Laying a Foundation for Solidification

Phase diagrams of aluminum alloys provide a picture of castability and expected properties before you pour the casting.

Geoffrey Sigworth, GKS Engineering Services, Dunedin, Florida

This article is the first in a three-part series on solidification in aluminum castings. While the series focuses on solidification principles in aluminum alloys, many can be applied to other metals, as well.

(Click here to see the story as it appears in April's Modern Casting.)

The metalcasting industry is concerned primarily with the solidification process, which essentially is a phase transformation from the hot, liquid state to a colder, solid state. Phase diagrams tell us a great deal about how this transformation occurs. This gives us clues about castability as well as the properties in the finished product. For example, they tell us about:

  • What phases form.
  • At what temperatures the phases form.
  • The composition of phases and how solute elements are distributed between the phases.
  • How difficult it will be to place a specific alloying element into aluminum.

If pure aluminum is slowly heated, it remains solid until it reaches 1,220F (660C). Then, it starts to melt but remains at 1,220F until all the metal is molten. Once it is fully liquid, it can be heated to higher temperatures.

This situation is akin to melting ice or placing ice cubes in a glass of water. Ice and liquid water coexist only at a single temperature: the melting point. The liquid temperature always is above this point, and the solid temperature always is below.

One way to describe the situation is the phase rule (p + f = n + 2); where p is the number of phases present, f is the number of degrees of freedom, and n is the number of components present).

For a pure metal, the number of components (n) is equal to one. When both solid and liquid are present, the number of phases (p) is equal to 2. Therefore, the number of degrees of freedom (f) must be equal to 1. However, in practice, the pressure is fixed by the prevailing atmospheric pressure, which uses up the one degree of freedom. In other words, the melting temperature is not free to vary or change, as long as two phases are present in a pure material.

If a pure metal was melted in a high pressure furnace in a lab, the melting point would increase. Aluminum exhibits about a 7% volume increase when it melts. Higher pressures would make it more difficult to melt metal by opposing this volume increase. The single degree of freedom means as long as the pressure is fixed, the melting point is fixed.

According to the phase rule, when a second element is dissolved in aluminum, we have an additional degree of freedom. In this case, the melting point can change. Those who live, or have lived, in cold climates are familiar with the practice of adding salt to icy sidewalks and driveways to melt ice in the winter. Salt dissolves in water, lowering its melting point. This makes it easier to remove the ice, as long as the temperature is not far below the freezing point of water.

The same thing happens in aluminum. 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. The two curves for the melting of aluminum and silicon meet at a eutectic at a composition of 12.6 weight percent silicon and a temperature of 1,071F (577C) (Fig. 1).

At the eutectic composition and temperature, the solidification phase transformation occurs when the liquid aluminum-silicon alloy transforms into solid aluminum and solid silicon. This transformation occurs at a single, constant temperature, 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 silicon dissolves in solid aluminum at higher temperature. The 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 silicon 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. These 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 system proposed in literature over the years disagree about the exact eutectic composition and temperature. This is because the formation of the Al-Si eutectic is sensitive to small amounts of impurities, especially potassium, sodium and other alkaline earth elements. The 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—These alloys have between 10% and 13% silicon, and consist mainly of Al-Si eutectic in the cast structure. They have a narrow freezing range, excellent fluidity and are easy to cast. They also have good wear resistance and are quite ductile when not alloyed and heat treated to high strength.

Hypereutectic alloys—These 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. These materials have remarkable wear resistance and are used where this characteristic is desired. They also have good high temperature strength, but are difficult to machine.

A more detailed look at the Al-Si phase diagram provides a better understanding of what these characteristics mean in practice. The 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 hypoeutectic alloy, containing 7% silicon. The molten metal alloy is taken from a furnace held at 1,400F (760C). This metal cools in the mold to a temperature 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. This segregation during solidification is best described by a distribution coefficient:

The phase diagram tells us that, at equilibrium, the silicon content in solid aluminum is 13% of that found in the surrounding liquid. The other 87% remains in the liquid, where it accumulates. And as the silicon content increases in the liquid, its melting point decreases. Hence, the composition and temperature of both solid and liquid phases follow the arrows in Fig. 2. This segregation continues until the liquid contains 12.6% Si and cools to the eutectic temperature. At this point, a eutectic mixture of solid Al and Si forms.

Another important factor that can be determined from the phase diagram is the depression of the melting point of aluminum. This is defined by the slope of the liquidus curve and by this equation for the Al-Si system:

For silicon in aluminum, m is equal to 11.9F (6.6C) per weight percent Si.

The last important factor is the solubility of the element in liquid aluminum at typical furnace temperatures. For silicon, this maximum concentration is equal to the eutectic composition, or12.6 weight percent Si.

These three factors have been tabulated for a number of important or interesting alloying elements and are shown in Table 1.

  • Several important and interesting things may be gleaned from Table 1.
  • Nickel, iron, silicon and copper segregate strongly during solidification.
  • Zinc and manganese segregate only moderately.
  • Manganese hardly segregates at all. The concentration of manganese in solid aluminum is 94% of the liquid. This is an important factor in the improved performance of diecasting alloys, where manganese replaces iron to prevent die soldering.

The elements below manganese have a value of k greater than one. This means there is a “negative” segregation—the equilibrium concentration in the solid is greater than that in the liquid. As a result, the melting point of aluminum increases.

When another element is added to a binary alloy, there is a ternary (three element) system. It is somewhat more complicated to read ternary phase diagrams, but it is often useful to consult them.

Figure 3 shows the liquidus surface for aluminum-rich alloys in the ternary Al-Zn-Mg system. This diagram is similar to a topographic map used for hiking or hunting outdoors. The contours show the temperature (C) at which solid aluminum begins to form during solidification.

A full ternary diagram is an equilateral triangle, but since the interest here is in aluminum-rich alloys, the top portion of the triangle (corresponding to magnesium-rich compositions) has been removed. The key to ternary diagrams is reading the composition coordinates. The diagram shows two ternary eutectics which will be used for instruction in the correct procedure.

Ternary eutectics are similar to the binary eutectics. However, the additional component adds another degree of freedom according to the phase rule. Thus, a ternary eutectic occurs only with this reaction:

The formation of three solid phases in the eutectic means the reaction occurs at a fixed temperature and composition.

At the top left of the phase diagram there is a ternary eutectic at 837F (447C). If a line is drawn from this point parallel to the sloping left edge, this line intersects the scale at the bottom at about 13% zinc. If a horizontal line is drawn parallel to the bottom edge, it intersects the left edge at about 31% magnesium. Thus, this ternary eutectic contains 13% zinc, 31% magnesium and (by difference) 56% aluminum.

There is a second ternary eutectic in the lower right-hand side of the diagram at a temperature of 887F (475C). A similar procedure shows this eutectic contains approximately 61% zinc, 13% magnesium and 26% aluminum.

Phase diagrams are useful to show what phases form during solidification and the relationship between the phases. Understanding what happens with multiple-element alloy systems will help the metalcaster derive practical conclusions about selecting, feeding, pouring and heat treating commercial castings.   

The next article in this series on solidification in aluminum castings will focus on dendritic solidification.  The paper this article is based on was originally presented at the 117th Metalcasting Congress.