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METAL CASTING-FUNDAMENTALS - PART 2
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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
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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
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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
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Riser
Gating System
Pouring Cup
Sprue
Runner Gate
Parting Line
Draft
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Sand Mold Features
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Figure 5.10 A Schematic illustration of a sand mold, showingvarious features.
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Mold Features
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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
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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
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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
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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
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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!