If you’re familiar with the process of gas metal arc welding, you know how important using the right shielding gas is. It can mean the difference between a strong and appealing weld or a weld that could be problematic. The hard part is knowing which gases will work with different welding applications and the practical difference between the different mixtures that can be used.
A shielding gas is used to keep the air atmosphere and contaminants away from the weld pool. If a shielding gas is not used, the weld pool reacts to the oxygen, nitrogen and hydrogen that are present in the atmosphere. A shielding gas also helps keep small particles that are in the atmosphere from interacting with the weld. If a shielding gas was not used the reaction from the atmosphere and the weld would result in porosity in the finished weld. Porosity is a term used to describe air bubbles that have become trapped inside the weld bead and can affect the strength of the weld.
The main purpose of using a shielding gas while welding is to protect the weld from air. It is also important in producing good weld penetration, arc stability, mechanical properties of the completed weld and transfer process. The use of a shielding gas is important in order to create a weld that is strong and free of contaminants. This is why choosing the right type of shielding gas for your job is very important.
There are four common gases that are used in gas metal arc welding; argon, helium, CO2 and oxygen. Each of these different gases produces different types of shielding gases which makes them suitable for a multitude of welding processes. The most common shielding gas used is CO2, which is the only of the four gases that can be used in its pure form without needing the addition of an inert gas. CO2 routinely produces very deep penetration but can produce an unstable welding arc and an excess of spatter.
Oxygen is the second most popular reactive gas metal arc welding gas. It is typically used in quantities of nine percent or less. It can be used in spray transfer welding on mild carbon, low-alloy and stainless steels. The use of oxygen as a shielding gas improves the weld pool fluidity, penetration and arc stability. Since it can cause oxidation of the weld metal surface, it is not recommended for use with aluminum, magnesium and copper.
Argon is the third most common type of shielding gas used by welders. It can be used as a shielding gas for almost any type of material, when used in its pure form; however, it should only be used with nonferrous metals such as aluminum, magnesium and titanium. Argon is known for producing a narrow arc cone and therefore a narrow penetration weld. When argon is combined with oxygen or CO2, it improves the stability of the arc and puddle control.
Helium is the more limited of the shielding gases. It is commonly used when welding stainless steel and is commonly mixed with argon and CO2. Helium produces a wide, deep weld bead and because of that it is limited to use on thick sections of aluminum and other types of nonferrous metals. When helium and argon are combined, it seems to allow for the best qualities of both gases to shine through. Helium tends to be more expensive than argon and because helium is lighter than air it requires the use of a higher flow rate to protect the quality of the weld puddle.
Knowing which gas will work the best in certain conditions is something that a welder will learn over time. Depending on the type of job you are doing, you may have to use all or a mixture of the four shielding gases to produce a high quality weld.
Reference: http://www.thefabricator.com/article/arcwelding/great-welds-need-the-right-gas
