MIG welding requires a shielding gas for a clean weld, and there are various gas mixes from which to choose. Each combination works slightly differently, so you can’t take a one-size-fits-all approach to these gasses. Some factors include the metal type, the transfer method, and the workpiece thickness.
A mixture of 75% argon and 25% CO2 is the most common gas for MIG welding, but it is not the only gas you might need since you could use Helium blends if you weld thick aluminum.
The shielding gas you choose will affect weld quality, costs, productivity, and arc behavior, so you should carefully pick the suitable gas for your application. Although it might initially sound confusing, MIG welding shielding gases are relatively straightforward, and this guide should answer your questions.
Let’s start with a list of the most commonly-used MIG welding gasses:
What gasses are used for MIG welding?
Four shielding gases are used for MIG welding: Oxygen (O2) and carbon dioxide (CO2) are reactive, and argon (Ar) and helium (He) are inert. Here they are in more detail:
The primary gas for MIG welding is argon, denser than air, settling over the weld pool to protect it from atmospheric gas contamination. Also, argon is easy to ionize, so it handles a long arc at low voltages.
A 100% argon gas creates favorable conditions for welding non-ferrous metals like aluminum, magnesium, titanium, copper, and nickel. Its cathodic cleaning removes surface oxides, making it an excellent choice for welding aluminum.
However, pure argon is not ideal for welding steel, including stainless. Because the arc’s outer edges remain too cold, it reduces penetration with less fusion, and the filler material remains on top of the steel, resulting in a taller bead. Instead, most welders use a mixture of Ar and CO2 or a blend of Ar and O2. Helium can also be added to increase the penetration and fluidity of the weld pool.
Carbon dioxide creates a less stable arc with more spatter, but it’s an inexpensive gas, making it the choice of hobbyists and DIYers working with a limited budget. Remember that CO2 or mixtures with CO2 only work with steel and will not work when welding non-ferrous metals.
Depending on the metal, oxygen may be blended into CO2 or argon, but the maximum percentage of oxygen in a shielding gas mixture is 9%. Because iron oxide attracts the arc when using 100% argon, creating arc irregularities, a small amount of oxygen significantly improves arc stability.
Adding oxygen to the mixture can improve penetration, weld pool fluidity, and arc stability with low alloy, mild steel, and stainless steel. However, oxygen causes metal oxidation and cannot be used with aluminum, copper, and magnesium.
Helium is an inert gas like argon, although it’s much lighter and has a higher ionization potential. Because of its low weight, helium floats away from the weld pool quickly, so it’s essential to double the shielding gas flow rate. Cranking up the flow is costly since helium is expensive, but blending it with argon can make welding more affordable.
Blending with argon gas for MIG welding
Here are the most common shielding gas mixtures used with argon:
75% Ar + 25% CO2: Offers an excellent balance of arc stability, spatter reduction, weld puddle control, and penetration when welding carbon and stainless steel.
Ar + 1%, 2%, or 3% O2: Peak arc control, but not as common as the 75/25 Ar/CO2 mixture. As a rule, 2-3% oxygen stabilizes the arc when welding carbon and low alloy steel, but 0.5-1% works better with stainless steel, preventing the refractory scale of chromium oxide.
10% Ar + 85-90% He + 2-5% CO2: Considered the best tri-mix shielding gas blend for stainless steel MIG welding.
Ar + 25%, 50%, or 75% He: Ideal for welding thick aluminum, magnesium, copper, and nickel. The higher the helium content, the hotter the arc and the thicker the material you can weld.
Ar + CO2 + O2: Recommended for welding low alloy steel and some stainless steel in all “metal transfer” modes.
Why use argon gas for MIG welding?
Argon is a chemical belonging to a group known as noble gases, which are odorless, colorless, and monatomic (composed of a single atom). Argon has these characteristics and two others: it has a property of relatively low chemical reactivity unless put under extreme conditions, and it can help prevent oxidization. Because of its low chemical reactivity, argon can push away other gases without interfering with the welding process.
Some of the most popular welding processes, including MIG (Metal Inert Gas) welding, use argon as a shielding gas to protect the weld zone from contamination during the welding process. The argon is often mixed with carbon dioxide, typically 75% argon and 25% CO2, to help stabilize the arc.
As a shielding gas, argon is diffused from the MIG welding gun and spreads around the surface of the metal, creating an atmosphere or ozone layer that insulates the weld from outside contaminants.
Choosing the MIG shielding gas for a specific metal type
Let’s look at the recommended shielding gasses to use with commonly welded metals:
Carbon steel: Using 100% Ar gas for carbon steel causes iron oxide present in the steel to attract the arc. The iron oxide emits electrons, and the arc follows them. Because iron oxides are distributed unevenly on the steel’s surface, the arc follows an irregular path, producing uneven weld deposits. Adding carbon dioxide improves arc stability; the preferred combination is 75% Ar and 25% CO2. Since CO2 has high thermal conductivity, it requires more voltage to keep a stable arc, providing deeper penetration.
Aluminum: Use 100% argon gas to MIG weld aluminum, and avoid reactive gasses such as CO2 or O2 since they will produce a dreadful joint. Most welders will succeed with pure argon gas, but if the aluminum piece is thick and you don’t want to buy a helium mixture for a single job, preheat the aluminum to 250F° maximum. Otherwise, Ar/He mixtures create excellent aluminum welds on thicker parts.
Stainless steel: The shielding gas choices for welding stainless steel are complex. However, there are gas selections that work in most cases, especially for DIYers and home hobbyists.
You can weld stainless steel with a 75% Ar/25% CO2 mixture, which works for most welders. However, for some applications, the 25% carbon dioxide might provide too much carbon in the joint, resulting in the loss of some corrosion resistance. If corrosion resistance is essential to you, keep the CO2 content below 5%. However, if you are a hobbyist welding a noncritical joint, the 75/25 mix will do fine, and you can save money by not buying another gas tank to reduce the carbon dioxide percentage.
Choosing a shielding gas for stainless is tricky because there are numerous grades and types of it. Various alloys require different shielding gasses, and your choice of shielding gas will influence the resulting joint characteristics. Professional applications may require a tri-mix formula.
Where can welders order argon gas?
Get PrimeWeld's industrial-grade argon welding gas delivered right to your door. Purchase an argon tank without worrying about costly rental fees or wasted trips to your local welding supply shop. Our argon tanks are filled and ready to use. With a 10-year date stamp, you'll never have to worry about your tank being turned away for a refill.
We offer a 100% argon full welding tank for TIG welding and a 75% argon 25% CO2 full tank for MIG welding. Questions? Use our handy Contact Us form, or call us at 856-537-4368. Our customer service professionals are standing by!