Chapter 10 - Fundamentals of Metal Casting

What is a chill in casting?

A chill is a heat sink placed to encourage rapid freezing in certain regions of the casting.

Explain why, as shown in Table 10.1, gray cast iron undergoes expansion rather than contraction during solidication.

As gray cast iron solidies, a period of graphitization occurs during the final stages. This causes an expansion that counteracts the shrinkage of the metal during solidification, and results in an overall expansion.

What practical demonstrations can you suggest to indicate the relationship of the solidifcation time to the volume and surface area of a casting?

By the student. As an example, consider the following: If a swimming pool is lled with water and an equal volume of water is spread on a road were both subjected to a temperature below freezing, it is easy to see which would freeze rst. Students should give other examples to illustrate this situation.

Utilizing the equipment and materials available in a typical kitchen, design an experiment to reproduce results similar to those shown in Fig. 10.11. Comment on your observations.

By the student. For example, a simple experiment can be performed with melted chocolate and a coffee cup. If a parting agent (oil) is rst sprayed inside the cup and then molten chocolate is poured into the cup, after a short while the molten center portion can be poured out of the cup, leaving a solidied shell. This effect can be made more pronounced by using cups that have been chilled in a freezer. It is also interesting to investigate object shapes with steps as those shown in Fig. 10.11 on p. 248.

In casting metal alloys, what would you expect to occur if the mold were agitated (vibrated) aggressively after the molten metal had been in the mold for a sufficient period of time to form a skin?

By the student. Several effects can occur. The most obvious is that any dendrites which may exist in the slushy phase will be broken up by the agitation. Also, agitation will aid in more rapid cooling, because of the increased contribution of convection and also because the mold/casting interface will have a lower thermal resistance.

Assume that you are an instructor covering the topics described in this chapter, and you are giving a quiz on the numerical aspects to test the under- standing of the students. Prepare two quantitative problems and supply the answers to them.

By the student. This is a good, open-ended question that requires considerable focus and understanding on the part of the students, and has, in the past, been found to be a very valuable homework problem.

Pure copper is poured into a sand mold. The metal level in the pouring basin is 10 in. above the metal level in the mold, and the runner is circular with a 0.4-in. diameter. What are the velocity and rate of the flow of the metal into the mold? Is the flow turbulent or laminar?

Equation (10.3) on page 243 gives the metal flow; assuming the pressure does not change appreciably in the channel and that there is no friction in the sprue, the flow is

Explain the effects of mold materials on fluid flow and heat transfer in casting operations.

Heat transfer and fluid flow have a direct effect on the formation or suppression of defects in metal casting. Defects such as porosity (due to either shrinkage or gas), hot tears, and misruns (i.e., when the molten metal in a casting freezes before the mold is completely lled, shutting of that portion of the mold) are all controlled by these factors. Furthermore, the grain structure (hence properties such as strength and toughness of a metal casting) is dependent upon the rate and direction of heat transfer.

Explain why a casting may have to be subjected to various heat treatments.

Heat treatments (described in Chapter 4) such as quenching and tempering, among oth- ers, are carried out to optimize the grain structure of metal castings, thereby controlling and enhancing mechanical properties. Heat treating can control microporosity, which is a main reason that castings are weak in tension.

It is known that pouring metal at a high rate into a mold can have certain disadvantages. Are there any disadvantages to pouring it very slowly?

If a metal is poured too slowly it may solidify while it is still in the gating system or before completely lling the mold cavities. This will result in an incomplete or partial casting. This situation can be overcome by using a mold with a lower thermal conductivity or a preheated mold, but these lead to reduced mold life and longer cycle times.

Describe the stages involved in the contraction of metals during casting.

The stages involved in the contraction of metals during casting are outlined in Section 10.5.2 on p. 248. The student is encouraged to elaborate further regarding these stages, also providing some data from the technical literature for purposes of comparison.

The downsprue leading into the runner of a certain mold has a length = 175 mm. The cross sectional area at the base of the sprue is 400 mm2. The mold cavity has a volume = 0.001 m3. Determine (a) the velocity of the molten metal flowing through the base of the downsprue, (b) the volume rate of flow, and (c) the time required to fill the mold cavity.

Solution: (a) Velocity v = (2 x 9815 x 175)0.5 = (3,435,096)0.5 = 1853 mm/s (b) Volume flow rate Q = vA = 1853 x 400 = 741,200 mm3/s (c) Time to fill cavity TMF = V/Q = 1,000,000/741,200 = 1.35 s

Are external chills as effective as internal chills? Explain.

The answer depends on the location of the chills in the mold (see Fig. 10.14 on p. 251). That is, if a surface needs to be chilled (say, for example, to directionally solidify a casting), then an external chill is as effective as an internal chill. Often, however, chilling is required at some depth beneath the surface of a casting. For this condition an internal chill would be more eective.

Explain why the constant C in Eq. (10.7) depends on mold material, metal properties, and temperature.

The constant C in Eq. (10.7) on p. 247 takes into account various factors such as the conductivity of the mold material and external temperature. For example, zircon sand (zirconium silicate) has a higher thermal conductivity than basic silica sand; as a result, a part cast in a zircon mold of equal volume and surface area to that of a part cast in silica will require less time to solidify.

How can you tell whether cavities in a casting are due to shrinkage or entrained air bubbles?

The simplest method is observing them under a microscope. Air bubbles will have sufficient surface tension while the metal is liquid to form a spherical cavity, whereas shrinkage pores will be far more jagged because they are formed by localized fracture of the solidied metal. There are other tests that can be performed as well, for example, shrinkage cavities will theoretically be under a vacuum, whereas an air bubble will be lled with gas. Therefore, casting can be performed in the presence of a gas that can be traced, such as argon or helium. The casting can be remelted in vacuum and outgassing of argon or helium can be measured.

Describe the drawbacks to having a riser that is (a) too large and (b) too small.

A riser that is too large wastes material, adds to the solidication time and will require additional nishing operations. Also, a large riser can adversely affect the solidication pattern and lead to voids or cold shuts in the casting. In addition, large risers may be difficult to locate in the sand mold. On the other hand, a riser that is too small may not provide sufficient molten metal to compensate for solidication shrinkage in the casting. Also, it may solidify prematurely, so that it fails to serve as a riser; it may not influence the solidication front; and it may require higher preheat levels, leading to more shrinkage pores and hence lower casting quality.

Which of the following considerations are important for a riser to function properly? Must it: (a) have a surface area larger than the part being cast, (b) be kept open to atmospheric pressure, and/or (c) solidify rst? Explain.

Both (a) and (c) would result in a situation contrary to a riser's purpose, that is, if a riser solidies rst, it cannot feed the mold cavity to avoid shrinkage in the part. Concerning (b), when the molten metal enters the mold cavity, the air which was in the mold has to be forced out. If a riser is not open to the atmosphere, either the gas will become dissolved into the metal (due to the increased pressure and depending on solubility), or sucient pressure will build up which may crack the mold. Thus, a riser should be kept open to atmospheric pressure in order for it to function properly.

Figure II.2 shows a variety of components in a typical automobile that are produced by casting. Think of other products, such as power tools and small appliances, and prepare an illustration similar to the gure.

By the student. Among many possible solutions, the following is an example showing cast parts. Students should be encouraged to develop their own designs, based on disassembly of a product, which can be a challenging project.

Can you devise fluidity tests other than that shown in Fig. 10.9? Explain the features of your test methods.

By the student. As a suggestion, tests could involve convergent sections, moving walls, or gravity assistance. Note that the tests should allow for a competition between pressure- driven flow and cooling of the molten metal along the path.

Describe the effects on mold design, including the required change in the size of the risers, runners, chokes, and sprues, for a casting of a given shape that is to be doubled in volume.

By the student. This is an open ended problem, and students may interpret this problem differently in their answer. Some of the considerations are: It must be realized that if the volume is doubled, with no changes in any other mold features, the molten metal still has to ow through the same sprues and at the samevelocity for twice as long as the initial design. This may be accomplished without any changes in sprue, runner, etc., size at all if the original design had large features to begin with or if a high superheat is prescribed. Otherwise, these features will have to be increased in size, but not by a factor of two - Eqs. (10.3)-(10.5) on pp. 243-244 give the design guidelines. Students may discuss the need for chills, or the use of vacuum casting or alternative processes to prevent entrained gases or shrinkage pores in larger castings.

The fluidity test shown in Fig. 10.9 illustrates only the principle of this test. Design a setup for such a test, showing the type of materials and the equipment to be used. Explain the method by which you would determine the length of the solidied metal in the spiral passage.

By the student. This is an open-ended problem with a large number of potential solutions, automated or manual and made of different materials to accommodate different metals. Students should be encouraged to pursue their own creative solutions to this problem. Note that if the spiral pattern is known, an angular measurement can be converted to length.

Is there any difference in the tendency for shrinkage void formation in metals with short and long freezing ranges, respectively? Explain.

Consider an alloy poured into a mold, where the exterior solidied and a solidication front progresses towards the casting center. In an alloy with a large freezing range, the presence of a large mushy zone is more likely to occur and, thus, the formation of microporosity. However, in an alloy with a short freezing range, the formation of gross shrinkage voids is more likely to occur near the center of the casting. The total porosity is the same in this case. With proper gating and riser

What are some of the limitations and disadvantages of casting?

Disadvantages include (1) limitations on mechanical strength properties; (2) porosity; (3) poor dimensional accuracy; (4) safety hazards due to handling of hot metals; and (5) environmental problems.

Describe the events depicted in Fig. 10.5.

Due to a greater freezing range [see Eq. (10.1) on p. 239], gray cast iron forms an extensive dendritic structure and requires a considerable amount of time to solidify (Fig. 10.5a on p. 240). Steel, on the other hand, has a shorter freezing range and, thus. has a less extensive dendritic structure. As the carbon in the steel is increased, however, there is a greater tendency to form dendrites and hence the time to solidify increases. The effect of a chill mold is to greatly decrease the time for the metal to solidify, and this limit dendrite formation.

It has long been observed that (a) low pouring temperatures (i.e., low super- heat) promote the formation of equiaxed grains over columnar grains and (b) equiaxed grains become ner as the pouring temperature decreases. Explain these two phenomena.

Equiaxed grains develop in castings near the mold wall where rapid cooling and solidi- cation takes place by heat transfer through the relatively cool mold. With low pouring temperatures, cooling to the solidication temperature is faster because of the lower heat capacity of the molten metal. With a high pouring temperature, cooling to the solidi- cation temperature is slower. The mold still dissipates the heat but the metal being poured remains molten for a longer period of time, thus producing columnar grains in the direction of heat conduction. As the pouring temperature is decreased, equiaxed grains become ner because the energy required to heat the mold is a larger fraction of the heat in the molten metal. Thus there is more rapid initial cooling as the mold temperature is increased.

Is there porosity in a chocolate bar? In an ice cube? Explain.

Maybe. A chocolate bar is cast into its shape, and it will have a liquid core and a solid edge at some point. Continued solidication and shrinkage invariably will lead to porosity. With ice cubes, the method of manufacture is relevant, as you may have seen. Ice expands when it shrinks, but there still are entrained gases that separate from the water as it freezes. However, some ice cubes are formed around a cylindrical tube, and do not have such porosity.

If you inspect a typical cube of ice, you are likely to see air pockets and cracks in the cube. Some ice cubes, however, are tubular in shape and do not have noticeable air pockets or cracks in their structure. Explain this phenomenon.

Note that this is not universally true; ice expands when it solidies, and needs to be cooled sufficiently before stresses develop that crack the ice. The reason for this is that the ice cube rst begins to solidify at its outside surfaces; the interior then contracts as it begins to cool. Since there is no riser or an equivalent means, the ice cube develops microcracks in the interior. The effect is actually less than for metals because water has a minimum specic volume at -4C, whereas most metals shrink further while undergoing phase changes during solidication. Tubular ice pieces are formed by the exposure of water to copper tubes that have a refrigerant pumped through them. Thus, they solidify from the inside outward. The pockets are gases that are soluble in the water but have lower solubility in ice.

Why does porosity have detrimental effects on the mechanical properties of castings?Would physical properties, such as thermal and electrical conduc- tivity, also be adversely affected by porosity? Explain.

Pores are, in effect, internal discontinuities that are prone to propagate under external stresses. Thus, the toughness of a material, for example, will decrease as a result of porosity. Furthermore, the presence of pores in a metal part under tension requires that the material around the pores support a greater load than if no pores were present; thus the strength and elastic modulus are also lowered. Considering thermal and electrical conductivity, porosity decreases both the thermal and electrical conductivity because of the presence of a vacuum or air.

List and explain the reasons why porosity can develop in a casting.

Porosity is basically caused by shrinkage, especially in the absence of effective risers. Porosity can also be caused by entrained gases that form bubbles as the molten metal cools and solidies.

How does solidification of alloys differ from solidification of pure metals?

Pure metals solidify at a single temperature equal to the melting point. Most alloys (exceptions are eutectic alloys) start to solidify at the liquidus and complete solidification occurs at the solidus, where the liquidus is a higher temperature than the solidus.

List the process variables that affect the fluidity index shown in Fig. 10.9.

Refer to Fig. 10.9 on p. 246, and realize the fluidity index increases the further the metal penetrates into the spiral channel. The process variables that influence the fluidity index include: The degree of superheat of the metal Viscosity of the molten metal Channel diameter Mold material Metal specic heat The height of the pouring cup, which can also be stated as the size of the pressure head The mold temperature

What is the influence of the cross-sectional area of the spiral channel, shown in Fig. 10.9, on fluidity test results? What is the effect of sprue height? If this test is run with the entire test setup heated to elevated temperatures, would the results be more useful? Explain.

Referring to Fig. 10.9 on p. 247, we can make the following observations: i. The greater the cross-sectional area of the spiral channel, the further the metal will flow in the mold. Consider Eq. (10.7) on p. 247, which describes the solidication time. ii. An increase in sprue height would increase the velocity of the metal that enters the spiral, thus allowing the metal to flow further into the spiral than for a lower sprue height. iii. Tests can be, and are, conducted with the test setup used at elevated temperatures, showing the effect of a preheated mold on the fluidity of the molten metal. Such as test is especially useful for the investment or die-casting processes described in Sections 11.3.2 on p. 271 and 11.4.5 on p. 278, respectively.

A spoked handwheel is to be cast in gray iron. In order to prevent hot tearing of the spokes, would you insulate the spokes or chill them? Explain.

Referring to Table 10.1 on p. 249, we rst note that gray iron undergoes an expansion up to 2.5% on solidication. Although this fact may suggest that hot tearing cannot occur (see Fig. 10.12 on p. 249), we should also consider contraction of the spokes during cooling. Since the hot tearing tendency will be reduced as the strength increases, it would be advisable to chill the spokes to develop this strength.

Would you be concerned about the fact that portions of internal chills are left within the casting? Explain.

The fact that a part of the chill remains within the casting should be considered in the design of the part. The following factors should be taken into consideration: i. Any gas entrained in the molten metal when it contacts the chill may not readily escape; the chill could be a location where gas bubbles are in locally high concen- tration, and this can be a stress concentration. ii. The chill may not fuse with the casting, developing regions of weakness. iii. The material from which the chill is made should be compatible with the metal being cast, i.e., it should have approximately the same composition of the metal being poured. If these factors are understood and provided for, the fact that a piece of the chill remains within the casting should generally not be a signicant concern.

Make a sketch of volume vs. temperature for a metal that shrinks when it cools from the liquid state to room temperature. On the graph, mark the area where shrinkage is compensated by risers.

The graph for specic volume versus temperature is shown to the left, including compensation for shrink-age. The risers can compensate for shrinkage from the superheat temperature to solidication temperature, and also, if properly designed, for shrinkage associated with solidication.

One method of relieving stress concentrations in a part is to apply a small, uniform plastic deformation to it. Make a list of your concerns and recom- mendations if such an approach is suggested for a casting.

The plastic deformations, if tensile or exural, need to be applied with great care on castings. The main reason is that castings are typically weak in tension, due to micropores which act as stress risers. While the intent may be to relieve residual stresses, the result may be to fracture the part.

Assume that you have a method of measuring porosity in a casting. Could you use this information to accurately predict the strength of the casting? Explain.

The porosity can be used to estimate the strength, but only if the size and distribution of pores is consistent. For example, a casting with one large pore or thousands of micropores will have different strengths, and certainly different strengths in different part of the casting (inhomogeneity). Thus, one needs to also know the size, shape, and location of the pores.

Review Fig. 10.8 and make a summary, explaining the purpose of each feature shown and the consequences of omitting the feature from the mold design.

These can be summarized as follows: Pouring cup; this serves to accept the molten metal and provide for a large cross sectional area to provide controlled metal flow into the sprue; the absence of a pouring cup leads to more entrained air. Sprue; this serves to transport material from the pouring cup, and is designed to provide a pressure head to assure proper lling of the mold. The absence of a sprue complicates mold design, as the casting must be fed by gravity. Well; this can be used instead of a sprue or in combination with one; the well traps dross and also leads to low Reynolds numbers in the gating system; in the absence of a well, the flow may aspirate. A runner serves to transport molten metal from the sprue and well to the casting; without a runner, the metal would be transported directly from the sprue, leading to high Reynolds numbers, increased aspiration and a casting that is more difficult to separate from the other features. Risers provide molten metal to account for shrinkage and also control the cooling across the mold; the absence of a riser will lead to shrinkage defects.

Design an experiment to measure the constants C and n in Chvorinovs rule, Eq. (10.7). Describe the features of your design, and comment on any di- culties that might be encountered in running such an experiment.

This is an open-ended problem and the students are encouraged to develop their own experiments. Some of the valuable concepts that can be applied are the following: It is useful to know when the casting is solidied. Embedded thermocouples can be used inside the mold for low melting point materials, and this will allow direct measurement of the temperature prole. Fig. 10.1 shows the temperature prole for a pure metal; the solidication time can then be easily measured as the time when the center of the casting has its temperature drop below the solication temperature. If the metal has a high melting temperature, then thermocouples cannot be embed- ded directly in the molten metal. However, they can be embedded in the mold, and similar measurements allow determination of the solidication time. It is useful to change aspect ratios of the molds. For expendable molds, this means producing castings with a variety of aspect ratios. For permanent molds, this can be achieved by having at least oe dimension of the mold adjustable.

Note the shape of the two risers shown in Fig. 10.8, and discuss your observations with respect to Eq. (10.7).

This is an open-ended problem, and a number of observations can be made. The side riser (at left in Fig. 10.8 on p. 243) has a greater volume than the top riser shown on the right. As a result and referring to Eq. (10.7) on p. 247, we would expect the side riser to require a longer solidication time than the top riser. This is, as one would expect,because the metal closest to the point of entry, i.e., sprue and runner, will be the hottest. The metal near the side riser should remain liquid longer than that near the top riser, thus requiring a larger riser because that portion of the casting is intended to be the last to solidify.

Referring to Fig. 10.11, explain why internal corners, such as A, develop a thinner skin than external corners, such as B, during solidication.

We note in Fig. 10.11 on p. 248 that the internal corner A has a larger volume of material near to its surface area than does the external corner B . This situation can be visualized even better by assuming that the angles at A and B are less than 90c. Consequently, a point at a certain distance inward from corner A will remain at a higher temperature than a point at the same distance inward from corner B. Therefore, during the same time period, corner A will develop a thinner skin than will corner B. This can also be explained by considering heat flow directions, and noting that the volume in a corner will solidify most quickly because heat can be quickly extracted from this area; at an internal corner such as at A, the mold material cannot conduct as much heat as quickly.