Flux Core Welding Vs MIG Welding Head to Head

By Bill Byers / January 27, 2019
Flux Core Welding Vs MIG Title Image

When new to welding you may wonder which is better? Flux core welding or MIG welding or

which one to use in a particular situation.

The truth is no one welding method is better than another.

Each welding process, and here we are focusing on flux core welding vs MIG, is orientated towards different things. Although there is some overlap, each is orientated different welding applications and situations.

Both flux core and MIG welding can produce sound welds.

Whether you use one rather than the other depends on what you're welding, where you are welding and the finish you would like to achieve. And to some extent which you use may come down to personal preferences.

Let me walk you through some key points and differences.

​Terminology

​The first thing I should say is that flux core welding's official title is FCAW - Flux Core Arc Welding.

And that MIG welding's official title is GMAW - Gas Metal Arc Welding.

If you are not sure what each of these welding processes is then I'd like to suggest checking out my articles "What is Flux Core Welding for Beginners and Everyone Else" and the one on MIG welding "What is MIG Welding The Essential Need to Knows" as they will both give you a great grounding on the subject before diving into this article.

​A YouTube Video Comparing the Two

​Here is a great 14 minutes 20 second video from weld.com that covers at a high level the difference between the two welding methods. Worth a watch or carry on and read the detail in this post.

MIG vs Flux Cored Welding and When to Use Each

Video Credit: weld.com

​Flux Core Welding vs MIG Welding the Wire Used

​The first important difference when comparing MIG welding and flux core welding is the electrode - the filler metal welding wire used.

Both use a spool of welding wire as the welding electrode.

Lincoln Electric Solid MIG Wire for Mild Steel

Lincoln Electric Solid MIG Wire for Mild Steel

​When MIG welding a solid wire is used. For flux core welding the welding wire is tubular. Not sure what that is?

A good way to understand this is to think of the flux core welding wire as a bit like a filled straw. The outer part of the tubular wire is metal but the center contains flux something called flux. And this flux is what gives rise to the name flux core wire.

Inside Flux Core Wire

​This is brief and high level description of flux core wire. Want to know more detail about flux core wire?

Look at my article all about it.

This difference in the construction of the two welding wires means the wires behave electrically in contrasting ways.

​Weld Polarity

​One way the difference shows up is in the welding polarity welded with.

The welding polarity setting set on the welder filled with each of the wires is generally the opposite.

Generally?

Let's talk specifics.

Self shielded flux core welding demands a DCEN (Direct Current Electrode Negative) setting when you weld.

What this means is when flux core welding, the electrical current to the welding gun (and the welding gun contains your welding wire), needs to be set to the negative terminal.

And the ground clamp attached to your welding project or the welding table, to the positive terminal.

MIG welding needs DCEP (Direct Current Electrode Positive).

The polar opposite. The MIG gun set to the positive terminal.

MIG welders that can also flux core weld have terminals that can be changed from one to the other.

Example of MIG Polarity Terminals

Example of MIG Polarity Terminals

​It is important for your weld that you have the right setting for your welding wire.  As you may well find the welding arc fails to form, or you have a really poor weld at the end.

Flux core only welders only use flux core welding wire and will be designed and manufactured set to DCEN when you buy.

It is worth noting some exceptions to the above.

Not all Flux core wire is welded with a DCEN setting.

Self shielded flux core wire for welding stainless steel needs a DCEP setting and so can only be used in a welder that allows you to change to this polarity.

Dual shielded flux core wire (also known as gas shielded) is welded with separate shielding gas, just like MIG welding.  This wire also needs a DCEP setting.

​Shielding Gas Compared

​Solid MIG wire has to be welded with shielding gas. This means having a high pressure tank of gas connected to a regulator or flow meter with a hose running into the MIG welder.

Argon/CO2 Shieding Gas Cylinder

Argon/CO2 Shieding Gas Cylinder and Regulator

​This means your welding machine needs to be capable of connecting to a separate tank of shielding gas.

Self Shielded Flux Core wire produces the protective gasses for the weld at the point of welding.  The flux inside the wire melts and produces gas as one of the things the flux does. This gas covers the weld pool.

The advantage of this way of welding is that no transportation or purchase of a cylinder of gas is needed for self shielded flux core welding.

Dual Shielded Flux core wire however demands a shielding gas is used and so in this aspect is similar to MIG welding. Where you need a regulator, hose and a MIG welder capable of feeding that gas.

In MIG welding the type of gases used are:

  • Argon,
  • Argon Carbon Dioxide mixes
  • Pure Carbon Dioxide
  • MIG welding uses Tri-Mix (a mix of Helium, Argon and Carbon Dioxide) to weld Stainless Steel.

Dual shielded flux core wire uses similar gas mixes as part of the dual shield of the welding wire. The gases are;

Argon, Argon Carbon Dioxide mixes and pure carbon dioxide.

​Flux Core or MIG Welding: Strength of the Weld

​Some swear that MIG welding gives a stronger weld.

Others will argue that flux core welding does.

The truth is both methods using this narrow measurement are the same - from a strength point of view. They will both produce a weld that will hold and a weld that is just as strong as each other - if welded well.

Lets take an example of welding 18 gauge mild steel, a thickness of metal that both MIG and Flux core wire is capable of welding.

And then we look at the filler wire electrode. Solid steel welding wire used for MIG welding compared to mild steel flux core. They both have to conform to the American Welding Society standard of a minimum of 70 ksi tensile strength.

Assuming the amperage and wire feed speed and gas coverage (in the case of MIG or gas shielded flux core) is correctly set for the wire.

And the skill of the person welding is the same.

All things being equal, no one wire will make the stronger weld. The strength will be the same.

It is not in this aspect will you see a difference.

Example of A MIG Weld

Example of A MIG Weld

​Metal Penetration

Flux core welding has the reputation of delivering better metal penetration compared to MIG welding. Is it true?

Again, the answer is it depends, as both welding methods used effectively will give great metal penetration.

The truth of this argument lies in the amperage or volts you have available to weld with compared to metal thickness.

Or put another way, what can you weld for the electrical input into the weld.

Flux core will allow you to weld thicker metal for the amount of amps - electrical power sent down the wire to form the arc.

Let me take you through an example. The Hobart 140 using flux core mild steel wire 0.030 size.

will weld 3/16th of an inch (4.8 mm) thick base metal at its setting number 4 and using 45 wire feed setting.

Compared to MIG welding with solid wire for mild steel. Taking the same sized welding wire 0.030 with C25 shielding gas, the Hobart 140 will weld 1/8 of an inch (3.2 mm) thick base metal at the welder's number 4 setting and 40 wire feed speed.

For the same welding setting on the Hobart 140 you can weld 3/16th flux core compared to 1/8 with MIG.

The same principal is true whether we are talking about any welder, Hobart, Lincoln, Everlast, ...

You can use less amperage for the wire size and gauge of metal to be welded with flux core welding.

Or think of it this way

Flux core welding gives more weld for your electrical buck.

If welding at the welding limit of your welder, using flux core wire will allow you to weld thicker metal. And when taking your flux core welder to a weld site you will not need as big a generator to weld compared to MIG welding.

​Flux Core Welding Compared to MIG: Thickness of Metal Welded

​Following on from the better penetration argument the difference in the two processes can be seen in the thickness of metal each can weld.

Flux core as the 'hotter' process generally is used for a minimum of 20 gauge metal as its thinnest metal welded. Or you risk blowing holes through your metal.

Versus MIG welding with solid wire and gas will weld down to 24 gauge.

​​Note: Bigger number gauges mean thinner metal.


This is partly helped by solid wire being available in smaller diameter wire sizes. But the 'cooler' MIG welding process is better suited to thinner gauges of metal.

At the opposite end of the spectrum, if you have a powerful enough welder and gas shielded flux core wire to weld with you can zip through welding half inch thick steel in a single pass with flux core wire.

Flux core wins when your looking at thick gauges of metal and it is why the construction and ship building industries have used flux core welding for many years.

Types of Metals Welded

​When you compare MIG welding and flux core welding there is some overlap in terms of metals welded and some notable differences.

Flux core is used to weld galvanized steel. It's the process that copes with that steel covering. ​

Flux Core Welding

Flux Core Welding Galvanized Steel

​MIG welding needs the galvanization completely cleaned off before welding.

Both MIG welding and flux core welding will weld mild steel and low alloy steel.

There is flux core wire available to weld stainless steel but MIG welding is used more often to weld stainless steel, as the results are more consistent. ​

MIG welding is the welding process of choice for welding Aluminum as there is no flux core wire that will weld Aluminum.

​MIG Versus Flux Core Weld Quality

​MIG is often claimed to produce the better quality weld. Is this true?

To answer the question a definition of quality is needed.

If the strength of the weld is a definition of quality then both welding methods produce a strong weld.

If the look of the finished weld is your measurement then with skilled hands both processes will produce good looking welds.

One issue with MIG welding is it seems easier to produce a quality weld. But in novice hands a great looking weld can be produced that has little metal penetration. Effectively the filler metal sits on top of the weld joint and doesn't penetrate at all well, leading to the 'good looking' weld failing over time.

So where does this argument come from?

​Slag and Spatter

​The argument that says MIG produces better quality welds is normally down to the production of slag and spatter in flux core welding.

I have documents that go in depth on slag and welding spatter if you want to know more about these two subjects. Head over to them and have a read. For now I will cover both briefly. ​

Flux core welding as part of its welding process produces a welding slag. A covering over the hot weld pool that needs to cool before it's removed to reveal the underlying weld.

Example Of A Flux Core Weld Slag On

Example Of A Flux Core Weld Slag On

The flux in the flux core welding wire also removes impurities from the weld into the slag.

The covering called slag in MIG welding is lighter and more easily brushed away.

Spatter is produced by the welding wire sending spots of molten metal all around the weld area.

Both flux core welding and MIG welding produces spatter. And the amount of spatter produced in MIG welding depends on the shielding gas used. For example MIG welding with Carbon Dioxide gas is well known for spatter production.

Yet flux core welding is expected to result in more spatter.

With the correct settings this spatter can be kept to a minimum in flux core welding .​

and products can be bought and used to reduce spatter's effect.

The truth is, that with the right settings used for your flux core welding the slag can pretty much peel itself off. Welding spatter minimized and brushed away using Anti Spatter products.

The argument has some merit and to some extent flux core welding does need more clean up of slag and spatter.

Dual Shielded flux core welding is even better than MIG welding for light weld coverage after welding and low spatter -  of course with the correct settings and gas used.

​Flux Core and MIG Compared on 'Dirty Steel'

​MIG welding is intolerant of 'dirty steel'. This means any steel that is rusty, or covered in mill scale or has paint or galvanization on it.

Metal to be MIG welded needs to be thoroughly cleaned down to the base metal. Or a poor weld with porosity within the weld and the weld rusting and failing will be the result.

Versus Flux core welding which is much more tolerant of 'dirty steel', because one feature of the flux core welding wire is that it contains scavengers. And

these scavengers lift impurities from the base metal into the slag where it is chipped off at the end.

Both MIG welding and flux core welding needs any oil removed from the weld point.

And it is always advised to have your metal as clean as possible before welding. But flux core welding is the method of choice where you need to weld an item that is galvanized. Or can't be easily cleaned because of where it is.

Example Of Flux Core Weld Slag Removed

Example Of Flux Core Weld Slag Removed

​Using Flux Core For Car Body Panels

MIG or indeed TIG welding is often chosen for the visible parts of car body repairs.

This is mainly because the MIG welding process is more effective for the thinner steel used in car panels.

Plus once the weld is made the weld needs to be primed and painted for the best finish.

Using flux core welding for body panels is firstly a problem as the metal is often thinner and flux core welding is just not suited to it.

An inexperienced welder thinking little cleaning of the weld is needed, may well find their flux core weld distributes the impurities through the weld area because they are welding quickly to prevent blowing through the metal,

Or alternative using short tack welds to prevent blowing holes in the metal.

But flux core welding needs good slag coverage, as it is in the welding slag that the impurities are gathered.

And good slag needs heat and enough time to form.

The rush to not blow through the metal or a lot of short tack welds with flux core welding will result in bad slag coverage and impurities and slag within the weld.

And no amount of grinding of the surface of the weld is going to cure that.

And then applying body filler and or primer on top is not going to work on a surface with impurities. Never mind the weld itself holding over time.

Because of this a complete clean of the surface and then MIG welding is often the way to go.

And if you are really pushing the envelope of MIG welding on thin metal body panels, a series of tack welds on thinner material is only really feasible with a MIG welder.

​Flux Core VS MIG Welding Which is Cheaper

​MIG welding is often said to be the cheaper welding method compared to flux core welding. Because the solid welding wire of MIG welding is cheaper to buy pound per pound compared to flux core welding wire. This is true.

But the argument is not quite as straight forward as that.

Flux core only welders tend to be cheaper to buy than a MIG welder that can also weld with flux core wire. Though of course you do have more flexibility with a welder that can weld with both methods - you can choose the method you want to use.

What is missing from the 'flux core welding is more expensive' argument is;

  • MIG wire needs shielding gas
  • You need to buy a tank of gas, with the implications of requiring different tanks filled with the type of gas needed for each type of metal welded
  • Size of tank and transportation of said tank of gas to and from where it is refilled
  • Safe storage of the gas cylinder, including safe lifting and moving it to where you weld
  • ​Buying the gas itself

When all these costs are taken into account you may well find the cost versus convenience of flux wire is much closer than you thought.

Flux Core Welding Outside

Flux Core Welding Outside

​Where You can Weld: Flux Core VS MIG

MIG welding and gas shielded flux core welding are inside processes. Meaning you will need a work shop, garage or work space to weld in.

As MIG welding and dual shielded flux core welding uses a tank of shielding gas. This gas is sensitive to wind and can be blown away, leaving the hot welded metal unprotected outdoors.

Yes, wind shields can be put up to protect the weld area if you do need to MIG weld out in a field. But is yet more equipment needed at and transported to the weld site.

Self shielded flux core welding is the welding process of choice when welding outside. Self shielded flux core welding is tolerant of quite windy conditions. And there is less equipment to take to an outdoor location. Take the welder filled with welding wire, a means of getting electricity and your safety wear and you are ready to weld.

​YouTube Video Demonstrating Welding In High Winds

​I found this 8 minute 38 second video (if you watch the first few minutes you'll get the idea) showing a flux core weld outdoors in pretty windy conditions.  It illustrates well the benefits of flux core for outdoors.

Flux Core Welding Thin Tubing

Video Credit: TheToolReview

​MIG vs Flux Core Other Considerations When Choosing Where To Weld

​Just because MIG welding is an indoor process this does not mean you do not need ventilation, whether MIG welding or flux core welding good ventilation is needed for the person welding.

Because: MIG welding and dual shielded flux core welding uses gasses that could suffocate the welder if leaked, killing silently. Good ventilation is a must.

Flux core welding produces more smoke and fumes while welding and can make it difficult to see the weld puddle.

Flux Core Welding Fumes

Flux Core Welding Fumes

​And some of these fumes are noxious and shouldn't be breathed in.

​A Round Up of Other differences

1. Flux core welding is more often used for out of position welding. This is where you are welding at any angle other than a flat, horizontal angle. Vertical, uphill and particularly for overhead welding flux core wins.

2. Both welding methods need a contact tip of the right size for the welding wire used. MIG needs a nozzle over the contact tip to direct the shielding gas to cover the weld pool.

3. Flux core welding needs greater welding wire stickout length. Around twice the stickout compared to MIG welding.

Welding Wire Stickout

Welding Wire Stickout

But always check the stickout recommendations from the manufacturer of the welding wire you're using.

4.Flux core wire needs knurled drive rolls installed inside the welder to feed the flux core wire over. The grooves in knurled drive rolls are U shaped and notched to gently grip the more delicate flux core wire. MIG welder drive rolls have V shaped grooves to feed the solid MIG wire.

Knurled Drive Roll

Knurled Drive Roll

​5. The welding gun angle for flux core welding is generally advised to be a drag angle, with the welding wire and gun angled ahead of the weld pool. This is because this angle helps build correct slag coverage in flux core welding.  For MIG welding the opposite a push angle is generally advised.

​Last Words

​I hope you enjoyed my head to head assessment of flux core welding vs MIG welding and now agree that the welding method you choose to use depends on what you are welding.

The type of metal, the kind of object you are welding. If it cannot be cleaned well or brought inside where you can MIG weld then your option when comparing the two is to flux core weld.

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