FUNDAMENTALS OF METAL CASTING 1. Overview of casting g technology gy 2. Heating and pouring 3. Solidification and Cooling ©2012 John Wiley & Sons, Inc. M P Groover, Introduction to Manufacturing Processes Solidification Processes Starting g work material is either a liquid q or is in a highly g y plastic condition, and a part is created through solidification of the material Solidification processes can be classified according to engineering material processed: Metals Ceramics, specifically glasses Polymers and polymer matrix composites (PMCs) ©2012 John Wiley & Sons, Inc. M P Groover, Introduction to Manufacturing Processes Casting of Metals Process in which molten metal flows byy gravity g y or other force into a mold where it solidifies in the shape of the mold cavity The term casting also applies to the part made in the process Steps in casting seem simple: 1. Melt the metal 2. Pour it into a mold 3. Let it freeze ©2012 John Wiley & Sons, Inc. M P Groover, Introduction to Manufacturing Processes Capabilities and Advantages of Casting R1 Can create complex p p part g geometries Can create both external and internal shapes Some casting processes are net shape; others are near net shape Can p produce very y large g p parts Some casting methods are suited to mass production ©2012 John Wiley & Sons, Inc. M P Groover, Introduction to Manufacturing Processes Disadvantages of Casting R2 Different disadvantages g for different casting g processes: Limitations on mechanical properties p p Poor dimensional accuracy and surface finish for some processes; e.g., sand casting Safety hazards to workers due to hot molten metals Environmental problems ©2012 John Wiley & Sons, Inc. M P Groover, Introduction to Manufacturing Processes Parts Made by Casting Big gp parts Engine blocks and heads for automotive vehicles,, wood burning g stoves,, machine frames,, railway wheels, pipes, church bells, big statues, pump housings Small parts Dental crowns, jewelry, small statues, frying pans ©2012 John Wiley & Sons, Inc. M P Groover, Introduction to Manufacturing Processes Overview of Casting Technology Casting g is usually yp performed in a foundry y Foundry = factory equipped for making molds, melting and handling molten metal, performing the casting process, and cleaning the finished casting R3 Workers who perform casting are called foundrymen ©2012 John Wiley & Sons, Inc. M P Groover, Introduction to Manufacturing Processes The Mold in Casting Contains cavityy whose g geometry y determines p part shape Actual size and shape p of cavity y must be slightly g y enlarged to allow for shrinkage of metal during solidification and cooling Molds are made of a variety of materials, including sand, plaster, ceramic, and metal ©2012 John Wiley & Sons, Inc. M P Groover, Introduction to Manufacturing Processes Open Molds and Closed Molds R4 Two forms of mold: (a) open mold and (b) closed mold for more complex mold geometry with gating system leading into the cavity ©2012 John Wiley & Sons, Inc. M P Groover, Introduction to Manufacturing Processes Two Categories of Casting Processes R5 1. Expendable p mold p processes – use an expendable p mold which must be destroyed to remove casting Mold materials: sand, p plaster, and similar materials, plus binders 2. Permanent mold processes – use a permanent mold which can be used to produce many castings Made of metal (or, less commonly, a ceramic refractory material ©2012 John Wiley & Sons, Inc. M P Groover, Introduction to Manufacturing Processes Advantages and Disadvantages More intricate g geometries are p possible with expendable mold processes Part shapes in permanent mold processes are limited by the need to open the mold Permanent mold processes are more economic in high production operations R6 ©2012 John Wiley & Sons, Inc. M P Groover, Introduction to Manufacturing Processes Sand Casting Mold ©2012 John Wiley & Sons, Inc. M P Groover, Introduction to Manufacturing Processes Terminology for Sand Casting Mold Mold consists of two halves: Cope = upper half of mold Drag = bottom half Mold halves are contained in a box box, called a flask The two halves separate at the parting line ©2012 John Wiley & Sons, Inc. M P Groover, Introduction to Manufacturing Processes Forming the Mold Cavity in Sand Casting Mold cavityy is formed byy p packing g sand around a pattern, which has the shape of the part] When the pattern is removed, the remaining cavity of the packed sand has desired shape of cast part The pattern is usually oversized to allow for shrinkage of metal during solidification and cooling Sand for the mold is moist and contains a binder to maintain its shape ©2012 John Wiley & Sons, Inc. M P Groover, Introduction to Manufacturing Processes Use of a Core in the Mold Cavity The mold cavityy provides p the external surfaces of the cast part In addition, a casting may have internal surfaces, determined by a core, placed inside the mold cavity to define the interior geometry of part R7 In sand casting, cores are generally made of sand ©2012 John Wiley & Sons, Inc. M P Groover, Introduction to Manufacturing Processes Gating System Channel through g which molten metal flows into cavity y from outside of mold Consists of a downsprue, through which metal enters a runner leading to the main cavity At the top of downsprue, a pouring cup is often used to minimize splash and turbulence as the metal flows i t downsprue into d R9 ©2012 John Wiley & Sons, Inc. M P Groover, Introduction to Manufacturing Processes Riser Reservoir in the mold which is a source of liquid q metal to compensate for shrinkage of the part during solidification The riser must be designed to freeze after the main casting in order to satisfy its function ©2012 John Wiley & Sons, Inc. M P Groover, Introduction to Manufacturing Processes Heating the Metal Heating g furnaces are used to heat the metal to molten temperature sufficient for casting The heat required is the sum of: 1. Heat to raise temperature p to melting gp point 2. Heat of fusion to convert from solid to liquid 3. Heat to raise molten metal to desired temperature for pouring ©2012 John Wiley & Sons, Inc. M P Groover, Introduction to Manufacturing Processes Pouring the Molten Metal For this step p to be successful,, metal must flow into all regions of the mold, most importantly the main cavity, before solidifying Factors that determine success Pouring temperature Pouring rate Turbulence ©2012 John Wiley & Sons, Inc. M P Groover, Introduction to Manufacturing Processes Solidification of Metals Transformation of molten metal back into solid state Solidification differs depending on whether the metal is A pure element or An alloyy ©2012 John Wiley & Sons, Inc. M P Groover, Introduction to Manufacturing Processes Cooling Curve for a Pure Metal Ap pure metal solidifies at a constant temperature equal to its freezing point (same as melting point) Superheat R8 ©2012 John Wiley & Sons, Inc. M P Groover, Introduction to Manufacturing Processes Solidification of Pure Metals Due to chilling g action of mold wall,, a thin skin of solid metal is formed at the interface immediately after pouring Skin thickness increases to form a shell around the molten metal as solidification progresses Rate of freezing depends on heat transfer into mold, as well as thermal properties of the metal ©2012 John Wiley & Sons, Inc. M P Groover, Introduction to Manufacturing Processes Solidification of Pure Metals Characteristic grain g structure in a casting of a pure metal, showing randomly oriented grains of small size near the mold wall wall, and large columnar grains oriented toward the center of the casting ©2012 John Wiley & Sons, Inc. M P Groover, Introduction to Manufacturing Processes Engineering Analysis of Pouring Continuity Equation Bernoulli Bernoulli’ss theorem R10 ©2012 John Wiley & Sons, Inc. M P Groover, Introduction to Manufacturing Processes Eutectic alloyy R13 Solidification of Alloys Most alloys freeze over a temperature range Phase diagram for a copper-nickel alloy system and cooling curve for a 50%Ni-50%Cu composition ©2012 John Wiley & Sons, Inc. M P Groover, Introduction to Manufacturing Processes Solidification of Alloys Characteristic grain structure in an alloy casting, showing segregation of alloying components in center of casting ti Eutectic alloy is a particular composition in an alloy system for which hi h solidus lid and d liquidus li id att the th same temperature. t t Hence, H ©2012 John Wiley & Sons, Inc. M P Groover, Introduction to Manufacturing Processes Solidification occurs at a constant temperature. R13 Solidification Time Total solidification time TTS = time required q for casting g to solidify after pouring TTS depends on size and shape of casting by relationship known as Chvorinov's Rule R14 n V ⎛ ⎞ TTS = Cm ⎜ ⎟ ⎝ A⎠ where TTS = total solidification time; V = volume of the casting; A = surface area of casting; n = exponent with typical value = 2; and Cm is mold constant. ©2012 John Wiley & Sons, Inc. M P Groover, Introduction to Manufacturing Processes Mold Constant in Chvorinov's Rule Mold constant Cm depends p on: Mold material Thermal properties of casting metal Pouring temperature relative to melting point Value of Cm for a given casting operation can be based on experimental data from previous operations carried out using g same mold material, metal, and pouring temperature, even though the shape of the part may be quite different ©2012 John Wiley & Sons, Inc. M P Groover, Introduction to Manufacturing Processes What Chvorinov's Rule Tells Us Casting g with a higher g volume-to-surface area ratio cools and solidifies more slowly than one with a lower ratio To feed molten metal to the main cavity, y TTS for riser must be greater than TTS for main casting Since mold constants of riser and casting will be equal, design the riser to have a larger volume-to-area ratio so that the main casting solidifies first This minimizes the effects of shrinkage ©2012 John Wiley & Sons, Inc. M P Groover, Introduction to Manufacturing Processes Shrinkage during Solidification and Cooling R15 (0) ( ) starting g level of molten metal immediately y after pouring; (1) reduction in level caused by liquid contraction during cooling ©2012 John Wiley & Sons, Inc. M P Groover, Introduction to Manufacturing Processes Shrinkage during Solidification and Cooling g (2) reduction in height and formation of shrinkage cavity caused by solidification; (3) further reduction in volume due to thermal contraction during cooling of solid metal ©2012 John Wiley & Sons, Inc. M P Groover, Introduction to Manufacturing Processes Solidification Shrinkage Occurs in nearlyy all metals because the solid p phase has a higher density than the liquid phase Thus, solidification causes a reduction in volume per unit weight of metal Exception: cast iron with high C content Graphitization during final stages of freezing causes expansion that counteracts volumetric decrease associated with phase change ©2012 John Wiley & Sons, Inc. M P Groover, Introduction to Manufacturing Processes Shrinkage Allowance Patternmakers correct for solidification shrinkage g and thermal contraction by making the mold cavity oversized Amount by which mold is made larger relative to final casting size is called pattern shrinkage allowance Casting dimensions are expressed linearly, so allowances are applied accordingly ©2012 John Wiley & Sons, Inc. M P Groover, Introduction to Manufacturing Processes Directional Solidification To minimize effects of shrinkage, g , it is desirable for regions of the casting most distant from the liquid metal supply to freeze first and for solidification to progress from f these regions toward the riser(s) ( ) Thus, molten metal is continually available from risers i tto preventt shrinkage hi k voids id The term directional solidification describes this aspectt off freezing f i and d methods th d by b which hi h it iis controlled ©2012 John Wiley & Sons, Inc. M P Groover, Introduction to Manufacturing Processes Achieving Directional Solidification Directional solidification is achieved using g Chvorinov's Rule to design the casting, its orientation in the mold, and the riser system that feeds it Locate sections of the casting with lower V/A ratios away from riser, so freezing occurs first in th these regions, i and d th the liliquid id metal t l supply l ffor th the rest of the casting remains open Chills - internal or external heat sinks that cause rapid freezing in certain regions of the casting ©2012 John Wiley & Sons, Inc. M P Groover, Introduction to Manufacturing Processes Riser Design Riser is waste metal that is separated p from the casting and remelted to make more castings To minimize waste in the unit operation, it is desirable for the volume of metal in the riser to be a minimum Since the shape of the riser is normally designed to maximize the V/A ratio, this allows riser volume to be reduced to the minimum possible value ©2012 John Wiley & Sons, Inc. M P Groover, Introduction to Manufacturing Processes 5.5 Molten metal can be poured into the pouring cup of a sand mold at a steady rate of 1000 cm3/s. The molten metal overflows the pouring cup and flows into the downsprue. The cross section of the sprue is round, with a diameter at the top = 3.4 cm. If the sprue is 25 cm long, determine the proper diameter at its base so as to maintain the same volume flow rate rate. •Solution: Velocity at base v = (2gh)0.5 = (2 x 981 x 25)0.5 = 221.5 cm/s Assuming volumetric continuity, area at base A = (1000 cm/s)/(221.5 cm/s) = 4.51 cm2 Area of sprue A = πD2/4; rearranging, D2 = 4A/π = 4(4.51)/π = 5.74 cm2 D = 2.39 cm ©2012 John Wiley & Sons, Inc. M P Groover, Introduction to Manufacturing Processes 5.8 A flat plate is to be cast in an open mold whose bottom has a square shape that is 200 mm by 200 mm. The mold is 40 mm deep. A total of 1,000,000 mm3 of molten aluminum l i iis poured d iinto t th the mold. ld S Solidification lidifi ti volumetric l t i shrinkage hi k iis kknown tto b be 6.0%. Table 5.1 lists the linear shrinkage due to thermal contraction after solidification to be 1.3%. If the availability of molten metal in the mold allows the square shape of the cast plate to maintain its 200 mm by 200 mm dimensions until solidification lidifi ti iis completed, l t d d determine t i th the fifinall di dimensions i off th the plate. l t Solution: The initial volume of liquid metal = 1,000,000 mm3. When poured into the mold it takes the shape of the open mold, mold which is 200 mm by 200 mm square square, or 40 40,000 000 mm2. The starting height of the molten metal is 1,000,000 / 40,000 = 25 mm. Volumetric solidification shrinkage is 6%, so when the aluminum has solidified its volume = 1,000,000(0.94) = 940,000 mm3. Because its base still measures 200 mm by 200 mm due to the flow of liquid metal before solidification, its height has been reduced to 940,000 / 40,000 = 23.5 mm. Thermal contraction causes a further shrinkage of 1.3%. Thus the final dimensions of the plate are 200(0.987) by 200(0.987) by 23.5(0.987) = 197.40 mm by 197.40 mm by 23.195 mm. ©2012 John Wiley & Sons, Inc. M P Groover, Introduction to Manufacturing Processes 5.15 A cylindrical li d i l riser i iis tto b be used d ffor a sand-casting d ti mold. ld F For a given cylinder volume, determine the diameter-to-length ratio that will maximize the time to solidify. Solution: To maximize TTS, the V/A ratio must be maximized. Cylinder volume V = πD2L/4. L = 4V/πD2 Cylinder area A = 2πD2/4 + πDL Substitute the expression for L from the volume equation in the area equation: A = πD2/2 + πDL = πD2/2 + πD(4V/πD2) = πD2/2 + 4 V/D Differentiate Diff ti t th the area equation ti with ith respectt tto D D: 2 dA/dD = πD – 4 V/D = 0 Rearranging, πD = 4V/D2 D3 = 4 V/π D = (4 V/π)0.333 From the previous expression for L, substituting in the equation for D that we have developed, L = 4V/πD2 = 4V/π(4V/π)0.667 = (4V/π)0.333 0 333, and Th Thus, optimal ti l values l are D = L = (4V/π)0.333 d th therefore f the th optimal ti l D/L ratio ti = 1.0 10 ©2012 John Wiley & Sons, Inc. M P Groover, Introduction to Manufacturing Processes
