Porosity is the defect that matters most in aluminium castings. It reduces strength, creates leak paths in pressure parts, and can blister during heat treatment. Controlling it starts long before the metal reaches the die, and the first step is knowing which kind you are looking at.
Two different defects, often confused
| Gas porosity | Shrinkage porosity | |
|---|---|---|
| Appearance | Round, smooth-walled holes | Jagged, branching voids |
| Location | Anywhere, often near surface | In thick sections and hot spots |
| Root cause | Hydrogen in the melt, or trapped air | Volume loss during solidification |
| Main cure | Degassing, venting, clean melt | Design, feeding, cooling control |
Where gas porosity comes from
Molten aluminium dissolves hydrogen readily, and its solubility drops sharply as the metal solidifies. The hydrogen has to go somewhere, and it comes out of solution as bubbles inside the casting.
The hydrogen comes from moisture, damp charge material, humid air, wet tools, and the combustion products of fuel-fired furnaces. Air entrained during high-velocity injection adds to the problem.
How we control it
- Electric resistance melting. We use electric furnaces rather than fuel-fired. With no hydrocarbon combustion, there are no combustion by-products introducing hydrogen into the melt.
- Rotary degassing. A mobile rotary degasser disperses inert gas through the melt, and the bubbles collect dissolved hydrogen and carry it to the surface.
- Density index measurement. A density index meter measures hydrogen content, so degassing is verified rather than assumed.
- Fluxing. Removes oxides and non-metallic inclusions that act as nucleation sites for bubbles.
- Die venting. Properly designed vents let displaced air leave the cavity rather than becoming trapped.
Burning fuel produces water vapour, and water vapour is a hydrogen source sitting directly above the melt. Electric resistance furnaces remove that mechanism entirely, which is one of the reasons we chose them.
Where shrinkage porosity comes from
Aluminium occupies less volume as a solid than as a liquid, roughly 6 percent less. If liquid metal cannot flow in to compensate as the casting freezes, the missing volume appears as a void.
This is why it concentrates in thick sections. They are the last to solidify, and by then the thinner surrounding material has frozen solid and cut off the feed path.
How it is controlled
- Uniform wall thickness in the design. The most effective control by far, and the reason we review drawings at quotation.
- Ribs and cored sections instead of solid masses.
- Gating and feeding arranged so solidification progresses toward the feed.
- Die cooling control to manage where hot spots form.
- Grain refining and modification to produce a finer structure that feeds more effectively.
How much porosity is acceptable?
That depends entirely on the part. A decorative cover can tolerate porosity that would scrap a hydraulic housing. What matters is agreeing the standard before production, not after.
Define the critical areas on the drawing, state whether the requirement is structural, pressure-tight or cosmetic, and specify the inspection method. We can then set the process to hit that target and verify it.
How it is verified
- X-ray inspection for internal soundness without cutting the part
- Density index testing on the melt, before casting begins
- Sectioning of sample parts during development
- Leak testing for pressure-tight components, arranged on demand
If a part must be pressure-tight
For components that must not leak, the options are gravity die casting rather than HPDC, careful feeding design, and where necessary vacuum impregnation to seal any residual porosity. Tell us at enquiry stage that the part is pressure-critical, it changes how we approach both the process and the tooling.
More detail on our inspection methods is on the quality page.
Have a part you need quoted?
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