ISO 6520-1 group 4 · Lack of fusion and penetration
ISO 6520-1 reference 401 is a planar imperfection that visual inspection cannot see. Cause matrix, why ultrasonic testing leads the detection routes, and the standards that govern acceptance.
Lack of fusion is the imperfection that most rewards process evidence, for one blunt reason: you cannot see it. It is planar, usually buried, and it sits under a bead that can look textbook. Every route that finds it after the fact is expensive — ultrasonic or phased array inspection, or a destructive check — and by then the part is finished.
The cause matrix collapses into a single physical statement: the sidewall or the previous run never reached fusion temperature at the moment metal arrived. Arc energy too low, travel too fast, the arc aimed in the wrong place, or a film of slag in the way. All four are conditions that exist while the weld is being made, and three of the four are measurable there.
That is the argument for recording arc energy per pass and watching the pool: not that monitoring detects lack of fusion, but that it detects the conditions that produce it, on the run that produced them, when a stop and a grind still cost minutes instead of a rejected assembly.
| Cause | Mechanism | Control |
|---|---|---|
| Arc energy below the qualified window | Insufficient energy per unit length fails to melt the sidewall or the previous run before the pool solidifies against it. | Compute arc energy from measured current, voltage and travel speed per pass instead of nominal figures. |
| Travel speed too high | The pool runs ahead of the melting front and lays metal over unfused material, leaving a bond that looks complete from above. | Measure travel speed rather than estimating it; it is the term that carries most of the error. |
| Torch angle, weave or gun position wrong for the joint | The arc is aimed at the pool instead of the leading edge, so the sidewall never reaches fusion temperature. | Qualify the technique and verify it visually in the cell, not only on the procedure sheet. |
| Slag or oxide left between runs | A residue film prevents metallurgical bonding even when the thermal conditions are correct. | Enforce interpass cleaning as a hold point and record that it happened. |
| Joint preparation too narrow or access too tight | The arc cannot reach the root or the sidewall of a deep, narrow groove at any parameter setting. | Treat repeated occurrences as a design or fit-up problem, not an operator problem. |
The column that matters most is the last one. An inspection family marked as not finding this imperfection will not find it however carefully it is applied, and planning around that is cheaper than discovering it during a final examination.
| Family | Method | Capability | What it sees |
|---|---|---|---|
| Visual | Visual testing to ISO 17637 | Does not find it | The imperfection is buried and planar. A visually perfect bead is entirely compatible with lack of fusion underneath it. |
| Thermal | In-process infrared thermography | Supporting signal | Sees the conditions that produce it — low arc energy, high travel speed, an abnormal cooling signature — rather than the imperfection itself. |
| Camera | High-speed weld pool imaging | Supporting signal | Shows sidewall wetting and the position of the arc relative to the leading edge while the run is being made. |
| NDT | Ultrasonic and phased array testing to ISO 17640 and ISO 13588 | Primary route | The leading route for planar imperfections. Radiography can miss one entirely when the plane is unfavourable to the beam. |
Therness pages never reproduce a standard's acceptance values: those tables are copyrighted and belong in the copy you hold. What follows is the pointer — which document governs, and which part of it to open.
| Standard | Clause topic | Note |
|---|---|---|
| ISO 5817 | Quality levels for planar imperfections, indexed by ISO 6520-1 reference number | Planar imperfections are treated far more strictly than volumetric ones at every level. |
| ISO 11666 | Ultrasonic acceptance levels for welds, paired with the quality level chosen | The detection standard and the quality-level standard are selected together, not independently. |
| AWS D1.1 | Structural welding acceptance, with separate criteria for statically and cyclically loaded work | Which criteria apply depends on the loading, so establish that before inspecting. |
| ASME BPVC Section VIII | Pressure vessel acceptance, with examination methods in Section V | Read the construction code, not the qualification code, for the limit. |
Most weld imperfections are produced by conditions that exist while the arc is running. Therness records those conditions on every weld, so a suspect run is known on the shift that made it rather than in a batch report.
Progetto cofinanziato nell'ambito del PR Piemonte FSE+ 2021-2027,
Priorità I, Obiettivo Specifico a), Azione 4 – "Sostegno alla nascita delle start up"
