How Does MIG Welding Work?

If you’re planning to get into welding, you must choose the welding method that fits your needs and helps you do your job well. MIG welding is arguably the easiest method to learn, especially if you’re just starting your welding journey.

You can weld sophisticated metal joints with MIG, using either shielding gas or flux-cored wire on a variety of metals. Whether you’re working in the automotive industry, heavy construction or basic fabrication, a MIG welder can do it all.

How MIG Welding Works

MIG stands for Metal Inert Gas, also known as “wire welding”. MIG welding joins two metals with an electrical arc that melts metal electrode wire that acts as a filler metal that fuses with the base metal to form a solid joint.

MIG welding has two main forms: Gas Metal Arc Welding (GMAW) and Flux-Cored Arc Welding (FCAW).

GMAW MIG welding joins two base metals using a solid wire electrode that is fed through a gun and melted with electricity. The electrical arc and molten weld pool are protected by inert gas shielding that flows through the gun, typically a mixture of argon and CO2 gas.

FCAW MIG welding joins two base metals using a hollow wire electrode filled with flux. This flux-cored wire is fed through a gun without an additional gas supply. When the electric arc forms it vaporizes the flux which expands into an inert gas “bubble” around the arc and weld pool.

MIG welders come in 220 volt and 110 volt sizes. The power supply and plug installation for a welder needs to be properly set up, using either 50 amp plug (for 220V) or 20-30 amp plug (for 110V).

MIG Welding Components

Here we break down the components of the MIG welding process and describe how each works:

Wire Feeding Process

With a MIG welding machine, wire is fed through a contact tip on the welding gun. Electrical current is passed through this wire which creates an arc when it touches or comes sufficiently close to the base metal.

The wire automatically feeds into the weld as the operator pulls the trigger on the gun. This forward wire movement acts as a filler in the weld joint, fusing the two pieces together.

The welding wire — whether it’s solid metal or flux-cored — feeds automatically from a spool mounted in the wire feeding mechanism in the MIG machine. The wire passes through a flexible tube with a steel or teflon liner into the welding gun, then out the nozzle. The manufacturer will provide instructions on how to feed the wire into the welding gun, the amount of tension required on the feed rollers and how much the wire should stick out of the nozzle.

Gas Shielding Process (GMAW MIG Welding)

The GMAW MIG welding process uses a layer of inert shielding gas to protect the molten puddle from oxygen, water vapor or any other contaminants in the atmosphere. For welding steel, this is typically a mix of CO2 and argon. For welding aluminum and aluminum alloys, argon is used and sometimes helium.

The shielding gas is held in high-pressure cylinders and flows through a pressure regulator to the welding machine, down a hose to the gun and out the gun tip. The pressure regulator has a valve to help you set the pressure to a usable level.

The inert shielding gas flows out of the gun when the trigger is pulled. The gas creates a “bubble” around the the arc, base metal, weld puddle and contact tip that protects the weld from oxidation and porosity.

Flux-Cored Shielding Process (FCAW MIG Welding)

The FCAW MIG welding process uses a hollow wire with flux in the center, rather than a separate supply of inert gas. When the superheated arc forms it vaporizes the flux core into a shielding gas mixture that protects the weld similar to GMAW shielding gas.

The effect is similar, in that this gas creates a “bubble” around the weld to protect it from oxidation and porosity. However, it also deposits a protective layer of flux on top of the weld that must be chipped away after the weld solidifies.

MIG Settings and Controls

MIG welders have dials and settings on the machine to control the electric polarity, voltage and wire feed speed. This is in addition to the gas regulator controls if using shielding gas. These controls help you better control the welding process on different metals and thicknesses using different electrode wires. It’s very important to match the wire feed speed depending on the type of metal you’re working on.

With MIG welding, the current is direct current (DC) where the workpiece is negative, and the electrode is positive. Mostly its DCEP, and in some cases, it’s DCEN for some types of metals. You have to connect a ground cable to the workpiece coming from the MIG welder to complete the circuit.

MIG Welding Procedure

The MIG welding procedure is fairly simple, and it’s easier for the beginners to get used to it. You feed a consumable electrode on the wire spool, attach the gas if using GMAW, adjust the regulator to the proper settings and run the welder.

The trigger on the welding gun under your pointing finger electrifies the wire, turns on the flow of shielding gas and executes the weld.

The operator must be careful to maintain the correct distance between the gun tip and weld pool at all times. MIG welding has the benefit of working with various weld patterns, including straight, push-pull, zig-zag, lazy E and other patterns. You can push or pull the weld across the joint. MIG welding also works at any angle, including horizontal, vertical or overhead. This makes MIG the easiest and most versatile welding method.

Advantages of MIG Welding

MIG welding can be used on almost any welding job to make solid joints on two metals. Here are the main advantages of MIG welding:

  • Controllability: MIG welding is way more versatile than the other welding methods when joining two metals quickly. You can adjust the wire feed speed, the voltage level or use both polarity settings for different applications.
  • Easy to use: MIG welding is the easiest welding method to learn, even if you’re a beginner to welding. It’s operable with a single hand and is easier to control and maneuver the welding gun around complex joints and angles.
  • Speed: MIG welding is faster than most other welding methods. This is due to the automatic wire feed that allows constant welding and does not require changing out welding sticks. FCAW is the fastest welding method and is widely used in production welding where output is critical.
  • Better quality:  When done properly with the right surface preparation, settings and consumables, MIG welds will be generally free of oxidation, porosity and slag inclusions. It’s fairly easy to create strong and clean welds with MIG.
  • Long passes:  If you have a long joint to lay down in a single pass, a MIG machine can do it. As you control the amount of electrode you’re using, you can use it as a bigger wire spool you want for a long pass.
  • Higher penetration: The MIG welding method is popular for its good penetration with rapid heat buildup. MIG welding can be done on thin or thick metals. Turning up the voltage and wire speed will “drive” the weld deep into the joint. As there are less starts and stops, you can easily get a solid, deep joint with higher penetration.

Disadvantages of MIG Welding

Although the MIG process works well in most situations, it does have some disadvantages:

  • Not good outdoors. MIG shielding gas has a tendency to blow away in the wind, causing the weld to be unshielded and resulting in oxidation that causes weak welds. So MIG welding must be done indoors or in a well-sheltered area away from any wind.
  • More complex than flux-cored welding. MIG welding requires gas canisters, lines and mixing that add a level of complexity that can be challenging for some welders. Failing to get the gas flow and wire flow matched up with the wire speed and power settings can cause your MIG welder to behave poorly.
  • More expensive than flux-cored welding. The MIG shielding gas, lines, valves, gauges, canisters and cart cost a significant amount of money, from an up-front standpoint and an ongoing cost of weld standpoint.
  • Not as mobile as flux-cored welders. A FCAW welder can just be picked up and put in the back of a truck or on an ATV rack to the job. A significantly MIG setup is less mobile and heavier.
  • Cannot weld dissimilar materials together well. MIG guns and wire are designed to join like pieces of metal together. The wire maintains a constant heat and arc, which makes it difficult to direct heat and welding material on one piece vs. the other. A TIG welder or brazing outfit is needed if you’re welding two different metals together.
  • Does not work well with dirty material. If your pieces are not well-prepared and cleaned of contaminants like oil and paint, then MIG welds will come out weaker and inconsistent. FCAW welders (and stick welders) tend to burn off contaminants as their shielding gas is both hotter and expands outward from the weld pool.

Hopefully you’ve gotten some good knowledge on how MIG welding works from this article.

If you have any questions check out our MIG welding FAQs page.

Good luck with your welding!

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Nick Klamecki, Author
About Nick Klamecki, Author

Nick Klamecki is a certified Fire and Workplace Safety expert with 15 years experience in product research and testing. He has a degree from U.C. Davis, is an active outdoorsman and spent years ensuring the safety of special needs children. Nick researches and tests workplace, industrial and safety products and provides advice on their safe use. Learn more about Nick here or connect with him on LinkedIn | Medium