Solid inclusions in welded joints have a negative effect on the integrity of a welded joint. If they are present in large quantities, they can even cause the joint to fail. There are four types of inclusions:
Slag inclusions can occur during slag-forming welding processes, such as arc welding with coated electrodes, MIG/MAG welding and submerged arc welding. In these welding processes, small slag particles (silicates) are formed which serve to shield the weld metal from the surrounding atmosphere immediately after welding. The slag can also influence the chemical composition of the weld metal. Especially when welding in multiple layers, this slag has to be removed properly.
There are various causes of slag inclusions:
- Welding with too low a current
- Incorrect positioning of the torch or electrode
- Bad tack welding
- Surfaces of the welded joints that are too rough
- Too convex a base layer and/or intermediate layers in multilayer welding
- An incorrect weld seam shape
- Incorrect waving pattern during welding
- When formed slag runs out in front of the weld pool
When submerged arc welding, flux may be trapped by the weld pool due to improper flux control. After solidification of the weld pool, this will be left behind as an inclusion.
Oxide inclusions occur particularly during gas arc welding of aluminium when
- the cleaning effect of the welding arc is insufficient,
- or too low a welding current is used,
- Or oxidised welding consumables are used,
- or the consumable is removed from the arc’s protective atmosphere.
The latter is particularly a risk in TIG or oxy acetylene (gas) welding.
Metallic inclusions occur during TIG welding when part of the tungsten electrode enters the weld pool. These tungsten inclusions are formed when
- the tungsten electrode touches the weld pool,
- or when the arc is started incorrectly (scratch starting),
- or when the welding consumable comes into contact with the electrode,
- or when the electrode is subjected to too high a current.
It can also happen that copper inclusions enter the weld pool. This is the case when
- copper welding consumables come into contact with the weld pool,
- or when the copper gas nozzle comes into contact with the weld pool or the seam preparation side.
Copper inclusions are extremely difficult to detect with non-destructive testing. These inclusions cause embrittlement of the weld.
Detection of solid inclusions
Solid inclusions on the surface of the weld can be detected by visual inspection. Internal inclusions (with the exception of copper inclusions, as mentioned above) can be detected well by radiographic testing. X-ray or gamma radiation is used, depending on the thickness of the material. With ultrasonic testing, using high-frequency sound pulses, it is more difficult to detect solid inclusions.