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METAL CASTING: DESIGN AND MATERIALS
3.2 Casting Design Considerations
3.3 Computer Modeling of Casting Processes
Increased competition for commercial casting products has highlighted the need to provide engineers and managers with a better understanding of the interrelationships between product design, material, and process selection and of economical methods of casting for specific applications. Successful casting practice requires the proper control of numerous variables. These variables pertain to the particular characteristics of the casting shape, the metals and alloys cast, the type of mold, the mold and die materials, and various process parameters.
3.2 CASTING DESIGN CONSIDERATIONS
A number of factors affect the successful final design of a cast component. Often, however, simple design changes can have a significant impact on the cost, speed of production, and usefulness of the part. For example, casting drill spots may be preferred to casting a cored-through hole. Even though an extra step is required, offhand drilling after casting may be simpler and less expensive than creating a complex mold with numerous cores.
All casting processes have the same characteristics: for example, the process begins before the pattern or die is made, at the point when an engineer decides on the best design for the component; the cost of materials is another important design consideration for all type of casting processes. Accurate comparisons require looking beyond the cost per pound or cost per cubic inch to fully analyze the advantages and disadvantages of each competing process. For instance, the relatively greater strength of metals generally allows thinner walls and sections and consequently requires fewer cubic inches of metals for a given application.
The one rule that covers every stage of good design is communication. While there are rules that govern how molten metal will solidify and take shape as a cast component, each casting process will affect the metal differently and will offer its own benefits. Before issuing a final drawing, it is imperative the design engineer consult a foundry team or patternmaker. The engineer must know how to design a casting that will actually have the requisite strength and functional properties, while a foundry team must be able to make the casting so that it has the strength and functional properties the engineer intended. From a foundry point of view it is more important to receive a component design that is practical and efficient than a “perfect” design that cannot be produced commercially without structural weakness.
Consultation will permit consideration of foundry problems that are likely to be encountered and will promote casting soundness. The time and cost of manufacture also should be considered in the preliminary stages of casting design. Hence, it is recommended that before the design or the order is finalized, the design engineer should communicate with the foundry to discuss pertinent issues such as the following:
•the proposed casting design and accuracy required;
•machining requirements;
•method of casting;
•the number of castings;
•the casting equipment involved;
•Any special requirements (for instance, datum target systems, individual dimensional tolerances, geometrical tolerances, fillet radii tolerance, and individual machining allowances);
•whether any other standard is more appropriate for the casting.
3.2.1 General Design Considerations for Casting
The responsibility of the casting engineer is to determine how to produce the part as a metal casting. He or she studies the original design and determines how best to design the part for manufacturability and casting. Once those considerations are defined, the engineer goes through a specific set of decisions in order to develop a casting design and a casting pattern for making the mold or die.
a) Design of Cast Parts
The following guidelines should be considered:
Corners and angles. Avoid designing parts with sharp corners; sharp corners produce a differential cooling rate, creating “hot spots,” the most common defect in casting design; mechanical weaknesses and stress concentration open the way for potential cracks.
In design of adjoining sections, sharp angles should be replaced with radii, and heat and stress concentration should be minimized. However, adding radii that are too large may also result in shrinkage defects. By incorporating small fillet radii, hot spots are avoided, assuring improved strength. Some examples of improved designs are shown in Fig. 3.1.
Fig. 3.1 Suggested design modifications to improve quality of castings: a) and c) incorrect; b) and d) are correct.
Fillets. Fillets (rounded corners) have three functional purposes:
•to reduce the stress concentration in a casting in progress;
•to eliminate cracks, tears, and draws at reentry angles;
•to make corners more moldable by eliminating hot spots.
The number of fillet radii should be the least possible, preferably only one. To fulfill engineering stress requirements and reduce stress concentration, relatively large fillets may be used with radii equaling or exceeding those of the casting section. Fillets that are too large are undesirable. The radius of the fillet should not exceed half the thickness of the section joined.
At an “L” junction, round any outside corner to match the fillet on the inside wall. In the case of “V” or “Y” sections and other angular forms, always design so that a generous radius eliminates localization of heat. Figure 3.2 shows suggested design modifications toward improving the quality of casting, avoiding filleting, and making corners more moldable.
Fig. 3.2 Redesigned part to avoid hot spots and fillet: a), c), e) and g) are incorrect; b), d), f), and h) are correct.
Section thickness. Maintain uniform cross-sections and thickness where possible. Thicker walls will solidify more slowly, so they will feed thinner walls, resulting in shrinkage voids. The goal is to design uniform sections that solidify evenly. If this is not possible, all heavy sections should be accessible to feeding from risers.
Figure 3.3 shows an example of a part that has been redesigned with uniform walls; the weight of the casting was reduced, lowering the manufacturing cost and remedying the shrinkage problem.
Fig. 3.3 Redesigned part with uniform walls: a) incorrect; b) correct.
The inner sections of castings cool much more slowly than the outer sections and cause variations in strength properties. A good rule is to reduce the inner sections to 0.9 of the thickness of the outer wall.
The inside diameter of cylinders and bushings should exceed the wall thickness of the castings. When the inside diameter of a cylinder is less than the wall thickness, it is better to cast the section solid, as holes can be produced by cheaper (and safer) methods than with extremely thin cores.
Section changes. The design should not contain abrupt section changes. Section changes should blend into each other (Fig. 3.4). The difference in the relative thickness of adjoining sections should not exceed a ratio of 2:1. If a section change of over 2:1 thickness ratio is unavoidable, the alternative is to design two separate castings to be assembled together, like machine tool beds that can be bolted.
Fig. 3.4 Suggested design modification to avoid abrupt section changes: a) incorrect; b) and c) correct.
Minimize the number of sections. The goal is to design the casting well with a minimum numberof sections together at one point. A simple wall section will cool freely from all surfaces, but simply adding a section forming a T or intersect shape also creates a hot spot at the junction, and it will cool as would a wall that was 50% larger.
The way to avoid this is to stagger the ribs (Fig. 3.5b) and thereby maintain uniform cross-sections. Staggering sections minimizes hot-spot effects, thus eliminating weakness and reducing distortion.
When two or more uniform sections intersect they create a region of heavy cross-section, resulting in the problems mentioned earlier. One way to minimize this is to core the intersection by a hole, similar to a hub hole in a wheel with spokes (Fig. 3.5c).
Fig. 3.5 Schematic illustrations of minimize number of section: a) incorrect; b) correct (stagger section); c) correct (core intersection by a hole).
Flat areas. Large flat areas (plane surfaces) should be avoided. They are difficult to feed and it is difficult to develop in them good directional solidification, and may wrap or develop poor finish surface because of uneven metal flow. Lightener holes eliminate the problem of isolated flat areas; they also save weight. But they need to be placed in “nonstructural” areas (Fig. 3.6).
Fig. 3.6 Suggested design modification to avoid flat areas: a) incorrect; b) correct.
Shrinkage. Molten metals contract during cooling. To avoid thermal stress and cracking of the casting during cooling, the cavity is usually made oversized. To account for shrinkage, pattern dimensions should also allow for shrinkage during solidification. These shrinkage allowances are only approximate, because the exact allowance is determined by the shape and size of the casting. Table 3.1 gives the approximate shrinkage allowance for metals that are commonly sand cast.
Table 3.1 Shrinkage allowance for some metals cast in sand molds
Draft. Draft is the amount of taper, or the angle, that must be allowed on all vertical faces of a pattern to permit its removal from the sand mold without permitting tearing of the mold walls; draft is also an issue in the removal of a casting from a die. (Fig. 3.7). The standard draft for sand casting is 1 degree per external side. The angles on the inside surfaces typically are twice this range.
Fig. 3.7 Draft angle: a) incorrect; b) correct.
Dimensional tolerances. Dimensional tolerances depend on the particular casting processes, row casting basic dimension, casting materials, and casting tolerance grades. The International Standard (ISO) is defined for16 casting tolerance grades designated CT1 to CT16. Table 3.2 gives dimensional casting tolerances for sand casting, machined molded.
Machining allowance. On raw castings, some extra material should be left to permit the removal of the effects of casting on the surface by subsequent machining and to allow the achievement of the desired surface texture and the necessary agreement with design dimensions.
Part identification. In the final cast drawing, the design engineer in final cast drawing needs to include some form of part identification. These features can be sunk into the casting or can protrude from the surface.
b) Locating the Parting Line
Parting in one plane facilitates the production of the pattern as well as the production of the mol. The term parting line is a bit of a misnomer. Perhaps the term parting surface would be better. However, the first term is commonly used, which is why we retain the term parting line in this book. Parting lines are the lines on the component where the mold or die halves come together. Parting in one plane facilitates the production of the pattern as well as the production of the mold.
Table 3.2 Dimensional Tolerances for Sand Casting, Machine Molded.
* The tolerance zone should be symmetrically disposed with respect to a basic dimension, i.e. with one half on the positive side and one half on the negative side
Patterns with straight parting lines (that is, with parting lines in one plane) can be produced more easily and at lower cost than those with irregular parting lines.
Casting shapes that are symmetrical about one centerline or plane readily suggest the parting line. Such casting design simplifies molding and coring and should be used wherever possible. The patterns should always be made as split patterns, which require a minimum of handwork in the mold, improve casting finish, and reduce costs.
Parting lines that cross a feature with tight tolerances may lead to decreased yield due to mold mismatch. Since parting lines will be noticeable as a bump on the surface of the part, they should not be located on a sliding surface. Similarly, parting lines should not be located on a sealing surface, where a bump or mismatch would prevent the seal from making complete contact. The placement of the parting line and orientation of the part determine the number of cores needed, and it is preferable to avoid the use of cores whenever possible.
c) Location and Design of a Gating System
The primary requirement for the gating system is that it accomplishes an extremely careful transport of the liquid metal to the mold cavity. Gating system location is defined on the basis of the geometry of the mold, method of casting, and economic aspects.
The gating system guides the poured liquid metal, prepared in the foundry, to the mold cavity and has direct and indirect effects on the quality of the casting process. Redundant the designer and the foundry team should take these aspects into account right from the design phase in order to guarantee trouble-free processing and constant casting quality.
In the design of a gating system the focus is on the casting part. Factors that influence the design of the gating system include:
•size
•targeted surface quality
•mechanical requirements
•complexity of the geometry
•economic efficiency.
The gating system can be divided into the following:
•pouring cup
•sprue
•runner
•riser
•gate.
Figure 3.8 shows a schematic illustration of a typical gating system for a gravity casting process.
Pouring cup design. At the top of the sprue, a pouring cup (another common name is pouring basin) is often used to minimize splash and turbulence as the metal flows into the sprue. The design of the pouring cup, for an optimal casting process, needs to be such that it can keep the sprue full of molten metal throughout the pour; also, if the level of molten metal is maintained in the pouring cup during pouring, then the dross will float and not enter the mold cavity. A constant level of molten metal in the pouring cup is important for a successful casting process because in some cases there is an automatic regulator of the molten metal going into pouring cup.
Fig. 3.8 Schematic illustration of a typical riser-gated casting.
Sprue and sprue base design. The sprue is a tapered vertical channel through which the liquid metal, from the pouring cup, enters a runner that leads into the mold cavity. The main goal in designing a sprue is to achieve the required flow rates. A sprue tapered to a smaller size at its bottom will create a choke (a restriction in the gating system that limits the flow rate of molten metal), which will help keep the sprue full of molten metal, but the choke will also increase the speed of the molten metal, which is undesirable. To solve this problem, the enginer needs to create an enlarged area at the bottom of the sprue, called a sprue base. The sprue base decreases the speed of the molten metal. There are two basic types of sprue bases: enlargement and well.
The general rules for designing an enlargement base are these:
•The diameter is roughly 2.5 times the width of the runner.
•The depth is equal to the depth of the runner.
The general rules for designing a well base are these:
•The depth of the wall base is twice that of the runner.
•A cross-section of the area of the base is five times the cross-sectional area of the sprue bottom.
The bottom of the sprue base should be flat, not round. If it is round, it could cause turbulence in the metal.
If sprue design is not used, there is a danger of turbulent filling of the gating system during pouring, as well as the possibility of having a nonuniform metalostatic pressure head. Turbulent filling can introduce oxides, which may produce defects. Filters are used in keeping oxides from the casting and make the flow more laminar.
Runner design. The runner is a horizontal distribution channel through which molten metal is guided from the sprue into the gate. The type of molding metal determines size and shape of the runner. A large runner allows better flow of the liquid metals; however, cooling takes longer. Thus, a runner is normally made smaller than the section of a casting and gradually made bigger while being tested. In any case, a runner should be basically smaller than the thickness area of the casting part. One runner is used for simple parts, but two runner systems can be specified for more complicated parts.
Gate design. The gate serves as the entrance to the cavity and should be designed to permit the mold cavity to fill easily. A cavity can have more than one gate. It is positioned at the thickest area of a part. A fillet needs to be used where a gate meets a casting. The gate length should be three to five times the gate diameter, depending on the metal being cast. The cross-section of the gate should be smaller than the runner cross-section.
Riser design. Risers are created in the gating system that will be filled with molten metal. Risers provide reservoirs of molten metal that can flow into the mold cavity to compensate for any material shrinkage that occurs during solidification. Any shrinkage should be in the risers and not in the final casting. The metal in the risers must stay liquid longer than the metal in the part being cast. There are two types of riser: open and blind. The blind riser is completely in the mold. This type of riser cools slower because it is not open to the air. The risers can be attached to the top or to the side of a part.
Risers may be attached in the runner instead of the casting. In this case the metal must flow through the riser prior to reaching the casting, and after the pouring is completed the metal in the riser will be hotter than the metal in the casting. It does not matter where the riser is located: the cross-sectional area of the gate just needs to be the same as the runner and the length no more than 0.5 the diameter of the riser. Theoretically, the best shape of the riser is spherical, but in practice the most useful (and easy shape to mold) is the cylinder. The height of the riser should be:
| h = (0.5 − 1.5)D | (3.1) |
where
| h | = | height of riser, m (in.) |
| D | = | diameter of cylinder, m (in.). |
If possible, the top of a blind riser should be spherical. This will help the metal stay molten longer. Risers, therefore, are not as useful for metals with low density, such as aluminum alloys, as they are for metals with a higher density, such as steel and cast iron.
3.3 COMPUTER MODELING OF CASTING PROCESSES
Computer modeling of metal casting processes has been very successful for companies around the world. Numerical simulation is widely used and accepted in manufacturing as a way to reduce hardware prototyping and to improve the parts design and manufacturing processes. The automotive industry and others are increasingly using computer simulations to attain their design objectives. As a result of rapid advances in computing technologies, three-dimensional (3-D) process modeling became practical during the last decade of the twentieth century. Today, many small- to midsized companies perform multiple simulations on a daily basis. Process modeling is no longer a luxury but a necessity for survival in the casting industry.
Cutting costs and reducing time to market are two of the most pressing issues in the foundry industry today. Development time can be very high in the conventional trial-and-error-based process design. In the current competitive environment, there is a need for foundry and casting units to develop the components and the process within quick response windows. Further, the costs of development also have to be kept low to be competitive. Casting process simulation helps engineers achieve these goals and is now widely used throughout the industry for process design and other various aspects of casting processes; tools such as simulation software systems help the designer to visualize the metal flow in the die cavity, the temperature variations, the solidification progress, and the evolution of defects such as shrinkage porosities, cold shuts, hot tears, and so on.
The application of casting simulation has been most beneficial toward avoidance of creating shrinkage scrap, improving cast metal yield, optimizing gating system design, optimizing mold filling, and finding the thermal fatigue life in permanent molds.
Many major foundries are using software based on powerful Finite Element analyses such as ProCAST, which covers a wide range of casting processes and alloys. However, several another commercial software programs are now available for modeling casting processes, such as Magma SOFT, Solidia, and AFS solid.
In the early centuries of casting, bronze and brass were preferred over cast iron as foundry materials. Iron was more difficult to cast due to its higher melting temperatures and workers lack of knowledge about its metallurgy. In the second part of the 16th century, the first cannons were cast from iron and after that a large demand for cast iron began to grow, first for military items and later for commercial products. Today, most commercial castings are made of alloys rather than pure metals. Pure metals like zinc, lead, tin, and copper are really good metals for casting but are used only for art, not for commercial products.
Casting alloys can be classified in two basic categories: ferrous and nonferrous. Ferrous casting alloys are subdivided into cast iron (gray iron, ductile iron, high-alloy white irons, malleable iron, compacted graphite alloy, and high-alloy graphitic irons) and cast steel (plain carbon steels, low-alloy steels, high-alloy steels, and cast alnico alloys).
Cast iron usually refers to gray cast iron but identifies a large group of ferrous alloys which solidify with a eutectic. In these ferrous alloys pure iron accounts for more than 93%, while the main alloying elements are carbon and silicon. The amount of carbon in cast irons is in the range of 2.11 to 4%, as ferrous alloys with less are denoted “carbon steel” by definition. Cast irons contain appreciable amounts of silicon, normally 1 to 3%, whose composition makes it highly suitable as a casting metal. Cast iron’s melting temperature is between 1150 to 1200°C (2100 to 2192°F), about 300°C (572°F) lower than the melting point of pure iron.
Cast iron tends to be brittle unless the name of the particular alloy suggests otherwise. The color of a fracture surface can be used to identify an alloy; carbide impurities allow cracks to pass straight through, resulting in a smooth, white surface, while graphite flakes deflect a passing crack and initiate countless new cracks as the material breaks, resulting in a rough surface that appears gray.
With their low melting point, good fluidity, castability, excellent machinability, and wear resistance, cast irons have become an engineering material with a wide range of applications, including pipes, machine parts, and car parts.
Cast iron is made by remelting pig iron, often along with substantial quantities of scrap iron and scrap steel, and taking various steps to remove undesirable contaminants such as phosphorus and sulphur.
a) Gray Cast Iron
Gray iron is one of the most easily cast of all metals in a foundry. It has the lowest pouring temperature of the ferrous metals, a characteristic that is reflected in its high fluidity and its ability to be cast into intricate shapes. Gray iron usually contains the following: total carbon, 2.75 to 4.00%; silicon, 0.75 to 3.00%; manganese, 0.25 to 1.50% percent; sulfur, 0.02 to 0.20%; phosphorus, 0.02 to 0.75%. One or more of the following alloying elements may be present in varying amounts: molybdenum, copper, nickel, vanadium, titanium, tin, antimony, and chromium, depending on the desired microstructure. As a result of a peculiarity during the final stages of solidification, the metal has very low and, in some cases, no liquid-to-solid shrinkage, so that sound castings are readily obtainable. For the majority of applications, gray iron is used in its as-cast condition, thus simplifying production. Gray iron has excellent machining qualities, producing easily disposed of chips and yielding a surface with excellent wear characteristics. The resistance of gray iron to scoring and galling with proper matrix and graphite structure is universally recognized. Figure 3.9 shows the form of graphite found in gray iron.
Fig. 3.9 Graphite form in gray iron.
Several molding processes are used to produce gray iron castings. Some of these have a marked influence on the structure and properties of the resulting casting. The selection of a particular process depends on a number of factors, and the design of the casting has much to do with it. Processes using sand as the mold medium have a somewhat similar effect on the rate of solidification of the casting, while the permanent mold process has a very marked effect on structure and properties. Products made from gray cast iron include automotive engine blocks and heads, motor housings, and machine-tool bases.
b) Ductile Iron
Ductile iron, also called ductile cast iron or nodular cast iron, is a cast iron that has been treated while molten with an element such as magnesium or cerium to induce the formation of free graphite as nodules or spherulites, which imparts a measurable degree of ductility to the cast metal. While most varieties of cast iron are brittle, ductile iron is quite easy to form with out braking, as the name implies. The main effects of the chemical composition of nodular (ductile) iron are similar to those described for gray iron, with quantitative differences in the extent of these effects and qualitative differences in the influence on graphite morphology. Fig. 3.10 illustrates graphite form in ductile iron structure.
The majority of applications of ductile iron have been made to utilize its excellent mechanical properties in combination with the castability, machinability, and corrosion resistance of gray iron.
Fig. 3.10 Graphite form in ductile iron structure.
c) White Cast Iron
White cast iron is unique in that it is the only member of the cast iron family in which carbon is present only as carbide. Due to the absence of graphite, it has a light appearance. The presence of different carbides, depending on the alloy content, makes white cast irons extremely hard and abrasion resistant, but very brittle. When fractured, the surface has a white crystalline appearance that gives the iron its name. An improved form of white cast iron is called chilled cast iron.
When a localized area of a gray cast iron is cooled very rapidly from the melt, cast iron is formed at the place that has been cooled. This type of white cast iron is called chilled iron. Adjusting the carbon composition of the white cast iron can produce a chilled iron casting, so that the normal cooling rate at the surface is just great enough to produce white cast iron while the slower cooling rate below the surface will produce gray iron. The depth of chill decreases and the hardness of the chilled zone increases with increasing carbon content.
Chromium is used in small amounts to control chill depth. Because of the formation of chromium carbides, chromium is used in amounts of 1 to 4% in chilled iron to increase hardness and improve abrasion resistance. These properties make white cast iron suitable for applications where wear resistance is required. Railway brake shoes are an example.
d) Malleable Iron
Malleable irons are cast white that is, their as-cast structure consists of metastable carbide in a pearlitic matrix. If cast iron is cooled rapidly, the graphite flakes needed for gray cast iron do not get a chance to form. Instead, white cast iron forms. This white cast iron is reheated to about 900 to 930°C (1650 to 1700°F) for long periods of time (50 hours), in the presence of materials containing oxygen, such as iron oxide; after that time it is slowly (60 hours) cooled. Upon cooling, the combined carbon further decomposes to small compact particles of graphite (instead of flake like graphite as seen in gray cast iron). If the cooling is very slow, more free carbon is released. This free carbon is referred to as temper carbon, and the process is called malleableizing. The new microstructures can possess significant ductility and good shock resistance properties. Fig. 3.11 shows the microstructure of malleable iron.
Fig. 3.11 Graphite form in malleable iron structure.
Malleable cast iron is used for connecting rods and universal joint yokes, transmission gears, differential cases and certain gears, compressor crankshafts and hubs, flanges, pipefittings, and valve parts for railroad, marine, and other heavy duty applications.
e) Compacted Graphite Iron
In 1949 a now well known material called ductile iron was patented. At the same time that ductile iron was patented, a lesser known material called compacted graphite iron (CGI) was also patented, although it was only considered a curiosity at the time. This material has higher strength than gray iron and better thermal conductivity than ductile iron.
The graphite of CGI is in the form of relatively short thick flakes with rounded ends and undulating surfaces. In compacted graphite, the graphite does not have the same weakening effect as flake graphite in grey iron, but it is still continuous and gives greater thermal conductivity than the discrete graphite nodules in ductile iron. Fig. 3.12 shows the form of graphite in compacted graphite cast iron.
Fig. 3.12 Graphite form in compacted iron structure.
CGI has risen to an important status in the automotive industry, particularly since 1996. The material has been used for manufacturing parts such as brake disk, exhaust manifolds, engine heads, and diesel engine blocks. The superior strength characteristics of CGI as compared to gray iron, allows the manufacturing of engines with higher pressure operating combustion chambers, which means these engines are more efficient, having lower emissions levels. Also, thinner walls are possible, which means lighter engines.
f) Alloy Cast Irons
Alloy cast irons is a term that designates casting containing alloying elements such as nickel, chromium, molybdenum, copper, and manganese in sufficient amounts to appreciably change the physical properties. They include high-alloy graphitic irons and high-alloy white irons.
One of the most important alloy cast irons is high-alloy white cast iron. Alloy cast irons are an important group of materials whose production must be considered separately from that of ordinary types of cast irons. In these cast iron alloys, the alloy content is well above 4%; and consequently such metals cannot be produced by ladle additions to irons of otherwise standard compositions. They are usually produced in foundries especially equipped to produce highly alloyed irons.
The high-alloy white irons are primarily used for abrasion-resistant applications and are readily cast into the parts needed in machinery for crushing, grinding, and handling of abrasive materials. The chromium content of high-alloy white irons also enhances their corrosion-resistance properties. The large volume fraction of primary or eutectic carbides in their microstructures provides the high hardness needed for crushing and grinding other materials. The metallic matrix supporting the carbide phase in these irons can be adjusted by alloy content and heat treatment to create the proper balance between resistance to abrasion and the toughness needed to withstand repeated impact. The high alloy graphitic cast irons have found special uses primarily in applications requiring corrosion resistance or strength and oxidation resistance in high-temperature service.
Steel is an alloy whose major component is iron with carbon content between 0.02 and 1.7% by weight. The mechanical properties of steel make it an attractive engineering material, and its utility in complex geometries makes casting an appealing process. However, the high temperature required to melt cast steels (for example carbon’s and low alloy steel’s pouring temperature is 1565 to 1700°C (2850 to 3100°F) means that casting them requires considerable experience. At these high temperatures, melting steel oxidizes very quickly; so special procedures must be used during melting and pouring. The high temperatures involved present difficulties in the selection of mold material. Carbon and low alloy steels approach the limit of temperature and ceramic refractors and titanium and zirconium alloys go beyond it, creating special situations. So, it is easy to see the abuse that sand and ceramic molds are subjected to when pouring temperatures approach the refractory limits. Also, molten steel has relatively poor fluidity, and this limits the design of thin sections in parts cast out of steel. Several advantages of cast steels are that steel castings have better toughness than most other casting alloys; also, steel castings’ process properties are isotropic, and depending on the requirement of the product, isotropic behavior of the material may be desirable. Steel castings can be welded, but after welding, castings need to be heat treated to restore to them their mechanical properties.
3.4.3 Nonferrous Casting Alloys
The nonferrous metals include metal elements and alloys not based on iron. Many nonferrous metals are used as engineering materials. Such metals include aluminum, magnesium, titanium, nickel, copper, zinc, refractory metals (molybdenum and tungsten), and noble metals. Although they are more expensive and cannot match the strength of the steels, nonferrous metals and alloys have important applications because of their numerous positive characteristics, such as low density, corrosion resistance, ease of fabrication, and color choice. For example, copper has one of the lowest electrical resistances; zinc has a relatively low melting point; aluminum is an excellent thermal conductor, and it is also one of the most readily formed metals.
a) Aluminum and Aluminum Alloys
Pure aluminum is a silver-white metal characterized by a slightly bluish cast. It has a crystal structure that is face-centered cubic (fcc), a melting point of 660°C, (1220°F), and density of 2700 kg/m3 (163.55 lb/ft3). Aluminum is thermodynamically the least stable of the main engineering metals, but a lucky property of aluminum is the formation of a dense, highly protective alumina film only 1 mm in thickness. This film can be reinforced by anodizing, and can be destroyed by salt. Aluminum is one of the few metals that can be cast by all of the processes used in casting metals. When aluminum is alloyed with other metals, numerous properties are obtained that make these alloys useful over a wide range of applications. Main alloying additions are copper, magnesium, manganese (Mn), silicon, lithium, and zinc.
Depending on the use to which the alloy will be put, different metals will be mixed in with the aluminum. For example, high magnesium content yields superior corrosion resistance. High copper or zinc adds superior strength. A large number of aluminum alloys has been developed for casting, but most of them are varieties of six basic types: aluminum-copper, aluminum-copper-silicon, aluminum-silicon, aluminum-magnesium, aluminum-zinc-magnesium, and aluminum-tin.
Aluminum-copper alloys. Aluminum-copper alloys that contain 4 to 5% of copper, with small impurities of iron, silicon, and magnesium, are heat-treatable and can reach yield strength of 450 MPa (65,266psi), with variations that depend on the composition and temper. These alloys after quenching will slowly harden when left at room temperature for several days. However, these alloys have poor castability and require very careful gating if sound castings are to be obtained.
Aluminum-copper-silicon alloys. The most widely used aluminum casting alloys are those that contain silicon together with copper. The amounts of both additions vary widely, so that the copper predominates in some alloys and the silicon in others. In these alloys the copper contributes to strength and the silicon improves castability and reduces hot shortness. Thus, the higher silicon alloys normally are used for more complex castings and for permanent mold and die casting processes, which cannot tolerate hot-short alloys.
Aluminum-silicon alloys. These alloys do not contain copper; silicon is used when good castability, weldability, and good corrosion resistance are needed. Because of their excellent castability, it is possible to produce reliable castings, even in complex shapes, in which the minimum mechanical properties obtained in poorly fed sections are higher than in castings made from higher-strength but lower-castability alloys. If high strength is also needed, magnesium additions make these alloys heat-treatable. Rapid cooling to increase strength and ductility can refine the microstructure.
Alloys with silicon content as low as 2% have been used for casting, but silicon content usually is between 5 and 13%. The strength and ductility of these alloys, especially the ones with higher silicon, can be substantially improved by “modification.” Modification can be effectively achieved through the addition of a controlled amount of sodium or strontium that refines the silicon eutectic. These alloys have high fluidity and are suitable for sand or die casting.
Aluminum-magnesium-alloys. These alloys, which contain 2 to 5% Mg and 0.1 to 0.4% Mn, have good weldability and high corrosion resistance, especially to seawater and marine atmospheres; this is the primary advantage of castings made of Al-Mg alloys. For best corrosion resistance, a low impurity content (both solid and gaseous) is required, and thus these alloys must be prepared from high quality metals and handled with great care in the foundry. The relatively poor castability of Al-Mg alloys and the tendency of the magnesium to oxidize increase handling difficulties and, therefore, cost.
Aluminum-zinc-magnesium alloys. These aluminum alloys contain 2 to 8 % zinc, 0.5 to 4% magnesium, and 0 to 3% copper. The total amount of zinc, magnesium, and copper controls the properties and consequently the uses. When the total amount is above 9%, high strength is greatest, and corrosion resistance, formability, and weldability are subordinate to it. Below a total of 5–6%, fabricability becomes greatest and stress corrosion susceptibility tends to disappear. Al-Zn-Mg alloys have the ability to naturally age, achieving full strength at room temperature 2 to 3 weeks after casting. This process can be accelerated by furnace aging.
Aluminum-tin alloys. Al-Sn alloys contain about 6% tin and a small amount of copper and nickel for improving strength; castability is good and they are used for cast bearings because of tin’s excellent lubrication characteristics.
b) Magnesium and Magnesium Alloys
Elemental magnesium is a fairly strong, silvery-white, lightweight metal; it is the world’s lightest metal. Its crystal structure is hexagonal close pocket (hcp), its melting point is 650°C (1202°F), and it has a density of 1740 kg/m3 (108.3 lb/ft3).
This lightness, combined with the good strength-to-weight ratio of magnesium, has made magnesium and its alloys very useful use in the airplane, missile, and automotive industries. Its compounds are used as refractory material in furnace linings for producing metals (iron and steel, nonferrous metals), glass, and cement.
Magnesium and its alloys are available in both wrought and cast forms. Magnesium is also the most electrochemically active metal. Therefore, in all processing of magnesium, small particles of the metal such as metal cutting chips oxidize rapidly, and care must be taken to avoid fire hazard.
Magnesium alloys. The main alloying elements are aluminum (Al), zinc (Zn), and manganese (Mn). Commercial cast alloy AZ81 (Mg-8, Al-0.5, Zn-03, Mn), contains 8% Al, 0.5% Zn, and 0.3% Mn. Magnesium alloy castings can be produced by nearly all of the conventional casting methods, namely, sand, permanent, and semipermanent mold and shell, investment, and diecasting. The choice of a casting method for a particular part depends upon factors such as the configuration of the proposed design, the application, the properties required, the total number of castings required, and the properties of the alloy. Typical applications include automotive wheels and air-cooled engine blocks.
c) Copper and Copper Alloys
The element copper is a reddish-yellow material and is extremely ductile. Copper has a face-centered cubic (fcc) crystal structure and a melting point of 1084.6°C (1984.6°F), with a density of 8920 kg/m3 (556.85 lb/ft3); it also has the second best electrical conductivity of the metals, second only to silver (redundant), with respect to which its conductivity is 97%.
Pure copper is extremely difficult to cast, and it is prone to surface cracking, porosity problems, and to the formation of internal cavities. The casting characteristics of copper can be improved by the addition of small amounts of elements, including beryllium, silicon, nickel, tin, zinc, chromium, and silver.
Copper alloys. Unlike pure metals, alloys solidify over a range of temperatures. Solidification begins when the temperature drops below the liquidus; it is completed when the temperature reaches the solidus. A wide variety of copper alloys are available including brasses, aluminum bronzes, phosphor bronzes, and tin bronzes.
Cast copper alloys are used for applications such as bearings, bushings, gears, fittings, valve bodies, and miscellaneous components for the chemical processing industry. These alloys are poured into many types of castings, such as sand, shell, investment, permanent mold, chemical sand, centrifugal, and die casting. The high cost of copper limits the use of its alloys.
REVIEW QUESTIONS
3.1Which rule covers every stage of good cast design?
3.2Describe general design considerations in metal casting.
3.3What are hot-spot effects?
3.4What is draft and is it necessary in all molds?
3.5What is shrinkage allowance?
3.6How many dimensional tolerances are defined in the ISO?
3.7What is the difference between shrinkage and machining allowance?
3.8What is a parting line and how it is determined?
3.9Name the factors that influence the design of the gating system.
3.10Describe a typical gating system for a sand-gravity casting process.
3.11Name the types of cast irons and describe their characteristics.
3.12What is casting steel? Describe its characteristics.
3.13List the most often used nonferrous alloys and describe their properties.
