2 Fundamentals of Casting Me206 t111

8/13/2019 2 Fundamentals of Casting Me206 t111

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METAL CASTING-FUNDAMENTALS - PART 2

October 2011 ME 206 Manufacturing Processes 1Dr Anwar K Sheikh 1


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Introduction

Casting melt the metal, pour into a mold and solidify

Advantages

Complex geometries

Can be net shaped

Can produce very large parts

Any metals

Can be mass-produced

Disadvantages

Limitation in mechanical properties, porosity,dimensional accuracy, surface finish

Safety Hazard



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Introduction (continued)

Dated back 6000 years

Ingot vs. Shape casting

Polymers and ceramics are cast as well.

Issues in casting

Flow

Heat Transfer

Selection of Mold Materials

Solidification- Nucleation and Growth

Depending on how we control solidification, theseevents influence the size, shape, uniformity andchemical composition of the grains.



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Fundamentals of Casting

Six basic factors involved in the casting process:

Mold cavity

Melting process

Pouring technique Solidification process

Part removal process

Post processing



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Overview

A Foundry is a casting factory.

Workers are Foundrymen.

Mold Materials sand, plaster,

ceramic and metals.

Open Molds Simple parts

Closed Molds Complex parts.

A passageway - the gatingsystem

Expendable or permanent molds


Flask

Mold

Runner

Downsprue

Parting line

Riser


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Process Factors Molten metal problems

Reaction of the metal and its environment can lead topoor quality castings. Oxygen and molten metal reactto form slag or dross. These impurities can become

trapped in castings to impair surface finish,machinability, or reduce the mechanical properties ofthe castings.

Fluidity

Molten metal must flow then freeze into the desired

shape. Incorrect flow characteristics can result inshort shots, incorrect part tolerances, cracks incastings, voids, etc.



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Process Factors


Gating System Correct design of the gating system is a must.

Gating system controls the speed, rate, and deliveryof molten material into the mold cavity.

Example: PIM general rule is gate depth is

equal to 1/3 its width Patterns

Shrinkage allowance

Cast Iron = 1/10 - 1/8 in/ft

Aluminum = 1/8 - 5/32 in/ft

Brass = 3/16 in/ft

Amount of draft

Finish material allowance

Final dimensional accuracy of the casting


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Basic features of Molds

Sand Casting Molds

Mold: cope (upper half) & drag (bottom half)

Flask

Parting line Pattern the mold cavity

The gating system pouring cup, (down) sprue, runner

Riser a source of liquid metal to compensate forshrinkage during solidification



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Casting Terms

Pattern

Flask

Cope

Drag

Core

Core Box

Core Print

Mold Cavity


Riser

Gating System

Pouring Cup

Sprue

Runner Gate

Parting Line

Draft


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Sand Mold Features


Figure 5.10 A Schematic illustration of a sand mold, showingvarious features.


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Mold Features


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


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MOLD



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Riser-Gated Casting


Schematic illustration of a typical riser-gated casting. Risersserve as reservoirs, supplying molten metal to the casting as itshrinks during solidification. Source: American FoundrymensSociety.


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Heating Metal for Casting-Problem

One cubic foot of a certain eutectic alloy will be heated in a crucible fromroom temperature to 2000above its melting point for casting. Theproperties of the alloy are density = 0.15 lbm/in.3. melting point = 1300 0F.specific heat of the metal = 0.082 Btu/ lbm 0F in the solid state and 0.071Btu/lbm 0F in the liquid state: and heat of fusion = 72 Btu/lbm. How muchheat energy must be added to accomplish the heating. assuming no losses. Solution: Assume ambient temperature in the foundry = 80 0Fand that the densities of liquid and solid states of the metal arethe same. Noting that 1 ft3 = 1728 in.3 and substituting the

property values into Eq. (H= V [Cs(Tm-To)+Hf + Cl (Tp-Tm)] ). we have

H = (0.15)(1728){0.082(1300 - 80) + 72 + 0.071(1500 - 1300)}

= 48,273.4 Btu

October 2011 ME 206 Manufacturing Processes 1Dr Anwar K Sheikh

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Foundry Practice

Furnace Cupolas

Direct Fuel-fired furnace

Crucible Furnace

Electric-arc Furnace Induction Furnace

Pouring with ladle

Solidification watch for oxidation

Trimming, surface cleaning, repair and heat treat,

inspection


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Commercial Melting Methods

Coke-fueled cupola (cast irons) - continuous

Electric

Induction (steels, cast irons, Ni, Al, Cu)

Coreless - batch Channel - continuous

Resistance (Al, Mg, Zn, Pb)

Crucible - batch

Reverberatory - continuous

Arc (steels, cast irons, Ti) - batch

Gas-Fired

Crucible (Al, Mg, Zn, Pb) - batch

Reverberatory (Al, Zn, Pb) - continuousOctober 2011 ME 206 Manufacturing Processes 1

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Types of Melting Furnaces


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


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FURNACES


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Gas Fired Furnace

Induction Furnaces

Electric ResistanceFurnace _MeltingAluminum


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Furnace Issues

Energy efficiency:Electric melting furnaces aregenerally about 3 times more efficient than gas-firedfurnaces. However, if the energy content of electricity(BTU/KWH) and natural gas (BTU/Cubic feet) are

equilibrated, for the same amount of energy electricityis historically about 3 times the cost of natural gas.

Melt losses: Gas-fired furnaces melting aluminumtypically generate about 3% dross by weight, whereaselectric furnaces generate about 1%.


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Pouring

Factors affecting pouring

Pouring temperature (vs.melting temp.)

Pouring rate

Too slow, metal freezes

Too high, turbulence

Turbulence

Accelerate the formation of

oxides Mold erosion

Voids?


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Pouring Analysis


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Figure 5.10 A Schematic illustration of a sand mold, showingvarious features.

Point 0 (A0, V0)Referencelevel h0= 0,distance fromreference level

Point 1 (A1, V1) -

h1distance fromreference level

Point x

(Ax, Vx) -hxxdistancefrom

referencelevel

Point 2 (A2, V2) -h2distance fromreference level

h1

hx

h2


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Pouring Analysis

Bernoullis theorem at any two points in a flowing liquid

h=head, p=pressure, =density, v=flow velocity, g=gravity,f=friction loss (Suffix x identified distance of point x from referencepoint( reference level) o. Applying Bernoulli's Theorem at Point 0 )and 1

Assuming point 0 is reference, no frictional loss and same pressure(p0=p1), Vo=0, and h0=0


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2 2

1 1 2 21 2

2 2

p v p vh h f

g g g g

g

v

g

ph

g

v

g

ph

22

2

111

2

000

12

1;

2

21

1 ghv

g

vh


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Pouring Analysis

Applying Bernoulli's Theorem at Point 0 and 2

Assuming point 0 is reference, no frictional loss and same pressure(p0=p2), Vo=0, and h0=0


24

g

v

g

ph

g

v

g

ph

22

2

222

2

000

222;

2

22

2 ghv

g

vh


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Pouring Analysis (continued)

Continuity law

Volumetric flow;

Mold fill time (MFT)=Time required to fill the cavity


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2211 AvAvQ

VMFTf Q

t


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Pouring Analysis (continued)

Similarly Applying Bernoulli's Theorem and Continuity Equation atPoint 0 and x , where point 0 is reference point, there is nofrictional loss and same pressure at all ponits in the mold(p0=px),Vo=0, and h0=0, we get

2

1

1

2

hh

AA

xxxx h

hAAAghAghQ 1111 22


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xxAvAvQ 11xghxvgxv

xh 2;

2

2

If hx=h2 then

Reference Point 0

Point 2

Point 1

Point x

h1

hx

h2


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Reynolds Number (Re)

Higher Re, greater tendency for turbulence flow

Turbulence and laminar flow

Re=vD/

Re 2,000(laminar)

2,000 to 20,000 (mixture of laminar-turbulence).

greater than 20,000 turbulence resulting in air

entrainment and dross(scum) formation

Minimize turbulence by avoiding a certain range in flowdirection


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Fluidity


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Molten metal must flow then freeze into the desired shape.Incorrect flow characteristics can result in short shots,incorrect part tolerances, cracks in castings, voids, etc.

Fluidity: A measure of the capability of a metal to flow intoand fill the mold before freezing.

Inverse of viscosity

Factors affecting fluidity

Pouring temperature

Metal composition

Viscosity Heat transfer to the surroundings

Heat of fusion

Solidification

Pure metals: good fluidity

Alloys: not as good


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Fluidity Test


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Figure 5.1X A test method forfluidity using a spiral mold. Thefluidity indexis the length of thesolidified metal in the spiralpassage. The greater the length of

the solidified metal, the greater isits fluidity.


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Solidification of Pure Metals

FIGURE 5.1 (a) Temperature as a function of time for the solidification of pure metals. Note that

freezing takes place at a constant temperature. (b) Density as a function of time.October 2011 ME 206 Manufacturing Processes 1

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Solidification (Pure Metals- Continued)

Undercooling

Solidification occurs at aconstant temperature andsupercooled Temperature

Actual freezing during the localsolidification time

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Pouring Temp.

Total

SolidificationTime

Local

Solidification

Time

Liquid

CoolingSolid

Cooling

Cooling Curve

Tm

timeendritic growth


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Temperature & Density for Castings


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FIGURE 5.1 (a) Temperature as a function of time for the solidification of pure metals.

Note that freezing takes place at a constant temperature. (b) Density as a function of

time.


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Solidification of Alloys

Most Alloys freeze over a temperature range, not at asingle temperature.

Chemical compositional gradiency within a single grain

Chemical compositional gradiency throughout the casting ingot segregation

Eutectic Alloys Solidification occurs at a singletemperature


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Two-Phased Alloys


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FIGURE 5.2 (a) Schematic illustration of grains, grain boundaries, and particles

dispersed throughout the structure of a two-phase system, such as lead-copper alloy.

The grains represent lead in solid solution of copper, and the particles are lead as a

second phase. (b) Schematic illustration of a two-phase system, consisting of two sets

of grains: dark and light. Dark and light grains have their own compositions and

properties.


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Phase Diagram for Nickel-Copper


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FIGURE 5.3 Phase diagram for nickel-copper alloy system obtained by a low rate of solidification. Note that pure nickel and

pure copper each have one freezing or melting temperature. The top circle on the right depicts the nucleation of crystals; the

second circle shows the formation of dendrites; and the bottom circle shows the solidified alloy with grain boundaries.


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Iron-Iron Carbide Phase Diagram


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FIGURE 5.4 (a) The iron-iron carbide phase diagram. (b) Detailed view of the microstructures above and below the

eutectoid temperature of 727C (1341F). Because of the importance of steel as an engineering material, this

diagram is one of the most important phase diagrams.


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Cast Structures


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Figure 10.5Schematicillustration of three

basic types of caststructures: (a)columnar dendritic;(b) equiaxeddendritic; and (c)equiaxednondendritic.Source: D. Apelian.

Figure 5.6 Schematic illustrationof cast structures in (a) plane front,single phase, and (b) plane front,two phase. Source: D. Apelian.


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Cast Structures


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FIGURE 5.9 Schematic illustration of cast

structures in (a) plane front, single phase, and

(b) plane front, two phase. Source: After D.

Apelian.

FIGURE 5.8 Schematic

illustration of three basic

types of cast structures:

(a) columnar dendritic;

(b) equiaxed dendritic; and

(c) equiaxed nondendritic.

Source:After D. Apelian.


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Cast Structures of Metals


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Figure 5.8 Schematicillustration of three cast

structures of metalssolidified in a square mold:(a) pure metals; (b) solid-solution alloys; and (c)structure obtained by usingnucleating agents. Source:

G. W. Form, J. F. Wallace,J. L. Walker, and A. Cibula.

Preferred Te t re


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Preferred Texture

Development


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Figure 5.9 Development of a preferred texture at a coolmold wall. Note that only favorably oriented grains growaway from the surface of the mold.


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Solidification Patterns for Gray Cast Iron


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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.


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Riser Design

Therefore for riser design: t riser > t casting allows moltenmetal to flow into the casting to compensate forvolumetric solidification shrinkage (risers must riseabove the casting to function).

Type of Risers-

Side Risersand Top Risers

Open riser and Blind Risers


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Casting/Mold Yield Risers can help eliminate shrinkage pore defects, but there

is a penalty to be paid in terms of yield (melt costs) andremoval costs (labor and cutting tools/supplies)

The ratio of saleable casting weight versus total weight ofmolten metal poured to produce the casting:

% Yield = [trimmed casting weight/pour weight] x100


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l l d f


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Volumetric Solidification Contraction

TABLE 5.1 Volumetric solidification contraction orexpansion percentages for various cast metals.


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Contraction (%):Aluminum 7.1Zinc 6.5

Al-4.5% Cu 6.3Gold 5.5White iron 4-5.5

Copper 4.9Brass (70-30) 4.5Magnesium 4.2

90% Cu- 10% Al 4Carbon steels 2.5-4

Al-12% Si 3.8Lead 3.2

Expansion (%):Bismuth 3.3Silicon 2.9

Gray iron 2.5

A idi Sh i k C iti 1


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Avoiding Shrinkage Cavities-1


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Casting Cross-Sections

Figure 12.2 Examples of designs showing the importance of maintaining uniform cross-sections in castings to avoid hot spots and shrinkage cavities.

Design Modifications to Avoid Defects in


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Design Modifications to Avoid Defects in

Castings

FIGURE 5.39 (a) Suggested design modifications to avoid defects in castings.Note that sharp corners are avoided to reduce stress concentrations. (b)-(d)Examples of designs that show the importance of maintaining uniform cross-sections in castings to avoid hot spots and shrinkage cavities.


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Alloy Solidification & Temperature


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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.


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Temperature Distribution


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FIGURE 5.11 Temperature distribution at the mold wall and liquid-metal interface during

solidification of metals in casting.


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Mold Filling and Solidification

FIGURE 5.45 Simulation of mold filling and solidification. (a) 3.7 secondsafter start of pour. Note that the mushy zone has been established beforethe mold is completely filled. (b) Using a vent in the mold for removal ofentrapped air five seconds after pour. Source: S. Shepel and S. Paolucci,University of Notre Dame.


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Solidification Time


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Figure 5.16 Solidified skin on a steel casting. The remaining molten metal is poured outat the times indicated in the figure. Hollow ornamental and decorative objects are madeby a process called slush casting, which is based on this principle. Source: H. F.Taylor, J. Wulff, and M. C. Flemings.


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Solidification Time of a Casting

Chvorinovs Empirical relationship: Solidification time as afunction of the size and shape

V=volume A=surface area and n=2

C=experimentally determined value

Used in riser design: the solidification time of the risermust be greater than the solidification time of the castpart.


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n

sA

V

Ct


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Slush Casting


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FIGURE 5.12 Solidified skin on a steel casting; the remaining molten metal is poured out at the

times indicated in the figure. Hollow ornamental and decorative objects are made by a process

called slush casting, which is based on this principle.Source:After H.F. Taylor, J. Wulff, andM.C. Flemings.

Chvorinovs Rule:

Cast Material


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Cast Material

Properties


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FIGURE 5.13 Mechanical properties for

various groups of cast alloys. Compare with

various tables of properties in Chapter 3.Source: Courtesy of Steel Founders'

Society of America.


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Properties & Applications of Cast Iron


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TABLE 5.4 Properties and typical applications

of cast irons.


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General Characteristics of Casting


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TABLE 5.2 General characteristics of casting processes.

T pical Applications &


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Typical Applications &

Characteristics


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TABLE 5.3 Typical applications for castings and casting

characteristics.


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Nonferrous Alloys


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TABLE 5.5 Typical properties of nonferrous casting alloys.


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Casting Quality

Casting defects Misruns

Cold shut

Cold shots

Shrinkage cavity

Microporosity

Hot Tearing


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Sand Mold defects

Sand blow

Pinholes

Sand wash

Scabs

Penetration

Mold shift

Core shift

Mold crack


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Shrinkage


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TABLE 5.1 Volumetric solidification contraction or

expansion for various cast metals.


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Directional Solidification

To minimize the damage during casting, the region mostdistant from the liquid metal supply needs to freeze firstand the solidification needs to process toward the riser.

Based on Chvorinovs rule, the section with lower V/A ratioshould freeze first.

Use Chills: Internal and External chills which encouragerapid cooling. (See Fig.5.17)


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Elimination of Porosity in Castings


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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 of maintaining uniform cross-sections in castings to avoid hot spots and shrinkagecavities.


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Avoiding Shrinkage Cavities -2


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Figure Examples ofdesign modifications toavoid shrinkage cavities

in castings. Source: SteelCastings Handbook, 5thed. Steel Founders'Society of America, 1980.Used with permission.

Various Types of Chills Used in Castings to


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yp g

Eliminate Porosity

FIGURE 5.17Various types of(a) internal and(b) external chills(dark areas atcorners), used in

castings toeliminate porositycaused byshrinkage. Chillsare placed inregions where

there is a largevolume of metal,as shown in (c).


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Chills


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FIGURE 5.35 Various types of (a) internal and (b) external chills (dark areas at corners), used in

castings to eliminate porosity caused by shrinkage. Chills are placed in regions where there is a

larger volume of metal, as shown in (c).


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Casting Defects


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Figure 5.17B Examples of common defects in castings. These defects can be minimized or eliminatedby proper design and preparation of molds and control of pouring procedures. Source: J. Datsko.

Hot Tears


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Hot Tears


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Figure 5.17A Examples of hot tears in castings. These defects occur because the castingcannot shrink freely during cooling, owing to constraints in various portions of the molds andcores. Exothermic (heat-producing) compounds may be used (as exothermic padding) tocontrol cooling at critical sections to avoid hot tearing.


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Solubility of Hydrogen in Aluminum


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Figure 5.18 Solubility of hydrogen inaluminum. Note the sharp decrease in

solubility as the molten metal begins tosolidify.


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Casting Processes


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PROCESS ADVANTAGES LIMITATIONS

Sand Almost any metal is cast; no l imit to size,shape or weight; low tooling cost.

Some finishing required; somewhatcoarse finishl wide tolerances.

Shell mold Good dimensional accuracy and surfacefinish; high production rate.

Part size limited; expensive patternsand equipment required.

Expendable pattern Most metals cast with no limit to size;complex shapes

Patterns have low strength and canbe costly for low quantities.

Plaster mold Intricate shapes; good dimensionalaccuracy and finish; low porosity.

Limited to nonferrous metals; limitedsize and volume of production; mold

making time relatively long.Ceramic mold Intricate shapes; close tolerance parts;

good surface finish.

Limited size.

Investment Intricate shapes; excel lent surface fini shand accuracy; almost any metal cast.

Part size limited; expensive patterns,molds, and labor.

Permanent mold Good surface finish and dimensionalaccuracy; low porosity; high productionrate.

High mold cost; limited shape andintricacy; not suitable for high-melting-point metals.

Die Excellent dimensional accuracy and

surface finish; high production rate.

Die cost is high; part size limited;

usually limited to nonferrous metals;long lead time.

Centrifugal Large cyl indrical parts wi th good qual ity;high production rate.

Equipment is expensive; part shapelimited.

TABLE 5.8 Casting processes, and their advantages and limitations.


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Austenite-Pearlite Transformation


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FIGURE 5.32 (a) Austenite to pearlite

transformation of iron-carbon alloys as a

function of time and temperature. (b)

Isothermal transformation diagram obtained

from (a) for a transformation temperature of675C (1247F). (c) Microstructures obtained

for a eutectoid iron-carbon alloy as a function

of cooling rate. Source: Courtest of ASM

International.

Phase Diagram for Aluminum-


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Phase Diagram for Aluminum-

Copper


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FIGURE 5.33 (a) Phase diagram for the aluminum-copper alloy system. (b) Various

microstructures obtained during the age-hardening process.


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Outline of Heat Treating


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TABLE 5.7 Outline of heat

treatment processes for

surface hardening.


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Heat Treatment Temperature Ranges


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

indicated on the iron-iron carbide phase diagram.


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Casting Processes Comparison


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TABLE 5.8 Casting Processes, and their Advantages and Limitations.


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Product Design Considerations Geometric simplicity

Corners

Section thicknesses Hot spot

Draft

Use of Cores Dimensional tolerances and surface finish

Machining allowance


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Casting Design Modifications FIGURE 5.40 Examples of casting design modifications. Source: Steel

Castings Handbook, 5thed., Steel Founders Society of America, 1980.Used with permission.


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Design Modifications


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FIGURE 5.38 Suggested

design modifications to

avoid defects in castings.

Source: Courtesy of TheNorth American Die Casting

Association.

Design Practices for Die Cast Parts


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Design Practices for Die-Cast Parts

FIGURE 5.41 Examples of undesirable and desirable design practicesfor die-cast parts. Note that section-thickness uniformity ismaintained throughout the part. Source: Courtesy of The NorthAmerican Die Casting Association.


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Economics of Casting


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FIGURE 5.39 Economic comparison of making a part by two different casting processes. Note that because of the

high cost of equipment, die casting is economical mainly for large production runs. Source: The North American Die

Casting Association.


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Summary

Successful casting engineering requires a holistic approachwhich includes concurrent consideration of:

Alloy selection/functional requirements/thermal treatments

Melting method/melt quality

Casting process/economics

Metal delivery system design

The location and amount of solidification shrinkage

Maximizing casting yield

Downstream processing requirements Each casting geometry, alloy, and process has its own unique

engineering challenges. Applying physical and chemicalprinciples to the problems yields the best results!

Documents

2 Fundamentals of Casting Me206 t111