What is MIG Welding?

MIG welding is an abbreviation for Metal 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 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.

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”.

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 manufactures of these electrodes use a standard code to identify the type of electrode. 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.

Common MIG Welding Gasses

Gas for MIG welding is what makes MIG welding possible. The name informs us of this; “Metal Inert Gas welding”. The gasses used are what shield the weld from oxygen in the air. When Carbon Dioxide or Oxygen is added to the mixture, MIG welding is not technically MIG welding anymore. This is due to the fact that both Carbon Dioxide and Oxygen are not inert gasses. The process then becomes GMAW or Gas Metal Arc Welding.

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. Or, one may utilize the welding wire manufacturer's recommendation. Typically, the manufacturer will provide a few choices ranging from the best choice, to something that will provide the minimum acceptable results. The final choice in the type of gas, is based on cost.

The four most common gasses used are;

• Argon
• CO2 / Carbon Dioxide
• O2 / Oxygen
• Helium (the least common)

Most of the times these gasses are used in a mixture form, typically consisting of carbon dioxide, and Argon or Oxygen. Oxygen is what causes most weld defects, however, in small percentages, mixed with other gasses, it improves the arc characteristics. Argon and Carbon Dioxide can be used by themselves. In some case there is tri-mix shielding gas containing Argon, Carbon dioxide, and Helium, or, Argon, Carbon dioxide, and Oxygen. I recently used this last mixture, when taking my

The more common mixtures and gasses are.

C2 or 2% Carbon Dioxide and 98% Argon C25 or 25% Carbon Dioxide and 95% Argon 100% Carbon Dioxide 100% Argon

Some basic guidelines for choosing the proper gas are as follows:

Welding of carbon steel can be done with Carbon Dioxide alone and it produces the deepest penetration, the most smoke and the roughest weld. A mixture of gasses ranging from 2% to 25% Carbon Dioxide and the rest Argon can be used. However, a higher percent of Argon will result in a smoother, better looking weld, and improves the arc characteristics.

Welding of stainless steel is typically done with C2 or 2% Carbon Dioxide and 98% Argon. In some cases there is tri-mix shielding gas containing 90% Helium, 7.5% Argon, and 2.5%Carbon dioxide.

Welding of aluminum is typically done with Argon alone, with one exception. If the aluminum being welded is thicker then ½ of an inch there may be Helium added to the mix.

Argon gas by itself works well on most exotic metals like:

Copper Copper Alloys Magnesium

Nickel Nickel Alloys Titanium

MIG Welding Transfer Types

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

• Short circuit
• Globular
• Spray
• Pulsed spray

Short Circuit Transfer

Short circuit transfer is a transfer used when a lower voltage is used for MIG welding. Short circuit transfer occurs when the wire arcs and contacts the metal creating short circuits. During this short circuit, the wire contacting the metal heats up and drips into the joint by creating a puddle. Then another arc begins and the process keeps repeating many times a second. The easiest way to tell if the transfer is short circuit is by the sound. The sound greatly resembles, an egg hitting an extremely hot frying pan. It is a very crisp and fast crackling sound. Typically short circuit transfer is used on thin metals or sheet metals.

Globular Transfer

Globular transfer is similar to short circuit transfer. Globular transfer like short circuit occurs when the wire arcs and contacts the metal creating a short circuit while melting the wire but the main difference is the wire melts for a longer period of time creating a glob that hangs off of the wire. Then the glob falls to the metal filling the joint and then the process begins again. Globular transfer has a popping sound to it. It has a few pops per second and many times you can actually see the glob being formed and dropped.

Spray Transfer

Spray Transfer is a transfer where a higher voltage is applied. The transfer is exactly like it sounds. The wire sprays or has very fine mist transferring to the metal. The way to tell if you using a true spray transferring are it has a fine hissing sound and no crackles or pops. You can also see a clean arc to the metal and will not have any spatter.

Pulsed Spray Transfer

Pulse transfer is a transfer that is done with special equipment. The equipment pulses the voltage many times a second, allowing the arc to go between spray transfer, to globular transfer, back to spray transfer and so on.

How to Set Different Transfer Types

If you have never MIG welded it would help to know that there are no settings on a MIG welding machine offering a choice of transfer type. The ways a different type of transfer are set have two ingredients. The first is the voltage settings and the second is the type of gas used.

Most short circuit transfer welding is done with lower voltage and usually using Carbon Dioxide or a Carbon Dioxide gas mixture.

Globular and spray transfer use a higher voltage setting and commonly use Aragon gas. The Argon gas may contain a percentage of Carbon Dioxide, Helium, and or Oxygen.

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.
• Stainless steel.
• Aluminum, with a special feeder because aluminum wire is very soft.

MIG Welding Carbon Steel

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 Stainless Steel

Stainless steel MIG welding does not require any special equipment. In the case of welding stainless steel, the biggest problem comes from the cord or liner of the welder. The trick when welding stainless steel is to keep the cord as straight as possible. Otherwise, the wire feed that feeds the weld joint will have too much friction due to stainless steel being stiffer then carbon steel. Think of trying to shove a wire hanger through a bent garden hose. Chances are if the hose is straight you can easily put it through. If the hose is bent, you will have difficulty. That is how stainless steel wire is in the liner of the welding cable. What happens is the wire is so stiff in the liner that it causes so much friction that the wire stops the feeding wheel from feeding the joint (literally spinning its wheels). The result is a fused MIG tip. That is when the wire stops or slows down to the point that the arc melts the wire up to the MIG tip and it welds itself to it. The other major problem is again the liner. If you bend the cord to much the friction stops the wire and the wheels that are feeding the MIG gun push the wire so hard that the wire having no place to go spooling up like a birds nest before it gets pushed into the liner.

MIG welding stainless steel is mostly done in a spray transfer or globular borderline spray. The reason behind this is that stainless steel does not do to well with short circuit transfers. The short circuit transfer and globular have the wire actually hitting the metal, splashing and splattering, and that also causes friction on the liner.

Welding stainless steel requires a high percentage of Argon used in the gas mixture. If it is a critical joint that will be x-rayed, any disturbance in the transfer will cause flaws. Spray is a smooth crackle-free transfer that has no metal being violently transferred. Spray transfer also keeps spatter to the bare minimum. With stainless steel spatter is extremely difficult to clean. The pictures below on the left is a weld I did using ER 308L wire, C2 gas, with a globular boderline spray transfer. I also used the same settings for the picture on the right. One of the great things about MIG welding is, I did not have many restarts. The long welds are about 3 feet between the restarts.

MIG Welding Aluminum

Aluminum MIG welding usually requires a double feed, referred to as, (a push and pull method). That is a spool of wire is pushed through the MIG handle and the handle itself has a pulley that pulls the wire. Welding aluminum is not very common for MIG welding. It is typically done when high production is needed.The first picture below is a double rollers push feed on a Millermatic 350P MIG Machine. The second and third pictures below are the bottom view, and top view of a Python pull feed gun.

Another type of feed that can be used is a spool feed MIG gun. It has a small spool of wire on the MIG gun itself. This type of feed is the most trouble free of the feeds when it comes to MIG welding Aluminum!

When MIG welding aluminum the technique used is always forehand and you need to push the puddle. These two pictures show the proper angle when MIG welding aluminum.

MIG welding aluminum is typically done with pure Argon gas using a spray transfer. On thicker joints the transfer has a fast crackle and the weld is very quick. Welding thicker aluminum out of position works well but the weld appearance is never perfect! The aluminum weld in the picture is a single pass and took about 10 seconds to do on a 3/8" bevel. It’s all about moving quickly and keeping the puddle from rolling over.

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.

MIG Welding Cons

MIG 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.

Joint Setup

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

Set-up of a MIG welder depending on what the type of wire transfer type is chosen. There are three main ingredients, first the voltage settings, then the wire speed, and finally the type of gas. Almost any type of gas will allow you to spray transfer, but the gas type is what changes the quality of the weld!

First set the gas to flow between 25 to 35 CFH. This is only a guideline! If you are welding indoors you may need less and if you are welding in a place that may have a draft coming through you may need more.

When setting a MIG welding machine there are two major settings and they are voltage and wire speed! Before setting up your MIG machine there should be some instructions on the machine itself for the proper voltage on the inside of the panel where the wire goes. The picture below on the left is from the inside of a Millermatic 250 and the picture to the right of it is a Lincoln Power MIG 255 setting chart. This comes into play when welding different thickness of metals. Most of MIG welding is done with the same wire diameter but what changes is the amount of heat the welder requires to heat that metal to get proper penetration of the weld.

In a past employer's shop, I welded 1/2 in. plate; I always used a minimum of 32 volts to weld with. The type of transfer is changed with the wire speed. If I wanted to spray I would just turn down the wire speed and then it would spray. If globular transfer is needed I increased the wire speed a little. Finally if short circuit is needed I crank the wire speed to a much higher level. The picture on the right is a horizontal filet weld I did on ½ plates to 12" channel using spray transfer at 36 volts, .035 wire, and the wire speed approaching 900 inches per minute (very hot). The welds are about 5 feet between restarts and the machine was running so hot that if I were to pause for too long it would blow a hole through the metal. MIG welding is execlent for production!

The settings on a MIG welding machine are best set within the voltage range of what the welding machines charts have suggested for that metal thickness. The transfer type mostly changes with the wire speed and gas type. When setting your wire speed to the transfer type you need or like to use it is always best to listen to the sound of the weld. Short circuit has a fast crackle sound; globular have a few crackles or pops a second and true spray transfer has just the hum of the welder or hissing sound. Once you learn the sound, you will know the settings.

MIG welding just like any other process it also relies on two major factors joint preparation and machine set up. Firsthand experience I had a welder who could not weld vertical up very well with a MIG welder. I set the machine to what my vertical up settings were and he did great! Do not discount skill verses machine set up and joint preparation. One of the secrets journeyman welders have to make solid welds, is based on how well they set up their welding machine!

MIG Welding Techniques

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.

There are two ways to MIG weld. The first and the most common is to push the MIG gun toward the direction of the weld, this is called forehand method. Forehand welding produces shallow penetration with a flat wide smooth surface. The second is backhand method where you drag the MIG gun like a Stick welder. This produces a deep penetration weld that is narrow and is high in the center.

To begin welding the wire stick out should be about 3/4 of an inch. Less is OK but a much longer wire stick out won’t allow the shielding gas to do its job properly.

Some general guidelines for MIG welding techniques are as follows and are only guidelines:

• Flat welding is the easiest. Typically the MIG gun will be pushed, the angle of the MIG gun can be pointing straight down to leaning toward the direction of the weld up to 35 degrees.
• Horizontal welding is a little harder. The MIG gun should be pointing upward between 35 to 45 degrees and tilting about 15 to 35 degrees toward the direction of the weld.
• Vertical down is also pretty easy. You start on the top and work your way down. The MIG gun needs to be tilted up between 35 to 45 degrees. There is a trick to welding vertical down. You need to stay ahead of the puddle and keep the electrode moving from side to side. Otherwise the weld will not penetrate properly.
• Vertical up is a bit harder. The MIG gun handle should be between 35 to 45 degrees. When welding vertical up you need to build a shelf of weld to work upward on. The vertical up weld in MIG is typically very convex. To help with welding vertical up it always a good idea to grind a small grove where you will be welding.
• Overhead MIG welding requires the MIG gun tilted 5 to 35 degrees toward the direction of the weld.

The general guidelines for MIG welding are whip when welding thin metal or making the first pass in a joint, circles for a both thicker materials and wider welds. Weave for vertical up and down. My personal preference is to have the voltage at the maximum recommendation and just hold the MIG gun steady letting the MIG gun do the work.

Trouble Shooting MIG Welders

Porosity in the weld:

• Check the gas to make sure there is enough left.
• Check the gas flow rate. First raise the flow rate and if that does not work try lowering it. Sometimes too much gas flow causes turbulence by pulling in air to the mix.
• Check the cup of the MIG gun for excessive splatter.
• Check your stick out. It’s common for people who stick weld to allow too much stick out when MIG welding.
• Check the MIG gun for leaks. Typically the MIG gun nozzle may be loose or worn out. Or the gas diffuser may be loose or have spatter blocking the gas flow.
• Change your travel angle. Too much lean toward the direction of the weld can cause air getting pulled into the weld area.
• Check the joint for dirt, water, or any contaminants that should not be there.
• Finally check all of the connections in the gas line for leaks.

Wire Feed Jams or Uneven Wire Speed:

• Check the MIG tip.
• Check the rollers for proper tension settings.
• Check the rollers for a birds nest.
• Check the wire spool to make sure there is wire left and nothing is catching on it. It’s common for the wire to jam on itself.
• Make sure you are not bending or twisting the liner too much while welding. If not you may need to replace the liner.
• Check the rollers to make sure they are the right size for the wire being used.
• Check the wire connector between the cabal and the wire feeder for a solid connection.

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.