Distributed porosity, Fig.1, is normally found as fine pores throughout the weld bead. Surface breaking pores, Fig.2, usually indicate a large amount of distributed porosity.
Porosity is caused by the absorption of nitrogen, oxygen and hydrogen in the molten weld pool which is then released on solidification to become trapped in the weld metal.
Nitrogen and oxygen absorption in the weld pool usually originates from poor gas shielding. As little as 1% air entrainment in the shielding gas will cause distributed porosity and greater than 1.5% results in gross surface breaking pores. Leaks in the gas line, too high a gas flow rate, draughts and excessive turbulence in the weld pool are frequent causes of porosity.
Hydrogen can originate from a number of sources including moisture from inadequately dried electrodes, fluxes or on the workpiece surface. Grease and oil on the surface of the workpiece or filler wire are also common sources of hydrogen.
Surface coatings like primer paints and surface treatments such as zinc coatings may generate copious amounts of fume during welding. The risk of trapping the evolved gas will be greater in T joints than butt joints especially when fillet welding on both sides. It should not be necessary to remove low zinc primers but if the primer thickness exceeds the manufacturer's recommendation, porosity is likely to result, especially when using welding processes other than MMA (SMA).
The gas source should be identified and removed as follows:
- seal any air leak
- avoid weld pool turbulence
- use filler with adequate level of deoxidants
- reduce excessively high shielding gas flow
- avoid draughts
- dry the electrode and flux in accordance with the manufacturer's recommendations
- clean and degrease the workpiece surface
- clean the join edges immediately before welding
- check that the weldable primer is below the recommended maximum thickness
For more information on porosity and how it can be prevented, see Job Knowledge for Welders 42.