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

Solid inclusions is solid foreign substances trapped in the weld metal.
Types of solid inclusions:

1. Slag inclusions

Slag trapped during welding which is an irregular shape so differs in appearance from a gas pore.  
Causes:
1. Incomplete slag removal from underlying surface of multi-pass weld
Prevention: Improve inter-run slag removal
2. Slag flooding ahead of arc
Prevention: Position work to gain control of slag Welder needs to correct electrode angle
3. Entrapment of slag in work surface
Prevention: Dress/make work surface smooth
A fine dispersion of inclusions may be present within the weld metal, particularly if the MMA process is used. These only become a problem when large or sharp-edged inclusions are produced.


2. Flux inclusions
Flux trapped during welding which is an irregular shape so differs in appearance from a gas pore. Appear only in flux associated welding processes (ie MMA, SAW and FCAW). 
Causes:
1. Unfused flux due to damaged coating
Prevention: Use electrodes in good condition
2. Flux fails to melt and becomes trapped in the weld (SAW or FCAW)
Prevention: Change the flux/wire. Adjust welding parameters ie current, voltage etc to produce satisfactory welding conditions
3. Oxide inclusions
Oxides trapped during welding which is an irregular shape so differs in appearance from a gas pore.
          Causes:
Heavy millscale/rust on work surface
Prevention: Grind surface prior to welding  
A special type of oxide inclusion is puckering, which occurs especially in the case of aluminium alloys. Gross oxide film enfoldment can occur due to a combination of unsatisfactory protection from atmospheric contamination and turbulence in the weld pool. 


4. Metallic inclusions (Tungsten, Copper, other metal)
Particles of tungsten can become embedded during TIG welding appears as a light area on radiographs as tungsten is denser than the surrounding metal and absorbs larger amounts of X-/gamma radiation.
Causes:
1. Contact of electrode tip with weld pool
Prevention: Keep tungsten out of weld pool; use HF start
2. Contact of filler metal with hot tip of electrode
Prevention: Avoid contact between electrode and filler metal
3. Contamination of the electrode tip by spatter from the weld pool
Prevention: Reduce welding current; adjust shielding gas flow rate
4. Exceeding the current limit for a given electrode size or type
Prevention: Reduce welding current; replace electrode with a larger diameter one
5. Extension of electrode beyond the normal distance from the collet, resulting in overheating of the electrode
Prevention: Reduce electrode extension and/or welding current
6. Inadequate tightening of the collet
Prevention: Tighten the collet
7. Inadequate shielding gas flow rate or excessive draughts resulting in oxidation of the electrode tip
Prevention: Adjust the shielding gas flow rate; protect the weld area; ensure that the post gas flow after stopping the arc continues for at least five seconds
8. Splits or cracks in the electrode
Prevention: Change the electrode, ensure the correct size tungsten is selected for the given welding current used
9. Inadequate shielding gas (eg use of argon-oxygen or argon-carbon dioxide mixtures that are used for MAG welding)
Prevention: Change to correct gas composition