Unlike wrought aluminum alloys, cast alloys have a much higher content of alloying elements.
Copper and silicon in aluminum
Strength and castability
Of all cast aluminum alloys, aluminum-copper-silicon system alloys have the widest application. These alloys have good casting properties. The combination of silicon and copper allows the use of heat treatment to improve mechanical properties.
The amount of both copper and silicon in these alloys varies widely. Therefore, in some aluminum-copper-silicon alloys, copper predominates, while in others, silicon predominates.
In these alloys:
- Copper provides strength, and
- Silicon improves castability and reduces hot cracking.
Effect of copper
Aluminum-copper-silicon alloys with a copper content of more than 3-4% are thermally hardenable. However, heat treatment is usually only used if these alloys also contain magnesium, which increases their susceptibility to heat treatment.
Effect of silicon
Alloys with a high silicon content (more than 10%) have low thermal expansion, which is an advantage for products operating at elevated temperatures. When the silicon content exceeds 12-13%, up to 22%, primary silicon crystals are present in the alloy. When these crystals are properly distributed, they provide the alloy with high wear resistance. These hypereutectic alloys are used, for example, to make automobile engine blocks.
Effect of magnesium
Many Al-Si-Cu system alloys also contain magnesium. This gives them high heat resistance with operating temperatures of 250-275ºC. These alloys are inferior to Al-Si and Al-Si-Mg alloys in casting properties, corrosion resistance and tightness. They do not require modification or crystallization under pressure.
Casting aluminum alloy 380.0
magnesium: 0.10% max;
manganese: 0.50% max;
iron: 2.0% max.$
Nickel: 0.50% max;
zinc: 3.0% max;
Tin: 3.5% max;
others: 0.50% max;
aluminum: the rest.
Impact of impurities
- High iron content reduces ductility.
- Impurity levels can reach relatively high levels before their significant effects begin to appear.
Typical mechanical properties:
tensile strength: 330 MPa;
yield strength: 165 MPa;
relative elongation: 3%;
poisson’s ratio: 0.33;
modulus of elasticity: 71.0 GPa.
density: 2.71 g/cm3;
liquidus temperature: 595 ºС;
solidus temperature: 540 ºС.
melting point: from 650 to 760 ºС;
casting temperature: from 635 to 705 ºС;
annealing temperature to increase ductility: 260-370 ºС,
exposure – 8 hours, cooling with a stove or in calm air;
tempering temperature for relieving internal stress: 175-260 ºС,
exposure – 4-6 hours, cooling in still air.
Aluminum-silicon and aluminum-copper phase diagrams
Fifure 1 – Aluminum-silicon phase diagram