Definition and Types of Lack of Fusion and Penetration according to BS EN ISO 6520-1

Lack of fusion is lack of union between the weld metal and the parent metal or between the successive layers of weld metal.
Types of lack of fusion:

1. Lack of sidewall fusion



Lack of union between the weld and parent metal at one or both sides of the weld.

Causes:
• Low heat input to weld
Prevention: Increase arc voltage and/or welding current; decrease travel speed
• Molten metal flooding ahead of arc
Prevention: Improve electrode angleg alend work position; increase travel speed
• Oxide or scale on weld preparation
Prevention: Improve edge preparation procedure
• Excessive inductance in MAG dip transfer welding
Prevention: Reduce inductance, even if this increases spatter

During welding sufficient heat must be available at the edge of the weld pool to produce fusion with the parent metal.


2. Lack of inter-run fusion



Lack of union along the fusion line between the weld beads.

Causes:
• Low arc current resulting in low fluidity of weld pool
Prevention: Increase current
• Too high a travel speed
Prevention: Reduce travel speed
• Inaccurate bead placement
Prevention: Retrain welder

Lack of inter-run fusion produces crevices between the weld beads and causes local entrapment of slag.


3. Lack of root fusion



Lack of fusion between the weld and parent metal at the root of a weld.

Causes:
• Low heat input
Prevention: Increase welding current and/or arc voltage; decrease travel speed
• Excessive inductance in MAG dip transfer welding
Prevention: Use correct induction setting for the parent metal thickness
• MMA electrode too large (low current density)
Prevention: Reduce electrode size
• Use of vertical-down welding
Prevention: Switch to vertical-up procedure
• Large root face
Prevention: Reduce root face
• Small root gap
Prevention: Ensure correct root opening
• Incorrect angle or electrode manipulation
Prevention: Use correct electrode angle; ensure welder is fully qualified and competent
• Excessive misalignment at root
Prevention: Ensure correct alignment

Types of Lack of Penetration:

1. Incomplete penetration



The difference between actual and nominal penetration.

Causes:
• Excessively thick root face, insufficient root gap or failure to cut back to sound metal when back gouging
Prevention: Improve back gouging technique and ensure the edge preparation is as per approved WPS
• Low heat input
Prevention: Increase welding current and/or arc voltage; decrease travel speed
• Excessive inductance in MAG dip transfer welding, pool flooding ahead of arc
Prevention: Improve electrical settings and possibly switch to spray arc transfer
• MMA electrode too large (low current density)
Prevention: Reduce electrode size
• Use of vertical-down welding
Prevention: Switch to vertical-up procedure

If the weld joint is not of a critical nature, ie the required strength is low and the area is not prone to fatigue cracking, it is possible to produce a partial penetration weld. In this case incomplete root penetration is considered part of this structure and not an imperfection This would normally be determined by the design or code requirement.


2. Incomplete root penetration



Both fusion faces of the root are not melted. When examined from the root side, you can clearly see both of the root edges unmelted.

Causes and prevention Same as for lack of root fusion.