This article continues the Welding Imperfections and Classification series in accordance with ISO 6520 1, following earlier discussions on Cracks, Cavities, and Solid Inclusions. Part 4 examines Group 4 imperfections, which include lack of fusion and lack of penetration. These defects are among the most structurally significant because they directly affect the continuity, load transfer, and mechanical integrity of welded joints.

Lack of fusion and lack of penetration occur when the molten weld metal fails to fully bond with the parent material or fails to penetrate the joint root as required. These discontinuities leave unbonded surfaces within the joint, creating internal planes of weakness that can trigger crack initiation, fatigue failure, and brittle fracture.

Understanding Lack of Fusion and Lack of Penetration

Lack of fusion refers to incomplete union between the deposited weld metal and either the base material or a previous weld bead. Lack of fusion can occur at side walls, between weld layers, or at the joint root. These areas remain unmelted, resulting in unwelded planar surfaces that behave like cracks when loaded.

Incomplete penetration refers to insufficient depth of weld metal through the joint thickness. In other words, the fusion penetration, (or depth of fusion) is the distance that fusion extends into the base metal or a previous weld pass from the surface melted during welding (see also BSI. PD ISO-TR 25901-1-2016) . The schematic below illustrates fillet welds showing fusion penetration, deep and incomplete penetration

Both imperfections share similar causes, including:

1. Insufficient heat input
2. Incorrect joint preparation
3. Improper torch or electrode positioning
4. Low travel speed or poor manipulation
5. Contaminants on the fusion faces
6. Incorrect welding parameters or polarity

These imperfections compromise joint strength because they leave unwelded surfaces inside the joint that cannot transmit structural loads.

ISO 6520 1 Group 4 – Lack of Fusion and Penetration

ISO 6520 1 classifies these imperfections under Group No. 4, with reference numbers beginning at 400.

400 – Lack of fusion and penetration (General Term)

A general description for any discontinuity caused by incomplete fusion or incomplete penetration during welding.

401 – Lack of Fusion – Lack of union between weld metal and parent material or between successive layers of weld metal. 

Subcategories – Lack of Fusion can be one of the following:

• 4011 – Lack of side wall fusion: Failure of the weld metal to fuse with one or both joint side walls.

• 4012 – Lack of inter run fusion: Incomplete fusion between successive weld beads (layers).

• 4013 – Lack of root fusion: Incomplete fusion at the joint root, leaving part of the root unfused.

• 4014 – Micro lack of fusion: Small scale lack of fusion detectable only under magnification.

Note: Lack of fusion imperfections are often referred to as cold laps.

Common causes:
• Low heat input or travel speed
• Incorrect manipulation or electrode angle
• Excessive oxide or contamination on joint faces
• Improper joint fit up or preparation

Consequences:
• Significant loss of strength
• Crack initiation under fatigue or dynamic loading
• Poor toughness and increased brittleness

402 – Incomplete Penetration (Lack of Penetration)

Difference between the actual and nominal penetration.

Nominal penetration is the required depth of fusion for the approved weld joint. Actual penetration is what occurs in practice. When actual penetration is less than required, the defect is classified as incomplete penetration.

Subcategories:

4021 – Incomplete root penetration: One or both fusion faces at the joint root are not melted. This creates a continuous internal notch extending along the root.

Common causes:
• Joint preparation too tight or misaligned
• Inadequate root gap
• Incorrect welding parameters (low current, incorrect polarity)
• Poor access or electrode angle
• Improper root backing

Consequences:
• Severe load carrying reduction at the root
• Potential for crack initiation under bending or tension
• Frequently unacceptable under most structural codes

403 – Spiking: Extremely non uniform penetration occurring in electron beam and laser welding, giving a sawtooth appearance.

Spiking is a fusion imperfection unique to high energy beam processes. It results from unstable keyhole behaviour, producing alternating deep and shallow penetration profiles. These spikes may include cavities, cracks, and shrinkage voids.

Common causes:
• Keyhole instability
• Incorrect focus or power level
• Weld pool turbulence
• Variations in beam energy or material reflectivity

Mechanisms Leading to Lack of Fusion and Penetration

These imperfections typically form because the heat source fails to generate adequate melting at the intended fusion surfaces. Contributing factors include:

1. Low heat input or heat loss through thick sections
2. Excessive travel speed reducing heat concentration
3. Inadequate joint fit up causing unpredictable root conditions
4. Poor welding technique and manipulation
5. Improper filler position relative to the arc
6. Contaminated surfaces reducing wetting and fusion
7. Use of incorrect consumables or polarity

These mechanisms leave internal unbonded planes that behave as cracks under service conditions.

Detection Methods

Lack of fusion and lack of penetration are planar imperfections and are often more harmful than volumetric discontinuities. Accurate detection is essential.

Effective methods include:

• Visual Testing (VT): for open root or surface breaking imperfections

• Radiographic Testing (RT): useful for root defects and major fusion gaps
• Ultrasonic Testing (UT):  excellent for detecting planar imperfections
• Phased Array UT:  highly effective for multi pass welds

Prevention Measures

1. Use appropriate heat input and controlled travel speed.
2. Maintain correct electrode or torch angle.
3. Ensure joint surfaces are clean and free of oxides or mill scale.
4. Provide correct root gap and proper joint preparation.
5. Follow welding procedures (WPS) specifying adequate penetration techniques.
6. Maintain adequate shielding and stable arc conditions.
7. Use appropriate access and electrode manipulation for full side wall fusion.

Designation Examples (ISO 6520 1)

• Imperfection ISO 6520 1 4011 (Lack of side wall fusion.)

• Imperfection ISO 6520 1 4013 (Lack of root fusion.)

• Imperfection ISO 6520 1 4021 (Incomplete root penetration.) • Imperfection ISO 6520 1 403 (Spiking.)

Summary Memory Cues

Reference	Description	Typical Cause	Detection Method
401	Lack of fusion	Low heat input, poor technique	UT, RT
4011	Side wall fusion lack	Incorrect angle	UT
4012	Inter run fusion lack	Poor cleaning or bead overlap	UT
4013	Root fusion lack	Poor root access	RT, VT
4014	Micro lack of fusion	Localised cold lap	Metallography
402	Incomplete penetration	Incorrect joint preparation	RT, UT
4021	Root penetration incomplete	Insufficient heat or root gap	RT
403	Spiking	Keyhole instability (EBW, LBW)	Metallography, UT

Letter Codes for Lack of Fusion and Penetration (Group 4)

Following the same system used for earlier parts

• Cracks → Ea series
• Cavities → Fa series
• Solid inclusions → Sa series

We now assign La series for Group 4 imperfections (Note that this convention is not in ISO 6520-1).

Letter Codes for Group 4: Lack of Fusion and Penetration

Letter Code	Imperfection Type	Typical Mechanism
La	Lack of side wall fusion (4011)	Inadequate melting of joint side walls due to insufficient heat or incorrect angle.
Lb	Lack of inter run fusion (4012)	Poor overlap or bead placement between weld passes.
Lc	Lack of root fusion (4013)	Inadequate melting at the joint root due to poor access or incorrect root parameters.
Ld	Micro lack of fusion (4014)	Localised cold laps or micro scale unfused areas.
Le	Incomplete penetration (402, 4021)	Actual penetration less than nominal due to improper joint preparation or low heat.
Lf	Spiking (403)	Unstable penetration profile in EBW or LBW from keyhole instability.

Full designation format:
Imperfection ISO 6520 1 [Reference Number] L[Letter]

Examples

• Lack of side wall fusion: Imperfection ISO 6520 1 4011 La
• Lack of root fusion: Imperfection ISO 6520 1 4013 Lc
• Incomplete root penetration: Imperfection ISO 6520 1 4021 Le
• Spiking in EBW or LBW: Imperfection ISO 6520 1 403 Lf

Closing Note

Lack of fusion and lack of penetration are critical welding imperfections that compromise joint integrity by interrupting complete fusion. Understanding their classification, causes, and detection according to ISO 6520 1 ensures accurate assessment and quality control in welding fabrication.

More lack of fusion and penetration illustrations

Frequently Asked Questions (FAQs)

Welding Imperfections and Classification – Part 4: Lack of Fusion and Penetration

Q1. What is lack of fusion in welding?

Lack of fusion is a planar imperfection where the weld metal fails to fuse with the parent material or with a previous weld bead. It leaves unmelted surfaces inside the weld, creating planes of weakness that behave like cracks under load.

Q2. What is incomplete penetration?

Incomplete penetration occurs when the weld metal does not reach the required depth of fusion. The actual penetration is less than the nominal penetration specified by the joint design or WPS, leaving an unfused gap at the joint root.

Q3. Why are lack of fusion and penetration considered dangerous imperfections?

Because they interrupt load carrying capability inside the weld. These internal unfused surfaces act as stress concentrators and are common sites for crack initiation, fatigue failure, brittle fracture, and loss of structural integrity.

Q4. What causes lack of fusion?

Common causes include:
• Low heat input
• Incorrect electrode or torch angle
• Poor bead placement or manipulation
• Excessive oxide or surface contamination
• Insufficient joint preparation or fit up
• Incorrect welding parameters

Q5. What causes incomplete penetration?

Incomplete penetration often results from:
• Too small or misaligned root gap
• Low welding current
• Incorrect polarity
• Poor access to the joint root
• Improper root backing or joint design

Q6. What is the difference between lack of root fusion and incomplete root penetration?

• Lack of root fusion means the root faces are melted but do not fuse.
• Incomplete root penetration means the root faces are not melted at all.

Both are serious planar imperfections but originate from different mechanisms.

Q7. What is spiking in welding?

Spiking is a penetration imperfection seen in electron beam and laser welding. It appears as a sawtooth pattern caused by unstable keyhole behaviour. These spikes may contain cavities, cracks, or shrinkage voids.

Q8. Which welding processes are most affected by spiking?

Spiking is specific to high energy beam processes:
• Electron beam welding
• Laser beam welding

These processes can produce deep, narrow penetration profiles that become unstable if parameters fluctuate.

Q9. What inspection methods are best for detecting lack of fusion?

Because lack of fusion is planar, the following methods are most effective:
• Ultrasonic Testing (UT)
• Radiographic Testing (RT)
• Phased Array UT for multi pass welds
• Visual Testing (VT) for surface breaking fusion issues
• Metallography for micro lack of fusion

Q10. Why are planar imperfections more harmful than volumetric imperfections?

Planar imperfections behave like cracks. They create continuous weak planes that reduce ductility, toughness, and fatigue life. Volumetric imperfections (like porosity) spread stress more diffusely and are usually less severe.

Q11. Can lack of fusion be detected visually?

Not always. Only surface connected lack of fusion can be seen during visual inspection. Most lack of fusion occurs internally and requires UT or RT for detection.

Q12. Is incomplete penetration acceptable under structural codes?

Incomplete root penetration is rarely acceptable. Most structural codes reject it because it significantly reduces load carrying capacity and creates a continuous internal notch.

Q13. How can welders prevent lack of fusion?

• Maintain correct heat input
• Use proper electrode angle and manipulation
• Clean joint faces thoroughly
• Ensure proper inter run cleaning
• Improve joint preparation and root access
• Follow the WPS for travel speed, current, and technique

Q14. How can incomplete penetration be prevented?

• Provide correct root gap and fit up
• Use correct current and polarity
• Ensure adequate access to the joint root
• Use appropriate backing bars or backing techniques
• Apply correct sequencing and heat control

Q15. Are lack of fusion and incomplete penetration the same defect?

No. They belong to the same ISO group but result from different mechanisms:
• Lack of fusion is a failure to bond.
• Incomplete penetration is a failure to reach the required depth.

Both compromise structural integrity.

Q16. How are Group 4 imperfections designated under ISO 6520 1?

Examples include:
• Lack of side wall fusion: Imperfection ISO 6520 1 4011
• Lack of root fusion: Imperfection ISO 6520 1 4013
• Incomplete root penetration: Imperfection ISO 6520 1 4021
• Spiking: Imperfection ISO 6520 1 403

Q17. Are these imperfections common in all welding processes?

They can occur in any fusion welding process if heat input, technique, or joint preparation is inadequate. However:
• Lack of fusion is common in SMAW, GMAW, FCAW, and GTAW.
• Incomplete penetration is common in root passes of multi pass welds.
• Spiking is unique to EBW and LBW.

Summary Statement

Lack of fusion and incomplete penetration are critical imperfections governed by ISO 6520 1 Group 4. Because they create internal planes of weakness, understanding their formation, detection, and classification is essential for maintaining weld quality and structural reliability.