The shielding gas you choose depends on the wire and the application. CO2 provides good penetration when welding thicker materials, and you can use it on thinner materials since it tends to run cooler, which decreases the risk of burn-through. For even more weld penetration and high productivity, use a 75 percent argon/25 percent CO2 gas mix. This combination also produces less spatter than CO2 so there is less post-weld cleanup. Use 100 percent CO2 shielding gas or a 75 percent CO2/25 percent argon mix in combination with a carbon steel solid wire. Aluminum welding wire requires argon shielding gas, while stainless steel wire works best with a tri-mix of helium, argon and CO2. Always reference the wire’s spec sheet for recommendations.