Flux-Cored Arc Welding (FCAW)

Flux-cored arc welding (FCAW) feeds a hollow wire filled with flux through a gun, much like MIG. The flux makes a slag and, on some wires, its own shielding gas. It lays down metal quickly and tolerates some mill scale, so it is a mainstay in structural steel fabrication, shipbuilding and heavy equipment. It also makes more fume than solid-wire MIG, and each wire has its own rules.

What You Will Be Able to Do

• Tell self-shielded from gas-shielded wire and choose between them.
• Read a flux-cored wire classification.
• Set polarity and stickout for the wire you are using.
• Run a drag-angle bead and clean slag between passes.
• Fix common flux-cored problems.

Two Families of Wire

FCAW-S has everything it needs in the wire. Wind cannot blow away a gas shield that comes from inside the arc, so it is common in field work. FCAW-G, also called dual-shield, uses an external gas, usually 75 percent argon and 25 percent CO2 or 100 percent CO2. It gives a smoother arc, and it is the usual choice for shop work.

Gas-shielded wire needs protection from drafts. AWS D1.1 sets a wind limit for gas-shielded processes unless you protect the weld, commonly 5 miles an hour. Windscreens or tents are normal on outdoor gas-shielded work.

Warning: Polarity changes between wires. Many self-shielded wires run DCEN, and most gas-shielded wires run DCEP. The wrong polarity gives a poor arc, spatter and porosity. Check the wire's data sheet before you weld.

Reading a Wire Classification

Take E71T-1C as an example:

You may see more letters after the classification, such as H8 for a limit on diffusible hydrogen, or a J or a D for toughness. When the WPS names a wire, use that one. Two wires that look alike can behave very differently. Newer AWS specifications use a longer naming system, so a shop may show either format.

Some common wire types:

Some self-shielded wires are limited to certain thicknesses or to single-pass welds. Check the data sheet.

Setting Up the Machine

Flux-cored runs on a constant-voltage power source, with the same basic feeder as MIG.

1. Set the polarity for the wire.
2. Use knurled drive rolls for flux-cored wire if the maker calls for them, because they grip without crushing the tube. Keep tension just high enough.
3. Fit the correct liner and contact tip.
4. For gas-shielded wire, set the flow by the maker's chart, often around 35 to 45 CFH.
5. Set the voltage and wire feed speed from the wire maker's chart.
6. Weld on a test plate and fine-tune.

The voltage controls the arc length and bead shape, and the wire feed speed controls the amps. If the voltage is too low, the bead turns ropy with lots of spatter. If it is too high, you get undercut, a flat bead and porosity.

Stickout Matters

Stickout, or electrical stickout, is the wire length from the contact tip to the arc. It heats the wire before it melts, and it affects fusion and porosity.

Check the data sheet. Self-shielded wires can be especially fussy: too short a stickout can cause porosity, and a stickout that wanders causes changes in penetration.

Running a Bead

Flux-cored makes slag, and that changes your technique.

1. Use a drag angle. Tilt the gun 10 to 20 degrees back, so the gun points at the weld and the arc stays ahead of the slag. The rule is: if there is slag, you drag.
2. Set your work angle. On a tee joint, aim into the corner at about 45 degrees. On a horizontal fillet, aim a little more at the bottom plate.
3. Keep the arc on the leading edge. Slag running ahead of the arc gets trapped under the bead.
4. Keep moving. The deposit rate is high, so bead size grows quickly.
5. Keep a steady stickout. A changing stickout changes your heat.
6. Fill the crater at the end of the bead.
7. Clean between passes. Chip all slag and brush the bead. Grind out anything that will not come loose.

On vertical-up welds, all-position wires allow a slight weave or a triangle motion. Keep the puddle small and pause at the toes.

Deposit Rate and Heat

A flux-cored wire can deposit far more metal in an hour than stick. That makes it productive, but heat builds up quickly. Watch interpass temperature on multi-pass work, and do not oversize welds, because extra metal adds distortion and cost.

Fume and Ventilation

FCAW is among the highest fume producers of the common arc processes, especially self-shielded wires at high amps. Fume extraction guns, hoods and good positioning matter a lot here. Fume from stainless flux-cored wires carries hexavalent chromium, and it needs stricter control than fume from carbon steel. Never rely on outdoor air to carry it all away. If you can see the plume at your face, your controls are not working.

Troubleshooting

Keep the wire dry. Flux inside the tube can absorb moisture, especially when the wire is left on the machine in humid air. Store it per the maker's instructions and cover it during breaks.

Common Mistakes

• Treating it like solid MIG. Pushing the gun traps slag.
• Using the wrong polarity. Check it each time you change wire.
• Skipping the wind check. Gas-shielded welds get porosity from drafts.
• Running a self-shielded wire off-label. Some wires are not rated for multiple passes.
• Leaving wire out on the feeder. Moisture causes porosity and hydrogen problems.

Check Your Understanding

You are on an open jobsite in a steady breeze, with no wind shelter. Which wire family suits this job better, and why?

Self-shielded wire, if the WPS allows it. Its shielding comes from inside the arc, so wind affects it less. Gas-shielded wire needs a windscreen.

You load a self-shielded wire on a machine set up for gas-shielded wire, and the arc is terrible. What is the first thing to check?

The polarity. Many self-shielded wires run DCEN, and gas-shielded wires usually run DCEP.

What does the 1 after the 7 in E71T-1C tell you?

It means all positions. A 0 in that spot would mean flat and horizontal only.

You see slag trapped under your bead on a multi-pass weld. What should you change?

Remove all slag between passes, use a drag angle, and keep the arc on the leading edge so the slag stays behind it.

Key Takeaways

• Flux-cored wire has flux inside, and it gives high deposition rates.
• Self-shielded wire needs no gas and handles wind better. Gas-shielded wire gives a smoother arc.
• Polarity depends on the wire, so check the data sheet.
• If there is slag, you drag.
• Hold a steady stickout and clean every pass.
• FCAW makes a lot of fume, so control it at the arc.