Introduction
The process of welding flux cored wire (or FCAW) is an arc welding that produces the coalescence of metals by heating them with an electric arc established between a continuous, consumable, tubular metallic electrode and the base metal.
Arc and bead protection is provided by a welding flux contained within the electrode, which can be supplemented by additional shielding gas provided by an external source.
Note: FCAW is the acronym in English: Flux Cored Arc Welding.
How the process works
Welding with cored wire was developed in order to combine the advantages of the MIG / MAG process (semi-automatic or automatic) with those of the coated electrode process (fusible coating that forms protective gases, slag, alloy elements, etc.).In this way, the solid electrode wire was replaced by another, composed of a flux-fusible flux core, similar to the one used in the submerged arc.
There are two types of cored wires:
Self-shielded
Where the protection of the arc and the weld pool is done solely by burning the powder flux, contained in the wire core. In other words, WITHOUT additional shielding gas.Some companies have placed limitations on the use of self-protected devices.
Additional gas protection
Where, in addition to the gases generated by the flow, an additional gas is used for protection, which flows through the same nozzle from which the tubular wire emerges. The gases normally used are:- 100% CO2 (More common).
- Ar + 2% of O2.
- Ar + 18 to 25% of CO2.
By using larger diameter wires and higher ranges of electric current, compared to the MIG/MAG process, high deposition rates, together with good penetration and high welding speeds, are obtained.
For this reason, it ends up being the most common in factories that use welding as the main manufacturing process.
Like solid wires used in MIG/MAG processes, cored wire is also packaged in a continuous form (coiled).
For this reason, they can be used in both semi-automatic and automatic processes. In both processes, the cored wire is automatically fed through a gun.
In the semi-automatic process, the welder controls the pitch and distance from the gun to the part, as well as the travel speed and handling of the arc.
The equipment for the flux cored wire process is very close to that used in the MIG/MAG process. However, the following caveats must be made:
- The source has the ability to generate greater current intensity;
- Pistols, in cases where the current intensity is high, are usually cooled with water or air;
- In the self-protected process, the shielding gas system is non-existent.
Types of metallic transfer
Metal transfers in the flux-cored wire process, in addition to being a function of the welding parameters used, are also a function of the gas or gas mixture used. In this process, there are the following types of transfers:Short circuit
Characterized by the constant process of extinguishing and re-igniting the electric arc. This type of transfer allows welding in all positions, with the disadvantage of generating a large amount of spatter.Globular
It is the typical metallic transfer produced by cored wires; occurs at lower currents than spray transfer. There is a high incidence of molten metal spatter.By spray
It occurs when high current intensities and high arc voltages are established in relation to a given wire diameter. As it produces a high deposition rate, spray transfer is restricted to the flat position only.A problem generated by this type of metallic transfer is the possibility of lack of fusion, due to the metallic jet being directed to regions that have not been sufficiently heated.
Spray transfer is the most common in FCAW because the intention is to weld faster and, for this, the current is normally increased.
Pulsed welding
It is a synthetic spray transfer obtained by pulsing the current between two pre-established levels: a base current low enough to keep the electric arc stable and cool the molten pool and a peak current, greater than the globular transition current – spray.For this reason the welding energy is low, making welding with large diameter wires out of the flat position easier.
Types and Functions of Consumables
In welding with cored wire the consumables used are:Electrodes
They are hollow cored wires with a core formed by a fusible flux of low hydrogen content. When the protective gas is of an active nature, deoxidizing elements such as Mn and Si must be present in the chemical composition of the electrode. In the case of self-protected wires, the presence of AI exists in the chemical composition of the flux. Note the inside of the wire in the figure below.
AWS specifications A5.20 and A5.29 classify cored wires for C-Mn and low alloy steels respectively. For stainless steel wires classified by AWS A5.22 are used.
Protection gases
Among the various gas options available, CO2 gas and mixtures of these with Argon are most frequently used. They are used if required by the electrode specification.Features and uses
Welding with flux cored wire has as its main characteristic the high deposition rate, which, combined with a good quality weld, has had a wide application in several areas of the industry.Special care must be taken by the welder when removing the slag formed on each deposited pass, in order to avoid inclusions in the welded joint.
Process-induced discontinuities
The welding inspector must pay attention to the following discontinuities:Lack of fusion
Connected to short-circuit transfer (due to low heat input).Lack of penetration
Also linked to short-circuit transfer, which can also arise from inadequate preparation of the chamfer or error in the configuration of the joint chosen by the project.Inclusion of slag
Welder deficiency in slag removal process, high speed, welding, inadequate joint design.Undercut (Resembles a bite)
Welder disability or high current combined with low welding speed.Porosity
They appear when the welding speed is high, not allowing the diffusion of gases through the bead. In gas shield welding, it can be caused by inadequate gas flow or wind at the welding site, which prevents effective protection of the weld pool.They can also occur when Argon-rich mixtures are used in welding thick plates. High voltages used in soldering.
Overlap
Linked to transfer by short circuit or inability of the welder.Cracks
They usually come from inadequate welding techniques and/or preparation. Base metal or consumable impurities can form components during soldering that lead to crack formation after cooling.Conditions for personal protection
Personal protective equipment (PPE) is the same as used in other electric arc welding processes. Because the radiation emitted is of greater intensity, the filters used must have a greater density.The process of welding with cored wire generates a lot of smoke. In this way, the environment must have good aeration, preferably through exhaust fans.
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