilize the arc and generate less spatter. The inertness of argon makes it desirable for reactive metals like aluminum and magnesium. You’d generally want an argon mix when working with soft metals. Pure argon isn’t commonly used in welding.

  • CO2 contributes to deeper penetration and increases the arc heat and energy. Since CO2 isn’t inert, it doesn’t prevent oxidation as effectively as argon or helium. It also creates a noticeable amount of spatter.

You can also go with 100% CO2 if you aren’t working on thin or exotic metals like aluminum. The arc is less stable than if you’d mix an inert gas, and there’s more cleaning up to do after each weld.

Text Box: It’s worth noting that with an argon-carbon mix like C25, you’d get a “V-shaped” penetration that is narrower at the bottom end. Whereas with pure C02, the penetration tends to be “U-shaped.”

The C50 mix (50% argon and 50% CO2) is not as wet as C25, and the weld tends to stick to where you put it. C50 is frequently used for short arc welding of pipes, especially when the surface contains contaminants.

By increasing the argon percentage to 80% or higher in an argon-carbon mix, you’d improve the weld penetration and reduce the spatter (and thus increase deposition rate). You can also use all four transfer modes at such high argon percentages.

If you want to weld sheet metal, it’s best to use an argon-based mixed (i.e., at least 75% argon) – the greater the argon content, the lesser the heat and spattering.

C15 (85% argon and 15% CO2) is popular with many businesses working with structural and carbon steel. If you are working with thick metals (around half an inch) or prefer to weld through mill scale, C15 is suitable.

C15 also supports all four modes of metal transfer. By alternating between the transfer modes, you can weld both thin and thick pieces of metals. However, due to 85% argon, the wetting might be more significant, though you can control the weld pool better with some practice.

The short-circuit transfer of C15 works well when welding sheet metal, and there’s less chance of burn-through than CO2-based mixes.

If you want to go for aesthetics, you can use a C10 mix (90% argon and 10% CO2). The resulting weld is smoother and flatter, and there is less spatter to clean up.

The penetration power of C10 is good but lower than that of C15 or C25. C10 is good for spray transfer and not usually suitable for short circuit transfer with many welding machines.

The arc gets less energetic as you move down to C5 (95% argon and 5% CO2). One significant advantage of C5 is that it allows a good pulse spray transfer. It’s suitable for welding through mill scale. However, if you’ve never used it before, you’ll have to adjust to the puddle control.

The C5 mix can also be used for short circuit transfer, though you should carefully adjust the welding parameters so the output heat isn’t too low.

MIG welding gas prices

Gas prices depend on the cylinder size, type of gas, and where you live. A 40, 80, or 125 cubic feet (CF) cylinder is sufficient for most welding hobbyists.

Inert gases like argon and helium are more expensive than CO2 or Oxygen. You can expect to pay anywhere from $150 to $350 if you buy a full cylinder for the first time. Refills are typically much cheaper. Refilling a 125 CF cylinder may cost up to $40 for pure argon, $30 for C25, and $20 for oxygen.

Volume (CF)Avg. cost of an empty cylinder
20$110
40$140
80$200
125$235

Conclusion

There are several welding gas mixes available in the market. Each combination gives a unique penetration profile and transfer mode. Generally, carbon dioxide and argon are either used independently or mixed, sometimes with added helium or oxygen. MIG welding gases generate less fumes and spatter than traditional stick welding.

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