Visual inspection identifies the casting defects that are visible on accessible surfaces. Dimensional inspection verifies that the casting's geometry meets its drawing requirements. Hardness testing confirms that the alloy has responded correctly to heat treatment. Each of these inspection methods is valuable and necessary — but none of them can detect the porosity and inclusion defects that exist below the casting's surface, invisible to the eye, undetectable by any surface measurement, yet capable of causing the casting to fail structurally under service loads or leak under fluid pressure. Subsurface defect detection requires methods that interrogate the casting's internal volume — and the three principal methods available to the industrial casting inspection function are radiographic testing, computed tomography scanning, and density measurement.
Understanding what each method can and cannot detect, at what cost and at what throughput, is practical knowledge for foundries producing quality-critical castings and for the engineers and procurement professionals who specify inspection requirements for casting programmes.
Radiographic Testing — The Industrial Standard
Radiographic testing — X-ray inspection — is the most widely used method for detecting internal casting defects, and it has been the industrial standard for casting quality verification since the mid-twentieth century. The principle is straightforward: a beam of X-rays passes through the casting, and the differential absorption of the beam by regions of different density is recorded on a film or digital detector. Voids within the casting — gas porosity, shrinkage porosity, or inclusion defects that are less dense than the surrounding metal — absorb less radiation and appear as darker regions on the radiographic image. Denser inclusions absorb more radiation and appear lighter.
The practical capability of radiographic testing for aluminium casting inspection is well-established. Gas porosity — from dissolved hydrogen precipitating during solidification — appears as rounded dark spots or clusters whose size and distribution can be assessed against reference radiograph standards such as ASTM E505 for aluminium castings. Shrinkage porosity — from solidification shrinkage in inadequately fed sections — appears as irregular dark regions, sometimes with a feathery or spongy texture that distinguishes it from gas porosity. Inclusions — oxide films, sand particles, or other non-metallic contamination — appear as dark or light irregularities depending on their density relative to aluminium.
The limitations of radiographic testing are primarily geometric. A two-dimensional radiograph collapses the three-dimensional internal structure of the casting onto a flat image, and the interpretation of the image requires the inspector to infer the three-dimensional location and extent of defects from their two-dimensional projection. A defect that is oriented parallel to the X-ray beam — a flat oxide film lying in the plane of the beam — may not produce a detectable density difference in the radiographic image, making it invisible to radiographic inspection despite being present and potentially damaging. Multiple radiographic exposures from different angles improve the detectability of orientation-sensitive defects but increase inspection time and cost proportionally.
The interpretation of radiographic images requires trained and certified inspectors — ASNT Level II certification is the standard qualification for radiographic testing in industrial casting inspection. Inspector qualification ensures that the defect indications visible in the radiographic image are correctly identified, correctly characterised as acceptable or rejectable against the applicable acceptance standard, and consistently assessed across different inspectors and different inspection sessions. Variability in radiographic interpretation — two inspectors assessing the same image differently — is a known quality risk in radiographic inspection programmes and is managed through inspector certification, regular proficiency testing, and clear written acceptance criteria that minimise the subjectivity in interpretation decisions.
Computed Tomography — Three-Dimensional Internal Mapping
Industrial computed tomography — CT scanning — is the advanced evolution of radiographic testing that addresses its fundamental two-dimensional limitation by reconstructing a full three-dimensional image of the casting's internal structure from a series of radiographic projections taken from multiple angles around the casting. The CT scanner rotates the casting through 360 degrees while recording thousands of individual X-ray projections, and specialised software reconstructs these projections into a three-dimensional volumetric model that can be examined from any angle, with any cross-section, at any depth within the casting.
The defect detection capability of CT scanning significantly exceeds that of conventional radiography for three reasons. The three-dimensional reconstruction eliminates the orientation-dependence problem that makes some defects invisible in conventional radiography — an oxide film that is invisible in one radiographic projection is detectable in the CT reconstruction regardless of its orientation. The volumetric data set allows defects to be precisely located in three-dimensional space within the casting — enabling assessment of how close a defect is to a critical surface, a bearing bore, or a pressure-containing wall section. And the quantitative analysis tools available for CT data — pore size distribution, total porosity percentage in a defined volume, largest defect size and location — enable statistical characterisation of casting quality that visual radiographic interpretation cannot provide.
The additional capability of CT scanning comes with additional cost and time requirements. A CT scan takes significantly longer than a conventional radiographic exposure — scan times range from minutes for simple small castings to tens of minutes for complex large ones — and the reconstruction and analysis software requires computing resources and trained operators. For production inspection of high-volume casting programmes, CT scanning is typically applied on a statistical sampling basis rather than to every casting — using it to characterise the internal quality distribution of the casting process and to verify that quality is maintained within defined bounds, rather than as a 100 percent inspection method for every casting produced.
CT scanning's most commercially significant capability for casting foundries is its use in process development — identifying where porosity occurs in a new casting geometry, verifying that feeding system changes have moved porosity to acceptable locations, and building the process understanding that eliminates internal defects before production begins. A foundry that uses CT scanning during die development — scanning trial shots to map the internal quality of the casting from the first production trial rather than waiting for customer complaints or hydraulic test failures to reveal internal defect patterns — compresses the process development cycle and reduces the number of tooling iterations required to achieve acceptable internal quality.
Density Measurement — Quantifying What You Cannot See
Density measurement — determining the mass per unit volume of a casting — provides a quantitative measure of the total void volume within the casting that correlates directly with the casting's overall porosity level. The principle exploits Archimedes' principle: a casting weighed in air and then weighed again while suspended in water will show a weight difference that precisely equals the volume of water displaced by the casting. The ratio of the casting's mass to its displaced water volume is its density, which can be compared to the theoretical density of the alloy at full density to calculate the percentage porosity.
Density measurement is faster and less expensive than radiographic or CT inspection — a digital balance and a water immersion fixture are the only equipment required, and the measurement of a casting takes less than two minutes. The limitation is that density measurement provides only a total porosity percentage for the entire casting — it cannot locate where the porosity is within the casting, distinguish between gas and shrinkage porosity types, or characterise the size distribution of individual pores. A casting with a total porosity of 2 percent may be acceptable if the porosity is distributed as small pores throughout a non-critical section, or unacceptable if the same total porosity is concentrated in a single large void adjacent to a critical bore or sealing face.
The Density Index — used specifically in melt quality assessment — applies the same density measurement principle to compare the density of samples solidified at atmospheric pressure and under vacuum, quantifying how much additional porosity the reduced-pressure solidification condition reveals relative to atmospheric conditions. This application of density measurement as a melt quality tool was covered in earlier content on melt quality management; its relevance in the context of casting inspection is that the same measurement principle applies at both the melt quality assessment stage and the finished casting inspection stage, with different applications but the same underlying physics.
The practical value of density measurement in production casting inspection is as a fast, economical screening tool that identifies castings whose total porosity exceeds an acceptable threshold for further investigation by radiography or CT. A production inspection protocol that density-measures every casting and radiographically inspects only those whose density measurement indicates elevated porosity achieves better coverage of the production output at lower cost than applying radiographic inspection to every casting — directing the more expensive inspection resource to the castings most likely to contain rejectable defects.
Choosing the Right Method — Or Combining Them
The choice between radiographic testing, CT scanning, and density measurement for a specific casting programme depends on the casting's functional requirements, the defect types that are critical to its application, the production volume, and the inspection budget available. For most production casting inspection programmes, the appropriate answer is a combination of methods rather than any single method applied in isolation.
A programme structure that uses density measurement as a 100 percent production screen, radiographic inspection on a statistical sample from each production batch, and CT scanning for process development and for investigation of borderline density measurement results provides comprehensive coverage of the casting's internal quality at a cost structure that is sustainable for most casting programmes. The density measurement identifies outlier castings for radiographic follow-up; the radiographic sample confirms that the process is producing castings within the acceptable internal quality range; and the CT scanning provides the detailed three-dimensional quality mapping that process development and complex defect investigation require.
The acceptance standards against which inspection results are evaluated — ASTM E505 reference radiographs for aluminium casting radiographic inspection, or customer-specific CT porosity limits — define what level of internal quality is acceptable for the specific application. Establishing these acceptance standards at the start of a new casting programme, before production begins, ensures that the inspection results can be interpreted consistently against defined criteria rather than subjectively assessed case by case — the foundation of an inspection programme that controls quality rather than simply documenting it.