Castdefect
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Casting Defects and Design IIssues
ver. 1
ME 6222: Manufacturing Processes and Systems Prof. J.S. Colton © GIT 2009
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OverviewOverview
• Processes• AnalysisAnalysis• Defects
D i l• Design rules• Economics
ME 6222: Manufacturing Processes and Systems Prof. J.S. Colton © GIT 2009
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Issues in CastingIssues in Casting
• Shrinkage• PorosityPorosity• Piping
Mi t t• Microstructure
ME 6222: Manufacturing Processes and Systems Prof. J.S. Colton © GIT 2009
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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
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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.
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Casting Defects
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Defects Hot TearsDefects - Hot Tears
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Casting Defects - Porosity
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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
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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
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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
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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
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• little piping because gas doesn t escape
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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
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Differential CoolingDifferential Cooling
• Transition between thicker and thinner sections can lead to porosityp y
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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
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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
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Chills
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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)
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Microstructure
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Grains on Willie B’s headGrains on Willie B s head
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Microstructure DendritesMicrostructure - Dendrites
• Finer structure at walls
• Grains / dendrites grow to center
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grow to center
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Microstructure DendritesMicrostructure - Dendrites
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MicrostructureMicrostructure
• Post-treatment may be necessary to get desired properties - grain structurep p g– annealing– temperingtempering– cold working
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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
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Sand Casting RulesSand Casting Rules
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Economics Example Optical BenchEconomics Example - Optical Bench
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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
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Alternative ProcessesAlternative Processes
• Sand castingg
• Low pressure casting• Low pressure casting
• Permanent mold casting
• Die casting
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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&
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= production raten
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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&
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Process Economics
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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.
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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
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piston m em ber transaxle case toe contro l hub,knuckle
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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
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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
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structure there
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SummarySummary
• Defects• Design rulesDesign rules• Economics
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