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6/4/2010

Bangladesh University of Engineering and Technology g y g g gy

DMME

MME 6203 Advanced Topics in Foundry Engineering

Lecture 4 Lecture 4

Casting Defects

2. Solidification Shrinkage

A.K.M.B. Rashid Professor, Department of MME BUET, Dhaka

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Today's Topics

General shrinkage behaviour Six rules of feeding Feeding mechanisms Initiation of shrinkage porosity Growth of shrinkage porosity Final form of shrinkage porosity

© B. Rashid, DMME, BUET

Lec #4: Casting defects ­ solidification shrinkage

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General Shrinkage Behaviour

Liquid contracts on freezing because of rearrangement of atoms from open "randomly p y packed" structure to a regular "denselypacked" structure.

FCC and HCP solids contract more during solidification.

What happens to a poorly fed casting?

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A sphere has been fed via an ingate of negligible size and the source of feed metal is cut off after a solid shell of thickness X is produced. Feed Liquid R0 R Liquid Solid X dx

What will happen during solidification of the next onionlayer of thickness dx ? Either a pore will form or the liquid will expand a little to compensate the volume difference. If no favourable nucleus available for pore formation, the liquid has to accommodate this by expansion, accommodate this by expansion creating a state of tension or negative pressure and sucking the solid shell inwards.

Solidification model for an unfed sphere

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Lec #4: Casting defects ­ solidification shrinkage

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A sphere has been fed via an ingate of negligible size and the source of feed metal is cut off after a solid shell of thickness X is produced. Feed Liquid R0 R Liquid Solid X dx

What will happen during solidification of the next onionlayer of thickness dx ? As more onion layers form, the tension in the liquid increases, the liquid expands, and the solid shell is drawn inwards. The liquid metal or the pasty liquid zone can accommodate a little of this pressure. this pressure Eventually, the pressure becomes large enough for the casting to collapse by plastic or creep flow and form shrinkage cavity.

Solidification model for an unfed sphere

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Whether the driving force for pore formation wins over the driving force for feeding will depend on whether nuclei for pore formation exist. If not (i.e. the metal is clean), then pore will not be able to nucleate and feeding is forced to continue until the casting is completely frozen. If favourable nuclei are present, then pores will be created at an early stage before the development of any significant hydrostatic pressure, with the result that little feeding will occur and the casting will develop its full percentage of porosity as defined by the physics of phase change.

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In most practical cases, the situation is somewhere between these two extremes, with castings displaying some internal porosity, together with some liquid some internal porosity together with some liquid feeding. In such cases feeding has continued under increasing pressure differences, until the development of a critical internal stress at which some particular nuclei, or surface puncture, can be activated at one or more p points in the casting. Feeding is then stopped at such g g pp locality, and pore growth starts.

© B. Rashid, DMME, BUET

Lec #4: Casting defects ­ solidification shrinkage

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When pores appear early

freezing contraction Steel Aluminium Al i i 3 vol.% 7 vol.% l% diameter of shrinkage cavity formed in 100 mm dia sphere casting 31 mm 41 mm

Theses considerable cavities require a dedicated effort to ensure that they do not appear in castings.

There are occasions when castings having defects of only 1 or 2 mm in size are scrapped!!

For vast majority of cast materials, therefore, shrinkage porosity j i f i l h f hi k i is the most common and most important defect in castings. Reducing/removing shrinkage porosity: 1. Clean melt 2. Effective feeding

Feeding ­ The Six Rules

1. 2. 3. 4. 5. 6.

Heat transfer requirement H t t f i t Volume requirement Junction requirement Feed path requirement Pressure differential requirement Pressure requirement

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Lec #4: Casting defects ­ solidification shrinkage

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Feeding Mechanisms

The gradual formation and growth of dendritic mass of solids during solidification presents increasing difficulties for the passage of feeding liquids. f h f f di li id During solidification, the pressure inside liquid also falls, causing and increased pressure difference between the inside and outside of the casting. Such negative pressure difference is undesirable in casting because it causes problems by providing the driving i b i bl b idi h d i i force for the initiation and growth of volume defects such as porosity.

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There appears to be at least five mechanisms by which such pressure difference (and the hydrostatic tension caused by it) can be reduced in solidifying material.

Schematic representation of the five feeding mechanisms in a solidifying casting

1. 2. 3. 4. 5.

Liquid feeding Mass feeding Interdendritic feeding Burst feeding Solid feeding

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Liquid feeding

Generally precedes other forms of feeding. For skinfreezing materials, this is the only method of feeding. Occurs at the early stage of solidification Wide feed path due to low liquid viscosity, and the pressure difference required to continue feeding is negligibly small (~ 1 Pa). When about 99 % solidification is completed, the pressure could reach up to about 100 Pa only (1 atm 105 Pa). p y( )

For all practical purposes, therefore, the hydrostatic stresses created during liquid feeding never causes a problem!!

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Liquid feeding

Inadequate feeding only resulted when inadequatesized feeder is used. Feeding terminates early and air is drawn into the casting. Two forms of porosity resulted: Skinfreezing alloys smooth shrinkage pipe, extending from the feeder into the casting as a long funnelshaped hole. Longfreezingrange alloys feeding occurs through interdendritic channels porosity resembles a mass of spongy, interconnecting shrinkage pipes.

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Mass feeding

Movement of slurry of solidified metal and residual liquid. This movement is arrested when the volume fraction of solid reaches anywhere between 0 and 50 % depending on Pressure differential that driving the liquid Amount of free dendrites in the liquid Role of mass feeding is of minor importance since the critical stages of feeding which most influence defects occur later after mass feeding comes to a stop. occur later after mass feeding comes to a stop Mass feeding period can be extended by grain refinement and formation of more equiaxed grains.

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Interdendritic feeding

Feeding of residual liquid through mushy zone. The pressure gradient required for interdendritic feeding of a cylindrical area of pasty zone.

2 2 2 L d P = 32 1- R 4D2

= viscosity of liquid d = dendrite arm spacing = solidification shrinkage R = radius of capillary L = length of past zone D = diam. of pasty zone = heat-flow constant (rate of freezing)

The pressure difference is the most sensitive to the size of flow p channel, R. P becomes extremely high as R becomes small. In the absence of suitable nuclei for pore formation, the high hydrostatic pressure is somewhat compensated by the inward collapse of the solid.

© B. Rashid, DMME, BUET Lec #4: Casting defects ­ solidification shrinkage Page 16 of 32

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Interdendritic feeding

Effect of the presence of eutectic Eutectics (shortfreezing range alloys) solidify in the ll ) lidif i h planer mode. In presence of eutectic, the interdendritic flow paths do not taper to zero, but finish abruptly trimmed.

Although most long-freezing-range alloys exhibit poor pressure tightness, why are 85Cu-5Sn-5Zn-5Pb alloys, being an extremely long-freezing-range alloy used for valves and pipe fittings applications?

Lead is practically insoluble in this alloy and freezes as pure lead at 326 C, and considerably eases freezing problem.

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Burst feeding

As solidification progresses, both hydrostatic stress inside liquid and strength of feeding barrier increase in a poorly fed region of casting, but at different rates. If stress grows at a faster rate, failure of casting is expected. If stress grows at a faster rate failure of casting is expected If the barrier is only a partial barrier, failure may not occur. Instead, feeding occurs in a burst when a sudden yield of feeding barrier occurs due to hydrostatic tension. g The internal stress will be reduced to allow the casting to remain free from shrinkage porosity. If the barrier is substantial, it may never burst, causing the resulting stress to rise and eventually exceeds the pore nucleation threshold. The stress is then released by forming the shrinkage cavity.

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Solid feeding

At the later stage of solidification, certain sections of casting may become isolated from feed liquid. Further solidification in this isolated region would cause a high hydrostatic stress in the remaining liquid, high enough to cause h d i i h i i li id hi h h the surrounding solidified shell to deform inwards by plastic or creep flow.

P = 2 Y ln (R0/R)

Y = Yield stress of solid R0 = Radius of spherical casting R = Internal liquid radius

Stress in liquid developed depends upon the plastic flow of solid that, in turn, is a function of yield stress and geometry of casting. For iron sphere of 20 mm in diameter, P can be reached up to 200 to 400 atm when the casting is 99.998 % solid. In the last liquid drops, this can reach up to 1000 atm !

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Initiation of Shrinkage Porosity

In absence of gas, and if feeding is adequate, no porosity will be formed. For large and/or complex castings, one or more regions of casting are not well fed and the liquid contains dissolved gasses. The internal hydrostatic stress reaches to a level when internal pore can form. If solid feeding occurs, internal pore will not occur, but the solidification shrinkage will appear at the surface of the casting.

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Lec #4: Casting defects ­ solidification shrinkage

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Internal porosity by surface initiation

If the liquid is connected to the outside surface, then the liquid can be sucked in, causing the porosity to form liquid can be sucked in causing the porosity to form connected to the surface. The sucking of liquid from surface also draws air which flows along the interdendritic channel causing air feeding (as opposed to liquid feeding) into the casting. The porosity formed in this way is indistinguishable from microporosity. microporosity

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Lec #4: Casting defects ­ solidification shrinkage

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Internal porosity by surface initiation

In thinsection castings, little or no feeding is necessary and sucking of surface liquid is negligible. For intermediate thickness, surface initiated pores can occur because of interdendritic feeding problems. This pore-forming mechanism is common in long freezing range alloys at a later stage of solidification, initiated often from a hot spot . Sometimes these pores can connect iinternally t opposite surfaces of thi t ll two it f f thin casting causing these alloy castings unsuitable for pressure-tight applications. A high enough positive internal pressure is necessary at all locations to prevent initiation of this type of surface-connected internal porosity.

Internal porosity by nucleation

Shortfreezing range alloys do not exhibit surface connected porosity. A sound, solid skin is formed at the early stage of solidification. Feeding is not a problem at this stage. hi At the end of freezing, pores are nucleated in the interior of liquid due to poor feeding which have no connection with the outside surface of casting. After nucleation, further solidification results growth of these pores. These "centreline porosity" are concentrated near the centre of the casting and do not d h f h i d d impair the leaktightness of the casting.

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Lec #4: Casting defects ­ solidification shrinkage

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Internal porosity by nucleation

In presence of surfaceactivated foreign particles, nucleating of these pores is not a problem. In absence of such particles, nucleation only occurs when the internal pressure accumulates and reaches to a certain threshold value, Pf , called the fracture pressure. The gas pressure, Pg , inside the liquid will join the negative pressure, Ps , to push the liquid away for the nucleation of the pore. Pf = Ps + Pg 2/r = Pi ­ Pe = P 2/r* = Pg ­ (-Ps) = Pf

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(condition for pore formation) (for pore of critical size)

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Internal porosity by nucleation

For well-fed castings with dissolved gases, Ps = 0. Freezing will proceed along the line ADCE. g y Gas pore will form heterogeneously at E on nucleus 1. Gas pressure goes back towards D. For poorly fed casting having no dissolved gases, Pg = 0. Internal pressure falls along the line AF. At F, fracture pressure for heterogeneous nucleation on nucleus 1 is met, and a shrinkage cavity forms. The hydrostatic tension is released and pressure returns to A. In practice, both gas and shrinkage will be present to some degree, and the freezing will progressed along the line ABCD. Both porosity and cavity will form.

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External porosity

If internal porosity is not formed, then the surface will "sink" due to solid feeding sink due to solid feeding. Adequate positive internal pressure would reduce or eliminate solid feeding and the casting would be sound and maintain its shape. Too high an internal pressure would reverse the movement of the surface and make the casting "swell". Examples: GCI, castings having high head of metal.

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Growth of Shrinkage Porosity

The internal pores nucleated within a stressed liquid grow explosively fast at the beginning. The subsequent growth of pore occurs leisurely and is controlled by the rate of solidification i.e., the rate of heat extraction. For surfaceinitiated pores, the rate is slow since the initial stress is lower and the puncture of surface will occur relatively slowly as the surface collapses plastically into the forming hole. into the forming hole When pore formed as shrinkage pipe, the growth is progressive and controlled by the rate of heat extraction from the casting.

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Final Form of Shrinkage Porosity

Shrinkage cavity or pipe

During liquid feeding, the solidification front progresses gradually towards the centre of the casting. The liquid level in the g feeder falls gradually, thus generating a smooth conical funnel shaped shrinkage pipe. The secondary shrinkage cavities are only an extension of the primary pipe. When shrinkage problem occurs in an isolated region inside the casting: For short-freezing-range alloys, shrinkage pipe would occur. The shape of this pipe is similar to those occur in case of liquid feeding. For long-freezing range alloys, layer porosity is formed in an isolated region inside the casting.

Layer porosity

Usually observed in all types of casting alloys. Conditions favourable for nucleation of layer porosity:

long pasty zone poor temperature gradient alloys with high thermal conductivity moulds with low rate of heat extraction

Initially layer porosity is believed to be caused by the same mechanism that causes hot tear. Presently, its formation can be explain by the equations that g governs the interdendritic feeding mechanism: g

P = 16 dR 1 N 1 - dt R

3 2 Lx - x 2

2 2 2 L d P = 32 4 2 1- R D

The hydrostatic tension increases parabolically with distance x though the pasty zone of length L.

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Layer porosity

The stress continues to increase parabolically with advancing solidification. Local stress exceeds the threshold value and a pore will form. The pore will spread immediately along the isobaric surface and form a layer. Local stress will dissipate instantly. The maximum stress will be at the centre of the remaining liquid and amounting about 1/4th of the original. Stress once again will increase with time and second llayer of porosity will form. d f it ill f Further nucleation and growth events will produce successive layers until the whole casting is solidified. The final state consists of layers of porosity with considerable interlinking.

Layer porosity

So it is clear that centreline porosity, layer porosity, and dispersed porosity transform imperceptibly from one to the other. the other Also, as the gas content of the alloy is increased, the shrinkage porosity changed gradually from layer porosity to dispersed pinhole porosity. In real castings, the nature of porosity is mixed in nature, allowing a complete spectrum of possibilities from pure shrinkage layer type to pure gasdispersed type.

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