Cavities in welded joints have a detrimental effect on the integrity of a welded joint. The presence of large cavities may even cause the joint to fail. Broadly speaking, there are three types of cavities:
- gas pores – visible holes in the surface
- porosity – holes in the material below and on the surface, so not always visible
- clustered porosity – large(er) gas deposits in the material, not visible
How cavities are formed
The solubility of gases is many times greater in the molten weld metal than in the solid metal. If these gases can no longer escape during the solidification process, porosity will develop. If the protection of the transferring droplets and the weld pool is insufficient, nitrogen and oxygen can be absorbed from the atmosphere. This can also apply to hydrogen originating from a humid atmosphere or humid welding material.
The absorbed gases must therefore be able to escape from the weld pool in order to prevent porosity. A rapidly solidifying weld pool will prevent this. Pollutants can also cause porosity. For example, sulphur that oxidises to SO2 and is absorbed as a gas in the weld pool.
What are the causes
The list of possible causes of cavities is long and diverse.
- Insufficient gas protection in the gas-arc welding processes
- Excessive gas flow during gas-arc welding processes (turbulence)
- Too long arc during welding
- Clustered porosity may form when starting or restarting the arc because the protection of the weld pool is still inadequate
- Paint, dirt, rust or grease on the weld seam preparation
- Damp welding consumables
- Incorrect position of torch or electrode
- Incorrect gas nozzle diameter
- Dirty gas nozzle
- Leakage of gas supply hose or its seal
- Water leakage in the welding torch
- Waving too wide
- Moisture in shielding gas
- Oxidation of welding filler material
In general, it can be said that the gas-arc welding processes are somewhat more sensitive to the occurrence of gas cavities.
The above-mentioned quantity and diversity means that definitive determination of the cause of porosity is not always easy. It requires a thorough evaluation.
Testing for cavities
Porosity on the surface of the weld can be demonstrated by visual inspection. Magnetic and penetrant testing are also very suitable for this purpose. In magnetic testing, cavities are detected by magnetising the workpiece and sprinkling it with magnetic particles. These particles will accumulate around the cavities and thus show them. In penetrant testing, a dye is used that soaks into the surface cavities and can be made visible using a developer. This also makes any cavities visible.
Radiographic testing is the most suitable method for demonstrating internal porosity. X-ray or gamma radiation is used, depending on the thickness of the material.