Aluminum-silicon casting alloys

Aluminum alloys with silicon as the main alloying element are the most important category of alloys used in the casting process, where the use of silicon in aluminum offers excellent properties such as castability, good weldability, good thermal conductivity, excellent corrosion resistance and good retention of physical and mechanical properties at elevated temperatures.  The addition of silicon as a dominant alloying element produces castings with physical and mechanical properties which are of great pertinence to industrial requirements. Aluminum-silicon (Al-Si) alloys experience an increase in mechanical resistance with the addition of copper, magnesium or nickel and by applying adequate heat treatments. These characteristics of Al-Si castings are the main reason for their acknowledged versatility in several fields of industrial application where Al-Si cast parts form as much as 85-90% of the total of cast aluminum pieces produced.

Commercial alloys of this type represent the full range of compositions, including hypoeutectic (5-10% Si), eutectic (11-13% Si) and hypereutectic (14-20% Si) .Thus, in accordance with the level of silicon present, the microstructural features of each group differ from one another. Consequently, the mechanical properties of these groups of alloys/castings are directly related to the specific features of the microstructure in each case. The main feature of Al-Si casting alloys is that a eutectic is formed between aluminum and silicon at a Si content of 11.5-12%.

In hypoeutectic alloys, the α-Al precipitates in the form of a dendritic network, followed by the precipitation of eutectic Al-Si in the interdendritic regions,In the case of eutectic alloys, the entire cast structure consists mainly of an Al-Si eutectic structure, Hypereutectic alloys are characterized by the precipitation of primary silicon cuboids followed by the solidification of the eutectic structure ,Hypoeutectic and  eutectic Al-Si casting alloys are the types most commonly used in industrial applications, whereas hypereutectic alloys are used principally for high wear resistance applications.

With regard to this type of alloy, it is necessary to control a series of parameters which have a direct effect on alloy properties. As mentioned before, these variables are related to the microstructure and depend as much on the alloy composition as on the casting process used. These include: (a) dendrite arm spacing (DAS), (b) level of modification of eutectic silicon, (c) grain size, and (d) gas porosity and shrinkage.

354 – Al-Si-Cu-Mg ALLOY SYSTEM 

it may be seen that the solidification process of Al-Si hypoeutectic alloys includes the formation of the α-Al dendritic network and the Al-Si eutectic reaction to produce the Al-Si eutectic.   Aluminum-silicon alloys containing copper and magnesium such as the permanent mold-cast 354-type alloys show a greater response to heat treatment as a result of the presence of both Mg and Cu. Within Al-Si alloys, Al-Si-Mg, Al-Si-Cu, and Al-Si-Cu-Mg are the three major alloy systems in the 3xx series, of which A356, A319, and B319 are typical examples. The main function of Mg and Cu is to form the Mg2Si and Al2Cu hardening precipitates.

Addition of Cu leads to a slight increase in alloy fluidity, and a depression in the Si eutectic temperature of ~1.8°C for every 1wt% Cu added. Also, a number of the mechanical properties, including YS and UTS, obviously benefit from the addition of Cu as an alloying element.  The presence of magnesium improves strain hardenability, while also enhancing the material strength by solid solution.  At~548°C, the amount of Cu in solid solution in Al is known to be about 5.7%; this value decreases with decreasing temperatures, reaching 0.1-0.2% at 250°C.  Copper forms an intermetallic phase with Al which precipitates during solidification either as block-like CuAl2 or in its eutectic form as (Al + CuAl2). In the 319 alloys, the copper intermetallic phase precipitates in both of these forms.  At the same time, because of the presence of certain impurity elements such as Fe and Mn, intermetallic phases also precipitate during solidification.