3.4rev.4~Casting.ppt

Section 3.4Section 3.4

CastingsCastings

Rev. 4Rev. 4

8/20/018/20/01

OverviewOverview

Casting FundamentalsCasting Fundamentals Pouring and Feeding CastingsPouring and Feeding Castings Foundry TechnologyFoundry Technology Inherent DiscontinuitiesInherent Discontinuities

Part 1- Casting FundamentalsPart 1- Casting Fundamentals

P A T T E R N

P A T T E R N IN S O L ID M O L D

The Casting ProcessThe Casting Process

Starts with a pattern representing the Starts with a pattern representing the finished part.finished part.

A mold is constructed from the pattern.A mold is constructed from the pattern.

M O L D C A V IT Y W IT H G A T IN G S Y S T E M

C O M P L E T E D C A S T IN G W IT HA T T A C H E D G A T IN G S Y S T E M

The Casting ProcessThe Casting Process

The mold cavity is filled with molten The mold cavity is filled with molten material.material.

After soldification, the mold is opened After soldification, the mold is opened and excess metal removed.and excess metal removed.

The Casting ProcessThe Casting Process

Process is then checked by inspection.Process is then checked by inspection.

SolidificationSolidification

First Phase:First Phase:– Heat is given up to the mold material.Heat is given up to the mold material.– Outside liquid is first to cool.Outside liquid is first to cool.– Crystal growth starts at the surfaceCrystal growth starts at the surface

SolidificationSolidification

Second Phase:Second Phase:– Solidification occurs at a slower rate.Solidification occurs at a slower rate.– The rate of temperature decrease slows.The rate of temperature decrease slows.– Crystals grow in a columnar shape toward Crystals grow in a columnar shape toward

the center of the heavy section of the the center of the heavy section of the casting.casting.

SolidificationSolidification

Third Phase:Third Phase:– As the wall thickness of frozen metal As the wall thickness of frozen metal

increases, the cooling rate of the remaining increases, the cooling rate of the remaining liquid decreases even further.liquid decreases even further.

– Grains grow to large size.Grains grow to large size.

Grain CharacteristicsGrain Characteristics

Influenced by cooling rates:Influenced by cooling rates:– Outside grains: fine equaxed.Outside grains: fine equaxed.– Outside grains to center grains: columnar Outside grains to center grains: columnar

and dendritic.and dendritic.– Center grains: weakest structure with large Center grains: weakest structure with large

equaxed grains.equaxed grains.

SegregationSegregation

Term used to describe the condition Term used to describe the condition wherein differences in chemical wherein differences in chemical composition occur during solidification composition occur during solidification of a casting or ingot.of a casting or ingot.– Microsegregation: within or around the Microsegregation: within or around the

grains themselves.grains themselves.– Macrosegregation: across the whole ingot Macrosegregation: across the whole ingot

or casting.or casting.

ShrinkageShrinkage

Three types:Three types:– Shrinkage in the liquidShrinkage in the liquid– Shrinkage during the transformation from Shrinkage during the transformation from

liquid to solid (solidification shrinkage)liquid to solid (solidification shrinkage)– Shrinkage that occurrs after solidification Shrinkage that occurrs after solidification

takes place (contraction in the solid state)takes place (contraction in the solid state)

ShrinkageShrinkage

1 .1 % 1 .7 %

S H R IN K A G E C A V IT Y

3 %

S H R IN K P E R C E N T A G E S A P P R O X IM A T E O N L Y F O R C A S T IR O N

T H R E E S T A G E S O F M E T A L C O N T R A C T IO N

ShrinkageShrinkage

Shrinkage in the LiquidShrinkage in the Liquid– Metal is superheated to a temperature Metal is superheated to a temperature

above the melting temperature. above the melting temperature. – Some superheat is lost during transfer of Some superheat is lost during transfer of

the liquid metal from the crucible to the the liquid metal from the crucible to the mold.mold.

– Loss of superheat results in contraction Loss of superheat results in contraction and increased density but usually causes and increased density but usually causes few serious problems in casting.few serious problems in casting.

ShrinkageShrinkage

Solidification Shrinkage Solidification Shrinkage – Occurs during the transformation from Occurs during the transformation from

liquid to solid.liquid to solid.– Localized shrinkage develops random Localized shrinkage develops random

voids called microporosity or voids called microporosity or microshrinkage.microshrinkage.

ShrinkageShrinkage

Solidification Shrinkage Solidification Shrinkage – As gas is evolved before and during As gas is evolved before and during

solidification, they may form pocket of their solidification, they may form pocket of their own or may enter voids to enlarge them.own or may enter voids to enlarge them.

– The evolved gas is usually hydrogen which The evolved gas is usually hydrogen which may combine with oxygen to form water may combine with oxygen to form water vapor. vapor.

– These randomly dispersed large openings These randomly dispersed large openings in the solid metal are called macroporosity.in the solid metal are called macroporosity.

ShrinkageShrinkage

Contraction in the Solid StateContraction in the Solid State– Occurs after solidification takes place.Occurs after solidification takes place.– Primary cause of dimensional change.Primary cause of dimensional change.

Part 2- Pouring and Feeding Part 2- Pouring and Feeding CastingsCastings

SolidificationSolidification

Progressive vs. directional solidification Progressive vs. directional solidification – Progressive: freezing of a liquid from the Progressive: freezing of a liquid from the

outside towards the center.outside towards the center.– Directional: freezing from one end to the Directional: freezing from one end to the

other end.other end.

Hot SpotsHot Spots

Focal points for solidification. Focal points for solidification. Defects are most likely at hot spots Defects are most likely at hot spots

created by section or geometry changes created by section or geometry changes and where gates and risers have been and where gates and risers have been connected to the castings.connected to the castings.

May disturb good directional May disturb good directional solidification and cause defects.solidification and cause defects.

Usually controlled by proper design.Usually controlled by proper design.

Pouring & Pouring RatePouring & Pouring Rate

If metal enters the cavity too slowly, it If metal enters the cavity too slowly, it may freeze before the mold is filled.may freeze before the mold is filled.

If the pouring rate is too high, it will If the pouring rate is too high, it will cause erosion of the mold walls.cause erosion of the mold walls.

The Gating SystemThe Gating System

Consists of:Consists of:– Pouring basin,Pouring basin,– Down sprues,Down sprues,– Runners,Runners,– Ingates, andIngates, and– Channels and openings.Channels and openings.

RisersRisers

Wells of materials, attached to the Wells of materials, attached to the outside of the casting, supply liquid outside of the casting, supply liquid metal as needed to compensate for metal as needed to compensate for shrinkage before solidification is shrinkage before solidification is completecomplete

ChillsChills

Initiate solidification. Initiate solidification. Choice of internal chills critical.Choice of internal chills critical.

Part 3- Foundry TechnologyPart 3- Foundry Technology

Sand moldingSand molding

Green sandGreen sand– ““green” refers to moisture.green” refers to moisture.– Mixture of sand, clay, and moisture.Mixture of sand, clay, and moisture.– Sand grains held together by clay.Sand grains held together by clay.

Dry Sand.Dry Sand.– Elimination of moisture reduces casting Elimination of moisture reduces casting

defects.defects.

Permanent Mold CastingPermanent Mold Casting

Permanent Mold CastingPermanent Mold Casting– Good accuracies and finishesGood accuracies and finishes– Metal molds (mostly cast iron and steel) Metal molds (mostly cast iron and steel)

used primarily for low melting point alloysused primarily for low melting point alloys– Used most for the shaping of aluminum, Used most for the shaping of aluminum,

copper, magnesium, and zinc alloyscopper, magnesium, and zinc alloys

Die CastingDie Casting

Die CastingDie Casting– Differs from the permanent molds casting Differs from the permanent molds casting

in that pressure is applied to the liquid in that pressure is applied to the liquid metal.metal.

– The mold is made of metal, has parting The mold is made of metal, has parting lines, and is constructed with small draft lines, and is constructed with small draft angles on the walls.angles on the walls.

– May be either May be either Hot chamberHot chamber (low pressure) (low pressure) or or Cold chamberCold chamber (high casting quality with (high casting quality with good surface finish and high material good surface finish and high material properties).properties).

Investment/Precision CastingInvestment/Precision Casting

Investment/Precision Casting Investment/Precision Casting – The working pattern destroyed during The working pattern destroyed during

casting – a new wax pattern is needed for casting – a new wax pattern is needed for every piece cast. every piece cast.

– Duplicate parts start with a master pattern.Duplicate parts start with a master pattern.– Process limited to small castings.Process limited to small castings.– High quality at high price.High quality at high price.

Part 3- Inherent Part 3- Inherent DiscontinuitiesDiscontinuities

Inherent DiscontinuitiesInherent Discontinuities

Discontinuities which are related to the Discontinuities which are related to the melting and original solidification of the melting and original solidification of the metal or ingotmetal or ingot

Inherent DiscontinuitiesInherent Discontinuities

Cold Shut Cold Shut – Cause: the meeting of two streams of liquid Cause: the meeting of two streams of liquid

metal that do not fuse togethermetal that do not fuse together– Location: surface or subsurfaceLocation: surface or subsurface

PipePipe– Cause: an absence of molten metal during Cause: an absence of molten metal during

the final solidification processthe final solidification process– Location: subsurfaceLocation: subsurface

Inherent DiscontinuitiesInherent Discontinuities

Hot TearsHot Tears– Cause: restraint from the core or mold Cause: restraint from the core or mold

during the cooling processduring the cooling process– Location: surfaceLocation: surface

Blowholes and PorosityBlowholes and Porosity– Cause: entrapped gases during the Cause: entrapped gases during the

solidification of metal.solidification of metal.– Location: surface or subsurfaceLocation: surface or subsurface

Inherent DiscontinuitiesInherent Discontinuities

Nonmetallic InclusionsNonmetallic Inclusions– Cause: contaminants introduced during the Cause: contaminants introduced during the

casting processcasting process– Location: surface or subsurfaceLocation: surface or subsurface

SegregationSegregation– Cause: localized differences in material Cause: localized differences in material

compositioncomposition– Location: surface or subsurfaceLocation: surface or subsurface

Inherent DiscontinuitiesInherent Discontinuities

MicroshrinkageMicroshrinkage– Cause: withdrawal of the low melting point Cause: withdrawal of the low melting point

constituent from the grain boundaries while constituent from the grain boundaries while the metal is in a plastic or semi-molten the metal is in a plastic or semi-molten statestate

– Location: subsurfaceLocation: subsurface Other Inherent DiscontinuitiesOther Inherent Discontinuities

– MisrunsMisruns– Unfused ChapletsUnfused Chaplets