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8/13/2019 ARTIGO Chapter 8 Solidification Shrinkage
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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%BA8/13/2019 ARTIGO Chapter 8 Solidification Shrinkage
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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.
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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.
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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.
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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
Figure 10 Gas-shrinkage map
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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Figure 10 Gas-shrinkage map
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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Figure 10 Gas-shrinkage map
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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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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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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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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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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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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* 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.