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Casting Defects and Design IIssues
ver. 1
ME 6222: Manufacturing Processes and Systems Prof. J.S. Colton © GIT 2009
1
OverviewOverview
• Processes• AnalysisAnalysis• Defects
D i l• Design rules• Economics
ME 6222: Manufacturing Processes and Systems Prof. J.S. Colton © GIT 2009
2
Issues in CastingIssues in Casting
• Shrinkage• PorosityPorosity• Piping
Mi t t• Microstructure
ME 6222: Manufacturing Processes and Systems Prof. J.S. Colton © GIT 2009
3
ShrinkageShrinkage
• Can amount to 5-10% by volume• Gray cast iron expands uponGray cast iron expands upon
solidification due to phase changes• Need to design part and mold to take• Need to design part and mold to take
this amount into consideration
ME 6222: Manufacturing Processes and Systems Prof. J.S. Colton © GIT 2009
4
ShrinkageShrinkage
TABLE 5.1
Metal or alloy
Volumetricsolidification
contraction (%) Metal or alloy
Volumetricsolidification
contraction (%)Metal or alloy contraction (%) Metal or alloy contraction (%)Aluminum 6.6 70%Cu–30%Zn 4.5Al–4.5%Cu 6.3 90%Cu–10%Al 4Al–12%Si 3.8 Gray iron Expansion to 2.5Carbon steel 2.5–3 Magnesium 4.2g1% carbon steel 4 White iron 4–5.5Copper 4.9 Zinc 6.5Source: After R. A. Flinn.
ME 6222: Manufacturing Processes and Systems Prof. J.S. Colton © GIT 2009
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Casting Defects
ME 6222: Manufacturing Processes and Systems Prof. J.S. Colton © GIT 2009
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Defects Hot TearsDefects - Hot Tears
ME 6222: Manufacturing Processes and Systems Prof. J.S. Colton © GIT 2009
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Casting Defects - Porosity
ME 6222: Manufacturing Processes and Systems Prof. J.S. Colton © GIT 2009
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Porosity
• Types– due to gases – smooth bubbles– due to shrinkage – rough voids
• Not a problem for ingots– parts that will be deformation processedp p– as long as it is not exposed to air
(corrosion)– can be healed
ME 6222: Manufacturing Processes and Systems Prof. J.S. Colton © GIT 2009
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Porosity due to Gasesy
• Smooth bubbleslt f t d– result from entrapped gases
– solubility in liquid is high, in solid is low, so gas is rejected d ring coolinggas is rejected during cooling
• Sievert’s lawS = kpg
0.5
– S = solubility– k = constant– pg = partial pressure of gas over melt
ME 6222: Manufacturing Processes and Systems Prof. J.S. Colton © GIT 2009
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pg p p g
Remedies for Gas Bubbles
• Control atmosphereControl atmosphere – vacuum
gases with less solubility– gases with less solubility• Proper venting to let gases out• Proper design of runners and gates to
avoid turbulence
ME 6222: Manufacturing Processes and Systems Prof. J.S. Colton © GIT 2009
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Remedies for Gas BubblesRemedies for Gas Bubbles
• Add metallic elements to react with gases– 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 doesn’t escapeME 6222: Manufacturing Processes and Systems
Prof. J.S. Colton © GIT 200912
• little piping because gas doesn t escape
Porosity due to ShrinkagePorosity due to Shrinkage
• Rough bubbles - voidsRough bubbles voids• Stages
li li id– cooling liquid– rejects latent heat at melting point
ll b l h li id d lid i t• alloys become slushy - liquid and solid co-exist– cooling solid
ME 6222: Manufacturing Processes and Systems Prof. J.S. Colton © GIT 2009
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Differential CoolingDifferential Cooling
• Transition between thicker and thinner sections can lead to porosityp y
ME 6222: Manufacturing Processes and Systems Prof. J.S. Colton © GIT 2009
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Porosity / Shrinkage SolutionsPorosity / Shrinkage Solutions• Risers allow molten metal to flow into
mold to make up for shrinkage• Design flow so no part freezes earlyg p y
– large channels• “Flexible” moldsFlexible molds
– allow metal to shrink, not hold metal
ME 6222: Manufacturing Processes and Systems Prof. J.S. Colton © GIT 2009
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Porosity / Shrinkage SolutionsPorosity / Shrinkage Solutions
• Heating or cooling certain areas to maintain uniform cooling (thermit or chills))
• Uniform part thickness– leads to uniform cooling– leads to uniform cooling,
less residual stress
ME 6222: Manufacturing Processes and Systems Prof. J.S. Colton © GIT 2009
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Chills
ME 6222: Manufacturing Processes and Systems Prof. J.S. Colton © GIT 2009
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Pipe DefectPipe Defect
D t h i k i i i t• Due to shrinkage giving rise to a funnel-like cavity
• Solutions– insulate top (glass wool)– heat top (exothermic mixture -
thermit)
ME 6222: Manufacturing Processes and Systems Prof. J.S. Colton © GIT 2009
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Microstructure
ME 6222: Manufacturing Processes and Systems Prof. J.S. Colton © GIT 2009
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Grains on Willie B’s headGrains on Willie B s head
ME 6222: Manufacturing Processes and Systems Prof. J.S. Colton © GIT 2009
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Microstructure DendritesMicrostructure - Dendrites
• Finer structure at walls
• Grains / dendrites grow to center
ME 6222: Manufacturing Processes and Systems Prof. J.S. Colton © GIT 2009
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grow to center
Microstructure DendritesMicrostructure - Dendrites
ME 6222: Manufacturing Processes and Systems Prof. J.S. Colton © GIT 2009
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MicrostructureMicrostructure
• Post-treatment may be necessary to get desired properties - grain structurep p g– annealing– temperingtempering– cold working
ME 6222: Manufacturing Processes and Systems Prof. J.S. Colton © GIT 2009
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Design Rules SummaryDesign Rules Summary
• Uniform wall thickness• Flat parting lines• Gradual thickness• Gradual thickness
transitionsD ft f l• Draft for removal– tapers: 0.5 to 2 degrees
• Surface of mold gives surface of part
ME 6222: Manufacturing Processes and Systems Prof. J.S. Colton © GIT 2009
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Sand Casting RulesSand Casting Rules
ME 6222: Manufacturing Processes and Systems Prof. J.S. Colton © GIT 2009
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Economics Example Optical BenchEconomics Example - Optical Bench
ME 6222: Manufacturing Processes and Systems Prof. J.S. Colton © GIT 2009
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RequirementsRequirements
• Casting of Al-Si alloyg y• Number
– one-offone off– preliminary run (100)
production run (10 000)– production run (10,000)• High precision required
hi i i d– machining required– pick cheapest casting method
ME 6222: Manufacturing Processes and Systems Prof. J.S. Colton © GIT 2009
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Alternative ProcessesAlternative Processes
• Sand castingg
• Low pressure casting• Low pressure casting
• Permanent mold casting
• Die casting
ME 6222: Manufacturing Processes and Systems Prof. J.S. Colton © GIT 2009
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Cost Equationq
CCCC Lc ++=nn
CC m &++=
• C = cost/part• C = cost/part• Cm = material cost
C i l• Cc = capital cost• CL = labor cost• n = number produced• = production raten&
ME 6222: Manufacturing Processes and Systems Prof. J.S. Colton © GIT 2009
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= production raten
Process CostsProcess Costs
Process SandCasting
LowPressure
PermanentMold
Die Casting
M t i l C ($) 1 1 1 1Material, Cm ($) 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&
ME 6222: Manufacturing Processes and Systems Prof. J.S. Colton © GIT 2009
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Process Economics
ME 6222: Manufacturing Processes and Systems Prof. J.S. Colton © GIT 2009
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Process SelectionProcess Selection
• Probably pick low pressure casting, as a preliminary run of 100 is assured.p y
• If production run is needed, die casting will probably be usedwill probably be used.
Th t h t i tti th• The tough part is getting the process cost data.
ME 6222: Manufacturing Processes and Systems Prof. J.S. Colton © GIT 2009
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Production of Aluminum Auto PartsM ain Casting
ALow-Pressure
BHigh-Pressure Squeeze
DCharacteristic GravityA D ie CastingB Die Casting(Pores Free)C
CastingD
Pouring/FillingM ethod
Ladle Air pressurethrough sta lk
H igh-speed andhigh-pressurein jection by
Relatively low-speed andhigh-pressure
hydraulic piston injectionFilling Tim e (s) 10-30 10-30 1 10OperatingPressure (atm .)
1 1+(0.2-0.5) 100-500 500-1,000
Cycle Tim e( i )
5-10 5-10 1-2 2(m in.)D ie/M oldTem perature
High High Low Low-medium
Dim ensionalAccuracy
+ ++ +++ +++
D i +++ ++ + +DesignAvailability
+++ ++ + +
Productivity + ++ +++ +++Quality + ++ +-+++ +++Cost + + +++ +++M achining M any M any Few FewM achiningRequired
M any M any Few Few
M ain Parts(other thanwheels)
Intake m anifo ld,cylinder b lockand head,piston
Cylinder block,cylinder head,suspension,m em ber
Cylinder block,oil pan, cylinderhead cover,transaxle case
Piston, d isk-brake caliper,power steeringtoe contro l hub
ME 6222: Manufacturing Processes and Systems Prof. J.S. Colton © GIT 2009
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piston m em ber transaxle case toe contro l hub,knuckle
Advantages of CastingAdvantages of Casting
• Near- or net- shape• Less scrapLess scrap• Intricate shapes
L h ll h• Large hollow shapes• No limit to size
ME 6222: Manufacturing Processes and Systems Prof. J.S. Colton © GIT 2009
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Disadvantages of Casting
• Shrinkage, porosity, cracksN i h d i• No strain hardening– can be brittle
• Tooling can be expensive– part shape depends on tool (mold)p p p ( )
• Microstructure can be difficult to control– non-uniform coolingnon uniform cooling– faster on outside produces finer grain
structure thereME 6222: Manufacturing Processes and Systems
Prof. J.S. Colton © GIT 200935
structure there
SummarySummary
• Defects• Design rulesDesign rules• Economics
ME 6222: Manufacturing Processes and Systems Prof. J.S. Colton © GIT 2009
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ME 6222: Manufacturing Processes and Systems Prof. J.S. Colton © GIT 2009
37