Which Gas is Used for MIG Welding?

Why Does MIG Welding Require Shielding Gas?

In the presence of heat, oxygen, nitrogen and other gases in the air bond with metal, creating new, less desirable materials. This causes fusion defects, porous welds, brittle joints and excessive spatter. These reactions can also contaminate the welding wire, causing repeated breaks in the bead.
 
The shielding gas creates a blanket of inert or nearly inert gas around the weld. This prevents reactions with atmospheric gases until the bead has time to cool down. The result is a solid, clean weld that’s easy to lay down, has a consistent bead, and creates minimal spatter and electrode contamination.

Which Gases are Used in MIG Welding?

MIG stands for Metal Inert Gas, but unlike TIG welding, MIG welding doesn’t require 100% inert gas. Technically, the use of non-inert gas makes this Gas Metal Arc Welding (GMAW.) Don’t be surprised if you see MIG and GMAW used interchangeably. Your MIG welder can handle both types of welding, as long as it’s using the correct settings, wire and gas.
 
Generally speaking, less chemical reaction means less spatter. However, by mixing inert, mostly inert and reactive gases, you can fine tune the characteristics of the weld. Commercial gas mixes use this to their advantage, targeting specific tasks and materials.
 
There are two completely inert gases used in MIG welding: argon and helium. Argon produces shallow welds with a wide, fluid arc. Gases are added to argon mixes to decrease cost or adjust the characteristics of the weld. Helium offers higher penetration than argon with higher welding temperatures. However, it requires more power, and it’s more liable to cause burn-throughs. Helium is used alone or mixed with other gases to increase arc temperature for better penetration.
 
Carbon dioxide is semi-inert during welding. This gas is inexpensive, so it’s mostly used to cut costs. CO2 causes more spattering than helium or argon, but it offers deep penetration with a narrow arc. It also works well for out of position welding.
 
Oxygen is reactive in high amounts. However, adding small amounts stabilizes the arc and deepens welds. When used, this gas is limited to 1-5% of the gas’s total volume.
 
Argon and helium are expensive, so pure gas is rarely used. The most common gas used in MIG welding is a blend of argon and carbon dioxide. These mixes are labeled by percentage of CO2 content, i.e. C25, C20, C10 and C2. Increasing carbon dioxide makes the gas better for short arc welding, while decreasing content improves globular transfer and spray arc welding by reducing spatter.

How Does Shielding Gas Work?

Your MIG welder feeds wire and gas through hoses to the gun. At the electrode, the gas is sent through a nozzle that surrounds the wire guide. This creates a jacket of gas that covers the welding wire and the weld pool.
 
Electricity from the electrode turns the gas into conductive plasma. This plasma is an excellent conductor of electricity and heat. This helps the transfer of both to the weld pool, helping create a solid weld. Gases with low ionization potential need less energy to ionize, which helps the arc start faster. This is most noticeable if you switch between argon and helium. That’s because argon needs 15.7 electron volts (eV) to ionize, while helium needs 24.5 eV.
 
Thermal conductivity affects weld penetration. The more conductive the gas is, the wider and shallower the arc will be. Argon is relatively poor at conducting heat, so it creates small, deep welds. Helium is highly conductive, so it makes a wide arc with shallow penetration. Carbon dioxide’s weld characteristics are closer to argon than helium.

What Determines the Best Gas Choice for Welding?

There’s more to choosing a gas than just the metal you’re working with. Several factors are at play, affecting the qualities you need from your gas.

Your MIG welder should come with suggestions for gases, alongside wires and settings for different types of metal. However, there are general recommendations that hold for different types of metal, no matter what you’re welding. These are the gas mixes you’ll see used to weld common materials.

Mild Steel

Mild steel is defined by its carbon content. This steel is 0.05-0.25% carbon, while high carbon steel has up to 2% carbon. Since mild steel is mostly iron, it’s easy to weld. However, the risk of oxidation increases as carbon content goes down.
 
There are two gases commonly used with this metal: 100% CO2 or 75% CO2 and 25% argon (C75.) While pure CO2 is cheap, C75 reduces burn-through and oxidation problems. Oxygen is also used in mild steel gas blends. Industrial welding of mild steel almost always uses argon and oxygen mixes, and it’s a common choice across the board thanks to reduced spatter and increased penetration.

Stainless Steel

A high carbon content and the addition of metals to form this steel alloy makes it harder to weld than mild steel. There are several formulas of stainless steel. Each formula reacts differently to shielding gas.
 
The most popular gas for this metal is “tri-mix.” This is a blend of helium, argon and either carbon dioxide or oxygen. Argon stops oxidation, helium helps with penetration, and both oxygen and carbon dioxide improve arc stability. Mixes of argon, CO2 and argon, and argon, carbon dioxide and oxygen are also common.
 
Next to tri-mix, argon with 25% CO2 (C25) is the most common shielding gas, but this is also available in several versions with small percentages of other gases added to target different characteristics, including high oxidation prevention and narrow, high penetration seams. A mix of argon, carbon dioxide and small amounts of nitrogen or hydrogen improve the color and form of the bead. This is useful for decorative welding, but the resulting welds don’t have the strength you’ll get with other gas mixes.
 
Ferritic stainless steels don’t contain nickel or chromium, which decreases their cost. These alloys are typically welded with argon mixed with 5-10% carbon dioxide. However, even the best combination of gas, welding wire and power results in slower welding and increased spatter compared to nickel and chromium alloys.

Aluminum

When aluminum is exposed to air, if forms aluminum oxide. Unlike iron oxide, aluminum oxide forms a protective layer that prevents further corrosion. Unfortunately, it melts at 3,700°F (2,050°C), while pure aluminum melts at 1,200°F (660°C.) Controlling oxidation is critical for aluminum welds. If you try to use heat to bust through aluminum oxide, it’s easy to burn through the piece once you reach pure aluminum.
 
Helium welding was invented to handle aluminum welds, but today’s welders use a helium, argon, or a combination of the two gases. Pure helium has excellent heat penetration, so it’s used to weld thick metal. Pure argon works on sheet aluminum up to a half inch (12.5 mm) thick. For thicker material, a mix of argon and helium are used. Like carbon dioxide and argon blends, these are offered in several formulas, which use labels based on the amount of helium used. Most blends are between 25% and 75% helium.

Other Metals

Argon is used to MIG weld almost every other metal. Using this gas, it’s possible to weld copper, magnesium, nickel, titanium and alloys of these metals.

Why Do Some Types of Flux Core Wire Need Shielding Gas, While Other Types Don’t?

Flux core wire is made from an outer jacket of welding wire surrounding a core of flux. This wire can be divided into two categories. FCAW-S is self-shielding, and requires no shielding gas. FCAW-G needs a shielding gas for welds. The flux in both types of wire has ingredients that help deal with oxidation and scale, deoxidizing and denitrating the metal’s surface. This makes flux core effective at welding contaminated metal.
 
FCAW-S works like a stick electrode turned inside out. The flux inside the wire produces slag, and the chemical reactions that take place when the flux melts generate a small amount of shielding gas around the wire. Most of the oxidation prevention comes from the slag layer that coats the bead. This wire tends to have a harsh arc with more spatter than gas shielded wire. FCAW-S is ideal for outdoor welding, since there’s no jacket of gas that can be blown away by the wind.
 
FCAW-G has the same penetration characteristics as self-shielding wire, but it produces a smoother weld with less spatter. This wire is typically used with 100% carbon dioxide gas, or a mix of 75 to 85% argon and 15-25% CO2.