Outline Metal Casting. Some Facts. Example Sand Casting. Introduction Process Constraints Green Sand Casting Other Processes

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1 2.008 Metal Casting Outline Introduction Process Constraints Green Sand Casting Other Processes Some Facts Example Sand Casting First casting: 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

2 Example Die Casting Example Investment Casting Casting Process Physics and Constraints Phase Change Density Solubility Diffusion rates High melting temperature Chemical activity High latent heat Handling 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

3 Mold Filling Cooling for Sand Mold Bernoulli s equation 2 P v h + + = const. ρ g 2g v 2gh 1.5m/s h T w AIR MOLD SOLID LIQUID Reynold s number vdρ Re = 5 10 µ Turbulence Injection Molding : Re ~ TEMPERATUR E T 0 DISTANCE T T METAL - MOLD INTERFACE MOLD - AIR INTERFACE Conductivity / Diffusivity Conductivity (W/mK) Cu ~ 400, Al ~ 200 Sand ~ 0.5, PMMA ~ 0.2 Sand Casting α sand < α metal Die Casting α tool metal ~ α metal Injection Molding α tool metal > α polymer Solidification Time : Sand Casting Transient 1-D heat transfer 2 T T = α 2 t s x Solution T T T T o M M Solidification time t s x = erf 2 α t 2 V = C A Chvorinov s rule s

4 Solidification Time : Die Casting Transient 1-D heat transfer T mc ( ) p = Ah T To t Solution mc t = Ah p T ln V = C A 1 inject Solidification time t s T + T eject sp T T mold mold Comparison: Sand Mold vs Metal Mold Sand Mold Sand casting t s ~ (V/A) 2 Metal Mold Die casting t s ~ (V/A) 1 Microstructure Formation Formation of Dendrites Liquid Temperature Solid L+S Solidus Liquidus T L T S Solid S + L Liquid Alloying element Mushy zone Solid Liquid Schematic illustration of three basic types of cast structures (a) Columnar dendritic (b) equiaxed dendritic (c) equiaxed nondendritic Mold wall Dendrites

5 Constitutional Supercooling Green Sand Casting T* C S * SOLID C L * LIQUID LIQUID COMPOSITION C L * C SOLUTE ENRICHED LAYER IN FRONT OF LIQUID-SOLID INTERFACE Core boxes Mechanical drawing of part Core halves pasted together Cope pattern plate Drag pattern plate DISTANCE, x* TEMPERATURE T* T ACTUAL (a) T LIQUIDS TEMPERATURE T* T LIQUIDS (b) T ACTUAL CONSTITUTIONALLY SUPERCOOLED REGION Cope ready for sand Cope after ramming with sand and removing pattern, sprue, and risers Drag ready for sand Drag after removing pattern DISTANCE, x* (c) DISTANCE, x* (d) Drag with core set in place Cope and drag assembled ready for pouring Casting as removed from mold; heat treated Casting ready for shippement Green Sand Mold Pattern Design Considerations (DFM) Dimensional, Thermal and Chemical stability at high T Size and shape Wettability by molten metal Compatibility with binder system Availability and consistency Shrinkage allowance Machining allowance Distortion allowance Parting line Draft angle

6 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.. 21 Magnesium alloys (25%) Manganese bronze. 21 Copper-nickel.. 21 Nickel Phosphor bronze Carbon steel Chromium steel Manganese steel. 26 Tin.. 21 Zinc Typical Pattern Machining Allowance Allowances, mm Pattern size, mm Bore Surface Cope side For cast irons Up to For cast steels Up to For nonferrous alloys Up to Gating System: Sprue, Runner, and Gate Riser: Location and Size Rapid mold filling Minimizing turbulence Avoiding erosion Removing inclusions Controlled flow and thermal conditions Minimizing scrap and secondary operations Casting shrinkage Directional solidification Scrap and secondary operation

7 Progressive Solidification in Riser Draft in Pattern Progressive solidification : Intermediate rate Slow rate Fast rate Patterns Mold Riser Temperature gradient rising toward riser Directional solidification Investment Casting Investment Casting (cont.) Injection wax or plastic patterns Wax pattern Ejecting pattern Pattern assembly (Tree) Heat Heat Autoclaved Heat Heat Casting Pattern Finished product Pouring Shakeout Slurry coating Stucco coating Completed mold Pattern meltout

8 Advantages of Investment Casting Die Casting Intricate geometry Close dimensional tolerance Superior surface finish High-melting point alloys Gas/oil accumulator Piston Shot sleeve Platen Toggle clamp Die Advantages of Die Casting Lost Foam Casting High production rates Closer dimensional tolerances Superior surface finish Improved mechanical properties

9 Lost Foam Casting Advantages of 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 flask with backlip medium Vibrate to compact medium Pour Shakeout castings Clean castings assembly Inspect castings Ship castings No parting line No cores One-piece flask Freedom of design Minimum handling of sand Ease of cleaning and secondary operation Semi-solid Casting Advantages of Semi-solid Casting Punch Die Induction furnace

10 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 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 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

11 Flexibility - Casting New Developments in 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 Computer-aided design Rapid (free-form) pattern making Die casting Low due to high die modification costs