Reading: Kalpakjian pp. 239-316 Outline
2.008 Introduction Process Constraints Metal Casting Green Sand Casting Other Processes
Some Facts Example – Sand Casting
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
1 Example – Die Casting Example – Investment Casting
Casting Process Physics and Analysis of Casting Processes Constraints
Phase Change •Density Fluid mechanics for mold filling • Solubility Heat transfer for solidification • Diffusion rates Thermodynamics, mass transfer and heat transfer for nucleation and growth High melting temperature Materials behavior for structure-property • Chemical activity relationships • High latent heat • Handling
2 Mold Filling Cooling for Sand Mold
Bernoulli’s equation
2 P v AIR MOLD SOLID LIQUID h + + = const. ρg 2g h v ≅ 2gh ≈1.5m/s Tw Reynold’s number METAL - MOLD vDρ ∆T Re = ≈ 5×104 INTERFACE µ TEMPERATUR E MOLD - AIR ∆T INTERFACE T0 • Turbulence DISTANCE • Injection Molding : Re ~ 10-4
Conductivity / Diffusivity Solidification Time : Sand Casting
Conductivity (W/mK) Transient 1-D heat transfer Cu ~ 400, Al ~ 200 ∂T ∂2T Sand ~ 0.5, PMMA ~ 0.2 = α ∂t s ∂x2 Sand Casting Solution T −T − x αsand < αmetal M = erf To −TM 2 α t Die Casting s αtool metal ~ αmetal Solidification time
2 Injection Molding ⎛ V ⎞ t s = C ⎜ ⎟ αtool metal > αpolymer ⎝ A ⎠ Chvorinov’s rule
3 Comparison: Solidification Time : Die Casting Sand Mold vs Metal Mold
Transient 1-D heat transfer ∂T mC = −Ah(T −T ) p ∂t o Solution mC ⎛ T + ∆T −T ⎞ t = p ln⎜ inject sp mold ⎟ ⎜ ⎟ Ah ⎝ Teject −Tmold ⎠ Solidification time Sand Mold Metal Mold 1 ⎛ V ⎞ Sand casting Die casting t s = C ⎜ ⎟ ⎝ A ⎠ 2 1 ts ~ (V/A) ts ~ (V/A)
Microstructure Formation Formation of Dendrites
Liquid TL Liq L+S uid T us Solid S S o l S id u + s Temperature L Solid Liquid
Alloying element Mushy zone
Solid Liquid
Schematic illustration of three basic types of cast structures Mold (a) Columnar dendritic (b) equiaxed dendritic (c) equiaxed nondendritic wall Dendrites
4 Constitutional Supercooling Green Sand Casting
SOLUTE ENRICHED LAYER IN FRONT OF CL* Mechanical LIQUID-SOLID drawing of part C * T* L INTERFACE CS* LIQUID
C∞ SOLID Core halves Core boxes pasted together Cope pattern plate Drag pattern plate LIQUID COMPOSITION DISTANCE, x* (b) (a) AL TU S C T IQUID T A L L A U T S ID Cope after ramming with Drag ready C U Drag after T A IQ sand and removing pattern, T L Cope ready for sand removing for sand sprue, and risers pattern CONSTITUTIONALLY T* T* SUPERCOOLED REGION TEMPERATURE TEMPERATURE
Casting as Casting ready removed from for shippement DISTANCE, x* DISTANCE, x* Drag with core Cope and drag assembled mold; heat treated set in place (c) (d) ready for pouring
Pattern Design Considerations Green Sand Mold (DFM)
Shrinkage allowance Machining allowance Distortion allowance Parting line
Dimensional, Thermal and Chemical stability at high T Draft angle Size and shape Wettability by molten metal Compatibility with binder system Availability and consistency
5 Typical Pattern Machining Typical Shrinkage Allowance Allowance Allowances, mm Metal or alloy Shrinkage allowances Pattern size, mm Bore Surface Cope side mm / m For cast irons Aluminum alloy ………………………………...... 13 Up to 152.……………………………….. 3.2 2.4 4.8 Aluminum bronze ……………………………...… 21 152 - 305………………………………… 3.2 3.2 6.4 Yellow brass (thick sections) ………...…....…… 13 305 - 510.………………………………... 4.8 4.0 6.4 Yellow brass (thin sections) …..……...….…...… 13 510 - 915………………………………… 6.4 4.8 6.4 Gray cast iron (a) …………………………….... 8 - 13 915 - 1524……………………………….. 7.9 4.8 7.9 White cast iron ………………………………..….. 21 Tin bronze …………………………………..……. 16 For cast steels Gun metal …………………………………...… 11 - 16 Up to 152.……………………………….. 3.2 3.2 6.4 Lead …………………………………………..…... 26 152 - 305………………………………… 6.4 4.8 6.4 Magnesium …………………………………..…… 21 305 - 510.………………………………... 6.4 6.4 7.9 Magnesium alloys (25%) ………………………... 16 510 - 915………………………………… 7.1 6.4 9.6 Manganese bronze …………………………….… 21 915 - 1524……………………………….. 7.9 6.4 12.7 Copper-nickel …………………………………….. 21 Nickel …………………………………………….... 21 For nonferrous alloys Phosphor bronze ……………………………… 11 - 16 Up to 76...……………………………….. 1.6 1.6 1.6 Carbon steel …………………………………… 16 - 21 76 - 152..………………………………… 2.4 1.6 2.4 Chromium steel ……………………………….….. 21 152 - 305………………………………… 2.4 1.6 3.2 Manganese steel ……………………………….… 26 305 - 510.………………………………... 3.2 2.4 3.2 Tin …………………………………………….……. 21 510 - 915………………………………… 3.2 3.2 4.0 Zinc …………………………………………….…... 26 915 - 1524……………………………….. 4.0 3.2 4.8
Gating System: Riser: Location and Size Sprue, Runner, and Gate
Rapid mold filling Minimizing turbulence Casting shrinkage Avoiding erosion Directional solidification Removing inclusions Scrap and secondary operation Controlled flow and thermal conditions Minimizing scrap and secondary operations
6 Progressive Solidification in Riser Draft in Pattern
Progressive solidification : Patterns Intermediate rate Slow Fast rate rate Mold
Riser
Temperature gradient rising toward riser Directional solidification
Investment Casting Investment Casting (cont.)
Wax pattern Injection wax or plastic patterns Ejecting pattern Pattern assembly (Tree)
Autoclaved Heat Heat Casting Pattern Heat Heat Finished product
Shakeout Pouring
Slurry coating Stucco coating Completed Pattern meltout mold
7 Advantages of Investment Casting Die Casting
Platen Toggle clamp Intricate geometry Gas/oil accumulator
Close dimensional tolerance Piston Superior surface finish High-melting point alloys Shot sleeve
Die
Advantages of Die Casting Lost Foam Casting
High production rates Closer dimensional tolerances Superior surface finish Improved mechanical properties
8 Lost Foam Casting Advantages of Lost Foam Casting
Receive raw polystyrene beads Invest assembly in flask with backlip medium No parting line Expand beads Vibrate to compact medium No cores
Mold component pattern, Pour One-piece flask including gating system Shakeout castings Freedom of design Join patters (if multipiece) Minimum handling of sand Clean castings assembly Ease of cleaning and secondary Coat pattern assembly Inspect castings operation
Dry assembly Ship castings
Semi-solid Casting Advantages of Semi-solid Casting
Punch
Die Induction furnace
9 Casting Process Comparison 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
Quality - Casting Rate - Casting
Sand casting Sand casting Tolerance (0.7~2 mm) and defects are affected by shrinkage Development time is 2~10 weeks Material property is inherently poor Production rate is depending on the cooling time : t~(V/A)2 Generally have a rough grainy surface Investment casting Investment casting Tolerance (0.08~0.2 mm) Development time is 5~16 weeks depending on the complexity Mechanical property and microstructure depends on the method Production rate is depending on the cooling time : t~(V/A)2 Good to excellent surface detail possible due to fine slurry Die casting Die casting Tolerance (0.02~0.6 mm) Development time is 12~20 weeks Good mechanical property and microstructure due to high Production rate is depending on the cooling time : t~(V/A)1 pressure Excellent surface detail
10 Flexibility - Casting New Developments in Casting
Sand casting High degree of shape complexity (limited by pattern) Computer-aided design Investment casting Ceramic and wax cores allow complex internal Rapid (free-form) pattern making configuration but costs increase significantly Die casting Low due to high die modification costs
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