lect2 casting

8/8/2019 Lect2 Casting

http://slidepdf.com/reader/full/lect2-casting 1/64

Manufacturing Processes for Engineering Materials, 5th ed.Kalpakjian • Schmid

© 2008, Pearson EducationISBN No. 0-13-227271-7

Metal-Casting Processes andEquipment





Solidification Processes

Starting work material is either a liquid or is in ahighly plastic condition, and a part is createdthrough solidification of the material

• Solidification processes can be classifiedaccording to engineering material processed:

• Metals

• Ceramics, specifically glasses• Polymers and polymer matrix composites(PMCs)





Casting

Process in which molten metal flows by gravity or otherforce into a mold where it solidifies in the shape ofthe mold cavity

• The term casting also applies to the part made in theprocess

• Steps in casting seem simple:

1. Melt the metal

2. Pour it into a mold

3. Let it freeze





Capabilities and Advantages ofCasting

• Can create complex part geometries

• Can create both external and internalshapes

• Some casting processes are net shape; others are near net shape

• Can produce very large parts• Some casting methods are suited to mass

production





Disadvantages of Casting

• Different disadvantages for different castingprocesses:

• Limitations on mechanical properties

• Poor dimensional accuracy and surfacefinish for some processes; e.g., sandcasting

• Safety hazards to workers due to hotmolten metals

• Environmental problems





Parts Made by Casting

• Big parts: engine blocks and heads forautomotive vehicles, wood burning stoves,machine frames, railway wheels, pipes,

church bells, big statues, and pumphousings

• Small parts: dental crowns, jewelry, small

statues, and frying pans• All varieties of metals can be cast, ferrous

and nonferrous





Typical Cast Parts

(a) Typical gray-iron castings used in automobiles, including the transmission valve body(left) and the hub rotor with disk-brake cylinder (front). Source : Courtesy of Central

Foundry Division of General Motors Corporation. (b) A cast transmission housing. (c) ThePolaroid PDC-2000 digital camera with a AZ191D die-cast high-purity magnesium case. (d)A two-piece Polaroid camera case made by the hot-chamber die-casting process. Source:

Courtesy of Polaroid Corporation and Chicago White Metal Casting, Inc.

(a)

(b)

(c)

(d)





The Mold in Casting

• Contains cavity whose geometry determinespart shape

•Actual size and shape of cavity must beslightly oversized to allow for shrinkage ofmetal during solidification and cooling

• Molds are made of a variety of materials,including sand, plaster, ceramic, andmetal





Temperature & Density for Castings



FIGURE 5.1 (a) Temperature as a function of time for the solidification of pure metals. Note thatfreezing takes place at a constant temperature. (b) Density as a function of time.





Two-Phased Alloys

FIGURE 5.2 (a) Schematic illustration of grains, grain boundaries, and particles dispersed throughout the structure of atwo-phase system, such as lead-copper alloy. The grains represent lead in solid solution of copper, and the particles arelead as a second phase. (b) Schematic illustration of a two-phase system, consisting of two sets of grains: dark andlight. Dark and light grains have their own compositions and properties.





Phase Diagram for Nickel-Copper

FIGURE 5.3 Phase diagram for nickel-copper alloy system obtained by a low rate of solidification. Note that pure nickeland pure copper each have one freezing or melting temperature. The top circle on the right depicts the nucleation ofcrystals; the second circle shows the formation of dendrites; and the bottom circle shows the solidified alloy with grainboundaries.





Irn-Iron Carbide Phase Diagram

FIGURE 5.4 (a) The iron-iron carbide phase diagram. (b) Detailed view of the microstructures above and below theeutectoid temperature of 727°C (1341°F). Because of the importance of steel as an engineering material, this diagramis one of the most important phase diagrams.





Texture in Castings

FIGURE 5.5 Schematic illustration of three cast structures of metals solidified in a square mold: (a) puremetals, with preferred texture at the cool mold wall. Note in the middle of the figure that only favorableoriented grains grow away from the mold surface; (b) solid-solution alloys; and (c) structure obtained byheterogeneous nucleation of grains.





Alloy Solidification & Temperature

FIGURE 5.6 Schematic illustration of alloy solidification and temperature distribution in the solidifying metal.Note the formation of dendrites in the semi-solid (mushy) zone.





Solidification Patterns for Gray CastIron

FIGURE 5.7 Schematic illustration of three basic types of cast structures: (a) columnar dendritic; (b)equiaxed dendritic; and (c) equiaxed nondendritic. Source: After D. Apelian.





Cast Structures

FIGURE 5.9 Schematic illustration of cast structures in (a) planefront, single phase, and (b) plane front, two phase. Source: After D.Apelian.

FIGURE 5.8 Schematic illustration of threebasic types of cast structures: (a) columnardendritic; (b) equiaxed dendritic; and (c)equiaxed nondendritic. Source: After D.Apelian.





Mold Features

FIGURE 5.10 Schematic illustration of a typical sand mold showing various features.





Temperature Distribution

FIGURE 5.11 Temperature distribution at the mold wall and liquid-metal interface during solidification of metals in casting





Skin on Casting

FIGURE 5.12 Solidified skin on a steel casting; theremaining molten metal is poured out at the times indicatedin the figure. Hollow ornamental and decorative objects aremade by a process called slush casting, which is based onthis principle. Source: After H.F. Taylor, J. Wulff, and M.C.

Flemings.

Chvorinov’s Rule:





Shrinkage

TABLE 5.1 Volumetric solidification contraction orexpansion for various cast metals.





Cast MaterialProperties

FIGURE 5.13 Mechanical properties for

various groups of cast alloys. Compare withvarious tables of properties in Chapter 3.Source: Courtesy of Steel Founders' Societyof America.





General Characteristics of Casting

TABLE 5.2 General characteristics of casting processes.





Properties & Applications of Cast Iron

TABLE 5.4 Properties and typical applications of castirons.





Nonferrous Alloys

TABLE 5.5 Typical properties of nonferrous casting alloys.





Microstructure for Cast Irons

FIGURE 5.14 Microstructure for cast irons. (a) ferritic gray iron with graphite flakes; (b) ferritic nodular iron, (ductile

iron) with graphite in nodular form; and (c) ferritic malleable iron. This cast iron solidified as white cast iron, with thecarbon present as cementite (Fe 3C), and was heat treated to graphitize the carbon.





Continuous-Casting

FIGURE 5.15 (a) The continuous-casting process for steel. Note that theplatform is about 20 m (65 ft) aboveground level. Source: AmericanFoundrymen's Society. (b) Continuousstrip casting of nonferrous metal strip.Source: Courtesy of Hazelett Strip-Casting Corp.





Production Steps in Sand-Casting

Outline of production steps in a typical sand-casting operation.





Sand Mold

Schematic illustration of a sand mold, showing various features.





Pattern Plate

A typical metal match-plate pattern used in sand casting.





Design for Ease of Removal fromMold

Taper on patterns for ease of removal from the sand mold





Sand Cores

Examples of sand cores showing core prints and chaplets to support cores.





Vertical Flaskless Molding

Vertical flaskless molding. (a) Sand is squeezed between two halves ofthe pattern. (b) Assembled molds pass along an assembly line for

pouring. (c) A photograph of a vertical flaskless molding line. Source :Courtesy of American Foundry Society.

(c)





SandCasting

FIGURE 5.16 Schematic illustration of the sequence of operations in sand casting. (a) A mechanical drawing of thepart, used to create patterns. (b-c) Patterns mounted on plates equipped with pins for alignment. Note the presence ofcore prints designed to hold the core in place. (d-e) Core boxes produce core halves, which are pasted together. Thecores will be used to produce the hollow area of the part shown in (a). (f) The cope half of the mold is assembled bysecuring the cope pattern plate to the flask with aligning pins, and attaching inserts to form the sprue and risers. (g) Theflask is rammed with sand and the plate and inserts are removed. (h) The drag half is produced in a similar manner. (j)The core is set in place within the drag cavity. (k) The mold is closed by placing the cope on top of the drag andsecuring the assembly with pins. (l) After the metal solidifies, the casting is removed from the mold. (m) The sprue andrisers are cut off and recycled, and the casting is cleaned, inspected, and heat treated (when necessary). Source: Courtesy of Steel Founders' Society of America.





Shell-Molding Process

FIGURE 5.17 Schematic illustration of the shell-molding process, also called the dump-box technique.





Caramic Mold Manufacture

FIGURE 5.18 Sequence of operations in making a ceramic mold.





Evaporative Pattern Casting

FIGURE 5.20 Schematic illustration of the expendable-pattern casting process, also known aslost-foam or evaporative-pattern casting.





Investment Casting

FIGURE 5.21 Schematic illustration of investment casting (lost wax process). Castings by thismethod can be made with very fine detail and from a variety of metals. Source: Steel Founders'Society of America.





Automated ShellProduction

A robot generates a ceramic shellon wax patterns (trees) for

investment casting. The robot isprogrammed to dip the trees andthen place them in an automateddrying system. With many layers,a thick ceramic shell suitable for

investment casting is formed.

Source : Courtesy of WisconsinPrecision Casting Corporation





Integrally Cast Rotor for a GasTurbine

Investment casting of an integrally cast rotor for a gas turbine. (a) Waxpattern assembly. (b) Ceramic shell around wax pattern. (c) Wax is meltedout and the mold is filled, under a vacuum, with molten superalloy. (d) Thecast rotor, produced to net or near-net shape. Source : Courtesy of Howmet

Corporation.





Rotor Microstructure

FIGURE 5.22 Microstructure of a rotor that has been investment cast (top) andconventionally cast (bottom). Source: Advanced Materials and Processes , October1990, p. 25. ASM International.





Pressure & Hot-Chamber Die Casting

FIGURE 5.23 The pressure casting

process, utilizing graphite molds for theproduction of steel railroad wheels.Source: Griffin Wheel Division of AmstedIndustries Incorporated.

FIGURE 5.24 Schematic illustration of the hot-

chamber die-casting process.





Cold-Chamber Die Casting

FIGURE 5.25 Schematic illustration of the cold-chamber die-casting process. These machines arelarge compared to the size of the casting, becausehigh forces are required to keep the two halves ofthe die closed under pressure.





Properties of Die-Casting Alloys

TABLE 5.6 Properties and typical applications of common die-castingalloys.





Types of Cavities in Die-Casting Die

Various types of cavities in a die-casting die. Source: Courtesy of American Die Casting Institute.





Centrifugal Casting

FIGURE 5.26 Schematic illustration of the centrifugal casting process. Pipes, cylinder liners,

and similarly shaped hollow parts can be cast by this process.





Semicentrifugal Casting

FIGURE 5.27 (a) Schematic illustration of the semicentrifugal casting process. Wheels with spokes canbe cast by this process. (b) Schematic illustration of casting by centrifuging. The molds are placed at theperiphery of the machine, and the molten metal is forced into the molds by centrifugal forces.





Squeeze-Casting

FIGURE 5.28 Sequence of operations in the squeeze-casting process. This process combinesthe advantages of casting and forging.





Turbine Blade Casting

FIGURE 5.29 Methods of casting turbine blades: (a) directional solidification; (b) method toproduce a single-crystal blade; and (c) a single-crystal blade with the constriction portion stillattached. Source: (a) and (b) After B.H. Kear, (c) Courtesy of ASM International.





Crystal Growing

FIGURE 5.30 Two methods of crystal growing: (a) crystal pulling (Czochralski process) and (b) floating-zone method. Crystal growing is especially important in the semiconductor industry. (c) A single-crystalsilicon ingot produced by the Czochralski process. Source: Courtesy of Intel Corp.





Melt-Spinning Process

FIGURE 5.31 (a) Schematic illustration of the melt-spinning process to produce thin strips of

amorphous metal. (b) Photograph of nickel-alloy production through melt-spinning. Source: Courtesy of Siemens AG.





Types of Melting Furnaces

Two types of melting furnaces used in foundries: (a) crucible,and (b) cupola.





Austenite-Pearlite Transformation

FIGURE 5.32 (a) Austenite to pearlitetransformation of iron-carbon alloys as afunction of time and temperature. (b) Isothermaltransformation diagram obtained from (a) for atransformation temperature of 675°C (1247°F).(c) Microstructures obtained for a eutectoidiron-carbon alloy as a function of cooling rate.Source: Courtest of ASM International.





Phase Diagram for Aluminum-Copper

FIGURE 5.33 (a) Phase diagram for the aluminum-copper alloy system. (b) Variousmicrostructures obtained during the age-hardening process.





Outline of Heat Treating

TABLE 5.7 Outline of heattreatment processes forsurface hardening.





Heat Treatment Temperature Ranges

FIGURE 5.34 Temperature ranges for heat treating plain-carbon steels, as

indicated on the iron-iron carbide phase diagram.





Casting Processes Comparison

TABLE 5.8 Casting Processes, and their Advantages and Limitations.





Chills

FIGURE 5.35 Various types of (a) internal and (b) external chills (dark areas at corners), used incastings to eliminate porosity caused by shrinkage. Chills are placed in regions where there is a largervolume of metal, as shown in (c).





Hydrogen Solubility in Aluminum

FIGURE 5.36 Solubility of hydrogen in aluminum. Note the sharp decrease in solubility as the molten metal begins to solid





Elimination of Porosity in Castings

FIGURE 5.37 (a) Suggested design modifications to avoid defects in castings. Note that sharp corners

are avoided to reduce stress concentrations; (b, c, d) examples of designs showing the importance ofmaintaining uniform cross-sections in castings to avoid hot spots and shrinkage cavities.





Design Modifications

FIGURE 5.38 Suggesteddesign modifications to avoiddefects in castings. Source: Courtesy of The NorthAmerican Die CastingAssociation.





Economics of Casting

FIGURE 5.39 Economic comparison of making a part by two different casting processes. Note that because of the highcost of equipment, die casting is economical mainly for large production runs. Source: The North American Die CastingAssociation.




Lost-Foam Casting of Engine Blocks

FIGURE 5.40 (a) An engine block for a 60-hp 3-cylinder marine engine, produced by the lost-foamcasting process; (b) a robot pouring molten aluminum into a flask containing a polystyrene pattern. Inthe pressurized lost-foam process, the flask is then pressurized to 150 psi (1000 kPa). Source: Courtesy of Mercury Marine

lect2 casting

Documents