Introduction to Metal Casting 2024/2025 PDF

‫ﻣﺤﻤﺪ ﺍﺣﻤﺪ ﻣﺤﻤﺪ ﺍﻟﺨﻄﻴﺐ‬ Tanta University Department of Production Engineering and Mech. Design 2024/2025 2024/2025 2024/2025...

‫ﻣﺤﻤﺪ ﺍﺣﻤﺪ ﻣﺤﻤﺪ ﺍﻟﺨﻄﻴﺐ‬ Tanta University Department of Production Engineering and Mech. Design 2024/2025 2024/2025 2024/2025 Introduction to Metal Casting Mahmoud Ahmadein Prof. Dr.-Ing. Melting and casting in ancient Egypt (~1450 B.C.) At top: Operating the fire bellows to heat up the crucible. At middle: Picking off the crucible. At bottom: Casting of a temple door and handling works ‫ﻣﺤﻤﺪ ﺍﺣﻤﺪ ﻣﺤﻤﺪ ﺍﻟﺨﻄﻴﺐ‬ Table of contents Tanta University Chapter 1 Department of Production Engineering and Mechanical Design Solidification of Metals...................................................................... 1 1.1. Grain nucleation...................................................................... 1 2024/2025 2024/2025 2024/2025 1.1.1. Physical phenomena.................................................... 2 Introduction to 1.1.2. Cooling of pure metals................................................... 6 1.1.3. Heterogeneous versus homogeneous nucleation.......... 8 1.1.4. Melt undercooling....................................................... 12 1.2. Solute partitioning.................................................................. 16 Metal Casting 1.3. Growth of solid...................................................................... 21 1.4. Segregation........................................................................... 29 Chapter 2 Metal Casting Technology................................................................ 33 2.1. Classification of casting processes.......................................... 39 2.2. Expendable mold casting......................................................... 45 Mahmoud Ahmadein 2.2.1. Sand casting.................................................................. 45 Prof. Dr.-Ing. 2.2.2. Shell mold casting.......................................................... 49 2.2.3. Plaster Molding.............................................................. 51 2024/25 2.2.4. Investment (precision) casting....................................... 52 2.2.5. Lost foam casting........................................................... 54 ‫ﻣﺤﻤﺪ ﺍﺣﻤﺪ ﻣﺤﻤﺪ ﺍﻟﺨﻄﻴﺐ‬ 2.3. Permanent Mold Casting......................................................... 56 4.2.1. Development of continuous casting:................................. 124 2.3.1. Permanent (gravity) mold casting................................... 56 4.2.2. Advantages of continuous casting:................................... 125 2.3.2. Slush Casting:................................................................ 58 4.2.3. Process description.......................................................... 125 2.3.3. Low-pressure casting..................................................... 58 2.3.4. Vacuum casting............................................................. 59 Chaper 5 2.3.5. Pressure die casting...................................................... 59 Casting Quality................................................................................ 131 2024/2025 2024/2025 2024/2025 2.3.6. Centrifugal casting......................................................... 63 5.1. Finishing of casting................................................................ 131 2.3.7. Semi-centrifugal casting................................................. 66 5.2. Casting Defects..................................................................... 134 5.3. Inspection of casting.............................................................. 142 Chapter 3 5.4. Modeling and Simulation of Casting....................................... 147 Sand Casting..................................................................................... 69 3.1. Pattern Making............................................................................. 71 Glossary.......................................................................................... 157 3.2. Core making................................................................................. 85 3.3. Mold Making................................................................................. 89 Appendices 3.4. Elements of Gating and Feeding Systems.................................. 110 A. Periodic table of the elements............................................. 163 B. Physical properties of some pure metals........................... 164 Chapter 4 C. Tables of approximate thermal data.................................... 166 Cast House Processes................................................................... 119 4.1. Ingot casting............................................................................... 119 References 167 3.1.1. Downhill casting............................................................... 120 3.1.2. Uphill casting.................................................................... 121 3.1.3. Ingot casting of nonferrous metals.................................... 123 4.2. Continuous casting..................................................................... 124 ‫ﻣﺤﻤﺪ ﺍﺣﻤﺪ ﻣﺤﻤﺪ ﺍﻟﺨﻄﻴﺐ‬ 2 1 Solidification of Metals essence of this process is the milestone to control the solidification morphology and reduce the defects arisen during solidification, which in turn improves the mechanical properties of the final Chapter 1 components and products. 1.1.1. Physical phenomena 2024/2025 2024/2025 2024/2025 Solidification requires an outflow of heat which changes the free energy and the relative thermodynamic stability of the present phases. The transformation from one equilibrium state to another (e.g. liquid to solid) requires the system to pass through an intermediate state of energy as shown in Fig.1.1. At some stage of production, the majority of metals and alloys are To minimize the amount of material in such intermediate high melted and then allowed to solidify as a casting. The latter may be energy state, the system transforms gradually, i.e. by nucleation and an intermediate product, such as a large steel ingot suitable for hot- growth. If the work required to form a stable nucleus is the same working, or a complex final shape, such as an engine cylinder block throughout the entire melt, a condition of homogenous nucleation of cast iron or a single-crystal gas-turbine blade of super-alloy. prevails. Otherwise, the existence of preferred nucleation sites (e.g. Solidification conditions determine the structure, homogeneity and mold walls) reduces the necessary work leading to a case of soundness of cast products and the governing scientific principles heterogeneous nucleation. find application over a wide range of fields. 1.1.Grain nucleation Grain nucleation is simply the birth of the solid phase. The consequences of this process can be seen in the final microstructure and mechanical properties of the solidified metal. Understanding the ‫ﻣﺤﻤﺪ ﺍﺣﻤﺪ ﻣﺤﻤﺪ ﺍﻟﺨﻄﻴﺐ‬ 1 Solidification of Metals 3 4 1 Solidification of Metals relatively high energy required to create the new surface (where surface-to-volume ratio is too large), however the others with radius more than a certain size ( o) will survive to form the stable nuclei that grow steadily. The phenomenon of nucleation of crystals from its melt depends mainly on two processes: thermal fluctuations which lead to the 2024/2025 2024/2025 2024/2025 creation of previously sized crystal embryos; and creation of an interface between the liquid and the solid. The total Gibbs free (a) ene G, resulting from the local formation of a unit volume of solid arises from two sources as given in equation ((1.1) Fig.1.1. (a) Energy and schematically represented in Fig.1.2. The first term, Gi [J/mol], relationship during a is the interface energy depends on free surface energy of the phase transformation and solid/liquid interface and is proportional to r2; and the second term, (b) advance of the high energy interface ,C , into Gv [J/mol] Gibbs free energy per mol, depends on the difference in the untransformed free energy between solid and liquid. The former is always positive, (b) region, A. while the later is zero at the equilibrium temperature, negative below, and positive above. Movement of the atoms during nucleation to form clusters or embryos requires irreversible activation energy (work). For this reason solidification can never be an equilibrium process. It is (1.1 ) necessary for the nucleation that clusters of solid atoms are formed, where the liquid atoms can jump and stick. These first clusters can exist randomly due to the thermal fluctuations, even over the melting point, but those will be unstable. Below the melting point too small clusters continue to be unstable and remelted again due to the ‫ﻣﺤﻤﺪ ﺍﺣﻤﺪ ﻣﺤﻤﺪ ﺍﻟﺨﻄﻴﺐ‬ 1 Solidification of Metals 5 6 1 Solidification of Metals values of both A and ro T increases as explained in Fig.1.3. (1.2 ) 2024/2025 2024/2025 2024/2025 Fig.1.2. Free energy of a crystal cluster as a function of its radius at temperature below the melting point. A stable nucleus is likely Fig.1.3. Total free G is starting to decease. energy change at different tempera- Where [J·m-2]: solid/liquid interface energy, [J·m-3]: latent tures below the melting point. heat of fusion per unit volume, Gibbs free energy per unit volume = , a: interface surface area, and : under- 1.1.2. Cooling of pure metals cooling. The critical radius, ro G starts to change its sign towards the stability can be obtained by differentiating both sides of The thermal history of a slowly cooling metal is depicted in Fig.1.4; equation (1.1) with respect to r and equating them with zero. The the plateau on the curve indicates the melting point, which is resulting radius is given in equation (1.2). The maximum energy pressure dependent and specific to the metal. Go, is called the work of Its value relates to the bond strength of the metal. The transition from nucleation or the activation energy, A. The value of A is infinity at a highly disordered liquid to an ordered solid is accompanied by a Tf although fluctuating clusters of atoms do exist. Below Tf the lowering in the energy state of the metal and the release of thermal energy (latent heat of solidification) as shown in Fig.1.5, forming ‫ﻣﺤﻤﺪ ﺍﺣﻤﺪ ﻣﺤﻤﺪ ﺍﻟﺨﻄﻴﺐ‬ 1 Solidification of Metals 7 8 1 Solidification of Metals the arrest on the cooling curve shown in Fig.1.4. This ordering has a marked and immediate effect upon other structure-sensitive properties of the metal; for instance, the volume typically decreases by 1 6% leading to increase of metal's density as shown in Fig.1.5, the electrical conductivity rises and the diffusivity or ability of the atoms to migrate falls. 2024/2025 2024/2025 2024/2025 Fig.1.5. Density, top, and latent heat, bottom, as a function of time. Fig.1.4. Temperature as a function of time for the solidification of pure metals. 1.1.3. Heterogeneous versus homogeneous nucleation Solidification is a classic example of a nucleation and growth process. In the general case of freezing within the bulk of pure molten metal, minute crystalline nuclei form independently at random points. After this homogeneous form of nucleation, ‫ﻣﺤﻤﺪ ﺍﺣﻤﺪ ﻣﺤﻤﺪ ﺍﻟﺨﻄﻴﺐ‬ 1 Solidification of Metals 9 10 1 Solidification of Metals continued removal of thermal energy from the system causes these As the nucleation takes place at the surface of these particles, the small crystalline regions to grow independently at the expense of the activation energy will be affected by the wetting angle, , formed surrounding melt. Throughout the freezing process, only nuclei between the new solid crystals and these particles as shown in which exceed a critical size, ro , are able to survive. Rapid cooling Fig.1.7a. The surface tension can be calculated as given in equation of a pure molten metal reduces the time available for nuclei (1.3) based on the balance between the three components: formation and delays the onset of freezing by a temperature interval between liquid and substrate, between crystal and substrate and of. This thermal undercooling (or supercooling) is depicted in between liquid and crystal as shown in Fig.1.7b. The free energy 2024/2025 2024/2025 2024/2025 Fig.1.8. However, commercial melts usually contain foreign matter equation for the embryo can be written as given in equation (1.4). (e.g. mold walls, oxide layers, or suspended insoluble particles from If the areas of the liquid/crystal interface, , and crystal/substrate refractory crucible or hearth) as shown in Fig.1.6, which act as interface, , and the volume of the embryo, , are substituted in seeding nuclei for so-called heterogeneous nucleation. This equation (1.4) as a function of the embryo radius, , the following contributes in reducing the activation energy, A, required for relation is obtained. nucleation. Therefore, the undercooling required for nucleation is much less likely under such conditions. Homogeneous nucleation is not encountered in normal foundry practice. Fig.1.6. Illustration of principle of (a) homogeneous, (b) heterogeneous nucleation. ‫ﻣﺤﻤﺪ ﺍﺣﻤﺪ ﻣﺤﻤﺪ ﺍﻟﺨﻄﻴﺐ‬ 1 Solidification of Metals 11 12 1 Solidification of Metals (1.5 ) From the above relation it is obvious that the free energy necessary for the heterogonous nucleation is reduced by a factor , which 2024/2025 2024/2025 2024/2025 affects the nucleation process as follows: Fig.1.7c explains that radius but rather in the three surface tension components. Fig.1.7. (a) Hypothetical precipitated solid crystal over a foreign 1.1.4. Melt undercooling substrate and the contact angle between each other, (b) the mechanical equilibrium between different surface tension The temperature-time curve of the solidification of a hypothetical components and (c) independence of on the radius of the embryo pure metal or eutectic alloy is demonstrated in Fig.1.9a. At certain but on the contact angle. undercooling (Fig.1.9a), the first solid fraction, , starts to build up (Fig.1.9c). As cooling continues, the nucleation rate, , increases sharply to a maximum value (Fig.1.9d) which corresponds (1.3 ) to a minimum temperature in Fig.1.9a. At this point the total grain (1.4 ) density, N, reaches the maximum and the growth rate, v, has a peak value. The subsequent increase in T (Fig.1.9a) is due to the internal heat flux arising from liquid-to-solid transformation and the release of the latent heat. At later stage and due to impingement of the grains ‫ﻣﺤﻤﺪ ﺍﺣﻤﺪ ﻣﺤﻤﺪ ﺍﻟﺨﻄﻴﺐ‬ 1 Solidification of Metals 13 14 1 Solidification of Metals and dendrites the growth rate, v, is reduced gradually to reach zero, (1.6 ) while the number of grains, N, remains constant. Melt undercooling is very crucial as a driving force for nucleation and further growth. It is reported that an undercooling of about is necessary for homogeneous nucleation, e.g. nucleation can be delayed for iron and nickel up to 300°C below the liquidus temperature. 2024/2025 However, in practice, few degrees can be sufficient to 2024/2025 2024/2025 impel the heterogeneous nucleation. T, can be calculated as the sum of the contribution of different sources as given in equation (1.6). Where : kinetic undercooling required to transport the atoms through liquid/solid interface, : curvature undercooling required to account for the formation of new surface with radius r, : constitutional undercooling resulting from solute rejection in liquid which changes the local liquidus temperature, : thermal undercooling required to maintain continuous remove of the superheat and the released latent heat during solidification and : pressure undercooling, where pressure variations change the free energy of liquid and solid. For metals is of the order of 10- Fig.1.8. Schematic cooling curve and the corresponding stages of 2 K/atm, and for a spherical solidification of eutectic alloy. metallic crystal with and inversely proportional to the radius of curvature. Thus the most significant undercooling values are and. ‫ﻣﺤﻤﺪ ﺍﺣﻤﺪ ﻣﺤﻤﺪ ﺍﻟﺨﻄﻴﺐ‬ 1 Solidification of Metals 15 16 1 Solidification of Metals 1.2. Solute partitioning 1.2.1. Partitioning Coefficient For any temporary binary phase diagram there are liquidus and solidus lines, where the concentration of the solute in the liquid and solid phases can be determined during solidification. As shown in 2024/2025 2024/2025 2024/2025 Fig.1.10, if an alloy with composition Co is cooled to a temperature T*, a solid phase with ws weight fraction and Cs solute concentration is formed. On the other hands the weight fraction and solute concentration in liquid are wl and Cl in respectively (ws and wl can be calculated from the lever rule). The partition coefficient, k, is Fig.1.9. Schematic illustration of the thermal history of equiaxed dendritic solidification. Fig.1.10. Part of a binary equilibrium phase diagram ‫ﻣﺤﻤﺪ ﺍﺣﻤﺪ ﻣﺤﻤﺪ ﺍﻟﺨﻄﻴﺐ‬ 1 Solidification of Metals 17 18 1 Solidification of Metals defined as the ratio of solute concentration in solid to solute concentration in liquid. Assuming straight liquidus and solidus lines, then k is constant and can be given as per equation (1.8). The material balance under equilibrium conditions can be calculated from equation (1.9). 2024/2025 2024/2025 2024/2025 (1.7 ) (1.8 ) (1.9 ) 1.2.2. Equilibrium Solidification Fig.1.11. Solute redistribution in equilibrium solidification of In case of equilibrium cooling the first solid has a composition koCo an alloy of composition Co at (Fig.1.11a) and begins to form at the liquidus temperature TL the beginning of solidification, a, at temperature T*, b, and (Fig.1.10). The solid/liquid interface moves so slowly that after solidification, c. equilibrium conditions at the interface are reached. If the solidification occurs at temperature T*, the solid composition CS* forms in equilibrium at the interface with liquid composition CL*. 1.2.3. Non equilibrium solidification Since diffusion in the solid and liquid is complete, the entire solid In practice, equilibrium will only be maintained at the interface, and liquid become of uniform composition as illustrated in where the compositions agree with the phase diagram. The Fig.1.11b. Any change in the solid and liquid composition occurring composition inside solid and in the liquid adjacent to the during the solidification process will vanish after the completion of solidification front is drastically influenced by the diffusion solidification (Fig.1.11c). coefficient in both phases. While the diffusion in solid phase is very limited, the diffusion in liquid decreases as temperature of the liquid ‫ﻣﺤﻤﺪ ﺍﺣﻤﺪ ﻣﺤﻤﺪ ﺍﻟﺨﻄﻴﺐ‬ 1 Solidification of Metals 19 20 1 Solidification of Metals falls. Therefore, a solute rich boundary layer with thickness, x, is built ahead of the advancing interface as shown in Fig.1.12. The boundary layer thickness in case of equiaxed grain growth can be calculated from equation (1.10), where D is the temperature dependent diffusion coefficient and v is the interface advance velocity. It is typically in the order of about 0.5 mm but can be just a few micrometers in rapid solidification processing. 2024/2025 2024/2025 2024/2025 (1.10 ) Solute partitioning at the solid/liquid interface causes a corresponding variation in the liquidus temperature. There is, however, a positive temperature gradient in the liquid, giving rise to a supercooled zone of liquid ahead of the interface (Fig.1.12). This is called constitutional supercooling because it is caused by composition changes. A small perturbation on the interface will Fig.1.12. The relationship between the constitutional phase therefore expand into a supercooled liquid. This leads to dendrite diagram for a binary alloy, solute concentration ahead of the solidification front, and constitutional undercooling on freezing (after the Greek word for tree, Dendros) formation. It is very difficult to avoid constitutional supercooling in practice because the velocity required to avoid is very small (when v 0, x. Directional solidification with a planar front is possible only at low growth rates, for example in the production of silicon single crystals. In most cases the planar interface is difficult to maintain stable and an unstable cellular or more stable dendritic one is formed (Fig.1.13). Fig.1.13. Schematic of the formation of cell, a, and dendrite, b. ‫ﻣﺤﻤﺪ ﺍﺣﻤﺪ ﻣﺤﻤﺪ ﺍﻟﺨﻄﻴﺐ‬ 1 Solidification of Metals 21 22 1 Solidification of Metals 1.3. Growth of solid more than one phase can be distinguished under the optical microscope. In case of complex alloys many phases and inter- The progress of solid metal into the melt will only occur if the heat metallic compounds coexist. In such cases EDX analysis (Energy- is continuously extracted through the freezing front to cool it below dispersive X-ray spectroscopy that is commonly connected to SEM the equilibrium freezing point. As the rate of heat extraction "Scanning Electron Microscope") is required to obtain more increases, the rate of the solid/liquid interface advance accurate information about the constituting phases. correspondingly increases. 2024/2025 2024/2025 2024/2025 For pure metals, as the solidification front is seen to go through a series of transitions. Initially it is planar; at higher rates of advance it develops deep protrusions, spaced regularly over the previously planer front. This type of growth is called cellular growth. At higher velocities still, the cells grow into rapidly advancing projections, sometimes of complex geometry. Their tree-like forms have given them the name dendrites. In case of alloys, the three growth forms are similarly present as schematically illustrated in Fig.1.14. However, the driving force for instability is the constitutional undercooling of liquid which arises because of the segregation of alloying elements ahead of the front. The presence of this extra concentration of alloying elements reduces the liquidus temperature of the newly developed alloy constitution in liquid. If this reduction is sufficient to reduce the melting point to below the actual temperature at that point, then the liquid is said to be locally constitutionally supercooled. Fig.1.14. The transition of growth morphology from planar, to The difference between the cellular grains with a planer front formed cellular, to dendritic, as compositionally (constitution) induced in pure metals and in alloys is shown in Fig.1.15. In case of alloys undercooling increases. ‫ﻣﺤﻤﺪ ﺍﺣﻤﺪ ﻣﺤﻤﺪ ﺍﻟﺨﻄﻴﺐ‬ 1 Solidification of Metals 23 24 1 Solidification of Metals growth rather than the nucleation of new crystals now predominates and grains grow into the melt perpendicular to the solid/liquid interface. The most rapid growth directions of dendrites are for FCC and BCC crystals and lie along the direction of heat flow. Thus the crystals develop a preferred orientation and a characteristic columnar form. 2024/2025 2024/2025 2024/2025 The growth form of the interface between the columnar crystals and the liquid varies from planar to dendritic, depending upon the particular metal (or alloy) and thermal conditions. As the columnar zone thickens, the undercooling becomes higher and the formation of new nuclei from the rest of liquid becomes more likely. Under these conditions a central zone of equiaxed, randomly oriented crystals can develop (Fig.1.16). Other factors Fig.1.15. Schematic illustration of grains and planer front of; (a) such as a low pouring temperature (low superheat), molds of low pure metals and single phase alloys, (b) two-phase system, thermal conductivity and the presence of alloying elements also consisting of two sets of grains, and. favor the development of this equiaxed zone. There is a related size effect, with the tendency for columnar crystals to form decreasing as The as-cast grain structure is usually not as uniform and the cross-section of the mold cavity decreases. straightforward as those discussed above. When solidification starts However, in the absence of these influences, growth of grains at the metallic mold interface there is usually an extreme dominates the nucleation and columnar zone may extend deeper to undercooling or chilling action which leads to the heterogeneous the center of the ingot to produce fully columnar grain ingots as nucleation of a thin layer of fine, randomly-oriented chill crystals shown in Fig.1.16b (e.g. pure metals). (Fig.1.16). The size of these equiaxed crystals is strongly influenced by the texture of the mold surface. As the thickness of the zone of e.g. titanium chill crystals increases, the rate of cooling decreases and crystal and/or boron (for aluminum alloys), zirconium or (for magnesium alloys) and aluminum (for steel), is a common and effective method ‫ﻣﺤﻤﺪ ﺍﺣﻤﺪ ﻣﺤﻤﺪ ﺍﻟﺨﻄﻴﺐ‬ 1 Solidification of Metals 25 26 1 Solidification of Metals for providing heterogeneous nucleation sites within the melt, The possible grain structures formed in castings are shown inhibiting undercooling and producing a uniform fine-grained schematically in Fig.1.17. The chill zone contains crystals nucleated structure. Refining the grain structure disperses impurity elements at the mold surface. There is then selective growth into the liquid as over a greater area of grain boundary surface and generally benefits heat is extracted from the mold. If the liquid in the center of the mold mechanical and founding properties (e.g. ductility, resistance to hot- is undercooled sufficiently there may also be equiaxed grains, either tearing). However, the need for grain refinement during casting dendritic or non-dendritic. The presence of melt agitation due to, e.g. operations is often less crucial if the cast structure can be free or forced convection reduces solute built up at the solid/liquid 2024/2025 2024/2025 2024/2025 subsequently worked and/or heat-treated. Nucleating agents must interface and reduces the boundary layer thickness, leading to the remain finely dispersed, must survive and must be wetted by the formation of equiaxed non-dendritic grains as shown in Fig.1.17c. superheated liquid. Fig.1.17. Schematic illustration of three basic types of cast structures: (a) columnar dendritic; (b) equiaxed dendritic; and (c) equiaxed non-dendritic A combination of the temperature gradient, G, and solidification rate (speed of solidification front advance), R, is very useful in predicting Mixed Columnar Equiaxed the resulting morphology of castings as given in Fig.1.18. The value (a) (b) G/R determines whether a particular region of the casting will have Fig.1.16. The macrostructure of an as-cast pure aluminum ingot, a, an equiaxed dendritic, columnar dendritic, or plane front structure. and schematic illustration of different structure zones, b. ‫ﻣﺤﻤﺪ ﺍﺣﻤﺪ ﻣﺤﻤﺪ ﺍﻟﺨﻄﻴﺐ‬ 1 Solidification of Metals 27 28 1 Solidification of Metals columnar structure or by addition of grain refiners to the melt to change it to fully equiaxed structure. 2024/2025 2024/2025 2024/2025 Fig.1.19. Example of turbine blades Fig.1.18. Effects of temperature gradient, G (°C/mm), and solidification rate, R (mm/s), on the as-cast grain morphology. Understanding the origin of the solidification structure is very helpful for controlling the as-cast structure of the casting components and correspondingly, their mechanical properties. Three basic types of as-cast structures are known: (1) the columnar growth that is favored e.g. in single crystal growth or in some turbine blades, (2) the equiaxed grain structure which exhibits more isotropic properties and characterizes with higher toughness, and (3) a mixed columnar-equiaxed structure with a columnar-to-equiaxed transition (CET) zone. The latter as-cast structure exhibits Fig.1.20. (a) Directional solidification (DC); (b) single crystal by undesirable anisotropic mechanical properties and have to be selection from DC base; (c) single crystal from a seed. avoided either by modification of cooling conditions to get a fully ‫ﻣﺤﻤﺪ ﺍﺣﻤﺪ ﻣﺤﻤﺪ ﺍﻟﺨﻄﻴﺐ‬ 1 Solidification of Metals 29 30 1 Solidification of Metals 1.4. Segregation Other forms of microsegregation may arise as a result of impurities and oxide inclusions dissolved in the liquid metal. During Segregation may be defined as any deviation from uniform solidification, such impurities are pushed progressively out of the distribution of the chemical elements in the alloy. Some variation in advancing solid front. The impurities content in liquid increases composition occurs on a microscopic scale, e.g. between dendrite gradually. At the end of solidification, the impurities either arms, known as microsegregation. It can sometimes be significantly entrapped at the inter-dendrite spacing or precipitated at the grain reduced by a homogenizing heat treatment because the diffusion boundaries as shown in Fig.1.22. This type of segregation is distance over which the alloying elements can 2024/2025 redistribute is 2024/2025 2024/2025 probably the most deleterious in its effect, since it will cause overall sufficiently small (usually in the range 10 -100 m). brittleness of the castings and, depending upon the nature of the Macrosegregation cannot be removed so simply. It occurs over impurity. It can be partially eliminated by subsequent forming distances ranging from 1 cm to 1 m, and so cannot be removed by diffusion. 1.4.1. Microsegregation There are many forms in microsegregation that may exist in castings as a consequence of solidification process. For instance, a cored structure is formed during non-equilibrium cooling of solid solutions with core rich in metal with higher melting point, while the outer of the crystals is rich in metal with lower melting point as shown in Fig.1.21a. Nevertheless, within such a 'cored' crystal, once diffusion is allowed; atoms migrate from higher concentration zones to the lower concentration areas. Diffusion is only appreciable at relatively high temperatures, for which reason a rapidly cooled alloy will be appreciably cored and a slowly cooled alloy only slightly cored. Prolonged annealing will remove coring completely by allowing Fig.1.21. Schematic illustration of the formation of the cored structure, a, and the homogenization of the grains by annealing diffusion to take place and produce a uniform distribution of solid heat treatment, b. solution (Fig.1.21b). ‫ﻣﺤﻤﺪ ﺍﺣﻤﺪ ﻣﺤﻤﺪ ﺍﻟﺨﻄﻴﺐ‬ 1 Solidification of Metals 31 32 1 Solidification of Metals processes, e.g. forging, where the impurities may homogeneously be 1. Decreasing the time available for their formation by increasing redistributed over the newly deformed grains. the rate of solidification. 2. Adjusting the chemical composition of the alloy to give a solute- rich liquid which has more nearly neutral buoyancy at the temperature within the freezing zone. 2024/2025 2024/2025 2024/2025 Fig.1.22. Segregation of impurities at grain boundaries 1.4.2. Macrosegregation When casting size gets bigger, the intensity of natural convection of the melt and the inter-dendritic fluid flow becomes stronger. Thus a macro-scale-segregation forms. The natural melt convection is considered the primary driving force for the formation of the A-type and V-type defects in steel ingots and sand mold roller castings as shown in Fig.1.23. The interaction between the effects of gravity driven natural convection and the solute type and concentration leads to the formation the various macrosegregation defects in the cast ingots. Fig.1.23. Macrosegregation defect in conventional large steel Channel (V or A) segregates can be controlled by: ingot ‫ﻣﺤﻤﺪ ﺍﺣﻤﺪ ﻣﺤﻤﺪ ﺍﻟﺨﻄﻴﺐ‬ 34 2. Metal Casting Technology casting process. For the first time, the core for making hollow Chapter 2 sockets in the cast objects was invented. The core was made of baked sand. Also the lost wax process was extensively used for making ornaments using the casting process. Casting technology was greatly improved by Chinese from around 1500 BC. For this there is evidence of the casting activity found in China. For making highly intricate jobs, a lot of time in making the perfect mold to the last 2024/2025 2024/2025 2024/2025 detail so hardly any finishing work was required on the casting made from the molds. Indus valley civilization was also known for their extensive use of casting of copper and bronze for ornaments, Casting is a process in which molten metal flows by gravity or other weapons, tools and utensils. But there was not much of improvement force into a mold where it is solidified in the shape of the mold in the casting technology. From various objects that were excavated cavity. The term casting is also used for the parts made by this from the Indus valley sites, they appear to have been familiar with process. all the known casting methods such as open mold and piece mold. Casting process is one of the earliest metal shaping techniques Although casting process seems too simple; yet there are many known to human being. It means pouring molten metal into a variables that could affect the quality of final products. Castings refractory mold cavity and allows it to solidify. The solidified object allow parts to be made at a rapid rate with controlled accuracy. is taken out from the mold either by breaking or taking the mold Castings replace parts that otherwise would be difficult or apart. The solidified object is called casting and the technique impossible to machine and very costly to manufacture. However, followed in method is known as casting process. The casting process castings cannot replace many types of machined parts because of was discovered probably around 3500 BC in Mesopotamia (Iraq material, configuration, and other physical considerations. Many region). In many parts of world during that period; copper axes, types of processes are used in the casting and foundry industries to wood cutting tools, and other flat objects were made in open molds produce cast parts in different materials and for various dimensional using baked clay. These molds were essentially made in single piece. accuracy requirements. The Bronze Age 2000 BC brought forward more refinement into ‫ﻣﺤﻤﺪ ﺍﺣﻤﺪ ﻣﺤﻤﺪ ﺍﻟﺨﻄﻴﺐ‬ 2. Metal Casting Technology 35 36 2. Metal Casting Technology Advantages of casting Strength and lightness in certain light metal alloys, which 1. The most intricate external or internal shapes may be cast. As can be produced only as castings. a result, many other operations, such as machining, forging, Good bearing qualities are obtained in casting metals. and welding, can be minimized or eliminated. 7. An economic advantage may exist as a result of any one or a 2. Some metals can only be shaped by casting since they cannot combination of points mentioned above. The price and sale be hot-worked into bars, rods, plates, or other shapes due to the factor is a dominant factor the competitive enterprises. 2024/2025 2024/2025 2024/2025 3. Construction may be simplified. Objects may be cast in a single Moreover casting can be used with other materials like some piece which would otherwise require assembly of several polymers and composites. pieces if made by other methods. Applications vary from very small parts of few grams to huge parts, 4. Metal casting is a process highly adaptable to the requirements e.g. dental crowns, jewelry, automotive engine blocks, machine of mass production. Large numbers of a given casting may be frames, etc. Table 2.1 summarizes the common applications and produced very rapidly. For example, in the automotive industry characteristics of different casting materials. hundreds of thousands of cast engine blocks and transmission cases are produced each year. 5. Extremely large, heavy metal objects may be cast when they would be difficult or economically impossible to produce otherwise. Large pump housings, valves, and hydroelectric plant parts weighing up to 200 tons illustrate this advantage. 6. Some engineering properties are obtained more favorably in cast metals. Examples are: More uniform properties; i.e., cast metals exhibit the same properties regardless of which direction is selected for the test piece. This is not generally true for wrought metals. ‫ﻣﺤﻤﺪ ﺍﺣﻤﺪ ﻣﺤﻤﺪ ﺍﻟﺨﻄﻴﺐ‬ 2. Metal Casting Technology 37 38 2. Metal Casting Technology 2024/2025 2024/2025 2024/2025 Table 2.1. Typical applications for castings and casting characteristics. Fig.2.1. Eight-cylinder grey cast iron engine block from Fig.2.2. Housing of wind power generator (in the front) and ship propeller (in the middle). ‫ﻣﺤﻤﺪ ﺍﺣﻤﺪ ﻣﺤﻤﺪ ﺍﻟﺨﻄﻴﺐ‬ 2. Metal Casting Technology 39 40 2. Metal Casting Technology There are also disadvantages associated with casting depending on the used casting technique, e.g. relatively poor dimensional accuracy and surface finish, porosity, limitations on mechanical properties, safety hazards to human and environmental problems. Casting is usually performed in a foundry which is a factory equipped for making molds, melting and handling molten metal, Fig.2.3. Classification of solidification processes performing 2024/2025 the casting process, and cleaning the finished casting. 2024/2025 2024/2025 Workers who perform casting are called foundrymen. 2.1. Classification of casting processes The solidification processes can be generally classified according to the engineering material that is processed as shown in Fig.2.3 into: metals; glasses (ceramics); and polymers and polymeric matrix composites. The casting processes of metals can in turn classified according to the parts produced into two main categories: Ingot casting, which is usually associated with the production of primary shapes, e.g. blocks or slabs, which are subsequently reshaped by other forming processes like rolling, forging, etc. This category of products lies out of the scope of this book. Shape casting aims at producing net shape or near net shape products of more complex geometries using various techniques. Fig.2.4 summarizes the different techniques of shape casting based on the mold type. Expendable molds of, e.g. sand plaster, etc. are those destroyed after pouring and solidification of the cast component to remove it. ‫ﻣﺤﻤﺪ ﺍﺣﻤﺪ ﻣﺤﻤﺪ ﺍﻟﺨﻄﻴﺐ‬ 2. Metal Casting Technology 41 42 2. Metal Casting Technology A brief comparison showing the advantages and limitations of each of the casting processes is given in Table 2.2. A more detailed quantified comparison about the characteristics and features of each process it given in Table 2.3. All casting processes follow up these steps: 1. Mold/die preparation. 2024/2025 2024/2025 2024/2025 2. Melting and treatment of the molten metal. 3. Pouring melt into a mold/die. 4. Solidification with certain rate under certain pressure 5. Finishing of the casting. Fig.2.4. Classification of metal casting processes Permanent molds are made of temperature resistant metals to allow production of numerous castings by the reuse of the molding tool. The selection of the casting method is mainly based on the size and number of parts, the required degree of surface finish. Because of the high cost of equipment, die casting is economical mainly for large production runs. Fig.2.5 shows an economic comparison between different casting processes. The costs per component are drastically reduces for permanent and die casting as the number of parts exceeds 100 pieces, whereas the costs of sand cast parts remains high and unchanged at this production rate. Fig.2.5. Economic comparison of making a part by two different casting processes.  ‫ﻣﺤﻤﺪ ﺍﺣﻤﺪ ﻣﺤﻤﺪ ﺍﻟﺨﻄﻴﺐ‬ Table 2.2. Typical applications for castings and casting characteristics. 2024/2025 2. Metal Casting Technology 2024/2025 43 2024/2025 44 Table 2.3. General characteristics of casting processes. Note: Relative rating, 1 best, 5 worst. For example, die casting has relatively low porosity, mid- to low shape complexity etc. 2. Metal Casting Technology ‫ﻣﺤﻤﺪ ﺍﺣﻤﺪ ﻣﺤﻤﺪ ﺍﻟﺨﻄﻴﺐ‬ 2. Metal Casting Technology 45 46 2. Metal Casting Technology 2.2. Expendable mold casting Sand cores may be placed in the cavity to produce holes in the part where required. Once the cope and drag are clamped together, the 2.2.1. Sand casting molten metal is poured into the gate of the mold. Vent holes placed In the sand-casting process, a wooden, plastic, or metal pattern is appropriately in the mold allow hot gases and water vapor to escape from the mold cavity during pouring. The pouring temperature of packed in special sand, which is dampened with water and then the metal is always made high enough over the melting point of the removed, leaving a hollow space having the part's negative shape. alloy so that the metal has good fluidity during the pour and does not The2024/2025 pattern is purposely made larger than the size2024/2025 of the cast part to 2024/2025 cool prematurely causing voids and defects in the part. allow for shrinkage of the casting as it cools. Some other allowances, e.g. shake, fillets and machining, are added too. A schematic illustration for the sequence of the operations used in sand casting is illustrated in Fig.2.7. (a) A mechanical drawing of The mold consists of two steel frames, which are called the cope (top half) and the drag (bottom half). Fig.2.6 is an illustration of a typical the part, used to create patterns. (b-c) Patterns mounted on plates equipped with pins for alignment. Note the presence of core prints sand-casting mold provided with the main features. designed to hold the core in place. (d-e) Core boxes produce core halves, which are pasted together. The cores will be used to produce the hollow area of the part shown in (a). (f) The cope half of the mold is assembled by securing the cope pattern plate to the flask with aligning pins, and attaching inserts to form the sprue and risers. (g) The flask is rammed with sand and the plate and inserts are removed. (h) The drag half is produced in a similar manner. (j) The core is set in place within the drag cavity. (k) The mold is closed by placing the cope on top of the drag and securing the assembly with pins. (l) After the metal solidifies, the casting is removed from the mold. (m) The sprue and risers are cut off and recycled, and the casting is cleaned, Fig.2.6. Schematic illustration of a typical sand mold showing inspected, and heat treated (when necessary). various features. ‫ﻣﺤﻤﺪ ﺍﺣﻤﺪ ﻣﺤﻤﺪ ﺍﻟﺨﻄﻴﺐ‬ 2. Metal Casting Technology 47 48 2. Metal Casting Technology Sand casting is the least expensive of all the casting processes on a part-to-part basis, but a need for secondary machining operations may indicate the use of one of the other casting processes. 2024/2025 2024/2025 2024/2025 Fig.2.7. Schematic sequence of sand casting operations. Fig.2.7. (Cont.) Schematic sequence of sand casting operations. ‫ﻣﺤﻤﺪ ﺍﺣﻤﺪ ﻣﺤﻤﺪ ﺍﻟﺨﻄﻴﺐ‬ 2. Metal Casting Technology 49 50 2. Metal Casting Technology 2.2.2. Shell mold casting The main disadvantages of shell molding are: It is relatively new technique discovered in Germany during the Higher pattern cost - not economical for small runs casting size Second World War. It is generally used for mass production of limitations relatively higher sand mixture costs. accurate thin castings with close tolerance of ± 0.02 mm and with smooth surface finish. The procedure of shell mold casting process is illustrated in Fig.2.8. Hot2024/2025 pattern (~225°C) and box is containing a mixture of sand and 2024/2025 2024/2025 resin. Pattern and box inverted and kept in this position for some time (~30 seconds). Now box and pattern are brought to original position. A hard layer (shell) of resin-bonded sand sticks to the pattern and the rest falls. Shell separates from the pattern with the help of ejector pins. Similarly, the second half of the shell-mold is prepared. Finally shell halves clipped together to form the mold cavity. The mold may be provided with cores in to form the internal features of the casting. The finished molds are supported in particulate rocks or permeable sand. The main advantages of shell molding are: (a) Suitable for thin-wall sections and. (b) Excellent surface finish and good dimensional accuracy. (c) Reduced post-machining and cleaning costs. (d) Occupies less floor space. (e) The produced molds can be stored for long time. (f) Mass production. Fig.2.8. Schematic illustration of the shell-molding process. (g) Low skilled labor can be employed. ‫ﻣﺤﻤﺪ ﺍﺣﻤﺪ ﻣﺤﻤﺪ ﺍﻟﺨﻄﻴﺐ‬ 2. Metal Casting Technology 51 52 2. Metal Casting Technology 2.2.3. Plaster Molding The mold material is gypsum or plaster of Paris. Additives like talc, fibers, asbestos, silica flour etc. are added in order to control the contraction characteristics of the mold as well as the settling time. This plaster slurry is poured over a metallic pattern (Fig.2.9) confined in a flask. Wood pattern are not used because the water in the2024/2025 plaster will swell up the wood. As the flask2024/2025 is filled with the 2024/2025 slurry, it is vibrated so as to bubble out any air entrapped in the slurry and to ensure good mold filling. The plaster material is allowed to set. The pattern is then withdrawn by blowing compressed air through the holes in the patterns leading to the parting outer surface of the pattern. The produced plaster mold is finally dried in an oven to ~200-700°C. Fig.2.9. Sequence of plaster molding process Advantages (a) Good surface finish compared to sand molds. (b) Slow and uniform rate of cooling because of low thermal 2.2.4. Investment (precision) casting conductivity of plaster and possibility hot cracking and distortion This process was first developed by ancient Egyptians, ~3500 years is reduced. ago. In this process an expendable pattern of wax is completely Limitations coated (invested) with refractory slurry that sets at ambient (a) Possibility of porosity formation as a result of the remaining temperature. The pattern is then melted out of the formed refractory water content. This can be avoided by mold dehydration at high shell. Ceramic cores are used when required. The finished parts are temperatures. However, the mold strength will be reduced dimensionally accurate and generally are used "as-cast". The process is used for high-accuracy mass-production of small size parts e.g. (b) The permeability of the plaster mold is low. jewelry accessories, dental crowns, turbine blades, complex ‫ﻣﺤﻤﺪ ﺍﺣﻤﺪ ﻣﺤﻤﺪ ﺍﻟﺨﻄﻴﺐ‬ 2. Metal Casting Technology 53 54 2. Metal Casting Technology machinery parts, etc. The economics of this process must be 2.2.5. Lost foam casting weighed against the complexity of the part. Simple parts are Unlike any other sand casting processes, no binders are used. Pre- generally not economical to be produced with this process. The steps forms of the parts to be cast are molded in expandable polystyrene of investment casting are illustrated in Fig.2.10. or special expandable copolymers. Complex shapes can be formed by gluing pattern parts together. The pre-forms are assembled into a cluster around a sprue then coated with a refractory paint. The cluster 2024/2025 2024/2025 2024/2025 is invested in dry sand in a simple molding box and the sand compacted by vibration. Metal is poured, vaporizing the pre-form and replacing it to form the casting (Fig.2.11). Fig.2.11. Schematic illustration of the expendable-pattern casting. Fig.2.10. Schematic illustration of investment casting (lost wax). ‫ﻣﺤﻤﺪ ﺍﺣﻤﺪ ﻣﺤﻤﺪ ﺍﻟﺨﻄﻴﺐ‬ 2. Metal Casting Technology 55 56 2. Metal Casting Technology Many problems hindered the development of the lost foam casting process. By working closely together, designers, foundries, 2.3. Permanent Mold Casting equipment engineers, polymer manufacturers and coating suppliers 2.3.1. Permanent (gravity) mold casting have now removed these barriers, making the process a cost Mold materials are basically made of dense, fine grained, heat effective way to manufacture quality castings. resistant cast iron, steel, bronze, anodized aluminum, graphite or Advantages other suitable refractoriness. The molten metal is poured into die a. The capital cost of lost foam foundry ~50% lower 2024/2025 than green 2024/2025 2024/2025 cavity under gravity. Thus, the operation is also known as gravity sand plant. die casting. When the molten metal is introduced into the die under b. Reduced grinding and finishing. pressure, the process is called pressure die casting. Fig.2.12 shows c. Design freedom and ability to make complex castings by gluing schematically the procedure of permanent mold casting. (1) Mold is patterns together. preheated and coated; (2) cores are inserted and mold is closed; (3) d. Casting configuration flexibility, e.g. thin walls, zero draft, molten metal is poured; (4) mold is opened and; (5) finished part backdrafts, undercuts and keyways. after release of the gating system. e. Cast-in metal inserts are easily added to the pattern. f. Coring required in most complex castings is no longer needed. Advantages g. Parting lines and the costs of grinding them off are eliminated. Challenges: (a)Finer grain structure (Fig.2.13) and better mechanical properties. a. Process is difficult to automate completely; cluster assembly and (b) No blow holes exist in castings. coating involves manual labor. (c) Economical for mass production. b. To obtain good casting, pre-experiments are required. (d) Close dimensional tolerance and good surface finish. c. (e) The process requires less labor. with gases produced. Disadvantages d. Deformation of patterns during compaction. (a) The metallic mold is more expensive compared to sand mold. e. Uniform vaporization of foam during casting and removal of (b) The process is impractical for large castings. combustion products. (c) Refractoriness of the high melting point alloys. ‫ﻣﺤﻤﺪ ﺍﺣﻤﺪ ﻣﺤﻤﺪ ﺍﻟﺨﻄﻴﺐ‬ 2. Metal Casting Technology 57 58 2. Metal Casting Technology 2.3.2. Slush Casting: Slush Casting is a special type of permanent mold casting, where the molten metal is not allowed to completely solidify. After the desired wall thickness is obtained, the not yet solidified molten metal is poured out. This is useful for making hollow ornamental objects such as candlesticks, lamps, statues etc. Zinc- or lead-base alloys 2024/2025 2024/2025 2024/2025 generally are used. The casting wall thickness is controlled by the solidification tim