What is GMAW (MIG Welding) & How Does it Work?

Gas metal arc welding (“GMAW”) is an arc welding process that uses a consumable wire electrode and shielding gas, which a welding gun feeds to the joint. An electrical arc between the wire and base metal generates heat that melts both while shielding gas protects the molten weld pool from harmful atmospheric gasses.

A constant voltage, direct current power source is most commonly used with GMAW, but constant current systems, as well as alternating current, can be used.

This article explains the basics of the MIG welding process works, when to apply it, the necessary equipment, GMAW career opportunities, and the pros and cons of MIG welding.

How Does MIG Welding Work?

A MIG welder automatically feeds an electrically charged, solid wire into the weld joint at a set speed, and the supplied gas shields the molten pool. The power source inside the welder energizes the wire, creating and driving the arc. 

GMAW always uses a direct current electrode positive (“DCEP”) polarity. That means the ground clamp is attached to the negative terminal, and the MIG torch is attached to the positive terminal. 

Attaching the ground clamp to the welded piece and pulling the MIG torch trigger to feed the wire into the joint creates an arc. The arc happens as the wire contacts the grounded metal, which closes and shorts the electrical circuit.

The arc quickly generates heat, and the wire at the tip melts, as does a small part of the base metal. But the wire feeder continually pushes solid wire into the joint, which creates another short circuit.

Shorting the circuit and melting the tip occurs many times per second, and it’s responsible for spatter, sparks, and the well-known, unique MIG sizzling sound.

Note that your wire acts as an electrode and filler metal material with the MIG process. It’s required to create an arc and deposit metal into the joint. Therefore, it is impossible to MIG weld without filler material.

The short circuit transfer method is the most commonly used. But there are four primary methods of metal transfer possible with GMAW:

• Short-circuiting
• Globular
• Spray
• Pulsed-spray

The metal transfer mode depends on the wire feed speed and the voltage setting. 

Read more on transfer modes here.

MIG Equipment

The minimum equipment needed for a typical GMAW setup includes a MIG welding machine, a power source, a shielding gas cylinder, consumable MIG wire, a welding gun, and a ground clamp.

It is also recommended to have basic safety equipment such as a welding helmet to protect your eyes and welding gloves to protect your hands from hot weld spatter.

The Shielding Gas

While the shielding gas protects the weld pool from atmospheric contamination, it’s imperative to understand how different shielding gasses affect the resulting weld. 

There are two classifications for shielding gasses:

• Active gasses like carbon dioxide (“CO₂”) and oxygen (“O₂”)
• Inert gasses like argon (“Ar”) and helium (“He”)

Technically, MIG welding requires the use of inert gas only. That’s why the process is called “metal inert gas” welding.

If the shielding gas mixture contains active gasses, the welding process is named “metal active gas” welding, or MAG for short. But, most people call both approaches MIG welding because there isn’t an apparent difference.

Active gasses influence the weld pool, spatter amount, and penetration depth. Inert gasses don’t interact with the welded metal. However, inert gasses also modify the welding characteristics to an extent. For example, helium is inert, but it improves heat input and penetration.

MIG welding carbon steel usually requires a mix of 75% argon and 25% CO₂. It’s also possible to MIG weld mild steel with 100% CO2 to achieve better penetration. But, welding aluminum requires a 100% argon or an Ar/He mixture.

You can learn more about MIG welding shielding gasses here.

Where is MIG Welding Used?

Various industries use MIG welding, from vehicle manufacturing to construction and general fabrication. It’s easy to master and provides the highest production rates of all manual arc welding methods. Therefore, professionals and hobbyists use MIG welding often.

Since it’s easy to automate and has a high degree of flexibility, MIG welding is the most common metal joining method in industrial applications. Most manufacturing plants perform more than 50% of their welded joints using the MIG process.

The most common MIG welding applications include:

• Cars and other vehicles are usually manufactured using automated (”robotic”) MIG welds, but auto body repair is usually performed manually, and most automotive shops use the MIG welding process.
• Manufacturing plants use MIG welding to join sheet metal and various metal sections.
• The construction industry depends on welding small and large load-bearing elements like steel beams and columns. MIG welding offers the necessary speed in quick-paced construction environments.
• Pipe welders often use the MIG welding process in the oil and gas industry and for infrastructure projects to join pipes.
• General repair is faster and more efficient with the GMAW. But, TIG welding is better suited for repair jobs if you need the highest weld quality. 

Read more: What is MIG Welding Used For?

What matters the most for weekend warriors and individual professionals is how portable and affordable manual MIG welding equipment has become. MIG welders are easy to set up and often work with a 110V input, making this welding process accessible to anyone just about anywhere. The ease of use and flexibility makes GMAW the usual go-to choice for most welding applications. 

Related: 4 Most Common Welding Processes (MIG, TIG, STICK FCAW)

Pros & Cons of MIG Welding

Advantages of MIG welding:

• Most straightforward arc welding process to master
• Inexpensive equipment
• High welding speed and productivity
• Excellent metal deposition rate
• Works with commonly welded metals
• Weld appearance is better than stick welding
• Supports numerous metal transfer modes, including pulsed MIG welding
• Can weld thin sheet metal
• Offers deep welding penetration for thick materials
• Lower spatter, spark, and smoke emission than stick welding
• Established welding process with lots of experience and information available
• No slag to remove which minimizes post-weld cleaning
• Works great as a low hydrogen alternative to stick welding

Disadvantages of MIG welding:

• Produces more spatter, sparks, and smoke than TIG welding
• You cannot use MIG welding outdoors in windy conditions because the shielding gas blows away
• The resulting weld appearance is not as pleasing compared to TIG welds
• Requires numerous consumables meaning replacement tips and gas nozzles need frequent replacement, which makes the process somewhat expensive
• MIG welding aluminum often requires additional, costly equipment like spool guns or push-pull systems (Note: the MIG welder must have a connection for these specialized torches)
• Fewer functions for arc control compared to TIG welding
• A spool gun to weld aluminum limits you to small spool diameters, increasing your wire costs since it’s cheaper to buy large spools and necessitating more frequent reloading
• MIG torch is often challenging to use in hard-to-access corners

Related read: MIG Welding vs TIG welding

What Equipment Do I Need to MIG Weld?

To MIG weld, you need the following equipment:

• MIG welder – a constant voltage power source with an integrated wire feeder. If the welder does not include a wire feeder, you’ll need a dedicated machine for the task. Most hobbyist-grade and portable professional welders integrate the wire feeder with the power source. We have a continuously updated, in-depth guide on recommended MIG welders on the market. Give it a read if you are considering your first MIG welder to avoid purchasing unnecessary features or overpriced units.
• MIG torch – usually comes with the welding machine. But, consider the duty cycle rating of the MIG gun. For example, a hobbyist-grade MIG torch rated for a 40% duty cycle at 150A will overheat when welding 1/2-inch thick steel with 250A output.
• Shielding gas tank – you can buy or rent a gas bottle, but we recommend buying it. Read our guide on shielding gas bottle sizes here. 
• Shielding gas regulator – dual-gauge, dial regulators are “standard,” but flowmeter regulators offer better readability and reliability. Our guide on MIG welding gas pressure settings covers this topic in-depth.
• MIG welding wire – you need a solid MIG wire for gas-shielded arc welding. Many MIG units can also use a flux-cored wire for FCAW, a different process that doesn’t require a shielding gas.
• Welding helmet – we suggest an auto-darkening welding helmet because it makes work easier and more enjoyable than a passive hood.
• Welding gloves – get a pair of robust and durable leather gloves. MIG welding spatter can injure you, so get heavy-duty gloves.
• Welding boots – never use regular sneakers to weld because the molten slag can melt synthetic materials and burn cloth, creating severe burns. Instead, get a pair of boots certified for welding. Boots with metatarsal guards are preferred because they offer additional protection from molten debris.

Careers in MIG Welding

Certified MIG welders can find work in almost every industry since the welding method is widely adopted. Plus, MIG welding is easy to learn, and it only takes a few weeks of training to pass the welding certification tests. So, this is an excellent career to consider for people who love hands-on work.

Almost every manufacturing facility needs certified MIG welders. If bending, shaping, cutting, and MIG welding raw materials sound interesting, a welder fabricator job may suit you. U.S. national median pay is $44,000, and the most dedicated fabrication welders earn above $75,000 yearly.

Construction ironworkers also need to know how to weld. While this job often relies on stick welding, MIG welding is preferred when the welding area is protected from wind drafts.

Creating your own welding shop is a great way to build a career based on MIG welding. You can repair cars, farm equipment, tools, boilers, and more. Welding shops also contract for the fabrication of construction parts, or you can work as a subcontractor for a larger manufacturing facility. 

Whatever industry you are interested in, the chances are high that it has a strong demand for certified MIG welders. We suggest learning how to weld in a recognized welding school like the Tulsa Welding School or Lincoln Electric Welding School. These educational institutions will teach you well and help you land an excellent job after graduation.

Wrapping It

MIG welding is a widely adopted metal joining method. It’s the least challenging arc welding process to master and provides high welding speeds. As a result, the MIG welding process offers the most efficient and productive choice for many applications.

Because the electrode wire is fed automatically, the MIG welding equipment is simple to set up and use. Plus, MIG welding machines are more affordable than TIG welders. But GMAW offers fewer features for arc control.

Almost every industry depends on MIG welding. While there are innovative variations of MIG welding like Fronius’s cold metal transfer (“CMT”), traditional MIG welding will remain the top metal joining method for the foreseeable future.