Aluminium–silicon alloys or Silumin is a general name for a group of lightweight, high-strength based on an Aluminium–silicon system (AlSi) that consist predominantly of aluminum – with silicon as the quantitatively most important alloying element. Pure AlSi alloys cannot be hardened, the commonly used alloys AlSiCu (with copper) and AlSiMg (with magnesium) can be hardened. The hardening mechanism corresponds to that of AlCu and AlMgSi.
AlSi alloys are by far the most important of all aluminum cast materials. They are suitable for all Castability and have excellent casting properties. Important areas of application are in car parts, including Engine block and Piston. In addition, their use as a functional material for high-energy heat storage in electric vehicles is currently being focused on.
Alloying elements
The addition of silicon to aluminum also makes it less viscous when in liquid form, which, together with its low cost (as both component elements are relatively cheap to extract), makes it a very good casting alloy. Silumin with good castability may give a stronger finished casting than a potentially stronger alloy that is more difficult to cast.
All aluminum alloys also contain iron as an admixture. It is generally undesirable because it lowers strength and elongation at break. Together with Al and Si it forms the -phase AlFeSi, which is present in the structure in the form of small needles. However, iron also prevents the castings from sticking to the molds in die casting, so that special die-casting alloys contain a small amount of iron, while iron is avoided as far as possible in other alloys.
Manganese also reduces the tendency to stick, but affects the mechanical properties less than iron. Manganese forms a phase with other elements that is in the form of globulitic (round) grains.
Copper occurs in almost all technical alloys, at least as an admixture. From a content of 0.05% Cu, the corrosion resistance is reduced. Additions of about 1% Cu are alloyed to increase strength through solid solution strengthening. This also improves machinability. In the case of the AlSiCu alloys, higher proportions of copper are also added, which means that the materials can be hardened (see Aluminum-copper alloy).
Together with silicon, magnesium forms the Mg2Si (magnesium silicide) phase, which is the basis of hardenability, similar to aluminum-magnesium-silicon alloys (AlMgSi). In these there is an excess of Mg, so the structure consists of aluminum mixed crystal with magnesium and Mg2Si. In the AlSiMg alloys, on the other hand, there is an excess of silicon and the structure consists of aluminum mixed crystal, silicon and Mg2Si. Aluminium-Taschenbuch – Band 1: Grundlagen und Werkstoffe. Aluminium-Verlag, Düsseldorf, 16. Auflage, 2002, S. 145–151.
Silicon powders are used in aluminum-silicon alloys for enhancing strength and castability, providing better durability under high-stress conditions. It also improves the fluidity of molten aluminum which allows easier casting of complex shapes with fewer defects.
Small additions of titanium and boron serve to refine the grain.Sebastian F. Fischer, Christian Oberschelp: Aluminiumbasis-Gusswerkstoffe in: Andreas Bühring-Polaczek, Walter Michaeli, Günter Spur (Hrsg.): Handbuch Urformen, Hanser, 2014, S. 21.
Pure aluminium–silicon alloys
Pure AlSi alloys are smelted from primary aluminium, while AlSi alloys with other elements are usually smelted from secondary aluminium. The pure AlSi alloys are medium strength, non-hardenable, but corrosion resistant, even in salt water environments.Friedrich Ostermann: Anwendungstechnologie Aluminium. 3. Auflage. Springer, 2014, S. 182.
The exact properties depend on whether the composition of the alloy is above, near or below the eutectic point. Castability increases with increasing Si content and is best at about 17% Si; the mechanical properties are best at 6% to 12% Si.
- The mold filling capacity reaches its maximum at 12% Si, but is also good with other contents.
- The tendency to form cavities is lowest at 6% to 8% Si and considered low overall.
- The tendency to hot cracking is low with less than 6% Si.
Otherwise, AlSi alloys generally have favorable casting properties: the shrinkage is only 1.25% and the influence of the wall thickness is small.Fritz, Schulze, 9. Auflage, S. 36.
Hypereutectic alloys, with a silicon content of 16 to 19%, such as Alusil, can be used in high-wear applications such as pistons, and internal combustion . The metal is etched after casting, exposing hard, wear-resistant silicon precipitates. The rest of the surface becomes slightly porous and retains oil. Overall this makes for an excellent bearing surface, and at lower cost than traditional bronze bearing bushes.
Hypoeutectic alloys
In pure AlSi alloys, the eutectic is often in a degenerate form. Instead of the fine structure that is otherwise typical of eutectics with its good mechanical properties, AlSi takes the form of a coarse-grained structure on slow cooling, in which silicon forms large plates or needles. These can sometimes be seen with the naked eye and make the material brittle. This is not a problem in chill casting, since the cooling rates are high enough to avoid degeneration.Handbuch Urformen, S. 62.
In sand casting in particular, with its slow cooling rates, additional elements are added to the melt to prevent degeneration. Sodium, strontium and antimony are suitable. Aluminium-Taschenbuch – Band 1: Grundlagen und Werkstoffe. Aluminium-Verlag, Düsseldorf, 16. Auflage, 2002, S. 101.Handbuch Urformen, S. 23, 62. These elements are added to the melt at around 720 °C to 780 °C, causing supercooling that reduces the diffusion of silicon, resulting in a common fine eutectic, resulting in higher strength and elongation at break. Aluminium-Taschenbuch – Band 1: Grundlagen und Werkstoffe. Aluminium-Verlag, Düsseldorf, 16. Auflage, 2002, S. 101.
Eutectic and near-eutectic alloys
Hypereutectic Alloys
Aluminium–silicon–magnesium alloys
Aluminium–silicon–copper alloys
Compositions of standardized varieties
Wrought alloys Aluminium-Taschenbuch – Band 1: Grundlagen und Werkstoffe. Aluminium-Verlag, Düsseldorf, 16. Auflage, 2002, S. 649ff
| EN AW-4004 | AlSi10Mg1.5 | 9.0–10.5 | 0.8 | 0.25 | 0.10 | 1.0–2.0 |
| EN AW-4014 | AlSi2 | 1.4–2.2 | 0.7 | 0.20 | 0.35 | 0.30–0.8 |
| EN AC-42000 | AlSi7Mg | 6.5–7.5 | 0.45 | 0.15 | 0.35 | 0.25–0.65 |
| EN AC-42200 | AlSi7Mg0.6 | 6.5–7.5 | 0.15 | 0.03 | 0.1 | 0.45–0.7 |
| EN AC-43400 | AlSi10Mg(Fe) | 9.0–11.0 | 1.0 | 0.10 | 0.001–0.4 | 0.2–0.5 |
| EN AC-45000 | AlSi6Cu4 | 5.0–7.0 | 1.0 | 3.0–5.0 | 0.20–0.65 | 0.55 |
| EN AC-47000 | AlSi12(Cu) | 10.5–13.5 | 0.8 | 1.0 | 0.05 | 0.35 |
Mechanical properties of standardized and non-standard grades
| AlSi7Mg |
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| AlSi7Mg0.6 | Sand cast, artificially aged | 230 | 190 | 2 | 75 |
| AlSi10Mg(Fe) | Die cast, cast condition | 240 | 140 | 1 | 70 |
| AlSi6Cu4 | Sand casting, cast condition | 150 | 90 | 1 | 60 |
| AlSi12(Cu) | Sand casting, cast condition | 150 | 70 | 6 | 45 |
| AlSi17Cu4Mg (A390) | Gravity die casting, cast condition | 200 | 200 | <1 | 110 |
4000 series
| +4000 series aluminium alloy nominal composition (% weight) and applications !Alloy !Al contents !Alloying elements !Uses and refs | |||
| 4006 | 98.3 | Silicon 1.0; Iron 0.65 | Work-hardened or aged |
| 4007 | 96.3 | Silicon 1.4; Manganese 1.2; Iron 0.7; Nickel 0.3; Chromium 0.1 | Work-hardened |
| 4015 | 96.8 | Silicon 2.0; Manganese 1.0; Magnesium 0.2 | Work-hardened |
| 4032 | 85 | Silicon 12.2; Copper 0.9; Magnesium 1; Nickel 0.9; | Forgings |
| 4043 | 94.8 | Silicon 5.2 | Rod |
| 4047 | 85.5 | Silicon 12.0; Iron 0.8; Copper 0.3; Zinc 0.2; Manganese 0.15; Magnesium 0.1 | Sheet, cladding, fillers |
| 4543 | 93.7 | Silicon 6.0; Magnesium 0.3 | architectural extrusions |
Applications
In general, AlSi alloys are mainly used in foundries, especially for vehicle construction. Wrought alloys are very rare. They are used as a filler metal (welding wire) or as a Soldering in brazing. In some cases, forged AlSi pistons are also built for aviation.Friedrich Ostermann: Anwendungstechnologie Aluminium. 3. Auflage. Springer, 2014, S. 152 f.
AlSi eutectic casting alloys are used for machine parts, cylinder heads, cylinder crankcases, impellers and ribbed bodies. Hypereutectic (high silicon) alloys are used for engine parts because of low thermal expansion and high strength and wear resistance. This also includes special piston alloys with around 25% Si.Fritz, Schulze: Fertigungstechnik, 11. Auflage, S. 40 f.
Alloys with additions of magnesium (AlSiMg) can be hardened by heat treatment. An example use-case are wheel rims produced by low-pressure casting because of their good strength, corrosion resistance and elongation at break. Alloys with about 10% Si are used for cylinder heads, switch housings, Inlet manifold, transformer tanks, Car suspension and oil pans. Alloys with 5% Si to 7% Si are used for chassis parts and wheels. At levels of 9%, they are suitable for structural components and body nodes.Sebastian F. Fischer, Christian Oberschelp: Aluminiumbasis-Gusswerkstoffe in: Andreas Bühring-Polaczek, Walter Michaeli, Günter Spur (Hrsg.): Handbuch Urformen, Hanser, 2014, S. 63.
The copper-containing AlSiCu alloys are used for gear housings, crankcases and cylinder heads because of their heat resistance and hardenability.Sebastian F. Fischer, Christian Oberschelp: Aluminiumbasis-Gusswerkstoffe in: Andreas Bühring-Polaczek, Walter Michaeli, Günter Spur (Hrsg.): Handbuch Urformen, Hanser, 2014, S. 63.
Almost eutectic AlSi melts are also used for hot-dip aluminizing. In the process of Galvanization, steel strips are finished with a heat-resistant metallic coating 10–25 μm thick. Hot-dip aluminized sheet steel is an inexpensive material for thermally stressed components. Unlike zinc coatings, the coating does not provide cathodic protection under atmospheric conditions.
Latent heat storage
Characteristics
- High castability, fluidity, corrosion resistance, ductility, and low density.
- Usable for large castings, which can operate under heavy load conditions.
- Considered to not be a heat-treatable alloy, but the addition of Mg & Cu can allow it to be heat treated, e.g. AΠ4 alloys.
- Strengthened by solution treatment, e.g. adding 0.01% sodium (in the form of sodium fluoride NaF and sodium chloride NaCl) to the melt just before casting.
- A disadvantage is a tendency for porosity in the casting, i.e. the casting can become foam-like. This can be avoided by casting under pressure in autoclaves.
Further reading
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(2026). 9783319583792, Springer International Publishing. ISBN 9783319583792
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