Castdefect

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


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Issues in CastingIssues in Casting

• Shrinkage• PorosityPorosity• Piping

Mi t t• Microstructure


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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


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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


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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


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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


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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

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

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

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

Prof. J.S. Colton © GIT 200935

structure there

SummarySummary

• Defects• Design rulesDesign rules• Economics


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Castdefect

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