0. Alloys and Phase Diagram FUNDAMENTALS OF • Pure Metals • Alloys Substitutional – Solid solutions Solid Solution METAL CASTING • Substitutional Solid Solution (Zn/Cn and Cu/Ni) – Atomic radii is similar – Lattice type is the same 0. Phase Diagram • Interstitial Solid Solution – Smaller atoms are interstitially located among 1. Overview bigger atoms 2. Heating & Pouring – Lattice type usually does not change 3. Solidification and Cooling – Intermediate Phases – The solubility of one Interstitial element in another element is limited. Solid Solution • Metallic compounds (Fe3C) • Intermetallic compounds(Mg2Pb) 1 2 Cu-Ni Phase Diagram Pb-Sn Phase Diagram (°C) T T 1600 T 600 Liquid Eutectic Composition Temperature, °F Temperature, and Temperature 1400 500 L L+S α+L 400 α β+L 1260 362 °F β 61.9% Sn 1200 300 Solid 200 α+β Substitutional Solid Alloys α 1000 100 Time Time Time Cu26% 36% 50% 62% Ni Pure Metals Pure Metals 0 Time L S 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 – Selection of Mold Materials α 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. Fe1 23456 C 5 6 1 Introduction Classification • Casting (process) – melt the metal, pour into a Sand Casting mold by gravity or other force and solidify. Shell Molding Vacuum Molding • Casting (Part) Metal casting Expandable-mold 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 – Limitation in mechanical properties, porosity, Polymers & PMC Centrifugal Casting – Dimensional accuracy, surface finish Processing Injection Molding – Safety Hazard Other Molding – 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 • Two categories -Expandable or permanent molds. refractory material – Part shapes are limited – Permanent mold processes are more economic in high production operations 9 10 (a) open mold (b) closed mold Basic features of Molds Casting Processes • Forming the Mold Cavity Downsprue • Sand Casting Molds Parting – Mold cavity is formed by packing sand around a pattern. line – The pattern usually oversized for shrinkage is removed. – Mold: cope (upper half) & Pouring Riser 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 cup, (down)sprue, runner Flask – 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 Pouring Analysis 2. Heating & Pouring • Bernoulli’s theorem at any two points in a flowing 2 2 • Sufficient to melt and raise the molten metal to a right liquid P1 v1 P2 v2 state h1 + + = h2 + + ρ 2g ρ 2g • Total Heat Energy required: • h=head, P=pressure, ρ=density, v=flow velocity, g=gravity, F=friction loss H=ρV[C (T -T )+H +C (T -T )] s m o f l p m – Assuming no frictional loss and same pressure where ρ=density, V=volume, Cs=specific heat for solid C =specific heat for liquid, T =melting temperature 2 2 l m v1 v2 T =starting temperature, T =pouring temperature h1 + = h2 + o p 2g 2g • Factors affecting ‘pouring’ 1 – Pouring temperature (vs. melting temp.) – Assuming point 2 is reference (h2=0) and v1=0, – Pouring rate 2 v2 • Too slow, metal freezes h1 = ; v2 = 2gh1 • Too high, turbulence 2g – Turbulence • Continuity law Q = v A = v A 2 • Accelerate the formation of oxides 1 1 2 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 • 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 Temperature 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 S • Rate of freezing depends on 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 Solidification of Alloy Solidification of Alloys • Most Alloys freeze over a temperature range, not at a single temperature. 4 3 2 ∆Gt = πr ∆Gv + 4πr γ • Nucleation 3 ∆Gt – Energy involved in homogeneous nucleation Pouring L Temperature temperature – Total free energy change: Liquid cooling where ∆ G v =volume free energy r L+S = radius of embryo 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 as a function of the size and shape Metal Volume Contraction Solidification Thermal n ⎛V ⎞ Contraction Aluminum TST = Cm ⎜ ⎟ 7% 5.6% ⎝ A ⎠ Al alloys 7 5 V=volume A=surface area and n=2 Gray cast iron 1.8 3 Gray Cast Iron 0 3 Cm=experimentally determined value that depends on with High C 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 part. during final stages causes expansion that counteracts volumetric 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 PROCESSES toward the riser. • 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 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 • Sand mold is used. • A new mold