Machinery's Handbook, 31st Edition
MATERIALS FOR IRON AND STEEL CASTING 1485 maximum hardness to ensure immediate hardness in use. Heat-treated austenitic manganese steel is machined only with great difficulty since it hardens at and slightly ahead of the point of contact of the cutting tool. Grinding wheels mounted on specially adapted machines are used for boring, planing, keyway cutting, and similar operations on this steel. Where grinding cannot be employed and machining must be resorted to, high-speed tool steel or cemented carbide tools are used with heavy, rigid equipment and slow, steady operation. In any event, this procedure tends to be both tedious and expensive. Austenitic manganese cast steel can be arc-welded with manganese-nickel steel welding rods containing from 3 to 5 percent nickel, 10 to 15 percent manganese, and, usually, 0.60 to 0.80 percent carbon. Table 3. Nominal Chemical Composition and Mechanical Properties of Corrosion-Resistant Steel Castings ASTM A743-17
Tensile Strength, min
0.2% Yield Strength, min
Percent Elonga tion in 2 inch, or 50 mm, min
Percent Reduction of Area, min
Nominal Chemical Composition, Percent a
Grade
ksi MPa ksi MPa 140 965 135 930 110 755 80 550 90 620 65 450 100 690 70 485 100 690 70 485 115 790 85 580 65 450 30 205 80 550 40 275 70 485 30 205 70 485 30 205 70 c 485 c 30 c 205 c 70 485 30 205 70 485 30 205 85 585 42 290 70 485 30 205 70 485 30 205 75 515 35 240 85 585 42 290 75 520 35 240 70 485 28 195 70 485 30 205
CA6N
11 Chromium, 7 Nickel
15 15 18
50 35 30 24 25 … 35 … … … … … … … … … … … … … … … … … … … … … … …
CA6NM 12 Chromium, 4 Nickel
CA15 and CA15M CA28MWV b
12 Chromium
12 Chromium, with Molybdenum, Tungsten, and Vanadium 140 965 110 760
10 15 12 16 10 35 30 35 25 30 30 30 25 30 25 30 25 35 30 35 30 30 35 35 35 20
CA40 CA40F CB6 CB6 CC50 CE30 CF3M CF3MN CF8 CF8C CF8M CF3
12 Chromium
12 Chromium, Free Machining 16 Chromium, 4 Nickel
20 Chromium 28 Chromium
… …
55 380 … … … …
29 Chromium, 9 Nickel 19 Chromium, 9 Nickel
19 Chromium, 10 Nickel, with Molybdenum
19 Chromium, 10 Nickel, with Molybdenum and Nitrogen 75 515 37 255
19 Chromium, 9 Nickel
19 Chromium, 10 Nickel, with Columbium 19 Chromium, 10 Nickel, with Molybdenum
CF10SMnN 17 Chromium, 8.5 Nickel with Nitrogen, 9 Nickel
CF16F and CF16Fa
19 Chromium, 9 Nickel, Free Machining
CF20 CG3M CG8M CG12
19 Chromium, 9 Nickel
19 Chromium, 11 Nickel, with Molybdenum
CG6MMN Chromium-Nickel-Manganese-Molybdenum d
19 Chromium, 11 Nickel, with Molybdenum
22 Chromium, 12 Nickel
CH10 and CH20 25 Chromium,12 Nickel
CK3MCuN 20 Chromium, 18 Nickel, with Copper and Molybdenum 80 550 38 260 35
CK35MN CK20 CN3M e CN3MN CN7M CN7MS
23 Chromium, 21 Nickel, with Molybdenum and Nitrogen 83 570 41 280
25 Chromium, 20 Nickel
65 450 28 195 63 435 25 170
---
21 Chromium, 24 Nickel, with Molybdenum and Nitrogen 80 550 38 260 20 Chromium, 29 Nickel, with Copper and Molybdenum 62 425 25 170 19 Chromium, 24 Nickel, with Copper and Molybdenum 70 485 30 205
HG10MNN 19 Chromium, 12 Nickel, 4 Manganese
76 525 33 225
a Remainder is iron. b These mechanical properties apply only when this material is heated to 1875–1925°F (1025–1050°C), quenched in air or oil, and tempered at 1150°F (620°C) minimum, or when annealed at 1400°F (760°C) minimum. c For low ferrite or non-magnetic castings of this grade, the following values shall apply: tensile strength, min, 65 ksi (450 MPa); yield point, min, 28 ksi (195 MPa). d Nominal Chemical Composition for CG6MMN as noted in the ASME standard. Other industry data indicates 20.5–23.5% Chromium, 11.5–13.5% Nickel, with 4–6% Manganese, 1.5–3% Molybdenum. e Nominal Chemical Composition for this grade is not noted in the ASME standard. Other industry data indicates 20–22% Chromium, 23–27% Nickel, with 4.5–5.5% Molybdenum.
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NONFERROUS CASTING MATERIALS Nonferrous Casting Metals Machinery's Handbook, 31st Edition
1486
Nonferrous metals include metal elements and alloys not based on iron. Offering a wide variety of material characteristics and mechanical properties, these casting materials are commonly specified for structural applications that require reduced weight, increased strength, higher melting points, nonmagnetic properties, and/or resistance to chemical and atmospheric corrosion. They also are suitable for electrical and electronic applica- tions. Such metals include aluminum, copper, magnesium, nickel, titanium, zinc, refrac- tory metals (molybdenum and tungsten), and noble metals. Important considerations in selecting material for a specific mechanical or structural application include how easily the material can be shaped into a finished part and how its properties may be altered in the process—either intentionally or inadvertently. De- pending on the end use, metal can be cast into a part and finished. Or it can be cast into an intermediate form (such as ingots), worked, or wrought, by forging, extruding, rolling, or applying other deformation processes, and then finished as needed. Following are the primary nonferrous metals and alloys used in casting. For more information on specific alloys, see NONFERROUS ALLOYS starting on page 510. Aluminum (Al).—Aluminum, also spelled aluminium, is a lightweight, silvery-white metal. It has a melting point of 1220°F (660°C) and a density of 163.55 lb/ft 3 (2700 kg/ m 3 ). The most abundant metallic element in the Earth’s crust, aluminum is the most widely used nonferrous metal. While it is thermodynamically the least stable of the main engi- neering metals, it has the advantage of forming a dense, highly protective alumina film only 20 μin.–60 μin. (0.5 μm–1.5 μm) thick. This film can be reinforced by anodizing and destroyed by salt. Aluminum is one of the few metals that can be cast by all of the processes. When alloyed with other metals, numerous properties are obtained that make such alloys useful over a broad range of applications. Many organizations publish specific standards for the manu - facture of aluminum alloy, including ASTM International and SAE International (specifi - cally, its aerospace standards subgroups). Alloys composed mostly of aluminum have been important in aerospace manufacturing since the introduction of metal-skinned aircraft. Classification of cast aluminum alloys is developed by the Aluminum Association of the United States. Each cast alloy is designated by a four-digit number with a decimal point separating the third and fourth digits: 1. The first digit indicates the alloy group, according to the major alloying element: 1xx.x aluminum (99.0% minimum) 2xx.x copper (4% to 4.6%) 3xx.x silicon (5% to 17%) with added copper and/or magnesium 4xx.x silicon (5% to 12%) 5xx.x magnesium (4% to 10%) 7xx.x zinc (6.2% to 7.5%) 8xx.x tin 9xx.x others 2. The next two digits indicate the alloy purity or identify the alloy: In the alloys of the 1xx.x series, these digits indicate the level of purity of the alloy—the same as the two digits to the right of the decimal point in the minimum concentration of aluminum (in percent): 150.0 means a minimum 99.50% of aluminum in the alloy, 120.1 means a minimum 99.20% of aluminum. In all other groups of aluminum alloys (2xx.x through 9xx.x), the second and third digits together signify different alloys in the group. 3. The last digit indicates the product form: casting (designated by 0) or ingot (designated by 1 or 2, depending on chemical composition limits). A modification of the original alloy or impurity limits is indicated by a serial letter before the numerical designation. The serial letters are assigned in alphabetical order, starting with A, but omitting I, O, Q, and X. (The letter X is reserved for experimental alloys.)
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NONFERROUS CASTING MATERIALS 1487 Aluminum-Copper (Al-Cu) Cast Alloys (2xx.x series): These alloys contain 4–4.6% copper, along with small impurities of iron, silicon, and magnesium. Characteristics are heat-treatable, high strength, low fluidity, low ductility, susceptibility to hot cracks, and low corrosion resistance. Aluminum-copper cast alloys are used for cylinder heads for automotive and aircraft engines, pistons for diesel engines, and exhausting system parts. Aluminum-Silicon-Copper (Al-Si-Cu) Cast Alloys (3xx.x series): Aluminum-silicon- copper alloys contain 5–17% silicon and 2–4.5% copper. Characteristics of these alloys are heat-treatable, high strength, good fluidity, low ductility, good machinability, good wear resistance, and decreased corrosion resistance. The copper contributes to strength, and the silicon improves castability and reduces hot shortness; thus, higher silicon alloys are suitable for more complex castings and for permanent mold and die casting processes, which cannot tolerate hot-short alloys. They are used for automotive cylinder blocks and heads, car wheels, aircraft fittings, casings, and other parts of compressors and pumps. Aluminum-Silicon (Al-Si) Cast Alloys (4xx.x series): These alloys contain 5–12% silicon but no copper. Characteristics are non-heat-treatable, moderate strength, moderate duc- tility, very good cast properties, and good wear and corrosion resistance. Rapid cooling to increase strength and ductility can refine the microstructure. These alloys are used for pump casings, thin wall castings, and cookware. Aluminum-Magnesium (Al-Mg) Cast Alloys (5xx.x series): Containing 4–10% magne- sium, characteristics of these alloys are non-heat-treatable, good machinability, and good appearance when anodized. The moderate to relatively poor castability of such alloys and the tendency of magnesium to oxidize increase handling difficulties and, therefore, cost. In general, these alloys are used for sand cast parts. Aluminum-Tin (Al-Sn) Cast Alloys (8xx.x series): Aluminum-tin alloys usually contain about 6% tin and a small amount of copper and nickel to improve strength. Though these compounds are not heat treatable, castability and machinability are good, and wear re- sistance is very good. They are used for cast bearings, due to tin’s excellent lubrication characteristics. Aluminum-Zinc (Al-Zn) Cast Alloys (7xx.x series): With the addition of 6.2–7.5% zinc, these alloys can be heat treated and have good dimensional stability, good machinability (if the alloy contains copper), and good corrosion resistance. They are used for abrasion- resistant parts under various operating conditions, such as axle bushes, shaft sleeves, and worm gears. Copper (Cu).—Copper is a reddish-yellow material with a melting point of 1984.6°F (1084.6°C) and a density of 556.85 lb/ft 3 (8920 kg/m 3 ). Usually a good conductor of elec- tricity and heat, copper is one of the most ductile metals, but it is not especially strong or hard. Pure copper is extremely difficult to cast. As copper forms alloys more freely than most metals, casting and other characteristics can be improved by adding small amounts of various elements, including beryllium, chromium, nickel, silicon, silver, tin, and zinc. Brass: This generic term refers to a range of copper-zinc (Cu-Zn) alloys with differing combinations of properties, including strength, hardness, ductility, machinability, resis- tance to wear and corrosion, and electrical and thermal conductivity. Bronze: Bronze made from copper and tin (Cu-Sn) was the first manmade alloy used thousands of years ago. In modern times, wrought bronzes have been developed with 4–8% tin. These alloys are harder, stronger, and stiffer than wrought brasses and, in strip and wire form, produce a combination of high yield strength and good corrosion resistance. Copper-Nickel (Cu-Ni) Alloys: This combination offers high conductivity, excellent resistance to marine corrosion, and low susceptibility to attachment of marine macro- organisms. The addition of nickel to copper improves strength and corrosion resistance, but good ductility is retained. The two main alloys are 90/10 (90% copper, 10% nickel) and 70/30 (70% copper, 30% nickel). Copper-Nickel-Silver (Cu-Ni-Zn) Alloys: Made from copper, nickel, and zinc but no silver, these alloys can be regarded as special brasses. Containing 10–20% nickel, they
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1488 NONFERROUS CASTING MATERIALS polish to an attractive silvery color, rather than the typical brassy color, which accounts for common use over several centuries. Beryllium-Copper (Be-Cu) Alloys: In the fully heat treated and cold-worked condition, this is the hardest and strongest copper alloy. It is similar in mechanical properties to many high-strength alloy steels but, compared to steels, has better corrosion resistance. Magnesium (Mg).— A shiny gray, solid, lightweight metal, magnesium has a melting point of 1202°F (650°C) and a density of 108.3 lb/ft 3 (1740 kg/m 3 ). As the world’s lightest metal, with good strength-to-weight ratio, magnesium and its alloys are prevalent in the au- tomotive, airplane, and missile industries. Its compounds also are used as refractory mate- rial in furnace linings for producing iron, steel, other nonferrous metals, glass, and cement. Magnesium is the most electrochemically active metal. Care must be taken in process- ing to avoid fire hazards, and small particles of the metal, such as metal cutting chips, oxidize rapidly. Magnesium and its alloys are available in both wrought and cast forms. Magnesium Alloys: These light alloys have received renewed interest as substitutes for some conventional structural materials to reduce weight in vehicles. Cast alloys, widely used in interior and power-train components, account for more than 99 percent of magne- sium alloys used today, while only a small number of wrought products are used. This is because magnesium alloys lack formability for wrought applications; their high cost also discourages use for some automotive applications. Rare earth (RE) elements, such as cerium (Ce), gadolinium (Gd), neodymium (Nd), and yttrium (Y), often are used as major alloying elements because of their relatively high solubility in magnesium and effectiveness in precipitation hardening and creep re- sistance. Commercial wrought magnesium alloys, such as ZK60 and AZ61, are based on magnesium-zinc (Mg-Zn) and magnesium-zinc-aluminum (Mg-Zn-Al) compounds, both of which are age-hardenable. ZK and AZ alloys attain good strength with hot extru- sion or rolling; however, age-hardening processes, such as T6 (solution heat treatment and aging) and T8 (cold-work and subsequent aging), do not add strengthening due to soften- ing by recrystallization at the temperature for artificial aging. CASTING OF METALS Casting is an age-old manufacturing process in which a liquid material usually is poured into a mold that contains a hollow cavity of the desired shape. As a result of new technological advances, this traditional form of manufacturing has become more essen- tial than ever. In metal casting, the mold generally includes runners and risers that facilitate metal filling the cavity. The mold and metal are cooled until the metal solidifies, then the solidi - fied part ( casting ) is recovered from the mold. Subsequent operations are used to remove excess material caused by the casting process (such as the runners and risers), after which the part may solidify further. Metal casting processes are divided into two broad types: expendable-mold casting processes and nonexpendable-mold casting processes. Expendable-mold casting includes sand, plastic, shell, plaster, and investment (lost-wax technique) molding processes, all of which rely on gravity to move the liquid material into casting cavities. The molds in which the molten material solidifies usually are made of non- metallic materials and are temporary—that is, they must be destroyed to remove the casting. Nonexpendable-mold casting refers to casting processes involving a reusable mold. Such molds are designed with two or more sections for easy, precise closing (for molding) and opening (to remove the casting) and can be used for multiple castings. For metal cast- ing, the mold, also called a die , is commonly made of metal or ceramic. Permanent mold casting refers to all casting technologies in which the mold cavity is reused many times and is made of a metallic material or graphite. This is the predomi- nant casting method for manufacturing metal shapes. Specifically, about 90 percent of all aluminum castings produced use metal molds, including gravity-fed, low-pressure, and high-pressure die castings.
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CASTING OF METALS 1489 Permanent metal molds are commonly made of steel or cast iron, with metal or sand cores, though it is desirable and generally more economical to use permanent steel cores to form cavities. When the casting has re-entrant surfaces or cavities from which one- piece permanent metal cores cannot be withdrawn, destructive cores made of sand, shell, plaster, or other materials may be used. This process is called semipermanent mold cast - ing . Sectional steel cores also may be used in some instances. An advantage of permanent metal molds is that they heat up and expand during the pour, so the cavity does not need to be expanded as much as in sand castings. Therefore, the cavity, with the gating system included, can be machined into halves that produce more precise parts with closer dimensional tolerances and smoother surfaces. Permanent mold castings also usually have better mechanical properties than sand castings, as solidifica - tion is more rapid and fill is more laminar. Permanent mold casting is used mostly for aluminum, copper alloys, magnesium, and gray iron, because of their generally lower melting points. Typical parts include automo- bile pistons, cylinder heads, gears, and kitchenware. Parts that can be made economically generally weigh less than 55 lb (25 kg), though special castings weighing a few hundred kilograms have been made using this process. It may not be economical for small, unique production runs (due to the cost of permanent molds) and generally is not suitable for cast- ing intricate shapes (because of difficulty of removing the part from the mold). Disadvantages of different types of castings include poor finish; wide tolerance (sand casting); limited workpiece size (shell molds and ceramic molds); patterns with low strength (expendable-pattern casting); expensive, limited shapes (centrifugal casting); porosity (all types); and environmental problems (all types). Heating and Pouring the Metal Pouring is the process by which molten metal is transferred to the mold for cooling and solidification to be converted into the intended shape. Pouring temperature ( T p ) is the temperature to which the molten metal must be heated before being poured into molds for cooling and setting. The heat energy required for heating metal to a pouring temperature is the sum of 1) the heat needed to raise the temperature of a unit mass solid to its melting point; 2) the heat of fusion required to convert it from a solid at its melting point to a liquid without an increase in temperature (see Fig. 1); and 3) the heat needed to raise the molten metal to the desired temperature for pouring. The pouring temperature must take into account heat lost in the transfer of metal through ladles, due both to the heat absorbed by the ladles and to the distance between the furnace and the mold. Molten metal must be poured carefully to avoid casting defects. For example, too rig- orous a stream could cause mold erosion; highly turbulent flows could result in air and inclusion entrapments; and relatively slow filling might generate cold shuts. Thus, design of the gating and venting overflow systems must take into consideration proper control of the liquid metal as it fills the mold.
Fig. 1. Phase Change Diagram of Process of Heating Metal Casting to Pouring Temperature.
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1490 Fluid Flow Whether the casting process takes place by expendable-mold casting (such as sand cast- ing) or nonexpendable-mold casting (using a permanent metal mold), the basic terminol- ogy of the casting process is the same. Fluid Flow Fluid flow is very important in casting. The molten metal is poured through a pour - ing basin. It then flows through the gating system (comprising sprue, risers, runners, and gates) to fill the mold cavity. (See the gravity casting system shown in Fig. 2.) The sprue is the vertical part of the gating system that connects the pouring cup or pour - ing basin and runners; liquid metal enters the mold through the sprue. The cope is the top half of the mold. The runners, which are cut in the drag (the lower part of the mold), form the horizontal portion of the gating system that connects the sprue to the gates. The gate is the portion of the runners through which molten metal enters the mold cavity; there may be one or more gates. The core is designed to leave an unfilled space in the part. Risers are the part of the gating system used to identify when enough liquid material has been poured to fill the mold; it also acts as a reservoir of extra material to compensate for shrinkage during solidification. The flask is the molding box or outside of the mold. Fig. 2. Cross Section of Typical Two-Part Sand Mold Gating Design: Successful casting requires proper design and control of the solidifi - cation process to ensure adequate fluid flow in the system. For example, an important function of the gating system in sand casting is to trap contaminants (oxides and other inclusions) in the molten metal by having such contaminants adhere to the walls of the gat- ing system, thereby preventing them from reaching the mold cavity. A properly designed gating system also avoids or minimizes problems such as premature cooling, turbulence, and gas entrapment. Two basic principles of fluid flow are relevant to gating design: Ber - noulli’s theorem and the law of mass continuity. Bernoulli’s theorem states that the sum of the energies in a flowing liquid is constant at any two points. This can be written as: (1) where h = elevation above a certain preference plane (datum); p = pressure at that eleva- tion; v = velocity of the liquid at that elevation; ρ = density of the fluid; and g = gravitation constant. This concept of conservation of energy requires that (between two different elevations) the following relationship be satisfied as: (2) where f = frictional losses in the liquid as it travels downward through the system. The law of mass continuity states that for incompressible liquids and in a system with impermeable walls, the volume rate of flow remains constant throughout the liquid. So, considering two different locations in the system: g 2 + + = + + + ρ ρ g g g 2 h p g + + = ρ 2 2 g v constant h p v h p v f 1 1 1 2 2 2 2 2
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Fluid Flow
1491 (3)
Q Av Av = = 1 1 2 2
where Q = volume rate of flow; A = cross-sectional area of the liquid stream; v = the aver- age velocity of the liquid at that cross-sectional location; and subscripts 1 and 2 indicate two different locations in the liquid flow. Sprue Design: The shape of the sprue can be calculated using Equation (2) and Equation (3). Assuming that pressure at the top of the sprue is equal to pressure at the bottom and that there are no frictional losses, the relationship between height and cross-sectional area at any point in the sprue is given by the parabolic relationship: (4) Flow Characteristics: A molten metal’s flow characteristics are an important consider - ation in the gating system because of the possible consequences of turbulence. There are two types of real fluid flow: laminar and turbulent. In laminar flow, fluid moves in layers called laminas . Laminar flow need not be in a straight line; the flow may follow curved surfaces. The fluid layers slide smoothly over one another, without fluid being exchanged between the layers. In turbulent flow , secondary random motions are superimposed on the principal flow, and there is an exchange of fluid from one adjacent sector to another. More important, there is an exchange of momentum: slow-moving fluid particles speed up, while fast-moving particles give up their momentum to slower-moving particles and are slowed down. The factor that determines which type of flow is present is the ratio of the inertial forces to the viscous forces within the fluid. This ratio is expressed by the dimensionless Reyn - olds number as: (5) where R e = Reynolds number; v = mean fluid velocity; L = characteristic length (equal to diameter if cross section is a circle); µ = dynamic fluid viscosity; ρ = density of the fluid. The higher the R e, the greater the tendency for turbulent flow to occur in the gating system: (1) laminar flow: 0 = R e < 2000; (2) transition flow: 20 < R e < 20,000; (3) turbulent flow: R e > 20,000. Turbulent flow leads to inclusions, and therefore a reduction in fluidity. Fluidity of Molten Metal The phrase fluidity of molten metal describes the capability of molten metal to fill mold cavities, based on two factors: characteristics of the molten metal and casting parameters. The following characteristics of molten metal influence fluidity: Viscosity: If viscosity (a measure of the metal’s deformation in a molten state) and sensi- tivity to temperature (viscosity index) increase, fluidity decreases. Surface Tension: High surface tension of the liquid metal reduces fluidity. Oxide films that develop on the surface of molten metal increase surface tension and thus can signifi - cantly reduce fluidity. Fluxing processes (treating metal with flux to promote melting) in the heating of metals are used to reduce or eliminate oxidation and to improve fluidity of metal surface layers. Inclusions: Metallic and non-metallic inclusions have long been recognized as one of the most important quality issues in metal casting. As insoluble particles, inclusions can have a significant adverse effect on fluidity; the presence of inclusions often is the cause of decreased fluidity. Solidification Pattern of the Alloy: Fluidity is inversely proportional to the freezing tem- perature range. In general, pure metals and alloys of eutectic composition have the highest values of fluidity. Solid-solution and other alloys have lower fluidity. Superheat: The difference between the melting temperature and the pouring temper- ature also influences fluidity. For given alloy compositions, if the melt temperature is higher, fluidity increases. The pouring temperature is often specified, rather than the su - perheat temperature, because it is easier to do so. A A h h 1 2 1 2 = R e = ρ μ vL
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1492 FLUIDITY OF MOLTEN METAL Test for Fluidity.—Although none is accepted universally, several tests have been devel- oped to quantify fluidity. One such test is the spiral test ( Fig. 3a), where the molten metal is made to flow along a channel at room temperature. The distance the metal flows before it solidifies (and stops flowing) is a measure of its fluidity. Obviously, this distance is a function of the thermal properties of both the metal and mold, as well as the design of the channel. Another test for fluidity is the vacuum test ( Fig. 3b), which measures the length the metal flows inside a narrow channel when sucked from a crucible by a vacuum pump. Such tests can be useful in simulating casting situations. B
Fig. 3. Test Method for Fluidity: a) Spiral Method; b) Vacuum Method Heat Transfer
An important consideration in casting is heat transfer during the complete cycle from pouring to solidification and cooling to ambient temperature. Heat flow at different loca - tions in the system is a complex phenomenon that depends on many factors relating to the casting material and mold and process parameters. For instance, when casting thin sec- tions, metal flow rates must be high enough to avoid premature chilling and solidification. However, the flow rate must not be so fast as to cause excessive turbulence, which can have detrimental effects. Solidification and Cooling of Metals At the macro level, solidification refers to the phase change of metal from liquid to solid. At the micro level, changes occur in the material as the disordered structure of the liquid transforms into an orderly arrangement of crystals. Once molten metal has been poured into the mold, it cools rapidly. When the tempera- ture of the liquid drops below the melting point of that metal or alloy, the solidification process begins. This usually takes less than a few minutes. As the temperature drops further, the molten metal loses energy, and crystals begin to form. (This process starts near the mold walls, where cooling occurs first.) These crystals eventually become grains within the final structure. Grain size refers to crystals ( dendrites ) formed during the solidification process. If the metal solidifies slowly, the grains are longer. If it cools quickly, the grains are visibly shorter. Crystals continue to form and harden, until the entire melt is solidified. Throughout the solidification process, the metal shrinks. It is important to compensate for such shrinking to ensure castings are free of voids and shrink defects. This is accom- plished by using risers. The cooling rate of a casting affects its microstructure, quality, and properties. The cool- ing curve illustrates how molten metals solidify. There is a fundamental difference be- tween the cooling curve observed during solidification of a pure metal and that of an alloy.
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SOLIDIFICATION AND COOLING OF METALS
1493
Fig. 4. Cooling Curve for a Poured Pure Metal During Casting
Solidification of Pure Metal.—Pure metal solidifies at constant temperature. It has a clearly defined melting (or freezing) point. Fig. 4 shows the cooling curve for a poured pure metal during casting. After the temperature of the molten pure metal drops to its freezing point, the tempera- ture remains constant while the latent heat of fusion is given off. The solidification front (solid–liquid interface) moves through the molten metal, solidifying from the mold walls toward the center. At the mold walls, the molten metal cools rapidly and first produces a solidified shell of fine, approximately equal-dimension-in-all-direction (equiaxed) grains. Starting with these grains, the grains grow upon themselves, in the opposite direction of the heat transfer out through the mold. Those with favorable orientations—that is, away from the surface of the mold—are columnar in shape; as the driving force of the heat transfer is re- duced (farther from the mold walls), the grains again become equiaxed, as well as coarse. Those grains that have substantially different orientations are blocked from further growth. When the heat is segregated rapidly during solidification, it leads to fine structures due to a decrease in diffusion rates. Solidification of Alloys.—Solidification begins when the temperature of the alloy drops below the point of liquidus, and it is complete when it reaches solidus. A phase diagram and a cooling curve for alloys during casting are shown in Fig. 5.
Fig. 5. Phase Diagram and Cooling Curve for Alloy Composition During Casting
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1494 SOLIDIFICATION AND COOLING OF METALS Within the temperature range where solidification begins ( T l ) and solidification ends ( T S ) , the alloy is in a “mushy” state, with columnar dendrites. The mushy metal is present between the dendrite arms. The width of this mushy zone is an important factor during solidification. It is described by the freezing range as: (6) Pure metals have no freezing range, and the solidification front moves in a plane without forming a mushy zone. In alloys with a nearly symmetrical phase diagram, the structure generally is lamellar, with two or more solid phases present, depending on the alloy system. For alloys, a short freezing range generally involves a temperature difference less than 122°F (50°C) and a long freezing range higher than 230°F (110°C). Ferrous castings generally have narrow mushy zones, whereas aluminum and magnesium alloys have wide mushy zones. Slow cooling rates approximately 10 2 K/s result in coarse dendritic structures with large spac- ing between the dendrite arms. For higher cooling rates, from 10 6 to 10 8 K/s, the structures developed are amorphous. Solidification Time.—Total solidification time is the time required for the casting to so - lidify from molten metal after pouring. Casting geometry, material, and process deter- mine solidification time. Chvorinov’s rule states that under the same conditions, a casting with a large surface area and small volume will cool more rapidly than a casting with a small surface area and large volume. Therefore, a large sphere solidifies and cools to ambient temperature at a much slower rate than a smaller sphere. The reason is that the volume of a sphere is proportional to the cube of its diameter, and its surface area is proportional to the square of its diameter. Similarly, molten metal in a cube-shaped mold will solidify faster than in a spherical mold of the same volume. According to this rule, solidification time is a function of the volume of a casting and its surface area: (7) where V is the volume of the casting; A is the surface area of the casting; k is the mold constant; and n is the exponent ( 1.5 2 < ≤ n , but usually taken as 2). The mold constant k depends on the properties of the cast metal (heat of fusion, specific heat, and thermal conductivity), mold material, and pouring temperature. The value of k for a given casting operation can be based on experimental data from previous operations carried out using the same mold material, metal, and pouring temperature, even though the shape of the workpiece might be complex. During the early stages of solidification, a thin, solidified skin begins to form at the cool mold walls; as times passes, the skin thickens. With flat mold walls, this thickness is pro - portional to the square root of time. Shrinkage.—Most materials contract or shrink during solidification and cooling. Shrink - age is the result of contraction of the liquid as it cools prior to solidification; contraction during the phase change from liquid to solid; and contraction of the solid as it continues to cool to ambient temperature. Sometimes, shrinkage can cause cracking in a component as it solidifies. Since the cool - est area of a volume of liquid is where it contacts a mold or die, solidification usually begins first at this surface. As the crystals grow inward, the material continues to shrink. If the solid surface is too rigid and will not deform to accommodate the internal shrink- age, the stresses can exceed the tensile strength of the material and cause a crack to form. Shrinkage cavitation also may occur as a material solidifies inward and shrinks to such an extent that not enough atoms are present to fill the available space, and a void is left. The amount of contraction during the solidification of metals is shown in Table 1. Note that gray cast iron expands, because graphite has a relatively high specific volume, and when it precipitates as graphite flakes during solidification, it causes a net expansion of the metal. Freezing range = T l – T S Solidification time = k ( V / A ) n
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Machinery's Handbook, 31st Edition
SOLIDIFICATION AND COOLING OF METALS 1495 Table 1. Volumetric Solidification Contraction or Expansion for Various Metals
Defects in Casting The International Committee of Foundry Technical Associations has developed a stan- dardized nomenclature, consisting of eight basic categories of casting defects: Metallic Projections: Such defects can include fins, flash, or massive projections, such as swells and rough surfaces. Cavities: Variously rounded or rough internal or exposed cavities include blowholes, pinholes, and shrinkage cavities. Discontinuities: These can include cracks, cold or hot tearing, and cold shuts. If the solidifying metal is constrained from shrinking freely, cracking and tearing can occur. Coarse grain size and the presence of low-melting-point segregates along the grain boundaries (intergranular) increase the tendency for hot tearing. Cold shut is an interface in a casting that lacks complete fusion because two streams of liquid metal meet from dif- ferent gates and do not completely fuse. Defective Surface: Surface defects include folds, laps, scars, adhering sand layers, and oxide scale. Incomplete Casting: Misruns can occur due to premature solidification, the molten metal being at too low a temperature, pouring metal too slowly, insufficient volume of metal being poured, or runout (loss of metal from a mold after pouring). Incorrect Dimensions or Shape: These undesirable results may be caused by factors such as an improper shrinkage allowance, pattern-mounting error, deformed pattern, irregular contraction, or warped casting. Inclusions: Inclusions form during melting, solidification, and molding and generally are non-metallic. They increase stress and reduce strength of the casting. Inclusions may form during melting, when the molten metal reacts with the environment (usually oxygen) or with crucible or mold material. Chemical reactions among components in the molten metal itself may produce inclusions. Spalling of the mold and core surfaces also produces inclusions, indicating the importance of the quality and maintenance of molds. Porosity: Porosity may be caused by shrinkage or gases or both. Thin sections in a cast- ing solidify sooner than thicker regions. As a result, molten metal can flow into thicker
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Machinery's Handbook, 31st Edition
1496 DEFECTS IN CASTING regions that have not yet solidified, and porous regions may develop at their centers due to contraction as the surfaces of the thicker region begin to solidify. Micro-porosity can develop when the liquid metal solidifies and shrinks between dendrites or between den - drite branches. Porosity caused by shrinkage can be reduced or eliminated by various means. Adequate liquid metal should be provided to avoid cavities caused by shrinkage. Internal or external chills used in sand casting also are an effective means of reducing shrinkage porosity by increasing the rate of solidification in critical regions. Internal chills usually are made of the same material as the casting and are left in the casting; external chills may be made of the same material or of iron, copper, or graphite. With alloys, porosity also may be re- duced or eliminated by making the temperature gradient steep. For example, mold materi- als with higher thermal conductivity may be used. Subjecting the casting to hot isostatic pressing is another method of reducing porosity. Most obvious porosity defects are caused by entrapment of gases within the molten solution. Because liquid metals have much greater solubility for gases than solid metals, when a metal begins to solidify, dissolved gases are expelled from the solution. Typi- cally, hydrogen precipitates into melt because of contact with the atmosphere or excessive moisture in the flux. Since hydrogen is highly soluble in molten metal, it is best to avoid superheating metals beyond their melting temperature and to avoid holding the material in a molten state any longer than necessary. To reduce absorption of gases from the atmosphere, which may leave slag or dross, cover molten metal until just prior to pouring it into the mold. Gases also may result from re- actions of the molten metal with the mold materials, either accumulating in regions of existing porosity or causing micro-porosity in the casting, particularly in cast iron, alu- minum, and copper. Dissolved gases may be removed from the molten metal by flushing or purging with an inert gas or by melting and pouring metal in a vacuum. If the dissolved gas is oxygen, the molten metal can be deoxidized. Steel is usually deoxidized with alu- minum, copper-based alloys with phosphorus, silicon, titanium, and zirconium-bearing materials. If the porosity is spherical and the walls are smooth, porosity usually is the result of gases. If the walls are rough and angular, porosity is likely the result of shrinkage between dendrites. Gross porosity is caused by shrinkage and usually is called a shrinkage cavity . The loss in casting properties measured by a tensile test may reflect the amount of poros - ity in a casting; because imperfections become areas of higher stress concentration, the percentage of property loss becomes greater when the strength requirement is higher. Porosity also is detrimental to the ductility of a casting and its surface finish, making it permeable and thus affecting the pressure tightness of a cast pressure vessel. A metallo- graphic examination can determine whether porosity exists in a casting. X-ray techniques also are used for nondestructive evaluations of porosity. METAL CASTING AND MOLDING PROCESSES Metal casting processes may be classified in four different ways: According to the Mold Type: (1) expendable mold (destroyed after each casting); or (2) permanent mold (reused many times). According to the Type of Pattern Used for Making a Sand Mold: (1) expendable pattern (melted for each mold), using wax as the pattern material; or (2) permanent pattern (reused for many molds), using wood or metal as the pattern material. According to the Type of Core Used for Producing a Hole in Casting: (1) expendable core (used in both sand and metal molds), using sand as the core material; or (2) permanent core (used with a permanent mold only), using metal as the core material. According to the Method by Which the Mold is Filled: (1) gravity (sand casting, gravity die casting); (2) pressure (low- and high-pressure die casting); or (3) vacuum (vacuum investment casting).
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Machinery's Handbook, 31st Edition
SAND CASTING
1497
Sand Casting Sand casting uses natural or synthetic sand in forming molds. Larger-sized molds use green sand (a mixture of sand, clay, and some water). Advantages of Sand Casting: Ferrous, nonferrous, and even non-metal materials can be cast in this process. Sand can be reused; excess metal poured is cut off and reused as well. Simple, inexpensive tools are required. Intricate shapes can be made by this process, as molten metal flows into small sections. Finally, this process can be used to produce many small components, with no limit on the size or weight of castings. Disadvantages of Sand Casting: Accuracy and surface finish are lacking, requiring addi - tional processing and finishing of parts. Overall, sand casting is a labor-intensive process. However, since sand casting usually is the least expensive way of making a component, its cost advantage over other methods makes it an attractive molding method. Sands.—Most sand casting operations use a refractory material called silica (SiO 2 ). Sand is inexpensive and has high melting point of 3110°F (1710°C). Sand can be naturally bonded (bank sand) or synthetic (lake sand). Because its composition can be controlled more accurately, synthetic sand is preferred by most foundries. Several factors affect the selection of sand for molds. The grains of the sand must be small enough so that it can be packed densely; sand having fine, round grains can be closely packed and form a smooth mold surface. Fine-grained sand enhances mold strength, but fine grains also lower mold permeability. The sand grains must be large enough for good permeability of molds (and cores), which allows the gases and steam that evolve during casting to escape through the pores of the mold. For proper functioning, mold sand must be clean and preferably new. And the sand mold should have good collapsibility to allow the casting to shrink while cooling, thus avoiding defects such as hot tearing and cracking. Types of Sand Molds.— Sand molds are characterized by the types of sand that compose them and by the methods used to make the molds. Green-Sand Molds: Clay sand is the mixture of natural silica sand, clay, additives, and water. It is the least expensive method of making sand molds, and the sand can be recycled for the next use. Bentonite clay is used to make wet clay sand. Sand in these molds is kept moist or damp while the metal is being poured. As the wet sand has a high moisture content, good air permeability, and low strength, castings can have issues with porosity, coarse, sticky sand, and sand expansion defects. Green-sand molds are commonly used in hand molding and machine molding. In hand molding, dimensional accuracy is low, so it is generally used only for production of small and medium-sized iron castings and nonferrous alloy castings. But in mechanical mold- ing, the castings have much higher dimensional accuracy, so it is widely used for high- volume production of castings. Skin-Dried Method: In this case, the mold surfaces need to be sprayed with a mixture of 10 percent water to one part molasses or lignin sulfonate after the surfaces are dried, either by storing the mold in air or by using torches. Cold-Box Mold: Various organic and inorganic binders are blended into the sand to bond the grains chemically for greater strength. These molds are dimensionally more accurate than green-sand molds, but they are more expensive. No-Bake Mold: A synthetic liquid resin is mixed with the sand; the mixture hardens at room temperature. Because bonding of the mold in this and in the cold-box process takes place without heat, these processes are called cold-setting processes . This type of mold has good dimensional control in high-production applications. Dried Molds: Sand molds that are oven dried (baked) prior to pouring the molten metal are stronger than green-sand molds and impart better dimensional accuracy and surface finish to the casting. However, this method has drawbacks: (a) distortion of the mold is greater; (b) castings are more susceptible to hot tearing because of the lower collapsibility of the mold; and (c) production rates are slower due to the drying time required.
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Machinery's Handbook, 31st Edition
1498 SAND CASTING Features of Sand Molds.—Major features of sand molds (most shown in Fig. 2) are:
Cope: The top half of the flask, mold, or core. Drag: The bottom half of the flask, mold, or core.
Core: A core part is inserted into the mold cavity to produce a hole within the mold. Core Print: The region added to the pattern, core, or mold to locate and support the core. The prints are made of such a size and shape that it would be impossible to set the core in any position except the proper one. Mold Cavity: The combined open area in the molding material and core, where the liquid metal solidifies to produce the casting. Gating System: The network of connected channels that allows molten material to flow into the mold cavity; it includes the sprue, riser, runners, and gates. Pouring Cup or Basin: The part of the gating system that initially receives the molten metal from the pouring vessels and controls its delivery to the rest of the mold. Sprue: The vertical part of the gating system that connects the pouring cup and runners. The liquid metal enters into the mold cavity through the sprue. Runners: The horizontal portion of the gating system, cut into the drab part of the mold, which connects the sprues to the gates. Riser: This part of the gating system normally is used to identify the filling position of the liquid metal in the mold. It also acts as a reservoir for extra molten metal to compensate for shrinkage during solidification. Gates: The controlled entrances from the runners into the mold cavities. Properly de- signed gates admit liquid metal into the mold cavity without turbulence. Vents: These small holes, in all parts of the mold, provide passage for gases to escape during pouring and solidification of mold metals. Parting Line or Parting Surface: The interface line between the cope and drag halves of the mold or flask. Draft: The taper on the pattern that allows it to be easily withdrawn from the mold. Core Box: This is the mold or die used to produce the cores. Patterns.—Patterns are used to mold the sand mixture into the shape of casting. They may be made of wood, plastic, or metal. Because patterns are used repeatedly to make molds, the strength and durability of the materials selected must reflect the number of castings that the mold will produce. Patterns usually are coated with a parting agent to facilitate their removal from the molds. There are four types of patterns: solid one-piece patterns, split patterns, match-plate patterns, and cope-and-drag patterns. One-Piece Patterns: Also called loose or solid patterns , one-piece patterns are used for simple shapes and low-quantity production. They usually are made of wood and are inexpensive. Split Patterns: These two-piece patterns are made so that each part forms a portion of the cavity for the casting, enabling casting of complicated shapes.
Fig. 6. Match-Plate Pattern Match-Plate Patterns: These two-piece patterns are constructed by securing each half of one or more split patterns to the opposite sides of a single plate (Fig. 6). The gating sys- tem can be mounted on the drag side of the pattern.
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