Aluminum-silicon alloy

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Aluminum-silicon alloys (AlSi) are aluminum alloys - alloys that mainly consist of aluminum - with silicon as the most important alloying element in terms of quantity . Pure AlSi alloys cannot be hardened, the frequently used alloys AlSiCu (with copper ) and AlSiMg (with magnesium ) can be hardened. The hardening mechanism corresponds to that of AlCu and AlMgSi . The rarely used wrought alloys in the 4000 series and the predominantly used cast alloys in the 40000 series are standardized . AlSi alloys are by far the most important of all cast aluminum materials . They are suitable for all casting processes and have excellent casting properties . Important areas of application are in auto parts, including engine blocks and pistons. The current focus is also on their use as a functional material for high-energy heat storage in electric vehicles.

Alloy elements

In addition to aluminum as the main component, the AlSi alloys contain silicon as the main alloying element . It ensures very good casting properties .

All aluminum alloys 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 AlFeSi phase , which is present in the structure in the form of small needles. However, iron also prevents the castings from sticking to the molds during die casting , so that special die-casting alloys contain a small amount of iron, while iron is avoided as far as possible with other alloys.

Manganese also lowers 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 globular (round) grains.

Copper occurs at least as an admixture in almost all technical alloys. From a content of 0.05% Cu, the corrosion resistance is reduced. Additions of about 1% Cu are added in order to increase the strength through solid solution strengthening. This also improves machinability . In the case of AlSiCu alloys, higher proportions of copper are also added, which means that the materials can be hardened (see aluminum-copper alloy ).

Magnesium forms together with silicon the phase Mg 2 Si ( magnesium silicide ), which is the basis of the hardenability, similar to the aluminum-magnesium-silicon alloys (AlMgSi). These have an excess of Mg, so the structure consists of aluminum mixed crystal with magnesium and Mg 2 Si. In the case of AlSiMg alloys, on the other hand, there is an excess of silicon and the structure consists of the aluminum mixed crystal, silicon and Mg 2 Si.

Small additions of titanium and boron serve to refine the grain .

Applications

In general, AlSi alloys are mainly used in foundries, there especially for vehicle construction. Wrought alloys are very rare. They are used as filler metal (welding wire) or as solder in hard soldering . Forged AlSi pistons are also sometimes built for aviation.

Eutectic casting alloys made of AlSi are used for machine parts, cylinder heads , cylinder crankcases , impellers and rib bodies. Hypereutectic (silicon-rich) alloys are used for engine parts because of their low thermal expansion and high strength and wear resistance. This also includes special piston alloys with around 25% Si.

Alloys with additions of magnesium (AlSiMg) can be hardened and are used because of their good strength, corrosion resistance and elongation at break for rims that are manufactured by low-pressure casting. Alloys with about 10% Si are used for cylinder heads, switch housings, intake manifolds , transformer tanks , wheel suspensions and oil pans. Alloys with 5% Si to 7% Si are used for chassis parts and wheels. With a content of 9%, they are suitable for structural components and body nodes.

The copper-containing AlSiCu alloys are used for gearboxes, crankcases and cylinder heads because of their high temperature strength and hardenability.

In addition to the use of AlSi alloys as a structural material, in which the mechanical properties are in the foreground, another area of ​​application is latent heat storage . In the phase change of the alloy at 577 ° C, thermal energy can be stored in the form of latent heat of fusion. AlSi can thus also as a metallic phase change material (English m etallic P hase C hange M aterial, mPCM ) may be used. Compared to other phase change materials, metals are characterized by a high specific energy density combined with high thermal conductivity. The latter is important for the rapid introduction and discharge of heat into the storage material and thus increases the performance of a heat storage system. These advantageous properties of mPCM such as AlSi are of particular importance for vehicle applications, since low masses and volumes as well as high thermal performance are key objectives here. By using storage systems based on mPCM, the range of electric cars can be increased by storing the thermal energy required for heating in the mPCM instead of taking it from the traction battery.

Almost eutectic AlSi melts are also used for hot-dip aluminizing. In the process of continuous strip galvanizing , steel strips are refined with a heat-resistant metallic coating with a thickness of 10-25 µm. Hot-dip aluminized sheet steel is an inexpensive material for components exposed to heat. In contrast to zinc coatings, the coating does not offer any cathodic protection under atmospheric conditions .

Pure aluminum-silicon alloys

Phase diagram of aluminum-silicon

Aluminum forms a eutectic with silicon , which is at 577 ° C, with a Si content of 12.5% ​​or 12.6%. Up to 1.65% Si can be dissolved in aluminum at this temperature. However, the solubility decreases rapidly with temperature. At 500 ° C it is still 0.8% Si, at 400 ° C 0.3% Si and at 250 ° C only 0.05% Si. Silicon is practically insoluble at room temperature. No aluminum can be dissolved in silicon, not even at high temperatures. Only in the molten state are both completely detachable. Strength increases due to solid solution strengthening are negligible.

Pure AlSi alloys are melted from primary aluminum , while AlSi alloys with other elements are usually melted from secondary aluminum. The pure AlSi alloys have medium strength, cannot be age-hardened, but are corrosion-resistant, even in an environment with salt water.

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 around 17% Si; the mechanical properties are most favorable with 6% to 12% Si.

Otherwise, AlSi alloys generally have favorable casting properties: The shrinkage is only 1.25% and the influence of the wall thickness is low.

Hypoeutectic alloys

Hypo-eutectic alloys (also hypo-eutectic) have silicon contents below 12%. With them , the aluminum solidifies first. As the temperature falls and the proportion of solidified aluminum increases, the silicon content of the residual melt increases until the eutectic point is reached. Then the entire residual melt solidifies as a eutectic. The structure is therefore characterized by primary aluminum, which is often in the form of dendrites , and the eutectic of the residual melt between them. The lower the silicon content, the larger the dendrites.

In pure AlSi alloys, the eutectic is often in a degenerate form. Instead of the fine structure with its good mechanical properties, which is otherwise typical for eutectics, with AlSi it is in the form of a coarse-grain structure when slowly cooled, in which silicon forms large plates or needles. Some of these can be seen with the naked eye and embrittle the material. This is not a problem with die casting , since the cooling speeds are high enough here to avoid degeneration.

In sand casting, in particular, with its slow cooling speeds, additional elements are added to the melt in order to avoid degeneration. Suitable: sodium , strontium and antimony . These elements are added to the melt at around 720 ° C to 780 ° C , resulting in supercooling that reduces the diffusion of silicon and thus leads to an ordinary fine eutectic, resulting in higher strengths and elongations at break.

Eutectic and near-eutectic alloys

Alloys with 11% Si to 13% Si are counted among the eutectic alloys. Annealing can improve elongation and fatigue strength. The solidification is shell-forming in untreated alloys and smooth-walled in refined alloys, which leads to very good castability. Above all, the flowability and the mold filling capacity are very good, which is why eutectic alloys are suitable for thin-walled parts.

  • Structure, unrefined

  • Structure, refined

Hypereutectic alloys

Alloys with more than 13% Si are called hypereutectic or hypereutectic. The Si content is usually up to 17%, with special piston alloys also over 20%. Hypereutectic alloys have a very low thermal expansion and are very wear-resistant. In contrast to many other alloys, AlSi alloys do not have the maximum flowability in the vicinity of the eutectic, but at 14 to 16% Si, in the case of overheating at 17% to 18% Si. The tendency to hot cracks is minimal in the range from 10% to 14%. With hypereutectic alloys, the silicon crystals first solidify in the melt until the residual melt solidifies as a eutectic. For grain refinement of copper-phosphorus alloys are used. The hard and brittle silicon leads to increased tool wear during subsequent machining , which is why diamond tools are sometimes used. (See also Machinability # Aluminum and Aluminum Alloys .)

Aluminum-silicon-magnesium alloys

AlSiMg alloys with small amounts of magnesium (less than 0.3 to 0.6% Mg) can be hardened both cold and warm. The proportion of magnesium decreases with increasing silicon content, which is between 5% Si and 10% Si. They are related to the AlMgSi alloys: Both are based on the fact that magnesium silicide Mg 2 Si is precipitated at high temperatures , which is present in the material in the form of finely divided particles and thus increases the strength. Magnesium also increases the elongation at break. In contrast to AlSiCu, which can also be hardened, these alloys are corrosion-resistant and easy to cast. However, in some AlSiMg alloys, copper occurs as an impurity, which reduces corrosion resistance. This is particularly true of materials that have been melted from secondary aluminum.

Aluminum-silicon-copper alloys

AlSiCu alloys are also heat-age hardenable and additionally high-strength, but susceptible to corrosion and worse, but still sufficiently, castable. It is often melted from secondary aluminum. The hardening is based on the same mechanism as the AlCu alloys. The copper content is 1% to 4%, that of silicon 4% to 10%. Small additions of magnesium improve the strength.

Compositions of standardized varieties

All data are in percent by mass . The rest is aluminum.

Wrought alloys

Numerically Chemically Silicon iron copper manganese magnesium
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

Casting alloys

Numerically Chemically Silicon iron copper manganese magnesium
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-standardized grades

Chemically Status Tensile strength [MPa] Yield strength [MPa] Elongation at break [%] Brinell hardness [HB]
AlSi7Mg
  • Sand casting, as cast
  • Sand casting, artificially hardened
  • Chill casting, as-cast state
  • Chill casting, artificially age hardened
  • 140
  • 220
  • 170
  • 260
  • 80
  • 180
  • 90
  • 220
  • 2
  • 1
  • 2.5
  • 1
  • 50
  • 75
  • 55
  • 90
AlSi7Mg0.6 Sand casting, artificially hardened 230 190 2 75
AlSi10Mg (Fe) Die-cast, as-cast 240 140 1 70
AlSi6Cu4 Sand casting, as cast 150 90 1 60
AlSi12 (Cu) Sand casting, as cast 150 70 6th 45
AlSi17Cu4Mg (A390) Chill casting, as-cast state 200 200 <1 110

literature

  • Friedrich Ostermann: Application technology aluminum. 3. Edition. Springer, 2014, ISBN 978-3-662-43806-0 .
  • Aluminum Pocket Book - Volume 1: Fundamentals and Materials. Aluminum-Verlag, Düsseldorf, 16th edition, 2002.
  • George E. Totten, D. Scott MacKenzie: Handbook of Aluminum Volume 1: Physical Metallurgy and Processes. Marcel Dekker, Yew York, Basel. 2003.
  • Sebastian F. Fischer, Christian Oberschelp: Aluminum-based cast materials in: Andreas Bühring-Polaczek , Walter Michaeli , Günter Spur (Eds.): Handbuch Urformen. Hanser, 2014, pp. 62–66.

Individual evidence

  1. ↑ Aluminum Pocket Book - Volume 1: Basics and Materials . Aluminum-Verlag, Düsseldorf, 16th edition, 2002, pp. 145–151.
  2. ↑ Sebastian F. Fischer, Christian Oberschelp: Aluminum-based cast materials in: Andreas Bühring-Polaczek, Walter Michaeli, Günter Spur (eds.): Handbuch Urformen, Hanser, 2014, p. 21.
  3. ^ Friedrich Ostermann: Application technology aluminum. 3. Edition. Springer, 2014, p. 152 f.
  4. ^ Fritz, Schulze: Manufacturing technology , 11th edition, p. 40 f.
  5. ↑ Sebastian F. Fischer, Christian Oberschelp: Aluminum-based cast materials in: Andreas Bühring-Polaczek, Walter Michaeli, Günter Spur (eds.): Handbuch Urformen, Hanser, 2014, p. 63.
  6. ↑ Sebastian F. Fischer, Christian Oberschelp: Aluminum-based cast materials in: Andreas Bühring-Polaczek, Walter Michaeli, Günter Spur (eds.): Handbuch Urformen, Hanser, 2014, p. 63.
  7. ↑ Increased range of electric vehicles in winter. In: Website of the German Aerospace Center. Retrieved May 17, 2018 .
  8. ↑ Characteristic features of 095: Hot-dip coated strip and sheet metal. In: Website of the steel trade association. Retrieved October 11, 2019 .
  9. ↑ a b Aluminum Pocket Book - Volume 1: Fundamentals and materials . Aluminum-Verlag, Düsseldorf, 16th edition, 2002, p. 100.
  10. ^ Friedrich Ostermann: Application technology aluminum. 3. Edition. Springer, 2014, p. 182.
  11. ^ Friedrich Ostermann: Application technology aluminum. 3. Edition. Springer, 2014, p. 182.
  12. ^ Fritz, Schulze, 9th edition, p. 36.
  13. ↑ Aluminum Pocket Book - Volume 1: Basics and Materials . Aluminum-Verlag, Düsseldorf, 16th edition, 2002, p. 100.
  14. ↑ Handbuch Urformen, p. 62.
  15. ↑ Aluminum Pocket Book - Volume 1: Basics and Materials . Aluminum-Verlag, Düsseldorf, 16th edition, 2002, p. 101.
  16. ↑ Handbuch Urformen, pp. 23, 62.
  17. ↑ Aluminum Pocket Book - Volume 1: Basics and Materials . Aluminum-Verlag, Düsseldorf, 16th edition, 2002, p. 101.
  18. ↑ Sebastian F. Fischer, Christian Oberschelp: Aluminum-based cast materials in: Andreas Bühring-Polaczek, Walter Michaeli, Günter Spur (eds.): Handbuch Urformen, Hanser, 2014, p. 63.
  19. ↑ Sebastian F. Fischer, Christian Oberschelp: Aluminum-based cast materials in: Andreas Bühring-Polaczek, Walter Michaeli, Günter Spur (eds.): Handbuch Urformen, Hanser, 2014, p. 66.
  20. ↑ Aluminum Pocket Book - Volume 1: Basics and Materials . Aluminum-Verlag, Düsseldorf, 16th edition, 2002, p. 146 f.
  21. ↑ Sebastian F. Fischer, Christian Oberschelp: Aluminum-based cast materials in: Andreas Bühring-Polaczek, Walter Michaeli, Günter Spur (eds.): Handbuch Urformen, Hanser, 2014, p. 63.
  22. ↑ Aluminum Pocket Book - Volume 1: Basics and Materials . Aluminum-Verlag, Düsseldorf, 16th edition, 2002, p. 149 ff.
  23. ↑ Sebastian F. Fischer, Christian Oberschelp: Aluminum-based cast materials in: Andreas Bühring-Polaczek, Walter Michaeli, Günter Spur (eds.): Handbuch Urformen, Hanser, 2014, p. 63 f.
  24. ↑ Aluminum Pocket Book - Volume 1: Basics and Materials . Aluminum-Verlag, Düsseldorf, 16th edition, 2002, p. 637
  25. ↑ Aluminum Pocket Book - Volume 1: Basics and Materials . Aluminum-Verlag, Düsseldorf, 16th edition, 2002, p. 637
  26. ↑ Sebastian F. Fischer, Christian Oberschelp: Aluminum-based cast materials in: Andreas Bühring-Polaczek , Walter Michaeli , Günter Spur (eds.): Handbuch Urformen , Hanser, 2014, pp. 64–65.