铸造缺陷及设计

ME 6222: Manufacturing Processes and Systems Prof. J.S. Colton 1 Casting Defects and Design Issues ver. 1 ME 6222: Manufacturing Processes and Systems Prof. J.S. Colton 2 Overview • Processes • Analysis • Defects • Design rules • Economics ME 6222: Manufacturing Processes and Systems Prof. J.S. Colton 3 Issues in Casting • Shrinkage • Porosity • Piping • Microstructure ME 6222: Manufacturing Processes and Systems Prof. J.S. Colton 4 Shrinkage • Can amount to 5-10% by volume • Gray cast iron expands upon solidification due to phase changes • Need to design part and mold to take this amount into consideration ME 6222: Manufacturing Processes and Systems Prof. J.S. Colton 5 Shrinkage TABLE 5.1 Metal or alloy Volumetric solidification contraction (%) Metal or alloy Volumetric solidification contraction (%) Aluminum 6.6 70%Cu–30%Zn 4.5 Al–4.5%Cu 6.3 90%Cu–10%Al 4 Al–12%Si 3.8 Gray iron Expansion to 2.5 Carbon steel 2.5–3 Magnesium 4.2 1% carbon steel 4 White iron 4–5.5 Copper 4.9 Zinc 6.5 Source: After R. A. Flinn. ME 6222: Manufacturing Processes and Systems Prof. J.S. Colton 6 Casting Defects ME 6222: Manufacturing Processes and Systems Prof. J.S. Colton 7 Defects - Hot Tears ME 6222: Manufacturing Processes and Systems Prof. J.S. Colton 8 Casting Defects - Porosity ME 6222: Manufacturing Processes and Systems Prof. J.S. Colton 9 Porosity • Types – due to gases – smooth bubbles – due to shrinkage – rough voids • Not a problem for ingots – parts that will be deformation processed – as long as it is not exposed to air (corrosion) – can be healed ME 6222: Manufacturing Processes and Systems Prof. J.S. Colton 10 Porosity due to Gases • Smooth bubbles – result from entrapped gases – solubility in liquid is high, in solid is low, so gas is rejected during cooling • Sievert’s law S = kpg0.5 – S = solubility – k = constant – pg = partial pressure of gas over melt ME 6222: Manufacturing Processes and Systems Prof. J.S. Colton 11 Remedies for Gas Bubbles • Control atmosphere – vacuum – gases with less solubility • Proper venting to let gases out • Proper design of runners and gates to avoid turbulence ME 6222: Manufacturing Processes and Systems Prof. J.S. Colton 12 Remedies for Gas Bubbles • Add metallic elements to react with gases – killed steels - highly deoxidized (Al, Si) • high shrinkage due to gas removal - piping – semi-killed steels - less deoxidized • less piping, porosity – rimmed steels - little deoxidization • blow holes in ring at rim (sometimes break through) • little piping because gas doesn’t escape ME 6222: Manufacturing Processes and Systems Prof. J.S. Colton 13 Porosity due to Shrinkage • Rough bubbles - voids • Stages – cooling liquid – rejects latent heat at melting point • alloys become slushy - liquid and solid co-exist – cooling solid ME 6222: Manufacturing Processes and Systems Prof. J.S. Colton 14 Differential Cooling • Transition between thicker and thinner sections can lead to porosity ME 6222: Manufacturing Processes and Systems Prof. J.S. Colton 15 Porosity / Shrinkage Solutions • Risers allow molten metal to flow into mold to make up for shrinkage • Design flow so no part freezes early – large channels • “Flexible” molds – allow metal to shrink, not hold metal ME 6222: Manufacturing Processes and Systems Prof. J.S. Colton 16 Porosity / Shrinkage Solutions • Heating or cooling certain areas to maintain uniform cooling (thermit or chills) • Uniform part thickness – leads to uniform cooling, less residual stress ME 6222: Manufacturing Processes and Systems Prof. J.S. Colton 17 Chills ME 6222: Manufacturing Processes and Systems Prof. J.S. Colton 18 Pipe Defect • Due to shrinkage giving rise to a funnel-like cavity • Solutions – insulate top (glass wool) – heat top (exothermic mixture - thermit) ME 6222: Manufacturing Processes and Systems Prof. J.S. Colton 19 Microstructure ME 6222: Manufacturing Processes and Systems Prof. J.S. Colton 20 Grains on Willie B’s head ME 6222: Manufacturing Processes and Systems Prof. J.S. Colton 21 Microstructure - Dendrites • Finer structure at walls • Grains / dendrites grow to center ME 6222: Manufacturing Processes and Systems Prof. J.S. Colton 22 Microstructure - Dendrites ME 6222: Manufacturing Processes and Systems Prof. J.S. Colton 23 Microstructure • Post-treatment may be necessary to get desired properties - grain structure – annealing – tempering – cold working ME 6222: Manufacturing Processes and Systems Prof. J.S. Colton 24 Design Rules Summary • Uniform wall thickness • Flat parting lines • Gradual thickness transitions • Draft for removal – tapers: 0.5 to 2 degrees • Surface of mold gives surface of part ME 6222: Manufacturing Processes and Systems Prof. J.S. Colton 25 Sand Casting Rules ME 6222: Manufacturing Processes and Systems Prof. J.S. Colton 26 Economics Example - Optical Bench ME 6222: Manufacturing Processes and Systems Prof. J.S. Colton 27 Requirements • Casting of Al-Si alloy • Number – one-off – preliminary run (100) – production run (10,000) • High precision required – machining required – pick cheapest casting method ME 6222: Manufacturing Processes and Systems Prof. J.S. Colton 28 Alternative Processes • Sand casting • Low pressure casting • Permanent mold casting • Die casting ME 6222: Manufacturing Processes and Systems Prof. J.S. Colton 29 Cost Equation n C n CCC Lcm &++= • C = cost/part • Cm = material cost • Cc = capital cost • CL = labor cost • n = number produced • = production raten& ME 6222: Manufacturing Processes and Systems Prof. J.S. Colton 30 Process Costs Process Sand Casting Low Pressure Permanent Mold Die Casting Material, Cm ($) 1 1 1 1 Labor, CL ($/hr) 20 20 20 20 Capital, CC ($) 0.9 4.4 700 3000 Rate, (#/hr) 6.25 22 10 50n& ME 6222: Manufacturing Processes and Systems Prof. J.S. Colton 31 Process Economics ME 6222: Manufacturing Processes and Systems Prof. J.S. Colton 32 Process Selection • Probably pick low pressure casting, as a preliminary run of 100 is assured. • If production run is needed, die casting will probably be used. • The tough part is getting the process cost data. ME 6222: Manufacturing Processes and Systems Prof. J.S. Colton 33 Production of Aluminum Auto Parts M ain Characteristic Casting GravityA Low-Pressure Die CastingB High-Pressure Die Casting (Pores Free)C Squeeze CastingD Pouring/Filling M ethod Ladle Air pressure through sta lk H igh-speed and high-pressure in jection by hydraulic piston Relatively low- speed and high-pressure in jection Filling Tim e (s) 10-30 10-30 1 10 Operating Pressure (atm .) 1 1+(0.2-0.5) 100-500 500-1,000 Cycle Tim e (m in.) 5-10 5-10 1-2 2 Die/M old Tem perature High High Low Low-medium Dim ensional Accuracy + ++ +++ +++ Design Availability +++ ++ + + Productivity + ++ +++ +++ Quality + ++ +-+++ +++ Cost + + +++ +++ M achining Required M any M any Few Few M ain Parts (other than wheels) Intake m anifo ld, cylinder b lock and head, piston Cylinder block, cylinder head, suspension, m em ber Cylinder block, oil pan, cylinder head cover, transaxle case Piston, d isk- brake caliper, power steering toe contro l hub, knuckle ME 6222: Manufacturing Processes and Systems Prof. J.S. Colton 34 Advantages of Casting • Near- or net- shape • Less scrap • Intricate shapes • Large hollow shapes • No limit to size ME 6222: Manufacturing Processes and Systems Prof. J.S. Colton 35 Disadvantages of Casting • Shrinkage, porosity, cracks • No strain hardening – can be brittle • Tooling can be expensive – part shape depends on tool (mold) • Microstructure can be difficult to control – non-uniform cooling – faster on outside produces finer grain structure there ME 6222: Manufacturing Processes and Systems Prof. J.S. Colton 36 Summary • Defects • Design rules • Economics ME 6222: Manufacturing Processes and Systems Prof. J.S. Colton 37