What is MIG Welding?

Inert Gas Welding

It is a process developed in the 1940’s, and is considered semi-automated. This means that the welder still requires skill, but that the MIG welding machine will continuously keep filling the joint being welded.

MIG welders consist of a handle with a trigger controlling a wire feed, feeding the wire from a spool to the weld joint. The wire is similar to an endless bicycle brake cable. The wire runs through the liner, which also has a gas feeding through the same cable to the point of arc, which protects the weld from the air.

MIG welding is most commonly used in fabrication shops where production is high, and the possibility of wind blowing away your gas shielding is unlikely.

MIG Welding Names

Those seeking a job as a welder would be advised to know all of the names by which this process is known. Employers may use other names in the classifieds or on a written test.

When it was first developed it was called (GMA) Gas Metal Arc. It is also known as; GMAW or Gas Metal Arc Welding. Technically the differences in the names are the type of gas used, Inert gas versus non-inert gas.

How MIG Welding Works

MIG weld welding requires three things, electricity to produce heat, an electrode to fill the joint, and shielding gas to protect the weld from the air. MIG welding is done using a very small electrode that is fed continuously, while the operator controls the amount of weld being done. In some cases when a robot takes over this process, it becomes automatic welding.

MIG Voltage Type and Welding Polarity

MIG welding unlike most other welding processes has one standard voltage type and polarity type. The voltage used is D/C direct current, much like the current in a car battery. Direct current flows in one direction, from the negative (-) to the positive (+).

The polarity used is also standard and that is D/C electrode (+) positive. This means that the handle is the positive side of the circuit, or it may be said, the electricity flows from the metal in to the welding handle.

The power source used for MIG welding is called a “constant voltage power supply”. In MIG welding the voltage is what is controlled and adjusted. When comparing MIG welding to Arc or TIG welding, MIG welding machines use voltage settings to set the machine. TIG and Arc welding machines use amperage to set the machine or a “constant amperage power supply”.

MIG Welders and Power Supplies

MIG Welding Equipment and Accessories

MIG Electrode Types

When choosing the proper MIG wire or electrode you need to match the type of wire to the type of metal being welded. Some other considerations are the type of transfer, position to be welded, and resistance to abrasion. Most of the times when working as a welder the welding engineers specify the weld size and electrode type to be used.

Typical MIG welding electrodes are a solid wire ranging from a thickness of .023 to .045. Some are much thicker for heavy industrial applications. The most common sizes are:

  • .023
  • .030
  • .035
  • .045

The most common wire used for welding carbon steel is ER 70S-6. In some cases you can weld two different metals together. An example of this is welding 304 stainless steel to A36 carbon steel using an electrode made of 309 stainless steel “ER 309L”. The manufactures of these electrodes use a standard code to identify the type of electrode.


GMAW ER70S-6 welding wire label

For example the code on the label ER 70S-6 represents the following:

ER– An electrode or filler rod that is used in either a wire feed or TIG welding.

70– A minimum of 70,000 pounds of tensile strength per square inch of weld.

S -Solid wire.

6– The amount of deoxidizing agent and cleansing agent on the electrode.


MIG Welding Gasses

MIG welding requires a shielding gas to be used. As the name states “Metal Inert Gas Welding” there is no shielding on the electrode or filler wire. MIG welding would not be possible without shielding gas. The way the shielding gas works is it is feed through the MIG gun and it literally suffocates the weld area from any air. This provides a air free zone where the welding arc and filler wire can do their work to get the joint welded.

MIG welding typically three types of gas for shielding and they are:

  • Argon
  • Carbon Dioxide
  • Helium

These three gases are typically used as a mixture depending on the metals that are being welded. The shielding gas needs to be matched to the electrode and base metal. If they are not compatible then the welds will either not be strong or it just won’t weld properly.

The type of gas uses also determines:

  • How deep the weld penetrates the metal welded
  • The characteristics of the welding arc
  • The mechanical properties of the weld.

When choosing the type of gas to be used, it is best to seek input from a welding supply store. The store will recommend the proper gas to match the welding wire to be used.

Transfer Types

MIG welding has four ways of transferring the wire to the joint.

  • Short circuit
  • Globular
  • Spray
  • Pulsed spray

The transfer types used to MIG weld are determined by the metal type, shielding gas used and machine settings. MIG welding transfer types are more of a machine set-up issue then a welding issue.

Almost any metal may be MIG Welded

MIG welding is a welding process that can weld almost any metal. It may not always be the best choice for weld quality but MIG welding is a fast, cost efficient, and produces results that are more than acceptable for most manufacturing and fabrication needs! Not everybody is building a space station. The three most common metals welded with a MIG welder are:

Carbon steel welds are almost flawlessly done with a MIG welder. There are very few problems, beside the downside of the design of a MIG welding machine. The wire stiffness is just right to pass through the liner from the machine with minimal friction to cause problems and has enough stiffness to be feed without coiling up. Depending on how much voltage the MIG welding machine is running at, the weld can be set to one of three transfer types, short circuit, globular, or spray.

MIG Welding ConsMIG welding does have a few problems!

  • The welder cannot go too far from the MIG machine.
  • Wind is a major factor outdoors.
  • Always need a bottle of gas.
  • The cable that transfers the spool of wire through it needs to be well maintained.
  • Contact tips getting spatter on them, then they seize up.
  • Needs a clean joint.
  • Finally there are many parts to a MIG welder that need to work properly and it can get very annoying sometimes trying to figure out what is wrong.

MIG Welding Pros

MIG welding has some very good advantages!

  • Welds are clean with very little smoke.
  • Production is cheap and fast.
  • Long welds with less restarts.
  • The skill level needed compared to other welding processes make it easy,
  • Welding wire runs continuously with less down time to replace electrodes.
  • Excellent for spot and tack welds.

Joint Setup and Preparation

tacked test coupon post inspection

Tacked test coupon Post Inspection

With MIG welding it is very critical that the weld area is clean. MIG welding will not be successful with a dirty joint. Unlike some stick welding / SMAW rods that can burn through rust, MIG welding has a lot of difficulties welding dirtier metals. It also does not have slag to protect the weld when the gas is gone. When MIG welding make sure you have a clean joint by removing any foreign substance. With MIG welding a slight bit of dirt or rust is OK but anything more is asking for trouble. MIG welding painted or coated metals does not work well at all.

MIG Welding Machine Set-Up

MIG Welding Techniques

Trouble Shooting MIG Welders

MIG Welding Summary

With MIG Welding just like all other welding processes it is the same techniques. Whip, circles, or weave for most joints. The best MIG welders begin as Shielded Metal Arc Welders. They learn the basics on a harder process then pick MIG welding, it then seems like child’s play to them. Even though MIG welding is very easy to do, if you do not have basic welding skills the machine set-up can very aggravating.

Carbon steel welds best with MIG because the stiffness of the wire is perfect for the liner. MIG welding is the best choice for spot welding and tack welds. When welding soft metals like aluminum there is special equipment that is needed to be added to the MIG welder. Harder metals like stainless steel work fine on any MIG welding machine as long as you pay attention to keeping the cord straight. Almost any metal can be MIG welded as long as the type of wire and gas are properly chosen.

Most welding techniques that are used for other welding processes still apply here. The whip of a weld, circles, and weaves for wider welds. What changes here is typically the forehand method is used but sometimes the backhand method may be needed. To be an excellent MIG welder the main thing is to master machine set-up. This is the biggest factor when it comes to MIG welding and the one thing many people overlook.


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