Porosity in castings and die castings: when industrial CT becomes necessary

The pore that scraps a die casting is rarely the one final inspection finds. It's the one the milling cutter finds, when machining a sealing face opens a cavity that sat half a millimeter below the surface. By then the part has absorbed its full cost in metal, casting cycle and machining time. A void becomes critical only in relation to what surrounds it, meaning the stock that will be machined away, the pressurized channel that has to hold, the section that carries fatigue load. Judging it takes knowing where it sits and how large it really is. That is the information a radiographic projection doesn't contain, and the information industrial computed tomography delivers.

One pore is not like another

None of the fates of a porous casting depends on total void volume alone. The most frequent is machining: a pore that is harmless in the raw part becomes a crater on a sealing face once the cutter removes the stock, and the discovery often happens on the assembly line or at the customer. Then there is leak-tightness, and in castings for hydraulic and cooling circuits what matters is interconnected porosity, the path a fluid can follow through the wall, far more than any isolated cavity. The third fate is fatigue.

On fatigue, the published evidence is unambiguous. Studying cast Al-Si alloys, Gao, Yi, Lee and Lindley (Fatigue & Fracture of Engineering Materials & Structures, 2004) showed that crack initiation depends on pore size and on how close the pore sits to the surface. Serrano-Munoz and co-authors (Scientific Reports, 2017) followed crack growth with synchrotron tomography and put a number on position. Surface and near-surface defects dominate initiation even when they are up to ten times smaller than internal ones. On the shop floor, a 300 µm pore in the core of a thick section may mean nothing, while a 100 µm pore just under a machined surface can set the life of the component. Judging porosity takes two coordinates, size and position, and the most widely used inspection method carries only one of them.

Where the projection stops

A radiograph is a shadow: everything the beam crosses through the wall collapses onto a single image. For detection that works remarkably well, and the reference standards have turned it into a mature contractual language. ASTM E505-22 provides reference radiographs for aluminum and magnesium die castings, organized in series of four increasing severity levels covering gas porosity, shrinkage porosity, cold shuts, inclusions and oxide films, and acceptance criteria are specified in terms of those radiographs. ASTM E2422 is its digital evolution for aluminum castings, with thirteen reference images across eight severity grades, distributed as 16-bit TIFF or DICONDE files. There are two thickness classes, 6.35 mm for walls up to 12.7 mm and 19.1 mm for walls from 12.7 to 51 mm. How radiographic testing of castings is set up and read is covered in our guide to radiography of welds and castings.

The limit lies in the kind of verdict a projection can return. These references grade the apparent severity of the image, and two castings with the same radiographic grade can hide very different porosity distributions, because indications at different depths overlap in the same shadow. A pore just beneath a machined surface looks identical to the same pore at mid-wall. As long as the criterion concerns the overall amount of defect, industrial radiography remains the fastest and cheapest screening there is. Once the drawing ties acceptance to position, the projection is missing the third coordinate.

The porosity limit is written by whoever signs the drawing

From hundreds of projections acquired around the part, CT reconstructs a volume in which every pore exists as a measurable object: a position in three coordinates, a shape, a volume, a distance to the nearest functional surface. What the volume does not contain is the verdict. Anyone looking for a standard that says how much porosity is allowed in a casting examined by CT will not find one, and it is worth knowing that before writing a specification. The ISO 15708 series, reissued in full between October 2024 and June 2025, is a technique standard. Part 2 covers principles, equipment and samples, Part 3 operation and interpretation, Part 4 qualification of the system against the inspection task. None of them sets acceptance levels. Part 2 even says something a purchasing department does not expect. It calls CT an indirect test procedure and requires absolute measurements, the size of an inhomogeneity for example, to be based on comparison with other measurement methods.

The limit lives elsewhere, and it has an author. The German guideline BDG P 203, December 2019 edition, valid for aluminum, magnesium and zinc castings, provides the grammar for writing the requirement on the drawing, the porosity key. Its companion sheet P 202 classifies porosity on section images instead. P 203 does not provide the values. The volumes over which porosity is measured have to be agreed between supplier and customer, oriented to local loads and shown on the drawing. The guideline itself shows how much that choice weighs, with an aluminum specimen: 0.34% porosity referred to the whole part, 0.38% in a hand-drawn region, 7.59% in the reference cube placed where the voids cluster. Same specimen, same scan, and a factor of more than twenty between the first number and the third.

From the drawing the criterion moves into the software, where it becomes three fields someone has to fill in. One is the reference volume, just discussed. Another is the size below which a pore is not counted, the letter U in the key. It is a customer requirement, and where it is missing, counting stops wherever the scan's resolution stops, which is a different point in every lab. The last is the algorithm that separates void from metal. The guideline lists several: the global threshold halfway between the air peak and the material peak, known as iso-50%, the Otsu and Kittler-Illingworth methods, and local thresholds that adapt to their surroundings. It also documents a case where the most common one fails, a stepped cylinder full of gas pores in which beam hardening brightens the upper steps and the iso-50% threshold cannot segment them. Whether segmentation worked is judged by eye by an experienced examiner, with a metallographic section as the tie-breaker.

Disputes start in that third field, for a documentary reason. The guideline's minimum test report contains the resolution achieved on the part, the contrast and an image of the same section with and without segmentation. Software name, version and algorithm sit in the internal documentation, which the lab hands over only if agreed. How two labs end up with two different percentages on the same casting, and which lines to add to the order to prevent it, is the subject of the article CT porosity: same casting, two different numbers.

For a foundry the most tangible consequence remains virtual machining. Overlaying the CAD model on the reconstructed volume simulates stock removal and shows in advance which pores will surface on the finished faces, before a machining center is occupied. Dimensional measurement is a different job with a different reference, the VDI/VDE 2630 Part 2.1:2015-06 guideline on task-specific uncertainty, and it is covered in the article on CT metrology of internal geometry.

How small can you see: the voxel rule, with numbers

Before writing "no pores above 100 µm" on a drawing, there is a calculation many specifications skip. The useful resolution of a scan is measured in voxels, the three-dimensional pixel of the reconstruction, and there are two thresholds to know. BDG P 203 sets the theoretical minimum: under ideal conditions a detail is recognizable at two to three voxels per side. Reliable detection takes more than that. Yosifov and co-authors (NDT&E International 162, 2026) scanned aluminum-alloy samples at 10, 20, 35 and 70 µm, using the finest scan as ground truth. The equivalent diameter detected with 90% probability at 95% confidence came to 100 µm at a 20 µm voxel size and 235 µm at 35 µm. That is five voxels in the first case, almost seven in the second. The same study shows that at lower resolution partial-volume effects inflate defect size and distort shape, with mean sphericity rising from about 0.80 to 0.87.

That gives the rule we use across all our CT articles: the maximum voxel size to write into the specification is the smallest pore you want to guarantee divided by five, or by seven if the voxel is coarse. A worked example shows what that means. A housing 60 mm across, framed in full on a 2,000-pixel detector, yields voxels of about 30 µm, so reliable detection starts at pores of 150 to 200 µm. With that scan a 100 µm requirement cannot be guaranteed. Getting there takes voxels between 15 and 20 µm, which means a field of view of 30 to 40 mm. Either the critical region is scanned on its own at higher magnification, at the price of longer cycle times, or the threshold gets renegotiated. Run this calculation while the criterion is being written, with the part geometry in front of you. Running it later, over a disputed report, costs far more.

Radiography and CT in the same specification, without mixing them

Moving to CT does not abolish the radiographic references, which remain the right criterion when the inspection is radiographic. It does require keeping the two languages apart. Specifying "porosity per ASTM E505" and then verifying by CT produces disputes, because severity in projection and segmented volume measure different things. A well-written CT criterion puts on the drawing, next to the P 203 key with zones and thresholds, the maximum scan voxel size derived with the rule above. For segmentation method, calculation rules and report contents it refers to the purchase specification. When the system's qualification for the task has to be demonstrated, ISO 15708-4:2025 is the reference.

In most foundries the two techniques live side by side. Digital radiography or industrial radioscopy controls production, while CT serves as the reference during die commissioning, qualification and disputed lots, and calibrates the radiographic judgment on the zones that matter. Whether scans should stay with an outside service or move to an in-house system depends on volumes and turnaround, and the calculation is in the article on industrial CT: buying a system or using a service.

The first step is a scan of a real casting

Any operation that produces or buys castings with machined faces, sealing zones or fatigue-loaded sections, and today judges them in projection only, has a simple way to learn what it is missing. Have a casting scanned that was rejected downstream, in machining or in a leak test, and compare the 3D porosity map with what the current inspection had seen. That is how DIPITECH sets up the assessment, through the contract CT scanning service. The service reviews the drawing and the contractual criteria in force and runs a demonstration scan on one of your castings, with a porosity report by functional zone and stated measurement conditions. When the radiography you already have is enough, the answer says so plainly.

What to send us for an assessment

To tell you which voxel size you need and how to set up the porosity analysis, we need a few precise details:

• drawing of the casting with its functional zones (sealing, machined surfaces, fatigue-loaded sections);
• alloy, process (high-pressure die casting, permanent mold, sand) and wall thicknesses;
• current porosity requirement, even if only as a radiographic reference (for example an ASTM E505 grade);
• smallest pore to guarantee in each zone;
• volumes, inspection frequency and the turnaround you need;
• any castings already rejected in machining or leak testing, to use as samples.

You can send them by email to info@dipitech.com or on WhatsApp, with drawings or photos attached.

A pore is judged by where it sits relative to what must hold, before it is judged by its size, and in a projection position does not exist. If your drawing ties acceptance to a zone, the next step is a scan at the right voxel size for that zone. For the fundamentals of the technique, from scan parameters to artifacts, see the industrial CT scanning guide.