Siluminami called a group of aluminum alloys with a relatively high silicon content. Often, silumin means a narrower group of alloys containing silicon 12-13 %. It:
- eutectic silumins, which are also called normal or normal silumin..
There are also:
- hypoeutectic silumins (containing silicon 4-10 % with copper, magnesium and manganese),
- wear-resistant hypereutectic silumins (with silicon content up to 20 % with copper, magnesium and nickel), and
- special silumins, for example, zinc silumin.
Eutectic silumins have a silicon content in the range of 10 to 13 %, moderate strength properties, but quite high elongation for cast alloys. Their main advantage over other cast aluminum alloys, including other types of silumins - very good casting properties and, first of all, high fluidity. These casting alloys are very suitable for thin-walled casting., complex in shape, sealed, resistant to vibration and shock loads.
Eutectic Silumine Casting
Of all the aluminum-silicon alloys, these alloys, containing about 13 % silicon, have the best fluidity. These alloys have their own technological features..
In the case of free hardening, these alloys form a dense, bell-shaped surface on top of the ingot. In this type of solidification, crystallization begins with the formation of a hard shell, which then grows deeper into the casting. This type of alloy has only two states – “Solid” and “liquid”. The cast hardens completely at a eutectic temperature of about 577 S.
These eutectic silumins can be modified with sodium. Sodium modification is commonly used in sand casting and chill casting., if increased demands are made on castings for elongation of the cast microstructure. Usually, foundry alloys for sand casting and chill casting are always used in a modified chemical composition.
The effect of iron on silumins
Chemical resistance, atmospheric resistance, including, nautical, increases with increasing purity of the alloy used. Therefore, in such applications as the food industry or shipbuilding, only primary aluminum alloys are used. The elongation of the molded microstructure substantially depends on the content of iron and other impurities. therefore, for, to guarantee high strength castings, only primary alloys with a minimum content of iron and other impurities are used.
Heat treatment of silumins
These alloys do not have the ability to heat harden due to the aging mechanism.. However, when casting castings in sand molds and casting in chill molds from cast alloys with a low content of copper and magnesium, sometimes an improvement in ductility can be achieved with . This is achieved by annealing at a temperature 520-530 C for the formation of a solid solution of alloying elements with subsequent cooling in cold water.
Silumin in EN standards 1676 и EN 1706
The chemical composition of cast aluminum alloys is set by two European standards:
- EN 1676 - for cast aluminum alloys in pigs and
- EN 1706 - for cast aluminum alloys in castings.
Basic eutectic silumins according to EN standards 1676 и EN 1706 presented in the figure 1. In EN standards 1676 и EN 1706 and to the digital designation, and to the chemical designation of the alloy are added "prefixes" EN AB- и EN AC-, respectively. for instance, alloy 44200 it looks like this:
EN AB-44200 and EN AC-44200;
EN AB-Al Si12 (a) and EN AC Al Si12 (a).
These standards allow a very wide range of the main silicon alloying element - from 10,5 to 13 %. The practical range of silicon content is from 12,5 to 13,5 %, as well as slightly hypereutectic interval from 10,5 to 11,2 %. Important, that alloys from these two intervals exhibit completely different solidification behavior. With an intermediate interval of silicon content from 11,5 to 12,5 % there is a high risk of shrinkage porosity. Therefore, the use of alloys in this interval is not recommended.
Eutectic silumins in accordance with GOST 1583-93
GOST 1583-93 defines the requirements for both types of aluminum casting alloys: and in ingots, and in castings.
All silumins in ingots have a silicon composition close to eutectic - from 10 to 13 %.
As the content of iron (and other impurities) increases, silumins in ingots (from the former GOST 1521-68) are in the following order:
- AK12och (SIL-00) - iron content up to 0,20 %;
- AK12pch (SIL-0) - iron content up to 0,35 %;
- AK12ch (SIL-1) - iron content up to 0,50 %;
- AK12zh (SIL-2) - iron content up to 0,7 %.
alloys, which are used in pigs, and in castings, alloy AK12 (AL2) belongs to eutectic silumin. The permissible iron content in this alloy depends on the type of product (ingot or casting), as well as casting method. When casting in sand molds and in investment casting, the permissible iron content should be no more than 0,7 %, and when casting under pressure - no more 1,5 %.
Properties of eutectic silumins
Technological, physical and mechanical properties of eutectic silumins, as well as typical technological parameters of their casting are presented in the figures 2, 3, 4 and 5.
A source: Aleris International, 2014