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Incomplete fusion: what it is and how to spot it

Fillet Gauge editorial teamSources checked 4 min read

Incomplete fusion is weld metal that lies against the base metal, or against an earlier pass, without melting into it. The result is a thin, flat unbonded boundary inside the weld. It can sit on a groove face (sidewall), between passes (interpass), or at the toe, and it behaves much like a crack under load.

A multi-pass V-groove weld in cross-section with a thin gap along the left groove face and a second thin gap on a pass boundary, while the root is fully fused
Two homes for incomplete fusion: the sidewall and the boundary between passes. Original drawing, not to scale.
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How to recognize incomplete fusion

  1. Look for a line, not a pocket. Incomplete fusion is thin and even. It follows a surface that should have melted: the bevel face, the top of an earlier bead, or the plate at the toe.
  2. Name where it sits. Along the bevel is sidewall fusion. Between two beads is interpass fusion. At the edge of the cap, weld metal can roll onto the plate without bonding, which shades into overlap.
  3. Check the root. If the root is fully melted through, the flaw higher up is incomplete fusion, not incomplete joint penetration. That check sorts most mix-ups.
  4. On a macro etch, the weld metal and plate meet at a sharp, unmixed line instead of a blended fusion zone.

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Welding Inspection and Discontinuities

A convex fillet weld replica is checked with a leaf-type fillet gauge. The drawing calls for a single fillet size. The vertical leg fits the 3/8 in blade and the horizontal leg the 5/16 in blade. What size is recorded?

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How it is found

Because it is planar and usually buried, visual inspection only catches incomplete fusion where it reaches the surface, such as at a cap toe or on the face of an intermediate pass that the inspector looks at before the next one goes on. Ultrasonic testing with an angle beam is well suited to it, because the sound hits the flat face square on when the angle matches the bevel. Radiography can miss it unless the beam lines up with the plane of the flaw. During welding, an inspector looks for beads that sit on the sidewall with a sharp re-entrant edge instead of wetting into it.

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Common mix-ups

  • Incomplete fusion or incomplete joint penetration. Fusion is about bonding to a surface. Penetration is about reaching the depth the joint needs. A fully melted root rules out incomplete penetration.
  • Incomplete fusion or slag. Slag is a chunky pocket of solid. Incomplete fusion is a clean, thin line. The two often appear together, because slag left on a sidewall stops the next pass from fusing.
  • Incomplete fusion or a crack. Both are planar. A crack has jagged walls and pointed tips; incomplete fusion has the smooth shape of the surface it failed to melt.

Why it forms

The arc doesn't put enough heat where it is needed: current too low, travel too fast, or an electrode angle that aims the arc at the middle of the groove instead of the sidewall. A narrow groove, mill scale or slag on the surface, a large pool rolling ahead of the arc, and arc blow all make it worse. GMAW in short-circuit transfer on thick sections is a well-known risk.

Incomplete fusion on the CWI exam

On Part B, incomplete fusion sits in Welding Inspection and Discontinuities, at least 34% of the practical, and in NDE Application when you pick the method that finds it. On Part A, it fits Visual Examination, NDE Fundamentals and Welding Application and Control. On Part C, find how the code treats fusion in its visual and NDT acceptance criteria, D1.1 Clause 8 for the D1.1 endorsement.

Practice it

Spot sidewall fusion against its look-alikes in the weld discontinuities quiz. Then read the root-side partner, incomplete joint penetration, and the flaw that often causes it, slag inclusion. The weld discontinuities guide lists every type.

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Facts checked 25 October 2026 against the sources listed above. If something has changed, tell us and we will correct it within 2 business days.

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