FCAW Welding Problems: How to Avoid Common Flux Core Defects

Self-shielded flux core welding, or FCAW-S, is one of the most dependable processes for structural steel erection, bridge construction, heavy equipment repair, and outdoor field work.

It runs without an external shielding gas supply and delivers high deposition rates, which is exactly why it holds up in the demanding, variable conditions that shop-based processes can’t always handle.

For the full process walkthrough, see Best Practices: Self-Shielded Flux-Cored Welding.

But every FCAW welding job runs into the same short list of problems; burnback, birdnesting, porosity, worm tracking, slag inclusions, fusion issues, and penetration trouble, and each one means grinding, rework, and lost arc time.

Nearly all of them trace back to a parameter, a technique, or a piece of equipment that’s easy to correct once you know what to look for. The table below diagnoses the most common flux core welding problems at a glance, the sections that follow give you the fix.

Flux Core Welding Problems at a Glance

Problem	Most likely cause	First fix to try
Burnback	Wire feed speed too slow; gun held too close	Raise wire feed speed; keep tip-to-work under 1-1/4 in.
Birdnesting	Wrong or worn drive rolls; bad tension; liner blockage	Use knurled V- or U-groove rolls; reset tension; check liner
Porosity	Dirty base metal; too much stick-out; drafts	Clean the joint; keep stick-out under 1-1/4 in.; block wind
Worm tracking	Voltage too high for the wire feed setting	Lower voltage in 1/2-volt steps until the marks stop
Slag inclusions	Poor bead placement; wrong angle/speed; low heat	Clean between passes; correct drag angle; raise heat input
Undercutting	Too much current/voltage; travelling too fast	Reduce current/voltage; slow down; pause at the toes
Lack of fusion	Incorrect work angle; dirty base metal	15–45° drag angle; arc on trailing edge; clean the joint
Penetration issues	Heat input too high or too low; poor joint access	Adjust voltage/feed/travel; prep the joint for groove access

flux core welding overhead on structural steel — Bernard Dura-Flux self-shielded FCAW gun — Flux-cored welding offers many advantages when welding on construction applications, including high disposition rates, and good chemical and mechanical properties.

Tip One: Avoid Wire Feeding Problems

Wire feed stoppages and malfunctions are common problems on many job sites, and they cause a considerable amount of downtime. The two most prevalent types, burnback and birdnesting, tend to extinguish the arc prematurely, which in turn can lead to weld defects.

flux core welding burnback at the contact tip — caused by slow wire feed speed — Prevent burnback, as shown here, by having the appropriate wire feed speed and MIG gun to work piece distance.

Burnback occurs when the wire melts into a ball at the end of the contact tip. It is most often the result of too slow a wire feed speed and/or holding the welding gun too close to the workpiece.

To prevent the problem, use the correct feed speed for your application and maintain a distance from contact tip to work of no further than 1-1/4 inch.

To prevent birdnesting, a tangle of wire that halts feeding, always use knurled V- or U-groove drive rolls in your wire feeder.

Compared with GMAW solid welding wire, FCAW wire is much softer due to its tubular design, so the wrong drive roll can easily compress it. Set the correct drive roll tension by releasing it, then increasing it while feeding the wire into your gloved palm, continuing about one half-turn past wire slippage.

fcaw wire birdnesting in the drive rolls — wrong drive roll type or tension — Using the correct drive rolls and tension settings can prevent birdnesting.

Additionally, setting the correct drive roll tension can prevent the wire from flattening and becoming tangled. To set the proper tension, begin by releasing the tension on the drive rolls. Increase the tension while feeding the wire into the palm of your welding glove and continue to increase the tension one half turn past wire slippage.

Other causes of birdnesting include blockages in the liner, improperly trimmed liners, or using the wrong liner. Replace a blocked liner, trim it to the manufacturer’s recommendation, and always use the correct size for your wire diameter.

Bernard’s E-Z Feed™ liners use a corrosion-resistant coating and a unique enhanced coil profile that reinforces rigidity and wire support, which keeps hard-to-feed wires moving smoothly and holds a consistent wire-feed path.

For more answers to the questions welders ask most about this process, see Your FAQs About FCAW.

Tip Two: Stop Porosity and Worm Tracking

Porosity and worm tracking are both common weld discontinuities that can weaken the integrity of your welds.

Porosity results when gas becomes trapped in the weld metal, and it can appear at any single point or along the full length of the weld.

To prevent it, remove rust, grease, paint, coatings, oil, moisture, and dirt from the base metal before welding. Filler metals with added deoxidizers help weld through light contaminants, but they should never replace proper pre-cleaning. Keep your electrode stick-out to no more than 1-1/4 inch beyond the contact tip.

To prevent worm tracking, marks on the surface of the weld bead caused by gas that the flux in the wire’s core creates, avoid excessive voltage for your given wire feed setting and amperage.

Follow the parameters recommended by the filler metal manufacturer for the specific wire diameter. Those parameters tie back to the wire’s classification under the American Welding Society’s AWS A5.20 specification for carbon steel FCAW electrodes.

If worm tracking does occur, reduce your voltage in increments of one half-volt until you eliminate the problem.

Tip Three: Eliminate Slag Inclusions

Slag inclusions occur when the slag generated by the molten flux in the wire’s core becomes trapped inside the weld. There are four major causes, all preventable with proper technique.

First, avoid incorrect weld bead placement, especially on multiple passes through thick sections such as root passes or wide V-groove openings, and leave enough room in the joint for additional passes.

Second, maintain the correct travel angle and speed: a drag angle of 15 to 45 degrees in the flat, horizontal, and overhead positions, 5 to 15 degrees vertical-up, with a steady travel speed so the puddle does not get ahead of the arc.

Third, maintain proper heat input, since too little heat traps slag rather than letting it float out.

Finally, clean thoroughly between passes, removing slag with a chipping hammer, wire brush, or grinder before the next pass.

flux core welding worm tracking on the weld bead — caused by excessive voltage — To prevent worm tracking, use the manufacturer’s recommended parameters for your given wire diameter and lower your voltage setting if necessary.

Tip Four: Prevent Undercutting and Lack of Fusion

Like other weld defects, undercutting and lack of fusion both affect weld quality, and preventing them reduces downtime and rework.

Undercutting occurs when a groove melts into the base metal next to the toe of the weld but is not filled by weld metal, creating a weak point that often leads to cracking.

Use the proper welding current and voltage, adjust to the right gun angle, and maintain a travel speed that lets the weld metal fill the melted-out areas completely.

If you are weaving, pause briefly at each side of the bead.

To prevent lack of fusion, the failure of the weld metal to fuse completely with the base metal or the preceding bead, maintain the correct work angle and heat input, keep the arc on the trailing edge of the puddle at a 15 to 45 degree drag angle, and clean the base metal thoroughly before welding.

These are some of the most useful mig welding tips for sound, multi-pass FCAW welds.

Tip Five: Avoid Excessive Penetration or Lack of Penetration

Maintaining the appropriate heat input is key to avoiding penetration problems. Excessive penetration occurs when the weld metal melts through the base metal and hangs underneath, most often from too much heat.

If it happens, select a lower voltage range, reduce wire feed speed, and increase travel speed.

Conversely, a higher wire feed speed, a higher voltage range, and/or reduced travel speed prevent lack of penetration, the shallow fusion between the weld metal and the base metal.

Prepare the joint to permit access to the bottom of the groove, and maintain proper welding wire extension and arc characteristics.

Frequently Asked Questions

Q: What causes porosity in flux core welding?

A: Most often, contamination on the base metal or too much wire stick-out. Clean off rust, oil, paint, and moisture before welding, keep stick-out under 1-1/4 inch, and shield the weld from drafts. Wire with added deoxidizers helps with light contamination but does not replace cleaning.

Q: How do I stop spatter when flux core welding?

A: Match your voltage and wire feed speed to the filler metal manufacturer’s settings for the wire diameter, keep the contact tip clean and correctly sized, and hold the right gun angle and stick-out. Excess voltage for the wire feed setting is the usual cause.

Q: Why does my flux core wire keep birdnesting?

A: Usually the wrong drive rolls or incorrect tension. Soft tubular FCAW wire needs knurled V- or U-groove drive rolls. Reset the tension to just past wire slippage, and inspect the liner for blockages or kinks.

Q: What causes worm tracking in FCAW welds?

A: Voltage that is too high for the wire feed speed setting. Lower the voltage in half-volt increments until the surface marks disappear, staying within the manufacturer’s recommended range for your wire diameter.

Q: Are flux core welds strong?

A: Yes. Self-shielded FCAW produces high-deposition, code-quality welds for structural steel, bridge, and heavy-equipment work when run with the correct parameters and technique. It is a standard process for demanding structural and field applications.