Met 402 mod_2
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The document discusses the fundamentals of casting processes including solidification of metals, fluid flow of molten metal, casting defects and quality, and various casting processes such as sand casting and shell molding. It also covers topics like heat transfer during solidification, grain structure formation, and classifications of different casting techniques based on the type of mold and pattern used.
Met 402 mod_2
- 1. MODULE 2 COURSE OUTCOMES Fundamentals of Casting process Solidification of metals Fluid flow of molten metal Various casting process Casting defects & quality
- 2. : Casting / Foundry Casting processes basically involve the introduction of a molten metal into a mold cavity, where upon solidification, the metal .takes on the shape of the mold cavity :Applications Cylinder blocks, liners, machine tool beds, pistons, piston rings, mill rolls, wheels, housings, water supply pipes, bells
- 3. Examples of Cast Parts Crank handle formed by casting; some areas were machined and assembled after casting
- 4. Examples of Cast Parts C-clamps formed by casting (left) and machining )(right
- 5. Examples of Cast Parts Complex part formed by casting
- 6. Aluminum piston for an internal : combustion engine .a) as-cast (b) after machining (
- 8. Flask : A molding flask is one which holds the sand mould intact Drag : Lower molding flask Cope : Upper molding flask Pattern : It is a replica of final object to be made with some modifications. The mold cavity is made with the help of pattern material Molding sand : It is the freshly prepared refractory material used for .making the mold cavity .Core : It is used for making hollow cavities in the castings Core Print : A region used to support the core Pouring basin : A small funnel shaped cavity at the top of the mold .into molten metal is poured
- 14. Parting Line / Parting Surface : Interface that separates the cope and drag Runner: The channel through which the molten metal is carried .from the sprue to the gate Gate: A channel through which the molten metal enters the mold cavity Chaplets: Chaplets are used to support the cores inside the mold .cavity . Riser: It is a reservoir of the molten metal provided in the casting Vent: Small opening in the mold to facilitate escape of air and .gases Sprue : The passage through which the molten material from the .pouring basin reaches the mold cavity
- 15. Pattern Material Wood, metals & alloys, plastic, plaster of Paris, plastic and rubbers, wax, and resins. Material selection depends on size and shape of casting. To be suitable for use, the pattern material should be: 1. Easily worked, shaped and joined 2. Light in weight 3. Strong, hard and durable 4. Resistant to wear and abrasion 5. Resistant to corrosion, and to chemical reactions 6. Dimensionally stable and unaffected by variations in temperature and humidity 7. Available at low cost
- 16. Sand Casting
- 17. Pattern Allowances Pattern allowance is a vital feature as it affects the dimensional characteristics of the casting The selection of correct allowances greatly helps to reduce machining costs and avoid rejections. 1. Shrinkage or contraction allowance 2. Draft or taper allowance 3. Machining or finish allowance 4. Distortion or camber allowance 5. Rapping allowance or shake allowances
- 18. Shrinkage or Contraction Allowance All most all cast metals shrink or contract volumetrically on cooling. The metal shrinkage is of two types: (1) Liquid Shrinkage: It refers to the reduction in volume when the metal changes from liquid state to solid state at the solidus temperature. To account for this shrinkage; riser, which feed the liquid metal to the casting, are provided in the mold. (2) Solid Shrinkage: It refers to the reduction in volume during the cooling of the cast metal to room temperature. To account for this, shrinkage allowance is provided on the patterns. The rate of contraction with temperature is dependent on the material. For example steel contracts to a higher degree compared to aluminum.
- 19. Shrinkage Metal Percent Contraction (-) Expansion(+) Aluminum -7.1% Zinc -6.5% Gold -5.5% Copper -4.9% Brass -4.5% Carbon Steel -2.5-4% Lead -3.2% Gray Cast Iron +2.5%
- 21. Draft or Taper Allowance Taper is provided on all vertical surfaces of the pattern so that it can be removed from the sand without tearing away the sides of the sand mold. Draft allowance varies with the complexity of the sand job. Inner details of the pattern require higher draft than outer surfaces. The amount of draft depends upon the length of the vertical side of the pattern to be extracted; the intricacy of the pattern; the method of molding; and pattern material.
- 22. Pattern Height of the Draft angle Draft angle material given surface (External (Internal (inch) surface) surface) 1 3.00 3.00 1 to 2 1.50 2.50 Wood 2 to 4 1.00 1.50 4 to 8 0.75 1.00 8 to 32 0.50 1.00 1 1.50 3.00 1 to 2 1.00 2.00 Metal and 2 to 4 0.75 1.00 plastic 4 to 8 0.50 1.00 8 to 32 0.50 0.75 Draft Allowances of Various Metals
- 23. Taper on patterns for ease of removal from the sand mold
- 24. Machining or Finish Allowance Machining or finish allowances are added in the pattern dimension to have good surface finish or dimensionally accurate The amount of machining allowance to be provided is affected by the method of molding and casting used viz. hand molding or machine molding, sand casting or metal mold casting. The amount of machining allowance is also affected by the size and shape of the casting; the casting orientation; the metal; and the degree of accuracy and finish required.
- 25. Machining Allowances of Various Metals Metal Dimension (inch) Allowance (inch) Up to 12 0.12 Cast iron 12 to 20 0.20 20 to 40 0.25 Up to 6 0.12 Cast steel 6 to 20 0.25 20 to 40 0.30 Up to 8 0.09 Non ferrous 8 to 12 0.12 12 to 40 0.16
- 26. Distortion or Camber Allowance .Castings get distorted, during solidification, due to their typical shape For example, if the casting has the form of the letter U, V, T, or L etc. it will tend to contract at the closed end causing the vertical legs to look . slightly inclined This can be prevented by making the legs of the U, V, T, or L shaped pattern converge slightly (inward) so that the casting after distortion will have its sides vertical . The distortion in casting may occur due to internal stresses These internal stresses are caused on account of unequal cooling of . different section of the casting and hindered contraction :To prevent the distortion in castings include i. Modification of casting design ii. Providing sufficient machining allowance to cover the distortion affect iii. Providing suitable allowance on the pattern, called camber or )distortion allowance (inverse reflection
- 28. Rapping Allowance Before the withdrawal from the sand mold, the pattern is rapped all around the vertical faces to enlarge the mold cavity slightly, which facilitate its removal. Since it enlarges the final casting made, it is desirable that the original pattern dimension should be reduced to account for this increase. There is no sure way of quantifying this allowance, since it is highly dependent on the foundry personnel practice involved. It is a negative allowance and is to be applied only to those dimensions that are parallel to the parting plane.
- 29. Fluid flow • 2 principles of fluid flow are relevant to gating design: Bernoulli’s theorem and the law of mass continuity.
- 30. Fluid flow Bernoulli’s theorem • Based on - principle of conservation of energy - frictional losses in a fluid system 2 h = elevation p v p = pressure at elevation h+ + = Constant ρg 2 g v = velocity of the liquid ρ = density of the fluid • Conservation of energy requires that, p1 v12 2 p 2 v2 h+ + = h2 + + +f ρg 2 g ρg 2 g
- 31. Fluid flow Mass continuity • States that for an incompressible liquid the rate of flow is constant. Q = A1v1 = A2 v2 Q = volumetric rate of flow A = cross-sectional area of the liquid stream v = velocity of the liquid • Subscripts 1 and 2 pertain to two different locations in the system.
- 32. Fluid flow Sprue profile • Relationship between height and cross- sectional area at any point in the sprue is given by A1 = h2 A2 h1 • Velocity of the molten metal leaving the gate is v = c 2 gh • When liquid level reached height x, gate velocity is v = c 2g h − x
- 33. Fluid flow Flow characteristics • Reynolds number, Re, is used to characterize aspect of fluid flow. • It represents the ratio of the inertia to the viscous forces in fluid flow and is defined as vDρ v = velocity of the liquid Re = D = diameter of the channel η ρ = density n = viscosity of the liquid.
- 34. Flow Characteristics 0 < Re < 2000 => laminar flow 2000 < Re < 20 000 =>mixture of laminar and turbulent flow , generally regarded as harmless in gating systems. Re > 20 000 => severe turbulence In gating systems, Re typically ranges from 2000 to 20,000 Techniques for minimizing turbulence • Dross or slag can be eliminated by vacuum casting • Use of filters eliminates turbulent flow in the runner system
- 35. Turbulence can be reduced by the design of a gating system that promotes a more laminar flow of the liquid metal. Sharp corners and abrupt changes in sections within the casting can be a leading cause of turbulence. Their affect can be mitigated by the employment of radii.
- 36. Fluidity of molten metal Fluidity of Molten Metal : The capability of molten metal to fill mold cavities is called fluidity. The following influence fluidity Characteristics of molten metal – Viscosity (How runny is it when hot) – Surface tension (Development of film ) – Inclusions – Solidification pattern of the alloy Casting parameters – Mold design (Risers, runners, gates, etc.) – Mold material and its surface characteristics – Degree of superheat – Rate of pouring – Heat transfer
- 37. Heat Transfer Important consideration in casting – Heat flow in the system • Complex • Depends of flow characteristics Solidification Time – A function of the volume of a casting and its surface area • Solidification time = C volume 2 surface area – Effects on solidification time • Mold Geometry • Skin thickness
- 38. Heat transfer • Temperature distribution in the mold- liquid metal interface is shown below.
- 39. Solidification of Metals Involves liquid metal turning back in to solid metal The process is different for Pure metals and alloys Can be divided into two steps: Formation of stable nuclei Growth of crystals Pure Metals • Have a clearly defined melting point • Temperature remains constant during freezing • Solidifies from the walls of the mold toward the center of the part
- 40. Grain Structure for Pure Metals • Two types of grains are formed for a pure metal – Fine equiaxed grains – Columnar • Rapid cooling at the walls produces fine equiaxed grains • Columnar grains grow opposite of the heat transfer throughout the mold following the chill zone Equiaxed Grains • If crystals can grow approximately equally in all directions – equiaxed grains will grow. • Large amounts of under cooling is needed near the wall of the mold.
- 41. Illustration of Cast Structures
- 42. Alloys • Solidification in alloys begins when the temperature drops below the liquidus TL and is complete when it reaches the solidus, TS.
- 43. Alloys • Within the TL and TS Temperature range, the alloy is like a slushy with columnar dendrites
- 44. Effects of Cooling Rates • Slow cool rates results in course grain structures (102 K/s) • Faster cooling rates produce finer grain structures (104 K/s) • For even faster cooling rates, the structures are amorphous (106 – 108 K/s) • Grain size influences strength of a material • Smaller grains have higher ductility and strength • Smaller grains help prevent hot tearing and/or cracks in the casting
- 45. Casting Process Classifications Expendable Mold / Reusable Pattern. 1 Sand Casting )Wood ,Plastic ,metal ( Shell molding Ceramic-Mold Casting Expendable Mold / Expendable Pattern. 2 Investment Casting ( Wax , Plastic ,Polystyrene Foam ) Evaporative-Foam Casting Permanent Mold / No Pattern. 3 Permanent Mold Casting Die Casting Centrifugal Casting
- 46. Shell molding
- 48. Shell molding – a mounted pattern, made of a ferrous metal or aluminum, is heated to 175-370 0 C, coated with a parting agent such as silicone, and clamped to a box or chamber containing a fine sand coated with a 2.5 - 4.0% thermosetting resin binder – the sand mixture is blown over the heated pattern, coating it evenly – the assembly is placed in an oven to complete the curing of the resin – the shell is formed by removing the pattern – two half shells are made and are clamped together in preparation for pouring
- 49. Shell molding Advantages • Better surface finish • Better dimensional tolerances. • Reduced Machining. • Less foundry space required. • Semi skilled operators can handle the process. • The process can be mechanized.
- 51. Shell Molding Applications • -Crankshaft fabrication • -Steel casting parts, fittings • -Molded tubing fabrication • -Hydraulic control housing fabrication • -Automotive castings (cylinder head and ribbed cylinder fabrication).
- 52. Expendable Mold Uses a polystyrene foam pattern which evaporates with molten metal to form a cavity for the casting. Polystyrene foam pattern includes sprue, risers, gating system and internal cores (if needed) Polystyrene inexpensive and easily processed into patterns
- 53. mo me support sand pattern polystyrene pattern
- 54. Advantages of expanded polystyrene process: 1.Pattern need not be removed from the mold 2.Simplifies and speeds mold-making, because two mold halves are not required as in a conventional green-sand mold Disadvantages: 1.A new pattern is needed for every casting 2.Economic justification of the process is highly dependent on cost of producing patterns
- 55. Evaporative Pattern Casting of an Engine Block (a ) (b ) a) Metal is poured into mold for lost-foam casting of a 60-hp. ( .3-cylinder marine engine; (b) finished engine block
- 58. Investment Casting – Characteristics • Advantages: Complex shapes possible Thin wall sections possible High production rates High dimensional accuracy • Disadvantages: Limited weight range Expensive Machinery & Dies Expensive Unit Cost, Labor Intensive Mold is not reusable
- 59. . Typical parts produced by investment casting Products such as rocket components, and jet engine turbine blades
- 60. Die casting The molten metal is injected into die cavity under high pressure Pressure maintained during solidification Die casting typically makes use of non-ferrous alloys. The four most common alloys that are die cast are Aluminum alloys, Copper alloys, Magnesium alloys, Zinc alloys
- 61. Hot chamber die casting
- 62. Cold chamber die casting
- 63. Advantages of die casting Excellent dimensional accuracy Smooth cast surfaces Thinner walls can be cast Inserts can be cast-in (such as threaded inserts, heating elements, and high strength bearing surfaces). Reduces or eliminates secondary m/c ing operations. Rapid production rates.
- 64. Disadvantages of die casting The main disadvantage - very high capital cost. To make die casting an economic process a large production volume is needed. Die casting is limited to high fluidity metals (Zinc, Aluminum, Magnesium, Copper, Lead and Tin) (Not applicable for high melting point metals and alloys (eg. steels) Casting weights must be between 30 grams and 10 kg Limited die life
- 65. Centrifugal Casting In this process, the mold is rotated rapidly about its central axis as the metal is poured into it. Because of the centrifugal force, a continuous pressure will be acting on the metal as it solidifies. The slag, oxides and other inclusions being lighter, get separated from the metal and segregate towards the center. This process is normally used for the making of hollow pipes, tubes, hollow bushes, etc., which are ax- symmetric with a concentric hole.
- 66. Centrifugal Casting Since the metal is always pushed outward because of the centrifugal force, no core needs to be used for making the concentric hole. The mold can be rotated about a vertical, horizontal or an inclined axis or about its horizontal and vertical axes simultaneously. The length and outside diameter are fixed by the mold cavity dimensions while the inside diameter is determined by the amount of molten metal poured.
- 67. Defects • Cavities – Internal or external • Metallic projections • Blow holes – Fins • Pin holes – Flash • Shrinkage cavities – Massive projections • Discontinuities • Swells • Rough surfaces – Cracks – Cold or hot tearing – Cold shunts
- 68. Casting Defects