Welding with Shielding Gas
That bottle of shielding gas behind your welder often goes overlooked but its importance can’t be overstated. In fact, your MIG and TIG joints wouldn’t be the same without it. There are a lot of components to welding, and your shielding gas plays a critical role in the process. Let’s take a look at what gases are used for shielding gas, the reasons for using them, and more.
What Do Shielding Gases Do?
The process of welding is the process of fusing like metals together. During this fusion, a molten ‘pool’ or weld pool is created as you weld. It is this pool of molten metal (and usually filler material) that we need to protect as it solidifies.
Why does it need protection? Simply, the atmosphere we breath is harmful to the weld joint and will cause a variety of problems if allowed to contaminate the joint. When you weld without a shielding gas, the molten metal is immediately exposed to the air. This brings it into contact with oxygen, carbon dioxide, and nitrogen. It also exposes the metal to any water vapor in the air.
These four gases can actually damage your weld. They can impact how fast the weld sets, how deep it penetrates, and what the final weld looks like. These gases can increase the porosity of the weld and cause more spatter as well.
How is Shielding Gas Applied?
Shielding gas is distributed over a weld via a special diffuser and out a nozzle contained in your MIG or TIG gun. Supply pressure is adjusted through a regulator at the gas cylinder and the gas is released as the MIG or TIG gun trigger is depressed.
As the shielding gas reaches the molten weld pool it acts as a shield protecting it from the other gases in the air. This ‘bubble’ of gas moves with the welding gun and follows the weld bead, maintaining the integrity of the joint. This shield is somewhat delicate and will be affected by the welder’s travel speed and environmental conditions (wind). Shielding gases can also have an impact on the arc heat and even the finished weld.
The Importance of the Nozzle
Nozzle selection is important as it directly affects how much or how little shielding gas is applied over the weld. Too narrow and there won’t be enough gas entering the weld pool for it to be effective. If it’s too wide, the shielding gas may not fully protect the molten metal. The right nozzle will evenly distribute gas over the weld while allowing the welder to maintain his/her travel speed.
Clean nozzles matter as well. A nozzle that’s clogged up with spatter will restrict the amount of shielding gas that reaches the weld. Nozzles should be replaced often during MIG welding as joint quality will suffer from dirty ones or fouled tips.
Two Types of Shielding Gases
There are two types of gases that can be used as shielding gases. We can broadly group them into inert gases and semi-inert gases. Gases classified as inert don’t react with most substances. ‘Noble gases’ such as helium and argon fall into this category. Semi-inert gases will have a slight reactivity and can be used to improve the characteristics of the arc and increase weld penetration. Each type has advantages for specific welding processes.
Inert Gases
The first inert gas used as a shielding gas is argon. Argon makes up about one percent of the air we breathe. In MIG welding, pure argon is good for nonferrous metals and aluminum, but its typically mixed with a few other gases for best results. When combined with a small amount of carbon dioxide, it stabilizes the arc, creates a more fluid weld pool and improves penetration. TIG on the other hand, has clear benefits from 100% argon. Using a pure inert gas in TIG welding aids in starting, promotes stable arcs, and keeps the TIG electrode clean.
Helium can also be used as shielding gas due to its thermal properties. Its very useful for welding aluminum and can be tailored for stainless steel as well. It excels in applications that require high heat or fast travel speed such as automated or computer-controlled seam welding. Since pure helium tends to create an erratic arc and doesn’t have as good ‘cleansing’ power, its usually mixed with argon for MIG welding. Helium’s high relative cost and recent scarcity have seen its use in the consumer sector decline however.
Semi-Inert Gases
Semi-inert or ‘active’ gases that are typically used in welding are carbon dioxide, hydrogen, nitrogen, and oxygen.
Carbon dioxide is a good shielding gas for welding steel. It works well in MIG welding providing good penetration and speed, but tends to cause more spatter in pure form. CO2 isn’t a good option for thin metals and performs best when combined with argon or another inert gas.
Hydrogen often isn’t used as a shielding gas on its own. Instead, it’s usually combined with other gases, including argon and carbon dioxide. Used in small quantities, it aids in penetration and welding speed. Many industrial applications have turned to hydrogen for stainless steel welding due to the high cost of helium. Hydrogen does require some precautions however, as there is the risk of porosity in mult-pass welds.
Nitrogen, like hydrogen is typically used as an additive to an existing shielding gas for welding special alloys. In low concentrations (1%-2%), it has a beneficial effect when TIG welding various stainless types such as duplex (Austenitic + Ferritic Stainless Steel). Higher concentrations can cause arc instability and increased electrode wear.
Oxygen is also used in small quantities with argon (or an argon/helium mix) for welding certain types of stainless steel. Benefits include arc stabilization and increased pool fluidity. Note that oxygen cannot be when welding certain metals such as aluminum, copper, or magnesium because it will cause oxidation.
Conclusion
Shielding gas plays a pivotal role in most types of welding. The type and mixture have far ranging affects on the welding process and the finished product. They can affect everything from the arc’s stability, to weld penetration and fusion characteristics.
The properties of these common gasses used for MIG and TIG welding are well understood as is there effects when combined in various percentages. This allows the welder or hobbyist to order a shielding gas that is best suited for a particular metal or joint. The gas can further be tailored by adding small amounts of semi-inert gasses for special welding conditions (automation) or TIG welding stainless steels.
Which shielding gas is right for your project? While there’s several variables to take into consideration (material thickness + type of wire for example), a good all-around gas for MIG welding is a 75%/25% Argon/CO2 mix or C25 as its commonly known. Less cleanup, less fumes and a better-looking weld.
Now for TIG, straight Argon is really the only way to go unless you want to practice sharpening tungsten electrodes. Whereas MIG welding can benefit a bit from adding in a semi-inert gas, TIG welding only requires and inert shielding gas. CO2 will react with the tungsten creating tungsten carbide and tungsten oxide and way more heat. The end result being flaws in your weld as well as an extremely short lifespan for your electrodes.
A basic understanding of how shielding gas affects a weld will make you a better welder. It gives yet another method of fine tuning your joints as long as you select the right one. For most of us, the basic argon mixes are all we need. Precise mixtures also exist for welding those exotic alloys or stainless, and anything in between.
