Definition and Types of Cavities according to BS EN ISO 6520-1

Cavities, is imperfection formed by entrapped gas, and also caused by shrinkage during solidification. Types of cavities:

1. Gas Pore 


Gas pore: A gas cavity of essentially spherical shape trapped within the weld metal.

Gas cavities can be present in various forms:
• Isolated porosity 
• Uniformly distributed porosity 
• Clustered (localised) porosity 
• Linear porosity 
• Elongated cavity
• Worm hole 
• Surface pore 

Causes:

1. Damp fluxes/corroded electrode MMA
Prevention: Use dry electrodes in good condition
2. Grease/hydrocarbon/water contamination of prepared surface
Prevention: Clean prepared surface

3. Air entrapment in gas shield (MIG/MAG, TIG)
Prevention: Check hose connections

4. Incorrect/insufficient deoxidant in electrode, filler or parent metal
Prevention: Use electrode with sufficient deoxidation activity
5. Too great an arc voltage or length
Prevention: Reduce voltage and arc length

6. Gas evolution from priming paints/surface treatment
Prevention: Identify risk of reaction before surface treatment is applied

7. Too high a shielding gas flow rate results in turbulence (MIG/MAG, TIG)
Prevention: Optimise gas flow rate


2. Worm Holes



Worm holes: Elongated or tubular cavities formed by trapped gas during the solidification of the weld metal which can occur singly or in groups.

Causes:
1. Gross contamination of preparation surface
Prevention: Introduce preweld cleaning procedures
2. Laminated work surface
Prevention: Replace parent material with an unlaminated piece
3. Crevices in work surface due to joint geometry
Prevention: Eliminate joint shapes which produce crevices

Worm holes are caused by the progressive entrapment of gas between the solidifying metal crystals (dendrites) producing characteristic elongated pores of circular cross-section. These can appear as a herringbone array on a radiograph and some may break the surface of the weld.


3. Surface Porosity


Surface Porosity: A gas pore that breaks the surface of the weld.

Causes:

1. Damp or contaminated surface or electrode
Prevention: Clean surface and dry electrodes
2. Low fluxing activity (MIG/MAG)
Prevention: Use a high activity flux
3. Excess sulphur (particularly free-cutting steels) producing sulphur dioxide
Prevention: Use high manganese electrodes to produce MnS. Note free-cutting steels (high sulphur) should not normally be welded
4. Loss of shielding gas due to long arc or high breezes (MIG/MAG)
Prevention: Improve screening against draughts and reduce arc length
5. A shielding gas flow rate that is too high results in turbulence (MIG/MAG, TIG)
Prevention: Optimise gas flow rate


4. Crater Pipe



Crater pipe: A shrinkage cavity at the end of a weld run usually caused by shrinkage during solidification.

Causes:


1. Lack of welder skill due to using processes with too high a current
Prevention: Retrain welder
2. Inoperative crater filler (slope out) (TIG)
Prevention: Use correct crater filling techniques


Crater filling is a particular problem in TIG welding due to its low heat input. To fill the crater for this process it is necessary to reduce the weld current (slope out) in a series of descending steps until the arc is extinguished.