Elements causing porosity
A major element causing or contributing to formation of porosity is hydrogen, supplied by:
1. The gas atmosphere surrounding the arc zone and weld deposit area;
2. The presence of moisture in the atmosphere, flux or electrode coating, or water in the area
being welded;
3. Cellulose or other hydrogen-forming constituents in the flux or electrode coating;
4. The base metal itself.
The welding arc’s intense heat dissociates the water vapour and other hydrogen-bearing
constituents. In the atomic state, hydrogen diffuses readily into the molten material. As the molten
metal cools, the solubility of hydrogen decreases for several common metals. As molten weld
material decreases, hydrogen tends to diffuse out of the weld deposit into the atmosphere and also
into the adjacent heat-affected zone of the base material.
Temperature decreases, however, generally occurs so rapidly that diffusion does not take place fast
enough to allow hydrogen content to remain within its solubility limits in the molten metal. The
resulting super-saturation causes the combination of hydrogen and porosity formation.
Being lighter the molten metal, gas bubbles tend to rise to the weld deposit’s surface. If the bubbles
fail to reach the surface before solidification, they will be entrapped as internal porosity. If they reach
the surface, they may appear as external or surface porosity. When weld solidification occurs as
dendritic growth, the voids may also become entrapped along the grain boundaries. Other impurities
may also precipitate out. Gas bubbles may also form by chemical reactions.
Thus, porosity tends to form when oxides in the weld puddle are reduced by hydrogen or carbon at
or slightly above their melting point. Thus, the presence of deoxidisers, when added to filler metals,
may reduce the oxide reducing actions, minimizing in turn the tendency toward porosity formation.
Worm holes or tunnelling can be caused primarily by turbulence in the weld pool, usually a result of
excessively high welding currents.
Among the welding processes, the tendency for porosity formation is greatest in manual metal arc
welding with coated electrodes. This is due to the moisture in the coating and the heavier than
normal surface oxidation of the core wire.
The tendency for porosity is somewhat less with Mig welding. Nevertheless, porosity can form,
particularly with the smaller diameter wires which have a high surface-to volume ratio, and may thus
contain significant oxidation and absorbed moisture. Grease and oil lubricants on the wire may also
provide the hydrogen resulting in significant porosity. The tendency for porosity formation is least
with Tig welding. To minimize porosity in aluminium welds, surface treatment is particularly
important to minimize the presence of oxides, moisture and other sources of hydrogen.
Very rarely has porosity been associated with actual service failures. No failures have been reported
in welds where the porosity was applicable code requirement. Quite likely, porosity limits two to four
times those now established in codes could be tolerated in the majority of critical service
applications. There is significant divergence among the major codes on the acceptability limits
applicable to porosity. Porosity, even under the most liberal acceptability limits of the standards, is
not likely to result in service failures so long as it is removed from the surface. Far more critical are
surface conditions or notches, which can trigger mechanical or metallurgical failures. Latest results
and codes should however be checked.
