In the process, we will also learn different types of shielding gases that are commonly used in MIG Welding.
A Brief Note on MIG Welding
Gas Metal Arc Welding or GMAW is the technical name to MIG Welding by the American Welding Society. Shortly after the development of the TIG Welding, engineers and scientists working on a new welding process. In this process, they used a continuous feed of metal wire as an electrode instead of Tungsten electrode in TIG Welding. Also, they protected the hot electrode and workpiece with the help of argon or helium, which we know are inert gases. Combining the “Metal” wire electrode and the “Inert” gas shielding, we have the Metal Inert Gas Welding or MIG Welding. Later, scientists found that using Carbon Dioxide for protecting the welding spot (either directly or in combination with Argon) provided better results. As the protection is no longer by just using an inert gas, American Welding Society renamed the welding process to Gas Metal Arc Welding or GMAW. But the name MIG Welding is still popular all over the world. The main difference between TIG Welding and MIG Welding is the type of electrode in the respective welding process. If you remember, we discussed the TIG Welding Process in the previous TIG Welding Gas Guide, we mentioned that the main electrode in TIG Welding Process is, well, Tungsten. This Tungsten Electrode is not a consumable i.e., its job is just carrying the current from the welding machine to create an arc. It doesn’t melt or reduce in size. This is where things differ with MIG Welding. MIG Welding also has an electrode that carries the electric current to create an arc. This electrode is a thin metal wire that is consumed while welding i.e., it melts and becomes a part of the welding process as a filer material. As a result, you need a continuous supply of the metal wire to perform MIG Welding.
Importance of Shielding Gas
We know that Arc Welding is a high-temperature process where we melt the metals to weld them. While the high temperature helps in melting the metals, it also causes an important problem that can ruin the process. What is this problem? Temperatures close to the melting point of a metal can accelerate the surrounding air and causes a reaction with the molten metal or the electrode. This can contaminate the welding process and might lead to welding deformities, welding cracks, porosity, formation of slags, making the welding spot brittle, or any other. The two main culprits in this contamination are oxygen and nitrogen. When a metal at a very high temperature reacts with oxygen, it forms oxides. This is bad for the longevity of the weld. There are two problems with nitrogen. One is as the weld cools and the molten metal solidifies, the trapped nitrogen gas in the form of bubbles escapes and this results in the formation of pores in the weld. When atmospheric nitrogen mixes with molten metal, it forms nitrides. Metal nitrides can make the weld brittle. So, what can we do to prevent this from happening? A simple and reliable approach is to shield the welding process from the surrounding atmosphere. There are two ways we can do this; Shielding Gas and Flux.
Gas Shielding
Shielding Gases form an envelope around the molten metal and prevent any contaminants to enter the welding zone and react with the metal. Gases essentially purge the surrounding gases from the metal and create a shield around the weld spot. Welding Torches or Guns are designed to direct the shielding gas around the metal and electrode to protect them. Both TIG Welding and MIG Welding use shielding gases. In the later section, we will see more details about MIG Welding Gas. In an earlier guide, we already saw the TIG Welding Gas.
Flux Shielding
Welding Flux is also another way to shield the molten pool of metal and the electrode. Shielded Metal Arc Welding, Submerged Arc Welding, Plasma Arc Welding, etc. use fluxes for shielding. There are a couple of ways in which you can use flux shielding. The first and simple approach is to apply a solid flux on the surface of the metal before welding. Next, you can use it with the electrode wire through a binding agent. Alternatively, some welding processes have flux as a core in the wire itself. Irrespective of how you apply it, welding flux provides shielding action in two ways. The solid (or semi-solid goop) flux will melt when it meets the heat of the arc. The liquid flux will react with the contaminants on the surface of the metal and forms solid slags on top of the molten metal. Once the metal solidifies, you have to clean the surface to remove the flux slags. The next way is quite interesting. When we start the welding process, the high-temperature arc will decompose the flux and releases carbon dioxide into the surroundings. This will push the surrounding air and prevents any contamination.
MIG Welding Gas
Shielding Gas is an important part of the MIG Welding Process. It forms a protective shield around the weld pool to prevent contaminants from entering the hot and molten welding electrode or the workpiece. Apart from protecting the welding spot from contamination, the MIG Welding Gas also influences the following:
Arc Characteristics during welding Depth of the weld (penetration) and width of the weld Shape of the weld as well as speed of the welding process
In the early days of MIG Welding i.e., in late 1940s, inert gases such as Argon and Helium were the primary and only types of Shielding Gases for MIG Welding. The choice of these two gases is very simple. As they are inert gases, they don’t react with any material, be it solid, liquid, or gas. But later, engineers and scientists realized that using plain inert gases isn’t suitable for welding different metals. So, after testing a bunch of different gases, they concluded that mixing Active or Reactive Gases with Inert Gases provides better MIG Welding result. As the shielding gas uses a portion of Active Gases, the welding process is known as Metal Active Gas Welding or MAG Welding. Gases such as Carbon Dioxide, Nitrogen, Hydrogen, and Oxygen are all suitable for MIG Welding either alone or as a mixture of inert gases.
Types of MIG Welding Gases
Let us now see some common and widely used MIG Welding Gases.
Argon
This is the most popular type of MIG Welding Gas for shielding. It is an inert gas i.e.; it has no chemical reaction with any material. As a result, Argon is the main choice of Shielding Gas for welding all kinds of metal (both ferrous and non-ferrous metals).
Helium
The next popular MIG Welding Gas is Helium. Helium is much lighter than Argon. Hence, when you use Helium alone, you need to increase the flow rate of the gas. But the thermal conductivity of Helium is much better. As a result, we often use Helium as shielding gas for thick sheets of Aluminum, Magnesium, Copper, etc.
Argon + Helium
When you combine the high density of Argon and high thermal conductivity of Helium, you can work on a wide range of metals. The arc action Argon + Helium combination is much better that either of them used individually. Hence, you can use this combination (usually higher % of Helium) to work with thicker metals such as aluminum or copper alloys. Welding Nickel Alloy with a mixture of 75% Argon and 25% Helium will help in better penetration and good puddle fluidity
Carbon Dioxide
After a stint of experiments with Argon and Helium, scientists and engineers found out that the Carbon Dioxide is a cheaper alternative to those two expensive inert gases. As Carbon Dioxide is an active gas at very high temperatures. Hence, the welding process is also known as Metal Active Gas Welding (MAG Welding). At high temperatures, carbon dioxide dissolves into individual elements i.e., carbon, oxygen, and carbon monoxide. Oxygen reacts with alloying elements such as manganese and silicon at the weld pool. As a result, oxides of these metals float on the surface of the weld pool. The benefit of this reaction is a reduction in the amount of alloying material in the molten part and increase in the amount of retained quantity of manganese or silicon. This improves the mechanical properties of the weld (better tensile strength and ductile strength).
Argon + Carbon Dioxide
Using Carbon Dioxide alone as MIG Welding Gas is very rare now a days. We usually mix Argon and Carbon Dioxide and this mixture is an extremely popular blend of MIG Welding Gas. Lower volumes of Carbon Dioxide (2%, 5%, 8%, or 10%) is preferrable for Axial Spray Transfer while higher percentages (20% or 25%) are preferrable for Short Circuit Transfer. For instance, a mixture of 75% Argon and 25% Carbon Dioxide (sometimes 80% Argon and 20% Carbon Dioxide) is very popular shielding gas for MIG (or MAG) welding of Carbon Steel. Blend of 98% Argon and 2% Carbon Dioxide is a good shielding gas for welding carbon steel, stainless steel, and even High-Speed Steel (HSS).
Argon + Oxygen
If we are trying to prevent formation of metal oxides during welding, how come Oxygen is in the list of Shielding Gases? This is because, we need some essential oxides to stabilize arc in the MIG/MAG welding process. Hence, we use minute quantities of Oxygen along with Argon as a shielding gas. The percentage of oxygen will be less than 5%. We can use Oxygen as a replacement to Carbon Dioxide. A bend of 99% Argon and 1% Oxygen as shielding gas is suitable for welding stainless steel. With slight increase in the volume of Oxygen (2%), you can use it for MIG or MAG Welding both stainless steel and carbon steel.
Hydrogen
When you blend small quantities of Hydrogen (typically between 1 to 5%) with Argon, it increases the thermal conductivity of the gas and it improves welding speed and toe wetting. You can use this blend for welding Stainless Steel, Nickel, and Nickel Alloys. Sometimes, a very small quantity of Carbon Dioxide is added to this mixture to improve the stability of the arc.
MIG Welding Gases for Different Metals
Conclusion
The choice of Shielding Gas in MIG Welding is very important as it helps in protecting the molten metal and electrode from contaminants. Depending on the type of Shielding Gas, we call the welding process MIG (Metal Inert Gas) Welding or MAG (Metal Active Gas) Welding. Helium 75 + Argon 25 Argon 75 + Helium 25 Helium 90 + Argon 7.5 + Carbon Dioxide 2.5 Helium 55 + Argon 42.5 + Carbon Dioxide 2.5 Argon 98 + Carbon Dioxide 2 Argon 98 + Hydrogen 2 Carbon Dioxide 100 (less frequent) Argon 98 + Carbon Dioxide 2 Argon 95 + Oxygen 5 Argon 90 + Carbon Dioxide 7.5 Oxygen 2.5 Argon 95 + Carbon Dioxide 5 Argon 98 + Oxygen 2 Argon 89 + Helium 10.5 + Carbon Dioxide 0.5 Helium 75 + Argon 25 Argon 75 + Helium 25 Argon 98 + Hydrogen 2 Additionally, the type of Shielding Gas is also responsible for high heat transformation. In this guide, we saw the basics of MIG Welding and looked at the essentials of MIG Welding Gas. After that, we understood the importance of Shielding Gas and also saw different types of MIG Welding Shielding gases. Finally, we listed out some popular choices of Shielding Gases for different types of metals. Comment * Name * Email * Website
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