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2.008 Metal Throwing

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  1. 2.008 Metal Casting Reading: Kalpakjian pp. 239-316

  2. Outline • Introduction • Process Constraints • Green Sand Casting • Other Processes

  3. 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

  4. Example – Sand Casting

  5. Example – Die Casting

  6. Example – Investment Casting

  7. Casting Process Physics and Constraints • Phase Change • Density • Solubility • Diffusion rates • High melting temperature • Chemical activity • High latent heat • Handling

  8. 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

  9. h Mold Filling • Bernoulli’s equation • Reynold’s number • Turbulence • Injection Molding : Re ~ 10-4

  10. MOLD LIQUID SOLID AIR Tw TEMPERATURE METAL - MOLD INTERFACE T MOLD - AIR INTERFACE T T0 DISTANCE Cooling for Sand Mold

  11. Conductivity / Diffusivity • Conductivity (W/mK) Cu ~ 400, Al ~ 200 Sand ~ 0.5, PMMA ~ 0.2 • Sand Casting asand < ametal • Die Casting atool metal ~ ametal • Injection Molding atool metal > apolymer

  12. Solidification Time : Sand Casting • Transient 1-D heat transfer Solution • Solidification time Chvorinov’s rule

  13. Solidification Time : Die Casting • Transient 1-D heat transfer Solution • Solidification time

  14. Comparison:Sand Mold vs Metal Mold Sand Mold Sand casting ts ~ (V/A)2 Metal Mold Die casting ts ~ (V/A)1

  15. Microstructure Formation • Schematic illustration of three basic types of cast structures • Columnar dendritic (b) equiaxed dendritic (c) equiaxed nondendritic

  16. Liquid TL L+S TS Liquidus Solid Temperature S + L Solidus Liquid Solid Mushy zone Alloying element Liquid Solid Mold wall Dendrites Formation of Dendrites

  17. SOLUTE ENRICHED LAYER IN FRONT OF LIQUID-SOLID INTERFACE CL* CL* T* CS* LIQUID LIQUID COMPOSITION C SOLID DISTANCE, x* (b) (a) TACTUAL TLIQUIDS TACTUAL TLIQUIDS TEMPERATURE TEMPERATURE CONSTITUTIONALLY SUPERCOOLED REGION T* T* DISTANCE, x* DISTANCE, x* (c) (d) Constitutional Supercooling

  18. Mechanical drawing of part Core halves pasted together Drag pattern plate Cope pattern plate Core boxes Cope after ramming with sand and removing pattern, sprue, and risers Drag ready for sand Drag after removing pattern Cope ready for sand Casting ready for shippement Casting as removed from mold; heat treated Drag with core set in place Cope and drag assembled ready for pouring Green Sand Casting

  19. 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

  20. Pattern Design Considerations (DFM) • Shrinkage allowance • Machining allowance • Distortion allowance • Parting line • Draft angle

  21. Metal or alloy Shrinkage allowances mm / m Aluminum alloy ………………………………...... 13 Aluminum bronze ……………………………...… 21 Yellow brass (thick sections) ………...…....…… 13 Yellow brass (thin sections) …..……...….…...… 13 Gray cast iron (a) …………………………….... 8 - 13 White cast iron ………………………………..….. 21 Tin bronze …………………………………..……. 16 Gun metal …………………………………...… 11 - 16 Lead …………………………………………..…... 26 Magnesium …………………………………..…… 21 Magnesium alloys (25%) ………………………... 16 Manganese bronze …………………………….… 21 Copper-nickel …………………………………….. 21 Nickel …………………………………………….... 21 Phosphor bronze ……………………………… 11 - 16 Carbon steel …………………………………… 16 - 21 Chromium steel ……………………………….….. 21 Manganese steel ……………………………….… 26 Tin …………………………………………….……. 21 Zinc …………………………………………….…... 26 Typical Shrinkage Allowance

  22. Allowances, mm Pattern size, mm Bore Surface Cope side For cast irons Up to 152.……………………………….. 3.2 2.4 4.8 152 - 305………………………………… 3.2 3.2 6.4 305 - 510.………………………………... 4.8 4.0 6.4 510 - 915………………………………… 6.4 4.8 6.4 915 - 1524……………………………….. 7.9 4.8 7.9 For cast steels Up to 152.……………………………….. 3.2 3.2 6.4 152 - 305………………………………… 6.4 4.8 6.4 305 - 510.………………………………... 6.4 6.4 7.9 510 - 915………………………………… 7.1 6.4 9.6 915 - 1524……………………………….. 7.9 6.4 12.7 For nonferrous alloys Up to 76...……………………………….. 1.6 1.6 1.6 76 - 152..………………………………… 2.4 1.6 2.4 152 - 305………………………………… 2.4 1.6 3.2 305 - 510.………………………………... 3.2 2.4 3.2 510 - 915………………………………… 3.2 3.2 4.0 915 - 1524……………………………….. 4.0 3.2 4.8 Typical Pattern Machining Allowance

  23. 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

  24. Riser: Location and Size • Casting shrinkage • Directional solidification • Scrap and secondary operation

  25. Progressive solidification : Intermediate rate Fast rate Slow rate Riser Temperature gradient rising toward riser Directional solidification Progressive Solidification in Riser

  26. Patterns Mold Draft in Pattern

  27. Wax pattern Injection wax or plastic patterns Ejecting pattern Pattern assembly (Tree) Autoclaved Heat Heat Heat Heat Stucco coating Completed mold Pattern meltout Slurry coating Investment Casting

  28. Casting Pattern Finished product Shakeout Pouring Investment Casting (cont.)

  29. Advantages of Investment Casting • Intricate geometry • Close dimensional tolerance • Superior surface finish • High-melting point alloys

  30. Toggle clamp Platen Gas/oil accumulator Piston Shot sleeve Die Die Casting

  31. Advantages of Die Casting • High production rates • Closer dimensional tolerances • Superior surface finish • Improved mechanical properties

  32. Lost Foam Casting

  33. Invest assembly in flask with backlip medium Expand beads Vibrate to compact medium Pour Mold component pattern, including gating system Shakeout castings Join patters (if multipiece) Clean castings assembly Coat pattern assembly Inspect castings Ship castings Dry assembly Lost Foam Casting Receive raw polystyrene beads

  34. 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

  35. Punch Die Induction furnace Semi-solid Casting

  36. Advantages of Semi-solid Casting

  37. Casting Process Comparison

  38. 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

  39. 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

  40. 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

  41. 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

  42. New Developments in Casting • Computer-aided design • Rapid (free-form) pattern making