11.4 Lack of Fusion & Lack of Penetration

11.4 Lack of Fusion & Lack of Penetration

Quick Answer: Lack of fusion (LOF) is failure of weld metal to fuse with parent metal or with previous weld beads (side-wall, inter-run, or root LOF). Lack of penetration (LOP) / incomplete penetration means the weld did not reach the required penetration depth or root fusion for a full-penetration joint. Both are planar (or near-planar) imperfections, typically not permitted at ISO 5817 levels B, C, and D. Causes include low heat input, poor technique, and unsuitable joint design/access. UT often detects planar LOF better than RT when orientation is unfavourable for radiography.

Module WI1.7 and acceptance practice under ISO 5817 require IWI-S candidates to distinguish LOF from LOP, explain planar risk, list causes and prevention, and understand NDT detectability differences—especially RT versus UT.

Definitions That Must Stay Sharp

Lack of fusion (LOF)

LOF means the fusion boundary that should have melted and bonded did not. Types:

1. Side-wall lack of fusion — weld metal fails to fuse to the preparation face (bevel face). Common in thick prep walls with wrong angle, too-low heat, or poor torch/electrode angle.
2. Inter-run (interpass) lack of fusion — a bead fails to fuse to the previous bead. Common when slag is left, when the previous bead is too convex/cold, or when the next bead is placed with insufficient energy or wrong placement.
3. Root lack of fusion — root bead fails to fuse to one or both root faces even if metal is present in the root area.

LOF surfaces are often smooth and tightly closed—hard to see by VT when subsurface, and easy to underestimate.

Lack of penetration / incomplete penetration (LOP)

LOP means the joint did not achieve the designed penetration. For a specified full-penetration butt, the root is not fully consumed/fused through the thickness as required. Related terms include incomplete root penetration and insufficient penetration depth on partial-penetration joints that fail their specified design throat/penetration.

Distinction for exams:

• LOF = no metallurgical bonding where bonding was required (fusion missing).
• LOP = penetration depth / root penetration requirement not met (geometry of fusion zone incomplete relative to design).

They can occur together at the root (incomplete root penetration with lack of root fusion).

Why Planar LOF/LOP Is Serious

Like cracks, LOF/LOP:

• Creates a sharp or crack-like notch along a plane.
• Reduces effective load-carrying section.
• Acts as a fatigue initiation site, especially at roots and side walls.
• Can be leak paths in pressure boundaries when continuous along the root.

Therefore ISO 5817 practice treats lack of fusion and incomplete penetration as not permitted at quality levels B, C, and D for the relevant joint categories—not as size-limited rounded pores.

Do not apply porosity acceptance logic to LOF.

Causes

Heat input and parameters too low

Insufficient current, excessive travel speed, or wrong polarity/settings for the process yield a cold weld pool that does not wet side walls or root. Thin, ropey beads and poor wash-in are visual warnings during welding—if the inspector is present.

Technique errors

• Incorrect electrode/torch angle (not pointing into the wall or root).
• Arc too long (energy spread, poor dig into root on MMA).
• Bead placement missing the side wall (especially in narrow gaps).
• Failure to clean slag/glass between runs → inter-run LOF.
• Inadequate root gap or excessive root face for the procedure (fit-up).
• Poor weave technique that fails to dwell on side walls.

Joint design and access

• Too narrow a groove for the process and diameter of electrode/wire.
• Excessive root face or insufficient root gap for full penetration without back gouging/backing as designed.
• Backing bar missing when required; ceramic backing misaligned.
• Restricted access in nozzles, branches, and fixed pipe positions (5G/6G challenges).

Process-specific contributors

• MMA: wrong electrode size for prep; slag traps in re-entrants.
• GMAW short-circuit: cold lap / LOF risk if used outside its thickness/position comfort zone without proper settings.
• SAW: misaligned head, wrong flux depth, too-fast travel.
• TIG root: insufficient root gap/face control, poor keyhole/root puddle practice, inadequate purge leading to cold, oxidised root that will not fuse properly on stainless.

Metallurgical / surface factors

Heavy oxide films, mill scale, and copper contamination can impede wetting. On aluminium, oxide must be cleaned and (for GTAW) arc polarity/cleaning action understood—link Chapter 8.

Prevention and Procedure Controls

Inspector hold points often include fit-up (gap/face/alignment), root pass visual (where accessible), and interpass cleanliness on multipass critical welds.

Detection and NDT Detectability

Visual testing

• May see root concavity, lack of root bead, or cold lap on accessible surfaces.
• Side-wall and inter-run LOF are often invisible once capped.
• Surface cold lap / overlap-related LOF can be seen as a smooth unfused ledge—do not confuse with acceptable reinforcement blend without checking fusion.

Radiographic testing (RT)

• LOF may appear as dark lines if the plane opens enough and aligns with the beam.
• Tight LOF parallel to the beam path can be missed or under-called.
• Root LOP often shows as a straight dark line in the root on butt welds—classic RT teaching image—but interpretation must follow written procedures and acceptance standards.
• RT is generally stronger on volumetric rounded flaws than on all tight planar flaws.

Ultrasonic testing (UT)

• Better suited to many planar side-wall and inter-run LOF orientations in butts when procedure, angle probes, and geometry are appropriate.
• Root regions need correct technique (and sometimes complementary methods).
• UT characterisation still requires qualified personnel (ISO 9712 context—Chapter 14).

Surface methods (PT/MT)

• Detect only surface-breaking LOF/cold lap openings—not buried inter-run LOF.

Destructive macros

• Macro sections in procedure qualification clearly show LOF/LOP; production cut-outs are rare and contractual.

Exam takeaway: for planar LOF, do not assume RT clears the joint; method selection should consider planar risk (Chapters 13–14). IWI-S identifies suitable techniques and verifies that specified NDT was applied—not every interpretation detail of every method.

Acceptance (ISO 5817 Lens)

For steel fusion welds to ISO 5817:

• Lack of fusion and incomplete penetration are typically not permitted at B, C, and D.
• Partial-penetration joints must still meet their designed penetration/throat—failure against design is still rejectable even if someone misapplies full-pen tables.
• Engineering critical assessment routes, if any, are formal contract tools—not informal inspector discretion.

Differentiation From Related Imperfections

Accurate naming drives correct repair: grinding out LOF to sound fused metal differs from simply blending undercut.

Repair and Corrective Action

1. Remove LOF/LOP completely to sound metal (grinding, gouging) with VT/NDT verification of excavation.
2. Correct the cause (parameters, fit-up, cleaning, welder technique coaching).
3. Re-weld with approved repair WPS / original WPS as allowed.
4. Re-apply full required NDT—not only VT of the cosmetic cap.
5. Trend LOF occurrences: if side-wall LOF repeats on one process/position, raise a process audit, not only welder blame.

Link Forward

Chapter 12 covers undercut, overlap, and shape imperfections that VT catches daily, and inclusions that often accompany poor interpass practice. Chapter 13–14 expand VT/PT/MT/RT/UT so you can defend method choice for planar versus volumetric risks. Chapter 15 returns to ISO 5817 tables so “not permitted” is applied from the standard, not from memory alone. Together, Sections 11.2–11.4 define the high-severity planar set: cracks, LOF, and LOP—the imperfections that most often stop a release stamp cold.