X Ray Inspection Internal Defects Casting: Symptom, Cause, Fix
A troubleshooting guide for engineers who need to know why a casting failed radiography and what to change. Written for buyers and process engineers working with sand, investment, and die castings. After reading, you can match a defect image to a process cause and decide whether to re-cast, re-machine, or accept under a written deviation.
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X ray inspection internal defects casting: symptom to cause to fix
Use this table to route a radiograph finding to the right process correction. Read the image first, then the cause, then the action.
| Symptom on the film | Likely cause | How to handle it |
|---|---|---|
| Round dark spots, scattered | Gas porosity from turbulent fill | Lower pour temperature 20-30 °C, slow the fill |
| Ragged dark areas at thick section | Shrinkage from feeding starvation | Add riser, raise pouring temperature |
| Sharp-edged holes near the gate | Cold shut or incomplete fill | Widen gate, raise metal temperature |
| Bright dense specks, heavier than base | Sand or slag inclusion | Improve filter, check mould wash |
| Hairline dark lines at corners | Hot tear from restrained contraction | Ease fillet radius, adjust knockout time |
| Worm-like dark channel | Gas from core binder or moisture | Bake cores, reduce binder, vent the core |
| Uniform mottled density | Microshrinkage in thick wall | Chill the section, refine grain |
| No defect, but density low | Wrong alloy or heavy segregation | Verify heat chemistry, check solidification |
| Cracks only after machining | Residual stress, not a casting flaw | Stress-relieve before finish cuts |
Fix the process, not the film
If the same defect keeps appearing in the same zone, the casting process is telling you something. Change one variable, re-scan five parts, and lock the fix into the process sheet. Send us a drawing and the failed radiograph and we will come back with a DFM note and a quote within 12 hours.
Why x ray inspection internal defects casting work starts with the image, not the part
A radiograph of a casting is a density map. Denser metal absorbs more radiation, so the film or detector reads bright where the wall is solid and dark where the wall is thin or missing. That is the whole principle. Every defect you will troubleshoot shows up as a variation in that density map, and the shape of the variation points to the process step that failed.
Shape matters more than size. Round, smooth-edged dark spots usually mean gas. Ragged, branching dark areas usually mean shrinkage. Sharp, angular dark spots with bright edges usually mean a foreign body like sand or slag. An engineer who skips the shape and only measures the size will chase the wrong correction half the time.
The same defect can look different depending on geometry. A gas pore in a 3 mm wall reads clearly; the same pore in a 25 mm wall may sit below the contrast threshold of a 2D DR scan. That is why wall thickness and section modulus belong in the inspection plan before the first shot, not after a failed batch.
One more limit to set early: radiography finds volumetric discontinuities. Tight planar cracks oriented parallel to the beam can pass undetected through a 2D scan. If the service environment is fatigue-critical, plan a CT scan or an angle-shot sequence rather than a single exposure.
- 1Dark and roundGas, typically from turbulence or moisture.
- 2Dark and raggedShrinkage, typically from feed starvation.
- 3Dark and angularInclusion, typically from mould or melt.
- 4Bright and linearHigher-density phase or segregation.
2D DR vs CT: which scan answers your question
Digital radiography (DR) projects the whole thickness onto one plane. It is fast, cheap per part, and good for a go or no-go call on porosity level. A typical DR setup for an aluminium casting runs 80-160 kV and resolves pores down to roughly 1-2% of wall thickness. For a 10 mm wall, that is about 0.1-0.2 mm.
Computed tomography (CT) reconstructs the volume. You get slice positions, defect depth, and true 3D size. CT costs more per part and takes longer, so it belongs on first-article qualification, on safety-critical parts, and on root-cause work when a 2D image is ambiguous. It does not belong on every part of a 10,000-piece run unless the drawing demands it.
Choose by the question you are asking. If the question is pass or fail against a porosity chart, DR is enough. If the question is where exactly the void sits relative to a machined bore, CT is the only honest answer. Mixing the two up wastes money at one end and misses defects at the other.
For most casting suppliers, the practical split is DR for production screening and CT for the first article plus any escalation. That keeps the per-part cost low and still gives you a full 3D record on the parts that set the process.
- 1DR strengthsFast, low cost per part, easy to automate.
- 2DR limitsDepth is lost, planar cracks can hide.
- 3CT strengthsTrue 3D position and size of every void.
- 4CT limitsHigher cost, longer cycle, size limits on the chamber.
Using acceptance standards instead of opinion
Two engineers can look at the same radiograph and disagree. The fix is a written acceptance standard referenced on the drawing before production. ASTM E505 covers aluminium and magnesium castings, ASTM E155 covers aluminium and magnesium too but with a different reference scale, and ASTM E446 covers steel castings. Each gives reference radiographs at severity levels so the call is a number, not a mood.
Severity levels run in steps, and the drawing should state which level is acceptable for which zone. A thick boss can often tolerate a higher level than a thin flange. Zone-based acceptance is normal in aerospace and medical work. If your drawing just says 'no porosity', you have created an argument, not a specification.
Radiographic technique matters as much as the standard. The beam angle, source-to-object distance, and film or detector class all change what you can see. A 2% sensitivity shot and a 4% sensitivity shot are not comparable. Ask the lab for the technique sheet before you compare two suppliers' results.
Document the technique in the inspection plan and keep it with the part record. When a defect appears six months into production, the first question is whether the scan changed or the casting changed. Without a technique sheet, you cannot answer that.
- 1ASTM E505Aluminium and magnesium, reference radiographs.
- 2ASTM E155Aluminium and magnesium, alternate scale.
- 3ASTM E446Steel castings up to 2 in thick.
- 4ZonesSet acceptance level per drawing zone, not globally.
When x ray inspection is the wrong tool
Radiography is not free and it is not always the best use of a budget. For a low-stress bracket in a non-critical assembly, visual and dimensional checks plus a dye penetrant test on suspect surfaces may be enough. Adding DR to that part raises unit cost without changing the risk profile.
Surface-breaking defects are better found by penetrant testing (PT) or magnetic particle (MT) on ferrous parts. Radiography looks through the wall; PT and MT look at the surface. If the failure mode is a surface crack, spend the money there.
Wall thickness above roughly 50 mm of steel or 150 mm of aluminium pushes the required energy up and the contrast down. At that point, 2D DR gets unreliable and CT, ultrasonic testing, or sectioning may be the better call. Sectioning destroys the part, but on a one-off suspect casting it can settle the question faster than any scan.
The honest rule: match the method to the failure mode and the acceptance standard. If neither the drawing nor the service condition justifies radiography, skip it and put the money into process control at the foundry instead.
- 1Use PT or MTFor surface-breaking cracks on any alloy.
- 2Use UTFor thick sections and planar flaws.
- 3Use sectioningOn a one-off to settle a root cause.
- 4Skip radiographyWhen the part has no structural role.
Step by step: from a failed radiograph to a corrected casting
Run these steps in sequence. Skipping a step is how a bad batch becomes a bad month.
- 1Confirm the technique sheet before you blame the castingPull the source-to-object distance, kV, current, and detector class. Re-shoot one known-good part from the last accepted lot. If the good part now fails, the scan changed, not the process. Re-qualify the technique before touching the foundry.
- 2Map the defect location onto the part drawingMark the defect position and depth on a drawing section. Group findings by zone: gate area, thick boss, thin web, or last-to-solidify region. This single step usually narrows the cause to one or two process variables.
- 3Classify by shape, not by size aloneRound and smooth: gas. Ragged and branching: shrinkage. Angular with bright edges: inclusion. Write the class on the report. If the class is unclear, cut one sample part along the defect plane and look at it under a low-power microscope.
- 4Change one process variable at a timeFor gas: drop the pour temperature by 20-30 °C and slow the fill rate. For shrinkage: add or enlarge the riser and raise the pour temperature slightly. For sand inclusions: check the mould wash and the gating filter. Change one thing, then re-shoot five parts.
- 5Re-scan five parts from the adjusted runFive parts give a quick read on whether the correction held. Use the same technique sheet as step one. If two or more still fail at the same zone, the change was too small or aimed at the wrong cause.
- 6Lock the corrected parameters into the process sheetWrite the new pour temperature, fill time, and riser geometry into the foundry process sheet with a revision number. Without this, the correction drifts back within a few weeks and the same defect returns on a later lot.
- 7Decide re-cast, rework, or accept under deviationRework only if the defect sits in a zone that will be machined away and the remaining wall still meets the drawing. Otherwise re-cast. If the part is serviceable at a lower load, write a deviation with the acceptance level and get it signed by the design owner.
- 8Add the defect to the inspection plan for the next runRecord the zone and class in the control plan so the next production run scans that zone first. This turns a one-time escape into a monitored characteristic.
Questions engineers ask after a failed scan
Can a casting pass x ray and still fail in service?
Yes. Radiography finds volumetric discontinuities above the technique's sensitivity. A tight planar crack oriented parallel to the beam can pass a 2D scan undetected.
If the service load is fatigue-critical, add a CT scan, an angle-shot sequence, or a surface method like penetrant testing. No single method covers every failure mode.
How small a pore can 2D DR actually see?
As a working figure, DR resolves pores down to roughly 1-2% of the local wall thickness. On a 10 mm aluminium wall, that is about 0.1-0.2 mm.
The real limit depends on kV, detector class, and geometry. A thick section hides small pores that a thin section reveals. Always state the sensitivity on the technique sheet.
Should we CT scan every part of a production run?
Usually no. CT is best on the first article, on safety-critical parts, and on root-cause work.
For a running lot, DR screening at the agreed acceptance level plus CT escalation on failures keeps cost sane and still catches process drift.
The foundry says the defect is normal for the alloy. Is that true?
Some microshrinkage is normal in most casting alloys. The question is whether the level on your drawing is met.
Ask for the reference standard and the severity level the part was judged against. 'Normal for the alloy' is not an acceptance criterion.
Can machining remove an internal defect?
Only if the defect sits inside the material that will be cut away and the remaining wall still meets the drawing after the cut.
Map the defect depth on a section before you decide. Cutting into a void and breaking through a wall is a worse outcome than re-casting.
What records should come with the parts?
Ask for the technique sheet, the reference standard and severity level, the radiographs or CT data, and a signed inspection report.
GreatLight keeps inspection reports on request and runs 100% inspection before shipment across raw material, in-process, and final checks.
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