What is MIG welding and how does it work?

Metal inert gas (MIG) welding joins together two pieces of metal, using an electrical supply to fuse the base metal with a consumable electrode filler.

Also known as gas metal arc welding (GMAW) or simply wire welding, the MIG process differs from TIG in that it uses a consumable wire that acts as both the electrode and the filler material. TIG welding relies on a tungsten tip to heat and join the metal surfaces directly.

In this blog, we answer some frequently asked questions, such as ‘What is MIG welding’, while exploring the pros and cons of the process.

What is MIG welding?

MIG welding is an arc welding process in which a continuous solid wire electrode is fed through a welding gun and into the weld pool, joining the two base materials together. A shielding gas is also sent through the welding gun, protecting the weld pool from contamination.

MIG stands for metal inert gas. The technical name for it is gas metal arc welding (or GMAW), and the slang name for it is wire welding.

The MIG process enables the home hobbyist, artist, farmer/rancher, motorsports enthusiast or DIY welder to make most types of fabrication and maintenance/repair welds on material from 24-gauge up to 1/2-inch thick.

In addition to flexibility, many people turn to MIG welding because they’ve heard that it’s an easy process to learn. Most people can become competent MIG welders by following some basic advice.

Difference Between MIG and MAG Welding

Both metal inert gas (MIG) and metal active gas (MAG) are fusion welding processes and belong to the GMAW family.

They’re often seen as one welding technique because apart from the shielding gas, the welding process is exactly the same. Both of these processes are performed using the same welding machine.

MIG welding uses inert shielding gases (argon, helium, nitrogen, or a mixture of the three). These inert gases are stable during welding, wherein it does not diffuse particles to the weld bead.

MIG is generally used for welding aluminum, magnesium, copper, titanium, and other non-ferrous metals and alloys.

MAG welding uses active shielding gases or a mixture of active and inert gases (CO₂, Ar + 2 to 5% O₂, Ar + 5 to 25% CO₂ and Ar + CO₂ + O₂). The two common active gases in MAG welding are oxygen and carbon dioxide.

Due to the extreme temperature during welding, these active gases disintegrate and alter the chemical composition of the weld bead. This type of welding is generally preferred for carbon steel (especially mild steel) and stainless steel.

Between the two, MAG welding is desired if you need to alter the chemical and mechanical properties of the weld.

How Does MIG Welding Work?

The MIG welding process is relatively straightforward, and most of it is automated too.

MIG welding is always done on direct current (DC), which means that the current only runs in one direction. Like a battery, it requires a completed electrical circuit to work. All the components of the welder work together to form this circuit.

First, a wire spool is loaded into the machine. This wire is fed through the driver rollers, into the torch liner, and out of the contact tip and nozzle. The wire will start feeding continuously when the torch trigger/button is pressed and stop when it’s released.

Pressing the torch trigger will also start the gas (if it’s needed), which is fed through the nozzle to shield the weld.

The wire plays two parts: the heat source and the filler material. When the wire passes through the copper contact tip, it becomes electrically charged with the welding current. The wire comes out of the torch and touches the base metal, creating an arc between the two.

That’s the first half of the electric circuit formed.

The arc melts both metals, depositing filler metal into the molten base metal to form a weld. There are several ways filler metal can be transferred from the torch into the base. Your normal MIG welding transfers metal via the short circuit mode.

The final piece of the puzzle is the earth clamp. Without it, your machine won’t arc. You can still pull the trigger and feed the wire through your torch, but all you’ll end up with is wasted wire.

The earth clamp completes the electric circuit that runs from the machine, through the torch, into the base metal, and then back up the earth clamp to the machine.

On top of that, getting a proper grounding is important because a bad earth can cause several issues, including burnback and a wandering arc.

MIG welding is considered a semi-automatic process. The wire feeding is automatic, but the torch movement is still manual, making it semi-automatic.

MIG welding setup

The first step before turning on the welder is to prepare your metal. Unlike stick and flux-cored electrodes, which have higher amounts of special additives, the solid MIG wire does not combat rust, dirt, oil or other contaminants very well.

Use a metal brush or grinder and clean down to bare metal before striking an arc. Make sure your work clamp connects to clean metal, too. Any electrical impedance will affect wire feeding performance.

To produce strong welds on thicker metal, bevel the joint to ensure the weld fully penetrates to the base metal. This is especially important for butt joints.

Next, it’s time to set up your machine:

• Check your cables. Before striking an arc, check your welding equipment to make sure all of the cable connections are tight and free of fraying or other damage.
• Select electrode polarity. MIG welding requires DC electrode positive, or reverse polarity. The polarity connections are usually found on the inside of the machine.
• Set gas flow. Turn on the shielding gas and set the flow rate to 20 to 25 cubic feet per hour. If you suspect leaks in your gas hose, apply a soapy water solution and look for bubbles. If you spot a leak, discard the hose and install a new one.
• Check tension. Too much or too little tension on either the drive rolls or the wire spool hub can lead to poor wire feeding performance. Adjust according to your owner’s manual.
• Inspect consumables. Remove excess spatter from contact tubes, replace worn contact tips and liners and discard the wire if it appears rusty.

Materials used in MIG welding

Let’s take a look at MIG equipment and materials and their role in the process:

Wire electrode

There are several types of wire electrodes available to tackle different projects and metals. As these electrodes run through the same wire feed unit, they behave differently during the welding process and leave distinguishable results.

Hard wire is the general wire used by most MIG users as it is affordable and easy to control. This wire usually comes in large reels and can be used at different angles. Typical hardwires used are in a combination of 72/25 argon and CO₂ ratio.

Flux-cored wire requires no shielding gas for the welding project as the flux is built into the wire itself. In fact, this is actually another type of welding process called flux-cored arc welding (FCAW) but this can often be performed on the same welding equipment.

Portability is a great bonus with flux-cored wires as there is no need to carry a gas tank around. The absence of an additional shielding gas makes flux-core more suited to working outdoors and in windy conditions.

This is extremely convenient and user-friendly but on the downside, these wires create slag during welding. Investing in a good metal brush will come in handy for the cleaning procedure.

Inert gas

As the metals fuse in the welding zone, MIG welding gas is supplied through the welding torch to keep the weld pool free from contamination. These inactive gases have no effect or reaction to the weld, keeping the metal’s properties intact.

The most commonly used gases in MIG welding are argon and helium. Sometimes they’re mixed with other gases as these two noble gases are quite expensive.

Additionally, semi-inert gases can be used that contain small percentages of carbon dioxide (CO₂). Cheaper than argon and helium, CO₂ allows for deeper penetration while resulting in more spatter in the weld pool. This means that more cleanup is required to clean the welds after gas metal arc welding.

In specific situations, non-inert gases are used in very small percentages to further increase metal penetration. The downside is that oxygen creates rust and oxidation to the weld metal, which can affect the weld quality.

Welding torch

A welding torch, or gun, is a specialized tool for fusing and melting metals. MIG torches are versatile in their application to a variety of metal types and thicknesses. MIG torches are divided into two groups:

Gas-cooled welding torches: overheating can become an issue so these torches are generally limited to minor projects.

Water-cooled welding torches: can be used at higher amperages and provide more power. They also offer smoother control over the contact tip of the nozzle. However, they cost 20-30% more than gas-cooled torches and require a water-cooled system to be installed in the welding machine.

There are 4 different nozzle types used in a welding torch: recessed, flush, protruding and adjustable. The choice is determined by the type of wire electrode being used.

Power source

Modern MIG welding equipment auto-corrects the current when the arc length and wire feed speed change, creating a stable weld puddle. These are your choices for both TIG and MIG welding:

DC positive polarity: in DCEP (direct current electrode positive) or reverse polarity, the electrons flow from the contact tip of the electrode to the base metal.

This is the most widely used setting since it offers a stable arc, ensuring better bead quality, weld penetration, and less spatter. Suitable for welding both thick and thin materials.

DC negative polarity: in DCEN (direct current electrode negative) or straight polarity, the electrons travel from the base metal to the tip of the electrode wire.

This method offers faster deposition rates than DCEP but it has several drawbacks such as lack of penetration and not enough heat in the weld pool. Not suitable for thicker materials but is sometimes used for welding thin metals.

AC power: AC is hardly ever used in MIG welding. TIG welding of aluminum requires AC in which the machine is constantly switching between positive and negative currents. The alternating current provides a cleaning action to remove the oxide film present on the surface of the metal.

Common MIG Fabrications and Applications

MIG welding can be used on a wide range of metals across a wide range of thicknesses. It’s also one of the fastest ways to weld. These factors combined make it the most commonly used welding method across several industries.

Some of the most common uses of MIG include:

• Home hobby use and DIY projects – the relative ease of MIG welding makes it a perfect option for first-time welders and weekend warriors
• Automotive repairs – MIG’s ability to weld sheet metal at speed makes it a preferred method for car repairs, as it’s a faster option than TIG
• Farm repairs and outdoor use – the gasless MIG option (otherwise called FCAW) means it’s practical for the outdoors and other off-grid applications
• General fabrication – frames, trailers, etc. are usually done with MIG
• Construction – its speed, ability to weld relatively thick metal (16mm @ 350A) and minimal clean up (compared to stick welding) makes it a time and money saver across construction applications

Advantages of MIG Welding

MIG (Metal Inert Gas) welding has many advantages, including:

• Speed: The continuously fed wire keeps both hands free for MIG welding, which improves the welding speed, quality of the weld, and overall control.
• Quality: Excellent visibility of the weld pool and the straightforward welding process will allow you to produce consistently good-looking welds. Due to its high degree of predictability, MIG is now among the most widely utilized welding techniques.
• Cleanliness: Since MIG uses a shielding gas to protect the arc, there is very little loss of alloying elements as the metal transfers across the arc. There is no slag to remove, which is typical for stick welding, and only minor weld spatter is produced. After a brief clean-up, MIG welders will be back on the job in a fraction of the time thanks to the minimal clean up it requires.
• Versatility: One of the most adaptable welding techniques is MIG welding, which can be used on several materials such as steel, stainless steel, cast iron, magnesium, and aluminum and can be done semi- or fully autonomously.
• Ease of use: The MIG torch’s simplicity of use and automated functions, along with its short learning curve, make it one of the first welding procedures that beginners learn. These elements free them from dealing with the technicalities of welding equipment so they can concentrate on learning to weld.
• It welds in every position. MIG welding works regardless of whether you’re in a flat, horizontal, vertical, or overhead position.
• Efficiency: Many welders will enjoy higher productivity due to time saved by not having to constantly change rods or chip away slag, as well as not having to brush the weld repeatedly. They’re able to work faster and cleaner.

Disadvantages of MIG Welding

Although using MIG for your welding projects provides many benefits, you should also be aware of a few drawbacks.

Cost

While MIG welding is highly productive and efficient, with good economic results, it is an expensive process for setup, which can also be complex.

There are many components to include, which can be costly, including shielding gas, replacement parts, nozzles, electrodes, and related tools. Personal protective equipment is also essential and can add to the cost, especially in extensive production facilities.

Despite the expenses and regular maintenance required, MIG welding’s efficiency is well worth the investment, though it can be challenging for smaller operations with a limited budget.

Limited Positions

The fluidity of the welding puddle and the high heat input of a MIG welder rules it out for vertical or overhead welding.

While some welders would never even think of welding in one of these positions, take this factor into consideration when deciding whether or not to purchase a MIG welder to make sure it can accomplish all of the jobs planned for it.

Unsuitable for Outdoor Welding

Besides not being portable, MIG welders are also unsuitable for working outside since they use a shielding gas to protect the purity of the weld. Wind will play havoc with the shielding gas and impact the quality of the weld.

A MIG welder also can’t be hauled out into a field to repair a tractor but works great in the garage for automotive work.

Fast Cooling Rates

The welded metal will cool at rates that are higher since they aren’t covered by slag when the weld is completed.

Unsuitable for Thick Metals

While MIG welding is suitable for thin metals, it does not deliver proper penetration for thicker steel that requires a solid weld.

Shielding Gas

The bottle of shielding gas can take time to replace and can get in the way of welding.

Metal Preparation Time

Before welding with a MIG welder, the material has to be free of rust or dirt in order to get a good weld and for safety’s sake.

5 Beginner’s Tips for Making a Good MIG Weld

Whether you are thinking of taking a welding class, want to build your knowledge base or just need a refresher, here are 5 introductory tips for making a good MIG weld:

#1. Get comfortable with the welding gun.

When you are just starting out, holding a welding gun may not yet be familiar territory.

Begin by holding the gun to get comfortable with its weight and position. Rest the barrel in one hand and place that hand on the table, while the other hand operates the trigger.

The wire should meet the weld at about a thirty-degree angle. Stand in a comfortable position with good posture, and practice moving the welding gun back and forth over the work surface.

Practice touching the wire very lightly to the surface, and gently pull the gun toward you in a steady motion at an even rate. Then, when you are ready, squeeze the trigger and repeat the motion with the welder on.

#2. Clean your metalwork surface.

This is very important because one of the most common reasons for bad MIG welds is porosity, which can be caused by a dirty, oily surface. Contaminants can get trapped in the weld, creating holes like a sponge.

So, to prevent porosity, always clean your work surface, which may involve removing paint, dirt, oil or rust, and grinding out the cracks in the surface. Another way to prevent porosity is to use a deoxidizer in your wire.

#3. Get a good ground.

Having a strong ground clamp can greatly improve your welds. Copper grounds conduct electricity the best and are designed to last. Lesser quality ground clamps, such as plated steel with copper jaws, are not as good as full copper grounds.

Bad ground clamps can make a horrible noise and possibly even cause damage to your welding machine. The ground should be placed close to the arc and firmly attached to the clean, bare metal to create a smooth flow of electricity.

#4. Set your MIG welder correctly.

You should adjust your welding machine every time you set up to weld. First, look at the chart on the welding machine and adjust the voltage and wire speed to the thickness of the metal you are welding.

While your machine will give you a rough guide, your settings should be adjusted to the weld you see.

So, you may need to make a few practice welds on scrap metal to test the wire. If the wire is feeding too quickly, it will spit. In that case, try turning your wire speed down or turning the voltage rate up.

If the wire is burning back to the tip or producing a glob, turn the voltage down or the wire speed up.

#5. Read your bead.

Your weld bead can tell you a lot. A convex-shaped, ropy bead usually indicates that the settings are too cold. You need more heat to penetrate the base metal. You can also adjust the welding angle by 5 or 10 degrees to help with penetration.

On the other hand, if you see a concave-shaped bead, this could mean you have a problem with your heat input.

FAQs.