In one of my previous companies, I acted as a welder coordinator, and my duty was to make the WPS and WPQ. I had to line up the welders, materials, documents, and the TUV Nord inspector.
By writing this article, I share my knowledge gained from the institute, industry, and my training.
Quite often, someone starts late in a race and finishes first. MIG is the most common industrial welding process today. What is the reason behind the popularity of MIG?
Find the table of contents that will give you an overview.
An old memory still flickers in my mind. Thirty-three-year ago, Mom and I walked past a gentleman creating arcs and sparks. The welder’s clothing seemed soiled and oily, and the helmet was old. But what my mom said still lingers and flickers in my mind. She said, ” Don’t look there, or else you’ll go blind.”
From that time, I developed a fear of welding until I joined engineering school. I learned some fundamentals that are worth reproducing.
Basic Building Blocks of Welding
To weld, you need a couple of things:
- Power supply
- Filler material
The AC mains supply differs from what welding needs, and the power supply converts it into a usable form.
You will find two kinds of electrodes. Consumable electrodes melt the workpiece and sacrifice themselves, while non-consumable electrodes remain unchanged.
Most steels melt at around 1500 degrees Celsius, but they are no match for the electrode-created electric arc. The melting of steel, casting it between the two workpieces, adding flux, and removing slag. Doesn’t it resemble a mini steel mill?
The filler metal unites the two workpieces into one structure. The more it resembles the base metal— chemically and physically—the better the joint.
Welding electrodes conduct the current and create the arc. But they can appear in a double role as well—filler metal.
What is the difference between an electrode and a filler metal?
Depending on the welding process, the electrode and the filler metal can be the same or different. Both are the same in SMAW and MIG welding.
But, in TIG welding, both are different. The electrode is a tungsten rod that does not melt, and a filler metal becomes necessary.
The biggest enemy of a healthy weld is atmospheric contamination. To avoid it, shield the weld pool, one way or another.
One way to shield is through a flux. When the arc strikes, it burns and produces shielding gas. During welding, it floats on the molten metal and protects it. Finally, it combines with the impurities to form the slag.
Another way to shield is to use a shielding gas cylinder. Or you can use both together. As we progress, we will see how it happens.
Welding Procedure Specification (WPS)
A weld requires two more things, and I deliberately kept them out to show their significance.
To start welding, you need a document such as Welding Procedure Specification (WPS). It shows how everything fits together—the type of joint, voltage range, current, filler wire, and shielding.
The WPS lives to tell the tale even after people have moved.
Among the items I have described above, I need to add another one that rarely gets a second glance: the ground clamp. Without a clamp, the circuit is incomplete, and no current flows.
What is MIG welding?
With the above information, we come to the agenda of our meeting—MIG welding.
The American Welding Society refers to MIG as Gas Metal Arc Welding (GMAW), but I will call it MIG as it is more common. Born around World War II, MIG got its name as inert gases were prevalent.
One thing special to MIG is a continuous feed of filler wire that makes it semi-automatic. You can make it automatic also—a boon for mass production.
What is MAG welding?
Often, we come across Metal Active Gas (MAG) welding. It is the same as MIG; the only difference is that the shielding gas is not inert but active.
What is gasless MIG?
Another term that confuses people is gasless welding. Earlier I said that shielding is mandatory, so what is gasless?
In Gasless welding, shielding gas is absent, so the only option is to use a flux-cored electrode. The cross-section of a flux-cored electrode is opposite to a stick one. Flux is inside, and the filler metal is outside.
It is better to call it self-shielded flux-cored arc welding rather than gasless. The shielding is present as a flux-cored electrode.
When to use flux-cored arc welding?
The plus point of using flux-cored electrodes is that you can weld outside in windy areas as the shielding gas may prove ineffective.
Flux is tolerant to dirt due to additives. You can weld on dirty surfaces as it combines with impurities to form a slag.
The setup for flux cored looks attractive, portable, and cost-effective. You do not have to buy, haul and store cylinders.
What is Dual Shielding?
In dual shielding, we use flux-cored wires along with shielding gas. It gives double protection and hence the name.
MIG welding – Natural successor
SMAW has two problems. You must stop once the electrode finishes, chip off the slag, and insert a new electrode. Not only is this time-consuming, but it causes an interruption.
TIG filler rods are almost twice as long as SMAW electrodes. Again, you have to stop when the filler rod finishes and use a new filler rod. You can make clean welds, weld root passes, or thin materials, but the process is slow.
MIG got over all these issues. With MIG, you can keep welding until the spool keeps going. Welding quality is good because the shielding gas does not form slag.
Overview of MIG equipment
The first thing is a power source, and there are only two options—AC or DC. MIG prefers DC due to its stable flow. DC does not reverse polarity in every cycle, as the electron flow is unidirectional.
The earth clamp connects with the negative terminal, while the MIG gun (electrode) goes to the positive one. You will find the term DCEP (direct current electrode positive).
The electrode (positive terminal) attracts the electrons (negative charge). They carry most of the heat to the filler wire and melt it, and the heat drops back to the joint. It results in deeper penetration.
But in flux-cored arc welding, the polarity is the opposite— DCEN (DC electrode negative). Heat moves from the electrode to the joint, leading to less penetration, and is suitable for small thicknesses. Read more
Do not worry about DCEP or DCEN. Refer to the WPS or the datasheet on the filler wire manufacturer’s website.
Many threads on a reel—this is what the MIG spool wire resembles. Allow the spool to rotate, and it will entangle. The alternative is to insert some resistance it must overcome. Who provides this force?
Between the spool and the welding gun lie the rollers. When they pull the wire from the reel, they push it through the welding gun.
Number of Rollers
We need two rollers to push the wire forward. One becomes the drive roller as a motor drives it, while the other becomes the driven. The drive roller has two grooves cut on it.
Rollers pinch the wire and force them into motion. Pinching (or tension) must be reasonable—high force will damage the wire, while the wire will slip if it is less.
Number of Grooves on a Roller
Each drive roller has two grooves to accommodate two different sizes of filler wire. As an example, you may find 1.2U or 1.0U on aluminum rollers.
Let us assume you are using the 1.2U groove, and you wish to change to the 1.0U groove. Take out the roller and rotate it 180 degrees. Remember that the filler wire diameter must match the groove size.
Shape of Grooves
Groove shape varies. V grooves are best for steel rollers, while flux-cored filler wires use V-knurled drive rolls. Aluminum wires are softer, so U grooves are perfect.
My boss used to say—The site team curses us (the designers) because their designs are tough to install at the site.
My boss was right. Our welders have a tough job as they weld in confined spaces, and their welding gun must reach even more confined areas. How can we design a MIG torch that can access difficult areas?
The long neck of a vulture allows it to reach the deep internals of the carcass. The swan uses its long neck to pluck vegetation while swimming. The MIG gun borrows this inspiration from Nature and solves the accessibility problem.
The torch shape looks like a swan neck and bears the name, thus allowing it to reach hard-to-reach places. You can use it for any welding position or joint type.
The welding gun is where the three meet: filler wire, welding current, and shielding gas. By pressing the gun trigger, the welder coordinates the start and stop of the welding process.
Due to the high heat, the torch needs cooling, so an air-cooled torch is enough for low amperages. Use a water-cooled one for high amperage.
Finally, the gas nozzle directs the shielding gas around the weld zone.
Aluminum spool guns
Aluminum is softer than steel and requires different rollers. Some guns are specific to aluminum, and you can recognize them by a spool present on top.
The good thing is it reduces the travel distance of the filler wire and the chances of damage. You don’t have to worry about changing rollers. But it puts more weight on the wrist of the welder.
The liner of the Welding Gun
The liner acts like a tunnel and provides a channel to the filler wire. It cruises unhindered till it reaches the welding gun.
Many times, the liner fails in its duty. If the liner is loose, there is more legroom, and the filler wire may jumble. And if the liner is a tight fit, it hinders movement and gathers dust.
Before feeding the filler wire, check if the liner can accommodate the filler wire. Steel and aluminum wires have different liners. During welding, try to keep the wire as straight as possible.
The contact tip
The contact tip (in the torch) is where the action is. It takes energy from the power cable and transfers it to the filler wire.
It also happens to be the place where the reaction is. As welding progresses, the weld spatter starts depositing on the tip. Keep this tip handy—change the contact tip if you notice a change in weld quality.
Nitrogen, oxygen, and water vapor are the culprits that cause contamination. They are everywhere, so how to remove them when we weld? One option is to use a heavier inert gas.
Another thing worth noting is the ionization potential. It is the voltage necessary to create the arc. An ideal shielding gas must have low ionization potential. (Here is a technical definition of ionization)
Let us see some common shielding gasses.
Argon is about 1.4 times heavier than air. Pure argon is suitable for shielding non-ferrous metals and stainless steel.
Even though Carbon dioxide is not inert and is an active gas, you can use it at 100% concentration. It provides deep penetration, but it causes more spatter.
Argon and Carbon dioxide
To reduce the spatter of carbon dioxide, companies combine it with argon. The resultant gas produces a smooth weld. 75% argon and 25% CO2 is a popular mixture today.
I know helium is lighter than air, courtesy of the helium-filled balloon images. To compensate for its low density, welders increase the flow rate.
Even though losing points on weight and cost, it scores in another area. Due to its higher ionization potential, the penetration is deep.
People also use a mixture of argon, carbon dioxide, and helium (Trimix). It takes more power and may cause a burn-through, so use it with thick metal.
Gas selection depends on several variables. Material and welding processes are two important ones. People get confused about which shielding gas to use, but there is no need to worry. Refer to the WPS, or consult the product datasheet on any reputed website.
Shielding gas cylinder
This not-so-good-looking cylinder longs for kindness. People haul, pull, roll, sometimes drop it, and forget it is pressurized.
It wants its valves and pressure cap in the closed position. It wants people to handle it using a handcart and to keep it chained. It wants to stand upright on an even surface. It just wants to say one thing, “Handle it with care.”
If someone wants to connect a new cylinder, it wants people to follow proper instructions. Attach the regulator to the gas cylinder, and attach the gas hose to the regulator. If they take the regulator off, put the cap on. The regulator gives people all the information they need: pressure and flow rate.
Types of metal transfer
During welding, the welder deposits filler metal between the workpieces. The formal name is metal transfer.
Short circuit transfer
At low voltages (around 12-20), the deposition is intermittent and resembles a short-circuit. It goes on and off, many times a second. Due to the reduced heat input, you can weld thin materials without worrying about distortion.
Globular metal transfer
If you increase the voltage, the material deposition changes from short-circuiting to globular. Material deposits as irregular globules with a lot of spatter, so it is not desirable.
Increase the voltage beyond this, and the deposition changes from globular transfer to spray. Material deposits as a continuous spray with more penetration and less spatter.
It causes more heat input and creates a large weld pool, so use it for welding in flat and horizontal positions. You can also weld thick materials.
But what if you want to weld thin materials? Increase the travel speed. However, you must avoid welding on open root welds as it will cause a burn-through.
Pulsed Spray Transfer
It uses a pulsing current that keeps fluctuating between high and low levels. Metal deposition occurs at the high level (peak current), then falls to a level enough to sustain the arc.
Due to these oscillating current levels, the average current is lower, and so is the heat input. As you can notice, the pulsed spray transfer process avoids the disadvantage of spray transfer.
Some MIG Filler Wires and their AWS designations
The manufacturer of filler wires follows the standard AWS code for identification. Let us see some examples to make it clear.
What does ER70S-3 mean?
The word ER stands for electrode or rod. We know that an electrode is a conductor of electricity, but what is a rod?
Rod applies to a straight length of filler metal that does not conduct the welding current. For example, TIG welding, so it is a filler metal. You can use this ER for TIG or MIG; it will come in both forms: reel and rod.
The weld metal must have strength equal to the members it is joining. The number 70 means that the weld material will have a tensile strength of 70 ksi, where 1 ksi=1000 psi. We use ksi to make big numbers manageable.
S stands for solid wire, and the filler wire is without flux coating. Carbon steel filler wires appear orangish due to a copper coating that prevents rust.
3 is a numerical number that shows the chemical composition and shielding gas. By the way, ER70S-3 works best on clean surfaces.
ER70S-6 resembles ER70S-3, and the only difference is the last number. 6 denotes the addition of deoxidizers in the flux coating to handle dusty surfaces.
Stainless steel MIG wires designation: what does ER308LSi mean?
Stainless steel filler wire designation differs from carbon steel. For example, ER308LSi is an electrode rod for stainless steel solid wire.
- 308 is a number that indicates it is suitable for welding type 304 stainless steel.
- L implies that it has low carbon content to prevent sensitization. If it was H, it means high carbon content.
- Si shows the presence of silicon.
Let me complete this section by telling you what sensitization is. Between 400 to 800 deg celsius (752 to 1472 degrees F), chromium comes out of the grain boundary. It mixes with carbon to form chromium carbide, so the prime element for corrosion resistance (chromium) is not there. To avoid this, we use stainless steel with low carbon content.
Flux-cored filler wire designation: What does E70T-1C mean?
The designation of flux-cored electrodes is different from that of solid filler wires. For example, take E70T-1C.
The first noticeable thing is that these electrodes have the letter T, where T stands for a tubular electrode.
- 7 gives an idea about the tensile strength. Multiply it by 10,000 to get the value in psi. Have you noticed the difference? We multiplied by 1000 for solid filler wires.
- 0 indicates welding positions such as flat and horizontal.
- T stands for a flux-cored tubular electrode.
- The number 1 gives the wire usability specification.
- C is 100% Co2. If it was M, it means mixed gas, 75%-80% Ar, balance CO2.
Flux-cored filler wire designation: What does E70T-1C-H8 mean?
Other things remain the same, H8 indicates hydrogen less than 8ml/100g.
If you cannot remember what the letters mean, go to the website.
Push and Pull Technique of Weld Deposition
In life, if you want to push something, you move back.
Do it the same with MIG, and move back (the gun) behind the direction of travel. The electrode looks forward, the gas moves ahead, and you get a better view.
Now, pull is the opposite, and to remove any confusion, let me explain it. In the pull technique, move your gun ahead of the travel direction.
You might not see any visual difference. But if you compare the weld samples, push gives a wider bead and hence, less penetration.
MIG Welding Defects and How to Avoid them
Just as creating sound welds is in our hands, so are defects. Let us take the major ones.
If the shield breaks down, atmospheric contaminants will arrive in the weld pool. It could be that the shielding gas cable is leaking. Or it could be that the wind force is high. Or there is too much weld spatter.
Lack of fusion
When the weld metal does not attach to the workpiece, it results in insufficient fusion.
There are simple reasons for the lack of fusion. It could be an improper gun orientation, fast gun speed, or less heat input.
Due to high heat, the weld metal penetrates the base metal and causes a burn-through. Thin base metals are more vulnerable. To avoid it, either reduce the heat input or travel faster.
You will be surprised to know we can put a stop to defects, and you can read in detail about welding defects.
Advantages of MIG welding
MIG can weld metals such as mild steel, stainless steel, and aluminum. It can also work in all positions. Due to the shielding gas, the welder gets more visibility and control of the weld.
MIG equipment was more costly and less portable. But now we have machines that perform TIG, MIG, and stick together.
Disadvantages of MIG welding
MIG is less suitable for outdoor welding because the wind will blow away the shielding gas. You can increase the gas flow or switch to flux-cored arc welding.
You need to prepare the surface as shielding gas is intolerant to dirt. Wire brush or grind until clean.
Safety in MIG Welding
Thirty-three years have passed since I and mom passed by that welder with soiled clothes and an old helmet. If I met him again, I would teach him about safety.
I like to outsource the first-stage inspection to the eyes and ears. I am surprised they have a natural-born talent for finding damaged wires, loose electrical connections, and uncanny sounds.
Prevent the Fire Triangle
Three things are mandatory to start a fire: source of ignition, oxygen, and combustible material. Fires do not start until a flammable material is present.
To identify flammable material, refer to its Material safety data sheets (MSDS). Our daily landscape is thick with messages such as warning signs on containers. Notice them.
Even clothing and sneakers can burn if there is oil and grease.
Lack of oxygen can be fatal. The continuous flow of shielding gas and welding fumes can cause asphyxiation (deprivation of oxygen).
It protects a priceless part—the eyes. Without a proper helmet, everything can automatically darken.
The helmet protects from sparks, arc eyes (inflammation of the cornea), and harmful rays. It rescues us from sunburns, depends whether welding is for business or pleasure.
I gave you a guide to MIG welding through my experience and education. You have a broader glimpse of what MIG is all about. And yes, if you happen to pass by a welder welding in soiled clothes and using an old helmet, request him to read this article so that he knows what my mom told me and what I told you.