The gas you need for welding depends on your welding process and the metal you’re working with. For MIG welding carbon steel, a 75% argon / 25% CO2 mix is the most popular all-around choice. For TIG welding, pure argon is the standard. And for flux-cored welding with self-shielded wire, you don’t need external gas at all. Here’s how to pick the right gas for your setup.
MIG Welding Gas for Carbon Steel
MIG welding (also called GMAW) requires an external shielding gas to protect the molten weld pool from contamination by the surrounding air. For carbon steel, you have two main options: a 75% argon / 25% CO2 blend, or straight 100% CO2.
The 75/25 argon/CO2 mix is the go-to for most hobby and professional welders. It produces less spatter, gives you better control of the weld puddle, and works well on thinner material (14 gauge and up) without burning through. It also delivers the best mechanical properties for any given wire, meaning your welds will be stronger and more consistent. If you’re welding out of position, like overhead or vertical, the 75/25 mix makes puddle control significantly easier.
Pure CO2 is cheaper per tank and gives you deeper penetration into the base metal. That deeper bite makes it useful on thicker steel or when you need fast welding speeds. The trade-off is noticeably more spatter, which means more cleanup time. CO2 also produces a broader, deeper penetration profile compared to argon-based mixes, which create a narrower, more tapered bead shape. If you’re doing rough structural work or welding thick plate where appearance doesn’t matter much, CO2 can save you money. For cleaner work on thinner stock, the 75/25 blend is worth the extra cost.
TIG Welding Gas
TIG welding (GTAW) almost always uses 100% pure argon. It’s economical, produces a stable arc, and keeps the weld clean. Argon is denser than air, so it settles over the weld area and provides excellent shielding coverage, especially in the flat position.
Helium is sometimes added to argon for specialized TIG work. Helium has higher thermal conductivity, meaning it transfers more heat to the base metal through the arc. This increases weld penetration and widens the root of the weld bead, which is useful on thick sections of aluminum, copper, or stainless steel where you need to push more heat into the joint. The downsides: helium/argon mixes cost more, require higher flow rates because helium is lighter and dissipates faster, and can cause more discoloration on aluminum as the higher arc temperatures burn off magnesium in the alloy. For most TIG work in a home shop or small fabrication setting, pure argon is all you need.
Gas for Welding Aluminum
Whether you’re MIG or TIG welding aluminum, pure argon is the standard shielding gas. It has good arc cleaning properties, which help break through the oxide layer that naturally forms on aluminum surfaces, and it produces a clean weld with minimal porosity. The argon should have a minimum purity of 99.997% with a dew point of -76°F or lower. Contaminated or wet gas is a common cause of porosity in aluminum welds.
For thicker aluminum sections, adding helium to the mix (typically 25% to 75% helium with the balance in argon) increases heat input and penetration. This reduces porosity in the weld bead and widens the fusion zone at the root. But the higher arc temperatures also burn more magnesium out of aluminum alloys, leaving more discoloration on the surface. Most welders start with pure argon and only move to helium blends when they’re struggling to get adequate penetration on heavy material.
Flux-Cored Wire: Gas vs. No Gas
Flux-cored arc welding comes in two varieties that handle shielding gas very differently. Self-shielded flux-cored wire (FCAW-S) generates its own shielding gas from the flux inside the wire as it burns. No tank, no regulator, no hose. This makes it popular for outdoor work and jobsite welding where wind would blow away an external shielding gas.
Gas-shielded flux-cored wire (FCAW-G) uses both the internal flux and an external gas supply, just like MIG. The two most common gases for FCAW-G are 100% CO2 or a 75% argon / 25% CO2 blend. The argon mix produces less smoke and spatter and gives better puddle control, while CO2 provides deeper penetration. If you’re buying flux-cored wire, check the packaging carefully. Self-shielded and gas-shielded wires are not interchangeable, and using the wrong type will give you a bad weld.
Flow Rate Settings
Getting the right gas is only half the equation. You also need to set the correct flow rate, measured in cubic feet per hour (CFH). Too little gas and the weld is exposed to contamination. Too much and you create turbulence that pulls air into the shielding envelope, which is just as bad.
- Indoor welding, no draft: 10 to 15 CFH
- Indoor welding, light draft: 20 to 30 CFH
- Outdoor welding: 30 to 35 CFH, though outdoor MIG welding is generally not recommended because wind easily disrupts the gas shield
Start at the lower end and increase if you see porosity (tiny holes) in your weld bead. If you’re going through gas faster than expected, check for leaks at every connection from the tank to the nozzle before cranking up the flow.
Tanks, Regulators, and Compatibility
Welding gas cylinders use standardized fittings from the Compressed Gas Association (CGA) to prevent you from accidentally connecting the wrong regulator to the wrong tank. Argon, helium, and argon/helium blends use a CGA-580 fitting. Pure CO2 tanks use a CGA-320 fitting. A regulator designed for one will not thread onto the other, which is a deliberate safety feature.
If you’re switching between pure CO2 and a 75/25 argon/CO2 blend, check what fitting your premixed tank uses. Many argon/CO2 blends come with a CGA-580 fitting, but some use CGA-320, so verify with your gas supplier before buying a regulator.
Storing Gas Cylinders Safely
High-pressure gas cylinders can be dangerous if mishandled. OSHA requires that cylinders be stored upright in a well-ventilated, dry location, at least 20 feet from highly combustible materials like oil or grease. Keep them secured so they can’t be knocked over by passing traffic or falling objects. Valve protection caps should be hand-tight whenever the cylinder isn’t actively connected to a regulator.
Oxygen cylinders, if you’re doing oxy-fuel cutting alongside your welding, must be stored separately from fuel gases like acetylene. Acetylene cylinders must always be stored valve end up. Empty cylinders should have their valves closed and ideally be marked as empty so no one tries to use them and wonders why their welds look terrible.