Different gasses can be used for different types of the weld. For example, carbon dioxide is often used and is the least expensive shielding gas option. Its problem is that it produces quite a messy arc. For high-quality welds, either argon or helium makes up the majority of the mix. Argon has a narrow profile, making it ideal for fillet and butt welds. Also, if you’re welding a non-ferrous material, such as magnesium or titanium, you’ll want to use 100% argon or helium. Oxygen can also be added to the mix to increase fluidity and arc stability. Helium is able to create the hottest arcs and is better for thick materials. The problem with helium is that it is more expensive and requires a higher flow rate. On beginner welders, however, sometimes, no gas is used at all. Instead, they will use flux-covered rods for the shielding. This can make them easier to use and is ideal if you're only going to be using your welder for small jobs.