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MIG welding stands for Metal Inert Gas and is a more common name for GMAW (Gas Metal Arc Welding). MIG welding is probably what you picture when you think of welding as it is the most common type of welding process due to being easy to learn and versatile.
As the name suggests MIG welding does require gas. The gas in MIG welding acts as a shield that protects the welding pool from reacting with elements in the surrounding environment. There are several different options for gases in MIG welding, and the choice of gas will impact the overall weld.
Gas is a major component of the MIG welding process. Keep reading to learn how and why gas is used in MIG welding, what types of gas can be used, and how to ensure that the gas in a MIG weld is being used effectively.
What Exactly is MIG Welding, and Why Does It Require Gas?
We already know that MIG (Metal Inert Gas) welding has gas in the name, but besides the name what does gas have to do with joining two pieces of metal together with more hot metal? To answer this question, we first need to understand exactly what MIG welding is.
What is MIG Welding?
MIG welding is a type of arc welding. Arc welding produces an electrical current between the parent metal (the metal being joined) and the filler metal (the metal being used as glue) which produces heat to melt the filler metal and create a welding pool of molten metal. This welding pool joins the parent metal and creates the weld.
MIG welding is fast and can be used on almost any type and thickness of metal. There are different choices of filler metal, which is usually in the form of a wire fed through the welding gun, as well as different choices of gas that impact the welding.
What Does the Gas Do in MIG Welding?
The gas used in MIG welding is commonly known as the shielding gas, which gives away its main function, shielding the weld. While hot molten metal is great for welding, it is also a highly reactive substance. When the arc and welding pool reacts with chemicals in the surrounding air it creates spatter and a porous weld.
Shielding gas creates a protective layer around the weld to prevent chemical reactions with the air. This is where the inert part of the name comes into play. The gases used as shielding gases are inert, meaning that they do not react easily. The inert gases thus form a non-reactive boundary between the weld and the reactive chemicals in the air.
Creating a shield is not the only thing that the gas does in MIG welding. The gas also has an impact on weld penetration and the stability of the arc. The shielding gas is just as important of a component of MIG welding as your choice of filler metal and the power of the electrical current.
What Kinds of Gas Are Used in MIG Welding?
There are a few different options when it comes to selecting a gas for your MIG welding project. Each of the gases has different advantages and disadvantages which include things like cost, amount of spatter, and weld penetration.
Before we get into the different kinds of gas, one point needs to be clarified. Although shielding gases are meant to prevent chemical reactions, there are not all technically inert gases. Truly inert gases are the nobles gases such as argon and helium.
You can use some of the nobles gases for welding, but a 100% noble gas will be costly. Therefore, MIG welding also makes use of non-inert gases, which are reactive with some chemicals but not those used in welding. Many gases used for MIG welding will be a mixture of inert and non-inert gases.
Now that you know the basics, we will look at the four types of gas most commonly used for MIG welding.
Carbon dioxide (CO2) is the most commonly used gas in MIG welding, mainly because it is the cheapest. Carbon dioxide is not actually an inert gas, but it is not reactive enough that it works well as a shielding gas for welding. This is the only non-inert gas that can be used as a shielding gas without adding some inert gas to the mix.
The advantage of CO2 as a shielding gas is that it allows for deeper weld penetration, which makes it a good choice for welding thick metal. The disadvantage when using pure CO2 is that it makes for less arc stability which increases the amount of spatter. When using 100% carbon dioxide you can expect a deep weld that will require a little more cleaning.
Argon is a noble gas, making it an inert gas. It makes up the majority of the most popular MIG welding gas which is a 75% argon to 25% carbon dioxide mixture.
Argon is more expensive than carbon but provides a higher quality weld. The spatter is greatly reduced with argon as compared to pure CO2 resulting in a much cleaner weld. Argon is also great for welding jobs that require a narrow penetration.
Because argon is expensive, it is common to mix ratios of argon and carbon dioxide to bring down the price while also preventing the high splatter rate of 100% CO2. However, if you are welding non-ferrous metals such as aluminum or titanium, you will need to use 100% argon.
If you remember anything from high school chemistry then you may be wondering why oxygen is on this list. Oxygen is actually a highly reactive gas, and in fact, is one of the chemicals from which the shielding gas is designed to protect the arc and welding pool. How then could it be used as a shielding gas?
Oxygen is never used in large quantities as shielding gas for MIG welding. However, in small percentages within the overall shielding gas mixture, it has notable benefits. It makes the weld pool more fluid, gives great stability to the arc, and allows for deeper penetration.
While oxygen can be helpful when welding stainless steel and carbon metals, it is not a good idea to use a shielding gas containing oxygen on metals that will easily oxidize. This includes metals such as copper, aluminum, magnesium, and others.
Besides argon, helium is the other true inert gas that is used in MIG welding. Helium has a wide and deep penetration making it ideal for thick metals. Helium is also good for preventing oxidation on metals such as aluminum and magnesium. It has a high burn rate which results in an overall faster welding speed.
The downside to helium is the expense. To reduce cost helium often comes in mixtures with argon, which can vary from 25% helium and 75% argon to 75% argon and 25% helium. The different ratios will cause changes in the beading of the weld and penetration levels.
Helium can also be combined with carbon dioxide and oxygen. This three-part shielding gas is typically used on stainless steel.
Besides the most common four, there are other possible gases that can be used during MIG welding. These less common gases tend to have specific uses.
- Propane: Propane is a cheap gas that is used when quality is not a factor. This gas is often used for cutting metal in scrap yards.
- Hydrogen: Hydrogen is suited for especially hot welds. It can often be found in use with stainless steel, and it can be mixed with argon.
- Nitrogen: Nitrogen is used as a purging gas. It tends to be used for welding stainless steel tubes. Like hydrogen, it can be mixed with argon.
MIG Welding Gases and Transfer Type
We have already mentioned that the types of gases you use to weld affects both penetration and arc stability, but the shielding gas you use will also determine what transfer type with which you are welding. There are four transfer types.
This transfer type has the consumable metal touching the base metal. This generates a short circuit that creates a low heat welding mode with a small weld pool. The low heat means that the filler solidifies quickly.
Short circuit transfer does not require a lot of shielding gas. Because of this, carbon dioxide is typically used as the most cost-effective option.
Globular transfer allows thick drops of the wire consumable to fall on the welding area at a slow rate. This occurs because the consumable is heated longer so that larger globes of molten metal form.
Argon shielding gas tends to be used with globular transfer. It does not have to be 100% argon, but at least 75% argon tends to work best.
Spray transfer uses tiny droplets of molten metal, which are sprayed onto the base metal in the form of a mist. The heat has to be quite high for this transfer type.
Like globular transfer the shielding gas for spray transfer is argon. You should have at least 75% argon in your shielding gas for spray transfer, and you may need 100% depending on the metal.
This is a combination of spray transfer and globular transfer. In pulsed transfer the voltage pulses create a drop of the molten consumable with each pulse. You will need a power source that can create a pulsed voltage to achieve pulsed transfer.
Because pulsed transfer has more to do with the voltage rather than gas choice, there is not a particular shielding gas that goes with this transfer type.
How Do You Ensure That You are Getting the Most from Your Shielding Gas?
As you have seen, gas is an essential component of MIG welding, which means that if you want your welds to be successful you need to understand how to use your gas properly. Here are some things you need to know about the gases when MIG welding.
Use Appropriate Consumables
A gas is only as good as the equipment which distributes it. Your nozzle and diffuser play a large role in how well the gas shields your weld.
You need a wide nozzle to ensure a large enough volume of gas is distributed to fully surround your welding pool and arc. You also need to make sure your diffuser is clean as the buildup of material will interrupt the flow of gas. Without a wide nozzle and clean diffuser air can reach your weld causing porosity, increased spatter, and ultimately ruin a weld.
Some MIG welding gear comes with a spatter guard which achieves even better shielding gas coverage.
Pair Your Shielding Gas with the Right Metal
One of the most important aspects of achieving a good MIG weld is pairing the correct gas and metal. If you use oxygen with a metal that oxidizes easily you will quickly have problems. Here are the most common metals, and the shielding gases that work best with them.
- Aluminum: 100% argon works best as a shielding gas for aluminum. You may also add helium to your argon if you need to work faster or require deeper penetration. Above all do not use oxygen with aluminum as it will create oxidation.
- Mild Steel: You can cut mild steel with 100% carbon dioxide. If you want a cleaner weld then argon is recommended. For thicker pieces, you may need to add oxygen in order to make argon effective.
- Carbon Steel: Carbon dioxide works well with this material. You can use either pure CO2, or you can add some argon.
- Stainless Steel: A majority argon mixture with either a little carbon dioxide or oxygen is usual for working with stainless steel. A trimix with helium, argon, and carbon dioxide is also suitable.
Proper Flow Rate
Another thing that affects the shielding gas is the flow rate. If the gas is coming out too slow it will not be able to provide full coverage. On the other side, if it comes out too fast, it will become messy instead of giving even coverage.
What the proper flow rate changes depending on the job. If you increase the weld pool or travel speed, you will also need to increase the gas flow rate to provide larger coverage. Finding the exact balance here may require some trial and error.
“Gasless” MIG Welding
Some people may claim that they MIG weld without gas. How is this possible? MIG welding has gas in the name! The answer is that it isn’t possible. MIG welding must have gas. However, that gas does not necessarily have to come in the form of a gas canister.
So where is the gas coming from if not from a canister? Gasless MIG welding uses flux core wiring which acts as a self-shielding material. When the wire melts so does the flux which turns into a gas. Thus gas is produced in this method but without the use of a gas cylinder.
“Gasless” MIG welding can be beneficial in both its reduced cost and smaller setup. However, you need a specific type of MIG welding machine that supports this type and it is not as applicable to as many metals as MIG welding with a gas canister.
In general “gasless” MIG welding is used for small projects by people who rarely need to weld. More avid welders usually rely on gas canisters as these MIG welding rigs have a wider range of uses and can handle bigger projects.
How Do I Know What Shielding Gas to Use with My MIG Welding Project?
All of this information about MIG welding and gases can be overwhelming, but there are only a few questions you need to ask yourself to decide what gas to use.
- Is cost an issue? If you are looking to cut costs then there is no doubt that carbon dioxide is the best choice. However, if the project calls for a higher quality gas you can still consider using a mixture to save money as well.
- What metal am I using? The metals you are working with make a big difference in the choice of shielding gas. You can consult our previous section to discover what metals pair well with which gases. It is also useful to consult the labels on your wire consumables and gas for information regarding the combinations of metals and gases as well.
- What transfer type will work best? If you plan to weld with globular or spray transfer types, then you will need to be using argon.
- How thick is the material you are welding? If you are welding a thick material you will need to achieve deeper welding penetration. Pure carbon dioxide has deep penetration, and adding helium to a gas mixture can increase penetration.
Gas is an essential part of MIG welding and requires some forethought and understanding to use effectively in a welding project. Being aware of what types of gas you can use and the advantages and disadvantages of each type are necessary knowledge for any MIG welder!