Shielding Gas for MIG Welding: What You Should Know
Shielding gas for MIG welding protects the molten weld pool from the oxygen, nitrogen and hydrogen in the surrounding air. Without it, those elements react with the pool and cause porosity, excessive spatter and weak welds. The gas you choose also sets penetration depth, arc stability, bead appearance and how much post-weld cleanup the job takes.
Consistent productivity, high quality and low costs are all key components in a successful welding operation. Gaining these advantages depends on everything from the equipment and filler metals to the skill of the welding operators and the techniques being used in the process. The shielding gas also plays a critical role.
Both the gas metal arc welding (GMAW) process (using solid or metal-cored wires) and the gas-shielded flux-cored arc welding (FCAW) process require the use of an external shielding gas, each type of which offers distinct characteristics. Knowing how to select the appropriate one for the application can go far in helping obtain the desired welding performance and minimizing the downtime for rework caused by poor weld quality. For the fundamentals of the process itself, see MIG Welding Basics: Techniques and Tips for Success.
To help, following are some basics of what you should know about shielding gases.
What Shielding Gas Does in MIG Welding
The primary purpose of shielding gas is to protect the molten weld pool against elements in the atmosphere, including oxygen, nitrogen and hydrogen. The reaction of these elements with the weld pool can create a host of problems, including porosity and excessive spatter.
Shielding gas also plays an important role in determining weld penetration profiles, helping maintain arc stability and achieving the desired mechanical properties in the finished weld. Shielding gas can also affect the transfer of the filler metal from the arc to the weld joint, which in turns contributes to the efficiency of the welding process and the quality of the weld. Other important factors that shielding gas help determine include the weld bead appearance, and weld toughness and strength.
Porosity is the clearest symptom of a shielding problem. When gas coverage breaks down, the pool absorbs atmosphere and the defect shows on the face and interior of the bead. The same failure mode turns up in gas-shielded flux-cored work, which is covered in Tips for Avoiding Common Flux-Cored Problems.
How to Choose the Right Shielding Gas for MIG Welding
The four most common shielding gases used in the welding process are carbon dioxide, argon, helium and oxygen. Each has specific characteristics, and factors such as cost, available labour for weld preparation and the weld properties desired are all considerations when selecting which shielding gas is best for a given welding application. The table below compares the four at a glance. The sections that follow explain each one in detail.
Shielding Gas at a Glance
| Gas | Typical mix | Penetration | Best suited to | Trade-off |
|---|---|---|---|---|
| Carbon dioxide (CO2) | 100% CO2 | Deep | Thick carbon steel, short circuit transfer | Less stable arc, more spatter, more post-weld cleaning |
| Argon | 75–95% argon with 25–5% CO2; 100% argon for non-ferrous | Narrow | Fillet and butt welds; aluminum, magnesium, titanium at 100% | Higher gas cost than CO2 |
| Helium | 25–75% helium with a balance of argon | Wide and deep | Non-ferrous metals; tri-mix with argon and CO2 for stainless | More expensive and needs a higher flow rate |
| Oxygen | 9% or less, added to argon | Improved fluidity and penetration | Carbon, low alloy and stainless steels | Oxidizes the weld metal; not for aluminum, magnesium, copper or exotics |
Carbon dioxide (CO2): This gas is the most common of the reactive gases used in the welding process and also the least expensive of the shielding gases. It is also the only one able to be used without the addition of an inert gas. One of the biggest advantages of pure CO2 is that it provides deep weld penetration, which is useful when welding thick material. It does, however, tend to create a less stable arc and more spatter than when it is mixed with other gases, including argon. This additional spatter can lead to downtime for post-weld cleaning. Pure CO2 is also limited to use in short circuit welding processes.
Argon: When welding aluminum, magnesium or titanium, it is common to use 100 percent argon as a shielding gas due to its stable arc features. Adding argon to a CO2 shielding gas is also an option for materials like carbon steel. It provides consistent weld quality and appearance and good weld pool control, and can help minimize post-weld cleanup. Argon also produces a narrow penetration profile, making it useful for fillet and butt welds.
Typical mixtures include a balance of 75 to 95 percent argon with 25 to 5 percent CO2. An argon/CO2 shielding gas mixture allows the use of a spray transfer process, which lends itself to high productivity rates and visually appealing welds. The 75/25 blend is common enough that it is often ordered by its shorthand, C25.
Helium: Helium is generally used when welding non-ferrous metals. It is also used in a tri-mix formula of argon and CO2 for welding stainless steels. The gas produces a wide, deep penetration profile, making it suitable for welding thick materials, and also creates a hot arc, which helps increase travel speeds and productivity rates. Helium is typically used in ratios of 25 to 75 percent helium with an appropriate balance of argon. Adjusting these ratios changes the weld penetration, bead profile and travel speeds.
It is important to note that helium is more expensive than other gases and requires a higher flow rate than argon, because it is so light. For this reason, it is imperative that companies calculate the value of the productivity increase against the increased cost of this gas.
Oxygen: Oxygen is a reactive gas typically used in ratios of 9 percent or less. The addition of the gas to a mixture with argon helps to improve weld pool fluidity, weld penetration and arc stability, particularly when welding carbon, low alloy and stainless steels. Because the gas causes oxidation of the weld metal, it is not recommended for use with aluminum, magnesium, copper or other exotic metals.
Filler metal classification and base metal chemistry both influence the final selection. The American Welding Society publishes the governing specifications, and the filler metal manufacturer’s data sheet will state the gases a given wire is qualified for.
Shielding Gas by Base Metal
| Base metal | Common shielding gas |
|---|---|
| Carbon steel | 100% CO2, or 75 to 95 percent argon with 25 to 5 percent CO2 |
| Stainless steel | Tri-mix of argon, helium and CO2, or argon with an oxygen addition of 9 percent or less |
| Aluminum | 100% argon; helium added to argon for a deeper profile on thicker sections |
| Magnesium and titanium | 100% argon |
| Copper and other exotic metals | Argon-based mixes without an oxygen addition |
How Front-End Consumables Protect Your Shielding Gas
The right gas only helps if it reaches the weld pool. The front-end consumables, meaning the gas diffuser, contact tip and nozzle, play a critical role in delivering the shielding gas to the weld pool and protecting it from the atmosphere. A worn or wrong-sized part will undo a correct gas selection.
Choose consumables with a smooth surface to help resist the spatter build-up that could block shielding gas flow and lead to issues such as porosity.
Choose an appropriate size nozzle for the application, because a nozzle that is too narrow can easily become clogged with spatter, again hindering its ability to deliver enough shielding gas to the weld pool to protect it.
Consider nozzles with a built-in spatter guard, a design that adds a second phase of shielding gas diffusion and results in even smoother, more consistent flow. And be certain to select quality gas diffusers to ensure smooth and balanced gas flow. A trusted authorized distributor can make recommendations for the specific application.
Consumable condition compounds across a shift. Bernard AccuLock™ S consumables lock and concentrically align the liner to both the contact tip and the power pin, which keeps the wire-feed path consistent and delivers up to 3x contact tip life compared with standard tapered contact tips. Fewer changeouts means fewer opportunities for a gas leak at the front end. Anatomy of a MIG Gun walks through how each front-end part contributes to gas coverage.
Frequently Asked Questions
Q: What is shielding gas?
A: Shielding gas is an externally supplied gas that protects the molten weld pool from the oxygen, nitrogen and hydrogen in the surrounding air. In MIG welding it also influences penetration depth, arc stability, spatter levels and bead appearance.
Q: What shielding gas is used for MIG welding?
A: The four most common are carbon dioxide, argon, helium and oxygen. Carbon steel is typically welded with 100 percent CO2 or a mix of 75 to 95 percent argon with 25 to 5 percent CO2. Non-ferrous metals such as aluminum, magnesium and titanium use 100 percent argon.
Q: Can you MIG weld without shielding gas?
A: Not with the standard GMAW process, which depends on an external gas supply. The alternative is self-shielded flux-cored welding, where flux inside the wire generates its own shielding. That is why self-shielded flux-cored guns hold up outdoors and in windy conditions where a gas envelope would blow away.
Q: Which shielding gas gives the deepest penetration?
A: Pure CO2 provides deep penetration on carbon steel at the lowest gas cost, though with a less stable arc and more spatter. Adding helium to argon produces a wide, deep profile and a hotter arc, which suits thicker non-ferrous material.
Q: What shielding gas do you use for aluminum?
A: 100 percent argon is standard for aluminum because of its stable arc characteristics. Helium can be added to argon for a deeper penetration profile on thicker sections. Oxygen is not recommended for aluminum because it oxidizes the weld metal.
Q: What is C25 shielding gas?
A: C25 is common shorthand for a blend of 75 percent argon and 25 percent CO2. It sits in the middle of the typical argon/CO2 range and is widely used on carbon steel because it balances the penetration of CO2 against the arc stability and cleaner bead appearance of argon.
Q: Why is my weld porous when the gas is turned on?
A: Flow at the regulator is not the same as coverage at the weld pool. Spatter build-up in the nozzle, an undersized nozzle, a worn diffuser or a loose front-end connection can all starve the pool while the gauge still reads normally. Check the front-end consumables before adjusting parameters.
Get the Mix Right, Then Protect It
Shielding gas for MIG welding is one of the least expensive variables in a welding operation and one of the most controllable. The mix decides what the arc can do. The front end decides whether that gas ever reaches the pool. Get both right and the weld is largely settled before the trigger is pulled.