The effects of silicon in cast aluminum
Aluminum casting alloys
The tabulated chemical composition of cast aluminum alloys can include up to ten specific alloying elements, not counting the “other” or “total impurities” columns. Not all of these elements are the primary alloying elements for every aluminum alloy: some elements are major or trace elements in some alloys and may be interchanged in others. Zinc, for example, is an impurity in most cast aluminum alloys, and only in some it is the main alloying element.
The main alloying elements of cast aluminum alloys in different standards include:
Other elements may be minor elements, structure modifiers, or impurities.
The effects of silicon in casting alloys
Silicon is without a doubt the most important alloying component in the vast majority of cast aluminum alloys. These alloys owe their so-called “good casting properties” to silicon, that is, the ability to easily fill molds and harden into castings without the formation of hot cracks.
The important role of silicon as an alloying element in cast aluminum alloys is as follows:
- The high latent heat of solidification of silicon ensures good or satisfactory fluidity of the alloy as a whole.
- 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 the fact that for an alloy, even with a content of several percent silicon, solidification occurs mainly in a regime close to isothermal.
- At the same time, the cast aluminum alloy achieves significant strength because it undergoes little or no thermal shrinkage, which is very important for preventing the formation of hot cracks.
- The more silicon an aluminum alloy contains, the lower its coefficient of thermal expansion.
- Silicon is a very hard phase, so it makes a significant contribution to the wear resistance of an aluminum alloy.
- Silicon compounds with other elements, such as magnesium, increase the strength of the aluminum alloy and make it thermally hardenable.
- High silicon content can lead to dimensional instability of the casting, especially at elevated temperatures.
Isothermal hardening
Pure aluminum hardens “isothermally,” that is, at a constant temperature. Eutectic compositions (aluminum and 12% silicon, such as normal silumin) also harden almost “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 casting cross-section. 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 for hot cracks to form.
Silicon heals hot cracks
The presence of silicon generally prevents hot cracking and also improves the fluidity of cast aluminum alloys. As little as 5% silicon in the alloy provides a sufficient degree of isothermal solidification to eliminate the formation of hot cracks and, at the same time, increase the fluidity of the alloy. Foundry workers often refer to aluminum alloys with a wide range of solidification temperatures as “difficult to cast.” However, what makes them difficult is not the wide temperature range of solidification, but rather the characteristic, non-isothermal shape of the cooling curves, as well as insufficient fluidity. Both of these problems are caused by a lack of sufficient silicon. American cast aluminum alloy 332 (9.5%Si-3.0%Cu-1.0%Mg) has a relatively wide solidification temperature range, but since it contains a significant amount of silicon, it has a good fluidity and near isothermal solidification.
Cast aluminum alloys with a high silicon content (American series 3xx and 4xx) spend a significant part of their solidification on the eutectic “platform” of the cooling curve. When cooling reaches temperatures below this “platform”, a large proportion of the solid alloy has already formed and only the phases with the lowest solidification temperatures still remain liquid (usually eutectics involving copper and/or magnesium). By this point, the alloys have already managed to form a sufficiently hard and durable structure. This structure is able to successfully resist shrinkage during the remaining cooling from the eutectic “platform” to complete solidification without the formation of hot cracks.
Silicon plus magnesium
- Silicon itself makes a very small contribution to the strength of cast aluminum alloys.
- However, when combined with magnesium in the form of Mg2Si, silicon provides a very effective strengthening mechanism in aluminium castings.
Some influences of silicon
- With increasing silicon content, the coefficient of thermal expansion of the alloy, as well as its density, decrease.
- Silicon increases the wear resistance of the aluminum alloy, which often makes aluminum-silicon alloy castings an attractive replacement for gray cast irons.
A downside
The importance of silicon’s contribution to improving the casting properties of aluminum alloys also has a downside:
- The more silicon there is in the alloy, especially in the hypereutectic range, the greater the wear of the cutting tool during machining.
- With polycrystalline diamond cutting materials, the problem has ceased when choosing a suitable casting alloy.
- When processing castings with cutting tools made of high-speed steels, carbide cutting tools and other less wear-resistant materials, this circumstance must be taken into account.
Apelian D. Aluminum Cast Alloys, NADCA, 2009
