Why Your MIG Welds Have Porosity: Causes and Fixes

Quick Answer

MIG weld porosity is almost always a shielding problem, not a machine problem. Check your gas flow rate first, then inspect the nozzle for spatter and the liner for contamination. If gas flow is correct and the nozzle and liner are clean, the cause is usually a leak between the regulator and the gun, or the wrong gas for the base metal.

Check Gas Flow Rate Before Anything Else

Set your flow rate at the regulator and confirm it at the nozzle, not at the cylinder. A flowmeter that reads 20 CFH at the regulator can deliver far less at the arc if the hose is kinked or the solenoid is partially blocked. For most short-arc MIG welding on steel, 15 to 25 CFH is a reasonable starting range. Too little flow lets atmosphere into the puddle. Too much flow creates turbulence that pulls air into the shielding envelope, which causes porosity that looks identical to a low-flow problem.

If you weld with CO2 rather than a mixed gas, remember that CO2 runs hotter and needs a flowmeter rated for it. A general-purpose argon regulator will not meter CO2 accurately and may freeze or creep. Using the right metering device is the difference between a repeatable setting and a guess.

Inspect the Nozzle for Spatter Buildup

A nozzle caked with spatter disturbs gas flow. The spatter narrows the opening and creates turbulence right where the shielding gas needs to be calm. Clean the nozzle with a reamer at every consumable change, and replace it when the spatter cannot be removed. Anti-spatter dip helps, but it is not a substitute for cleaning.

Also check that the nozzle is the right size and seated fully. A loose or wrong-length nozzle changes the distance between the gas exit and the arc, which changes how well the shielding gas covers the puddle.

Check the Liner and Gun Cable for Contamination

A dirty liner is an underrated cause of porosity. Liner contamination from rust, dirt, or wire shavings creates drag on the wire, which causes inconsistent feed and erratic arc behavior. When the wire stutters, the arc length changes and the shielding gas envelope becomes unstable. Blow out the liner with dry compressed air, and replace it if the wire still feeds rough.

If you weld aluminum, liner setup matters even more. Some machines do not use a guide tube for aluminum, and the liner must be cut flush so the wire exits cleanly. A liner that is too long or not trimmed correctly will drag and produce the same erratic feed that shows up as porosity.

Look for Leaks Between the Regulator and the Gun

A small leak at a hose fitting, solenoid, or gun connection pulls air into the gas stream. You will not always hear it. Pressurize the system and check every connection with soapy water, or watch the flowmeter ball for drift when the gun is not triggering. A flowmeter that creeps upward with the trigger released points to a leak or a bad solenoid.

The gun itself can leak at the base of the neck where the cable enters. If the gun has been dropped or the cable has been kinked repeatedly, the internal gas passage can crack. In that case, no amount of regulator adjustment will fix the porosity.

Match the Gas to the Metal and the Transfer Mode

Porosity can also come from using the wrong shielding gas for the process. CO2 is economical and works well for short-arc welding on carbon steel, but it does not perform the same as a 75/25 argon-CO2 mix in spray transfer. If you switch transfer modes or base metals without changing gas, porosity is a predictable result.

Drafty shops make this worse. A fan or open bay door can blow the shielding gas away no matter how well the regulator is set. If porosity appears only in one part of the shop, move the work or build a simple screen before you change any settings.

Porosity Diagnosis: Symptom, Cause, Fix, Stop Point

• Symptom: Porosity scattered evenly along the bead: Likely cause is low shielding gas flow or a leak between the regulator and the gun. Check flow at the nozzle, then soap-test every fitting. If flow is correct and no leak is found, move to liner and nozzle inspection.
• Symptom: Porosity at the start of the weld only: Likely cause is gas pre-flow that is too short or a nozzle partially blocked by spatter. Increase pre-flow and ream the nozzle. If the nozzle cannot be cleaned back to bare metal, replace it.
• Symptom: Porosity with erratic arc and inconsistent wire feed: Likely cause is liner contamination or a liner that is too long or not trimmed for the application. Blow out the liner; replace it if feed is still rough. If the gun cable is kinked or the neck is damaged, stop adjusting the machine and replace the gun.
• Symptom: Porosity only in one area of the shop or only on one base metal: Likely cause is drafts or the wrong shielding gas for the metal and transfer mode. Screen the work area and confirm gas selection. If the correct gas and a screened area still produce porosity, the problem is in the gun or cable, not the settings.

Parts That Solve Common Porosity Causes

• HTP® AMERICA CO2 Flowmeter: HTP America CO2 Flowmeter is designed for MIG welding with CO2 only, with a 0 to 4000 psi cylinder gauge and adjustable output from 10 to 60 CFH. If you weld with CO2, a dedicated CO2 flowmeter gives you a repeatable flow setting and a clear read on how much gas is left, which removes the guesswork that leads to low-flow porosity.
• 9' MIG Welding Gun: 9' MIG Welding Gun for aluminum with Pro Spot MIG welders; the liner is cut flush on the replacement gun, and if your machine does not include a guide tube, HTP will install a new liner with stick out before shipping at no cost. For aluminum MIG, correct liner trim and stick out prevent the wire drag and erratic feed that show up as porosity. Noting your liner requirements at checkout gets the gun set up for your machine.