Guide
Why does die casting porosity show up after machining?
In a high-pressure die casting, die casting porosity is the network of small voids, gas pockets and shrinkage cavities, left inside the metal as it freezes in the die. Most of it sits below a dense outer skin, so a casting can look sound as cast and then show pinholes, open pores or a leak path the moment a cutter faces a sealing surface or drills a port through that skin.
By Saola Manufacturing engineering team · Published · Updated
This guide is for engineers and sourcing managers who buy die-cast and then machined parts such as valve bodies, housings, manifolds and covers with O-ring grooves. It covers why machining exposes hidden porosity, how to tell the two main kinds apart, and what to write on the drawing so porosity is judged against an agreed rule.
The NADCA Product Specification Standards and the ASTM reference radiographs are paid documents; this page uses only what their publishers make public and does not reproduce their acceptance tables.
On this page
Why does machining expose porosity that the casting did not show?
Because the die chills the outside of the casting first, forming a dense skin and pushing the voids toward the middle of each wall, and machining removes that skin. The cutter then opens pores that were sealed inside the part.
The North American Die Casting Association describes this as the skin effect. Metal against the die steel solidifies fastest, with a fine grain and higher strength than the metal underneath, so the voids end up concentrated in the core of each wall rather than near the surface. The dense layer is only a few hundredths of an inch thick; NADCA puts it at roughly 0.38 to 0.50 mm (0.015–0.020 in) (NADCA, structural criteria), citing a NADCA transaction paper on the skin of ADC12 die castings by Borland and Tsumagari.
Two consequences follow for anyone machining a casting. First, an as-cast surface that passes visual inspection says little about the interior; the skin hides what is below it. Second, the deeper a cut goes into a wall, the closer it gets to the zone where voids collect. A face that is skimmed lightly usually stays in dense metal, while a face that is milled heavily, a bore that is opened up, or a port drilled into solid material cuts straight into the core of the section.
Porosity also collects where metal freezes last: thick bosses, rib-to-wall junctions and heavy pads left for machining, which are often the places chosen for a sealing face or a threaded port.
Is it gas porosity or shrinkage porosity?
Gas porosity looks like round, smooth-walled bubbles, while shrinkage porosity looks like ragged, branching cavities with a crystalline surface. The shape tells you whether the cause is mostly in the process or mostly in the part geometry.
Gas porosity comes from air in the die cavity and shot sleeve that was not vented out before the metal sealed it in, and from vapor given off by die lubricant. Because the metal is injected at high speed, the gas is trapped as rounded pockets, often in clusters. Its fixes are mainly on the caster’s side: venting, overflows, shot profile and lubricant practice, and, for demanding parts, vacuum assistance.
Shrinkage porosity forms because metal contracts as it solidifies. When a thick section freezes after the gate around it has already frozen, no liquid metal can flow in to make up the lost volume, and the section pulls apart internally. The result is irregular and often interconnected, which is why it is the kind most likely to create a leak path between two machined surfaces. Its fixes are mainly on the designer’s side: remove mass, even out walls and move hot spots away from critical faces.
On a machined face the two can look alike; a polished cross-section under a low-power microscope usually settles it, and a radiograph shows where the voids concentrate.
Which design choices keep porosity away from machined faces?
Keep walls as uniform as the function allows, core out heavy sections, and leave as little machining stock as possible on faces that must seal. Each of these keeps the machined surface in the dense skin instead of the porous core.
NADCA’s design guidance says wall thickness should be as consistent as possible, with gradual transitions where it has to change (NADCA, uniform walls). For a cast-and-machined part, the practical reading is to look at every face and hole that will be machined and ask whether it sits on a thick pad or a mass junction. If it does, try to hollow the pad from the back, add a core, or move the feature to a thinner wall.
Machining stock is the second lever: every extra amount left on a sealing face takes the finished surface further from the skin. Hold the cast dimension close enough that the pass only cleans up the face, and flag any critical face that needs a heavier cut, because it is the most likely to open porosity.
Holes follow the same logic. A hole cast close to size by a core pin keeps a skin on its wall, and a light finishing pass or a tap then works in dense metal. A hole drilled into a solid boss goes straight through the last metal to freeze. NADCA notes that cored holes need more draft than outside walls because the casting shrinks onto the pin (NADCA, draft), so a cored hole that has to end up straight usually still gets a light reaming or boring pass. The tolerance guide covers how to dimension those as-cast and machined features on the drawing.
What should the drawing say about porosity?
Divide the part into zones and give each zone its own porosity requirement, with the strictest one only on the surfaces that seal, carry a thread or take a finish. A blanket “no porosity” note cannot be met by a conventional die casting and will simply be ignored or argued about.
A workable zone callout has three parts. The map: letter the areas on the drawing, for example the O-ring groove and sealing face as the tightest zone, threaded ports next, the rest as a general zone. The criterion per zone after machining: the largest pore accepted, how many per area, and how close one may sit to an edge or sealing land, values only you can set from the part’s function. The method: visual inspection at a stated magnification, a sectioned sample, a radiograph or a leak test.
For internal porosity the recognized language is the ASTM reference radiograph sets. ASTM E505 covers aluminum and magnesium die castings, with graded discontinuity categories shown at four levels of increasing severity, and it states that use outside its illustrated cases depends on agreement between purchaser and manufacturer (ASTM E505-22). In practice that means the drawing names the standard, the category and the severity level accepted in each zone. The NADCA Product Specification Standards include a quality assurance section that die casters use when agreeing those provisions (NADCA, Product Specification Standards).
| Method | What it answers | Reference | What the scope says |
|---|---|---|---|
| Film radiography | How much internal porosity, and where | ASTM E505-22 | Reference radiographs for aluminum and magnesium die castings; graded categories at four severity levels; other uses by purchaser and manufacturer agreement |
| Digital radiography | Same question, with digital X-ray | ASTM E2973-22 | Digital reference images for aluminum and magnesium die castings; requires a prior purchaser and supplier agreement on attributes and acceptance criteria |
| Radiography of other castings | Porosity in aluminum castings that are not die castings | ASTM E155-20 (reapproved 2026) | Reference radiographs for aluminum and magnesium castings in general, not die castings specifically |
| Hydrostatic leak test | Does a pressurized part leak through the wall | ASTM E1003-13 (reapproved 2022) | Components pressurized inside with a liquid; procedures limited to leakage indications of 1 × 10−4 std cm3/s or larger |
| Choosing a leak method | Which leak test fits the part and the sensitivity needed | ASTM E432-91 | Guide to selecting a leak testing method, with methods ordered by increasing sensitivity |
Read the table as a menu, not a specification. None of these documents tells you what is acceptable on your part. They give you a shared vocabulary and a test method, and the acceptance level comes from your drawing and purchase order.
When should a cast-and-machined part be leak tested?
Leak test after machining, because machining is what opens the paths that leak. A casting that holds pressure as cast can fail once its sealing face and ports have been cut.
Match the test to the medium and leak rate that matter in service. ASTM E432 exists to help choose among leak methods in order of sensitivity (ASTM E432-91, reapproved 2022), and ASTM E1003 describes the hydrostatic method in which the part is filled with liquid under pressure (ASTM E1003-13 (reapproved 2022)). Air-decay and immersion bubble tests are common for housings; tracer-gas methods serve very small allowed leaks. Put the test pressure, medium and pass criterion in writing.
For a part whose job is to hold pressure, testing every part after machining is the normal expectation, and NADCA’s cost guidance assumes pressure testing all castings before deciding which to seal (NADCA, secondary processing).
Can impregnation fix porosity found after machining?
Impregnation can seal fine, interconnected porosity so that a part stops leaking, but it does not restore strength or fill an open pore on a cosmetic or sealing surface. Treat it as a way to recover leak-test rejects, not as a cure for a bad casting.
NADCA describes impregnation as forcing an organic sealant into the surface pores of a casting, and notes that cost can be kept down by pressure testing every casting and impregnating only those that fail (NADCA, secondary processing). The sealant fills the connected micro-voids that form the leak path; it cannot make a large shrinkage cavity load-bearing, and it will not make a visible pore on an O-ring land acceptable if your zone criteria reject it.
Three drawing decisions go with it. State whether impregnation is permitted at all, since some applications, such as parts in contact with certain fluids, may rule it out. If it is permitted, state the sealant standard or type you accept. And state the sequence: impregnation normally follows machining, because machining opens the pores the sealant has to reach, and it comes before any plating, powder coat or other finish. The finishing guide explains which finishes suit die castings.
Does the choice of alloy change porosity risk?
It changes pressure tightness at the margin, but geometry and process control matter more than the alloy. NADCA rates alloy 413 best for pressure tightness and places 380 and 383 in the next group.
NADCA’s comparison of aluminum die-casting alloys, drawn from ASTM B85, ASM and SAE data, rates each alloy from 1 (most desirable) to 5 on pressure tightness and on machining (NADCA, Table 4.3). The table below reproduces those two rows. The same page notes that as silicon content approaches the eutectic, around 11.7 percent, fluidity rises, solidification shrinkage falls and pressure tightness improves.
| Alloy | Pressure tightness | Machining |
|---|---|---|
| 360 | 2 | 3 |
| 380 | 2 | 3 |
| 383 | 2 | 2 |
| 384 | 2 | 3 |
| 390 | 3 | 5 |
| 413 | 1 | 4 |
| 443 | 3 | 5 |
| 518 | 5 | 1 |
ADC12, the JIS grade often quoted by Asian die casters, sits in the same aluminum-silicon-copper family and leans toward 383 in chemistry; the ADC12 vs A380 guide sets out the registered limits. For zinc, NADCA notes that precision features such as zero draft can often be cast rather than machined (NADCA, alloy selection), and every face that does not need machining is a face that keeps its skin.
What should you agree with an offshore supplier before tooling?
Agree the porosity zones, the inspection method, the leak test and the impregnation policy in writing before the die is built, because each of them can change the die design. Finding out after a shipment has crossed an ocean is the most expensive way to settle the question.
When parts come from Vietnam or anywhere far from your plant, the checks are the same but a late surprise costs more. Ask for a design-for-manufacturing review that marks likely hot spots against your machined faces, a first-article sample sectioned through the critical features, and a control plan that states whether leak testing is after machining and on every part.
It also helps when one party manages both casting and machining, because a pore that shows up at the machine can be traced back to a die, a cavity and a shot while the cause is still fresh. Saola Manufacturing sources die casting in ADC12 aluminum and Zamak 3 zinc from partner foundries and runs CNC milling and turning at its lead CNC plant; the quote names the production site and origin; the quality page covers inspection, and ISO 9001 certification is in progress, not yet held. The RFQ preparation guide lists what to include in the package.
Frequently asked
Does porosity mean the die casting is defective?
Not by itself. Conventional high-pressure die castings contain some porosity, and a part is defective only when the porosity breaks a rule on the drawing, such as a pore on a sealing face or a leak above the allowed rate.
Can I take a deeper cut to get below the porosity?
Usually not. The skin is the dense layer and the voids collect toward the middle of the wall, so a deeper cut tends to open more porosity, not less.
Should the leak test come before or after impregnation?
Both. Test after machining to find the leakers, impregnate only the parts that fail if the drawing allows it, then test those parts again before they are accepted.
Are Zamak 3 zinc castings less prone to porosity after machining?
Zinc can often hold precision features as cast, so fewer faces need machining and more of the skin stays intact. The same rules still apply to any face that is machined.
What should an RFQ for a pressure-tight casting include?
The drawing with porosity zones, the inspection method per zone, the leak test pressure, medium and pass criterion, and whether impregnation is allowed. Send it for a quote and flag the sealing faces.
Sources
- Structural Criteria: Mechanical Properties and the Skin Effect, NADCA Design, North American Die Casting Association, accessed September 2026 (citing Borland and Tsumagari, NADCA Transaction T06-031, 2006)
- Uniform Walls, NADCA Design, accessed September 2026
- Draft, NADCA Design, accessed September 2026
- Secondary Processing Costs, NADCA Design, accessed September 2026
- Aluminum Alloys, including Table 4.3, NADCA Design, accessed September 2026
- Alloy Selection, NADCA Design, accessed September 2026
- Product Specifications Standards for Die Castings (table of contents), NADCA Design, accessed September 2026
- ASTM E505-22, Standard Reference Radiographs for Inspection of Aluminum and Magnesium Die Castings (scope), ASTM International, 2022
- ASTM E2973-22, Digital Reference Images for Inspection of Aluminum and Magnesium Die Castings (scope), ASTM International, 2022
- ASTM E155-20 (reapproved 2026), Reference Radiographs for Inspection of Aluminum and Magnesium Castings (scope), ASTM International, 2020, reapproved 2026
- ASTM E1003-13(2022), Standard Practice for Hydrostatic Leak Testing (scope), ASTM International, 2013, reapproved 2022
- ASTM E432-91, Standard Guide for Selection of a Leak Testing Method (scope), ASTM International, reapproved 2022