An inert gas in welding is a shielding gas that does not chemically react with the molten metal during the welding process. Argon and helium are the two inert gases used in welding. Their job is to form a protective blanket around the weld pool, keeping oxygen and moisture in the surrounding air from contaminating the joint. Without this shield, hot metal would oxidize almost instantly, producing weak, porous, corroded welds.
Why Shielding Gas Matters
When metal melts during welding, it becomes extremely reactive. Oxygen, nitrogen, and water vapor in the air will bond with the molten metal and create defects: porosity (tiny gas bubbles trapped inside the weld), brittleness, and surface oxidation that weakens the joint. Shielding gas flows out of the welding torch and surrounds the arc and weld pool, displacing the atmosphere and preventing these reactions.
The word “inert” is the key distinction. Inert gases have a full set of electrons in their outer shell, which means they have no chemical incentive to bond with anything, even at the extreme temperatures inside a welding arc. This makes them ideal for protecting metals that are sensitive to contamination, like aluminum, titanium, and stainless steel.
Argon vs. Helium
Argon is by far the more common choice. It’s abundant (about 1% of Earth’s atmosphere), relatively cheap, and works well across a wide range of metals and processes. A large cylinder of argon costs roughly $20, while the same size helium cylinder runs closer to $37. For ultra-high-purity grades (99.999%), argon is about $68 per cylinder compared to $78 for helium. That price gap adds up quickly in production environments.
Beyond cost, the two gases behave differently in the arc. Argon produces a stable, smooth arc that’s easy to control, with a relatively narrow penetration profile. Helium has much higher thermal conductivity, meaning it transfers more heat into the workpiece. Adding helium to argon shielding gas increases heat flow and current density, producing deeper weld penetration, roughly 13% deeper on average in aluminum welding. The tradeoff is that helium arcs run at higher voltage and can be harder to manage.
For most general welding, pure argon does the job. Helium or argon-helium blends come into play when you need deeper penetration on thick aluminum or copper, or when you want to increase travel speed on automated production lines. Research on aluminum TIG welding has shown that adding helium also reduces porosity and improves the overall weld profile, likely because the hotter arc does a better job of driving dissolved gases out of the molten pool.
Inert Gas vs. Active Gas
Not all shielding gases are inert. Carbon dioxide and oxygen are “active” gases, meaning they do react with the molten metal. This is the difference between MIG (Metal Inert Gas) and MAG (Metal Active Gas) welding, though both fall under the broader GMAW category.
MIG welding uses only non-reactive gases: pure argon, pure helium, or a blend of the two. MAG welding uses CO₂, or CO₂ and oxygen mixed with argon. Active gases are cheaper and work well on carbon steel and construction-grade metals, where a small amount of oxidation is tolerable and actually helps with weld pool fluidity. But the intense heat of the arc can split CO₂ into carbon monoxide and oxygen, causing partial oxidation. That’s why MAG welding isn’t used on aluminum, titanium, or other metals that are sensitive to contamination.
TIG welding (GTAW) almost always uses pure argon. Because TIG produces the cleanest, most precise welds, there’s no room for reactive gases that could introduce impurities.
Which Gas for Which Metal
The base metal you’re welding determines your gas choice:
- Aluminum, copper, and nickel alloys: Pure argon or argon-helium blends. These metals oxidize aggressively, so only a truly inert gas will protect them.
- Stainless steel: Pure argon for TIG welding. For MIG, argon with a small addition of CO₂ (around 2%) is common, though this technically makes it an active mix.
- Mild steel: Can use CO₂, argon with 2 to 5% oxygen, or argon with 5 to 25% CO₂. Pure inert gas isn’t required here, and active mixes are more cost-effective.
- Titanium: Requires pure argon with high purity, often 99.996% or higher. Titanium is so reactive at welding temperatures that even trace contaminants in the gas can ruin the weld. Some titanium applications also require trailing shields and back-purging to protect the cooling metal after the torch passes.
Flow Rates and Purity
Shielding gas is measured in cubic feet per hour (CFH). For mild steel MIG welding indoors with no drafts, 10 to 15 CFH provides solid coverage. Aluminum needs more gas, with a minimum of 20 CFH and an optimal range of 25 to 35 CFH. Stainless steel falls in between at 20 to 30 CFH. If you’re seeing porosity in your welds (small holes or a rough, spongy surface), increasing flow to 20 to 30 CFH often fixes the problem.
Too little gas leaves the weld exposed to air. Too much gas creates turbulence that actually pulls air into the shielding envelope, defeating the purpose. Drafts and wind make outdoor welding particularly tricky, since even a light breeze can blow the gas shield away from the weld pool.
Gas purity matters most on reactive metals. Standard welding-grade argon is typically 99.996% pure. For critical aerospace or nuclear work on titanium and specialty alloys, ultra-high-purity argon at 99.999% is sometimes specified. The small fraction of a percent difference might sound negligible, but at welding temperatures, even parts-per-million levels of oxygen or moisture can discolor titanium or cause micro-cracking.
Safe Handling of Gas Cylinders
Inert gases aren’t toxic or flammable, but they can displace oxygen in enclosed spaces. A leaking argon cylinder in a small welding booth can silently lower the oxygen concentration to dangerous levels. Because argon is heavier than air, it pools near the floor, which makes it especially hazardous in pits or confined spaces.
OSHA requires that gas cylinders be stored in well-ventilated, dry locations at least 20 feet from highly combustible materials. They should be secured upright so they can’t be knocked over, and stored away from elevators, stairs, and high-traffic areas. Fuel gas and oxygen manifolds need to be in accessible locations and never inside enclosed, unventilated spaces. Even though inert gas cylinders don’t pose a fire risk the way acetylene or oxygen cylinders do, the high pressure inside (typically 2,000+ PSI) makes a damaged valve or dropped cylinder a serious projectile hazard.