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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 and
Constraints 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 TEMPERATUR METAL - MOLD INTERFACE METAL - AIR
DISTANCE
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Conductivity / Diffusivity
Conductivity (W/mK) Cu ~ 400, Al ~ 200 Sand ~ 0.5, PMMA Sand Casting αsand < αmetal Injection Molding αtool metal ~ αmetal Die Casting αtool 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 Mold Metal Mold Sand casting
Die 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 Liquidus Temperature Solid Solidus Solid Liquid Mushy zone Alloying element Solid Liquid Dendrites
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Constitutional Supercooling
SOLUTE ENRICHED LAYER IN FRONT OF LIQUID-SOLID INTERFACE Liquid LIQUID COMPOSITION Solid Temperature Temperature CONSTITUTIONALLY SUPERCOOLED REGION
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Green Sand Casting Mechanical drawing of part Core halves
pasted together Core boxes Cope pattern plate Drag pattern plate Cope after ramming with sand and removing pattern, sprue, and risers Drag ready for sand Cope ready for sand Drag after removing pattern Casting as removed from mold; heat treated Casting ready for shippement Drag with core set in place Cope and drag assembled ready for pouring
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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 Allowance
Metal or alloy Shrinkage allowances mm / m Aluminum alloy Aluminum bronze Yellow brass (thick sections) Yellow brass (thin sections) Gray cast iron (a) White cast iron Tin bronze Gun metal Lead Magnesium Magnesium alloys (25%) Manganese bronze Copper-nickel Nickel Phosphor bronze Carbon steel Chromium steel Manganese steel Tin Zinc
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Typical Pattern Machining
Allowance Allowances,mm Pattern size, 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 secondary operations
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Riser: Location and Size
Casting shrinkage Directional solidification Scrap and secondary operation
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Progressive Solidification in
Riser Progressive solidification Intermediate rate Slow rate Fast rate Riser Temperature gradient rising toward riser Directional solidification
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Draft in Pattern Patterns Mold
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Investment Casting Wax pattern Injection wax or plastic patterns
Ejecting pattern Pattern assembly (Tree) Autoclaved Heat Heat Heat Heat Slurry coating Stucco coating Pattern meltout Completed mold
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Investment Casting (cont.)
Casting Pattern Finished product Shakeout Pouring Pouring
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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
Platen Toggle clamp Gas/oil accumulator 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 Sand Casting Shakeout
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Lost Foam Casting Invest assembly in flask with backlip medium
Vibrate to compact medium Pour Shakeout castings Clean castings assembly Inspect castings Ship castings Receive raw polystyrene beads Expand beads Mold component pattern, including gating system Join patters (if multipiece) Coat pattern assembly Dry assembly
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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 Induction furnace
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Advantages of Semi-solid Casting
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Produciotn rate (Pc/hr)
Casting Process Comparison Cost * Produciotn rate (Pc/hr) Process Die Equipment Labor Sand Shell-mold Plaster Investment Permancnt mold Die Centrifugal
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Cost - Casting Sand casting Investment casting Die 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 ‧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 Investment casting Die 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 t~(V/A)1
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Flexibility - Casting Sand casting Investment casting Die 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
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