How is Argon Used in MIG Welding and how do MIG and MAG Welding Differ?

The acronyms MIG (Metal Inert Gas), MAG (Metal Active Gas) and GMAW (Gas Metal Arc welding) all describe the same basic welding process. In this welding process, an arc is struck between a continuously fed consumable electrode and the workpiece as depicted in Figure 1. The consumable electrode is a bare wire. The heat generated by the arc melts the electrode and part of the base metal in the weld area. The arc itself transfers molten metal from the tip of the melting electrode to the workpiece, and here it combines with the melted base metal to form a weld deposit.

The solid electrode comes in the form of a small diameter wire that is wound on a wire-pool. During welding, this wire electrode is continuously fed, via a wire feeder, to the welding zone and supplies the required filler metal to fill the joint gap.

Figure 1. Principles of the MIG/MAG/GMAW Process

The composition of the consumable electrode is chosen based on the components being joined and it often has a similar composition to that of base metals. Since the process employs a bare electrode, without any form of fluxing, a shielding gas is required to be supplied to the welding zone. This shielding gas is necessary in order to protect the molten weld metal from undesirable oxidation and contamination.

In effect the shielding gas is a key to how and where the process is used and how it is defined. One form of the process is to use inert gases to perform the shielding. An inert gas does not undergo any chemical reactions under a given set of conditions as they are non-reactive. As the name suggests, inert gases such as argon and helium are used for shielding purpose in the process derivative defined as (MIG), Metal Inert Gas Welding. Here the shielding gas remains stable during the welding and thus does not diffuse any external element into the weld bead

Metal Inert Gas (MIG) welding was first developed for welding aluminum. In the initial development, the arc and weld pool were formed using a bare wire electrode which was protected by helium gas. Later developments substituted argon. Argon and helium gases are currently typically used for the MIG welding of non-ferrous metals such as aluminum, copper and nickel alloys.

On the other hand, the shielding gas can contain an inert gas but this can also be mixed with other gases that are termed active. Oxygen and carbon dioxide are two commonly used active gases for mixing with argon and /or helium gases. In these cases, when an active gas mix is used to shield the arc, the process is defined as (MAG), Metal Active Gas Welding. Active gases can break up in the heat of the arc and induce chemical reactions in the weld metal. They thus have the ability to alter the mechanical and chemical properties of the weld metal and affect the of transfer of filler metal across the arc.

MAG welding is usually used for the joining of steels with shielding gases typically based around.
•    100% CO2
•    argon +2 to 5% oxygen
•    argon +5 to 25% CO2

We can re-define the MIG and MAG processes below. These two terms are mostly used in Europe whereas, in North America, we have defined these particular processes with the more generic term that essentially covers them both. That term is (GMAW), Gas Metal Arc Welding,

MIG Welding

• Uses an inert gas as shielding, Argon and Helium being the most common but sometimes nitrogen is introduced.
• The shielding gas, being inert, does not induce any eternal chemical elements to the weld pool. As such it does not influence the properties of the weld bead.

MAG Welding

• A mixture of active gases, commonly oxygen and/or carbon dioxide, with inert gases. Also, in the MAG mode, 100% carbon dioxide can be used.
• The active gases breakdown in the arc and thus certain elements, e.g. oxygen and carbon may be introduced into the weld pool. Thus, MAG gases can influence the final properties of the weld metal.

A further property of all these gases, whether in the MIG or MAG mode, is that they do influence the way the metal droplets are transferred across the arc. Metal transfer can be by, 1) short circuit, 2) globular and 3) spray. The type of metal transfer is affected by the shielding gas, amongst other factors, and this may be discussed in a future article.