Intermittent Arc Stubbing and Restart Porosity on a 1.2 mm GMAW Cell
A six-axis robot welds 6 mm mild-steel fillets with 1.2 mm ER70S-6 wire and a 90Ar/10CO2 shield. Parameters are 260 A, 28 V, 6.4 mm/s travel, 15 mm stickout and a 10 degree push angle. The 4.5 m conduit has a gentle S-bend at the feeder inlet, the drive-roll tension is at the highest setting, and the contact tip is a 1.2 mm bore. Over one shift, operators report intermittent arc stubbing, audible popping and clustered porosity at each restart. The wire is a 250 kg drum mounted 1.2 m above the feeder, and the liner is a steel liner with roughly 40,000 arc hours on it. Shop ambient is 38 C and the cell is rated 60% duty at 300 A. The weld schedule was copied from a 1.0 mm wire job, and the drive rolls were not changed.
1. Rank the three most probable root causes of the stubbing and restart porosity, and give the evidence that supports each ranking.
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1. Drive-roll groove mismatch (most probable). The schedule came from a 1.0 mm job, so 1.2 mm wire is running in a 1.0 mm groove. The wire is squeezed into an oval, which changes contact-tip fit and feed resistance. That produces periodic feed hesitation, current fluctuation and the audible popping described. Highest tension makes it worse by flattening the wire further.
2. Worn liner plus conduit routing. A steel liner with 40,000 arc hours, combined with an S-bend at the inlet, accumulates metallic debris and drag. The symptom pattern fits: feed is fine until friction builds, then the wire stalls and the arc stubs.
3. Drum brake/spool drag and thermal margin. A 250 kg drum 1.2 m above the feeder needs a correctly set brake; too much drag adds load at the drive rolls. At 38 C ambient on a 60% duty, 300 A cell running 260 A, the feeder motor and gun cable are already near thermal limits.
Restart porosity is secondary: short preflow after a stop lets air reach the puddle, so also check gas preflow and postflow, not just feed.
2. The WPS limits heat input to 1.0 kJ/mm. Using the stated parameters, calculate the actual heat input and the minimum travel speed needed to comply.
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Heat input in J/mm is (volts x amps) / travel speed in mm/s.
- Arc power = 28 V x 260 A = 7,280 W.
- Heat input = 7,280 / 6.4 = 1,137.5 J/mm, or 1.14 kJ/mm.
That is 14% above the 1.0 kJ/mm limit, so the procedure as run is out of compliance even before the feed problem is fixed.
Required travel speed = 7,280 / 1,000 = 7.28 mm/s (about 17.2 in/min, versus the current 15.1 in/min).
The speed increase is modest, but confirm it still gives adequate penetration and sidewall fusion on the 6 mm fillet. The alternative is to reduce current: at 6.4 mm/s, compliant current is 1,000 x 6.4 / 28 = 229 A. Choose one route and re-qualify, because changing both at once confuses the cause if the bead profile later fails.
3. List the corrective actions and the verification steps required before the cell is released back to production.
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Corrective actions
- Fit 1.2 mm drive rolls and set tension per the feeder manufacturer, typically mid-scale rather than maximum; confirm by checking for wire deformation with a micrometer after the rolls.
- Replace the steel liner, trim the conduit so the S-bend is removed, and blow out debris.
- Set drum brake drag so the wire can be pulled by hand without slack spooling.
- Verify contact tip bore and replace if worn or spattered; 1.2 mm bore is correct only if the wire stays round.
- Set preflow of roughly 0.5-1.0 s and postflow of 0.3-0.5 s to cover restarts.
Verification
- Command a wire-feed speed and measure actual feed over 10 s; deviation should be small and repeatable.
- Run a 1 m test bead and listen for popping; no stubbing should occur.
- Cut and etch a cross-section to confirm fusion and measure heat input again.
- Run first-off visual inspection and a restart test with at least five stop/starts to prove porosity is gone before releasing the cell.