Burn Baby, Burn
If you are an off-roader, it is more than likely that you have had to weld something on to your vehicle to make it perform better or suit a task that you have determined necessary. There are three common types of welding methods used in commercial application: SMAW, GMAW and GTAW—or more commonly known as, Stick, MIG, and TIG.
All three of these processes use a shielding agent to protect the raw weld from oxygen and water vapor. This is necessary because the atmospheric gasses and contaminants can reduce the quality of the weld or make the process more difficult. To make matters more confusing, the individual metal choice requires certain blends of different welding gasses to ensure the best weld.
But, lets back up for a moment: It is important to point out that not all shielded welding processes use gasses for their shielding agent. Stick welding and some MIG processes use shielded wire, which is a coating on the stick/wire itself that burns as the wire melts creating a protective pocket in which the weld can fully mature and then crusts over. While its effective at its job, especially in less than ideal conditions like dirty surfaces, wind, or extremely thick plate; it comes with the faults of having less control and more clean-up work. When shielded wire/stick welds are finished, they’re covered in what is called slag. This is the protective crust that was the by-product of the shielding on the wire/stick burning. This slag needs to be knocked off or removed to expose the finished weld.
Shield gas welding is a separate method that uses actual inert gas from a cylinder, being pumped to the electrode. This is the preferred method for MIG and TIG welding due to the control, low weld spatter, and the ability to make welds works of art.
The common shielding gasses are based in two categories, inert and semi-inert. Only two of the noble gasses are produced enough to be cost effective for shielding, helium and argon. These two are used in TIG and MIG welding for non-ferrous metals. The semi-inert categories include carbon dioxide, nitrogen, oxygen and hydrogen. These four are used for welding ferrous metals. The reasons why these gasses are chosen is based on their ability to conduct heat, its transfer, and their density relative to air and their ionization process.
Argon is extremely common and is widely used as a base for more specialized gas mixes. For example, standard low-carbon steel welding uses a mixture of 75-percent argon, and 25-percent carbon dioxide. The CO2, despite being the least expensive gas, is used in moderation because, while it provides deep penetration, it reduces the arc stability when welding and increases spatter.
Helium is used in situations that require its very high thermal conductivity and its resistance to ionization. Because of its high ionization resistance, it requires a high voltage to strike the initial arc, and burns much hotter due to its high voltage. This provides deep penetration, which is a big advantage for non-ferrous metals like aluminum, magnesium and copper alloys. Generally, due to the cost of helium and the need for other specific traits, other gasses are blended with helium, like argon and carbon dioxide. This blend of three is known as tri-mix, and is the go-to for welding stainless-steel.
Oxygen is used, although rare as its presence encourages rapid oxidation and can lead to brittleness. As such it’s not used when welding aluminum, magnesium, copper and other exotics. Generally, oxygen is injected for special scenarios for mild carbon, low alloy and stainless steels.
The last major gas to be used is hydrogen, and it is used for the welding of nickel and thicker stainless-steel plates.
If you would like to know more about the various blends of shielding gas and their uses, Lincoln Electric has a fantastic write-up here.
