Solidification Of Castings

Module-I of Manufacturing Science-I

1.8.1 SOLIDIFICATION OF CASTINGS After molten metal is poured into a mould, a series of events takes place during the solidification of the casting and its cooling to ambient temperature. These events greatly influence the size, shape, uniformity, and chemical composition of the grains formed throughout the casting, which in turn influence its overall properties. The significant factors affecting these events are the type of metal, thermal properties of both the metal and the mould, the geometric relationship between volume and surface area of the casting, and shape of the mould. Nucleation and Grain Growth When the free energy of a parent phase is reduced by means of temperature or pressure then there is a driving force leading to crystallization. At the melting point, the thermal fluctuations result in the formation of tiny particles (containing only a few atoms) of the product phase within the parent volume. Such a tiny particle has an interface that separates it from the parent matrix. It grows by transfer of atoms across its interface. The process of formation of the first stable tiny particle is called nucleation. And the process of increase in the sizes of these particles is called grain growth. The grain size in the product phase depends on the relative rates of nucleation and grain growth. Each nucleating particle becomes a grain in the final product. So a high nucleation rate means a larger number of grains. Also, when this is combined with a low growth rate, more time is available for further nucleation to take place in the parent phase that lies between slowly growing particles. A combination of high nucleation rate with low growth rate yields a fine grain size. On the other hand, a low nucleation rate combined with a high growth rate yields a coarse grain size. The temperature of maximum rate of nucleation is lower than that of maximum growth rate. An increase in cooling rate lowers the effective transformation temperature and results in the combination of high nucleation rate and a relatively slow growth rate and yields a fine grain size.

Figure 1.8.1: Grain representation Solidification of Pure Metal or Eutectic Alloy

Figure 1.8.2: Cooling curve for metal

Lecture Notes of Chinmay Das

Because a pure metal or eutectic alloy has a clearly defined melting or freezing point, it solidifies at a constant temperature. After the temperature of the molten metal drops to its freezing point, its temperature remains constant while the latent heat of fusion is given off. The solidification front (solid-liquid interface) moves through the molten metal, solidifying from the mould walls in toward the centre. Once solidification has taken place at any point, cooling resumes. The solidified metal, called casting, is

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Module-I of Manufacturing Science-I taken out of the mould and is allowed to cool to ambient temperature. At the mould walls, which are at ambient temperature, the metal cools rapidly. Rapid cooling produces a solidified skin or shell. The grains grow in a direction opposite to that of the heat transfer through the mould. Those grains that have favourable orientation will grow preferentially and are called columnar grains. As the driving force of the heat transfer is reduced away from the mould walls, the grains become equiaxed and coarse. Those grains that have substantially different orientations are blocked from further growth. This grain development is called homogeneous nucleation, meaning that grains grow upon themselves, starting from the mould wall.

Figure 1.8.3: Grain structure for pure metal Control of Solidification for obtaining Sound Castings The solidification which starts from the mould wall toward the centre line of the cavity is called Lateral or Progressive Solidification. The longitudinal or Directional Solidification occurs at right angles to lateral solidification at the centre line and is shown in the figure 1.8.4. The casting shown is a simple bar or plate and the metal is a pure metal, or a skin forming alloy.

Figure 1.8.4: Solidification of a plate In order to obtain a sound casting with no shrinkage void along the centerline, two requirements must be satisfied as follows: 1. The longitudinal solidification must be progressive toward the riser from the point, or points, most distant from the riser. 2. The temperature gradient, in addition to being properly directed, must be sufficiently steep so that liquid metal can pass through the wedge-shaped channel to compensate for shrinkage as it occurs at the centerline. If the temperature gradient is not sufficiently steep, the included angle of the wedge-shaped channel will be too small and proper passage of feed metal is not possible. If there were no temperature gradient, the lateral solidification at all points would reach the centerline at the same time. The result in either case is a lack of metal at the centerline, which causes an elongated narrow void known as centerline shrinkage. In other casting sections, voids of various shapes are caused by the shrinkage of skin forming type of alloy.

Lecture Notes of Chinmay Das

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Module-I of Manufacturing Science-I

Solidification of Alloys

Figure 1.8.5: Solidification curve of alloy Solidification in alloys begins when the temperature drops below the liquidus temperature and is complete when it reaches the solidus temperature. Within this temperature range, the alloy is in a mushy or pasty state with columnar dendrites. The mushy zone is described in terms of a temperature difference, known as the freezing range, as follows: Freezing Range = TL – TS The control of solidification of alloys which solidify throughout a temperature range is more complicated. It has been determined that steeper temperature gradients with these alloys produce sounder castings. Let us consider the entire casting and riser consist of mushy alloy for a period of time. During the early stage, the mushy alloy is quite fluid, and there is no problem. Then the solid dendrites gradually become thicker, surrounded by only a small amount of liquid metal. At this stage, whole sections may move to accomplish what is known as mass feeding. Later near the end of solidification, the mushy alloy becomes rigid, so it will be no longer move as a body. Some liquid metal still surrounds some dendrites, but since it is practically impossible to supply feed metal through the narrow passageways, small voids in the form of porosity are formed. This is known as shrinkage porosity or micro shrinkage and it is dispersed through out Figure 1.8.6: Solidification of alloy the metal in which the temperature gradient is not sufficiently steep. Riser Feeding or Centreline Feeding Distance For steel castings, a riser as shown in figure 1.8.4 will promote proper solidification if the distance L is no greater than 4.5 times the minimum thickness T. This maximum distance for L is known as the feeding distance for the riser. An effective metal chill located at the end most distant from the riser has been found to add about 50 mm to the riser feeding distance. Centreline Feeding Resistance The freezing patterns of a chilled and an ordinary mould are shown in figure 1.8.7. The solidification starts at the centre line of the mould before the solidification is completed even at the mould

Lecture Notes of Chinmay Das

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Module-I of Manufacturing Science-I face. In the chilled mould, on the other hand, due to rapid heat extraction, a narrow liquid zone quickly sweeps across the molten metal. The difficulty of feeding a given alloy in a mould is expressed by a quantity, called centre line feeding resistance (CFR).

CFR =

Time interval between start and end of freezing at centre line Total solidification time of casting

x 100 %

Figure 1.8.7: Performance of ordinary sand and chilled moulds In the above figure, we have

CFR =

AC x 100 % OC

Normally, feeding is considered to be difficult if CFR > 70 %. Chills These are provided in the mould so as to increase the heat extraction capability of the sand mould. A chill normally provides a steeper temperature gradient so that directional solidification as required in a casting is obtained. These are metallic objects having a higher heat absorbing capability than the sand mould. The chills can be of two types: external and internal.

Lecture Notes of Chinmay Das

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Module-I of Manufacturing Science-I

Figure 1.8.8: Chills

The external chills are placed adjoining the mould cavity at any required position. Providing a chill at the edge may not normally have the desired effect as the temperature gradient is steeper at the end of the casting since heat is removed from all sides. However, if it is placed between two risers it would have maximum effect. The chills when placed in the mould should be clean and dry, otherwise gas inclusions be left in the castings. Also, after placing the chills in the mould, they should not be kept for long since moisture may condense on the chills causing blow holes in the casting.

The internal chills are placed inside the mould cavity where an external chill cannot be provided. The material of chill should approximately resemble the composition of the pouring metal for proper fusing. Cleanliness of the internal chills is far more important because they are surrounded on all sides by the molten metal. Chaplets Chaplets are metallic support often kept inside the mould cavity to support the cores. These are of the same composition as that of the pouring metal so that the molten metal would provide enough heat to completely melt them and thus fuse with it during solidification.

Figure 1.8.9: Types of chaplets

Lecture Notes of Chinmay Das

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Module-I of Manufacturing Science-I Though the chaplet is supposed to fuse with parent metal, in practice it is difficult to achieve and normally it forms a weak joint in the casting. The other likely problems encountered in chaplets are the condensation of moisture which finally ends up as blow holes. So chaplets Figure 1.8.10: Core supported by chaplets before they are placed in the mould should be thoroughly cleaned of any dirt, oil or grease. Because of the problems associated with chaplets, it is desirable to redesign the castings, as far as possible. Forces acting on the Core and Moulding Flask The main force acting on the core when metal is poured into the mould cavity is due to buoyancy. The buoyant force can be calculated as the difference in the weight of the liquid metal to that of the core material of the same volume as that of the exposed core. It can be written as

P = V (ρ – d) Where

P = buoyant force, N V = volume of the core in the mould cavity, cm3 ρ = weight density of the liquid metal, N/ cm3 -2 d = weight density of the core material, N/ cm2 = 1.65 x 10 N/ cm3 The above equation is valid for horizontally placed core. But for vertically placed core the following equation has to be used.

P = 0.25 π ( D12 – D2 ) H ρ - Vd Where V = total volume of the core in the mould.

Figure 1.8.11: Horizontal and vertical core In order to keep the core in position, it is empirically suggested that core print will be able to support a load of 3.5 N / cm2 of the surface area. Hence to fully support the buoyant force, it is necessary that the following condition is satisfied.

P ≤ 350 A Where A = core print area, mm2 If the above arrangement is not satisfied, then it is would be necessary to provide additional support by way of chaplets. The buoyancy force is transmitted by the core to the cope and would tend to lift the cope away from the drag. There is another force termed as metallostatic force which is also present inside the mould cavity. This force is exerted by the molten metal in all directions of the cavity. However, we will consider the forces exerted in the upward direction.

Lecture Notes of Chinmay Das

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Module-I of Manufacturing Science-I The force Fm can be estimated by taking the area of cross section of the casting on which this is acting. The projected area, AP of the casting in the parting plane is the area on which the metal pressure will be acting. The head of the metal is given by (h-c). The head of the metal shown is given by (h – c). Hence the force,

F3 = AP ρ (h – c)

Figure 1.8.12: Core supported Reference 1. Manufacturing Technology by P.N.Rao, TMH , page 119 -123 2. Manufacturing Engineering and Technology by Kalpakjian and Schmid, Pearson Education, page 242-245 3. Manufacturing Science by A. Ghosh and A.K.Mallik , East-West Press Private Limited, page 60-63.

Review Questions 1. What do you mean by progressive and directional solidification? 2. Why fine grained skin is formed just adjacent to the mould wall in solidification of casting? 3. Why columnar grains are formed in casting? 4. To generate equi-axed grains in casting what conditions one has to create? 5. If nucleation rate is slow but growth rate is fast, what type of grain structure will form? 6. Differentiate between homogeneous and heterogeneous nucleation. 7. How dendrites are formed in alloys? 8. What do you mean by Centre line Feeding Resistance? 9. How feeding distance from riser can be increased? 10. What are the functions of chills and chaplets? 11. Explain external and internal chills. 12. What are the common materials used in chills and chaplets? 13. Find the weights that need to be kept to compensate for the forces during the pouring in a sand casting of a cast iron pipe of 12.5 cm OD and 10cm ID with a length of 180 cm. The metal head is to be about 20 cm, while the moulding flask size used for the purpose is 200 x 25 x 20 cm in size. Take the density of the core sand to be 0.0165 N/ cm3 and the liquid metal density to be 0.0771 N/ cm3. 14. The volume of a sand core is 160 cm3. Find the buoyant force on the core if poured with the following alloys: a) cast iron b) cast steel c) aluminium. 15. What is the freezing range for steel? 16. How to fill very thin section of the casting by liquid metal? 17. If the core print area is not sufficient to support the buoyant force, then what steps one will consider? 18. Which types of grain contribute toward high strength of the material? 19. Why centre line cavity is formed in cooling of casting? 20. How thermal properties of mould affect grain sizes of casting?

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