Lack of Fusion (LOF)
Definition
A planar weld defect where the weld metal fails to fuse to the base metal or to the adjacent weld pass. Essentially a crack in the weld joint. Among the most serious weld discontinuities because it acts directly as a fatigue initiation site.
- Defect type
- Planar weld discontinuity
- Location
- Weld metal to base metal or between weld passes
- Severity
- Among the most serious weld defects
- Failure mechanism
- Acts as a fatigue crack initiation site
Common questions
How is lack of fusion typically detected before it causes failure?+
Radiographic testing, ultrasonic testing, or destructive sectioning during quality inspection can reveal LOF, since it often shows no surface indication and is invisible to visual inspection alone.
Why is lack of fusion treated as more serious than porosity in a weld?+
Porosity is a rounded, isolated void, while LOF is a planar, crack-like defect that concentrates stress sharply at its edges, making it far more effective at initiating and propagating a fatigue crack under cyclic loading.
Related terms
- Hot Cracking
- Solidification cracking or liquation cracking that occurs at elevated temperature during or just after welding. Driven by liquid films of low-melting impurity...
- Hydrogen Cold Cracking (HCC)
- Delayed intergranular cracking in the heat-affected zone or weld metal, caused by atomic hydrogen absorbed during welding, susceptible martensite microstructure, and residual...
- Laminar Tearing
- Step-like fracture in the base plate beneath a restrained weld, propagating along planes of MnS inclusion stringers aligned with the rolling direction...
- Phased-Array Ultrasonic Testing (PAUT)
- Advanced ultrasonic inspection using a multi-element probe whose beam angle and focal depth are electronically controlled to scan a weld volume systematically....
- Porosity
- Gas-filled voids in weld metal or castings, spherical or elongated (wormhole/piping), caused by dissolved gas evolving during solidification. Reduces cross-section and acts...