Table 1. Alloy Constants for Several Elements Calculated from Phase Diagrams

to remove the ice, as long as the temperature is not far below the freezing point of water.

Te same thing happens in alu-

minum. Adding a second element to pure aluminum usually lowers the melting point, as illustrated in the aluminum-silicon system. Silicon lowers the melting point of aluminum, but aluminum also lowers the melting point of silicon. Te two curves for the melting of aluminum and silicon meet at a eutectic at a composition of 12.6 weight percent silicon and a tem- perature of 1,071F (577C) (Fig. 1). At the eutectic composition and temperature, the solidifica- tion phase transformation occurs when the liquid aluminum-silicon alloy transforms into solid aluminum and solid silicon. Tis transformation occurs at a single, constant tempera- ture, as anticipated by the phase rule: f = n + 2 - p = 2 + 2 - 3 = 1. At constant pressure, three phases

can coexist in a binary (two element) system only at a single temperature and at a single composition, so the eutectic temperature is fixed (constant). An appreciable amount of sili-

con dissolves in solid aluminum at higher temperature. Te maximum solubility is seen to be 1.65 weight percent at the eutectic temperature. However, only a negligible amount of aluminum dissolves in silicon. Liquid aluminum and liquid sil-

icon are completely soluble in one another and form a single phase field represented by the “L” in Fig. 1. Using the standard terminology for this behavior, the two liquids are said to be miscible (mixable). At temperatures below the melting point of the pure metals, but above the eutectic temperature, two phase fields of solid are in contact with liquid. Tese are labeled “L+S.” On the left-hand side, solid aluminum is in contact with liquid. On the right, solid silicon is in contact with aluminum. At temperatures below the eutectic temperature, there is another two-phase field containing two solids: aluminum and silicon. Phase diagrams for the Al-Si

Element Nickel

Iron

Silicon Copper Zinc

Distribution Coefficient

0.007 0.02 0.13 0.17 0.4

Magnesium 0.51 Manganese 0.94 Niobium

1.5

Chromium 2 Hafnium

Molybde- num

2.4

Tantalum 2.5 2.5

Zirconium 2.5 Vanadium 4 Titanium

9

Melting Point Depression

-3.3 -3

-6.6 -3.4 -1.6 -6.2 -1.6 13.3 3.5 8

70 5

4.5 10

30.7

% max 6

1.8

12.6 33.2 50 34

1.9

0.15 0.4 0.5 0.1 0.1

0.11 0.1

0.15

system proposed in literature over the years disagree about the exact eutectic composition and temperature. Tis is because the formation of the Al-Si eutectic is sensitive to small amounts of impurities, especially potassium, sodium and other alkaline earth elements. Te phase diagram in this article is based on a study conducted at Alcoa.

Foundry alloys are grouped into

three classes based upon silicon content. Hypoeutectic alloys—These

alloys have a silicon content less than the eutectic composition. Most of the common alloys have between 5% and 10%. These alloys are designed primarily for high strength applications where good ductility is also required. Eutectic alloys—Tese alloys

have between 10% and 13% silicon, and consist mainly of Al-Si eutectic in the cast structure. Tey have a narrow freezing range, excellent fluidity and are easy to cast. Tey also have good wear resistance and are quite ductile when not alloyed and heat treated to high strength. Hypereutectic alloys—Tese alloys

have between 15% and 20% silicon, so their cast structure is composed of primary silicon particles imbedded in a matrix of Al-Si eutectic. Tese materi- als have remarkable wear resistance and are used where this characteristic is desired. Tey also have good high temperature strength, but are dif- ficult to machine. A more detailed look at the

Al-Si phase diagram provides a better understanding of what these characteristics mean in practice. Te most important portion of the Al-Si phase diagram for the metalcaster is shown in Fig. 2. Consideration is given to the

solidification of a typical hypoeutec- tic alloy, containing 7% silicon. Te molten metal alloy is taken from a furnace held at 1,400F (760C). Tis metal cools in the mold to a tem- perature of about 1,139F (615C). At this temperature the first solid forms in the shape of aluminum crystals containing 1% silicon. As solidification continues, the

silicon concentration in the liquid portion of the casting increases. Silicon segregates to and accumulates in the liquid phase. Tis segregation during solidification is best described by a distribution coefficient:

Fig. 2. Detail from the aluminum-silicon phase diagram is shown. The composition and temperature of both liquid and solid phases follow the arrows.

April 2014 MODERN CASTING | 35

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