Welding process
Flux cored arc welding (FCAW)
A continuously fed tubular wire filled with flux, run either with additional shielding gas or entirely self-shielded, combining wire-feed productivity with the slag and forgiveness of a covered electrode.
How it works
The electrode is a hollow metal sheath filled with flux and alloying compounds. It feeds continuously like a solid wire, but as the arc burns the core ingredients decompose, generating shielding gas, deoxidising the pool, adding alloy, and forming a slag that covers the finished bead exactly as an electrode covering does.
In the gas-shielded variant an external gas, usually carbon dioxide or an argon blend, supplements the core's own shielding. This gives a smoother arc, less fume, and better mechanical properties, at the cost of needing gas at the joint. In the self-shielded variant the core alone protects the pool, using strong denitriders so the process can run in wind without a nozzle.
Because the current flows through a thin tube rather than a solid cross-section, current density in the sheath is high and deposition rates exceed those of a solid wire at the same amperage. The resulting slag freezes over the bead and supports it out of position, which is why flux cored wires handle vertical and overhead work better than most spray-transfer solid wires.
Strengths
- High deposition rates, often the highest of the common semi-automatic processes
- Self-shielded wires weld outdoors in wind where gas-shielded processes fail
- Slag support makes out-of-position welding on heavy plate practical at high current
- More tolerant of mill scale and light surface contamination than solid wire
Limitations
- Slag must be removed from every pass, reintroducing the inclusion risk that GMAW avoids
- Generates considerably more fume than solid wire, so ventilation and respiratory protection matter
- Wire cost per kilogram is higher than solid wire, and the flux fill is not deposited metal
- Self-shielded deposits can have lower toughness than gas-shielded ones unless the wire is chosen carefully
Key variables
- Electrical stickout
- Controls preheating of the tubular wire. Excessive stickout raises deposition but drops current, penetration, and on self-shielded wires the shielding itself.
- Wire feed speed
- Sets current and deposition rate on a constant-voltage machine, and drives how much flux is delivered to the pool per unit length.
- Voltage
- Governs arc length, bead width, and slag behaviour. Too high burns out core ingredients and can cause porosity in self-shielded deposits.
- Polarity
- Gas-shielded wires generally run electrode positive while many self-shielded wires require electrode negative, and reversing it ruins the arc.
- Shielding gas
- On gas-shielded wires, carbon dioxide gives deeper penetration and more spatter while argon blends give a smoother arc and better toughness.
What an inspector watches for
- Confirm the wire classification matches the procedure, since gas-shielded and self-shielded wires of similar diameter look identical on the spool
- Check that gas is actually flowing on gas-shielded wires and is absent on self-shielded ones, because running one as the other degrades the deposit
- Inspect interpass cleaning carefully, since heavy slag in deep grooves is a persistent inclusion source
- Watch electrical stickout, as this process is unusually sensitive to it and long stickout starves penetration