Choosing the proper shielding gas for MIG welding can greatly improve your weld quality as well as save you time and money. In order to choose the proper gas or gas mixture, you must take into consideration various factors including the material, transfer method, cost, and the amount of cleanup required.
What gas is used for MIG Welding? Inert gas is used for MIG welding or metal inert gas welding, use inert gas as a shielding gas. The shielding gas prevents unwanted contaminants like oxygen and nitrogen in the air from entering your welding area.
Shielding gases are used in certain welding processes and are composed of inert gas and/or semi-inert gas. Often semi-inert gasses or non-inert gasses are used in small percentages and technically turns the process into a GMAW welding process rather than a MIG welding process (“MIG Welding Gases”, n.d.).
Depending on whether you are welding ferrous or nonferrous metals, your most common options will range from a mixture of argon and carbon dioxide to pure forms of either. More expensive mixes are also available for welding stainless steel. Among the negative effects of choosing the wrong type of gas are burnouts, inconsistent bead patterns, excessive fumes, and spattering.
With all the options available, it might seem overwhelming at first. However, in this article, I will outline your best options for MIG welding based on several different applications. I will start with a basic overview of what a shielding gas is and what these gasses are composed of.
Inert Gasses: Argon and Helium
An inert gas, or a noble gas, is a gas that’s highly resistant to chemical change under certain circumstances. Argon and helium are the two inert gasses used in both MIG and TIG welding. Argon is the most commonly used inert gas for MIG welding, either on its own or mixed with one or two other gasses. Argon leads to shallow but wider weld penetration and stable arc, while helium burns much hotter than argon and is more expensive (“MIG Welding Gases”, n.d.).
Both argon and helium tend to be mixed with other gasses since both are comparatively expensive. Also, mixes made up only of inert gases like argon and helium are normally limited to use on nonferrous metals like aluminum and copper. Since inert gasses are resistant to a chemical reaction, they result in less spatter than semi-inert gasses. Another consideration is that while helium reduces the porosity of the weld, it also uses more power and more caution has to be exercised to avoid burnouts and overheating (“MIG Welding Gases”, n.d.).
Semi-Inert Gas and Non-Inert Gas
A semi-inert gas is still resistant to chemical change but to a lesser degree than inert gasses. Carbon dioxide (CO2) is a semi-inert gas that is frequently used in MIG welding, either by itself or mixed in small percentages with an inert gas. The main advantages of CO2 are that it’s cheaper than inert gas and it allows for deeper penetration of the metal. The main disadvantage of CO2 is that its harsher arc characteristics result in more spattering, which in turn will require more cleanup around the weld (“MIG Welding Gases”, n.d.).
Oxygen is a non-inert gas that is used in very small percentages to increase weld penetration for thicker metals and stainless steel. This sounds counterintuitive since oxygen is also responsible for the oxidation and rust found in faulty welds. However, in very small percentages (one to five percent), it helps to stabilize the arc and is less expensive than helium (“MIG Welding Gases”, n.d.).
The Advantages of C25 and Argon-CO2 Mixes
Each of these gasses with their various properties has its advantages and disadvantages, and there are various mixtures designed for specific purposes. One of the most common gas mixtures for MIG welding is a mix of 75 percent argon to 25 percent carbon dioxide known as C25. Mixtures of 80 percent argon and 20 percent CO2 as well as 90 percent argon and 10 percent CO2 are also common (“MIG Welding Gases”, n.d.).
While the C25 is more expensive than the 100 percent CO2, it is less expensive than the 100 percent argon. The higher the argon content, the higher the price. At the same time, the welding profile of the C25 is much narrower and results in less spatter and cleanup than 100 percent CO2. For most applications in MIG welding, C25 will likely be your go-to gas (“MIG Welding Gases”, n.d.).
While C25 is the most common option, your welding transfer method will also determine the optimal gas for you to use. Shielding gasses with 25 percent carbon dioxide or higher are best suited for short-arc or short-circuit welding. Higher levels of argon are better for globular transfer and spray arc welding since they help limit spatter. Again, the big tradeoff here is cost (“MIG Welding Gases”, n.d.).
The Best Gas for Welding Mild Steel
C25 and 100 percent CO2 are the most commonly used gasses for carbon steel with some mixture of argon, carbon dioxide, and oxygen often recommended as well (“MIG Welding Gases”, n.d.). Mild steel, or low-carbon steel, is easier to weld or machine and is more affordable than high carbon steel. Mild steel’s low carbon content and lower levels of other metal alloys while making it more affordable, also make it more prone to oxidation and rust (“What is Mild Steel?”, 2016).
Shielding gases composed purely of inert gas, like argon, are not well-suited for MIG welding steel as they frequently result in an
undercut and an ugly, inconsistent weld (“MIG Welding Gases”, n.d.). An undercut is a small groove at the top of a weld bead that was cut into the parent metal due to inconsistent travel speed or high voltage.
Mixtures of argon and anywhere from one to five percent oxygen are common for industrial applications like in the automotive industry for carbon steel and stainless steel (“Argon Oxygen Welding Mixtures”, 2012). The risk of oxidation and rust from these oxygen mixtures obviously rises with the lower carbon content of mild steel.
Tri-mix and Stainless Steel
For stainless steel, a tri-mix of helium, argon, and carbon dioxide or helium, argon, and oxygen is sometimes used. One example would be a tri-mix of 90 percent helium, 7.5 percent argon, and 2.5 percent CO2. Again, the added levels of helium increase heat while oxygen helps to penetrate thicker metals and stabilize the arc. Both CO2 and helium require higher voltages to sustain a stable arc, and helium is the most expensive of the inert gases. A more affordable and common option for stainless steel is C2, which is composed of 98 percent argon to 2 percent carbon dioxide (“MIG Welding Gases”, n.d.).
Will the same gas work for both MIG and TIG welding?
TIG welding, or tungsten inert gas welding, is another form of welding that uses an inert gas, however, TIG welding uses only inert gas. Also called GTAW (gas tungsten arc welding), TIG welding relies only on argon, helium, or a mixture of the two. While it is technically possible to weld with mixtures that contain gasses like carbon dioxide or oxygen, it is not at all recommended. Carbon dioxide will lead to flare-ups, overheating, and make quite a mess (Nguyen, 2015).
Since TIG welding requires 100 percent inert gas, you could potentially use 100 percent argon gas for TIG welding while being able to use the same gas to MIG-weld thin aluminum (less than ½ inch). For most other MIG welding applications, especially for steel, 100 percent argon gas is not recommended. Also, since TIG welding provides a better weld for aluminum using the same gas, it’s difficult to see why you wouldn’t just use the 100 percent argon to TIG weld the aluminum. If you do a lot of TIG welding and MIG welding, having one cylinder of C25 and one cylinder of 100 percent argon could cover a wide range of applications.
To learn more about the differences between mig and tig welding, please click here!
MIG Welding Gas Cylinders
Gas tanks or cylinders are available in various sizes such as 20, 40, 80, 125, 150, 250, and even 330 cubic feet. For something that you intend to move frequently, you’re probably looking at something in between 20 to 125 CF. Gas cylinders range in height from 14 inches to 55 inches, and the smaller ones can weigh anywhere from 13 lbs to 71 lbs full, so it’s important to consider how often you intend to move the cylinder (Jones, 2019).
For high-volume work, another option is to buy or lease a larger gas tank or cylinder. Many welders do this for cylinders larger than 125 CF. One major advantage of the larger cylinders is that you will pay much less per cubic foot of gas than you would for the smaller cylinders (Jones, 2019).
Gas cylinders are stamped with the month and the year that they were last certified. This normally lasts for five years unless there is a star stamped next to the date as well. This indicates that the certification is good for 10 years (Byers, 2019).
MIG Welding Gas Pressure
For most gas regulators, the gas pressure (PSI) from your gas flow regulator or pipeline is set at a minimum of 25 PSI and as high as 80 PSI. Regulators designed for CO2 will usually range from 50 to 80 PSI. On its way to your welding gun hose, this pressure is reduced through a needle valve or orifice of around 0.025-inch in diameter, which is usually factory preset somewhere between 3 to 8 PSI. The amount of pressure needed is dependent on restrictions in the gun nozzle caused by spattering or restrictions in the gun cable (Uttrachi, 2019).
When such restrictions occur in most gas flow systems that use such a “choked flow” or “critical flow” system, the automatic gas flow compensation adjusts to maintain the gas flow rate at around the average of 5 PSI. This is due to the gas pressure behind the choke point being at least 25 PSI (Uttrachi, 2019).
The downside of this is when you stop welding while the gas is till flowing, gas pressure will build up in the delivery hose to the same level as your gas regulator. Then, once you start welding again, gas will blast out of the welding gun nozzle. Not only is this a waste of gas, but it can also pull contaminants like moist air into the gas stream and weld area (Uttrachi, 2019).
Attempts to prevent this waste of gas led to the development of lower pressure gas flow regulators based on the 3 to 8 PSI needed in the delivery hose. What this does, however, is to eliminate the automatic gas flow compensation provided by having the choked flow system and can cause inferior starts. Unlike the relatively constant flow rate achieved by conventional systems, tests on the lower-pressure regulators revealed a fluctuating gas flow rate ranging between 16 and 37 CFH (Uttrachi, 2019).
MIG Welding Gas Flow Rate
Not to be confused with gas pressure (PSI), the gas flow rate is measured in cubic feet per hour (CFH). The gas flow rate needs to be just high enough to shield the weld, but a setting that’s too high can actually suck air into the weld (“MIG Welding Gases”, n.d.).
While in an enclosed area, the setting can be as low as 15 CFM, although a draft may require something toward the higher recommended end of the spectrum of around 50 CFH. The proper gas flow also varies based on the diameter of the nozzle. Always check the manufacturer recommendations for your welding equipment.
A MIG welding chart showing gas flow recommendations can usually be found in the welding machine. Still, these are guidelines, and finding the best gas flow setting involves some degree of trial and error (“MIG Welding Gases”, n.d.).
Among the other factors to take into consideration when setting your flow rate are your welding surface and welding speed. Welding flat surfaces require a higher gas flow than welding grooved materials. Fillet welds typically require the lowest flow rates since they are shielded from drafts, while butt welds require higher gas flow since they are not. Increasing your welding speed will also require a higher gas flow as will welding a thicker material.
How long your gas will last will depend heavily on your gas flow rate. It’s relatively easy to calculate your welding time by dividing your cylinder volume in cubic feet (CF) by your regulator flow rate in cubic feet per hour (CFH).
Let’s say you’re indoors and at the low CFH setting of 15. With a 20 cubic foot cylinder, you’re looking at about an hour and 18 minutes of MIG welding time. With a larger tank of 125 CF at the same setting, that would be about 8 hours and 18 minutes. On a higher setting like 40 CFH, that would be only a half an hour for the 20 CF cylinder and three hours for the 125 CF cylinder (Jones, 2019).
Pick What Works Best for You
If you’re looking for the best option available for a MIG welding shielding gas with the broadest application, the 75 percent argon and 25 percent CO2, or something close to it like an 80/20 mix, is likely to be your best bet. If you’re on a budget and you don’t mind cleaning up a little extra spatter, carbon dioxide is cheap and great for hobby welding or experimentation. For MIG welding aluminum, or for TIG welding in general, 100 percent argon will be the way to go.
Stainless steel is where things tend to get more expensive with higher levels of helium mixed with argon, and CO2 or oxygen. Even there, you have the option of the less expensive C2 with a 98/20 mix.
Always consider carefully what materials you are trying to weld and be sure to make the most of your gas flow rate. Remember that your choice of gas and your gas flow rate is key to a consistent bead pattern and avoiding overheating the material. Consult the manufacturer’s recommendations for gas flow settings and experiment to see what works best for you.
Argon Oxygen Welding Mixtures. (2012). Retrieved from https://www.praxairdirect.com/Industrial-Gas-and-Welding-Information-Center/Welding-and-Fuel-Gases/StarGold/Argon-Oxygen.html
Byers, B. (2019). Argon Tank Sizes for MIG Welding, the Best Tips and Tricks. Retrieved from https://welditmyself.com/argon-tank-sizes/
Jones, D. (2019). What Size Gas Cylinder for MIG Welding? It’s Decision Time. Retrieved from https://welditu.com/welding/tips-mig/what-size-gas-cylinder/
MIG Welding Gases. (n.d.). Retrieved from https://gowelding.org/welding/mig-gmaw/gasses/
Nguyen, O. (2015). What is GTAW (Gas Tungsten Arc Welding)? Retrieved from https://www.weldingschool.com/blog/welding/what-is-gtaw-gas-tungsten-arc-welding/
Uttrachi, J. (2019). MIG Shielding Gas Control and Optimization. Retrieved from http://netwelding.com/Shielding_Gas_Control_Download.pdf
What is Mild Steel? (2016). Retrieved from https://www.metalsupermarkets.com/what-is-mild-steel/