What's inside
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Lack of fusion is a welding defect in which the weld metal does not properly bond to the base metal, the previous weld pass, or both. The bead may look full and reasonably smooth while a narrow unfused line remains below the surface. That makes it one of the more difficult defects to identify by eye.
A visual inspection can flag suspicious welds, but it cannot prove that fusion exists through the full joint. If the weld is load-bearing, safety-critical, or governed by a welding code, visual inspection should be followed by the required non-destructive test, such as ultrasonic, radiographic, magnetic-particle, or dye-penetrant inspection.
What Lack of Fusion Looks Like
Look for a continuous or intermittent line at the weld toe, along the centerline between passes, or at the root of the joint. It may appear as a dark slit, sharp groove, or unusually straight boundary that does not blend into the surrounding metal. Unlike a normal weld ripple, the line tends to follow the joint geometry rather than the movement pattern of the torch or electrode.
Other visual clues include:
- A weld toe that sits on top of the base metal instead of flowing smoothly into it.
- Cold-looking, convex weld metal with little evidence that the edges melted.
- Visible sidewall lines between a multi-pass weld and the previous pass.
- Overlap, where molten metal rolls onto unmelted parent metal.
- A root bead that appears narrow, irregular, or disconnected from the joint edges.
- Sudden changes in bead width or profile that correspond to pauses, starts, or poor torch positioning.
Do not confuse a visible weld line with a defect automatically. A clean, consistent line can be a normal boundary between passes. The concern increases when it is sharp, open, undercut, associated with overlap, or accompanied by a raised bead that shows little wetting.
Where to Inspect First
Clean the weld before judging it. Remove slag from stick and flux-cored welds, brush away spatter, and degrease TIG or MIG welds if oil or smoke residue obscures the toes. Inspect under bright, angled light rather than relying on a dim shop fixture. An inexpensive LED inspection light can make shallow grooves and toe lines easier to see.
Use a viewing angle of roughly 30 to 45 degrees to the weld surface. Look from both sides and inspect the start and stop areas closely. Lack of fusion often occurs where the welder hesitated, restarted, changed direction, or failed to reach the sidewall.
For a fillet weld, examine both toes and the root. For a butt weld, inspect the edges, crown, and any accessible root surface. On a multi-pass joint, each exposed pass boundary deserves attention before the next pass is deposited. Once buried, a lack-of-fusion defect becomes much harder to evaluate visually.
Signs by Welding Process
| Process | Common visual warning | Likely contributing cause |
|---|---|---|
| MIG or flux-cored | Convex bead, poor toe wetting, overlap, or a straight dark line at the sidewall | Travel too fast, low heat input, excessive wire speed, incorrect gun angle, or inadequate cleaning |
| TIG | Narrow bead sitting on the joint, unmelted edge, or visible boundary between passes | Low amperage, excessive travel speed, a large tungsten-to-work distance, or filler added before the puddle reaches both sides |
| Stick | Slag trapped along the toe or between passes, sharp bead edges, or poor tie-in at starts and stops | Incorrect electrode angle, low current, long arc length, inadequate slag removal, or poor work position |
These clues are not substitutes for procedure qualification. A bead can have an attractive profile and still contain internal lack of fusion, especially with low heat input or poor joint access.
Check the Weld Profile and Dimensions
Use a weld inspection gauge to check fillet size, reinforcement, undercut, and uneven legs. A gauge will not detect hidden fusion failure, but it can identify a profile that makes lack of fusion more likely.
Pay attention to the relationship between bead size and joint edges. In a fillet weld, the bead should flow into both members rather than form a rounded strip on one member. Excessively convex welds, abrupt toes, and overlap are warning signs. In a butt weld, excessive reinforcement does not prove a sound root; it can simply mean that metal was deposited without sufficient penetration or sidewall melting.
Compare the actual weld with the drawing or welding procedure. For example, a specified 6 mm fillet weld is not automatically acceptable because it measures 6 mm at one point. Uneven legs, undercut, and poor toe transition can still indicate a defective joint.
Distinguish It From Other Defects
Lack of fusion is often confused with cracks, undercut, porosity, and slag inclusions. A crack is usually sharper and may branch or continue beyond the weld toe. Undercut is a groove melted into the base metal beside the weld; it can contribute to failure but is not itself lack of fusion. Porosity appears as rounded holes or surface pinholes. Slag inclusions are generally hidden, although dark pockets may be visible between stick or flux-cored passes.
A straight, continuous interface is more suspicious than scattered porosity. However, visual appearance alone cannot reliably separate a tight crack from an unfused boundary. Do not grind aggressively to “see if it opens.” That can remove evidence, reduce the joint cross-section, and make a later inspection less useful.
What to Do When You Find a Suspect Area
Mark the location, photograph it with a scale, and record the weld position, process, electrode or wire, and visible dimensions. Do not simply cover the area with another pass. Additional metal may hide the defect without correcting it.
If the joint is noncritical and you are permitted to repair it, follow the applicable procedure: remove defective metal to sound material, clean the groove, and re-weld using the specified preheat, current, travel speed, and interpass limits. A small angle grinder with metal grinding discs is suitable for many repair preparations, but do not use grinding as a substitute for inspection. For code work, the repair method and acceptance criteria should come from a qualified welding inspector or engineer.
Improve Inspection Accuracy
Use clean safety glasses or a properly fitted auto-darkening welding helmet with a cheater lens for close visual work. Magnification helps with fine toe lines, but it does not reveal defects beneath an intact surface. A helmet with a clear grind mode is useful when inspecting and cleaning, while a basic fixed-shade helmet is adequate for occasional fabrication if it fits well and meets the required safety standard.
Finally, treat visual inspection as a screening step. If the weld supports a vehicle component, pressure vessel, lifting point, structural member, or anything where failure can injure someone, arrange the appropriate qualified inspection instead of accepting a good-looking bead as proof of fusion.