Reference Information
Casting methods, influence of alloying elements, and phase diagrams
Aluminum — Silicon (Al-Si) Phase Diagram
1 Eutectic Point
An alloy containing 12.6% Si melts at a minimum temperature of 577°C. This allows for alloys with excellent fluidity.
2 Limited Solubility
The maximum solubility of silicon in solid aluminum is 1.65% at 577°C and drops almost to zero at room temperature.
3 Microstructure
Upon cooling in hypereutectic alloys, hard silicon crystals precipitate first, followed by a fine mechanical mixture of aluminum and silicon at the eutectic temperature.
Casting Methods
Sand Casting
Forming in a mixture of sand and binder.
- •Low tooling cost
- •Any dimensions
- •Low cost
- •Low accuracy and surface finish
- •Slow cooling (lower Rm)
Gravity Die Casting (Permanent Mold)
Pouring metal into a metal mold under gravity.
- •Fine grain (high Rm, A)
- •Good accuracy
- •High productivity
- •Mold cost
- •Complexity limitations
High Pressure Die Casting (HPDC)
Injecting melt into a steel die under high pressure.
- •High productivity
- •Thin walls (up to 1.5mm)
- •High accuracy
- •Expensive equipment
- •Gas porosity (unsuitable for T6)
Alloying Elements
Main alloying element. Improves casting properties (fluidity), increases hardness, reduces density and CTE. Forms eutectic at 12.6%.
Increases strength (especially after heat treatment) and hardness, improves machinability. Reduces corrosion resistance.
Increases strength, corrosion resistance, and weldability. In combination with Si (Mg2Si), makes the alloy heat-treatable.
Neutralizes the harmful effects of iron (converts needle-like β-AlFeSi crystals into less harmful "hieroglyphs"). Increases high-temperature strength.
Main impurity. Reduces ductility (A) and impact toughness due to brittle phase formation. In die casting, prevents sticking to the mold.
Increases strength (in Al-Zn-Mg system). In Al-Si alloys, it is usually an impurity.
Refines grain (Type 1 modifier), which improves mechanical properties and casting pressure tightness.
Modifies the Al-Si eutectic (converts Si plates into fibers), significantly increasing ductility.
Similarly modifies the eutectic. Quickly burns out from the melt.