Iron in the foundry aluminum alloys

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The main factors influencing iron

Iron is the main impurity, which is responsible for the low level of toughness of conventional aluminum alloys.
iron-containing phase, which is formed from a liquid in high-silicon aluminum alloys is β-FeSiAl₅.
Β-FeSiAl particles are commonly referred to as needle-shaped or needle-shaped, although in fact they are plates.
Additives in manganese alloy in quantity, equal to half the iron content, change the β-FeSiAl phase to the α-FeSiAl phase with the chemical formula (Fe, Mn)₃And₂Al₁₅. This phase is no longer needle-shaped, but something resembles a written font. She is not so harmful, like a needle, although it still remains an embrittling phase (Figure 2).

Figure 1 - β-FeSiAl needles

Figure 2 - Particles α-FeSiAl - less harmful, than β-FeSiAl needles. Raft A357

Β-FeSiAl needle length is a function of cooling rate. A measure of the cooling rate is the interdendritic distance in the alloy structure.. Higher cooling rate, the smaller the interdendritic distance.
With an increase in the iron content in the alloy, the length of the β-FeSiAl needles also increases (Fig. 3).

Figure 3 - Dependence of the length of β-FeSiAl needles on the interdendritic distance of secondary dendrides (Source - Biswal et al.)

The presence of β-FeSiAl needles in the microstructure of aluminum alloys of aluminum reduces their mechanical properties.
Most β-FeSiAl needles reduce the viscous properties of aluminum alloys, especially the toughness of secondary alloys with a high iron content (figure 4).

Figure 4 - Comparison of the mechanical properties of the A357 alloy with high (0.093 %) and low (0.055 %) iron content (Source: F. Major, Alcan)