MIG welding with shielding gases produces cleaner and faster welds, and removes the need to frequently stop to replace electrodes like you would in stick welding. Reduced clean up and increased efficiency also come with using shielding gases, but it helps to understand the role these gases play in the welding process, as well as the different gases available and their specific properties.
The main intent of using a shielding gas is to avoid exposing the molten weld pool to the oxygen, hydrogen, and nitrogen in the air around you. Various problems can arise from the reaction of these elements in the weld pool, including excessive spatter, and holes in the weld bead, known as porosity, that results in weaker welds.
Technically, when carbon dioxide or oxygen is used, it is no longer MIG, or Metal Inert Gas, welding. It is then MAG, or Metal Active Gas, welding. This is because neither carbon dioxide nor oxygen is an inert gas. MIG welding utilizes inert shielding gases, such as helium or argon, where as MAG uses active gases instead.
Choosing the Right Gas
Different gases play different roles in the weld process, from weld penetration to arc stability to the finished weld itself. Choosing the consumables that provide continuous and even gas delivery is also a very important aspect to consider in your MIG welds.
Be sure to evaluate your project goals in order to select the right gas for the weld at hand. Things to keep in mind when selecting are cost, what preparation entails, the base material you’ll be welding, the finished weld properties, and what needs to be done during the post-weld clean up.
The four most common shielding gases used in MIG welding are carbon dioxide, argon, oxygen, and helium. Each has their unique benefits, and drawbacks, in any given implementation.
Of course, it is always a good idea to consult with your supplier for recommendations on gases that match the welding wire you’ll be using. You can even consult with the wire’s manufacturer for suggestions too. They will most likely provide several options, ranging from the best gas option to gas that will provide the minimum acceptable welds, as well as their prices. However, your MIG welder may have an electrode and gas recommendation guide on the inside panel which will provide you a list of several options.
Carbon Dioxide (CO2)
CO2 is, by far, the most common and is one of the only gases that can be used in its pure form without needing the addition of an inert gas, such as argon or helium. Because of this, CO2 is the most cost effective option and a good choice if project costs are a priority.
Pure CO2, also known as 100% CO2, provides a deep weld penetration, making it handy when needing to weld thick materials. That being said, pure CO2 is limited to only the short circuit welding process, and produces a less than stable arc as well as more spatter than when it is combined with other gases (also known as ‘mixed gases’). Pure CO2 is good for projects where the aesthetics of the weld are either not important, or the weld cannot be seen, such as on the underside of a car. Post weld clean up is also a little more involved.
Argon allows for narrower penetration, which is handy for butt and fillet welds. It also boasts a smooth and relatively fluid arc. If you are going to be welding non-ferrous metals, like titanium, aluminum, or magnesium, you’ll need to use pure argon. Argon is also often mixed with hydrogen, helium, or oxygen. This helps intensify arc characteristics and aid in metal transfer.
If weld quality and aesthetic are important, mixed gases are good to use. You have several options that vary from between 75-95% argon to 5-25% CO2. They produce better arc stability and reduce spatter compared to 100% CO2. Mixed gases can also be used in the spray transfer process that, in turn, provides more visually appealing welds as well as increased productivity. Argon/CO2 mixtures are good for welding low-alloy, some stainless steels, and carbon metals. Be aware, however, that higher CO2 levels may cause increased spatter.
A reactive gas, oxygen is typically used in small amounts when added to shielding gases, usually between 1-9%. This improves weld pool fluidity, as well as arc stability and penetration in stainless steel, mild carbon, and low alloy metals. It is not recommended to use oxygen with aluminum, copper, magnesium, or other exotic metals since it can cause oxidation.
Oxygen/argon blends are typically used on stainless steel and plain carbon metals. It produces a stable arc with limited spatter. Higher levels of oxygen, however, may make out-of-position welding hard due to the fact that it will increase puddle fluidity.
Generally used on non-ferrous metals, helium can also be used on stainless steels. It works well with thick metals due to its wide and deep penetration abilities. It is usually used in ratios of 25-75% helium to 75-25% argon. By adjusting these ratios, you can alter the penetration and bead profile. When using on stainless steels, helium is usually used in a tri-mix gas combination with CO2 and argon. Helium is also used to prevent oxidation during the welding of metals like stainless steel, aluminum, magnesium, and copper alloys.
Helium does create a hotter arc, which provides faster travel speeds and, thus, increased productivity. That being said, helium is more expensive and does require a higher flow rate than argon does. Weighing out the value of the cost of the gas against productivity rates is important to keep in mind when considering using helium.
Hydrogen serves as a shielding gas in high-temperature applications, such as for stainless steels. It is often mixed with argon for use on austenitic stainless steel.
Nitrogen is used as a purging gas for welding stainless steel tubes. Added to argon in small amounts, it can also be used as a shielding gas for stainless steels.
Propane is typically used in scrap yards for cutting carbon steel where cut quality is not important. If your application does not require high cut quality, propane is a rather cost-effective option.
What consumables you attach to your MIG gun are just as important as selecting the correct gas to use. The diffuser, contact tip, and nozzle all play an important role in making sure the weld pool is adequately protected from the air around you. If your diffuser is clogged with any spatter or if your nozzle is too narrow, you run the risk of too little shielding gas getting out to protect the weld pool. This allows pockets of air into the gas, which can lead to spatter, porosity, and even contaminated welds.
Ensure you are selecting MIG gun consumables that can resist the build up of spatter and also have a wide enough nozzle bore to be confident in your shielding gas protection. Some manufacturers makes nozzles with a built in spatter guard, which can double your gas diffusion, which will give you a much more consistent gas flow. The selection of your consumables does demand careful evaluation of the pieces, as well as the project at hand, and your operational priorities.