TL;DR

Porosity in MIG welds is almost always a shielding gas problem: leak in the hose or regulator, wrong flow rate, or wind displacing the gas shield. Check flow rate at the nozzle first.

What you might see

visible pores or voids in the weld bead surface
gas porosity in weld radiograph or cross-section
bead surface pocked with pinholes after grinding
porosity worsens outdoors or when fans are running nearby

Likely causes

Tap to mark as ruled out

Shielding gas leak in the hose, regulator, or solenoid valve, reducing coverage at the weld puddle
Gas flow rate too low (under 15 CFH) or too high (over 50 CFH, causing turbulence)
Wind or drafts displacing the shielding gas cloud from the weld zone
Base metal contamination with oil, mill scale, moisture, or primer that generates gas in the puddle

Required tools

Gas flow meter or rotameter at the nozzle
Soapy water solution and brush for leak check
Solvent wipe cloths
Wire brush
Wind shield panels or welding curtain

Safety first

Shielding gas cylinders are high-pressure vessels. Secure them upright at all times and use the cylinder cap when not in use.
Opening the welding arc produces UV radiation, ozone, and fumes. Use a proper welding helmet and ensure adequate ventilation.

Procedure

7 steps · ~30 min

1
Set the gas flow rate to the middle of the recommended range for the wire and process. For most 75/25 Ar/CO2 processes, 20-25 CFH at the regulator is a good starting point.[1]
2
Hold a clean gloved hand near the nozzle during a no-arc gas check and verify a steady, smooth flow of shielding gas. Turbulent or pulsing flow suggests a restriction or solenoid problem.
3
Inspect the gas hose from the cylinder regulator to the machine, and from the machine to the gun. Look for kinks, pinched sections, or loose fittings. A soapy water test on all fittings will identify leaks.
4
Inspect the contact tip-to-nozzle relationship. The contact tip should recess into the nozzle per the Lincoln recommendation for the process. An extended or protruding tip disrupts gas coverage.[1]
5
Clean the weld zone on the base metal with a wire brush and solvent wipe to remove oil, moisture, and mill scale before welding.
6
If welding outdoors or near ventilation fans, erect a wind shield around the weld zone. Even 5 mph of airflow can blow away the shielding gas.
7
Make a test weld on clean scrap at the corrected parameters. Cut and inspect a cross-section or polish and etch the bead to confirm porosity is eliminated.

Sources

Lincoln Electric Power MIG Industrial MIG Welder Operator Manual, Lincoln Electric
Lincoln Electric Power MIG Operator's Manual, shielding gas setup and recommended flow rates (general)
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