Insider’s Guide to Flux-Cored Arc Welding (FCAW)

Flux-cored arc welding (FCAW) is an arc welding process that has many of the advantages of shielded metal arc welding (SMAW) process as well as gas metal arc welding (GMAW) process. And, unlike SMAW, FCAW is amenable to automation due to its continuous flux-cored wire electrode feed.

This article takes you through various subtopics viz. what is flux-cored arc welding and how it works, types of FCAW process, equipment used in FCAW, advantages and disadvantages or limitations of FCAW, application of FCAW, differences between FCAW and GMAW, differences between FCAW and SMAW, shielding gas and electrodes used in FCAW, and safety.

What is flux-cored arc welding, and how it works?

Flux-cored arc welding (FCAW) is an arc welding process. The heat required for the coalescence of workpiece metals (faying surfaces) is generated by striking an arc between the continuously fed consumable flux-cored wire electrode and the workpiece. Shielding of the weld pool and the arc is provided by the gasses produced by the burning of flux during welding. There is a provision for using an additional shielding gas supplied from an external source.

The arc is struck between a continuously fed flux-cored wire electrode and the workpiece, and the arc melts the wire electrode and the faying surfaces to form the weld pool. The weld pool and the arc are protected by the gasses produced by the burning of flux and/or by an external supply of shielding gas. The welding gun used for FCAW has a continuous feed of flux-cored wire and optional shielding gas. FCAW process has a high productivity rate and can be used for the welding of mild steel, plain carbon steels, alloy steels, stainless steel, duplex steels, and hard facing (surfacing). However, FCAW is normally not used for welding nonferrous metals like aluminum.

FCAW process has many features common with SMAW and GMAW/MIG/MAG welding process. FCAW uses a flux-cored wire electrode with a flux similar to the flux coated welding rod used in the SMAW process (basic or rutile type of flux); however, unlike in SMAW, the flux-cored wire is fed continuously (as in MIG/MAG). FCAW normally uses a constant voltage power supply and equipment similar to GMAW/MIG/MAG.

FCAW has a provision for using an active gas as a shielding gas (similar to metal active gas (MAG) welding), and FCAW is better than MAG welding in terms of productivity. FCAW process was developed during the 1950s, and it was meant to be an alternative for SMAW/MMA. One of the major issues with SMAW was the stoppage of replacing the consumed electrode with a fresh electrode, and FCAW overcomes this issue with a continuous feed of flux-cored wire electrodes.

Good penetrative properties of flux-cored wire electrodes, along with high deposition rates, make it good for extensive use for outdoor welding and welding contaminated metals. FCAW can be used as a semiautomatic machine, as well as an automatic process. In a semiautomatic FCAW process, the flux-cored wired is fed continuously, and the power source maintains the arc length. The welder moves the welding gun manually and adjusts the welding parameters as required. In the case of the machine FCAW process, in addition to the continuous flux-cored wire feed and arc length control, the machine feeds the weld joint to the welding gun. The welder monitors the welding parameters. A completely automated FCAW process is used when the rate of production required is very high.

FCAW process is particularly helpful when welding metals with contamination, viz. scale, rust, etc. Excellent welding properties can be achieved (for different metals) by selecting flux-cored wire electrodes, shielding gas, and other parameters. The outcome of the FCAW process depends on the following variables viz. type of flux cored wire electrode and its feed speed, arc voltage, electrode extension or stick-out length, travel speed and angle of the welding gun, and the shielding gas (if used).

Types of the flux-cored arc welding process

The FCAW process can be divided into two types, the first type is FCAW that does not use a shielding gas and solely depends on the gas produced by the burning flux for the protection of weld pool and arc from atmospheric air and contaminations, and the second type is FCAW that depends on the external supply of active gas for shielding. In this case, the flux in the core can be a mixture of flux and the alloying elements and grain refiners.

When the FCAW process is used without external shielding gas, it is called self-shielded FCAW, and when used with an external shielding gas, it is called “dual shield FCAW.” Let us understand these two types of FCAW.

1. Self-shielding flux-cored arc welding process

The self-shielding FCAW process does not use an external shielding gas. The flux-cored wire electrode contains flux material (similar to the flux coating on the electrode used in SMAW) that burns during welding and gives out gases required to protect the weld pool and the arc from atmospheric air and contaminants; it also forms a protective slag on the weld pool. The self-shielding FCAW process has the advantage of high penetration, and since it does not use a shielding gas, it is suitable for indoor and outdoor welding.

However, there are some disadvantages, like it produces excess smoke, which makes it difficult for the welder to see the weld pool, and also the smoke is harmful to human health. Self-shielding FCAW process is used for indoor welding viz. fabrication in a workshop, maintenance works, shipbuilding, and outdoor welding at construction sites, heavy structural steel weldments, offshore fabrication, etc.

2. Dual-shield flux-cored arc welding process

This type of FCAW uses a shielding gas provided by an external source and the flux contained in the flux-cored wire electrode; hence welders call this process a “dual-shield” FCAW process. This method was principally developed for welding structural steel. When FCAW is used with a shielding gas, the shielding gas protects the weld pool and the arc from atmospheric air and contamination, and the shielding gas is supplied from a cylinder. The weld joint has the additional shielding of the slag formed by the molten flux. When FCAW is used with external shielding gas, the flux in the wire electrode provides de-oxidation of the weld pool. The flux can contain more alloying elements and less flux, contributing to a better quality of weld metal with higher mechanical strength and properties.

Carbon dioxide or a mixture of argon and carbon dioxide (75% argon and 25% carbon dioxide) are normally used as the shielding gas in the FCAW process. Under similar conditions, the FCAW process gives a higher weld deposition rate, better consistency of mechanical properties, and fewer welding defects than SMAW or MIG/MAG welding. This method is preferred for welding thick metals and out-of-position weldments. However, the dual shielding FCAW process is not suitable for welding at an open site since the turbulence in the flow of wind disturbs the protection of the shielding gas.

Few welding tips from experienced welders

• Use V or U-groove (or knurled) drive rollers for feeding the wire, and this prevents twisting of the flux-cored wire electrode.
• Keep a wire brush and a chipping hammer handy for chipping the slag and cleaning the weld bead between weld passes.
• Take care to set all the welding parameters correctly for the weldment being done.
• FCAW can be used for welding metals with contaminations like surface scales and rust, to some extent. However, you have to ensure the metals do not have oil, grease, and paint on their surface since they can result in a porous weld.
• Maintain proper stick-out length for the flux-cored wire electrode.
• Take all the recommended safety precautions and wear the recommended welding gear.
• If you have bad welding, refer to the manufacturer’s troubleshooting chart on the machine panel, and take corrective actions (the issue can be wire feed speed, travel speed of the welding gun, welding polarity, etc.).
• The flux-cored wire electrode forms a slag, and you have to ensure to drag the wire electrode and facilitate the slag formation. Mistakes in doing this can lead to slag inclusions in the weld.
• Also, when you are welding on a flat surface, the working angle can be 90 degrees, and for a lap or T joint, it can be 45 degrees.

Equipment used in the flux-cored arc welding process

The equipment used in flux-cored arc welding are:

• Source of power.
• Flux-cored wire feeding unit.
• Welding gun.
• Welding cables.
• Wire brush and chipping hammer.
• Safety gear.

1. Source of power

The power source and the welding machine for FCAW and GMAW/MIG/MAG welding are very similar, and the major difference between them is the type of wire electrode used and the shielding provided for the molten weld pool and the arc. FCAW process uses direct current (DC); both generator type and transformer-rectifier type are used. Many of the power sources used for the FCAW process are capable of continuous use (100% duty cycle); however, machines with a 60% duty cycle are also available. FCAW process can be operated with either straight (DCEN, direct current electrode negative) or reverse (DCEP, direct current electrode positive) polarity.

The flux-cored wire electrode used in the dual shielding (with external shielding gas) FCAW process is normally designed to operate with DCEP. Self-shielding flux-cored wire electrodes are available for operation with both DCEP and DCEN. Welding current with DCEP polarity gives deeper penetration into the weld joint. Welding current with DCEN polarity gives a faster deposition rate, and since the weld penetration is less, it can be used for welding thinner metals. Compared to DCEP, the weld produced by DCEN is wider and shallower.

The generator-type machines used in the FCAW process can be powered by an electric motor for indoor applications and by an internal combustion engine for outdoor applications. Generators produce a stable arc; however, they are noisy, expensive, and require more maintenance compared to the transformer-rectifier type of machine.

2. The flux-cored wire feed unit

The wire feed unit ensures a continuous supply of the flux-cored wire electrode to the welding gun. The selection of the wire feed system depends on the welding application. The majority of the wire feed systems used in the FCAW process are of constant speed type, using constant voltage power sources. However, when a variable speed wire feeder is used, it will have a circuit for voltage sensing to ensure the required arc length by varying the wire feed speed (variation in arc length results in an increase or decrease of wire speed).

3. Welding gun

The welding guns can be different for self-shielded FCAW and dual-shield FCAW. Depending on the current rating of the welding gun, the FCAW welding guns can be air-cooled or water-cooled. The surrounding air mainly cools the air-cooled welding guns used in the FCAW process, and when used with shielding gas, the shielding gas also helps in cooling the gun. Water-cooled welding guns have passages for cooling water (in and out).

FCAW welding guns are rated at their maximum current capacity and for continuous welding. Water-cooled welding guns are recommended for welding when the current required is more than 600 amperes, and it is a normal practice to use water-cooled welding guns when the current is 500 amperes or more. The cooling is more efficient in water-cooled welding guns. Air-cooled welding guns are lightweight and easier for the welder to manipulate and are preferred for welding when the current is less than 500 amperes.

4. Welding cables

There will be two welding cables, the first welding cable connects the power source and the workpiece (work lead), and the second welding cable connects the welding gun with the power source (electrode lead). The cables are normally of copper material and consist of several copper strands (wires) covered by a thick layer of good quality synthetic rubber.

In the semiautomatic FCAW process, a cable assembly consists of electrode lead, shielding gas hose, and a pipe for wire electrode feed. In the case of the machine and automatic FCAW process, the electrode lead may be separate. The work lead (cable connecting the workpiece) is normally connected to the workpiece using a clamp or a bolt. The size of the welding cable depends on the duty cycle, maximum current used, and the distance between the welding machine and the workpiece.

Shielding gas and electrodes used in the FCAW process

Shielding gas

The shielding gas is supplied from a gas cylinder, and the gas cylinder has pressure gauges and a valve to control the flow of gas. A pressure hose connects the gas cylinder and the welding gun. Many active gasses are not used for shielding; however, carbon dioxide has certain advantages for welding steel, viz. deeper weld penetration and low cost.

When carbon dioxide is used as a shielding gas, it can lead to carburizing or decarburizing the weld metal. Carburizing means the carbon is diffused into the surface of the metal (this may happen when the metal is very low carbon steel), and decarburizing means reduction of the amount of carbon in the metal (this may happen with medium or high carbon steels). When decarburization happens, carbon monoxide can get trapped in the weld metal leading to porosity in the weld.

The carbon dioxide used as a shielding gas breaks down into carbon monoxide and oxygen. Since carbon dioxide is an oxidizing gas, the flux filled in the wire electrode has deoxidizing elements to take out the oxygen. The oxides formed (by the deoxidizing elements in the flux) become part of the slag. A part of the carbon dioxide breaks down to carbon and oxygen, and if the carbon content of the weld pool is very low (below 0.05%), the carbon can get diffused into the weld pool.

The added carbon will reduce some stainless steel’s corrosion resistance, which will affect their end-use application. Added carbon can also affect the ductility and toughness of some low alloy steels. When the carbon content of the weld pool is more (higher than 0.10%), the carbon dioxide will affect by taking out the carbon from the weld pool (reduction in carbon percentage). The loss of carbon may happen due to the formation of carbon monoxide.

A mixture of argon (inert gas) and carbon dioxide (active gas) (75% argon and 25% carbon dioxide) is also used as a shielding gas. This results in the reduction of oxidation (compared to 100% carbon dioxide). The weld deposition with argon-carbon dioxide mixture as the shielding gas normally has higher tensile and yield strengths. The argon and carbon dioxide shielding gas mixture is frequently used for out-of-position welding, and the arc characteristics are better. This is also used for the welding of carbon steel. Argon oxygen mixtures (1 to 2% oxygen) are used in some special applications, like, welding of stainless steel where carbon dioxide may not be desirable (due to possible corrosion issues caused by carbon dioxide).

The choice of the shielding gas in the FCAW process depends on the type of metal being welded, desired mechanical properties, rate of deposition, weld penetration, weld bead shape, etc.

Flux-cored wire electrode

FCAW uses a hollow tube cross-section metal wire electrode filled with the flux (a mix of deoxidizers, flux, alloying elements, and grain refiners as required, and Ferroalloys). Flux-cored wire electrodes are available in coil form. The manufacturing process starts with bending the sheet metal strip of the required width and thickness to a U-shape by passing it through rollers. The U-shaped metal strip is then filled with a measured quantity of the flux. The U-shaped strip filled with flux is then passed through the die to form it into a circular or other required cross-section. The seam looks like a fine line and is not joined. The diameter of the flux-cored wire can be 0.9mm to 3.2 mm. The flux-cored wire electrodes are classified according to the strip metal and the contents of the flux core.

The sheath or the casing constitutes 75 to 90% of the weight of the wire electrode. Self-shielded flux-cored wire electrodes will have a higher proportion of fluxing elements compared to gas-shielded flux-cored wire electrodes. The flux cores of carbon steel and low alloy flux-cored wire electrodes normally contain fluxing compounds. Few low alloy flux-cored wire electrodes may have more alloying elements in their core and less flux; such electrodes need gas shielding.

The cross-section of the flux-cored wire can be a seamless tube, butt seam, joggle seam, or a complex section (all have flux filled in their hollow section). The seamless tube, as the name indicates, is a seamless tube filled with flux. A butt seam is a tube formed from a metal strip and filled with flux (the seam is not joined). Joggle seam is also a tube formed from a metal strip, and here one side of the strip is folded towards the inside, and the hollow section is filled with flux. This design increases the metal content of the tube. A complex section is created by forming the metal strip, and the purpose is to increase the metal content per unit length of the wire electrode. In one design, both the ends are folded inwards.

The function of the flux in flux-cored wire electrode is similar to the flux coated electrodes used in SMAW, and they are:

• The flux contains deoxidizers and denitrifying agents to protect the weld pool from oxidation and improve weld quality.
• Forms a protective slag over the weld pool.
• Adding the required alloying elements to the weld pool to improve its mechanical strength and other properties.
• To form shielding gasses during welding for protecting the weld pool and the arc from atmospheric air and contaminants. In the case of dual-shielding FCAW, the gasses produced by the flux supplements the shielding gas.
• Help for arc stabilization and minimize the spatter.

The FCAW process flux is designed to support the weld pool and increased penetration depth when doing out-of-position welding. Stainless steel flux cored wires are available for welding stainless steel metals with or without using shielding gas. Flux-cored wires are also available for hard-facing (surfacing) base metals with another metal to impart a hard and wear-resistant surface.

The American Welding Society (AWS) has developed a system for easy identification of flux-cored wire electrodes. This coding helps to know, apart from other things, whether the welding with a certain electrode requires an external shielding gas or not.

The code system is: E X X T-X

• The first digit, (E) indicates it is an electrode.
• The second digit (X) will be a numerical number indicating the tensile strength of the weld metal.
• The third digit (X) will also be a numerical number to indicate the welding position.
• The fourth digit (T) indicates that this is a tubular wire electrode.
• The next digit (X) indicates chemical and operating characteristics viz. numerical number 3, 4,6,7,8,10,11,13, and 14 indicates self-shielded electrodes, and numbers 1,2,5,9, and 12 indicates the requirement of external shielding gas.

There can be additional digits (numerical number or alphabet) to indicate additional details.

An example E70T6

(E) indicates electrode, (7) indicates the minimum tensile strength of the weld metal 70,000 pounds/square inch, (0) indicates weld positions (flat and horizontal), (T) indicates tubular wire electrode, and (6) indicates it is self-shielded.

Advantages and disadvantages of the flux-cored arc welding process

Advantages of FCAW

• FCAW process is portable and can be used for outdoor application since shielding gas is not mandatory for this process. This comes with the advantage of higher flexibility with alloying composition, higher wire deposition rates, and improved arc stability.
• FCAW process has a continuous feed of flux-cored wire electrodes similar to GMAW, but unlike in GMAW/MIG/MAG welding, the FCAW process can be used with or without external shielding gas.
• FCAW is an all-position welding process, and the skill level required is less than GMAW/MAG.
• Pre-cleaning the workpiece is not mandatory for the FCAW process since FCAW can be used for welding metals with slight contaminations like mill scale and rust.
• When the FCAW process is done properly, the chances of porosity are minimized to a great extent.
• FCAW process has higher deposition rates compared to similar processes (GMAW/MAG and SMAW), and it is economical.
• FCAW gives better mechanical properties, weld joints are strong, and welding defects are fewer.
• The bead appearance of the FCAW process is good and, in many cases, does not need secondary finishing operations.

Disadvantages or limitations of FCAW

• FCAW process produces more smoke than SMAW or GMAW/MAG, and the smoke obstructs the visibility of the weld pool to the welder. Also, smoke is harmful to human health.
• Porosity can be an issue if the gasses are not able to escape before the weld pool solidifies.
• Flux-cored wire electrode needs better storage facility and better handling compared to solid metal wire electrodes. Also, the flux-cored wire electrode can be costlier than the solid metal wire electrode due to the flux.
• The FCAW process forms a slag on the weld bead, and this needs to be chipped off at the end of the welding.
• FCAW process is not recommended for welding thin metals (less than 20 gauge).
• The complete flux-cored wire spool is to be changed when there is a change in work.
• The equipment used in the FCAW process is expensive compared to SMAW (however, increased productivity of FCAW makes up for this).
• FCAW process is limited to welding ferrous-based metals and a few nickel-based alloys.
• There are chances of the wire electrode tip getting damaged when it touches the workpiece.

Application of flux-cored welding process

• Welding of structural steels.
• Surfacing or hard facing of the required metal onto the base metal.
• The shipbuilding industry and repair of heavy equipment.
• For nuclear engineering and petroleum pipelines.
• Welding of structures at offshore construction sites and also for shallow underwater welding.
• In the automobile industry and other engineering industries, high productive welding of alloy steels, mild steel, carbon steels, and stainless steels are needed.
• Welding of heavy pipelines at outdoor sites or indoor sites.
• FCAW process is extensively used in the construction field due to its fast welding and ease of portability.
• FCAW is preferred for welding ferrous metals and also for applications that require high deposition rates.

Differences between FCAW and GMAW process

There are some similarities between FCAW and GMAW viz. both FCAW and GMAW use a continuous feed of electrode, and like GMAW, FCAW can use an external gas as the shielding gas as an option. The differences are compiled below.

Differences between FCAW and SMAW

There is one similarity between FCAW and SMAW processes. Both use flux in their electrodes, albeit in different forms (FCAW has a flux-filled tubular wire electrode, and SMAW has a flux-coated metal electrode). Both are portable and can be used for outdoor applications. The differences are compiled below.

Safety considerations

As in other arc welding processes, in the FCAW process, the welder has to deal with electric current, welding arc, hot workpieces, welding fumes, thermal radiations, etc., and the welder must take all the safety precautions and must wear safety gear for protection. FCAW produces more harmful fumes compared to other arc welding processes. A welder has to wear leather hand gloves, long sleeve jackets, shoes, good-quality welding helmets (with flip-able welding glasses), and a mask (if there is no provision of built-in protection from fumes in the helmet).

The welding enclosure should have good ventilation for the quick exit of the toxic gases formed during welding. Also, the welding enclosure must not contain inflammable/combustible items like fuel, oil, paper, etc. The welder may often have to sit within a portion of the part being fabricated, like in the shipbuilding industry. In such cases, proper ventilation is very important for the safety of the welder.

Conclusion

Our detailed discussion of FCAW in this article shows that it is a highly productive process, has many advantages of both SMAW and GMAW. One important advantage is that it can be used for outdoor welding without sacrificing its efficiency. All this indicates that the FCAW process is here to stay.

Reference:

• TWI Global Website.
• Wikipedia Encyclopedia.
• New England Tech (NEIT). 
• ScienceDirect Website.