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ME 6222: Manufacturing Processes and SystemsProf. J.S. Colton GIT 2011
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Casting Defects and Design
Issues
ver. 2
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ME 6222: Manufacturing Processes and SystemsProf. J.S. Colton GIT 2011
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Overview
Processes
Analysis
Defects
Design rules
Economics
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ME 6222: Manufacturing Processes and SystemsProf. J.S. Colton GIT 2011
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Issues in Casting
Shrinkage
Porosity
Piping
Microstructure
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ME 6222: Manufacturing Processes and SystemsProf. J.S. Colton GIT 2011
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Shrinkage
Can amount to 5-10% by volume
Gray cast iron expands upon
solidification due to phase changes
Need to design part and mold to take
this amount into consideration
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ME 6222: Manufacturing Processes and SystemsProf. J.S. Colton GIT 2011
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Shrinkage
TABLE 5.1
Metal or alloy
Volumetric
solidification
contraction (%) Metal or alloy
Volumetric
solidification
contraction (%)
Aluminum 6.6 70%Cu30%Zn 4.5Al4.5%Cu 6.3 90%Cu10%Al 4
Al12%Si 3.8 Gray iron Expansion to 2.5
Carbon steel 2.53 Magnesium 4.2
1% carbon steel 4 White iron 45.5
Copper 4.9 Zinc 6.5
Source: After R. A. Flinn.
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ME 6222: Manufacturing Processes and SystemsProf. J.S. Colton GIT 2011
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Casting Defects
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ME 6222: Manufacturing Processes and SystemsProf. J.S. Colton GIT 2011
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Defects - Hot Tears
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Steam Engine Flywheels
ME 6222: Manufacturing Processes and SystemsProf. J.S. Colton GIT 2011
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Car Rims
ME 6222: Manufacturing Processes and SystemsProf. J.S. Colton GIT 2011
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Stamped Cast
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ME 6222: Manufacturing Processes and SystemsProf. J.S. Colton GIT 2011
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Casting Defects - Porosity
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ME 6222: Manufacturing Processes and SystemsProf. J.S. Colton GIT 2011
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Porosity
Types due to gasessmooth bubbles
due to shrinkagerough voids
Not a problem for ingots parts that will be deformation processed
as long as it is not exposed to air
(corrosion) can be healed
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ME 6222: Manufacturing Processes and SystemsProf. J.S. Colton GIT 2011
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Porosity due to Gases
Smooth bubbles result from entrapped gases
solubility in liquid is high, in solid is low, so
gas is rejected during cooling
Sieverts law
S = kpg0.5
S = solubility k = constant
pg= partial pressure of gas over melt
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ME 6222: Manufacturing Processes and SystemsProf. J.S. Colton GIT 2011
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Remedies for Gas Bubbles
Control atmosphere
vacuum
gases with less solubility
Proper venting to let gases out
Proper design of runners and gates to
avoid turbulence
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ME 6222: Manufacturing Processes and SystemsProf. J.S. Colton GIT 2011
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Remedies for Gas Bubbles
Add metallic elements to react withgases
killed steels - highly deoxidized (Al, Si)
high shrinkage due to gas removal - piping semi-killed steels - less deoxidized
less piping, porosity
rimmed steels - little deoxidization
blow holes in ring at rim (sometimes break
through)
little piping because gas doesnt escape
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ME 6222: Manufacturing Processes and SystemsProf. J.S. Colton GIT 2011
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Porosity due to Shrinkage
Rough bubbles - voids
Stages
cooling liquid
rejects latent heat at melting point
alloys become slushy - liquid and solid co-exist
cooling solid
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ME 6222: Manufacturing Processes and SystemsProf. J.S. Colton GIT 2011
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Differential Cooling
Transition between thicker and thinner
sections can lead to porosity
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ME 6222: Manufacturing Processes and SystemsProf. J.S. Colton GIT 2011
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Porosity / Shrinkage Solutions
Risers allow molten metal to flow intomold to make up for shrinkage
Design flow so no part freezes early
large channels
Flexible molds
allow metal to shrink, not hold metal
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ME 6222: Manufacturing Processes and SystemsProf. J.S. Colton GIT 2011
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Porosity / Shrinkage Solutions
Heating or cooling
certain areas to maintain
uniform cooling (thermitor chills)
Uniform part thickness
leads to uniform cooling,less residual stress
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ME 6222: Manufacturing Processes and SystemsProf. J.S. Colton GIT 2011
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Chills
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ME 6222: Manufacturing Processes and SystemsProf. J.S. Colton GIT 2011
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Pipe Defect
Due to shrinkage giving rise to
a funnel-like cavity
Solutions insulate top (glass wool)
heat top (exothermic mixture -
thermit)
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ME 6222: Manufacturing Processes and SystemsProf. J.S. Colton GIT 2011
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Microstructure
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ME 6222: Manufacturing Processes and SystemsProf. J.S. Colton GIT 2011
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Grains on Willie Bs head
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ME 6222: Manufacturing Processes and SystemsProf. J.S. Colton GIT 2011
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Microstructure - Dendrites
Finer structure at
walls Grains / dendrites
grow to center
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ME 6222: Manufacturing Processes and SystemsProf. J.S. Colton GIT 2011
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Microstructure - Dendrites
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ME 6222: Manufacturing Processes and SystemsProf. J.S. Colton GIT 2011 25
Microstructure
Post-treatment may be necessary to get
desired properties - grain structure
annealing
tempering
cold working
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ME 6222: Manufacturing Processes and SystemsProf. J.S. Colton GIT 2011 26
Design Rules Summary
Uniform wall thickness
Flat parting lines
Gradual thickness
transitions
Draft for removal
tapers: 0.5 to 2 degrees
Surface of mold gives
surface of part
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ME 6222: Manufacturing Processes and SystemsProf. J.S. Colton GIT 2011 27
Sand Casting Rules
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ME 6222: Manufacturing Processes and SystemsProf. J.S. Colton GIT 2011 28
Economics Example - Optical Bench
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ME 6222: Manufacturing Processes and SystemsProf. J.S. Colton GIT 2011 29
Requirements
Casting of Al-Si alloy
Number
one-off
preliminary run (100)
production run (10,000)
High precision required
machining required
pick cheapest casting method
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ME 6222: Manufacturing Processes and SystemsProf. J.S. Colton GIT 2011 30
Alternative Processes
Sand casting
Low pressure casting
Permanent mold casting
Die casting
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ME 6222: Manufacturing Processes and SystemsProf. J.S. Colton GIT 2011 31
Cost Equation
n
C
n
CCC Lc
m
C = cost/part
Cm= material cost
Cc= capital cost
CL= labor cost
n = number produced
= production raten
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ME 6222: Manufacturing Processes andSystems Prof. J.S. Colton GIT 2011 32
Process Costs
Process SandCasting
LowPressure
PermanentMold
Die Casting
Material, Cm($) 1 1 1 1
Labor, CL($/hr) 20 20 20 20
Capital, CC($) 0.9 4.4 700 3000
Rate, (#/hr) 6.25 22 10 50n
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ME 6222: Manufacturing Processes and SystemsProf. J.S. Colton GIT 2011 33
Process Economics
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ME 6222: Manufacturing Processes and SystemsProf. J.S. Colton GIT 2011 34
Process Selection
Probably pick low pressure casting, as a
preliminary run of 100 is assured.
If production run is needed, die castingwill probably be used.
The tough part is getting the processcost data.
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ME 6222: Manufacturing Processes andSystems Prof. J.S. Colton GIT 2011 35
Production of Aluminum Auto PartsMainCharacteristic
CastingGravity
ALow-PressureDie Casting
BHigh-PressureDie Casting(Pores Free)
C
SqueezeCasting
D
Pouring/Filling
Method
Ladle Air pressure
through stalk
High-speed and
high-pressureinjection byh draulic iston
Relatively low-
speed andhigh-pressureinjection
Filling Time (s) 10-30 10-30 1 10OperatingPressure (atm.)
1 1+(0.2-0.5) 100-500 500-1,000
Cycle Time(min.)
5-10 5-10 1-2 2
Die/MoldTemperature
High High Low Low-medium
DimensionalAccuracy
+ ++ +++ +++
DesignAvailability
+++ ++ + +
Productivity + ++ +++ +++Quality + ++ +-+++ +++Cost + + +++ +++
MachiningRequired
Many Many Few Few
Main Parts(other thanwheels)
Intake manifold,cylinder blockand head,piston
Cylinder block,cylinder head,suspension,member
Cylinder block,oil pan, cylinderhead cover,transaxle case
Piston, disk-brake caliper,power steeringtoe control hub,knuckle
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ME 6222: Manufacturing Processes and SystemsProf. J.S. Colton GIT 2011 36
Advantages of Casting
Near- or net- shape
Less scrap
Intricate shapes
Large hollow shapes
No limit to size
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ME 6222: Manufacturing Processes and SystemsProf. J.S. Colton GIT 2011 37
Disadvantages of Casting
Shrinkage, porosity, cracks No strain hardening
can be brittle
Tooling can be expensive part shape depends on tool (mold)
Microstructure can be difficult to control
non-uniform cooling faster on outside produces finer grain
structure there
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ME 6222: Manufacturing Processes and SystemsProf. J.S. Colton GIT 2011 38
Summary
Defects
Design rules
Economics
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ME 6222 M f t i P d S t 39