2.008 metal casting. outline introduction process constraints green sand casting other processes
TRANSCRIPT
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2.008
Metal Casting
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Outline
Introduction
Process Constraints
Green Sand Casting
Other Processes
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Some Facts
First casting: 5000-3000 BC
Bronze, iron age, light metal age?
Versatility• Many types of metals
• Rapid production
• Wide range of shapes and sizes
• Complex parts as an integral unit
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Example – Sand Casting
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Example – Die Casting
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Example – Investment Casting
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Casting Process Physics andConstraints
Phase Change
• Density
• Solubility
• Diffusion rates
High melting temperature
• Chemical activity
• High latent heat
• Handling
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Analysis of Casting Processes
Fluid mechanics for mold filling Heat transfer for solidification Thermodynamics, mass transfer and heat
transfer for nucleation and growth Materials behavior for structure-property relationships
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Mold Filling
Bernoulli’s equation
Reynold’s number
• Turbulence• Injection Molding : Re ~ 10-4
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Cooling for Sand Mold
METAL - MOLDINTERFACE
METAL - AIRINTERFACE
DISTANCE
TE
MP
ER
AT
UR
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Conductivity / Diffusivity
Conductivity (W/mK) Cu ~ 400, Al ~ 200 Sand ~ 0.5, PMMA
Sand Castingαsand < αmetal
Injection Moldingαtool metal ~ αmetal
Die Castingαtool metal > αpolymer
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Solidification Time : Sand Casting
Transient 1-D heat transfer
Solution
Solidification time
Chvorinov’s rule
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Solidification Time : Die Casting
Transient 1-D heat transfer
Solution
Solidification time
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Comparison:Sand Mold vs Metal Mold
Sand MoldSand casting
Metal MoldDie casting
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Microstructure Formation
Schematic illustration of three basic types of cast structures(a) Columnar dendritic (b) equiaxed dendritic (c) equiaxed nondendritic
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Formation of Dendrites
Liquid
Liquid
Liquid
Mushy zoneAlloying element
Tem
per
atur
e
Solid
Solid
Solid
Dendrites
LiquidusSolidus
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Constitutional Supercooling
SOLUTE ENRICHEDLAYER IN FRONT OFLIQUID-SOLIDINTERFACE
CONSTITUTIONALLYSUPERCOOLEDREGION
Te
mp
era
ture
Te
mp
era
ture
Liquid
Solid
LIQ
UID
CO
MP
OS
ITIO
N
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Green Sand Casting
Mechanicaldrawing of part
Core boxesCore halves
pasted together Cope pattern plate Drag pattern plate
Cope readyfor sand
Cope after ramming withsand and removing pattern,
sprue, and risers
Drag readyfor sand Drag after
removingpattern
Drag with coreset in place
Cope and drag assembledready for pouring
Casting asremoved from
mold; heat treated
Casting readyfor shippement
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Green Sand Mold
Dimensional, Thermal and Chemical stability at high T Size and shape Wettability by molten metal Compatibility with binder system Availability and consistency
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Pattern Design Considerations(DFM)
Shrinkage allowance Machining allowance Distortion allowance Parting line Draft angle
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Typical Shrinkage AllowanceMetal or alloy Shrinkage allowances mm / m
Aluminum alloyAluminum bronzeYellow brass (thick sections)Yellow brass (thin sections)Gray cast iron (a)White cast ironTin bronzeGun metalLeadMagnesiumMagnesium alloys (25%)Manganese bronzeCopper-nickelNickelPhosphor bronzeCarbon steelChromium steelManganese steelTinZinc
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Typical Pattern MachiningAllowance
Pattern size, mm
Allowances,mm
Bore Surface Cope side
For cast irons
For cast steels
For nonferrous alloys
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Gating System:Sprue, Runner, and Gate
Rapid mold filling Minimizing turbulence Avoiding erosion Removing inclusions Controlled flow and thermal conditions Minimizing scrap and secondaryoperations
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Riser: Location and Size
Casting shrinkage Directional solidification Scrap and secondary operation
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Progressive Solidification inRiser
Progressive solidificationIntermediate
rateSlowrate
Fastrate
Riser
Temperature gradientrising toward riser Directional
solidification
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Draft in Pattern
Patterns
Mold
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Investment Casting
Injection wax orplastic patterns Ejecting pattern
Waxpattern
Patternassembly
(Tree)
Slurry coating Stucco coating Completedmold
Pattern meltout
AutoclavedHeat
Heat
Heat
Heat
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Investment Casting (cont.)
PouringPouring
Shakeout
Finished product
Casting Pattern
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Advantages of Investment Casting
Intricate geometry Close dimensional tolerance Superior surface finish High-melting point alloys
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Advantages of Investment Casting
Gas/oil accumulator
Platen Toggle clamp
Piston
Shot sleeve
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Advantages of Die Casting
High production rates Closer dimensional tolerances Superior surface finish Improved mechanical properties
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Lost Foam Casting
Pottern molding
Cluster Assemble Coating
Compacted in SandCasting Shakeout
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Lost Foam Casting
Receive raw polystyrene beads
Expand beads
Mold component pattern,including gating system
Join patters (if multipiece)
Coat pattern assembly
Dry assembly
Invest assembly in flaskwith backlip medium
Vibrate to compact medium
Pour
Shakeout castings
Clean castings assembly
Inspect castings
Ship castings
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Advantages of Lost Foam Casting
No parting line No cores One-piece flask Freedom of design Minimum handling of sand Ease of cleaning and secondary
operation
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Semi-solid Casting
Punch
Die
Inductionfurnace
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Advantages of Semi-solid Casting
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Casting Process Comparison
Process Die Equipment Labor
Produciotn rate (Pc/hr)
Cost *
Sand Shell-moldPlasterInvestmentPermancnt moldDieCentrifugal
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Cost - Casting
Sand casting ‧Tooling and equipment costs are low ‧Direct labor costs are high ‧Material utilization is low ‧Finishing costs can be high Investment casting ‧Tooling costs are moderate depending on the complexity ‧Equipment costs are low ‧Direct labor costs are high ‧Material costs are low Die casting ‧Tooling and equipment costs are high ‧Direct labor costs are low to moderate ‧Material utilization is high
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Quality - Casting
Sand casting ‧Tolerance (0.7~2 mm) and defects are affected by shrinkage ‧Material property is inherently poor ‧Generally have a rough grainy surface Investment casting ‧Tolerance (0.08~0.2 mm) ‧Mechanical property and microstructure depends on the method ‧Good to excellent surface detail possible due to fine slurry
Die casting ‧Tolerance (0.02~0.6 mm) ‧Good mechanical property and microstructure due to high ‧pressure ‧Excellent surface detail
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Rate - Casting
Sand casting ‧Development time is 2~10 weeks ‧Production rate is depending on the cooling time : t~(V/A)2
Investment casting ‧Development time is 5~16 weeks depending on the complexity ‧Production rate is depending on the cooling time : t~(V/A)2
Die casting ‧Development time is 12~20 weeks ‧Production rate is depending on the cooling time : t~(V/A)1
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Flexibility - Casting
Sand casting ‧High degree of shape complexity (limited by pattern)
Investment casting ‧Ceramic and wax cores allow complex internal configuration but costs increase significantly
Die casting ‧Low due to high die modification costs
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New Developments in Casting
Computer-aided design Rapid (free-form) pattern making