ARTIGO Chapter 8 Solidification Shrinkage

8/13/2019 ARTIGO Chapter 8 Solidification Shrinkage

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Materials Science & Engineering

Henan Polytechnic University

Outlines:

General shrinkage behavior

Solidification shrinkage

Feeding criteria

Feeding --- the five mechanisms

Initiation of shrinkage porosityGrowth of shrinkage pores

Final forms of shrinkage porosity

Chapter 8 Solidification shrinkage


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References:

John Campbell Castings().,, 2000.., 1994..., 2008..., 1998,. ,,1990.

Kurz W.Fisher D.J.Fundamentals of solidification1987.

.22000.,.,2002..2004.. 2007..19831982M.C. Fleming..1981.G. J..., 1981.
http://www.douban.com/book/search/%E5%BC%A0%E4%BC%9F%E5%BC%BAhttp://www.bestwebbuys.com/G_J_Davies-author.html?isrc=b-compare-authorhttp://www.bestwebbuys.com/G_J_Davies-author.html?isrc=b-compare-authorhttp://www.douban.com/book/search/%E5%BC%A0%E4%BC%9F%E5%BC%BA


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Objectives:

Understand and master the concepts of shrinkage void and

porosity

Know the formation and effective factors of shrinkage void and

porosityKnow the protective measures for shrinkage void and porosity


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8.1 General shrinkage behavior

ShrinkageThe molten metal in the furnace occupies

considerably more volume than the solidified

castings that are eventually produced.

metal Crystal

structure

Melting

point/ Liquid

density/kgm-3

Solid

density/kgm-3

Volume

change/%

Ref.

Al

Cu

Ni

Pb

Fe

K

Rb

Cd

Mg

Fcc

Fcc

Fcc

Fcc

Bcc

Bcc

Bcc

Bep

Bep

660

1083

1453

327

1536

64

303

321

651

2368

7938

7790

10 665

7035

827

11 200

7998

1590

2550

8382

8210

11 020

7265

-

-

-

1655

7.14

5.30

5.11

3.22

3.16

2.54

2.2

4.00

-4.10

1

1

1

1

1

4.5

2

2

3

Table 1 Solidification shrinkage for some metals


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*Regimes of shrinkage

Figure 1 Schematic illustration of three

shrinkage regimes

In the liquid

It is the first contraction in theliquid state, the normal thermal

contraction. The volume reduces

linearly with falling temperature.

During freezing

The contraction of solidification:

occurs at freezing point because

of the greater density of the solid

compared to that of liquid

In the solid

As cooling progresses, the

casting attempts to reduce its size

in consequence


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*Consequences of contractions

Regime I: in the liquid

The shrinkage of liquid metal is not troublesome

Regime II: during freezing

The solidification contraction may cause a number

of problems:

(i) the requirement for feeding

(ii) shrinkage porosity


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8.2 Solidification shrinkage

In general, liquids contract on freezing because of

the rearrangement of atoms from a rather open

random close-packed arrangement to a regular

crystalline array of significantly denser packing.The greatest values for contractionon solidification are

seen for the densest solids are those that have cubic close-

packed(fcc and hcp) symmetry.

The exceptionsto this general pattern are those materialsthat expand on freezing, including water, silicon and bismuth,

and perhaps the most importantalloy, such as cast iron.

See next table.

*Solidification shrinkage cases


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Table 2 Solidification shrinkage for some metals

fcc(face-entral-cubic), cubic

close-packed

symmetry.


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Figure 2 Volume change

on freezing of Fe-C alloys.

The relations up to 4.3 per

cent carbon are due to

Wray (1976); the relations

for higher carbon have

been calculated by J.Campbell.

Graphitic cast irons with carbon equivalent above

approximately 3.6 expand because of the precipitation of

the low density phase, graphite.


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As the solidification continues with the freezing of the following

onion layer of thickness dx, the reduced volume occupied by the

layer dx compared to that of the original liquid means that either a

pore has to form, or the liquid has to expand a little, and thesurrounding solid correspondingly has to contract a little.


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If there is no favorable nucleus available for the creation

of a pore, the liquid has to expand and thus create a state

of tension, or negative pressure.

As more solid layers form, the tension in the liquidincreases, the liquid expands, and the solid shell is drawn

inwards by plastic collapse though a creep process.

It seems that negative pressures of -100 to -1000 atm

may be expected under ideal conditions. The liquid andliquid/solid interface is easily able to withstand such

stresses.

More details:


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These hydrostatic stress is a driving force for the formationof shrinkage porosity. However, at the same time, of course, the

pressure gradient between the outside and inside of the casting is

also the driving force for the varies feeding mechanisms that help

to reduce the porosity.

Whether the driving force for pore formation wins over the driving

force for feeding will depend on whether nuclei for pore formation

exist.


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For the vast majority of cast materials, shrinkageporosity is one of the most important and common

defects in castings.

The internal porosity is more troublesome than the

porosity occurring on the outside of the casting, thus

the elimination of internal porosity and its

displacement to the outside of the casting is a

powerful technique that is strongly recommended.

The shrinkage porosity can be reduced by

(i) improvements to the cleanness of the metals(ii) the design and provision of good feeding.


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8.3.1 The seven feeding rules

In this list in the following text, the order is modified

slightly to group the thermal requirements (1 and 3)together, followed by the geometrical requirements, and

finally the pressure requirements.


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Rule 1 Do not feed unless absolutely necessary

Its the main question relating to the provision of a

feeder on a casting. The avoidance of feeding is to be

greatly encouraged. Probably half of the small and

medium-sized castingsmade today do not need to be fed.

The first reason is cost

Many casting are actually impaired by the inappropriate

placing of a feeder.

It is easy to make an error in estimation of the

appropriate feeder size, with the result that the casting canbe more defective than if no feeder were used at all.


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Rule 2 The feeder must solidify at the same time as,

or later than, the casting

This is the heat-transfer criterion, attributed

to chvorinov.


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Rule 3 The feeder must contain sufficient liquid to

meet the volume-contraction requirements of thecasting.

This is usually known as the volume criterion.

However, there are additional rules which are also

often overlooked, but which define additional thermal,

geometrical and pressure criteria that are absolutely

necessary conditions for the casting to freezesoundly, see next page.


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Rule 4 The junction between the casting and the

feeder should not create a hot spot, i.e. has afreezing time greater than either the feeder or the

casting.

This is a problem that, if not avoided, leads tounder-feeder shrinkage porosity.


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Rule 5 There must be a path a allow feed metal toreach those regions that require it.

The reader can see why this criterion has been often

overlooked as a separate rules: the communicationcriterion appears self-evident, as the

communication criterion.


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Rule 6 There must be sufficient pressure differentialto cause the feed material to flow, and the flow needs

to be in the correct direction.

Rule 7 There must be sufficient pressure at all

points in the casting to suppress the formation and

growth of porosity.


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Its essential to understand that all the rules must be

fulfilled if truly sound casting scale to be produced.The reader must not underestimate the scale of this

problem. The breaking of only one of the rules may result

in the ineffective feeding and a porous casting. The wide

prevalence of porosity in castings is a sobering reminderthat solutions are often not straightforward.

Of the remainder of castings that do suffer feeding

demand, many could avoid the use of a feeder by the

judicious application of chills or cooling fins.


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8.4 Feeding --- the five mechanisms

During the solidification of a casting, often in the

form of a tangled mass of dendrites, presents increasing

difficulties for the passage of feeding liquid.

There appear to be at least five mechanismsbywhich hydrostatic tension can be reduced in a solidifying

materials, although, of course, not all five processes are

likely to operate in any single case.


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Figure 3 Schematic representation of the five feeding mechanisms in a

solidifying casting (Campbell 1969)

Liquid feeding

Mass feeding

Interdendritice feeding

Burst feeding

Solid feeding


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8.4.1 Liquid feeding

Liquid feeding is the most open feeding

mechanismand generally precedes other forms of

feeding. It should be noted that in skin-freezingmaterialsit is normally the only method of feeding.


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Figure 4 Hydrostatic tensions in the residual liquid

calculated for the various feeding regimes during

the freezing of a 20mm diameter aluminium alloy

cylinder (Campbell 1969)

The liquid has low viscosity,

and for most of the freezingprocess the feed path is wide,

so that the pressure difference

required to cause the process

to operate is negligibly small.

Results of theoretical model ofa cylindrical casting only 20mm

diameter (see left figure)

indicate that pressures of the

order of only 1 Pa are

generated in the early stages.


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Inadequate liquid feedingis often seen to occurwhen the feeder has inadequate volume. Thus liquid

flow from the feeder terminates early, and subsequently

only air is drawn into the casting.

For skin-freezing alloys, inadequate liquid feeding will resultin a smooth shrinkage pipe extending from the feeder into the

casting as a long funnel-shaped hole.

Long- freezing-range alloys will be filled with a mesh of

dendrites in a sea of residual liquid. Liquid feeding effectively

becomes interdendritic feeding. Inadequate liquid feeding will

result in a sponge shrinkage pipe.


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Figure 5 Porosity in the long-freezing-range alloy Cu-10Sn bronze, cast with an

inadequate feeder resulting in a spongy shrinkage pipe.


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8.4.2 Mass feeding

Mess feeding is the term coined by Baker (1945) to

denote the movement of s slurry of solidified metal

and residual liquid, which is arrested when the volumefraction of solid reaches anywhere between 0 and 50%,

depending on the pressure differential driving the flow,

and depending on what percentage of dendrites are

free from points of attachment to the wall of the casting.


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The important criterion to assess whether mass flow will occur

is the ratio of casting section thickness to average grain diameter.

In large sections, or where grains have been refined, there

may be 20 to 100 grains or more, so that the flow of the slurry

can become an important mechanism to reduce the pressuredifferential along the flow direction.

Grain refinement is useful in reducing porosity in castings.


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8.4.3 Interdendritic feeding

Interdendritic feeding is to describe the flow of

residual liquid through the pasty zone.

At a point at which the grains in liquid/solidmixtures finally impinge strongly and stop is the point

at which mass feeding starts to become appreciably

more difficult. This is the regime of interdendritic

feeding.

S &


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The pressure differential required to cause a fluid to flow

along a capillary is controlled by a number of factors such as

viscosity, the radius of the capillary, the dendrite arm spacing

and the length of pasty zone.

The pressure is most sensitive to the size of the flow

channels.

M t i l S i & E i i


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8.4.4 Burst feeding

Where hydrostatic tension is increasing in a

poorly fed region of the casting, it seemsreasonable t expect that any barrier might suddenly

yield, like to dam bursting, allowing feed metal to

flood into the poorly fed region.

M t i l S i & E i i


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Figure 6 Gas-shrinkage map

Left figure shows the pathof development to early pore

nucleation at P.

In a contrasting case, slow

mechanical collapse of the casting

delays the build-up of internaltension, culminating in complete

plastic collapse in the form of burst

feeding processes at A and B.

This delay is successful in

avoiding pore nucleation, since

freezing is complete at C.

M t i l S i & E i i


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Where hydrostatic tension is increasing in a poorly fed

region of the casting, it seems reasonable to expect that

any barrier may be suddenly yield, like a dam bursting,allowing the feed metal to flood into the poorly fed region.

For small and intermediate barriers, bursts will reduce

the internal stress and allow the casting to remain free from

porosity.If the feeding barrier is substantial then it may never

burst, causing the result stress to rise and eventually

exceed the nucleation threshold.

On a microscale, a type of burst feeding is the rupture of

the casting skin, allowing an inrush of air or mould gases.

However, this is a gaseous burst that corresponds to the

growth of cavity, not a feeding process.

M t i l S i & E i i


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8.4.5 Solid feeding

At a late stage in freezing it is possible that sections of

the casting may become isolated from feed liquid by

premature solidification of an intervening region.

In this condition the solidification of the isolated region will be

accompanied by the development of high hydrostatic stressin thetrapped liquid: sometimes high enough to cause the surrounding

solidified shell to deform, sucking it inwards by plastic or creep flow.

This inward flow of the solid relieves the internal tension, like any

other feeding mechanism. In analogy with liquid feeding, the author

called it solid feeding or self feeding



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For a solidifying iron sphere of diameter 20mm, the elastic limit at the

inner surface of the shell was reached at an internal stress of about -40 atm.

And by the time the sphere solidify completely, the internal pressure is about

-1000 atm

Figure 7 Plastic zones spreading from isolated volumes of residual liquid in a

casting, showing localized solid feeding in action (Campbell 1969)

When solid feeding starts to operate, the stress in the liquid

becomes limited by the plastic yielding of the solid, and so is a

function of the yield stress and the geometrical shape of the solid.



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Figure 8 Cross-section of 25 mm diameter wax castings injected into a an

aluminum die at various temperature

It is evident that sound casting can, in principle, be

produced without any feeding in the classical sense.

In this case, feeding has been successfullyaccomplished by skillful choice of mould temperature to

facilitate uniform solid feeding.



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8.5 Initiation of shrinkage porosity

Ideal situation:In the absence of gas, and if feeding isadequate, then no porosity will be found in the casting.

In the real world:many castings are sufficiently

complex that one or more regions of the casting are not well fed,with the result that the internal hydrostatic tension will increase,

reaching a level at which an internal pore may form a in a

number of ways.

Conversely, if the internal tension is kept sufficiently low by

effective solid feeding, the mechanisms for internal poreformation are not triggered; the solidification shrinkage appears

on the outside of the casting.



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8.5.1 Internal porosity by surface initiation

Figure 9 Schematic representation of the origin of porosity (a) thin section

If the pressure inside the csting falls, then liquid that is

still connected to the outside surface may be drawn from the

surface, causing the growth of porosity connected to the

surface.

In thin-section castings, the withdrawal of surface liquid is

negligible, that explains why thin sections require little feeding, oreven no apparent feeding, but automatically exhibit good soundness.



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Figure 9 Schematic representation of the origin of porosity (b) intermediate section

In a section of intermediate thickness the experienced caster

will often notice a local frosting of the surface. This dull patch is a

warning that interdendritic liquid is being drained away from thesurface indicating an internal feeding problem that requires attention.



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Figure 9 Schematic representation of the origin of porosity (c) large section



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8.5.2 Internal porosity by nucleation

Short-freezing-range alloys, such as aluminum bronze and

Al-Si eutectic,do not normally exhibit surface-connected

porosity. They form a sound, solid skin at an early stage

of freezing, and liquid feeding continues unhindered

through widely open channels.

In castings of large length to thickness ratio this is

widely referred to as centreline porosity. Thus unless

subsequent machining operations cut into the porosity,

castings in such alloys are normally leak-tight.



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* The nucleation of shrinkage pores


Gas-shrinkage map showing the path of conditions within

the residual liquid in the casting in relation to the nucleationthreshold for pore formation.

For a well fed casting, Ps=0. As

freezing proceeds, the gas

progressively concentrated in theresidue liquid, as showed in figure

along the line ADCE.

At point E, the conditions for

heterogeneous nucleation of a gas

pore on nucleus 1 are met.The initial rapid growth of the gas

bubble will exhaust its surroundings

of excess gas in solution to D.



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If the casting is free fromgas, but is poorly fed. Theinternal pressure in the

casting falls, progressing

along the line AF.

At F the fracture pressurefor nucleation on

heterogeneous nucleus 1 is

met, and a cavity forms.

The hydrostatic tension isexplosively released and

conditions in the casting

shoot back to point A.



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In practice, both gas andshrinkage will be present to

some degree in the averagecasting, and both will cooperate,

causing the conditions to

progress along a curve AB. The

combined gas and shrinkage

pore will form at B on nucleus 1.

On the formation of a mixedpore at B, the pressure in the

liquid immediately reverts to

point C.

Subsequent slower diffusion

of gas into the pore will depletethe immediate surroundings of

the pore, causing the local

environment to progress to D.



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8.5.3 External porosity

If internal porosity is not formed (either by surface-

linked initiation or by nucleation events) then the lowering

of the internal pressure will lead to an inward movement

of the external surface of the casting.

If the movement is severe and localized, then it

constitutes a defect known as a sink or a draw. The

feeding of the internal shrinkage by the inward flow of

solid is solid feeding or self feeding.



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Adequate internal pressurewithin the casting will

reduce or eliminate solid feeding, so maintaining the shape

of the casting and keeping it sound.

In such favorable feeding conditionsneither internal norexternal porosity will occur.



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8.6 Growth of shrinkage pores

For internal pores that are nucleated within a stressed

liquid, the initial growth is extremely fast. The elastic stress

in the liquid and the surrounding solid can be dissipated at

the spread of sound. The tensile failure of a liquid is like the

tensile failureof a strong solid.

Then the subsequent growth of the poreis controlled by

the solidification or the rate of heatextraction by the mould.

For pores that are surface initiated, the initial stress isprobably lower, and the puncture of the surface will occur

relatively slowlyas the surface collapse plastically into the

forming hole.



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Figure 11 stages in the development of a primary shrinkage pipe. Stage (4) is the

appearance of stage (3) on a planar cut section if the central pipe is not exactly

straight.

8.7 Final forms of shrinkage porosity

8.7.1 Shrinkage cavity or pipe

During liquid feeding, the gradual progress of the solidification

front towards the centre of the casting is accompanied by the steady

fall in liquid level in the feeder. These linked advances by the solid

and liquid fronts generate a smooth conical funnel, which is a

shrinkage pipe.



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ate a s Sc e ce & g ee g


In the situation where the shrinkage problem is in an

isolated central region of the casting, a narrow-freezing-

range materialwill give a smooth single cavity. This isoccasionally called a macroporeto distinguish it from

microporosity.

There is no fundamental difference between

microporosity and macropore.

In the case of the single isolated area of macroporosity,

its final location will not be in the thermal centre of the

isolated region.

The shape and the position of the porosity can be

altered by changing the angle of the casting, since the

pore floats.



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g g


Figure 12 Stages in the development of an internal shrinkage cavity.

Stage (4) is again the equivalent cut section to stage

(3). Note that the porosity is not concentrated in the

thermal centre, but is offset from the centre of the trapped

liquid region, outlined by the broken line, by gravity.



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g g


Figure 13 Shrinkage cavity in short-freezing-range alloy

Shrinkage cavity in short-freezing-range alloy as a function or orientation

a, b and c. Porosity shown in d illustrates some other source of porosity (it can

not be a shrinkage type because of its random form, not linked to the casting

geometry).

Note:the long parallel walls of the casting give a

corresponding long tapering extension of the shrinkage

cavity.



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g g


8.7.2 Layer porosity

Alloys of long freezing range are particularly

susceptible to a type of porosity that is observed to

form in layers parallel to the supposed positions ofthe isotherms in the solidifying casting, known as

layer porosity.



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g g


Figure 14 Radiograph of

interdendritic porosity in a

carbon steel (Campbell 1969)

Conditions favorable to theformation of layer porosity

appear to be a wide pasty zone

arising from long freezing range

and/or poor temperaturegradients.

Given these favorable conditions, layer porosity has been observed inpractically all types of casting alloys, including those based on magnesium, copper,

steel, aluminum and high-temperature alloy based on nickel and cobalt.



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g g


* A new explanation of layer porosity --- Campbell (1968)

It avoids the difficulties mentioned above because it

is based not on thermal contraction in the solid as a

driving force, but on the contraction of the liquid on

solidification. The mechanism of formation of this defect is

easily understood. The sequence of events in the

solidifying casting is shown in next figures.



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Figure 15 Schematic representation of the formation of layer porosity (part I)

The stresses in liquid of pasty zone continue to increase with

advancing solidification until the local stress at some point exceedsthe threshold at which a pore will form.

As soon as a pore is created, it will immediately spread along the

isobaric surface, forming a layer and instantly dissipating the local

hydrostatic tension.



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Figure 15 Schematic representation of

the formation of layer porosity (part II)

The new layer-shaped pore

effectively provides a free liquid

surface, adjacent to which no large

stresses can occur in the liquid.

The maximum stress in the liquid at

this stage falls dramatically since

the length of pasty zone has now

approximately halved.

Because of the progressive

decrease of the size of the flow

channels, stress once again

gradually increases with time until

another pore-formation event

occurs as at stage 4.



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Figure 15 Schematic representation of

the formation of layer porosity (part III)

Further nucleation and growth

events produce successive

layers until the whole casting issolidified.

The final state consists of layers

of porosity that have

considerable interlinking.

Although these arguments have

been presented for the case of

porosity being formed only by

the action of solidification

shrinkage, the action of gas and

shrinkage in combination alsocontribute to the formation of

layers.



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Note: consideration of bifilm

It seems likely to suppose that bifilms would interfere with the

flow of residual liquid through the dendrite mesh.

The close spacing of dendrites, providing support for the films,would ensure that they would be capable of resisting large

pressure differences across their surface.

Bifilms transverse to the flow would halt the upstream flow, but

be sucked open by the downstream demand, creating a series of

shrinkage cavities arranged generally transverse to the flowdirection.



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8.7.3 Summary of shrinkage cavity morphologies

Without exception, all the morphologies are dictated by:

(i) the geometry of the casting

(ii) gravity

These two key features allow shrinkage-dominated porosity to beclearly differentiated from other sources of porosity.

If the porosity is clearly not strongly influenced by the shape of the

casting and by gravity (for instance, porosity in random corners, or

well away from the thermal axis of the casting) we can conclude it is

not shrinkage porosity. (Most often it will be bubble damage.)



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(a) Centreline porosity is formed

in a skin-freezing alloy that hassuffered an inadequate supply of

liquid from the feeder. The

geometry dictates that the pore is

closely parallel to the thermal axis

of the casting.

(b) Sponge porosity formed in

a long-freezing-range alloy,

with adequate temperaturegradient, but inadequate feed

liquid from the feeder.



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(c) Layer porosity is the result

of inadequate interdendritic

feeding in a poor temperature

gradient. The nucleation ofinternal porosity indicate a poor

clearness of the liquid metal.

Geometry dictates that the

pores are closely at right

angles to the axis of the

casting.(d) Surface-initiated

porosity generated in a

long-freezing-range alloy

in conditions of poor

temperature gradient, butgood clearness of the

melt.



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(e) Surface sink (external shrinkage porosity) formed in

conditions of no liquid available from the feeder, but good

clearness of melt, resulting in good solid feeding. Notice

gravity dictates that the sink is usually sited on the cope

surface of the casting.



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Assignment

1. Under what conditions the volumes of shrinkage void and porosity initiated in

casting would be large relatively?

2. Analyze the tendency of the initiation of shrinkage void and porosity in gray cast-

iron and spheroidal graphite iron.

3. When hupoeutectic white cast iron (C%=4%) and gray cast iron (Si%2%)crystallizing, what are the values of volume contraction for each other during

solidification? And whats the main cause of the different between both?

4. Repeat the main feeding rules.

5. What the main mechanism of feeding?

6. Summarize the final shrinkage cavity morphologies.

ARTIGO Chapter 8 Solidification Shrinkage

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