Casting Process

In the casting process (Fig. 5.1), the material is first liquefied by properly heating it in a suitable furnace. Then the liquid is poured in a mould cavity where it solidifies. Subsequently, the product is taken out of the mould cavity, trimmed and cleaned. A successful casting operation has to focus on the following areas:

1. Preparation of pattern and mould 2. Melting and pouring liquefied metal

3. Solidification and further cooling to room temperature.

(i) Pattern. A pattern is a replica of the part to be cast and is used in developing the mould cavity. Patterns are made of wood or metal. The mould cavity is the negative of the desired product and contains secondary cavities for pouring and channelling the liquid material into the main cavity. (ii) Flask. It is a sided frame in which a sand mould is made. If the mould is made in, say, two parts, the top portion is termed as cope, and the bottom one as drag.

(iii) Core. A core is usually a piece of a specific sand which rests on core prints in the mould cavity. It is used to produce hollow castings.

(iv) Greensand mould. The material for a greensand mould is a mixture of sand, clay, water, organic additives, e.g. wood flour and coal. Ratio is 70–85% sand, 10–20% clay, 3–6% water, and 1–6% additives.

Fig. 5.1 Sand casting steps.

(v) Dry sand moulding. As already mentioned, green sand moulds contain up to 7% water, depending on the type and percentage of the binding material. Therefore, this type of mould can be used only for smaller castings with thin walls, since large casting with thick walls would heat the mould, resulting in vapourization of water. This would, in turn, lead to bubbles in the castings. For this reason, moulds for large castings should be dried after they are made in the same way as greensand moulds. The drying operation is carried out in ovens at a temperature ranging from 300 to 650°F (150 to 350°C) for 8 to 48 h, depending upon the type and amount of binder used.

(vi) Core sand moulding. When the mould is too big to fit in an oven, moulds are made by

assembling several pieces of sand cores. Consequently, patterns are not required and core boxes are employed instead, to make the different sand cores necessary for constructing the mould. Since core- sand mixture (which has superior moulding properties) is used, a very good quality and dimensional accuracy of the casting is obtained.

(vii) Cement bonded sand moulds. A mixture of silica sand containing 8–12% cement and 4–6% water is used. When making the mould, the cement-bonded sand mixture must be allowed to harden first, before the pattern is withdrawn. The mould obtained is then allowed to cure for about 3–5 days. Large castings with intricate shapes, accurate dimensions and smooth surfaces are usually produced by this method. The only shortcoming being the long time required for the moulding process.

solution of sodium silicate (water glass). After the mould is remmed, CO₂ is blown through the sand mixture. As a result, the gel of silica bonds the sand grains together, and no drying is needed. Since the moulds are allowed to harden while the pattern is in position, higher dimensional accuracy of moulds is obtained.

(ix) Plaster moulds. A plaster mould is appropriate for casting silver, gold, magnesium, copper and aluminum alloys. The moulding material is a mixture of fine silica sand, asbestos and Plaster of Paris which is used as a binder. Water is added to the mixture until a creamy slurry is obtained, which is then used in moulding. The drying process should be very slow to avoid cracking of the mould.

(x) Loam moulds. The loam mould is used for very large jobs. The basic shape of the desired mould is constructed with bricks and mortar (just like a brick house). A loam mixture is used as a moulding material to obtain the desired intricacies of a mould. Templates, sweeps and the like are employed in the moulding process. The loam mixture used in moulding consists of 50% or more loam, with the rest being mainly silica sand. Loam moulds must be thoroughly dried before pouring the molten metal. (xi) Shell moulding. It is a process in which, a thin mould is made around a heated metallic pattern plate. The moulding material is a mixture of dry, fine silica sand (clay content should be kept very low), and 3–8% of a thermosetting resin like phenol formaldehyde or ureaformaldehyde.

Conventional dry mixing techniques are used for obtaining the moulding mixture. Specially prepared resin coated sands are also used (see Fig. 5.2).

When the moulding mixture drops on to the pattern plate, which is heated to a temperature of 35 to 700°F (18 to 375°C), a shell of about 6 mm thickness is formed. In order to cure the shell completely, it must be heated to 440 to 650°F (230 to 350°C) for about 1–3 min. The shell is then released from the pattern plate by ejector pins. To prevent sticking of the baked shell (sometimes called a biscuit) to the pattern plate, a silicone release agent is applied to the latter before the moulding mixture drops on to it. Figure 5.2 shows a typical sequence used in shell moulding.

Shell moulding is suitable for mass production of thin walled, grey cast iron (and aluminum alloy) castings having a maximum weight between 35 and 45 pounds (15 to 20 kg). However, castings

weighing up to 1000 pounds can be made by shell moulding on an individual basis. The advantages of shell moulding include good surface finish, fewer restrictions on casting design, besides rendering itself suitable for automation.

(xii) Ceramic moulds. The moulding material is actually a slurry consisting of refractory grains, ceramic binder, water, alcohol and an agent to adjust the pH value. The slurry is poured around the permanent pattern and is allowed to harden before the pattern is withdrawn. Next, the mould is left to dry for some time and then is fired to gain strength. In fact, ceramic moulds are usually preheated before pouring the molten metal. For this reason, they are suitable for casting high pouring

temperature alloys. Excellent surface finish and very close tolerances of the castings are among the advantages of this moulding process. Apparently, this leads to the elimination of the machining operations which are usually performed on castings. Therefore, ceramic moulds are certainly

advantageous when casting precious or difficult to machine metals as well as for castings with great intricacies.

(xiii) Precision moulding (investment moulding). Precision moulding is used when castings with intricate shapes, good dimensional accuracy and very smooth surfaces are required. The process is especially advantageous for high melting, point alloys as well as for those metals which are difficult to machine. It is also most suitable for producing small castings

Fig. 5.2 Shell moulding steps.

having intricate shapes as shown in Fig. 5.3, which shows a group of investment castings. A non- permanent pattern, which is usually made of wax must be prepared for each casting. Therefore, the process is sometimes referred to as lost-wax process. Generally, the precision moulding process involves the following steps:

Fig. 5.3 Lost-wax process.

1. A heat-disposable pattern, together with its gating system, is prepared by injecting wax of plastic into a die cavity.

2. A pattern assembly which is composed of a number of identical patterns is made. Patterns are attached to a runner bar made of wax of plastic, in much the same manner as leaves are attached to

branches. A ceramic pouring cup is also attached to the top of the pattern assembly which is sometimes referred to as the tree or cluster.

3. The tree is then invested by separately dipping it into a ceramic slurry, which is composed of silica flour suspended in a solution of ethyl silicate, and then sprinkling it with very fine silica sand. This enables a self-supporting ceramic shell mould of about 6 mm in thickness to be formed all around the wax assembly. Alternatively, a thin ceramic coating is obtained first (precoating); then a cluster is placed in a flask and thick slurry is poured around it as a backup material.

4. The pattern assembly is then baked in an oven or steam autoclave to melt out the wax (or plastic). Hence the dimensions of the mould cavity precisely match those of the desired product.

5. The resulting shell mould is fired at a temperature ranging between 1600 and 1800°F, in order to eliminate all traces of wax and to gain reasonable strength.

6. The molten metal is poured into the mould while the latter is still hot and a cluster of castings is obtained.

Today, the lost-wax process is used in manufacturing larger objects, like cylinder heads and camshafts. The modern process which is known as the lost-foam method, involves employing a

styrofoam replica of the finished product, which is then coated with refractory material and located in a box, where sand is moulded around it by vibratory compaction. When the molten metal is finally poured into the mould, the styrofoam vapourizes, allowing the molten metal to replace it.

(xiv) Graphite moulds. Graphite is used in making moulds to receive alloys such as titanium, which can be poured only into inert moulds. The casting process must be performed in a vaccum to eliminate any possibility of contaminating the metal. The graphite moulds can be made either by machining a block of graphite to create the desired mould cavity or by compacting a graphite base aggregate around the pattern and then sintering the obtained mould at a temperature 1800 to 2000°F in reducing atmosphere (see Chapter 6). In fact, graphite mould liners have found widespread industrial

applications in the centrifugal casting of brass and bronze.

(xv) Permanent moulds. A permanent mould can be used repeatedly for producing casting of the same form and dimension. Permanent moulds are usually made of steel or grey cast iron. Each mould is generally made of two or more pieces, which are assembled together by fitting and clamping. Although the different parts of the mould can be cast to their rough contours, subsequent machining and finishing operations are necessary to eliminate the possibility of sticking of the casting to the mould. Simple cores made of metal are frequently used. When complex cores are required, they are made (usually) of sand or plaster, and the mould is said to be semi-permanent.

Different metals and alloys can be successfully cast in permanent moulds. They include aluminum, magnesium, zinc, lead, copper alloys and cast iron. It is obvious that the mould should be preheated to an appropriate temperature, prior to casting. In fact, the mould operating temperature, which depends on the metal to be cast, is a very important factor in successful permanent mould casting.

Based on the foregoing discussion, we expect the mould life to be dependent on a number of

interrelated factors, including the mould material, the metal to be cast, and the operating temperature of the mould. Nevertheless, it can be stated that the life of a permanent mould is about 100,000 pourings or more when casting zinc, magnesium or aluminum alloys, and not more than 20,000 pourings for copper alloys.

to be cast, castings are either ferrous or non-ferrous. The ferrous casting includes cast steels and the family of cast irons, whereas the non-ferrous casting includes all other metals such as aluminium, copper, magnesium, titanium and their alloys. Each of these metals and alloys is melted in a particular type of foundry furnace, which may not be appropriate for melting other metals and alloys. Also, moulding method and material, as well as fluxes, degassers and additives depend on the metal to be cast. Therefore, this classification method is popular in foundry practice. The following is a brief discussion of each of these cast alloys.

(i) Ferrous metal. Cast steel is smelted in open-hearth furnaces, converters, electric arc furnaces and electric induction furnaces. It can be either plain-carbon, low alloy or high alloy steel. However, plain carbon casting steel is by far the most commonly produced type. As compared with cast iron, steel certainly has poorer casting properties, namely, higher melting point, higher shrinkage and poor fluidity. Steel is also more susceptible to hot and cold cracks after the casting process. Therefore, cast steel is mostly subjected to heat treatment to relieve the internal stresses and improve mechanical properties.

In order to control the oxygen content of molten steels, either aluminium, silicon or manganese is used as a deoxidizer. Aluminium is the most commonly used of these elements because of its availability, low cost, and effectiveness.

There is a major difference between cast steel and the wrought product. This involves the presence of a ‘skin’ of thin layer, just below the surface of a casting, where scales, oxides and impurities are concentrated. Also, that layer may be chemically or structurally different from the base metal.

Therefore, it has to be removed by machining in a single deep cut, which is achieved through reducing the cutting speed to half of the conventionally recommended value.

Grey cast iron is characterized by the presence of free graphite flakes, when its microstructure is examined under the microscope. This kind of microstructure is, in fact, responsible for the superior properties possessed by grey cast iron. For instance, this dispersion of graphite flakes acts as a lubricant during machining of grey cast iron, thus eliminating the need for matching lubricants and coolants. As compared with any other ferrous cast alloys, grey cast iron certainly possesses superior machinability. The presence of these graphite flakes enables absorbing of vibrations that characterize grey cast iron. The compressive strength is better—normally four times its tensile strength. Grey cast iron finds widespread applications, mainly because of its two properties: for machine tool beds and the like. On the other hand, it has some disadvantages such as its low tensile strength, brittleness and poor weldability. Nevertheless, it has the lowest casting temperature, least shrinkage, and the best castability of all cast ferrous alloys.

The cupola is the most widely used foundry furnace for producing and melting grey cast iron. The chemical composition, microstructure, as also the properties of the obtained casting are determined by the constituents of the charge of the cupola furnace. Therefore, the composition and properties of the grey cast iron are controlled by changing the percentages of the charge constituents and by adding inoculants and alloying elements. Commonly used inoculants include calcium silicate, ferro silicon and ferro manganese. An inoculant is added to the molten metal (either in cupola spout or the ladle) and usually amounts to 0.1–0.5% of the molten iron by weight. It acts as a deoxidizer and hinders the growth of precipitated graphite flakes. It is also important to remember as a product designer that the

properties of grey cast iron are also dependent on the dimensions (in fact, the thickness of the walls) of that product because the cooling rate is adversely affected by the cross-section of the casting. Actually, the cooling rate is high for small castings with thin walls, sometimes yielding white cast iron. For this reason, grey cast iron must be specified by the strength of critical cross-sections. (ii) White cast iron. When the molten cast iron alloy is rapidly chilled after being poured into the mould cavity, dissolved carbon does not have enough time to precipitate in the form of flakes.

Instead, it remains chemically combined with iron in the form of cementite. This material is primarily responsible for the white crystalline appearance of a fractured surface of white cast iron. Cementite is also responsible for high hardness, extreme brittleness, and excellent wear resistance of such cast iron. Industrial applications of white cast iron include components subjected to abrasion. Sometimes grey cast iron can be chilled to produce a surface layer of white cast iron in order to combine to advantageous properties of the two types of cast iron. In such a case, the product metal is usually referred to as chilled cast iron.

(iii) Ductile cast iron. Ductile cast iron is also called nodular cast iron and spheroidal graphite cast iron. It is obtained by adding small amounts of magnesium to a very pure molten alloy of grey iron that has been subjected to desulphurization. Sometimes, a small quantity of cerium is also added to prevent the harmful effects of impurities like aluminium, titanium and lead. The presence of

magnesium and cerium causes the graphite to precipitate during solidification of the molten alloy in the form of small spheroids, rather thin flakes, as in the case of grey cast iron. This microstructural change results in marked increase in ductility, strength, toughness and stiffness of ductile iron, as compared with such properties of grey cast iron, because the stress concentration effect of a flake is far higher than that of a spheroid (remember what you learned in fracture mechanics). Ductile cast iron is used for making machine parts like axles, brackets, levers, crank shaft housing, die pads and die shoes.

(iv) Compacted graphite cast iron. Compacted graphite cast iron falls between grey and ductile cast irons, both in its microstructure and mechanical properties. The free graphite in this type of iron takes the form of short, blunt and interconnected flakes. The mechanical properties of compacted graphite cast iron are superior to those of grey cast iron but are inferior to those of ductile cast iron. The thermal conductivity and damping capacity of compacted graphite cast iron approach those of grey cast iron. Compacted graphite cast iron has some applications in the manufacture of diesel engines. (v) Malleable cast iron. Malleable cast iron is obtained in two stages: by heat treatment and

graphitizations. Heat treatment of white cast iron having an appropriate chemical composition. The hard white cast iron becomes malleable after heat treatment due to microstructural changes. The combined carbon separates as free graphite, which takes the form of nodules. Since the raw material for producing malleable cast iron is actually white cast iron, there are always limitations on casting design. Large cross-sections and thick walls are not permitted, since it is difficult to produce a white cast iron part with these geometric characteristics.

There are two basic types of malleable cast iron: pearlitic and ferritic (black heart). Although the starting alloy for both types is the same (white cast iron), the heat treatment cycle and the atmosphere of the heat treating furnace are different in each case. Furnaces with oxidizing atmospheres are

employed for producing pearlitic malleable cast iron, whereas furnaces with neutral atmospheres are used for producing ferritic malleable cast iron. When we compare the properties of these two types, we find that the ferritic grade normally has higher ductility and better machinability, but lower

strength and hardness. Pearlitic grades can, however, be subjected to further surface hardening where the depth of the hardened layer can be controlled.

Figure 5.4 shows the heat-treating sequence for producing malleable cast iron, which is usually referred to as the malleabilizing cycle. As can be seen from the figure, the malleabilizing cycle