Flux Cored Arc Welding (FCAW) uses the heat generated by a DC electric arc to fuse the metal in the joint area, the arc being struck between a continuously fed consumable filler wire and the workpiece, melting both the filler wire and the workpiece in the immediate vicinity. The entire arc area is covered by a shielding gas that protects the molten weld pool from the atmosphere.
FCAW is a variant of the MIG process and, while there are many common features between the two processes, there are also several fundamental differences.
As with MIG, direct current power sources with constant voltage output characteristics are normally employed to supply the welding current. With flux cored wires, the terminal that the filler wire is connected to depends on the specific product being used (some wires run electrode positive and others run electrode negative). The work return is then connected to the opposite terminal. It has also been found that the output characteristics of the power source can have an effect on the quality of the welds produced.
The wire feed unit takes the filler wire from a spool, and feeds it through the welding gun, to the arc at a predetermined and accurately controlled speed. Normally, special knurled feed rolls are used with flux cored wires to assist feeding and to prevent crushing the consumable.
Unlike MIG, which uses a solid consumable filler wire, the consumable used in FCAW is of tubular construction, an outer metal sheath being filled with fluxing agents plus metal powder. The flux fill is also used to provide alloying, arc stability, slag cover, de-oxidation and, with some wires, gas shielding.
In terms of gas shielding, there are two different ways in which this may be achieved with the FCAW process:
- Additional gas shielding supplied from an external source, such as a gas cylinder.
- Production of a shielding gas by decomposition of fluxing agents within the wire (self-shielding)
Gas Shielded Operation
Many cored wire consumables require an auxiliary gas shield in the same way that solid wire MIG consumables do. These types of wire are generally referred to as ‘gas shielded’.
Using an auxiliary gas shield enables the wire designer to concentrate on the performance characteristics, process tolerance, positional capabilities and mechanical properties of the products.
In a flux cored wire, the metal sheath is generally thinner than that of a self-shielded wire. The area of this metal sheath surrounding the flux cored wire is much smaller than that of a solid MIG wire. This means that the electrical resistance within the flux cored wire is higher than with solid MIG wires and it is this higher electrical resistance that gives this type of wire some of its novel operating properties.
One often quoted property of flux cored wires is their higher deposition rates in comparison to solid MIG wires. What is often not explained is how they deliver these higher values and whether these can be utilised. For example, if a solid MIG wire is used at 250 A, then exchanged for a flux cored wire of the same diameter, and welding power source controls are left unchanged, then the current reading would be much less than 250 A, and perhaps as low as 220 A. This is because of Ohm’s Law, which states that as the electrical resistance increases (and if the voltage remains stable) then the current must fall.
To bring the welding current back to 250 A, it is necessary to increase the wire feed speed, effectively increasing the amount of wire being pushed into the weld pool to make the weld. It is this effect that produces the ‘higher deposition rates’ that the flux cored wire manufacturers claim for this type of product. Unfortunately, in many instances, the welder has difficulty in utilising this higher wire feed speed and must either increase the welding speed or increase the size of the weld. Often in manual applications, neither of these changes can be implemented and the welder simply reduces the wire feed speed back to where it was and the advantages are lost. However, if the process is automated in some way, then the process can show improvements in productivity.
It is also common to use longer contact tip to workplace distances with flux cored arc welding than with solid wire MIG welding, which has the effect of increasing the resistive heating on the wire further accentuating the drop in welding current. Research has also shown that increasing this distance can lead to an increase in the ingress of nitrogen and hydrogen into the weld pool, which can affect the quality of the weld.
Flux cored arc welding has a lower efficiency than solid wire MIG welding, because part of the wire fill contains slag forming agents. Although the efficiency varies by wire type and manufacturer, it is typically between 75 and 85%.
Flux cored arc welding does, however, have the same drawback as solid wire MIG in terms of gas disruption by wind, and screening is always necessary for site work. It also incurs the extra cost of shielding gas, but this is often outweighed by gains in productivity.
There are also self-shielded consumables designed to operate without an additional gas shield. In this type of product, arc shielding is provided by gases generated by decomposition of some constituents within the flux fill. These types of wire are referred to as ‘self-shielded’.
If no external gas shield is required, then the flux fill must provide sufficient gas to protect the molten pool and to provide de-oxidisers and nitride formers to cope with atmospheric contamination. This leaves less scope to address performance, arc stabilisation and process tolerance, so these tend to suffer when compared with gas shielded types.
Wire efficiency are also lower, at about 65%, in this mode of operation than with gas shielded wires. However, the wires do have a distinct advantage when it comes to site work in terms of wind tolerance, as there is no external gas shield to be disrupted.
When using self-shielded wires, external gas supply is not required and, therefore, the gas shroud is not necessary. However, an extension nozzle is often used to support and direct the long electrode extensions that are needed to obtain high deposition rates.