What Gases Are Used for MIG Welding?

MIG welding primarily uses argon, carbon dioxide (CO2), or mixtures of these gases as shielding agents. The most common option is a mix of 75% argon and 25% CO2 for mild steel applications.

Still, the specific gas choice depends on the metal being welded, so in this guide, we’ll explore the different shielding gases used in the process. We’ll then cover their specific applications and explain how to select the right gas for your welding projects.

Understanding Shielding Gas Functions in MIG Welding

Shielding gas serves as a protective barrier that prevents atmospheric contamination of the weld pool. Without it, oxygen and nitrogen from the air would create porosity and oxide inclusions in the weld, significantly weakening the joint.

Beyond protection, shielding gases directly influence:

• Arc stability
• Penetration depth
• The overall appearance of your welds

These gases affect how metal transfers from the wire to the workpiece, determining whether you achieve spray transfer, globular transfer, or short-circuit transfer.

The right gas choice can mean the difference between a solid-looking weld with minimal spatter and a rough, porous weld that requires extensive cleanup. This makes gas selection one of the most critical decisions in MIG welding.

Argon: The Foundation of MIG Welding Gases

Pure argon is the baseline shielding gas for MIG welding non-ferrous metals. This inert gas creates a clean, stable arc that’s perfect for gases like:

• Aluminum
• Copper
• Titanium

Argon produces a narrow, focused arc cone that results in deep, concentrated penetration at the center of the weld. This makes it ideal for thin materials where excessive heat input could cause burn-through.

Pure argon is also the standard choice for aluminum welding due to its ability to clean surface oxides through the cathodic cleaning action of the arc. This results in bright, clean welds without contamination.

The cost of argon falls in the mid-range of welding gases, which makes it accessible for most welding operations. While it’s more expensive than CO2, its superior performance on non-ferrous metals justifies the additional expense.

Carbon Dioxide: The Economical Option

Carbon dioxide (CO2) is the most cost-effective shielding gas option for MIG welding. At roughly half the cost of argon, it appeals to high-volume operations and budget-conscious welders.

Purse CO2 provides deeper penetration than argon-based mixes, which makes it suitable for welding thick sections of mild steel. The increased heat input helps ensure fusion throughout the joint.

Still, CO2 produces a less stable arc with more spatter compared to argon mixtures. This results in rougher weld appearances and often requires additional post-weld cleanup.

CO2 can only be used with ferrous metals like carbon steel and is not suitable for non-ferrous materials due to its reactive nature. The gas reacts with the molten metal, which is beneficial for metals like steel but detrimental to options like aluminum or copper.

Most professional welders use CO2 mainly for:

• Outdoor applications
• Structural steel
• Situations where cosmetic appearance is secondary to strength and cost-efficiency

Argon/CO2 Mixtures: The Industry Standard

The 75% argon/25% CO2 mixture (often called C25) is the most widely used shielding gas in MIG welding. It strikes an optimal balance between the clean arc characteristics of argon and the enhanced penetration of CO2.

C25 significantly reduces spatter compared to pure CO2 while providing better wetting action and bead appearance. This translates to less post-weld cleanup and more aesthetically pleasing results.

For thinner materials, a higher argon content mix like 80/20 or 90/10 can provide even better arc characteristics and less heat input. These mixes are particularly valuable when warping is a concern.

In production environments, the reduction in cleanup time and improved weld quality often justify the higher cost of argon/CO2 mixtures compared to pure CO2. The efficiency gains typically outweigh the increased gas cost.

Most manufacturers also recommend C25 as the starting point for new welders working with mild steel. It offers the most forgiving arc characteristics and produces consistently good results across a wide range of machine settings.

Specialty Gases for Stainless Steel and Aluminum

Stainless steel welding typically requires a tri-mix containing argon, CO2, and a small amount of oxygen. A common formulation is:

• 90% argon
• 5% CO2
• 5% oxygen

This blend promotes excellent fluidity and wetting, which makes it such a popular choice.

For stainless steel, CO2 content should be kept below 5% when possible to minimize carbon pickup and maintain corrosion resistance. Higher levels of CO2 can compromise the material’s inherent corrosion-resistant properties.

Aluminum welding demands pure argon or an argon/helium mixture for thicker sections. Helium increases heat input, improving penetration on thick aluminum plates while maintaining the inert protection needed.

Helium additions typically range from 25% to 75% depending on material thickness. While expensive, helium dramatically improves performance on thick sections of highly conductive metals like aluminum and copper.

Premium stainless applications sometimes use argon with 1-2% hydrogen, which increases arc temperature and fluidity. This specialized mix is particularly useful for automated welding systems where maximum speed is required. 

Adding even small amounts of hydrogen can significantly increase arc temperature, improving welding speed and penetration.

That said, hydrogen must be used carefully as it can cause porosity or cracking in some applications if not properly controlled.

Choose the Right Gas for Your MIG Welding Projects

Selecting the best shielding gas for MIG welding involves balancing material compatibility, desired weld characteristics, and budget considerations. While C25 serves as an excellent all-around option for mild steel, specialized applications may require pure gases or custom mixtures.

Understanding the strengths and limitations of each gas option helps welders make informed decisions that optimize both weld quality and operational efficiency. Whether you’re focusing on speed, appearance, strength, or cost, there’s an ideal gas solution for your project specifics.