Silicon in casting aluminum alloys

Casting aluminum alloys

Tabular chemical composition foundry aluminum alloys can include up to ten specific alloying elements, not counting the column "others" or "sum of impurities". Not all of these elements are the main alloying elements for every aluminum alloy: some elements are basic or impurities in some alloys and can be swapped in others.. Zinc, for example, in most cast aluminum alloys is an impurity, and only in some - the main alloying element.

The main alloying elements of cast aluminum alloys in different standards include:

• silicon,
• copper,
• magnesium,, and
• zinc i
• lead.

Other elements can be secondary elements, structure modifiers, as well as impurities.

The role of silicon in the casting alloys

Silicon undoubtedly the most important alloying component in the vast majority casting aluminum alloys. These alloys owe their so-called "good casting properties" to silicon., that is, the ability to easily fill molds and solidify into castings without hot cracking.

Important the role of silicon as an alloying element of cast aluminum alloys is as follows:

1. The high latent heat of solidification of silicon ensures good or satisfactory fluidity of the alloy as a whole.
2. Silicon has limited solubility in solid solution (maximum 1,65 %) and forms a eutectic with aluminum at a fairly high content (12 %). This leads to, that the alloy, even with a content of several percent silicon, solidification occurs mainly in the regime, close to isothermal. In this case, the cast aluminum alloy reaches significant strength., because they undergo little or no thermal shrinkage, which is very important to prevent hot cracking.
3. The more silicon the aluminum alloy contains, the lower its coefficient of thermal expansion.
4. Silicon is a very hard phase, therefore it makes a significant contribution to the wear resistance of the aluminum alloy.
5. Silicon compounds with other elements, for example, with magnesium, increase the strength of the aluminum alloy and make it thermally hardened.
6. High silicon content can lead to dimensional instability of the casting, especially at elevated temperatures. therefore, for example, when using casting aluminum alloy AL2 for high-precision instrument parts, provide a special stabilizing heat treatment.

Almost isothermal solidification

Pure aluminum hardens "isothermally", that is, at constant temperature. Eutectic compounds (aluminum and 12 % silicon, as, for example, normal silumin) also solidify practically "isothermally", that is, in a very narrow temperature range.

Eutectic aluminum alloys solidify gradually from the surface of the casting mold towards the thermal center of the cross section of the casting. They are characterized by a very small front thickness between the already solidified part of the casting and the remaining liquid metal.. This hardening minimizes the tendency to hot cracking..

Silicon heals hot cracks

The presence of silicon usually prevents hot cracking, and also improves the fluidity of cast aluminum alloys. Only 5 % silicon in the alloy provides a sufficient degree of isothermal solidification, to avoid hot cracking and, in the same time, increase the fluidity of the alloy. Castors often refer to aluminum alloys with a wide solidification range as "difficult to cast.". However, it is not the wide temperature range of solidification that makes them difficult., rather a characteristic, non-isothermal, cooling curves, as well as insufficient fluidity. Both of these problems – from lack of sufficient silicon. American Casting Aluminum Alloy 332 (9.5% Si-3.0% Cu-1.0% Mg), its closest domestic analogue is AL25, has a relatively wide temperature range of solidification, but since it contains a significant amount of silicon, it has good fluidity and close to isothermal solidification.

Cast aluminum alloys with a high silicon content (American series 3xx and 4xx, groups I and II according to GOST 1583-93) a significant part of their solidification is "spent" on the eutectic "area" of the cooling curve. When cooling reaches temperatures below this "area", a large proportion of the hard alloy has already formed and only the phases with the lowest solidification temperatures are still liquid (usually eutectics with the participation of copper and / or magnesium). By this time, the alloys have already managed to form a sufficiently hard and durable structure.. This structure is able to successfully resist shrinkage with the remaining cooling from the eutectic "site" until complete solidification without the formation of hot cracks.

Silicon and magnesium in aluminum alloys

Silicon itself makes very little contribution to the strength of cast aluminum alloys.. However, in combination with magnesium in the form of Mg₂Si silicon provides a very effective hardening mechanism in aluminum castings.

Influence of silicon on the properties of aluminum alloys

With an increase in the silicon content, the coefficient of thermal expansion of the alloy, like its density, decrease.

Silicon improves wear resistance of aluminum alloy, which often makes aluminum-silicon alloy castings an attractive replacement for gray cast irons, for example, in the automotive industry. for instance, hypereutectic American alloy 390 widely used for the manufacture of engine parts, pumps, compressors, pistons and gearboxes.

Silicon and cutting tool. The importance of the contribution of silicon to improving the casting properties of aluminum alloys also has a downside.. The more silicon in the alloy, especially in the hypereutectic interval, the greater the wear of the cutting tool during its machining. With the advent of polycrystalline diamond materials, the problem of cutting tool wear has ceased to be so urgent when choosing a suitable casting alloy.. However, when processing castings with a cutting tool made of high-speed steels, carbide cutting tools and other less wear-resistant materials, this circumstance must be taken into account.

 

Metallic silicon

The source: Apelian D. Aluminum Cast Alloys, NADCA, 2009