Metalurgi pengecoran

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Kalpakjian • Schmid Manufacturing Engineering and Technology © 2001 Prentice-Hall Page 11-1

PROSES PENGECORAN




Introduction

Casting

Penuangan logam cair ke cetakan sesuai dengan bentuk yang diinginkan. Setelah penuangan,dilanjutkan dengan proses pembekuan padacetakan dan benda dikeluarkan dari cetakan




Introduction

-Dapat digunakan untuk menghasilkan geometri yangkompleks baik internal maupun eksternal

- Beberapa proses pengecoran dapat mengasilkankomponen net shape (tidak memerlukan proses

pengerjaan manufaktur lanjutan)- Dapat menghasilkan komponen yang sangat besar (berat

lebih dari 100 ton)

- Dapat digunakan untuk berbagai jenis logam yang dapat

dipanaskan sampai fasa cair- Beberapa tipe proses casting cocok untuk mass

production




RingkasanProses – proses

Pengecoran

TABLE 11.1

Process Advantages LimitationsSand Almost any metal cast; no limit

to size, shape or weight; low

tooling cost.

Some finishing required;

somewhat coarse finis h; wide

tolerances.

Shell mold Good dimensional accuracy and

surface finish; high production

rate.

Part size limited; expensive

patterns and equipment

required.

Expendable pattern Most metals cast with no limit

to size; complex s hapes

Patterns have low strength and

can be costly for low quantities

Plaster mold Intricate shapes; good

dimensional accuracy and

finish; low porosity.

Limited to nonferrous metals;

limited size and volume of

production; mold making time

relatively long.

Ceramic mold Intricate shapes ; clos e

tolerance parts; good surface

finish.

Limited size.

Investment Intricate shapes; excellent

surface finish and accuracy;

almost any metal cast.

Part size limited; expensive

patterns, molds, and labor.

Permanent mold Good surface finish and

dimensional accuracy; low

porosity; high production rate.

High mold cost; limited shape

and intricacy; not suitable for

high-melting-point metals.

Die Excellent dimensional accuracyand surface finish; high

production rate.

Die cost is high; part sizelimited; usually limited to

nonferrous metals; long lead

time.

Centrifugal Large cylindrical parts with

good quality; high production

rate.

Equipment is expensive; part

shape limited.




Contoh komponen hasil pengecoran

Crank handle setelah castingpada beberapa area dilakukanpemesinan dan perakitan

C-clamps formed by casting(left) and machining (right)

Complex part formed by castingCourtesy of Toth Industries




Contoh produk hasilpengecoran

Figure 11.2 Typical gray-iron castings used inautomobiles, includingtransmission valve body(left) and hub rotor withdisk-brake cylinder (front).

Source: Courtesy of CentralFoundry Division of GeneralMotors Corporation.

Figure 11.3 A casttransmission housing.




Contoh : Die-Casting

(a) (b)

Figure 11.1 (a) The Polaroid PDC-2000 digital camera with a AZ91D die-cast, high puritymagnesium case. (b) Two-piece Polaroid camera case made by the hot-chamber die casting

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




Karakteristik Umum Proses Pengecoran

TABLE 11.2

Typical Weig ht (kg)

Typical

surface Section thic kness (mm)

Process

materials

cast Minimum Maximum

finish

(mm, R a) Porosity*

Shape

complexity*

Dimensional

accuracy* Minimum Maximum

Sand All 0.05 No limit 5-25 4 1-2 3 3 No limit

Shell All 0.05 100+ 1-3 4 2-3 2 2 --

Expendable

mold

pattern All 0.05 No limit 5-20 4 1 2 2 No limit

Plaster

mold

Nonferrous

(Al, Mg, Zn,

Cu) 0.05 50+ 1-2 3 1-2 2 1 --

Investment

All

(High melting

pt.) 0.005 100+ 1-3 3 1 1 1 75

Permanent

mold All 0.5 300 2-3 2-3 3-4 1 2 50

Die

Nonferrous

(Al, Mg, Zn,

Cu) <0.05 50 1-2 1-2 3-4 1 0.5 12

Centrifugal All -- 5000+ 2-10 1-2 3-4 3 2 100

*Relative rating: 1 best, 5 worst.

Note : These ratings are only general; significant variations can occur, depending on the methods used.




Persyaratan:

- Rongga cetakan sesuai dengan bentuk dan ukuran yangdiinginkan

-Proses pencairan untuk menghasilkan logam cair

- Proses penuangan untuk mengantarkan logam cair kecetakan

- Proses kontrol pembekuan untuk mencegah cacat

-Melepaskan produk dan cetakan

- Pembersihan, finishing dan inspeksi

Hal penting dalam pengecoran




Logam murni dan paduanLovgam munri membeku pada konstan temperatur dan paduan

membeku pada range temperatur




Proses pembekuan

A nucleating agent (inoculant) adalah senyawa yangberfungsi sebagai tempat nukleasi butir agar prosespengintian terbentuk pada waktu yang bersamaan di

seluruh bagian




Laju pendinginan

Rapid cooling cenderung menghasilkan butirequiaxed (bulat)

Slow cooling cenderung menghasilkan butirkolumnar (memanjang) yang tumbuh denganarah ke bagian tengah




Pembekuan logam

Dendrites

Tree-like structures that form during the solidification of alloys

Slow cooling rates produce dendrites with larger branch spacing;faster cooling rates produce finer spacing; very fast cooling rates produce no dendrites or grains




Istilah dalam Casting

FlaskThe box containing the mold

Cope

The top half of any part of a 2-part mold

Drag

The bottom half of any part of a 2-part mold

Core

A shape inserted into the mold to form internal cavities

Core Print

A region used to support the core




Istilah dalam Casting (lanjutan)

Mold Cavity

The hollow mold area in which metal solidifies into the part

Riser

An extra cavity to store additional metal to prevent shrinkage

Gating System

Channels used to deliver metal into the mold cavity

Pouring Cup

The part of the gating system that receives poured metal

Sprue

Vertical channel

Runners

Horizontal channels




Istilah dalam Casting (lanjutan)

Parting Line / Parting Surface

Interface that separates the cope and drag of a 2-partmold

DraftTaper on a pattern or casting that allows removal fromthe mold

Core Box

Mold or die used to produce coresCasting

The process and product of solidifying metal in a mold




Cetakan Pasir

Figure 11.4 Schematic illustration of a sand mold, showing various features.




Fungsi pada cetakan

• Vents sebagai pembuangan udara untuk mengeluarkan gas yang dihasilkanpada saat logam cair masuk dan kontakdengan cetakan.

•

Risers menahan dan men-supply logamcair agar tidak terjadi penyusutan selamaproses pembekuan

• Gates didesain untuk mencegah

kontaminan yang dapat masuk ke ronggacetakan




Figure 11.5 Outline of production steps in a typical sand-casting operation.

Tahapan Sand Casting




Karakteristik material untukpola/cetakan

TABLE 11.3

Ratinga

Characteristic Wood Aluminum Steel Plastic Cast iron

Machinability E G F G G

Wear resistance P G E F E

Strength F G E G GWeightb E G P G P

Repairability E P G F G

Resistance to:

Corrosionc E E P E P

Swellingc P E E E E

aE, Excellent; G, good; F, fair; P, poor.

bAs a factor in operator fatigue.

cBy water.

Source : D.C. Ekey and W.R. Winter, Introduction to Foundry Technology. New York.

McGraw-Hill, 1958.




Patterns Untuk SandCasting

Figure 11.6 A typical metalmatch-plate pattern used insand casting.

Figure 11.7 Taper on patterns forease of removal from the sand mold.




Examples of Sand Cores and Chaplets

Figure 11.8 Examples of sand cores showing core prints and chaplets to support cores.




Squeeze Heads

Figure 11.9 Various designs

of squeeze heads for moldmaking: (a) conventionalflat head; (b) profile head;(c) equalizing squeeze

pistons; and (d) flexiblediaphragm. Source: ©Institute of BritishFoundrymen. Used with

permission.




Tahapan pengerjaan dalam Sand Casting

Figure 11.11 Schematic illustration of the sequence of operations for sand casting. Source: Steel

Founders' Society of America. (a) A mechanical drawing of the part is used to generate a design for the

pattern. Considerations such as part shrinkage and draft must be built into the drawing. (b-c) Patternshave been mounted on plates equipped with pins for alignment. Note the presence of core prints designed

to hold the core in place. (d-e) Core boxes produce core halves, which are pasted together. The cores will

be used to produce the hollow area of the part shown in (a). (f) The cope half of the mold is assembled by

securing the cope pattern plate to the flask with aligning pins, and attaching inserts to form the sprue and

risers. (continued)




Figure 11.11 (g) The flask is rammed with sand and the plate and inserts are removed. (g) The drag half is produced in a similar manner, with the pattern inserted. A bottom board is placed below the drag and alignedwith pins. (i) The pattern, flask, and bottom board are inverted, and the pattern is withdrawn, leaving theappropriate imprint. (j) The core is set in place within the drag cavity. (k) The mold is closed by placing thecope on top of the drag and buoyant forces in the liquid, which might lift the cope. (l) After the metal solidifies,the casting is removed from the mold. (m) The sprue and risers are cut off and recycled and the casting iscleaned, inspected, and heat treated (when necessary).

Tahapan pengerjaan Sand Casting (lanjutan)




Surface Roughness for Various Metalworking Processes

Figure 11.12 Surface roughness in casting and other metalworking processes. See also Figs. 22.14 and26.4 for comparison with other manufacturing processes.




Ceramic Molds

Figure 11.16 Sequence of operations in

making a ceramic mold. Source: Metals Handbook , vol. 5, 8th ed.

Figure 11.17 A typical ceramicmold (Shaw process) for castingsteel dies used in hot forging.Source: Metals Handbook , vol.5, 8th ed.

Figure 11 18




Figure 11.18Schematicillustration ofinvestmentcasting, (lost-wax process).Castings by this

method can bemade with veryfine detail andfrom a variety ofmetals. Source:Steel Founders'Society ofAmerica.

InvestmentCasting

• Prosesinvestmentcasting disebut

juga dengan lost-

wax process

• pattern dibuatdari wax atauplastic sepertipolystyrene




Investment Casting of a Rotor

Figure 11.19 Investment casting of an integrally cast rotor for a gas turbine. (a) Wax pattern assembly.(b) Ceramic shell around wax pattern. (c) Wax is melted out and the mold is filled, under a vacuum,with molten superalloy. (d) The cast rotor, produced to net or near-net shape. Source: HowmetCorporation.




Investment and Conventionally Cast Rotors

Figure 11.20 Cross-section andmicrostructure of tworotors: (top)investment-cast;(bottom) conventionallycast. Source: Advanced

Materials and Processes, October1990, p. 25 ASMInternational




Vacuum-Casting Process

Figure 11.21 Schematic illustration of the vacuum-casting process. Note that the mold has a bottom gate. (a) Before and (b) after immersion of the mold into the molten metal. Source:From R. Blackburn, "Vacuum Casting Goes Commercial," Advanced Materials and Processes,February 1990, p. 18. ASM International.




Pressure Casting

Figure 11.22 (a) The bottom-pressure casting process utilizes graphite molds for the production of steelrailroad wheels. Source: The Griffin Wheel Division of Amsted Industries Incorporated. (b) Gravity-

pouring method of casting a railroad wheel. Note that the pouring basin also serves as a riser. Railroadwheels can also be manufactured by forging.




Hot- and Cold-Chamber Die-Casting

Figure 11.23 (a) Schematic illustration of the hot-chamber die-casting process. (b) Schematicillustration of the cold-chamber die-casting process. Source: Courtesy of Foundry Management andTechnology.

(a) (b)




Cold-Chamber Die-Casting Machine

(a)

Figure 11.24 (a) Schematic illustration of a cold-chamber die-casting machine.

These machines are large compared to the size of the casting because large forces arerequired to keep the two halves of the dies closed.




Figure 11.24 (b) 800-ton hot-chamber die-casting machine, DAM 8005 (madein Germany in 1998). This is the largest hot-chamber machine in the worldand costs about $1.25 million.

(b)

Hot-Chamber Die-Casting Machine




Die-Casting Die Cavities

Figure 11.25 Various types of cavities in a die-casting die. Source: Courtesy ofAmerican Die Casting Institute.

Figure 11.26 Examples ofcast-in- place inserts in diecasting. (a) Knurled

bushings. (b) Groovedthreaded rod.

Properties and Typical Applications of




Properties and Typical Applications ofCommon Die-Casting Alloys

TABLE 11.4

Alloy

Ultimatetensile

strength

(MPa)

Yield

strength

(MPa)

Elongation

in 50 mm

(%) ApplicationsAluminum 380 (3.5 Cu-8.5 Si) 320 160 2.5 Appliances, automotive components,

electrical motor frames and housings

13 (12 Si) 300 150 2.5 Complex shapes with thin walls, parts

requiring s trength at elevated

temperatures

Brass 858 (60 Cu) 380 200 15 Plumbing fiztures, lock hardware,

bushings, ornamental castings

Magnesium AZ91 B (9 Al-0.7 Zn) 230 160 3 Power tools, automotive parts, sporting

goods

Zinc No. 3 (4 Al) 280 -- 10 Automotive parts, office equipment,

household utensils, building hardware,

toys

5 (4 Al-1 Cu) 320 -- 7 Appliances, automotive parts, building

hardware, business equipment

Source : Data from American Die Casting Institute




Centrifugal Casting Process

Figure 11.27 Schematicillustration of the centrifugalcasting process. Pipes,cylinder liners, and similarlyshaped parts can be cast withthis process.




Semicentrifugal Casting

Figure 11.28 (a) Schematic illustration of the semicentrifugal casting process. Wheels with spokes can be cast by this process. (b) Schematic illustration of casting by centrifuging. The molds are placed atthe periphery of the machine, and the molten metal is forced into the molds by centrifugal force.



Kalpakjian • Schmid

Manufacturing Engineering and Technology © 2001 Prentice-Hall Page 11-40

Squeeze-Casting

Figure 11.29 Sequence of operations in the squeeze-casting process. This process combines theadvantages of casting and forging.

Si l C l C i f T bi Bl d





Single Crystal Casting of Turbine Blades

(c)

Figure 11.30 Methods of casting turbine blades: (a) directional solidification; (b) method to produce

a single-crystal blade; and (c) a single-crystal blade with the constriction portion still attached.Source: (a) and (b) B. H. Kear, Scientific American, October 1986; (c) Advanced Materials and

Processes, October 1990, p. 29, ASM International.





Single Crystal Casting

Figure 11.31 Two methods ofcrystal growing: (a) crystal

pulling (Czochralski process)and (b) the floating-zonemethod. Crystal growing isespecially important in thesemiconductor industry.

Source: L. H. Van Vlack, Materials for Engineering .Addison-Wesley PublishingCo., Inc., 1982.

M lt S i i





Melt Spinning

Figure 11.32 Schematicillustration of melt-spinning to

produce thin strips ofamorphous metal.





Types of Melting Furnaces

Figure 11.33 Two types of melting furnaces used in foundries: (a) crucible, and (b) cupola.

Casting Defects





Casting Defects

Casting Defects





Casting Defects

Porosity may be caused by shrinkage and/or gases

Thin sections solidify faster than thick sections; therefore

the molten metal cannot be supplied to thick regions thatare solidifying

Gases become less soluble in a metal as it cools andsolidifies, causing it to be expelled and sometimes form

or expand porosity

Casting Defects





g

Casting Defects





g

Chills

Pieces of material placed in the mold to speed upheat transfer in thicker areas of the part to

prevent shrinkage porosity

Internal chills are left within the cast part; externalchills are removed

Chills





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Metalurgi pengecoran