Aluminium Foam for Automotive Applications
Aluminium foam is a cellular metal, typically 75 – 92 % air by volume, that crushes at a nearly constant stress and so absorbs impact energy very efficiently for its weight. In vehicles it is used as a filler inside crash boxes and pillars, as the core of stiff, light sandwich panels, as underbody and battery protection, and to damp noise and vibration. This guide explains how it works, where it pays off and how to specify it.
- Density
- about 0.2 – 0.8 g/cm³
- Main role
- Crash energy absorption
- Other roles
- Stiffness, NVH, heat shielding
- Test standard
- ISO 13314 compression
What aluminium foam does in a vehicle
Under compression, a solid aluminium part either resists or buckles. A foam behaves differently: after a short elastic phase its cell walls bend and collapse progressively, so the stress stays almost flat over a large strain, often up to 50 – 70 %, before the cells are fully crushed and the material densifies. The area under that flat "plateau" is the energy it absorbs.
That property is exactly what crash structures need. An energy absorber should limit the peak force transmitted to occupants and to the battery or passenger cell while converting as much kinetic energy as possible into deformation. Because foam density can be chosen, engineers can tune the plateau stress to the load a structure must not exceed.
Foam is not a replacement for sheet, extrusions or castings. It works best combined with them: inside a thin-walled tube, between two face sheets, or as a local insert where packaging space is tight and a progressive crush is wanted.
Types of aluminium foam used in automotive parts
| Foam type | How it is made | Typical density | Cell structure | Best suited to |
|---|---|---|---|---|
| Closed-cell, melt route | Molten aluminium thickened with calcium or ceramic particles and foamed with a gas-releasing agent or injected gas, then cast into blocks and sliced | about 0.2 – 0.7 g/cm³ | Closed cells, roughly 2 – 10 mm | Panels, cores, energy absorbers, noise barriers |
| Closed-cell, powder route | Aluminium alloy powder mixed with a blowing agent, compacted to a precursor and foamed inside a mould or between face sheets | about 0.3 – 0.8 g/cm³ | Closed cells with a dense outer skin | Shaped parts, crash-box fillers, foam sandwich panels |
| Open-cell (reticulated or infiltrated) | Metal cast around a removable pattern or salt space-holder, which is then leached out | about 0.2 – 0.5 g/cm³ | Interconnected pores, fine to coarse | Sound absorption, heat exchange, filtration |
Closed-cell foams dominate structural and crash uses because the closed cells give higher stiffness and plateau stress at a given density. Foams made by the powder route can be expanded directly inside a tube or between sheets, producing a metallurgical bond to the skin. Open-cell foams are mainly used for acoustic and thermal duties.
Where aluminium foam is used in cars, trucks and EVs
| Application | Foam form | Why foam helps |
|---|---|---|
| Front and rear crash boxes | Foam-filled aluminium extrusions | Filling raises the mean crush force and stabilises folding, so a shorter box absorbs the same energy |
| A, B and C-pillar and sill reinforcements | Foam inserts | Resists local buckling of thin walls in side impact with little added mass |
| EV battery underbody protection | Foam sandwich or foam layer under the tray | Spreads bollard and road-debris loads and absorbs energy before it reaches cells |
| Floor, roof and bulkhead panels | Aluminium foam sandwich (AFS) | High bending stiffness per kilogram with good damping |
| Engine and motor mount brackets | Foam-cored castings or sandwich | Stiffness plus vibration damping |
| Heat and noise shields | Thin closed- or open-cell plate | Non-combustible, absorbs sound and slows heat transfer |
| Special vehicles: buses, rail, armoured | Panels and blast-mitigating layers | Energy absorption, fire safety and low weight |
A typical example is a foam-filled crash box: a 6xxx extrusion of square or octagonal section is filled with closed-cell foam of moderate density. During a frontal impact the tube folds in regular lobes, and the foam supports the walls between folds, so more energy is absorbed per unit length than with the empty tube plus the foam on its own. The benefit is largest when the space available for the crash zone is short.
Properties that matter to automotive engineers
| Property | What to expect | Design note |
|---|---|---|
| Density | About 0.2 – 0.8 g/cm³, against 2.70 g/cm³ for solid aluminium | Specify a density band, typically ±10 % |
| Compressive plateau stress | From around 1 MPa at low density to over 10 MPa at high density | Rises steeply with density (roughly with density to the power 1.5 – 2) |
| Densification strain | Typically 50 – 70 % | Leave this crush length available in the package |
| Energy absorption | Plateau stress × usable strain | Measure on the actual foam with ISO 13314 tests |
| Stiffness | Elastic modulus well below solid aluminium, increasing with density | Use as a core between stiff skins for panels |
| Fire behaviour | Metallic and non-combustible; melts near 600 – 660 °C | No toxic smoke from the foam itself |
| Recyclability | Fully recyclable with aluminium scrap | No polymer separation needed for unbonded foam |
Values are indicative ranges from the published literature. Foam properties scatter more than those of solid alloys, so design data should come from compression tests on the specified density and cell size.
Design and integration guidelines
- Match plateau stress to the force limit: choose density so that the crush force stays below the load that would damage the structure or the battery behind it.
- Allow for densification: only about half to two-thirds of the foam thickness is usable crush stroke.
- Consider strain rate and temperature: plateau stress changes slightly with impact speed and noticeably with temperature; test at representative conditions.
- Join correctly: foam can be bonded, mechanically fixed or foamed in place. Fusion welding directly to foam is impractical; weld the skins or brackets instead.
- Protect against corrosion: cut cell surfaces have a large exposed area; coat or seal foam in wet underbody zones and avoid contact with carbon fibre or steel without isolation.
- Check cost per function: foam is more expensive per kilogram than extrusions; it pays off where it removes mass elsewhere or shortens a crash zone.
Aluminium foam compared with honeycomb and polymer foam
| Criterion | Aluminium foam | Aluminium honeycomb | Polymer (PU/EPP) foam |
|---|---|---|---|
| Energy absorption | Good in all directions (isotropic) | Very high, but mainly along the cell axis | Moderate, rate-sensitive |
| Fire behaviour | Non-combustible | Non-combustible core; adhesives may burn | Combustible |
| Temperature range | Wide; stable well above engine-bay temperatures | Wide | Limited at high temperature |
| Shape freedom | Can be foamed into complex shapes | Mainly blocks and flat panels | Very high |
| Cost | Higher | Medium | Low |
Honeycomb is the better choice where loads come from one known direction, such as a dedicated impact attenuator. Foam is preferred where load direction varies, where the part must fill an irregular cavity, or where fire and heat rule out polymers, for example near batteries and exhausts. See honeycomb core for energy absorption for the alternative.
How to specify aluminium foam for automotive use
- Define the functionCrash absorber, stiffener, damping layer or shield, with the loads, temperatures and package space.
- Set density and cell sizeState a density band and, where relevant, a cell-size range and skin requirement.
- Agree testsCompression tests to ISO 13314, density checks and dimensional inspection; add any customer-specific crash or corrosion tests.
- Choose form and finishMachined blocks, sliced plate, foam-filled tubes or sandwich panels, bare or coated.
- Approve samplesRun prototype parts through your validation before volume supply.
References and standards
- ISO 13314 – Mechanical testing of metals: compression test for porous and cellular metals
- M.F. Ashby, A.G. Evans, N.A. Fleck, L.J. Gibson, J.W. Hutchinson, H.N.G. Wadley – Metal Foams: A Design Guide (Butterworth-Heinemann)
- J. Banhart – Manufacture, characterisation and application of cellular metals and metal foams, Progress in Materials Science 46 (2001)
- European Aluminium – Aluminium in cars
Aluminium (aluminum) foam for automotive: FAQs
Why is aluminium foam good for crash protection?
It collapses at a nearly constant stress over a large strain, so it absorbs a lot of energy while limiting the peak force passed to the vehicle structure, battery or occupants.
Is aluminium foam strong enough to be structural?
On its own it is weak in tension and bending. It becomes structural when combined with skins or tubes, as in foam sandwich panels or foam-filled extrusions.
How much does aluminium foam weigh?
Typical automotive foams are 0.2 – 0.8 g/cm³, roughly one-tenth to one-third of the density of solid aluminium.
Can aluminium foam be used in electric vehicles?
Yes. It is used for battery underbody protection, side-impact inserts and heat shields, where its non-combustible nature and energy absorption are both valuable.
Is aluminium foam recyclable?
Yes. Unbonded foam is remelted with aluminium scrap. Sandwich panels with adhesive layers need separation or are recycled as mixed scrap.
Evaluating aluminium foam for a vehicle part?
Tell us the function, density target and size. We will advise on foam type, test data and supply.
