A first casting that passes inspection has proved less than it appears to. It has shown that one casting, on one day, from one mould, met the requirements that were checked. It has not shown why it met them, whether it will meet them again, or what is happening inside the sections that were not examined. The only way to answer those questions is to cut the casting up.
Sectioning is the step where a casting stops being an object and becomes information. The cut faces show the internal structure directly: where the metal is sound, where shrinkage has concentrated, how the risers performed and whether the feeding path stayed open. That information cannot be obtained from the outside, and it cannot be inferred reliably from a radiograph alone.
The exercise is destructive by definition, which is why it is often skipped under schedule pressure. The saving is immediate and the cost is deferred, exactly the wrong trade on a programme that intends to run for years.
The first thing a section shows is the distribution of shrinkage. Shrinkage appears as a darker, more porous region at the centre of heavy sections and at junctions, and its position tells you whether the feeder reached the last region to freeze. If the shrinkage sits where the thermal map predicted, the model behind the feeding plan was right. If it sits somewhere else, the model was wrong and the plan needs revisiting.
The second thing a section shows is the effectiveness of the riser. A riser that fed properly leaves a sound region beneath its attachment and a clear transition into the fed section. A riser that froze early leaves a shrinkage cavity just below the neck, which is diagnostic and would never be visible from outside.
The third is the condition of the walls. Thin sections cut through show whether the metal filled completely, whether there are cold shuts, and whether the wall thickness is what the drawing intended. On a thin walled exhaust component this is often the most informative part of the exercise.
A sectioning plan is not a random selection of cuts. It targets the places where the analysis says defects are most likely, plus the places where a defect would be most damaging. Those are usually not the same place, and both are worth cutting.
The first category is the predicted last region to freeze, the heavy junctions and the riser necks. The second is the machined sealing faces, the pressure boundary and any feature that carries the fit of the component. A plan that covers both gives a picture of what the process is doing and what the consequences would be.
Photographing and recording each cut face before it corrodes or is handled is part of the exercise. A section that is cut and looked at without being recorded has produced an opinion rather than data, and opinions do not survive a change of personnel.
The value of sectioning is realised only if the findings change something. A programme that sections a casting, finds the shrinkage where it was predicted, and files the report has confirmed its model and can proceed. A programme that finds shrinkage somewhere unexpected and proceeds anyway has spent the cost of the exercise without taking the benefit.
Where the findings contradict the model, the correction is usually in the feeding plan or in the section design, and it is cheaper to make it now than after the pattern is in production. This is the point of the exercise: it buys information at the stage when information is still cheap to act on.
The cost of sectioning is one casting and a few hours of cutting and inspection. The cost of not sectioning is a defect that emerges in service, on a part that has already been through every downstream operation and possibly into a customer's hands. The comparison is not close.
Beyond the immediate technical findings, a sectioning report is a qualification record. It documents that the internal condition of the casting was examined and found acceptable at a defined point in the programme, against a defined plan. That record has value later, when a question arises about whether the process was ever validated or whether a change since then has affected the result.
The same logic applies to the welded alternative. A fabrication such as the work in the TIG aluminium welding range is qualified by procedure and by inspection of the joints, which is a different kind of evidence serving the same purpose. Both routes need a record that says the process was understood rather than merely observed to work once.
Cutting up the first casting is therefore not a curiosity or a tradition. It is the moment when a casting programme converts an assumption into knowledge. Everything after that point rests on whether the conversion was done.
Thin walls are best sectioned along the flow path rather than across it, because the question is whether the metal reached the end of the path rather than whether one point is sound. aluminium fuel tank range are cut this way when a wall is in question.
The cut has to be made where the structure can be read, with enough of the surrounding section left to show the context. A cut that removes the transition tells you less than one that includes it. aluminium water tank range and custom parts work is checked on that basis.
A section shows one plane, and a defect lying between planes will not appear in it. That is why sectioning complements radiography rather than replacing it, and why the TIG stainless steel welding and titanium fabricated components routes rely on a different kind of evidence.
