Aluminium Cast Materials – Influence of Addition Elements

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PGF 2019
Taschenbuch der GIESSEREI PRAXIS
Materials

Influence of the addition elements

Silicon
Structure – The eutectic of the Al-Si dual-substance system is at 11.7 % Si and a temperature of 577 °C. There is no bond between aluminium and silicon in the system. The solubility of the addition element is limited in stable aluminium and is 1.65 % in equilibrium at 577 °C, while only 0.22 % is dissolved in solution at 300 °C. The decrease in solid solubility leads to the precipitation of silicon in the aluminium solid solution during crystallization. Adding small amounts of sodium metal or sodium salts, also strontium, to eutectic or near-eutectic A-Si alloys results in a particularly fine eutectic and therefore significantly improves the mechanical properties of the material. The small amount of sodium at about 0.1 % causes the eutectic concentration to shift to a higher Si content and leads to supercooling due to the impairment of crystallisation, the extent of which is determined by the cooling rate.

Pouring properties – Silicon has the most favourable influence in comparison to all addition elements. The solidification shrinkage in particular is affected (it is only about 3 % for the eutectic composi- tion compared to 6.5 % for pure aluminium). A positive influence on the shrinkage tendency and shrinkage is achieved in this way. The fluidity and mould filling capacity are changed significantly by the addition of silicon. Fluidity refers to the flowability of a melt while filling a mould. Starting with the pure metal, the fluidity of the Al-Si alloy decreases up to the maximum solubility, increases up to the eutectic and continues rising with the increasingly hypereutectic melt.
These casting technology particularities have a significant influence on the fabrication of thin-walled, complicated castings. Therefore, the Si content is in the range from 5 to 12 %. A higher content is only seen with piston alloys.

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