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What porosity is—and why shielding gas matters
Porosity is gas trapped in solidifying weld metal. It can show up as scattered pinholes, a line of pores, or a cavity exposed when you grind the weld. In gas-shielded MIG welding, the usual culprits are air entering the shielding envelope, contamination on the joint or wire, and gas-flow problems. A weld can look acceptable on the surface and still contain subsurface pores, so a clean-looking bead is not proof of a sound weld.
Porosity weakens a joint by reducing its effective cross-section. On structural, pressure-containing, or safety-critical work, follow the applicable welding procedure and inspection requirements rather than trying to fix a defect by laying another bead over it. For ordinary shop work, the goal is to find the cause before making the next pass.
Check the gas supply and delivery
Start at the cylinder and work toward the gun. Confirm the cylinder contains the gas specified for the job, the regulator is set correctly, and the hose and connections are not damaged or loose. A typical starting flow for short-circuit MIG is about 20–30 cubic feet per hour (cfh), but the right setting depends on nozzle size, joint access, draft, and the process. Check the machine or procedure guidance, then verify flow at the gun if you have a suitable flowmeter.
More gas is not always better. Excessive flow can create turbulence that pulls surrounding air into the shielding envelope. A flow setting of 40 cfh may be appropriate in some conditions, but it can make shielding worse in a sheltered indoor weld. Avoid aiming a fan directly at the arc; even moderate air movement can disrupt coverage.
Inspect the nozzle for spatter and blockage. Check that the gas diffuser is installed, the nozzle seats properly, and the gun liner or hose is not kinked. Listen for leaks, but do not rely on sound alone. If a leak is suspected, use an approved leak-detection solution on accessible connections—never use a flame. Replace damaged hoses or fittings rather than compensating by turning the flow up.
Clean the joint and wire
Remove rust, mill scale, paint, oil, moisture, and other residue from the weld area. Clean both sides of the joint where practical, not just the visible bead path. Use a dedicated brush or abrasive suitable for the base metal; a brush previously used on carbon steel can contaminate stainless steel. If a solvent is needed, use one appropriate for the material, let it evaporate fully, and keep it away from ignition sources.
Keep wire dry and protected from dust or oil. Dirty wire can carry contaminants directly into the weld. Store opened spools in a dry place, and inspect the wire for rust or residue before feeding it. If the outer layers are corroded, replacing the spool may be more reliable than trying to clean the wire as it feeds.
Set the process and travel technique
Use wire, shielding gas, polarity, and machine settings suited to the material and transfer mode. For many solid-wire MIG setups, a common shielding gas is 75% argon and 25% carbon dioxide, but material, thickness, and procedure can call for something else. Flux-cored wire may require a different gas or no external gas at all; check the wire label rather than assuming solid-wire settings apply.
Make a short test weld on clean scrap of similar thickness. If the arc is erratic or the bead is unusually convex, adjust voltage and wire-feed speed in small steps within the machine’s recommended range. Settings that are too cold can cause poor fusion and an unstable pool; settings that are too hot or too fast can produce an irregular puddle and make shielding harder to maintain. Neither appearance alone confirms the cause of porosity.
Keep a consistent gun angle and stickout. A push angle of roughly 5–15 degrees is a useful starting point for many MIG applications, though joint position and procedure can require another technique. Excessive stickout moves the arc farther from the nozzle’s effective gas coverage. Avoid whipping or making a wide weave unless the procedure calls for it; a steady travel speed and a controlled puddle are easier to shield.
Match the symptom to the likely cause
| What you see | Likely causes to check | First corrective action |
|---|---|---|
| Scattered surface pinholes | Dirty base metal, drafts, poor flow, or a leaking connection | Clean the joint, shield it from air movement, and check flow and fittings |
| A line or cluster of pores | Contamination along the joint, blocked nozzle, or inconsistent gas coverage | Clean the full joint and inspect the nozzle and diffuser |
| Porosity at starts or stops | Gas preflow or postflow issue, rushed starts, or contamination at the crater | Check the machine’s gas timing and use a controlled start and stop |
| Porosity appears only outdoors | Wind disrupting shielding | Use a windbreak or move the work indoors; do not rely on excessive gas flow |
These are clues, not a diagnosis. If the defect remains after the obvious checks, change one variable at a time and make another test bead. Changing gas flow, voltage, wire speed, and technique all at once makes it harder to identify the problem.
When equipment is worth buying
If you weld regularly, a gun-end flowmeter can help confirm actual delivery instead of relying only on the regulator reading. Choose one compatible with your gas and torch. It is a diagnostic tool, not a substitute for sound connections or proper technique. A MIG gun flowmeter is most useful when flow readings are uncertain or a machine has a long hose run.
For indoor work, a basic welding screen can reduce drafts without enclosing the area completely. Keep ventilation in mind: shielding gas displaces air, and welding fumes must be controlled. Do not create an unventilated enclosure. A welding screen can be a practical fix for air movement around a bench, but it will not correct contaminated metal or a leaking gas line.
On occasional indoor repairs, you may not need to buy anything. Clean the metal, clear nearby fans, inspect the nozzle, and verify the cylinder and fittings first. Replace worn consumables only when inspection points to them. If porosity persists on a joint where failure would matter, stop and get the setup checked rather than hiding the defect beneath another pass.