Shielding Gas — Selection Guide

Shielding gas protects the molten weld puddle from nitrogen, oxygen, and hydrogen in the atmosphere. Without it, you get porosity, oxidation, and welds that look like Swiss cheese. The gas you choose also directly affects arc characteristics, penetration depth, spatter levels, and bead shape.

For MIG welding mild steel, 75% argon / 25% CO2 is the industry default. This blend gives a stable spray-transfer arc above 18-19 volts, manageable spatter, and good penetration. Straight CO2 is cheaper and digs deeper into thick plate, but it runs in globular transfer with heavy spatter. For thin sheet metal, 90/10 argon/CO2 reduces heat input and spatter at the cost of some penetration.

TIG welding uses 100% argon for steel, stainless, and most alloys. The inert gas provides a stable, focused arc without reacting with the tungsten or base metal. For aluminum TIG, some shops add 25-50% helium to argon for deeper penetration and faster travel speeds on thick material. Helium raises the arc voltage and increases heat input, which helps overcome aluminum’s high thermal conductivity.

Stainless steel MIG typically runs a tri-mix blend: 90% helium / 7.5% argon / 2.5% CO2 is the classic formulation. This reduces heat input compared to standard argon/CO2 mixes and helps prevent carbide precipitation. Some shops use 98% argon / 2% CO2 as a simpler alternative with good results on thin stainless.

Flow rate matters as much as gas selection. Too low and you lose coverage, getting porosity. Too high and you create turbulence that sucks in outside air. Start at 20-25 CFH for MIG and 15-20 CFH for TIG. Drafty shops need higher flow or wind screens. Outdoors, switch to flux-cored wire instead of fighting the wind with gas flow.

The guides below cover gas selection charts by process and material, cylinder sizes, regulator setup, and flow rate optimization.