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19casting - 2.008 Metal Casting Outline Introduction ...

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Unformatted text preview: 2.008 Metal Casting Outline ¡ Introduction ¡ Process Constraints ¡ Green Sand Casting ¡ Other Processes 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 Example – Sand Casting 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 Mold Filling . 2 2 const g v g P h = + + ρ 4 10 5 Re × ≈ = µ ρ vD m/s 5 . 1 2 ≈ ≅ gh v ¡ Bernoulli’s equation ¡ Reynold’s number • Turbulence • Injection Molding : Re ~ 10-4 h Cooling for Sand Mold ∆ T ∆ T METAL - MOLD INTERFACE MOLD - AIR INTERFACE DISTANCE T T w TEMPERATUR E MOLD AIR SOLID LIQUID Conductivity / Diffusivity ¡ Sand Casting α sand < α metal ¡ Die Casting α tool metal > α polymer ¡ Injection Molding α tool metal ~ α metal ¡ Conductivity (W/mK) Cu ~ 400, Al ~ 200 Sand ~ 0.5, PMMA ~ 0.2 Solidification Time : Sand Casting ¡ Transient 1-D heat transfer 2 2 x T t T s ∂ ∂ = ∂ ∂ α t x erf T T T T s M o M α 2 − = − − Solution 2 ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ = A V C t s ¡ Solidification time Chvorinov’s rule Solidification Time : Die Casting ¡ Transient 1-D heat transfer ) ( o p T T Ah t T mC − − = ∂ ∂ ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ − − ∆ + = mold eject mold sp inject p T T T T T Ah mC t ln Solution 1 ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ = A V C t s ¡ Solidification time 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 Schematic illustration of three basic types of cast structures (a) Columnar dendritic (b) equiaxed dendritic (c) equiaxed nondendritic Formation of Dendrites Temperature Alloying element Solid Liquid T S T L Solid Solid Liquid Liquid L+S S + L Mushy zone Dendrites Mold wall S o l id u s L iq u id u s Constitutional Supercooling...
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19casting - 2.008 Metal Casting Outline Introduction ...

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