What is MIG Welding? The Essential Need to Knows

By Bill Byers / December 22, 2018
What is MIG Welding Title Image

What is MIG Welding? I tell you through words, images, and a MIG welding introduction video what MIG welding is.

Its advantages and disadvantages, along with a little bit of history so you know how MIG welding came about.

​First Let Me Define the Term MIG Welding

​The term MIG stands for Metal Inert Gas.

MIG welding is a form of welding that joins metals together, using electricity and started with Inert gas as protection for the weld.

In the early days of MIG welding Inert gases such as Argon and Helium were employed to produce successful welds. And electricity was utilized to raise the temperature of the metals to be welded to melting point.

I will cover a bit of MIG welding's history later on in this post which will give you some more context and background to the name but for now let me also cover off the term GMAW.

The terms GMAW and MIG welding are often used interchangeably? Why?

Well ...

GMAW stands for Gas Metal Arc Welding.

GMAW Welding

​Later on in the history of MIG welding when further discoveries were made, it was found that Carbon Dioxide could also be successfully applied to protect welds. Then the term MIG didn't really fully apply, because Carbon Dioxide isn't an Inert gas at all.

Gas Metal Arc Welding was a better descriptive term because gases other than Inert gases can be drawn on to protect the weld.

MIG welding also takes advantage of electricity. An electrical arc enables welding.

But as the name MIG had been in use for quite a while by then, well it just stuck. And both terms are now used.

MIG welding is also a form of arc welding as electricity creates an arc and the arc generates the heat to melt the metals.

​Why has MIG Welding Become so Popular?

Because for those new to the skill of welding;

MIG welding is often the most recommended as it is relatively easy to learn and as MIG welders have become smaller and cheaper over time MIG welding became accessible for those not in industry.

And is now popular among home welders wanting to repair and mend metals around their home and yard.

​First let me Explain the Welding part of MIG Welding

​At its simplest level, MIG welding is the process of joining two separate pieces of metal.

There are in fact many ways you can join one piece of metal with another, you can glue, or stick different types of materials together such as in soldering or brazing.

The reason welding is called welding is that in welding the aim is to create one solid piece of metal.

You would weld two pieces of mild steel at a point and the weld would be so strong that it was as if the two pieces of mild steel were one.

Two Separate Pieces Of Mild Steel

Two Separate Pieces Of Mild Steel In Preparation for Welding

​In the picture above two pieces of mild steel are laid one on top of the other in preparation for joining by welding.

In fact a well welded joint can be as strong if not stronger than the separate pieces of metals individually. I'll tell you why later.

The way you join metals together so that they appear as one is by raising the temperature of the metals in a controlled way to liquefy the metals at the point you want to make the join.

Bringing metals to a molten state requires significant energy input. For example steel melts at greater than 2500 degrees Fahrenheit.

As a lot of energy is needed to weld joints together the aim is to concentrate that energy at a particular point - the point at which you want the weld the metals together.

The picture below illustrates.

Welding Base Metals

Welding Base Metals

​The metals you're looking to weld are called the base metals.

The area where the temperature is raised and the metals liquefy is called the weld pool - A pool of molten metal.

The weld in MIG welding is made by travelling the welding wire along the joint melting each point in turn to form the weld pool.

As the weld pool solidifies the weld is formed.

Melted In Turn To Form The Join

Each Point Melted In Turn To Form The Join

​Why the Welding Wire?

Let me explain a little more

If only the base metals were melted and nothing else the welded joint would be thinner than the thickness of the base metals.

So you need some more components to make all this happen.

​The Filler Metal

​After all you have the joint - a gap to fill and extra metal is added called the filler metal to fill that small gap to create a join that is strong and designed to take stress and strains.

The filler metal is a long thin length of wire of specific diameter in size. This wire sits on spools held inside the MIG welder.

Spool of Wire Inside the Hobart MIG Welder

Spool of Wire Inside the Hobart MIG Welder

​In MIG welding the filler metal is fed out from the spool held inside the welder via the MIG gun.

This is why MIG welding can also be referred to as wire welding.

The other thing unique about MIG welding is that those spools of wire are solid wire manufactured to be compatible with the base metals to be welded.

For example if you are welding mild steel. You will need filler metal made of solid mild steel.

And those spools of wire are designed to melt at a high temperature and enter the weld pool providing the filler metal - hence its name.

​The Wire is also the Electrode

​Remember earlier I said that to weld you raise the temperature of the base metals in a controlled way at the joint? In MIG welding electricity is used to generate those high temperatures.  And the filler metal, the spool of compatible wire is how the electricity is delivered to the joint. The wire filler metal is also known as your electrode.

A powerful arc is formed at the end of the wire electrode - your filler metal - and the power of that arc is what is used to raise the temperature of the base metals to their melting point.

You can think of the arc as being like having a lightening bolt in miniature at the end of your spool of wire.

When the melting point is achieved and the weld pool is formed the filler metal electrode is fed into the joint and as the arc travels along the joint the weld pool solidifies behind the filler metal electrode allowing the base metals to fuse and meld with the filler metal.

And that is why the welded joint can be stronger than the base metals because the filler metal helps fill up the joint as though the joint wasn't there.

​The Problem With Air

There is a major obstacle to this happy story. The atmosphere that surrounds us which we breathe and take life from.

The components that make up the air we breathe such as water, oxygen, and other constituents of air were found to enter the weld pool causing holes to form. These holes and defects are known as porosity.

Porosity leads to welds that fail and fall apart. Not something you want.

And welding in a room with no atmosphere wasn't for the masses.

So some convenient way was needed to protect the weld pool from the surrounding air.

​Inert Gas as Shielding Gas

​Now this is where we get to the I in MIG.

In the early days of MIG welding it was found that protecting the weld pool from the air using a shield was what was needed. And that Inert gas fed over the weld pool at a certain speed acted as a shield and protected the weld.

Hence the gas used in MIG welding is also known as shielding gas.

Gas is convenient because it can be brought to the welding site held in high pressure cylinders.

And devices such as regulators can adjust the speed the gas is fed out at to cover the weld pool.

Argon/CO2 Shieding Gas Cylinder

Argon/CO2 Shieding Gas Cylinder and Regulator

​Inert gas is fed up to the weld pool from a gas cylinder. The gas is fed to the MIG welding machine using a gas hose.

The gas feeds through the welder and into the cable that also supplies the welding wire electrode to the MIG gun. The gas is fed out at a speed that shields and protects the weld puddle from the contaminants in the air.

​Shielding Gasses that are Used

There are many mixes of shielding gasses used now.

Popular gasses and mixes are;

  • 100% Argon an Inert gas.
  • or C25 which is 75% Argon and 25% Carbon Dioxide
  • or Tri-mix - 90% Helium + 7.5% Argon + 2.5% CO2
  • or 100% Carbon Dioxide.

Though not an Inert gas, Carbon Dioxide can also perform this shielding function. It is also popular because it is cheaper than Argon gas mixes and can aid in the metal penetration of the weld of thicker metals.

Although it is renown for producing more weld spatter.

Spatter are bits of molten metal that are a by product of MIG welding.

I am not going to go into what Spatter is, and its causes and prevention here because I have two documents on the site that go into this subject in much more detail.

The first, "What is Spatter in Welding? And Why it Sucks" describes what spatter is all about and why it is a nuisance in welding.

The second document "How to Stop Welding Spatter and Tactics to Reduce It" covers the tips and tricks to keeping welding spatter to the absolute minimum and both are worth a read.


​​The gas you choose as your shielding gas depends on the metals you're welding and what you want to achieve. For example; a clean, neat finish with no spatter or maybe deeper metal penetration where the look of the weld isn't quite so important.

​MIG Welding and Its Use of Electricity

​In answering the question what is MIG welding an important aspect of what makes MIG welding MiG welding is its use of electricity to bring the base metals to their melting point.

MIG welding is essentially an electrical process and you provide your MIG welder with electricity from an electrical outlet.

Electricity of course needs a circuit.

Electricity runs through your welder using the welding wire as the electrode (the arc that generates the heat that is formed at the end of the welding wire). The circuit passes through your welding project and then the electricity runs through to ground through your grounding clamp, back through your welder to the ground provided by your electrical outlet.

MIG Process Positive And Negative Polarity

MIG Process Positive And Negative Polarity

​If you are familiar with the + and - terminals on a battery, you have positive and negative terminals on your MIG welder.

MIG welding needs the wire electrode coming out of the MIG gun to be positive.

You'll see this referred to as MIG gun positive or DCEP - Direct Current Electrode positive.

This is called the welding polarity.

Your welder's grounding clamp is the negative part of the circuit.

MIG welding uses Direct Current and constant voltage. The MIG welder itself controls the constant voltage. Constant voltage is needed because it is very important that the arc generated is stable and this is achieved partly through the voltage being as consistent as possible.

That is the path to the metal being melted in an even fashion as the welder travels along welding.

And while you are welding, your welding project - conducts electricity.

It is why in MIG welding you aim to get the joint as clean as possible, no oil, rust or paint to distort the voltage or prevent good conductivity.

If your welder is able to weld using other welding processes, inside there will be connections for changing the polarity for your weld.

To successfully MIG weld ensure the MIG gun is connected to the positive terminal for positive MIG welding polarity and your work clamp is set to the negative terminal for negative polarity.

Example of MIG Polarity Terminals

Example of MIG Polarity Terminals

By Products of MIG Welding

MIG welding produces a number of by products the welder needs to be aware of and mitigate.

1. Spatter

​Earlier in the article I mentioned spatter - bits of molten metal that leave the weld pool looking a bit like fireworks. And pointed you to a document where I detail exactly what spatter is​.

The aim in MIG welding is to keep spatter to a minimum and wear protective clothing to protect yourself while welding from burns from spatter.

2. Slag

​Another by product of MIG welding is something called slag.

As the welder moves along the joint welding the weld pool cools. A protective covering forms on top of the weld pool and this covering is often called slag.

Actually in reality for MIG welding it is not a true slag overlay but a covering consisting of metal oxides.

Quality MIG welding wire will include deoxidizers and scavenger compounds to help lift small contaminants from the molten weld pool to the top of the weld.

Slag and Direction Of Travel

Slag and Direction Of Travel

Then trapped in the covering along with the metal oxides will be small contaminants from the weld. A good slag covering in MIG welding is so perfect it simply peels away with little effort once the weld pool is cool.

Taking away particles that if they stayed in your weld would cause porosity.

​I have an article that goes into more detail on slag as it is an interesting by product. And you can follow this link to know more.

3. Welding Fumes

Another by product of MIG welding is the fumes given off as you weld. The plume - the smoke that rises up from your weld shouldn't be breathed in.

Experienced welders angle their heads so that they are well out of the weld plume and or wear a respirator,

or have ventilation or a fan at head height that directs the weld plume away from the welder.

​4. UV Rays

​MIG welding uses an electrical arc and that arc produces UV rays, just like UV rays from the Sun. Sunshine will give you sunburn over time if you do not wear clothing or cream.

The UV rays from welding are at a more intense level.

The rays will quickly burn the welder operator.

And believe me no one wants the mother of all sunburns.

Any exposed skin should have clothing protection and eyes particularly need special protection with a welding helmet, plus goggles if you can. Eye damage through welding can be permanent and protective gear must be worn.

Even when watching someone else weld.

Or being in the same work shop as someone welding as arc burn to the eyes can happen as the rays bounce of surfaces in the workshop.

Safety Clothing

​What is MIG Welding - Putting it All Together

​MIG welding is an electrical process creating an arc to bring metals to be welded to their melting point. MIG welding uses a spool of welding wire as the electrode.

When a robot in a factory is doing the welding - that is called automated MIG welding.

Welding using a MIG welding machine with a person controlling the MIG gun is called semi automatic MIG welding.

The automatic component is the welding machine. It is provided with power, the human welder sets the welding voltage, wire feed speed on the welder via dials.

Knowing the correct settings for the welding machine is determined partly through the manufacturer recommendations, and partly due to the human welder knowing the machine and the materials they are welding on and with.

Once the dials are set, the welding machine then controls that aspect.

The semi automatic part is the human welder directing the MIG gun, using their skill to position the gun the right distance and angle from the joint. And they control the direction and speed of welding travel through their skill.

The human welder also develops the skill of listening for the right crackle.

The right type of crackle tells the welder that they have the right voltage, the right wire feed speed, they’re the right distance from the weld and are getting the correct penetration of the metal.

And then can be confident they are getting a good weld.

The crackle sound is often compared to the sound made by fried bacon.

Thus an equally important aspect of MIG welding is the skills the person MIG welding.

​What is MIG Welding YouTube Video

​For more info on what is MIG welding I found a great introductory welding video.

It covers a few welding processes but at 5 minutes 37 seconds its a great primer on the fundamentals of welding.

Welding Introduction

Video Credit: Weldnotes.com

​So Why Use MIG welding? Advantages and Disadvantages

​The advantages of MIG welding are;

  • That it is straight forward and flexible and can be used to weld a wide variety of metals
  • Good looking secure welds are produced
  • A MIG welder is used for mild steel, stainless steel and Aluminum. Which lets face it are the most common types of materials welded.
  • The price of MIG welders has fallen so it's not just for the professional working in industry. MIG welders are within the reach of the home hobbyist and DIY'er as well
  • The skills needed to MIG weld well are within the capability of a hobbyist with sufficient practice
  • A MIG welder can use different types of welding wire depending on the metals to be welded.
  • Can run on 120 volt home voltage so it convenient for home use
  • MIG welding allows for relatively fast reliable welding

The disadvantages are;

  • That the metals to be welded must clean free from dirt, rust, paint and any other contaminants such as chemicals and oil
  • It not for thick steel. Half inch thick steel and 5/16th are really its limit.
  • MIG welding is really an indoor process as the wind outdoors can blow away the shielding gas unless good quality wind blocks are used.

​History of MIG Welding

​I promised that I would cover an overview of the history of how MIG welding came about.

It's worth understanding where welding has come from as it's the key to understand what it is.

Until the end of the 19th century the method of joining metals together and the molding of metals was done in a forge by a Blacksmith. The forge heated the metal and the blacksmith used the Thunk, Thunk Thunk of their hammer.

For hundreds of years if not thousands this was the only method available.  The Blacksmith's job was to generate enough heat in his forge to bring metals to a molten state and as the metals cooled he could manipulate, bond and mold the metals through hammering.

With the invention of electricity, experimentation by scientists found that an electrical arc could also bring metals to a molten state. With technology advancement in the 1900's scientists were able to stabilize that arc of electricity enough to prove it's practical use in welding.

Except the air around us was the enemy, causing problems with the strength and quality of the weld. With the technological invention of metal protection - called flux - used to shield the base metal from reacting with the air they found it could also stabilize the electrical arc.

World War I really started the race to find which welding method was the better. Why? Strong ships and aircrafts were needed in large numbers and progress was made. Arc welding was used to build those much needed ships and aircrafts.

By World War II more major advances were made with the automatic welding using an electrode wire.  Shielding gasses were all that was talked about as the oxygen and nitrogen in the air affected welds, and made them porous and brittle.  It was discovered that Argon, Helium or Hydrogen could be used to provide a good atmosphere to weld in.

Excitement as Shielded Metal Arc Welding arrived in the 1950's using a flux coated electrode - what we now know as stick welding. The flux created gasses that shielded the welded metal from the contaminants in the air and thus Stick welding became a very popular method of welding.

Further developments through the 50's, 60's, culminated in the 70's when a mathematical formula was discovered where by a wire electrode could be fed through at a certain speed to coincide with electrical pulses. All controlled by a transistor producing cleaner, more reliable welds and MIG welding was born.

​Last Words

​I hope you have enjoyed my article "What is MIG Welding? The Essential Need to Knows" and now have a far better grasp on what exactly it is along with a bit of history and its advantages and some of its disadvantages.

You can follow on and read about "What is a MIG welder, A Guide for Extraordinary People" ​

Or "What Is MIG Welding Used For? Have an Idea in 10 Minutes".

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