Aluminum casting alloys should contain, in addition to the reinforcing elements sufficient amount of elements, which form a eutectic. This is necessary in order, to casting alloys have high fluidity for feeding and filling the pores and cracks during shrinkage, which inevitably arise in all castings, Besides, may be, the simplest.
Silumin – alloys of aluminum-silicon
Alloys aluminum-silicon, which do not contain additives copper, then apply, When you want good casting properties and high corrosion resistance. Often they are called silumin. Metallographic structure pure components, aluminum and silicon, and a number of intermediate chemical compositions are shown below. Intermediate compounds are aluminum mixture, which contains about 1 % dissolved silicon and the particles of almost pure silica. The alloys with a silicon content of less than 12 % called eutectic, with a silicon content of about 12 % – eutectic, more 12 % – hypereutectic.
If you need high strength and hardness, in these alloys are added magnesium, which makes them thermally reinforcing. Alloys with a low content of silicon, such as 2 %, sometimes used for castings, but usually the content of silicon in the casting alloys ranges from 5 to 10 %.
The strength and ductility of these alloys, particularly with high silicon content, It can be significantly improved by modifying the eutectic aluminum-silicon.
modification eutectic alloys (silicon content less 12 %) are especially often used in sand casting. This modification is achieved by introducing a controlled melt of sodium or strontium additives, eutectic phase which is ground. As builders are also used calcium and antimony. The hypereutectic aluminum-silicon alloys for grinding proeutectoid silicon particles and improve the quality of castings phosphorus additives are used.
The silicon content in the range of 4 to 12 % reduces losses from casting marriage, and also enables more complicated castings cast with a large difference in thickness, and also provides a higher surface quality and the inner quality of the metal. This occurs due to the influence of silicon on improving the fluidity of the alloy, reduce cracking and improve the filling of forms.
Optimal eutectic aluminum-silicon
The required amount of eutectic is also dependent on the characteristics of the casting process. Alloys for sand casting tend to have lower eutectic, than those who poured into metal molds. This is due to the fact, that the "soft" casting sand molds is less crack, than rigid metal. Increased resistance to cracking during casting contributes to a small solidification temperature range. When the content of silicon 1 % he is 78 oC, and when the content 12 % is reduced to zero.
The aluminum-silicon alloys without other elements under nonequilibrium conditions casting volume fraction of eutectic increases linearly from 0 to 1 with increasing content of silicon 1 to 12 %.