Die casting porosity: reading the defect, choosing the test and setting acceptance

Three families, three signatures

Porosity in a die casting is not one defect. Gas porosity comes from air or hydrogen trapped in the metal. Shrink porosity comes from the volume change as the last liquid solidifies. A cold shut comes from two flow fronts meeting after the metal has lost too much heat to weld. Each has a different shape, a different location and a different fix.

The distinction matters because the tests and the acceptance criteria are different. A gas pore and a shrink cavity can both show as a dark spot on a radiograph, but one is cured by venting and the other by wall uniformity. A cold shut may not show as a void at all, because the metal is present but not bonded.

Read the indication before choosing the response. A supplier who treats every dark spot as gas will keep changing the shot profile while the real problem sits in a thick boss.

Gas porosity

Gas porosity forms when air is trapped during the fast fill or when hydrogen comes out of solution as the metal freezes. Air entrapment produces rounded pores, often clustered near the last point to fill, near a core or under a surface that was not vented. Hydrogen produces finer, more evenly distributed pores, sometimes called pinholing.

The shape is the clue. Gas pores are round or spherical because the gas bubble has a surface tension that pulls it into a ball. They are smooth inside, and they may sit just below the skin where the bubble migrated before the skin froze. After machining, they appear as small bright cavities with rounded edges.

The process fix depends on the source. Air entrapment needs better venting, a slower slow-shot, a different gate or a vacuum assist. Hydrogen needs a cleaner melt, a proper degas and a controlled holding temperature. Adding intensification pressure will not remove gas that is already sealed in the cavity; it only compresses it.

Shrink porosity

Shrink porosity forms in the last region to solidify. Aluminium contracts as it changes from liquid to solid, and if there is no liquid metal nearby to feed the contraction, a cavity opens. It is irregular, often elongated or spongy, and it follows the shape of the hot spot rather than the shape of a bubble.

The location is the tell. Shrink porosity sits in thick sections, at the centre of a heavy boss, at the junction of a rib and a wall, or in a section that is isolated from the gate. It can break through to the surface as a sink mark, or it can stay internal until a machining cut exposes it.

The fix is thermal, not pneumatic. Reduce the section, add a core, move the gate, add a chill or adjust the die cooling so the thick section solidifies while the gate is still feeding it. More pressure helps only while the gate is open and the section is still liquid. Once the skin closes, the cavity is set.

Cold shuts and flow defects

A cold shut forms when two flow fronts meet and fail to fuse. The metal is there, but the interface is a crease with an oxide film in it. It usually appears as a line or a seam on the surface, often at the end of a long flow or around a core, and it may be visible only after machining or shot blasting.

A cold shut is not a void in the same sense as a gas pore. A pressure test may pass it if the crease is tight, and a radiograph may show only a faint line. That makes it easy to miss and easy to argue about. The functional risk is a weak plane that can open under stress or leak under pressure.

The fix is to keep the metal hot and moving: raise the die temperature, increase the fill speed, move the gate, add an overflow at the meeting point or reduce the flow length. It is a filling problem, so the answer is in the gate and the thermal layout rather than the intensification pressure.

The three defects compared

The table is a reading aid for a defect review. It is not a substitute for sectioning a sample and looking at the actual morphology.

Choosing the detection method

Radiography is the standard internal check. It shows density differences, so it finds gas and shrink cavities and some cold shuts, but it flattens a three-dimensional defect into a two-dimensional image. A small pore can be hidden behind a rib, and the size on the film depends on the orientation and the source distance.

Computed tomography gives a three-dimensional map and is useful for a complex part or a first article. It is slower and more expensive, so it usually sits in the qualification phase rather than in production. Sectioning is destructive, but it shows the true morphology and is the best way to settle an argument about whether a defect is gas or shrink.

A pressure or leak test is the functional check, and it is the one that matters for a sealing part. It does not tell you which defect caused the leak, but it tells you whether the part works. Use it alongside the internal method, not instead of it. A part can pass a radiograph and leak through a cold shut that the film did not resolve.

Acceptance has to be zoned

A porosity specification that says no porosity anywhere is unbuildable. Every die casting contains some porosity, and a part that is rejected on a generic limit will be expensive and inconsistent. The workable approach is to divide the part into zones by function and set a limit for each zone.

A critical zone is a sealing face, a pressure boundary, a machined contact face or a thread. A controlled zone is a structural section or a cosmetic face. A general zone is a non-functional surface. The critical zone gets the tightest limit and the most inspection. The general zone may need no internal inspection at all.

Write the limits as a size, a count and a distribution, with a reference to a radiograph standard such as ASTM E505 for aluminium and magnesium die castings. Say whether the limit applies before or after machining, because a pore that is acceptable as-cast may open into a sealing face after the cut. A drawing that says only maximum porosity has not said anything an inspector can use.

Machining exposes what the die hid

A machining cut removes the dense skin and opens the subsurface. A pore that sat 1 mm below the surface is invisible on the casting and obvious on the finished part. That is why the machining stock, the gate position and the porosity zones have to be reviewed together.

The practical controls are to keep the critical machined faces out of the last-to-fill and last-to-solidify regions, to leave enough stock to cut past the skin without cutting into a defect, and to inspect the first parts after each machining setup. If a defect appears only after machining, the drawing needs a note on the acceptable condition of the cut face, not just the as-cast surface.

Process control is the long-term answer. Record the shot profile, the die temperature, the metal temperature and the vacuum level, and trend the reject rate against them. A porosity problem that is fixed by a one-off adjustment will return when the die temperature drifts or the melt quality changes.