What is Flux Core Welding for Beginners and Everyone Else

By Bill Byers / January 7, 2019
What Is Flux Core Welding Title Image

Curious and want to know what is flux core welding? I give a full explanation for those beginners and for everyone else to want to learn. Enjoy!

What is Flux Core Welding?

Flux core welding also known as FCAW (Flux Core Arc Welding) is a popular form of welding that joins two separate pieces of metal together, most commonly mild steel.

The arc part of flux core arc welding shows that electricity is crucial to this welding method. An intense and focused electrical arc is formed at the end of a continuously fed spool of welding wire.

The welding wire is special as it has a flux core - hence the name. In the flux core welding process, the welding wire is known as the electrode and is tubular. You have essentially a small tube of metal a bit like a straw with metal on the outside and the flux that gives the flux core its name on the inside.

Flux Core Wire

Flux Core Wire

​The electrical arc is stuck and melts the flux core wire. The heat generated by the arc also melts the base metals (the metals to be welded) and a small pool of molten metal is formed.

The flux core within the wire forms a protective gas known as a shielding gas to protect the molten pool of metal. Part of the flux also drops into the molten pool and creates a covering over the weld pool known as slag.

This slag covering is removed once the weld has cooled.

Flux core welding can be an automatic or semi automatic process. It can also use a separate tank of high pressure gas and then it is known as dual shielded flux core welding.

​YouTube Video on Flux Core Arc Welding

​This YouTube video is a really good and clear introduction to the principals of flux core welding. For those of us who are more visual and appreciate seeing a video this  5 minutes 6 seconds it is well worth a watch.

Video Credit: WeldNotes.com

​Why is Flux Core Welding Popular?

​As welding machines have reduced in price having your own welder at home is an option for even the most basic of workshop environments. Flux core welders, using self shielding flux core wires are one of the cheapest welding machines available.

And these machines are great for beginner welders as all you need is the welder, some flux core welding wire, an electrical supply and some basic safety equipment and your good to go.

Self shielded flux core welding is ideal for newbie welders as it is very tolerant of the lack of skill of the welder. And with a few hours of practice a novice welder can produce a good weld that will hold.

Self shielded flux core wire is great for welding outdoors as the process tolerates all but the very windiest of conditions. And the process doesn't demand that the metal being welded is scrupulously clean.

Though clean metal (cleaning with a metal brush or grinder) is always recommended. You can get away with an amount of dirty metal because the flux within the flux core welding wire helps cleans them out.

This means that it lifts to the surface contaminants in the weld pool, where those contaminants then sit as part of the slag and can be chipped off after welding.

Flux core welding has some of the highest deposition rates of welding wires. (Don't worry I'll explain more about what that means later.) It is also better at welding thicker gauges of metals and is better for out of position welds.

I'll explain what out of position welds are too later on in the post.

​What is Flux Core Welding Used For?

​Most often is it used for welding mild steel - also known as low alloy steel. Specialist flux core wires are used for stainless steel, though these wires are not as reliable, it is possible to get some good results.

Certain types of flux core wire can weld some high nickel alloys.

Aluminum cannot be welded with flux core wire. If you want to find out why I have an article dedicated to the subject called "Flux Core Aluminum Welding Wire, I Want It Now". ​

The right type of welding wire electrode needs to be chosen for the metal being welded. This means that if you are welding mild steel then the flux core wire needs to be for mild steel.

INETUB Flux Core Wire For Mild Steel

​If welding stainless steel then you need flux core wire rated for stainless steel.

Looking for a longer explanation on the typical uses of flux core welding? I have a document all about it on the site.

​Out of Position Welds

I promised to explain for those new to welding what welding out of position means.

Welding flat on a horizontal plane is the easiest welding as the weld can be seen from the top and gravity keeps the molten metal in the weld pool.

Welding a weld that is overhead is an example of an out of position weld.

Unless you have suitable welding wire that penetrates the metal quickly, forming a good weld, filled correctly with filler metal, you risk the filler metal dropping out of the weld. Dripping metal onto you, burning you and causing a poor weld.

Because flux core welding has higher deposition rates, it allows you the welder to move quickly, producing a strong weld that keeps the filler metal where it should be -  in the weld.

That is why flux core wire is often preferred for welding out of position.

​What is Welding?

​Before we go any further I want to be sure for those newbies here that you understand what is the aim of welding.

At the most basic level welding is the act of joining together two separate pieces of metal so that they act as one piece.

So whether you are building something new out of metal or carrying out a repair the goal is once you have joined the metals they are as one.

And the joint is as strong if not stronger than the individual pieces.

​Arc Welding using Flux Core Wire

​Flux core welding uses electricity in the form of an electrical arc to bring two separate pieces of metal to a liquid state at a particular point.

Welding Base Metals

Welding Base Metals

The two separate pieces are called the base metal and in flux core welding the base metal can be mild steel, low alloy steel, stainless steel or low alloy nickel.

An electrical arc is created at the end of the flux core wire when the wire is brought within 1/2 of an inch of the joint.

​Where the base metal melts is called the weld pool. The arc also melts metal in the flux core wire and that metal drips into the molten weld pool.

A weld along a joint is made by travelling the flux core wire with its arc on the end along the joint. The electrical arc only melts a small area at a time and as the arc travels along, the weld pool solidifies behind the arc and the weld forms.

Melted In Turn To Form The Join

Melted In Turn To Form The Join

​This gives you a high level view of the process but there are other key features of flux core wire welding.

​What is Flux Core Wire?

​I have a document on the site that goes into much more detail on what flux core wire is so I will only give a brief overview here. 

Flux core wire is welding wire designed to go into and be used in a flux core only welder or a MIG welder capable of using flux core wire. The wire itself consists of a long tube with metal on the outside and compounds called flux on the inside.

Inside Flux Core Wire

​You choose your flux core wire with metal on the outside that matches and is suitable for welding the base metal you're looking to weld.

The compounds inside the flux core wire include deoxidizers, alloys and fluxing agents that when melted at high temperatures form a gas, plus more solid matter that drops into the weld pool.

If you want the low down, head over to "What is Flux Core Wire, When to Use and Why". 

The flux core wire itself sits on a spool of wire held inside the flux core welder. It feeds out through a drive mechanism within the welder, up a hose to the gun or torch.

The intensity of the welding arc and the heat generated is set by a voltage or amperage dial or button the flux core welder. The speed that the wire is fed out at is also controllable and is at a constant speed.

Forney Easy Weld 125FC Front Dial and Switch

Forney Easy Weld 125FC Front Dial and Switch

​Why is the Flux Core Wire Melted into the Weld?

When you weld, whether you are using two fresh pieces of metal butted end to end or repairing gap in a car panel with a metal patch there is the join - the joint.

If only the base metals were melted then the joint would be thinner - after all if nothing else there is an air gap to fill.

That is why you need something called filler metal and in flux core welding this filler metal is provided by the flux core wire itself.

Which is why this kind of welding is sometimes called flux core wire welding.

There are other terms for this method of welding.

You may have also heard the term 'consumable electrode'. The flux core welding wire is a consumable electrode. The arc forms at the end of the wire so it is an electrode and is used up by the weld, so it is consumable.

To round up this point, the flux core wire provides the filler metal, it is a consumable electrode and is the means by which the arc is delivered to the metals welded.

​Does Flux Core Welding Require Gas?

Yes, but I need to explain further as all types of arc welding require the protection of gas as ...

The air that surrounds us and provides us with the oxygen we breathe is reactive with and destructive to welds. Oxygen and the Nitrogen in the air are particular problems. If the melted metal came into contact with these gasses the metal would react causing a weld that would be weak, and fail.

To prevent the contact with air the metal must be protected. And in flux core welding gas is used to provide that protection.

In fact there are two types of flux core wire welding.

One is called self shielded and is also referred to as FCAW-S and the other is called dual shielded or gas shielded referred to as FCAW-G.

​The Protective Gas of Self Shielded Flux Core Wire

​In self shielded flux core welding this protective gas is produced and is provided by the flux in the core of the wire.

As the wire melts, part of the flux melts to form a protective gas over the molten weld pool. This gas shields the weld pool from the gasses in the atmosphere we breathe.

FCAWS Shielding Gas

And because self-shielded flux core welding produces its shielding gas as the wire is melted into the weld it is ideal for beginners as there is no separate tank of gas to source, set up and use.

You only have to contend with setting up the flux core welder correctly for the gauge and type of metal you are welding and honing your skill.

And flux core welding is great for welding outdoors, because you have the convenience of simply bringing your welder along, and no worry about setting up protective barriers to protect your weld from the wind blowing your shielding gas away.

Note: Flux core wire described as self shielding should not be used with a separate tank of high pressure shielding gas. I explain why later on in the article.

​Gas Shielded Flux Core Wire

Gas shielded flux core wire uses separate shielding gas, just like MIG welding. The wire used is flux core of course, rather than solid wire but a cylinder of gas is  also needed. The gas is fed from a high pressure gas tank and is used to protect the weld pool.

This is the reason this form of flux core welding is called double shielded or dual shielded flux core welding because you not only have the flux in the flux core wire protecting the weld you have extra protection from the atmosphere by using a separate shielding gas.

The gas is controlled through a regulator.

Millermatic 135 Gas Regulator

​Regulator

​The regulator sets the gas pressure and the speed the gas is fed out at.

Just like MIG welding this form of welding needs a work shop environment otherwise the shielding gas can be blown away by the wind.

And really this form of welding can be seen as a combination of MIG welding and flux core wire welding.

Which means that the welding machine that is used must be one that can be set up to feed flux core wire and has the ability to feed gas up to the welding gun.

It is about as difficult a welding method to learn and use as MIG welding.

The flux core wire chosen must be one suitable for using with separate shielding gas as the chemical make up of self-shielded compared to dual shielded flux core wire is quite different. If you try and use gas with self shielded flux core wire you risk getting defects with your weld.

Dual shielded flux core welding is used for welding thicker metals because of its better weld penetration and it is good for out of position welds.

Gas shielded flux core wire has the ability to lay down the filler metal at an extremely high rate so you get much greater metal deposition rates, meaning faster welds.

Dual shielding flux core wire tends to produce fewer weld defects when compared to stick welding or MIG welding

The slag created by dual shielded welding is also easier to remove.

​What Kind of Shielding Gas is Used with Dual Shield

​Typically they are;

  • 100% Carbon Dioxide
  • Carbon Dioxide and Argon mixes such as C25, which is 75% Argon and 25% Carbon Dioxide
  • 100% Argon
  • Argon and Oxygen

It is important to check the welding parameters chart for your chosen dual shielded flux core wire to match the gas you are using to the wire.

Welding parameter charts are produced by the wire manufacturers and details voltages, wire feed speeds, stickout and shielding gasses to use to get the best result. They are found on the manufacturers websites.

Otherwise you may well get poor results and cracking of your weld.

Which is really not worth it when you have gone to the cost of purchasing dual shielded flux core and the cost of shielding gas, only to get bad results at the end.

​Flux Core Welding Use of Electricity

Flux core wire welding whether self shielded or dual shielded uses an electrical arc to melt the base metals with the filler metal in the welding wire.

This means that the flux core welder is provided with electricity. Electricity only works within a circuit. When the trigger on the flux core wire welder is pressed, electricity is fed to the flux core wire, which is run up through your welder's hose to the torch.

The electrical arc is generated at the end of the flux core wire arcing across to the base metals you are welding. The base metals also conduct electricity. Your base metals - your welding project - needs to be connected to a grounding clamp.

This clamp takes the electrical circuit back to your welder and out to the ground provided by the socket powering your welder.

Flux Core Electrical Circuit

​Flux Core Electrical Circuit

​As good electrical conductivity is needed it is important to clean the metals you are welding as much as you can.

Have your grounding clamp connected as close to your welding gun as practical. Connect the ground clamp to your project if you can and check the ground clamp is of good quality so it can conduct the electricity really well and has good clean connections.

​Flux Core Welding Polarity

Similar to using a battery where you have a positive terminal and a negative terminal the electrical circuit for flux core welding needs to be set correctly for successful welding.

Self shielded flux core wire needs what is called straight polarity. Sometimes it is referred to as MIG gun negative or you may hear the term DCEN used. DCEN stands for Direct Current Electrode Negative.

This is the opposite setting for MIG welding and for dual shielded flux core wire where reverse polarity is needed. Reverse polarity is also referred to as DCEP - Direct Current Electrode Positive.

I mentioned earlier in the article that it is best not to use shielding gas with self shielded flux core wire. This is because the chemical make up of the wire is different to dual shielded and the chemical make up means that is also reacts electrically in a different way and needs a different electrical set up.

If the welder you are using is flux core only, as some of the cheapest flux core welders are, then by default it will be designed to use self shielded flux core wire, and set to Direct Current Electrode Negative.

It will not be able to feed gas or use dual shielded flux core wire.

If you have a MIG welder that can feed both flux core wire and solid MIG wire then be sure to check and change the polarity when you switch wires as this is something often missed and causes failed welds.

​Feeding Flux Core Wire Through Your Welder

If your welder is a flux core only welder then the wire rollers will already be knurled. Knurled rollers are designed to grip and feed the flux core wire without crushing the wire and damaging it. Flux core wire is more delicate because its center contains flux compared to the solid center of MIG wire.

If your welder is a MIG welder that can also feed flux core wire then the odds are you will need to change over the rollers. Rollers designed for feeding solid wire have a smooth v shape to the groove the wire travels along.

Knurled rollers have a U shape and indents in the groove. This offers a surface that is designed to grip the wire but not crush.

Knurled Drive Roll

Knurled Drive Roll

​And on the subject of crushing your flux core welding wire check the wire tension, too tight a tension setting on your welder can crush your flux cored wire and cause a bad weld.

​What Thicknesses of Metals Can Flux Core Weld

Flux core welding is quite a 'hot' process and isn't suitable for the thinnest metals. Say 20 gauge and below.

At the other end of the scale flux core wire welding can easily go up to 24 gauge,

Quarter inch, half inch and five sixteenths of an inch may well need beveling and joint preparation depending on the power your welder can produce.

Assuming your welding machine is up to it and can produce a strong enough arc then flux core welding is great for thicker metals.

Flux core welding is often used when greater weld penetration on thicker metals is wanted, as that is when the higher deposition rate of flux core wire is needed.

​What does High Deposition Rate Mean?

​I said earlier in the article that I would explain what high deposition rate is.

Each brand and size of flux core wire, if you check the welding parameters chart, will declare what the deposition rate is of the wire.

Deposition rate is the amount of filler metal the wire is able to deposit into the weld.

Flux core wire is better is depositing more filler metal for the amperage supplied.

Voltage and Amps are needed to produce the arc that melts your flux core wire.

When compared to MIG welding with solid wire, flux core wire welding can be powered with the same amperage, yet melt more wire and put that wire into your weld joint.

This results in faster more productive welding.

And gas shielded flux core welding is even more productive than self shielded flux core welding and so is loved by the heavy construction industries.

​Flux Core Welding Slag

​Flux core wire welding produces slag as by product. It is a key feature of flux core welding. The flux core wire not only melts to produce the shielding gas. Part of the wire falls into the weld pool. The flux scavenges for contaminants and lifts them to the top of the weld pool where the flux core wire forms a covering called slag.

The slag covers the cooling weld pool protecting it from the surrounding air. Once the weld is cool the slag is chipped off.

If you want to know more about slag, have a look at my doc "Why Great Welding Needs Good Welding Slag".

​Why Does Flux Core Welding Produce More Spatter?

​And it does, it is one of the feature to control with flux core welding. The flux core wire itself tends to produce spatter - tiny droplets of molten metal jumping out or off your weld pool.

Welding Producing Spatter

Welding Producing Spatter

​Correct settings on your welder for the flux core wire you are using will minimize spatter.

Take a look at my document "What is Spatter in Welding And Why it Sucks" if you are not sure what spatter is.

And if you are looking to control spatter take a look at my tips for doing that in "How to Stop Welding Spatter and Tactics to Reduce It". 

​Flux Core Welding Fumes

​Along with more spatter flux core wire welding also produces thicker and more potent welding fumes. Again a feature of flux core wire.

In fact the fumes from flux core welding can be so thick that the fumes obscure the joint, wire and weld pool.

Welder Welding Mild Steel With Flux Core

Welder Welding Mild Steel With Flux Core

​To mitigate the problems cause by fumes;

  • Look for the ideal weld angle, of 90 degree angle with a travel angle of 10-15 degrees.
  • You can use the pull or drag technique - positioning the flux core wire angled away from the direction of travel by 10 to 15 degrees.  Like you are 'dragging', the wire with the weld pool positioned behind the wire.
Angle Flux Core Welding Wire Away from Direction of Travel

Angle Flux Core Welding Wire Away from Direction of Travel

​Keep your head out of the fumes or wear a respirator and or have a fan or good ventilation at head height.

​The UV Rays Produced by Flux Core Arc Welding

Flux core welding produces UV rays from the welding arc. And just as the arc itself is intensive enough to melt metal the UV rays are intense enough to give you the welder a good dose of sunburn.

Eye protection is a must not only from the UV rays, also from the light of the arc itself damaging your eyes and from potential stray spatter.

Wear protective clothing on your body and head plus UV rated goggles. The neck area is often forgotten and clothing that comes up to and cover the neck is a great idea.

Flux core welding will quickly give you the worlds worst sunburn without proper protection.

​Automatic Versus Semi Automatic Flux Core Welding

Flux core welding can be both automatic - this means that a robotic welder in a factory setting is set up to use flux core wire as part of the welding process.

Where everything is remotely controlled by a human operator that may control the set up of and settings of multiple robots in an automatic process.

Or Semi Automatic, which is most seen in a work shop or home shop setting.

Forney Easy Weld 125 FC Front

​Example of Flux Core Welding Machine

​The flux core welding machine is set using dials and or buttons. The skills of the human welder controls the;

  • The type of flux core electrode (whether the welding wire is self shielded or gas shielded)
  • Distance from the weld
  • Travel speed and welding angle
  • They listen for the right sounds coming from the weld
  • The operator develops feel. The feel that they are getting the right level of metal penetration and good slag production and coverage for their weld.

The human welder turns the dials or put the setting on the flux core welding machine to control

  • The voltage - the heat intensity produced by the arc
  • Polarity - straight polarity for self shielded. Reverse polarity for dual shielded flux core.
  • The speed the wire is out fed at
  • The cubic feet per hour on the regulator for the gas - if using

Once these settings are set the flux core welder itself keeps them constant and is the automatic part of the semi-automatic process.

The human welder develops their skill to correctly manage the flux core welder for a good weld so that porosity or a cracked weld isn't a problem.

​Isn't Flux Core Welding Expensive?

​When you compare the cost per lb, of buying self shielded flux core welding wire it is generally more expensive when compared to the costs of buying solid welding wire that is used in MIG welding. 

The thing to bear in mind with self shielded flux core wire is there is no cost and potential inconvenience of using a tank of shielding gas.

Once the cost of shielding gas is added to the expenses of MIG welding you may well find the costs even out.

With Dual Shielded flux core welding yes, it is an overall more expensive process. You have the more expensive dual shielded flux core wire and a cylinder of high pressure gas to buy. But what you get in return is the ability to weld much thicker gauges of metal, and more quickly because of its higher deposition rates. A boon, particularly when you have a lot to weld in a short time.

And that is why industry and professional fabricators use this method.

​Problems to Watch For When Flux Core Welding

​Whether using self shielded or gas shielded flux core wire the welder needs to watch for;

Burn back - This is where the electrode wire melts into a ball at the end of the contact tip and then the wire can fuse to the contact tip.

It's a pain to clean up and means a new contact tip is needed.

It is often caused by too close working of the end of the welding wire to the base metal or too slow a wire feed speed.

It is solved by maintaining a distance of a 1/2 inch to 5/8th of an inch from the base metal.

Stickout - This is literally the amount of wire sticking out the end of your contact tip. Too short a wire stickout will also cause burn back.

Welding Wire Stickout

Welding Wire Stickout

​Maintain a half inch to 3/4 inch stickout or follow the manufacturers recommendation for the length of stickout they recommend for their flux core wire. You'll find this information in the welding parameters chart for the wire.

And clip off the little bulb of wire at the end of the welding wore that forms when you pause welding. This ensures you have as smooth an arc as possible when you restart welding.

​Short History of Flux Core Welding

​Flux Core Welding was first developed in the 1950's. A process invented by Bernard and announced in 1954. The patent was lodged in 1957.

As a process it was positioned as an alternative to stick welding - SMAW.

Flux core arc welding overcame many of the restrictions associated with stick welding, as the operator had no need to pause to find and use a fresh stick of wire as the wire was continuously fed.

And it resulted in higher metal deposition rates and metal penetration.

The non shielded gas type of flux core arc welding was liked because it was more portable and regarded as an easier welding process as it is more tolerant of the rough skills of the beginner welder.

Yet still needed some skill to get the settings correct.

​Flux Core Wire Welding Advantages and Disadvantages

​The Advantages of Flux Core Welding Are;

  • that self shielded flux core wire needs no cylinder of shielding gas
  • Can be an all position welding process
  • Flux core welders are generally portable and cost effective
  • The process is more tolerant of out door conditions so you can bring the welder to the job rather than the other way around
  • the skills needed are within the reach of a hobbyist and DIY'er to learn
  • Good strong welds can be produced quickly
  • There is higher deposition rate of the welding wire compared to MIG welding
  • Flux core wire welding is more tolerant of dirt and rust while you weld. Less pre cleaning of the metal to be welded is needed so saving time.
  • Can run on 120 volt home voltage so it convenient for home use
  • Often the same equipment will flux core weld as well as MIG weld

The Disadvantages of Flux Core Welding Are;

  • That it doesn't always produce welds that are attractive to look at
  • More clean up is needed afterwards of your weld because the compounds used in the flux produces slag
  • Excess spatter can be a problem
  • More fumes are produced from flux core welding sometimes making it difficult to see the weld pool well
  • Is not recommended for very thin gauges of steel - thinner than 20 gauge
  • Flux core wire is more expensive pound per pound than solid wire

​Last Words

To round off my post, what is flux core welding? It is an electrical welding process that uses a powerful arc to join two pieces of metal. It uses flux core wire - wire that has a chemical compound called flux in the center and the filler metal around the outside. Flux core welding is ideal for welding outside, welding metals out of position and welding metals that can't be easily completely cleaned.

Beginner welders that are looking for a straight forward welding process to learn their skills on often choose flux core welding. Because with a few hours of practice you can produce good welds that will hold.

​Resource for History of Welding : MillerWelds The History of Welding