Porosity

Molten metal holds far more dissolved gas than solid metal does. As the weld pool cools and freezes, that solubility drops sharply, and any gas the metal cannot shed in time is pushed ahead of the advancing solid front until it is trapped as a bubble. Porosity is what that trapped bubble becomes once the metal is fully solid: a rounded cavity, or a cluster of them, sitting inside or on the surface of the weld.

Several distinct events feed gas into the pool, and ISO 6520-1 gives the resulting cavities separate reference numbers by their form — isolated pore, uniformly distributed, clustered, linear, elongated — and ISO 5817 then applies different limits to those forms. A single isolated gas pore is the least severe form. Clustered or aligned porosity along a pass indicates a sustained rather than momentary problem — commonly a loss of shielding gas coverage from draught, a blocked or spattered nozzle, or a flow rate mismatched to the torch. Contamination is the other major route: moisture in flux or on the joint, oil or cutting fluid residue, and surface coatings such as mill scale, paint or zinc all dissociate in the arc and add gas the shielding cannot compensate for.

Because porosity forms below the surface as often as on it, and because a scattered cluster can be small individually while still weakening a section collectively, visual inspection alone does not clear it — radiography and ultrasonic testing are the routes that reliably find internal porosity, which is why procedures for pressure-retaining and fatigue-critical joints specify volumetric NDT rather than relying on the surface appearance of the finished weld.

The practical control point is upstream of any inspection: a stable, correctly shielded weld pool with clean, dry consumables and a prepared, coating-free joint gives the gas nowhere to come from in the first place, which is cheaper than finding it after the weld has cooled.