Shielding gas mixture
Shielding gas mixture is a combination of argon, CO2, or helium. It controls the arc and heat input and reduces spatter and porosity. This lets the welder adjust penetration exactly to material and plate thickness.
Physical-Chemical Mechanism of Action
In the arc, the gases ionize. Argon ionizes easily, the arc burns quietly and the penetration is shallow. CO2 breaks down at high temperature into CO and O, which costs energy from the arc. The arc gets hotter and digs deeper into the workpiece. Helium is hard to ionize but produces a very hot arc with high heat transfer. The mixing ratio determines the energy density and the weld pool behavior.
Metallurgical Relevance & Risks in Welding
CO2 in the shielding gas breaks down in the arc. The released oxygen oxidizes alloying elements like manganese and silicon. The carbon from the CO2 can enter the weld and increase hardness. Too much CO2 (over 25 %) makes the weld brittle and crack-prone. Too little CO2 (under 5 %) leads to shallow penetration and poor fusion at the toes. Helium greatly increases heat input, causing distortion on thin sheets. Wrong gas for the material causes porosity or oxidation.
Workshop Practice & Action Instructions
Choose the mixed gas according to the material: For mild steel use 82 % argon + 18 % CO2. For stainless steel use 97.5 % argon + 2.5 % CO2. For aluminum use pure argon or 70 % argon + 30 % helium. Set the gas flow to 10 to 15 liters per minute. Check the gas nozzle for blockage before each weld. Store gas cylinders upright and dry.