2.008 Metal CastingOutline Introduction Process Constraints Green Sand Casting Other ProcessesSome 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 unitExample – Sand CastingExample – Die Casting Example – Investment CastingCasting Process Physics and Constraints Phase Change•Density• Solubility• Diffusion rates High melting temperature• Chemical activity• High latent heat• HandlingAnalysis 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 relationshipsMold Filling.22constgvgPh =++ρ4105Re ×≈=µρvDm/s5.12 ≈≅ ghv Bernoulli’s equation Reynold’s number• Turbulence• Injection Molding : Re ~ 10-4hCooling for Sand Mold∆T∆TMETAL - MOLD INTERFACEMOLD - AIR INTERFACEDISTANCET0TwTEMPERATUREMOLDAIRSOLIDLIQUIDConductivity / Diffusivity Sand Castingαsand< αmetal Die Castingαtool metal> αpolymer Injection Moldingαtool metal~ αmetal Conductivity (W/mK)Cu ~ 400, Al ~ 200Sand ~ 0.5, PMMA ~ 0.2Solidification Time : Sand Casting Transient 1-D heat transfer22xTtTs∂∂=∂∂αtxerfTTTTsMoMα2−=−−Solution2⎟⎠⎞⎜⎝⎛=AVCts Solidification timeChvorinov’s ruleSolidification Time : Die Casting Transient 1-D heat transfer)(opTTAhtTmC −−=∂∂⎟⎟⎠⎞⎜⎜⎝⎛−−∆+=moldejectmoldspinjectpTTTTTAhmCt lnSolution1⎟⎠⎞⎜⎝⎛=AVCts Solidification timeComparison:Sand Mold vs Metal MoldSand MoldSand castingts~ (V/A)2Metal MoldDie castingts~ (V/A)1Microstructure FormationSchematic illustration of three basic types of cast structures(a) Columnar dendritic (b) equiaxed dendritic (c) equiaxed nondendritic Formation of DendritesTemperatureAlloying elementSolidLiquidTSTLSolidSolidLiquidLiquidL+SS+LMushy zoneDendritesMoldwallSolidusLiquidusConstitutional SupercoolingSOLIDLIQUIDCS*CL*T*(a)(b)SOLUTE ENRICHED LAYER IN FRONT OF LIQUID-SOLID INTERFACEC∞CL*LIQUID COMPOSITIONDISTANCE, x*T*TEMPERATUREDISTANCE, x*TACTUALTLIQUIDS(c)T*TEMPERATUREDISTANCE, x*TACTUALTLIQUIDS(d)CONSTITUTIONALLY SUPERCOOLED REGIONGreen Sand Casting Mechanical drawing of part Core boxes Core halves pasted together Cope pattern plate Drag pattern plate Cope ready for sand Cope after ramming with sand and removing pattern, sprue, and risers Drag ready for sand Drag after removing pattern 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 Dimensional, Thermal and Chemical stability at high T Size and shape Wettability by molten metal Compatibility with binder system Availability and consistencyPattern Design Considerations (DFM) Shrinkage allowance Machining allowance Distortion allowance Parting line Draft angleTypical Shrinkage AllowanceMetal or alloy Shrinkage allowancesmm / mAluminum alloy ………………………………...... 13Aluminum bronze ……………………………...… 21Yellow brass (thick sections) ………...…....…… 13Yellow brass (thin sections) …..……...….…...… 13Gray cast iron (a) …………………………….... 8 - 13White cast iron ………………………………..….. 21Tin bronze …………………………………..……. 16Gun metal …………………………………...… 11 - 16Lead …………………………………………..…... 26Magnesium …………………………………..…… 21Magnesium alloys (25%) ………………………... 16Manganese bronze …………………………….… 21Copper-nickel …………………………………….. 21Nickel …………………………………………….... 21Phosphor bronze ……………………………… 11 - 16Carbon steel …………………………………… 16 - 21Chromium steel ……………………………….….. 21Manganese steel ……………………………….… 26Tin …………………………………………….……. 21Zinc …………………………………………….…... 26Typical Pattern Machining AllowanceAllowances, mmPattern size, mm Bore Surface Cope side For cast ironsUp to 152.……………………………….. 3.2 2.4 4.8 152 - 305………………………………… 3.2 3.2 6.4305 - 510.………………………………... 4.8 4.0 6.4510 - 915………………………………… 6.4 4.8 6.4915 - 1524……………………………….. 7.9 4.8 7.9For cast steelsUp to 152.……………………………….. 3.2 3.2 6.4 152 - 305………………………………… 6.4 4.8 6.4305 - 510.………………………………... 6.4 6.4 7.9510 - 915………………………………… 7.1 6.4 9.6915 - 1524……………………………….. 7.9 6.4 12.7For nonferrous alloysUp to 76...……………………………….. 1.6 1.6 1.6 76 - 152..………………………………… 2.4 1.6 2.4152 - 305………………………………… 2.4 1.6 3.2305 - 510.………………………………... 3.2 2.4 3.2510 - 915………………………………… 3.2 3.2 4.0915 - 1524……………………………….. 4.0 3.2 4.8Gating System: Sprue, Runner, and Gate Rapid mold filling Minimizing turbulence Avoiding erosion Removing inclusions Controlled flow and thermal conditions Minimizing scrap and secondary operationsRiser: Location and Size Casting shrinkage Directional solidification Scrap and secondary operationProgressive Solidification in RiserTemperature gradient rising toward riserRiserSlow rateFast rateIntermediate rateProgressive solidification :Directional solidificationDraft in