MME 345

Lecture 09

Solidification and Crystallisation6. Segregation and gas porosity

Ref:

[1] A. Ohno, The Solidification of Metals, Chijin Shokan Co. Ltd., 1976

[2] J. Campbell, Castings, Butterworth-Heinemann, 2001

Topics to discuss today ….

1. Segregation

2. Gas porosity

1. Segregation

Segregations are compositional differences arising during solidification and

persisting in some cases as a permanent feature of the cast structure.

Since equilibrium and homogeneity require prolonged time at temperatures

high enough for diffusion, segregation persists in the solidified structure.

Tem

pera

ture

Per Cent Solute

CS CL

(a) k0 < 1

Tem

pera

ture

Per Cent Solute

CL CS

(a) k0 > 1

part of equilibrium diagram for solid solution system.

(a) Freezing point lowered by solute, (b) freezing

point raised by solute.

The inherent segregation

tendency in an alloy can

be represented by the

equilibrium distribution

coefficient, k0, where

k0 = CS / CL

For k0>1 or, for k0<1,

segregation of second

phases would result.3/24

Constitutional factors which produce a strong segregation tendency are:

1. Long freezing range

2. Gentle liquidus slope

3. Low solid solubility

Besides k0, other solidification conditions determining the degree

and pattern of segregation are:

1. Freezing rate

2. The mode of development of grain structure

3. Motion of crystal and residual liquid

4/24

1.1 Classification of segregations

To a large extent the compositional variations that occur adjacent to

the solid-liquid interface during solidification determine the nature and

extent of segregation.

In general, segregation occurs in two general types of distribution:

micro-segregation and macro-segregation.

Micro-segregation Macro-segregation

1. Cellular segregation

2. Dendritic segregation

3. Granular segregation

1. Occurring prior to solidification(a) Gravity segregation

2. Occurring during solidification(a) Normal segregation

(b) Inverse segregation

(c) Negative segregation

(d) Freckle defects

(e) Banding segregations

5/24

Micro-segregation

Resulted due to non-equilibrium solidification and controlled primarily by diffusion

in the liquid phase

the scale of micro-segregation is controlled by D/R, where D = diffusion coefficient

(cm2/s) and R = growth velocity (cm/s)

Micro-segregation is a short-range phenomenon and extends over dimensions of

the order of a single grain or less (~ 10 – 100 micron).

Mechanical properties of castings are sensitive to micro-segregation.

Can usually be removed by homogenisation treatment.

Example: cellular, dendritic, and grain boundary segregation.

6/24

Macro-segregation

Macro-segregation refers to zonal segregation of solute in different parts of casting

occurring over a long distance ranging from 1 cm to 1 m and, therefore, can not be

removed.

Result either from changes that occur in the liquid before the solidification front has

proceeded or as the result of fluid motion in the mushy zone behind the solidification

front

Thus, the processes leading to zonal segregation may include:

1. diffusion of impurities from two phase zone of solidifying casting within the still

liquid alloy,

2. convection of alloy streams in the liquid portion of the casting,

3. floating of low density impurity, and

4. action of centrifugal forces.

Example: normal or positive segregation (or, coring), V-segregation, inverse

V-segregation, negative segregation, surface segregation.

7/24

Normal / Positive Segregation

Castings contain high concentrations of low-melting

solute at the central portion of ingot due to solute

rejection at the interface and accumulation at the centre.

This inhomogeneous structure is often called coring

Freezing of quiescent liquid:

During initial stages, solids form with a composition

k0C0, and solute builds up ahead of the front in the

liquid.

Subsequent freezing to solid of composition C0

takes place in a steady, continuous fashion.

In the final stages, segregation of solute in both solid

and liquid increases.

distribution of solute during solidification when there is no diffusion in solid and when solute moves only by diffusion in the liquid

8/24

Freezing of turbulent liquid:

Solid continues to freeze at its original

low concentration k0C0.

Any solute build up at the interface is

sweeped away by the moving liquid.

In the final stages, slow rise in

concentration of solute in both solid

and liquid occurs due to the bulk liquid

becoming progressively more

concentrated.

Example of Normal segregation:

Formation of sub-surface porosity in steel casting due to segregation of O (k0 = 0.05),

H, N and C (k0 = 0.25), resulting rimming action in steel.

distribution of solute during solidification when

there is no diffusion in solid and when there is

complete mixing in the liquid

9/24

Dendritic Segregation

It is a type of normal segregation or coring,

which is resulted between the centre and

outside of a dendrite arm during solidification

Dendritic segregation arising as a result of the

combined actions of solute rejection and shrinkage

during solidification in a temperature gradient

Alloys having wider freezing range are

susceptible to this type of segregation.

Dendritic segregation is more severe

1. across and between primary dendritic arms

than secondary/tertiary arms, and

2. low-alloy contents rather than high-alloy

contents

10/24

Crystals having a narrow necked shape roots are formed before the formation of a

stable solid skin. When crystals come in contact with the adjacent crystal, the liquid of

high low-melting solute concentration is trapped at the crystal root, and form inverse

segregation.

Inverse segregation can be reduced by:

1. Forming fine grained equiaxed crystals (no large dendrites are formed).

2. Rapid cooling (a stable solid skin is formed immediately).

Inverse Segregation

Examples: Cu-Sn alloys

contraction of casting or evolution

of dissolved gases causes almost

near-eutectic liquid to be forced

out on to the surface of the

casting. This phenomenon is

called “tin sweat”

11/24

Gravity Segregation

Produced due to the differences in densities of solid and liquid phases, or of

two non-mixing liquids, which lead to differential movements within the liquid.

Example:

1. Sb-Sn alloys – the first crystals are Sb-rich, which floats on top of the casting

2. Cu-Pb alloys – Cu-rich crystals remain on top of Pb-rich crystals

12/24

formation of string-type segregation:

(a) remaining liquid trapped between crystals,

(b) crystal growth and concentration of solute in the

remaining liquid, and (c) final stage or segregation

Different types of channel or gravity segregations can be controlled by:

1. Decreasing time available for their formation

by increasing rate of solidification.

2. Adjusting chemical composition of the alloy to

give a solute-rich liquid which has a more

nearly neutral buoyancy at the temperature

with the freezing zone.

In tool steels (containing W, Mo) and nickel-base

alloys (containing Ni, W, Hf, Mo, etc.), the highly

dense segregated liquid sinks downwards,

creating channel segregates which flow in the

opposite direction to those in conventional

steels. This type of segregation is called “freckle

defect.”

13/24

Development of segregation in a killed steel ingot

14/24

In steel casting, residual liquid containing light element (C, S, P) rises upwards

and tends to dissolve dendrites in its path. As the stream progresses, it

reinforces its channel, as a flooding river carves obstructions from its path.

The “A” segregates (or, inverse V segregates) are formed in this way.

The separated or fragmented purer crystals carried away by the upward stream to

the centre of ingot grows as they fall at the base of ingot. The coned shaped heap

of higher melting solutes forms the region of negative segregation.

The “V” segregations are found in the centre of the ingot. They are formed at the late

stage of freezing of ingot when the segregated pool of liquid in the top is being drawn

downwards to feed the solidification shrinkage in the central lower parts of the ingot.

15/24

r* = critical radius of gas pore

g = surface tension

DP = pressure difference

Homogeneous nucleation of gas pore

r* = – 2g/DP*

DG = gA + PeV – PiV

Energy required to nucleate gas bubble inside liquid metal

LiquidSurface tension,N/m

Atomic diameter,mm

DP* from equation,atm

DP* from experiment,atm

DP* forcomplex inclusion, atm

Water 0.072 - - 1320 16

Mercury 0.5 0.30 16700 22300 200

Aluminium 0.9 0.29 31000 30000 360

Copper 1.3 0.26 50000 50000 600

Iron 1.9 0.25 76000 70000 850

It is clear that, the pressure

required for nucleation are

extremely high, and reflect

the apparently real difficulty

of homogeneous nucleation

of pores in liquid metals.

= 4pr2 g + (4/3)pr3 (Pe – Pi)

2.1 Nucleation of gas pore

2. Gas Porosity

16/24

The problem of nucleation is reduced by the presence of surface-active

non-metallic impurities (O, S, P) in iron.

Presence of only 0.2 % oxygen reduces the value of surface tension from

1.9 to 1.0 N/m.

Partition coeff. of oxygen is 0.05. This means that if the liquid contains only

about 0.01 % O, due to segregation, the bulk liquid can have about 0.2 % O.

If the oxygen content is sufficiently high to produce FeO, the value of surface

tension is reduced further to about 0.6 N/m.

So to nucleate a gas bubble a pressure of only about 17000 atm is required in

presence of FeO

17/24

q = contact angle

Heterogeneous nucleation of gas pore

Heterogeneous nucleation of gas bubble on the surface of solid substrate

liquid metal

gas bubble

solid substrate

Nucleation is easier by a factor

DP*het / DP*hom = 1.122 + 3cosq - cos3q

4

1/2

18/24

For complete wetting

q = 0°, cos q = 1

DP*het = 1.12 DP*hom

For complete non-wetting

q = 180°, cos q = -1

DP*het = 0

Good nuclei for pores must be

non-wetted by the liquid

Example: most oxide inclusions

We must remind ourselves that solids wetted by liquid (borides, carbides,

nitrides, etc.) are FAVOURABLE for nucleating solid particles !!

DP*het / DP*hom = 1.122 + 3cosq - cos3q

4

1/2

q0o

q180o

nucleation on solid surfaces

does not become favourable

until the contact angle

exceeds 60 or 70 degrees.

19/24

This is especially troublesome for the casting technologist because

many commercially important alloys contain highly oxidisable

components such as aluminium and titanium.

Alloys difficult to cast without porosity:

high-temperature nickel- and cobalt-based alloys

aluminium- and magnesium-based alloys

aluminium bronzes, and ductile cast iron due to their magnesium content

All surface oxides give problems when they are entrained inside liquid

metal by surface turbulence

the oxides retain air as a surface film and, even without the entrained air,

decohere easily from the melt, and thus nucleate porosity readily

20/24

Controlled by the rate of diffusion of

gases through the liquid metal

the rate of growth is dominated by the

rate of arrival of the fastest diffusing gas

diffusion coefficients for elements

in aluminium and copper

2.2 Growth of gas porosity

Hydrogen has a diffusion coefficient

almost 10 times higher than that of any

other element in solution

Diffusion distance, d (Dt) 1/2

Radius of H bubble 101/2 3 times greater

Volume of H bubble 33 30 times greater

this is why [H] has the dominant influence

over the growth of gas pores

Remember:

[O] is the dominant element for

nucleating a gas pore !!21/24

The final amount of gas pore can be determined by (1) the time available for gas to diffuse

into the pore and (2) the well-known gas law:

PV = nRT

Final volume of gas porosity (V) is

1. directly proportional to the amount of gas (n)

present in the liquid and the temperature (T)

of the liquid, and

2. inversely proportional to the liquid pressure (P)

3. directly proportional to the time available for gas

to diffuse into the pore.

gas porosity at various percentage

levels in sectioned samples from the

reduced pressure test

Reduced pressure test (RPT)

• Volume of porosity is inversely proportional to the pressure

applied to it during its growth.

• For instance, the percentage porosity is commonly expanded

by a factor of 10 by freezing at 0.1 atm (76 mmHg) residual

pressure rather than at normal atmospheric pressure (760

mmHg).

• For the most sensitive tests, a reduced pressure of 2-4 mmHg

is recommended. 22/24

Methods of control of gas porosity:

1. Reduce moisture content in charge, furnace lining, tools, etc.

2. Melt and pour at as low a temperature as possible

3. Use adequate metal head pressure to suppress nucleation of gas pore

4. Pour the liquid metal inside the mould at a low pressure (to readily form

and eliminate the gas bubbles), but raise the pressure during solidification

(to suppress the formation of gas bubble).

2.3 Control of gas porosity

23/24

Next ClassMME 345, Lecture 10

The Feeding Design1. Necessity and requirements of feeding