Geoffrey Sigworth, GKS Engineering Services, Dunedin, Florida
This article is the first in a two-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.
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.
Dendrites’ Clues for Castability
A book on snowflakes at the Carnegie Library in Pittsburgh depicts many photographs of individual snowflakes. In the introduction, the author claims each snowflake is unique, and no two crystals are alike. This claim may be true, in spite of the incredibly large number of snowflakes that form each winter. The variety of snowflakes shown in the book is mind boggling.
Something similar happens every time metal solidifies in the mold. The liquid-to-solid transformation involves the formation of many small, individual crystals of solid aluminum. This is a fascinating area, one which has received a great deal of study. A brief overview will be given here, touching on the aspects of solidification most important to the casting industry.
Earlier, this article described the use of phase diagrams to see the sequence of phases forming during solidification, which influences final casting properties and can provide insight into castability issues. Even more about alloy properties can be gleaned from delving deeper into how the structure of a metal changes as it freezes. This knowledge is an important tool in choosing alloying combinations for your desired result.
The solid aluminum crystals forming during solidification are like snowflakes. The metallurgists first observing these crystals thought they resembled trees and called them dendrites, after the Greek word for tree (δένδρον or déndron). Dendrites were first observed by polishing metal samples or by etching the polished surface. More recently, real-time X-ray studies have observed the in situ formation of dendrites in Al-Cu alloys. Because the aluminum crystal contains much less copper than the surrounding liquid, they appear lighter in X-ray images. Examples are shown in Fig. 4.
The formation of dendritic crystals is a curious phenomenon, and many scientists have studied them. The technical literature in this area is extensive; however, a relatively simple explanation will suffice to understand what is happening.
One important clue is that pure metals do not form dendrites. But when silicon or other elements are alloyed to aluminum, dendrites appear. From the Al-Si phase diagram, only 13% of the silicon in the liquid metal remains in the first solid. This means that the silicon atoms pile up in front of the growing solid crystals. The situation is shown schematically in Fig. 5. In keeping with the snowflake analogy, the growing aluminum grain is represented by a snow plow.
Consider the act of shoveling snow. When a shovel is pushed, the snow quickly piles up in front, so one can go no farther. Years ago, sidewalks in some cities were cleared of snow by a horse-drawn plow. The plow would use a “V”-shaped blade the width of the sidewalk which easily cut through the snow, pushing it to the sides of the walkway. This is shown schematically in Fig. 6. Dendrites act much like this “V”-shaped plow. In other words, growing aluminum crystals adapt a dendritic shape as a response to the alloy composition.
Growing solid crystals adapt a planar or a non-planar (dendritic) shape depending on the interaction of two factors.
The growth rate of the crystal. This is usually defined as the velocity of motion of the solid/liquid interface, in microns per second (R), and is controlled by the thermal gradient in front of the crystal (G).
The rate at which the “piled up” solute elem
ents can be removed, by diffusion, from the solidifying front.
The shape of the solidifying aluminum depends on the amount and type of solute dissolved in the alloy. The grain size also is influenced by the presence of growth-restricting solutes, like Si and Cu. This may be seen by comparing the grains of different Al-Cu alloys in Fig. 7. These alloys were solidified at an average cooling rate of 1.8F (1C) per second. All four figures are shown at the same magnification. Compare this to the crystals in Fig. 4, which are new and just forming. The arms on the branches of the dendrites are fine, much like needle-shaped leaves on a Christmas tree. Also, the dendrites are growing freely into liquid metal. They are still largely unimpeded by neighboring grains.
At some point, however, the “trunks” of the dendrites come in contact with neighboring grains. (This type of contact is called “dendrite coherency”.) After this time, any further solidification (and growth of dendrites) can occur only by thickening of the leaves and branches on the dendrite. As a result, the dendrites in the final casting are thicker. The spacing between arms also becomes larger.
It has long been known that the spacing of arms of the dendrite in the casting depends on the solidification time. One of the first detailed studies was published in 1963 by Alcoa researchers who related dendrite cell size to the solidification time.
Many of the early papers reported cell size in their studies. However, it is now known that a better measure is the secondary dendrite arm spacing (SDAS). The easiest way to measure SDAS is to use the linear intercept method. This is illustrated in Fig. 8 for a modified Al-7% Si alloy. Lines are drawn on a micrograph where well defined dendrite arms can be observed, and the average spacing between the centers of adjoining arms is measured. Typically, a number of measurements are made and the results averaged.
The SDAS can be used to determine the local solidification time at any point in a casting. The results of many commercial and laboratory measurements on Al-Cu alloys have been reviewed. Results from castings made from 356 and 319 alloys are also shown in Fig. 9, where measurements of SDAS are plotted versus the local solidification time (as measured by thermocouples in the casting).
It can be seen that, for a given freezing rate, the copper-containing 319 alloy has a somewhat smaller SDAS than the 356 alloy. The correlation for most other foundry alloys would probably lie somewhere between these two curves. The ability to measure SDAS, and the correlations shown in Fig. 9, represents a useful tool. It can help in learning about the thermal history of a sample from an “unknown” casting (e.g., a competitor’s product) or from one’s own castings. It may not always be convenient to place thermocouples in the mold, but the solidification time at various points in the casting can be estimated from the SDAS.
The dendritic structure is often visible if you look carefully into pores on the fracture surface of tensile bars. An example is shown in Fig. 10. The rounded ends of the secondary dendrite arms are sticking out from the left hand-side of this picture. The SDAS in the sample appears to be between 40 and 50 microns, which corresponds to a local solidification time of about two minutes (for an A356 alloy). ■
The paper this article is based on and was originally presented at the 117th American Foundry Society Metalcasting Congress.