I-1 Fundemantals of Metal Casting

8/2/2019 I-1 Fundemantals of Metal Casting

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FUNDAMENTALS OF METAL CASTING

1. Overview of Casting Technology2. Heating and Pouring3. Solidification and Cooling

Solidification Processes

Starting work material is either a liquid or is in ahighly plastic condition, and a part is createdthrough solidification of the materialSolidification processes can be classified accordingto engineering material processed:

MetalsCeramics, specifically glassesPolymers and polymer matrix composites(PMCs)


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Classification of solidification processes.

Casting Process

Process in which molten metal flows by gravity orother force into a mold where it solidifies in theshape of the mold cavity. Once the mold and cavityinside the mold is prepared, the metal is melted,poured into the mold and let solified (freezed) insidethe mold.

The term casting also applies to the part made inthe process.


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Basic Requirements of CastingProcesses

Six basic steps of casting

1. Mold cavity is produced having the desiredshape and size of the part

Takes shrinkage into accountSingle-use or permanent mold

2. Melting processProvides molten material at the proper temperature

3. Pouring the molten metalMolten metal is poured into the mold at a proper rate toensure that erosion and or defects are minimized

Six Basic Steps of Casting4. Solidification process

Controlled solidification allows the product to have desiredpropertiesMold should be designed so that shrinkage is controlled

5. Casting removalThe casting is removed from the mold

Single-use molds are broken away from the castingPermanent molds must be designed so that removal of thecasting does not damage the mold and itself. How? Mold shouldbe opened during the removal of the casting and closed duringthe filling of the cavity with the molten metal.

6. Cleaning, finishing, and inspection operationsExcess material along parting lines may have to be machined


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Capabilities and Advantages of Casting

Can create complex part geometriesCan create both external and internal shapesSome casting processes are net shape ; othersare near net shapeCan produce very large partsSome casting methods are suited to massproduction

Disadvantages of Casting

Different disadvantages for different casting processes:

Limitations on mechanical propertiesPoor dimensional accuracy and surface finish for someprocesses; e.g., sand castingSafety hazards to workers due to hot molten metalsEnvironmental problems


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Parts Made by Casting

Big partsEngine blocks and heads for automotivevehicles, wood burning stoves, machine frames,railway wheels, pipes, church bells, big statues,pump housings

Small partsDental crowns, jewelry, small statues, frying pans

All varieties of metals can be cast, ferrous andnonferrous

Overview of Casting Technology

Casting is usually performed in a foundryFoundry = factory equipped for making molds,

melting and handling molten metal, performingthe casting process, and cleaning the finishedcastingWorkers who perform casting are calledfoundrymen


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The Mold in Casting

Contains cavity whose geometry determinespart shape

Actual size and shape of cavity must beslightly oversized to allow for shrinkage ofmetal during solidification and coolingMolds are made of a variety of materials,including sand, plaster, ceramic, and metal

Casting Terminology

Pattern - approximate duplicate of the part to be castMolding material - material that is packed around thepattern to provide the mold cavityFlask - rigid frame that holds the molding aggregateCope - top half of the moldDrag - bottom half of the moldCore - sand or metal shape that is inserted into themold to create internal features


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Casting Terminology

Mold cavity - combination of the mold materialand coresRiser -additional void in the mold that providesadditional metal to compensate for shrinkageGating system - network of channels that deliversthe molten metal to the moldPouring cup - portion of the gating system thatcontrols the delivery of the metalSprue - vertical portion of the gating systemRunners - horizontal channelsGates - controlled entrances

Gating System

Typical gating system for a horizontal parting plane mold, showing key components involved in controlling the flow of metal into themold cavity.


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Casting Terminology

Parting line - separatesthe cope and dragDraft - angle or taper ona pattern that allows foreasy removal of thepattern from the moldCasting - describesboth the process andthe product whenmolten metal is pouredand solidified Cross section of a typical two-part sand mold, indicating various

mold components and terminology.

Cross Section of a Mold


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Two forms of mold: Open Molds and Closed Molds

(a) open mold , simply a container in the shape of the desired part;

(b) closed mold , in which the mold geometry is more complex andrequires a gating system (passageway) leading into the cavity.

Two Categories of Casting Processes1. Expendable mold processes uses an expendable mold

which must be destroyed to remove castingMold materials: sand, plaster, and similar materials, plusbinders.More intricate geometries are possible with expendable moldprocesses

2. Permanent mold processes uses a permanent mold whichcan be used over and over to produce many castings

Made of metal (or, less commonly, a ceramic refractorymaterialPart shapes in permanent mold processes are limited by theneed to open the moldPermanent mold processes are more economic in highproduction operations


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THERE ARE ALSO TWO DIFFERENTTYPES IN SAND CASTING

There are two different methods by which sand castingscan be produced. Classified according to the type of patternused:(1) Removable pattern (kalc model) : In the method employing aremovable pattern, sand is packed around the pattern which is laterwithdrawn from the sand. The cavity produced is filled with molten metalto create the casting.

(2) Disposable (expandable) pattern (kalc olmayan model) :Disposable patterns are made from polystyrene. The polystyrene

pattern, including the gating and pouring system are left in the mold.Molten metal is poured rather rapidly into the sprue; the polystyrenevaporizes; and the metal fills the remaining cavity. Also, wax typepattern is melted out before casting.

Sand Casting Mold Pictures

Mold consists of two halves:Cope = upper half of moldDrag = bottom half

Mold halves are contained in a box,calleda flask The two halves separate at the parting line

Riser --tamalk (to compensateshrinkage).Sprue --dikey yolluk Gate/runner ---yatay yolluk (runner is thename of the gate used in large castings todistribute the metal to several gatepassageways).Pouring cup (basin) --dkme azSprue pin--yolluk pimi Parting plane- -ayrma dzlemi Molding board--dkm tablasCore--maa


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Forming the Mold Cavity In Sand CastingMold cavity is formed by packing sand around a pattern ,which has the shape of the partWhen the pattern is removed, the remaining cavity of thepacked sand has desired shape of cast part. The mold cavityprovides the external surfaces of the cast partThe pattern is usually oversized to allow for shrinkage ofmetal during solidification and coolingSand for the mold is moist and contains a binder to maintainits shapeIn addition, a casting may have internal surfaces, determinedby a core , placed inside the mold cavity to define the interiorgeometry of part. In sand casting, cores are generally made ofsand

WHAT HAPPENS WHEN THE MOLTENMETAL IS POURED INTO THE MOLD?

The molten metal goes through the passageway into themold cavit. It is also known as the gating system (yolluk sistemi).

The passageway is usually made up of a pouring cup(basin) (dkme az) which is used to minimize splash andturbulence as the metal flows into a vertical passage knownas a downsprue (dikey yolluk) through which metal enters arunner leading to the main cavity , and a gate (yatay yolluk)through which the metal flows from the sprue base to themold cavity. In large castings a runner may be used whichtakes the metal from the sprue base and distributes it toseveral gate passageways around the cavity.


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Gates,Risers and Solidification CharacteristicsThe design of the gating system is important and involves anumber of factors:

DIRECTIONAL SOLIDIFICATION:

Volumetric shrinkage usually occurs when metal solidifies,and a shrinkage cavity (void- ekme boluu ) in the castingresults if the solidification is not directed. The shrinkage, and sodoes the shrinkage cavity, occurs in the area where the metalstays molten the longest. The solidification should progress

from the mold cavity to outside of the cavity to compensate forshrinkage. Cavity should solidify first so that no defects occur inthe casting. The shrinkage cavities (voids) should take place inthe gate, risers, feed head or sprue where metal solidifies later.

Directional Solidification

To minimize damaging effects of shrinkage, it isdesirable for regions of the casting most distant fromthe liquid metal supply to freeze first and forsolidification to progress from these remote regionstoward the riser(s)

Thus, molten metal is continually available fromrisers to prevent shrinkage voidsThe term directional solidification describes thisaspect of freezing and methods by which it iscontrolled


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Achieving Directional Solidification

Desired directional solidification is achieved usingChvorinov's Rule to design the casting itself, itsorientation in the mold, and the riser system thatfeeds itLocate sections of the casting with lower V / A ratiosaway from riser, so freezing occurs first in theseregions, and the liquid metal supply for the rest ofthe casting remains openChills - internal or external heat sinks that cause

rapid freezing in certain regions of the casting

RISERS AND FEEDER HEADS TO PREVENTSHRINKAGE CAVITIES IN THE CASTINGS

Risers (tamalk) and feeder heads (ilave kafa) areoften provided in molds to feed molten metal into themain casting cavity to compensate for the shrinkage.They should be large in section, so as to remain moltenas long as possible, and should be located near heavysections that will be subject to large shrinkage.


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Risers and Riser Design

Riser is a reservoir in the mold which is a source of liquidmetal to compensate for shrinkage of the part duringsolidificationThe riser must be designed to freeze after the main casting inorder to satisfy its function. Riser must be separated from thecasting upon completion so the connection area must be assmall as possible.

Schematic of a sand casting mold, showing a) an open-type top riser and b) a blind-type side riser. The sideriser is a live riser, receiving the last hot metal to enter the mold. The top riser is a dead riser, receiving metalthat has flowed through the mold cavity.

Riser Design

Riser is waste metal that is separated from the castingafter solidifacation and remelted to make more castingsTo minimize waste in the unit operation, it is desirablefor the volume of metal in the riser to be a minimumSince the geometry of the riser is normally selected tomaximize the V / A ratio, this allows riser volume to bereduced to the minimum possible value


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TO HELP DIRECTIONAL SOLIDIFICATION INCASTINGS

Metal inserts called chills (soutucu) are sometimes used tocontrol solidification by carrying heat away from the solidifyingmetal at a more rapid rate speeding the solidification.External chills

Masses of high-heat capacity material placed in the moldAbsorb heat and accelerate cooling in specific regions

Internal chillsPieces of metal that are placed in the mold cavity and promote rapidsolidificationUltimately become part of the cast part

Oppositely, exothermic chemical compounds may bepacked next to a part of the casting so that heat is retained inthat area slowing down the solidification .

External Chills

(a) External chill to encourage rapid freezing of the molten metalin a thin section of the casting; and (b) the likely result if theexternal chill were not used.


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LET US LOOK AT SOLIDIFICATION IN MORE DETAIL

Heating the Metal

Heating furnaces are used to heat the metal tomolten temperature sufficient for castingThe heat required is the sum of:1. Heat to raise temperature to melting point2. Heat of fusion to convert from solid to liquid3. Heat to raise molten metal to desired

temperature for pouring

LET US LOOK AT SOLIDIFICATION IN MORE DETAILPouring the Molten Metal

For this step to be successful, metal must flowinto all regions of the mold, most importantlythe main cavity, before solidifyingFactors that determine success

Pouring temperaturePouring rateTurbulence


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LET US LOOK AT SOLIDIFICATION IN MORE DETAIL

Solidification of Metals

Transformation of molten metal back into solidstateSolidification differs depending on whether themetal is

A pure element orAn alloy

Cooling Curve for a Pure Metal

A pure metal solidifies at a constant temperatureequal to its freezing point (same as meltingpoint)

Cooling curve for a pure metal during casting.


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Solidification of Pure MetalsDue to chilling action of mold wall, a thin skin of solid metal is formed atthe interface immediately after pouring. Skin thickness increases to forma shell around the molten metal as solidification progresses. Rate offreezing depends on heat transfer into mold, as well as thermalproperties of the metal

Characteristic grain structure in a casting of a pure metal, showing randomlyoriented grains of small size near the mold wall, and large columnar grainsoriented toward the center of the casting.

Solidification of AlloysMost alloys freeze over a temperature rangerather than at a single temperature

(a) Phase diagram for a copper-nickel alloy system and (b)associated cooling curve for a 50%Ni-50%Cu composition duringcasting.


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Characteristic grain structure in an alloy casting, showing segregationof alloying components in center of casting.

Solidification of Alloys

Solidification TimeSolidification takes timeTotal solidification time T TS = time required forcasting to solidify after pouringT TS depends on size and shape of casting byrelationship known as Chvorinov's Rule

where TST = total solidification time; V = volume ofthe casting; A = surface area of casting; n =exponent with typical value = 2; and C m is mold constant.

n

m A

V

C TST


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Mold Constant in Chvorinov's Rule

Mold constant C m depends on:Mold materialThermal properties of casting metalPouring temperature relative to melting point

Value of C m for a given casting operation canbe based on experimental data from previousoperations carried out using same moldmaterial, metal, and pouring temperature, eventhough the shape of the part may be quitedifferent

What Chvorinov's Rule Tells Us

A casting with a higher volume-to-surface area ratiocools and solidifies more slowly than one with a lowerratio

To feed molten metal to main cavity, TST for risermust greater than TST for main casting

Since mold constants of riser and casting will beequal, design the riser to have a largervolume-to-area ratio so that the main castingsolidifies first .

This minimizes the effects of shrinkage and preventsformation of shrinkage cavities in the castings.


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Shrinkage in Solidification and Cooling

Shrinkage of a cylindrical casting during solidification and cooling:(0) starting level of molten metal immediately after pouring; (1)reduction in level caused by liquid contraction during cooling(dimensional reductions are exaggerated for clarity).

Shrinkage in Solidification and Cooling

(2) reduction in height and formation of shrinkage cavity caused bysolidification shrinkage; (3) further reduction in height and diameter due tothermal contraction during cooling of solid metal (dimensional reductionsare exaggerated for clarity).DO NOT FORGET THAT SHRINKAGE CAVITIES IN CASTINGS(SHOWN IN (3)) SHOULD BE PREVENTED BY USING DIRECTIONALSOLIDIFICATION.


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Solidification Shrinkage

Occurs in nearly all metals because the solidphase has a higher density than the liquid phaseThus, solidification causes a reduction in volumeper unit weight of metalException: cast iron with high C content

Graphitization during final stages of freezingcauses expansion that counteracts volumetricdecrease associated with phase change

Shrinkage Allowance

Patternmakers account for solidificationshrinkage and thermal contraction by making moldcavity oversizedAmount by which mold is made larger relative tofinal casting size is called pattern shrinkage allowance Casting dimensions are expressed linearly, soallowances are applied accordingly

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I-1 Fundemantals of Metal Casting