What Different Welding Gases are for!
Shielding Gas is necessary for Welding because it's essential to keep Oxygen from the weld pool. Shielding gas does this by replacing the Oxygen containing air that's all around the weld pool and, of course, all around us!
Shielding Gases for Mig Welders & Tig Welders fall into two basic categories "Inert" & "Active".
The most commonly used Inert gas is Argon. Argon is the main shielding gas used for Tig Welding most metals and Mig Welding Aluminium or Mig Brazing.
The most common Active gases are 100% Co2 and a mixture of Argon+Co2. Active gases are primarily used for Mig Welding most metals (except Aluminium and Mig Brazing).
Co2 is the cheapest of the Active Gases, but is far from the best. Using Co2 produces a cooler, coarser, more spattery arc and a slightly harder weld deposit. Co2 is a particularly challenging gas to use on thin material and not all Mig Welders perform well on thicker material with 100% Co2 as a shielding gas! An Argon/Co2 mix produces far superior results as the arc is softer, and smoother and the resulting weld deposit is slightly softer and more malleable.
There are several different mixes of Argon/Co2 on the market, typically:
95% Argon + 5% Co2, which is ideal for Mig Welding up to 8mm Steel
90% Argon + 10% Co2, which is ideal for Mig Welding from 8-25mm Steel
80% Argon + 20% Co2, which is ideal for Mig Welding 20mm plus
Disposable Cylinders are generally an 86% Argon + 14% Co2 mix, which is not ideal for the market that they're aimed at, but its an OK mix!
So what's the difference between "Inert" and "Active" gases"
An Inert gas, such as Argon, has no effect, or reaction, on or with the welding process, it simply performs the essential task of replacing Air/Oxygen from around the weld pool. Active gases do have an effect on the welding process. The effect of an active gas on Mig Welding is two fold. Firstly, the Co2 content in an Argon/Co2 mix makes the gas slightly electrically conductive, this in turn raises the arc voltage, which increases penetration. The second effect is the Co2 content breaks down the surface tension of the molten weld pool (this is the same type of surface tension that allows water to form a drip). Using Co2 to break the surface tension of the molten weld pool allows the weld to flow and flatten slightly for the correct weld deposit profile. The use of Active Gases for most Mig Welding applications means that most people are actually MAG welding (Metal Active Gas) rather than MIG (Metal Inert Gas) welding, however, MIG has become the generic term, despite being inaccurate!
If an active gas is used for Tig Welding the raised arc voltage will increase hole blows and cause excessive burning of the Tungsten Electrode.
If an Inert gas is used for Mig Welding (except Aluminium and Mig Braze), higher machine power output will be needed to achieve penetration and the finished weld will look too tall because the surface tension has not been broken.
Disposable Gas Cylinders
Although Disposable Cylinders of Co2 are usually the same price as Argon+Co2 and the cylinders are the same physical size, you should be aware that because 100% Co2 is stored as a liquid and Argon+Co2 is stored as a compressed gas, a disposable Cylinder of 100% Co2 will last approximately twice as long as an Argon+Co2 bottle. Our advice to DIY welders using disposable cylinders of gas is to use Argon+Co2 for mild steel where weld quality is important and for Stainless Steel and use 100% Co2 for welds where quality is of secondary importance.