There are 3 common types of weld cavities; They are gas pore, wormholes and surface porosity (including Crater pipe).
Let us explore them.
1. Gas Pore
A gas pore is a trapped cavity within the weld metal that has a basic round shape and comes in a form of isolated, elongated cavity and surface pore.
Meanwhile, a group of gas pore that is uniformly distributed, clustered (localized) or linear is called porosity which comes in a form of localized or finely dispersed.
2. Worm Holes
A wormhole does not contain worms in it. Just kidding.
During the solidification process, trapped gas may form elongated or tubular cavities in the weld metal. This can either be in a single form or in groups.
Eventually, the entrap gases build up progressively between the solidifying metal crystals (dendrites), forming characteristic elongated pores of circular cross-section.
In certain cases, some of the trapped gas can even break through to the weld surface thus looking like a wormhole.
Guess which one is the wormhole?
3. Surface Porosity
Surface porosity is simply a gas pore that breaks the surface of the weld. The origins of surface porosity are similar to those for uniform porosity.
Shrinkage during weld solidification can cause a surface porosity called Crater pipe.
It is usually formed at the end of a weld run and often occur while welding with current too high or inoperative crater filler (slope out using TIG).
Crater filling is a particular problem due to the low heat input of TIG welding which needs to reduce the weld current (slope out) in a series of descending steps until the arc is extinguished.