AAS vs. SAS: sandwich with aluminum or steel cover layers?
AAS and SAS differ in their cover layers: in the aluminum foam sandwich (AAS) they are made of aluminum, in the steel-aluminum foam sandwich (SAS) of DC01 (1.0330) steel. In both, the core is aluminum foam, bonded entirely metallically without adhesive. AAS is the lighter choice: in the 10-1-1 build-up (1 mm cover layers, foam core 0.7 g/cm³) a plate of 1,000 × 1,000 × 10 mm³ weighs about 11 kg as AAS and 21.3 kg as SAS. SAS is the stiffer one: at 210 GPa the steel cover layers have three times the Young's modulus of the aluminum cover layers (70 GPa) and determine the stiffness of the composite.
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How do AAS and SAS differ in their material values?
Both sandwiches are calculated for the 1/8/1 reference build-up (cover layer/core/cover layer, core: aluminum foam PM; SAS example with a core density of 0.7 g/cm³; the mass applies to AAS 10-1-1 and SAS 10-1-1 with a foam core of 0.7 g/cm³). The cover layer material determines the differences: steel cover layers give the higher Young's modulus (210 GPa versus 70 GPa) and therefore the higher stiffness; aluminum cover layers give the lower weight and the higher thermal conductivity. All values are analytically determined guide values.
| Property | Unit | AAS (aluminum cover layers) | SAS (steel cover layers) |
|---|---|---|---|
| Density | g/cm³ | 0.5 to 0.8 (core) · 2.7 (cover layer) | 0.7 (core) · 7.85 (cover layer) |
| Young's modulus | GPa | 0.33 to 0.76 (core) · 70 (cover layers) | 0.6 (core) · 210 (cover layers) |
| Compressive strength | N/mm² | 2.5 to 10 (core) | 8.5 (core) |
| Tensile strength | N/mm² | 120 to 200 | 270 to 410 (cover layer) |
| Bending stiffness | mm/1,000 N | 6.8** | 2.4 |
| Thermal conductivity | W/mK | 10 to 15 (core) · 170 to 220 (cover layer) | 25 (core) · 48 to 58 (cover layer) |
| Mass 1,000 × 1,000 × 10 mm³ | kg | about 11 (AAS 10-1-1, core 0.7 g/cm³)* | 21.3 (SAS 10-1-1, core 0.7 g/cm³) |
The foam core is regularly produced at 0.5 to 0.8 g/cm³, higher densities by agreement. Young's modulus and compressive strength of the foam core are given by density according to the AAS, SAS and Havel Lite® product data sheets (0.5 g/cm³ → 330 N/mm² / 2.5 N/mm² · 0.6 → 450 / 6.5 · 0.7 → 600 / 8.5 · 0.8 → 760 / 10); the SAS column gives the value for the core density of 0.7 g/cm³ of this build-up. SAS cover layer of DC01 (1.0330) steel to DIN EN 10130: tensile strength 270 to 410 N/mm², yield strength 140 to 280 N/mm². *Strongly dependent on the material density. **Strongly dependent on the geometry in question. Sources for the calculation: Tabellenbuch Metall, 43rd edition 2005; Fraunhofer Institute for Machine Tools and Forming Technology IWU. All values were determined analytically and serve as guide values; for specific applications the values must be determined individually.
When is AAS the right choice?
AAS is the choice when weight is the deciding criterion: as AAS 10-1-1 (1 mm aluminum cover layers, foam core 0.7 g/cm³) the reference plate of 1,000 × 1,000 × 10 mm³ weighs about 11 kg, against 21.3 kg as SAS 10-1-1 (1 mm steel cover layers, foam core 0.7 g/cm³) and 78.5 kg for solid steel. In addition, where heat has to be conducted deliberately, the aluminum cover layers offer a high thermal conductivity of 170 to 220 W/mK. The tensile strength is 120 to 200 N/mm². The bending stiffness depends strongly on geometry and can therefore be influenced through the design; the material values table gives the guide value for the reference build-up.
When is SAS the right choice?
SAS is the choice when stiffness and strength come first: the steel cover layers (DC01, 1.0330, Young's modulus 210 GPa) make the composite much stiffer than aluminum cover layers, as SAS 10-1-1 (1 mm steel cover layers, foam core 0.7 g/cm³) at 21.3 kg against 78.5 kg for solid steel. At the core density of 0.7 g/cm³ of this build-up, the compressive strength of the core is 8.5 N/mm²; the DC01 cover layer has a tensile strength of 270 to 410 N/mm² and a yield strength of 140 to 280 N/mm². There is also a manufacturing advantage: in the SAS process, fastening points, fastening rails or nuts can be foamed in or welded in directly.
What do AAS and SAS have in common?
Both designs share the same core and the same bonding technique: the aluminum foam core is bonded entirely metallically to the cover layers, without adhesive; material testing of AAS and SAS to DIN EN 45545-2 resulted in classification as HL3; no certificate was applied for. For both, customer-specific alloys and coatings (paints, marble, wood, wallpaper) are an option, and both can be processed: drilling, welding and milling are possible, as are curved sandwiches.
- Aluminum foam core (powder metallurgical, PM)
- Entirely metallic bond without adhesive
- DIN EN 45545-2: classified HL3
- Customer-specific alloys and coatings (paints, marble, wood, wallpaper)
- Processing: drilling, welding, milling; curved sandwiches
Choosing the cover layer
What separates a sandwich with aluminum cover layers from one with steel cover layers?
Also asked: What is the difference between AAS and SAS? · How do the two sandwich designs differ?
AAS and SAS share the same core and differ only in their cover layers. Havel metal foam produces both designs with an aluminum foam core that is bonded entirely metallically to the cover layers, without adhesive. In AAS the cover layers are aluminum (EN AW-6082), in SAS steel (DC01, 1.0330). Everything else follows from this one decision: at 210 GPa, steel cover layers have three times the Young's modulus of aluminum (70 GPa), and because the cover layers carry the bending load in a sandwich, they determine the stiffness of the composite. Aluminum cover layers keep the component lighter and conduct heat better. For the design there is a third point, which often decides first: fastening points, fastening rails and nuts can only be foamed in or welded in with the SAS process. The foam core itself is the same material in both designs, which is why its material values apply equally to AAS and SAS.
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Which cover layer suits which requirement?
Also asked: When is AAS the right choice, and when SAS? · How do you decide between aluminum and steel cover layers?
AAS is the right fit when weight is the deciding criterion; SAS when stiffness, a hard surface or foamed-in fastenings are needed. Havel metal foam supplies the reference plate of 1,000 × 1,000 × 10 mm³ as AAS 10-1-1 (1 mm aluminum cover layers, foam core 0.7 g/cm³) at about 11 kg and as SAS 10-1-1 (1 mm steel cover layers, foam core 0.7 g/cm³) at 21.3 kg; solid steel comes to 78.5 kg. For stiffness, the steel cover layers are the choice: at 210 GPa they have three times the Young's modulus of the aluminum cover layers (70 GPa). For heat management, conversely, the aluminum cover layers are the better choice. In practice, three questions usually settle it beforehand. If the design needs foamed-in nuts or fastening rails, only SAS is possible. If the component is to be welded to a steel structure, SAS is the more direct route. And if the depth is tight or the format large, it is worth looking at the available dimensions, which differ between the two designs.
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Build-up and dimensions
Which thicknesses and formats are available for each design?
Also asked: How thick can an aluminum foam sandwich be? · What is the maximum format of a sandwich with steel cover layers?
AAS and SAS differ in both total thickness and format. Havel metal foam supplies AAS at 6 to 60 mm total thickness, with cover layers of 1 to 8 mm and a maximum format of 2,800 × 1,400 mm. The company supplies SAS at 8 to 40 mm total thickness, with cover layers of 1.5 to 5 mm and a maximum format of 2,950 × 1,450 mm. For the design this means: the thinnest and the thickest build-ups are available only as AAS, the largest plate format only as SAS. The cover layer alloys are EN AW-6082 for AAS and DC01 for SAS. The density range of the foam core is the same for both designs: regular production covers 0.5 to 0.8 g/cm³, higher densities by agreement.
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Do the material values of the foam core differ between the two designs?
Also asked: Is the aluminum foam core the same in AAS and SAS? · Do the core values apply to both sandwich designs?
No. The aluminum foam core is the same material in AAS and SAS, so its material values apply equally to both designs. Havel metal foam regularly produces in the density range of 0.5 to 0.8 g/cm³, and the product data sheets for AAS, SAS and Havel Lite® list the same density-dependent values for this core: Young's modulus 330 to 760 N/mm² and compressive strength 2.5 to 10 N/mm², both rising with density. What differs between the designs are the properties of the composite, and those come from the cover layers. The full table of values per density grade is on the material values page. For the comparison, the consequence is what counts: choosing between AAS and SAS means choosing not the core but the cover layer; the density is chosen to suit the application, independently of that.
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Design and joining
In which design can fastening points and nuts be foamed in directly?
Also asked: Can nuts be foamed in directly? · Can fastening rails be integrated into a sandwich?
Only in SAS, the sandwich with steel cover layers. Havel metal foam can foam in or weld in the necessary fastening points, fastening rails or nuts with the SAS process; both, foaming in and welding in, are limited to this design. With aluminum cover layers (AAS) neither is possible. The reason lies in the cover layer material: the steel cover layer of SAS (DC01, 1.0330) can be welded, the aluminum cover layer of AAS (EN AW-6082) not by the same means. This is the difference with the greatest consequences for the design, because it often decides the choice of design before any material value: if a component needs foamed-in bolts or nuts, the cover layer is settled. Conversely, it is worth asking early whether the fastening really has to be integrated or whether it can be fitted after joining; then the lighter design stays in play. It should also be borne in mind that foam in the fastening area costs additional preparation time in manufacturing.
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How are the two designs connected to a steel structure?
Also asked: Can an aluminum foam sandwich be welded to steel? · How is a sandwich with aluminum cover layers joined to a steel structure?
AAS and SAS are connected differently, and this point fixes a design early. Havel metal foam joins SAS to a steel structure by MIG/MAG welding (gas metal arc), the process that is in use in steel construction anyway. For AAS to steel, on the other hand, explosion-clad transition profiles are needed, because aluminum and steel are not welded directly to each other. Here too the cover layer is what matters: DC01 steel for SAS, EN AW-6082 aluminum for AAS. Anyone integrating a component into a welded steel structure therefore has the shorter route with SAS; anyone working in an aluminum structure has it with AAS. Regardless of the design, both can be drilled, welded and milled, and curved sandwiches are also possible.
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Limits and open points
How do the two cover layers behave in the corrosion test?
Also asked: Does a sandwich with steel cover layers corrode? · How was the corrosion behavior of the sandwiches tested?
The cover layer determines the corrosion pattern, not the core. Havel metal foam had sandwich samples tested at Fraunhofer ICT in salt spray to DIN EN ISO 9227 NSS, 240 h at (35 ± 2) °C; a second accredited test in an SO₂ atmosphere to DIN EN ISO 6988 shows the same pattern. What was observed: the steel cover layer corroded, the bond between foam and cover layer did not. This statement comes with two limitations. The cover layers tested were EN AW 6060 and DC01, so the aluminum cover layer was not the EN AW-6082 of the data sheets. And neither of the two test reports states a result; the assessment is that of Havel metal foam, not that of the institute. Corrosion protection is as necessary for both designs as it is for steel.
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What do the two sandwich designs have in common?
Also asked: What is the same in AAS and SAS? · Which properties apply to both sandwich designs?
AAS and SAS share the core, the bonding technique and the processing. In both designs Havel metal foam foams an aluminum foam core made by powder metallurgy, which is bonded entirely metallically to the cover layers; neither contains adhesive. From this follows what applies to both: the build-up is purely metallic and adds nothing to the fire load, the material is 100 % recyclable, and material testing of AAS and SAS to DIN EN 45545-2 resulted in classification as HL3; no certificate was applied for. Customer-specific alloys and coatings are an option for both, from paints to marble, wood or wallpaper. Drilling, welding and milling are possible with both, as are curved sandwiches, and the density range of the core is the same for both: regular production covers 0.5 to 0.8 g/cm³, higher densities by agreement.
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