Weld pool

The weld pool is where welding actually happens. Everything the process parameters are meant to control — penetration, fusion, dilution, the final bead shape — is decided while the metal is still liquid, in the few seconds it takes the arc or beam to pass. By the time an inspector looks at a finished weld, the pool is gone; only its outcome, the solidified bead, remains.

Its shape is governed jointly by the heat source (current, voltage or beam power, and their distribution), travel speed, torch or beam angle, joint geometry, and gravity relative to welding position. A pool that is too long and shallow signals excess travel speed or insufficient energy; one that is wide and stagnant risks running ahead of the fusion front. Because these relationships are consistent for a given process and joint, pool geometry — width, length, and the symmetry of its outline — is the most direct real-time signal available for penetration and dilution, both of which are otherwise only confirmed after the joint has cooled and been sectioned or scanned.

Monitoring the pool directly is harder than monitoring the finished weld. The arc itself is several orders of magnitude brighter than the pool surface, so a camera has to be filtered or gated to see pool geometry instead of arc glare, and any conversion from measured radiance to an actual pool temperature depends on knowing the surface emissivity of liquid metal under an arc — a harder number to fix than the emissivity of a cooled, oxidised surface. This is why most production monitoring reads the pool for shape and stability rather than for an absolute temperature: geometry and its frame-to-frame consistency survive an uncertain emissivity in a way an absolute reading does not.

Porosity also originates in the pool: gas dissolved in the liquid metal has to escape before the surface solidifies over it, and a pool that freezes too fast — a common effect of travel speed set too high — traps more of it. A stable, well-formed pool is therefore not a cosmetic goal; it is upstream of several distinct failure modes at once.