If your flux-cored welds look worse than your MIG welds, the problem is not the wire. Flux-cored arc welding (FCAW) is a different process with its own defect patterns, and the troubleshooting charts you use for MIG will not help you diagnose worm tracks or slag inclusions. Self-shielded (FCAW-S) and gas-shielded (FCAW-G) wire have separate causes for similar-looking defects, and the fix that works for one type can make the other type worse.
This guide covers the most common FCAW-specific defects organized by symptom. Each section explains what the defect looks like, lists the likely causes separately for FCAW-S and FCAW-G, and gives the fixes to try in priority order. If you have never used FCAW before, start with the FCAW basics and process overview to understand the polarity and wire type differences that make troubleshooting possible. For a side-by-side comparison with other welding processes, see MIG vs TIG vs Stick welding.
Safety note: FCAW produces more fume than MIG because the flux generates additional airborne particles. Always weld with adequate ventilation or fume extraction. The slag produced by FCAW must be chipped off, and hot slag chips can cause eye injury – wear safety glasses under your helmet. Spatter from FCAW can travel further than MIG spatter, so clear all flammable materials from the work area. If you are running FCAW-G, follow standard gas cylinder safety procedures for securing and transporting cylinders.
Quick Reference: Polarity and Setup First Check
Before you start troubleshooting a specific defect, verify the most common setup mistakes first. A polarity, contact-tip-to-work distance (CTWD), or technique error can cause symptoms that look like a different problem entirely.
- Polarity: Self-shielded FCAW (FCAW-S) typically uses DC electrode negative (DCEN) for many wires. Gas-shielded FCAW (FCAW-G) typically uses DC electrode positive (DCEP) for many wires. Running the wrong polarity for your wire type causes severe spatter, poor fusion, porosity, and erratic arc behavior. This is one of the first setup errors to check in FCAW. Always verify polarity against the wire manufacturer’s data sheet printed on the spool or available online.
- Contact-tip-to-work distance (CTWD / stickout): Many flux-cored wires use a longer stickout than short-circuit MIG, but the exact value varies by wire type, diameter, and manufacturer recommendation. Typical starting ranges from manufacturer charts are often 3/4 to a range specified on the wire manufacturer data sheet (commonly longer for FCAW-S than FCAW-G, but verify each wire type). If the stickout is too short, feeding problems and arc instability can occur; if too long, the arc becomes erratic and shielding may be compromised. Check your wire data sheet.
- Travel technique: A drag (backhand) angle is commonly recommended for many flux-cored wires, typically in the range of 10 to 30 degrees from perpendicular depending on wire type, joint configuration, and position. Pushing with some wire types can increase the risk of slag entrapment and porosity. The drag technique is common but not universal – follow the wire manufacturer’s guidance for your specific wire.
If you have verified these three items and the defect persists, move to the symptom that matches your weld appearance.
Worm Tracks (Wormhole Porosity)
Worm tracks are elongated gas pockets that form channels in the weld bead, often visible on the bead surface or just under the slag layer. They look like short worm-like tunnels. Wormhole porosity is especially associated with flux-cored arc welding and is commonly discussed in FCAW troubleshooting – it can occur in other processes but is a signature defect of FCAW.
Causes
- FCAW-S: Travel speed too fast (puddle outruns the gas envelope), damp wire (flux absorbs moisture), arc voltage set too high for the wire, wrong contact-tip-to-work distance.
- FCAW-G: Same causes as FCAW-S, plus gas coverage issues: low gas flow, gas leaks, draft blowing shielding gas away, wrong gas mix or flow rate for the wire.
Fixes (Priority Order)
- Reduce travel speed to keep the puddle within the gas envelope (FCAW-S) or gas shield (FCAW-G).
- Check wire storage: flux-cored wire absorbs moisture faster than solid wire. Store opened spools in dry conditions, ideally in a dehumidified cabinet or warm storage area. Discard wire that has been exposed to humid conditions for extended periods.
- Adjust voltage downward in small increments (starting from the manufacturer’s recommended range for your wire) and check if worm tracks reduce.
- Verify CTWD is within the wire manufacturer’s recommended range for your specific wire and diameter.
- For FCAW-G: check gas flow rate (verify against the wire manufacturer data sheet, as flow varies by wire type, nozzle, and position), check for gas line leaks, and shield the weld area from drafts.
Porosity
Porosity appears as small holes or pockmarks on the weld bead surface or scattered throughout the weld cross-section. Unlike worm tracks, porosity consists of rounded or irregular cavities rather than elongated channels.
Causes
- FCAW-S: Damp or contaminated wire, dirty base metal (rust, oil, paint, mill scale), arc length too long (excessive voltage), windy conditions that disrupt the flux-generated gas envelope, base metal moisture.
- FCAW-G: All of the above, plus gas system problems: gas cylinder empty or low, gas flow too low or too high, gas leak in lines or fittings, wrong gas type for the wire, draft blowing gas away, clogged or spattered nozzle restricting gas flow.
Fixes (Priority Order)
- Clean base metal thoroughly before welding – remove rust, oil, paint, and mill scale.
- Check wire condition: if the spool has been exposed to humidity, the wire may be contaminated. Replace with fresh, dry wire.
- For FCAW-G: verify gas flow at the nozzle with a flow meter, check for gas line leaks, increase flow rate slightly (stay within manufacturer recommendations), add a wind screen if working in drafty conditions. Check that the nozzle is clean and gas ports are not blocked by spatter.
- Adjust voltage to an appropriate arc length for your wire type – excessive voltage lengthens the arc and can draw in air.
- For FCAW-S in outdoor conditions: use wind screens in sustained wind that disturbs the shielding gas; use wind control or a suitable process for outdoor conditions.
Slag Inclusions
Slag inclusions are trapped flux residue within the weld metal. They appear as elongated or irregular non-metallic particles in the weld cross-section and can reduce joint strength. Slag that is difficult to remove after welding is a related symptom indicating poor slag system performance.
Causes
- Wrong travel angle: pushing the gun (rather than dragging) can push molten slag ahead of the puddle, where it becomes trapped in solidifying weld metal.
- Travel technique that lets the puddle get ahead of the slag.
- Poor interpass cleaning: slag from the previous pass was not fully removed before the next pass.
- Narrow groove or tight joint geometry that prevents slag from floating to the surface.
- Travel speed too low, causing excessive slag buildup.
Fixes
- Use a drag (backhand) angle, which is commonly recommended for many FCAW wires. The drag angle keeps the slag behind the puddle where it can float out and solidify on top of the bead.
- Adjust travel speed so the weld puddle is positioned ahead of the slag, not buried in it.
- Clean thoroughly between passes: use a chipping hammer and wire brush (or grinding for stubborn slag) to remove all slag before laying the next bead.
- For narrow grooves or deep joints, consider a wider included angle or a different joint preparation to allow slag to escape.
Lack of Fusion
Lack of fusion (also called incomplete fusion) appears as a gap between the weld metal and the base metal or between adjacent weld beads. The weld metal sits on top of the base metal without properly bonding.
Causes
- FCAW-S: Wrong polarity (running self-shielded wire on DCEP instead of the typically recommended DCEN), wire feed speed too high for the voltage (cold settings), travel speed too fast (insufficient heat input), stickout too long (reduces current at the arc), base metal too cold for the wire diameter.
- FCAW-G: Wrong polarity (running gas-shielded wire on DCEN instead of the typically recommended DCEP), voltage too low for the wire feed speed, travel speed too fast, stickout too long, base metal contamination.
Fixes
- Verify polarity against the wire data sheet: this is a common cause of fusion problems in both FCAW-S and FCAW-G.
- Check that voltage and wire feed speed are matched within the manufacturer’s recommended range for your wire and diameter. If the weld sounds cold (stuttering arc, minimal penetration), increase voltage or decrease wire feed speed.
- Reduce travel speed to allow more heat input into the base metal.
- Ensure CTWD is within the manufacturer’s recommended range – excessive stickout reduces current and penetration.
- Consider preheat only when specified by the WPS, procedure, material requirement, or qualified supervision.
Undercut
Undercut is a groove melted into the base metal at the toe of the weld that is not filled by the weld metal. It weakens the joint by reducing the effective cross-section at the weld toe.
Causes
- Voltage too high for the travel speed (excessive arc force washes away base metal at the weld toe).
- Travel speed too fast (the weld metal does not fill the groove created by the arc).
- Wrong gun angle: if the gun is angled too steeply, the arc can concentrate on one side of the joint.
- Wire feed speed too high for the voltage (excessive current erodes the side walls).
Fixes
- Reduce voltage in small increments until the undercut disappears, staying within the manufacturer’s recommended range.
- Consider a slightly slower travel speed within the manufacturer range to help the weld metal fill the weld toes.
- Adjust gun angle: for a drag technique, reduce the angle (move closer to perpendicular) so the arc does not wash metal from the leading edge.
- Check that voltage and WFS are balanced – if the arc sounds harsh or digging, voltage may be too high or WFS too low.
Excessive Spatter
Spatter is the ejection of molten metal droplets from the weld pool that land on the base metal around the joint. While some spatter is normal in FCAW (especially self-shielded), excessive spatter wastes wire, increases cleanup time, and can indicate incorrect settings.
Causes
- Voltage/wire-feed-speed mismatch: typically voltage too low for the WFS, causing the arc to struggle and eject droplets.
- Wrong polarity: this is a common cause of severe spatter – verify against the wire data sheet.
- Contact-tip-to-work distance too short or too long for the wire type.
- Damp or contaminated wire (moisture causes the arc to sputter).
- For FCAW-G: running on CO2 with parameters outside the recommended range can increase spatter compared to argon/CO2 blends. Also check for gas coverage issues.
Fixes
- Verify polarity first – it is quick to check and can eliminate spatter immediately if wrong.
- Adjust voltage upward in small increments while maintaining WFS; if spatter decreases, the voltage was too low for that feed speed.
- Check CTWD against the manufacturer’s recommended range for your wire.
- Ensure wire is dry and the spool has been stored properly.
- For FCAW-G: verify gas type is correct for the wire, check flow rate, and ensure the nozzle is clean and properly installed.
Burnback
Burnback occurs when the wire melts back into the contact tip, fusing the wire and tip together. It stops wire feeding and requires removing and replacing the contact tip. Flux-cored wire is more prone to burnback than solid MIG wire because the tubular construction carries current differently and the wire is softer, which can cause feeding irregularities.
Causes
- Wire feed speed too slow for the voltage setting: the arc melts the wire faster than the feeder can push it, and the melt migrates up into the tip.
- Worn or oversized contact tip bore: poor electrical contact slows the wire feed or causes intermittent contact.
- Liner friction: a dirty, kinked, or undersized liner creates resistance that slows wire travel.
- Drive roll pressure too tight: deforms the soft flux-cored wire, causing it to bind in the liner or tip.
- Incorrect polarity can also contribute to burnback by changing melt-off rate – verify voltage and WFS per the wire chart for your specific wire type and diameter.
Fixes
- Increase wire feed speed or decrease voltage to reduce the burn-off rate relative to the feed rate.
- Replace the contact tip with the correct size for flux-cored wire (FCAW contact tips typically have a slightly larger bore than solid-wire tips of the same nominal diameter).
- Check the liner for debris, kinks, or correct size for your wire diameter. Clean or replace if needed.
- Adjust drive roll pressure: just enough tension to feed without slipping. Overtightening deforms tubular wire.
- For persistent burnback, see the MIG welding burnback guide for common causes that overlap with FCAW.
Poor Bead Shape
Poor bead shape includes beads that are too convex (ropey, tall and narrow), too concave (flat with sunk-in center), or inconsistent in width.
- Excessively convex bead: Voltage too low for the wire feed speed (the arc does not flatten the puddle enough), travel speed too slow (excessive buildup of metal per unit length), or CTWD too long (reduced arc force).
- Excessively concave bead: Voltage too high for the wire feed speed (arc flattens the puddle excessively), travel speed too fast (insufficient fill), or CTWD too short.
- Ropey or inconsistent bead: Unsteady travel speed, inconsistent wire feeding (see the wire feed problems guide), or an unstable arc from incorrect settings.
Fixes
- For a convex bead: increase voltage slightly or reduce wire feed speed, staying within the manufacturer’s recommended range. Slow down travel speed slightly (within manufacturer range) to allow the bead to flatten.
- For a concave bead: decrease voltage slightly or increase wire feed speed. Consider a slightly faster travel speed within the manufacturer range to reduce heat input per unit length.
- For a ropey bead: practice a steady, consistent travel motion. Check for wire feed stability (see wire feed section).
Silica Island Formation
Silica islands are glassy deposits that form on the weld bead surface, particularly with gas-shielded FCAW wire. They are caused by silicon from the flux reacting at … [OUTPUT TRUNCATED – 9954 chars omitted out of 59954 total] … en self-shielded and gas-shielded wire. Many defects that look similar have different root causes depending on which type you are running.
When you encounter a weld defect, use this checklist in order:
- Verify polarity against the wire data sheet. This is the fastest fix for the widest range of symptoms.
- Check that CTWD (stickout) is within the manufacturer’s recommended range for your specific wire and diameter.
- Confirm that voltage and wire feed speed are balanced based on the manufacturer’s chart.
- Inspect wire storage conditions and replace damp wire.
- Use the diagnostic table above to match your symptom to the most likely cause.
For related reading: see the MIG defects visual identification chart for defects that overlap between processes, stick welding defects for troubleshooting stick electrodes, and MIG vs flux core welding for a comparison of the two processes. For more on how flux works in welding, see flux in welding.
