FUNDAMENTALS OF METAL CASTING

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FUNDAMENTALS OF METAL CASTING

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Casting (process) – melt the metal, pour into a mold by gravity or other force and solidify.

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  1. 0. Alloys and Phase Diagram • Pure Metals FUNDAMENTALS OF • Alloys Substitutional METAL CASTING – Solid solutions • Substitutional Solid Solution (Zn/Cn and Cu/Ni) Solid Solution – Atomic radii is similar – Lattice type is the same 0. Phase Diagram • Interstitial Solid Solution 1. Overview – Smaller atoms are interstitially located among bigger atoms 2. Heating & Pouring – Lattice type usually does not change 3. Solidification and Cooling – Intermediate Phases – The solubility of one Interstitial Solid Solution element in another element is limited. • Metallic compounds (Fe3C) • Intermetallic compounds(Mg2Pb) 1 2 Cu-Ni Phase Diagram Temperature, °F Pb-Sn Phase Diagram (°C) T T 1600 T Liquid 600 Eutectic Composition and Temperature 1400 500 L L+S α+L 400 α 362 °F β+L 1260 β 300 61.9% Sn 1200 Solid 200 α+β Substitutional Solid Alloys α 1000 100 Cu Ni Time 26% 36% 50% 62% Time Time Pure Metals Pure Metals 0 L S Time Pb (lead) Sn (Tin) 3 4 Fe-C Phase Diagram Introduction • Dated back 6000 years 1800 δ • Ingot vs. Shape casting L • Polymers and ceramics are cast as well. 1400 L+θ • Issues in casting γ+L γ 1130 – Flow 1000 γ+θ – Heat Transfer α – Selection of Mold Materials 723 – Solidification- Nucleation and Growth 600 • Depending on how we control solidification, these α+θ events influence the size, shape, uniformity and 200 chemical composition of the grains. Fe 1 2 3 4 5 6 C 5 6 1
  2. Introduction Classification • Casting (process) – melt the metal, pour into a Sand Casting mold by gravity or other force and solidify. Shell Molding Expandable-mold Vacuum Molding • Casting (Part) Metal casting Casting Expanded Polystyrene • Advantages Investment Casting – Complex geometries – external and internal Plaster-Mold Casting – Can be net-shaped or near net-shaped Solidification – Can produce very large parts Process Ceramic-Mold Casting permanent-mold Permanent-Mold Casting – Any metals Glassworking Casting – Can be mass-produced Variations of Permanent-Mold Casting – Size variety – big and small Die Casting • Disadvantages Extrusion Centrifugal Casting – Limitation in mechanical properties, porosity, Polymers & PMC Injection Molding – Dimensional accuracy, surface finish Processing Other Molding – Safety Hazard – Environmental problems Special Molding for PMC 7 8 1. Overview Two Main Categories • A Foundry is a casting factory which equipped for making molds, melting and handling molten metal, 1. Expendable mold processes –A mold after process performing the casting process, and cleaning the must be destroyed in order to remove casting finished casting – Mold materials: sand, plaster and similar materials + binders – Foundrymen are workers. – More intricate geometries • Open Molds – Simple parts 2. Permanent mold processes – A mold can be used • Closed Molds – Complex parts. many times to produce many castings – A passageway - the gating system leading into the cavity – Mold: made of metal and, less commonly, a ceramic refractory material • Two categories -Expandable or permanent molds. – Part shapes are limited – Permanent mold processes are more economic in high production operations (a) open mold 9 10 (b) closed mold Basic features of Molds Casting Processes • Forming the Mold Cavity Downsprue • Sand Casting Molds Parting line – Mold cavity is formed by packing sand around a pattern. Riser – The pattern usually oversized for shrinkage is removed. – Mold: cope (upper half) & Pouring cup – Sand for the mold is moist and contains a binder to maintain shape drag (bottom half) • Cores in the Mold Cavity – Flask - containment – The mold cavity - the external surfaces of the cast part – Parting line Cope Mold – A core, placed inside the mold cavity to define the interior geometry of part. In sand casting, cores are made of sand. – Pattern – the mold cavity • Gating System - Channel through which molten metal – The gating system – pouring Drag flows into cavity Flask cup, (down)sprue, runner – A downsprue, through which metal enters a runner – Riser – a source of liquid – At top of downsprue, a pouring cup to minimize splash and Core turbulence metal to compensate for Runner shrinkage during solidification • Riser - Liquid metal reservoir to compensate for shrinkage during solidification – The riser must be designed to freeze after the main casting 11 solidify. 12 2
  3. Pouring Analysis 2. Heating & Pouring • Bernoulli’s theorem at any two points in a flowing • Sufficient to melt and raise the molten metal to a right liquid P v2 P v2 state h1 + 1 + 1 = h2 + 2 + 2 ρ 2g ρ 2g • Total Heat Energy required: • h=head, P=pressure, ρ=density, v=flow velocity, g=gravity, F=friction loss H=ρV[Cs(Tm-To)+Hf+Cl(Tp-Tm)] where ρ=density, V=volume, Cs=specific heat for solid – Assuming no frictional loss and same pressure Cl=specific heat for liquid, Tm=melting temperature v12 v2 To=starting temperature, Tp=pouring temperature h1 + = h2 + 2 2g 2g • Factors affecting ‘pouring’ – Pouring temperature (vs. melting temp.) – Assuming point 2 is reference (h2=0) and v1=0, 1 – Pouring rate v2 • Too slow, metal freezes h1 = 2 ; v2 = 2 gh1 • Too high, turbulence 2g – Turbulence • Accelerate the formation of oxides • Continuity law Q = v1 A1 = v2 A2 2 V • Mold erosion • Mold fill time (MFT) MFT = • Voids? 13 Q 14 Fluidity 3. Solidification(Pure Metals) • Fluidity: A measure of the capability of a metal to flow into and fill the mold before freezing. (Inverse of viscosity) Transformation of molten metal Pure Metals • Factors affecting fluidity - Pouring temperature, Metal composition, Viscosity, Heat transfer to the surroundings, Heat of fusion and Solidification into solid state Temperature • Higher Re, greater tendency for turbulence flow • Solidification differs Pouring – Turbulence and laminar flow depending on a pure temperature Reynold’s number: Re=vDr/h element or an alloy Liquid cooling Re ranges 2,000(laminar) to 20,000 (mixture of laminar-turbulence) • For Pure Metal Freezing greater than 20,000 turbulence resulting in air entrainment and dross Tm (scum) formation – Super(Under)cooling temperature Local • Minimize turbulence by avoiding a certain range in flow direction – Solidification occurs at a solidification constant temperature and time Solid cooling Pure metals: good fluidity supercooled Temperature Total solidification time Alloys: not as good – Actual freezing during the local solidification time Time Tests for fluidity [Schey, 2000] 15 16 Solidification of Pure Metals Dendrite Growth • A thin skin of solid metal is formed at the cold mold wall immediately after pouring • Skin thickness increases to form a shell around the molten metal as solidification progresses • Rate of freezing depends on S L heat transfer into mold, as well as thermal properties of the metal Randomly oriented grains of small size Temp. Temp. Temp. near the mold wall, and large columnar grains oriented toward the center of the Slower casting (Dendritic growth) Heat Transfer 17 18 3
  4. Solidification of Alloy Solidification of Alloys • Most Alloys freeze over a temperature range, not at a Temperature single temperature. 4 ∆G = πr 3∆G + 4πr 2γ t v • Nucleation 3 ∆Gt – Energy involved in homogeneous nucleation L Pouring temperature – Total free energy change: Liquid cooling where ∆Gv =volume free energy r = radius of embryo L+S Freezing γ temperature = specific surface free energy r r* Solid cooling • Chemical compositional gradiency within a single grain S Total solidification time • Chemical compositional gradiency throughout the casting – ingot segregation Time Ni Cu • Eutectic Alloys – Solidification occurs at a single temperature Dendritic Growth cooling curve for a 50%Ni-50%Cu composition during casting 19 20 pattern shrinkage allowance Solidification Time Shrinkage Simplification • Chvorinov’s Empirical relationship: Solidification time Metal Volume Contraction as a function of the size and shape Solidification Thermal n ⎛V ⎞ Contraction TST = Cm ⎜ ⎟ Aluminum 7% 5.6% ⎝ A⎠ Al alloys 7 5 V=volume A=surface area and n=2 Gray cast iron 1.8 3 Cm=experimentally determined value that depends on Gray Cast Iron with High C 0 3 mold material, thermal properties of casting metal, Low C Cast Steel 3 7.2 and pouring temperature relative to melting point Copper 4.5 7.5 • A casting with a higher volume-to-surface area ratio Bronze 5.5 6 solidifies more slowly than one with a lower ratio • Used in riser design: the solidification time of the riser • Exception: cast iron with high C content because of graphitization must be equal to the solidification time of the cast during final stages causes expansion that counteracts volumetric part. 21 decrease associated with phase change 22 Directional Solidification • To minimize the damage during casting, the region most distant from the liquid metal supply needs to METAL CASTING freeze first and the solidification needs to procede toward the riser. PROCESSES • Based on Chvorinov’s rule, the section with lower V/A ratio should freeze first. 1. Sand Casting 2. Other Expandable Mold Casting Processes • Use ‘Chills’: Internal and External chills which 3. Permanent Mold Casting Processes encourage rapid cooling. 4. Foundry practice 5. Casting Quality External Chills 6. Metals for Casting Internal Chills Made of metal 7. Product Design Consideration 23 24 4
  5. Introduction 1. Sand Casting • Casting of Ingot and Shape casting • Most widely used casting process. • Major Classification • Parts ranging in size from small to very large • Production quantities from one to millions – Expandable Mold • A new mold is required for each new casting • Sand mold is used. • Production rate is limited except Sand casting • Patterns and Cores • Sand Casting, Shell Molding, Vacuum Molding, Expandable – Solid, Split, Match-plate and Cope-and-drag Patterns Polystyrene, Investment Casting, Plaster Molding, Ceramic – Cores – achieve the internal surface of the part Mold Casting • Molds – Permanent Mold – Sand with a mixture of water and bonding clay – Typical mix: 90% sand, 3% water, and 7% clay • Mold is made of durable materials – to enhance strength and/or permeability • Ideal for a product with a high production rate 25 26 Molds Steps in Sand Casting The cavity in the sand mold is formed by packing sand around a pattern, • Sand – Refractory for high temperature separating the mold into two halves – The mold must also contain gating and riser system • Size and shape of sand – For internal cavity, a core must be included in mold – Small grain size -> better surface finish – A new sand mold must be made for each part – Large grain size -> to allow escape of gases during pouring 1. Pour molten metal into sand mold 2. Allow metal to solidify – Irregular grain shapes -> strengthen molds due to 3. Break up the mold to remove casting interlocking but to reduce permeability 4. Clean and inspect casting • Types 5. Heat treatment of casting is sometimes required to improve metallurgical properties – Green-sand molds - mixture of sand, clay, and water; “Green" means mold contains moisture at time of pouring – Dry-sand mold - organic binders rather than clay and mold is baked to improve strength – Skin-dried mold - drying mold cavity surface of a green-sand mold to a depth of 10 to 25 mm, using torches or heating lamps 27 28 Types of patterns used in sand Internal Cavity with Core casting: (a) solid pattern (b) split pattern (a) Core held in place in the mold cavity by chaplets (c) match-plate pattern (b) possible chaplet design (d) cope and drag pattern (c) casting with internal cavity 29 30 5
  6. Desirable Mold Properties and 2. Other Expendable Mold Casting Characteristics • Strength - to maintain shape and resist erosion • Shell Molding • Permeability - to allow hot air and gases to pass • Vacuum Molding through voids in sand • Expanded Polystyrene Process • Thermal stability - to resist cracking on contact with molten metal • Investment casting • Collapsibility - ability to give way and allow • Plaster and Ceramic Mold casting casting to shrink without cracking the casting • Reusability - can sand from broken mold be reused to make other molds? 31 32 Steps in shell-molding Shell Molding • Advantages: – Smoother cavity surface permits easier flow of molten metal and better surface finish on casting – Good dimensional accuracy – Machining often not required – Mold collapsibility usually avoids cracks in casting – Can be mechanized for mass production • Disadvantages: – More expensive metal pattern – Difficult to justify for small quantities 33 34 Expanded Polystyrene Casting Expanded Polystyrene Casting • Advantages: – Pattern need not be removed from the mold – Simplifies and expedites mold-making, since two mold halves (cope and drag) are not required as in a conventional green-sand mold – Automated Mass production of castings for automobile engines • Disadvantages: – A new pattern is needed for every casting – Economic justification of the process is highly dependent on cost of producing patterns 35 36 6
  7. Investment Casting Investment Casting • Advantages: – Parts of great complexity and intricacy can be cast – Close dimensional control and good surface finish – Wax can usually be recovered for reuse – Additional machining is not normally required - this is a net shape process • Disadvantages – Many processing steps are required – Relatively expensive process 37 38 Plaster Molding 3. Permanent Mold Casting • Basic Permanent Mold Process • Similar to sand casting except mold is made of plaster of Paris (gypsum - CaSO4-2H2O) – Uses a metal mold constructed of two sections designed for easy, precise opening and closing • Plaster and water mixture is poured over plastic or metal pattern to make a mold – Molds for lower melting point alloys: steel or cast iron and Molds for steel: refractory material, due to • Advantages: the very high pouring temperatures – Good dimensional accuracy and surface finish – Capability to make thin cross-sections in casting • Variations • Disadvantages: – Slush Casting – Moisture in plaster mold causes problems: – Low-pressure Casting • Mold must be baked to remove moisture – Vacuum Permanent Mold Casting • Mold strength is lost when is over-baked, yet moisture content can cause defects in product • Die Casting – Plaster molds cannot stand high temperatures • Centrifugal Casting 39 40 Permanent Mold Casting Basic Permanent Mold Process Process • Metals - Al, Mg, Copper alloy and Cast Iron • Basic Steps – Preheated Mold (metals to flow) – Coatings are sprayed – Pour and solidify – Mold is open and casting is removed • Advantage - Good surface finish and dimensional control and Fine grain due to rapid solidification. • Disadvantage - Simple geometric part, expensive mold. • Example - automobile piston, pump bodies castings for aircraft and missiles. 41 42 7
  8. Permanent Mold Casting Die Casting • Advantages: • The molten metal is injected into mold cavity – Good dimensional control and surface finish (die) under high pressure (7-350MPa). – More rapid solidification caused by the cold metal Pressure maintained during solidification. mold results in a finer grain structure, so stronger castings are produced • Hot Chamber (Pressure of 7 to 35MPa) – The injection system is submerged under the molten • Limitations: metals (low melting point metals such as lead, zinc, – Generally limited to metals of lower melting point tin and magnesium) – Simple part geometries compared to sand casting because of the need to open the mold • Cold Chamber (Pressure of 14 to 140MPa) – High cost of mold – External melting container (in addition aluminum, • Due to high mold cost, process is best suited to brass and magnesium) automated high volume production 43 44 Die Casting Hot-Chamber Die Casting • Molds are made of tool steel, mold steel, maraging steel, tungsten and molybdenum. • Single or multiple cavity • Lubricants and Ejector pins to free the parts • Venting holes and passageways in die • Formation of flash that needs to be trimmed • Advantages – High production, Economical, close tolerance, good surface finish, thin sections, rapid cooling 45 46 Cold Chamber Die Casting Die Casting • Advantages: – Economical for large production quantities – Good dimensional accuracy and surface finish – Thin sections are possible – Rapid cooling provides small grain size and good strength to casting • Disadvantages: – Generally limited to metals with low metal points – Part geometry must allow removal from die cavity 47 48 8
  9. Centrifugal casting 4. Foundry Practice • True centrifugal casting • Furnace – Cupolas (Fig. 11.18) • Semicentrifugal casting – Direct Fuel-fired furnace • Centrifuge casting – Crucible Furnace (Fig. 11.19) – Electric-arc Furnace – Induction Furnace • Pouring with ladle • Solidification – watch for oxidation • Trimming, surface cleaning, repair and heat treat, inspection 49 50 5. Casting Quality • Casting defects a) Misruns Three types : (a) lift-out crucible, (b) stationary pot, from which molten b) Cold shut metal must be ladled, and (c) tilting-pot furnace c) Cold shots d) Shrinkage cavity e) Microporosity f) Hot Tearing Induction furnace 51 52 Electric Arc Furnace Sand Mold defects 6. Metals for Casting • Ferrous casting alloys: cast iron – Gray Cast Iron, Nodular iron, White Cast Iron, Malleable Iron, Alloy cast iron (b) Pin hole • Ferrous casting alloys: Steels (c) Sand wash – Melting temperature is higher that casting alloys. Thus they are more reactive. – Less Fluidity – Higher strength, Tougher (d) Scabs – Isotropy and weldable (e) Penetration (f) Mold shift • Nonferrous casting alloys – Aluminum, Magnesium, Copper, Tin-based, Zinc, Nickel and Titanium Alloys 53 54 (g) Core shift (h) Mold crack 9
  10. 7. Product Design Considerations • Geometric simplicity • Corners • Section thicknesses – Hot spot • Draft (Fig. 11.25) • Use of Cores • Dimensional tolerances and surface finish • Machining allowance • Tolerance and Surface Roughness for Various Casting Processes – See Table 11.2 55 10

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