solidification

1. CONTENTS
 Introduction
 Solidification
 Driving force for solidification
 Nucleation and growth
 Solidification defects
 Design considerations in casting process
 References

2. INTRODUCTION
Solidification ?
solidification, is a phase transition in which
a liquid turns into a solid when its temperature is
lowered below its freezing point.
Why to study in detail ?
The solidification of metals and their alloys is
important in various industrial process
 Most of the components produced from metals are
by casting process.
 When a metal is welded a small portion of metal
near the weld melts and resolidifies.

3. SOLIDIFICATION
During solidification, the liquid changes in to solid as cooling proceeds.
The energy of liquid is less than that of the solid above the melting point. Hence
liquid is stable above the melting point.
But below the melting point, the energy of liquid becomes more than that of the
solid.
Hence below the melting point, the solid becomes more stable than the liquid.

4.  Thermodynamically, both liquid and solid have equal energy at melting point
and therefore both are equally stable at melting point.
 Freezing is almost always an exothermic process, meaning that as liquid
changes into solid, heat is released.
 This heat must be continually removed from the freezing liquid or the freezing
process will stop.
 The energy released upon freezing is a latent heat and is the entropy part.
 Some under-cooling will be essential for solidification.
 Solidification occurs by two process : nucleation and growth.
SOLIDIFICATION

5. COOLING CURVES
A cooling curve is a graphical plot of the changes in temperature with time for a
material over the entire temperature range through which it cools.
Total heat to be removed for solidification, Q = m ( SH + LH )
= m (Cp (Tm – Tf ) + L )

6. COOLING CURVE WITH UNDERCOOLING

7. DRIVING FORCE FOR SOLIDIFICATION
Solidification is undoubtedly the most important processing route for metals and
alloys
For a pure metal at the fusion temperature Tf , ΔG = 0 so that
ΔGf = ΔHf − Tf ΔSf = 0 ( or ) ΔHf = Tf ΔSf
For any temperature other than Tf ,
ΔG = ΔH − TΔS
~ ΔHf− TΔSf
= ΔSf (Tf− T) = ΔSf . ΔT
_
undercooling
The driving force is therefore proportional to the
undercooling assuming that the latent heat and the
entropy of fusion do not vary much with temperature.

8. NUCLEATION
The first step of metal solidification is the creation of tiny, stable, nuclei in the
liquid metal.
 Cooling the liquid below its equilibrium freezing temperature, or undercooling,
provides the driving force for solidification.
 Once a cluster reaches a critical size, it becomes a stable nucleus and continues to
grow.
The mold walls and any solid particles present in the liquid make nucleation easier.
Cluster of atoms Embryo Nuclei Crystals Grains
r > r’
r < r’ r’ = critical radius

9. NUCLEATION
The volume free energy ΔGV – free energy
difference between the liquid and solid
Δ GV = 4/3πr3ΔGv (- ve)
The surface energy ΔGs – the energy needed
to create a surface for the spherical particles
ΔGs = 4πr2γ (+ ve)
γ → specific surface energy of the particle
Total free energy Change, ΔGT = ΔGV + ΔGs
At low temperatures atoms form small cluster
or groups.
 Embryo’s formed may either form into stable
nuclei or may re-dissolve in the liquid.
 beyond the critical radius of the nuclei it will
remain stable and growth occurs

10. NUCLEATION
Nucleation and formation of grains

11. TYPES OF NUCLEATION
Nucleation is of two types-
 Homogeneous nucleation:
Homogeneous Nucleation – Formation of a critically sized solid from the liquid by
clustering together of a large number of atoms at a high undercooling.
 Heterogeneous Nucleation :
Formation of a critically sized solid from the liquid on an impurity surface.
heterogeneous nucleation occurs in a liquid on the surface of its container,
insoluble impurities and other structural materials that lower the critical free
energy required to form a stable nucleus.
In practice, homogeneous nucleation rarely takes place and heterogeneous
nucleation occurs either on the mould walls or on insoluble impurity particles.

12. TYPES OF NUCLEATION

13. GROWTH
 Once solid nuclei form, growth occurs as atoms are attached to the solid surface.
Growth is the physical process by which a new phase increases in size. In the case
of solidification, this refers to the formation of a stable solid particle as the liquid
freezes.
 Nature of growth of solid depends on how heat is removed from the system.
 Sensible heat and latent heat is to be removed
The manner in which the latent heat is lost determines the growth mechanism
There are 2 growth mechanisms:
1. Planar growth
2. Dendritic growth
The differences in planar and dendritic growth arises
because of the differences in sink for the latent heat.

14. GROWTH
Planar growth :
The heat is dissipated through the crystal, i.e.
the growing crystal is colder than the melt.
Here a solid bulge into the liquid would melt
again because the temperature in the bulge is
above Tm . Therefore, one obtains a stable flat
solidification front
Any small protuberance that begins to grow
on the interface is surrounded by liquid above
the freezing temperature.
The growth of the protuberance stops until
the remainder of the interface catches up with
it.
This planar growth occurs by the movement of
a smooth solid-liquid interface into the liquid.

15. GROWTH
Dendritic growth :
For a strongly undercooled melt the heat of
crystallization can also be dissipated through
the melt.
Under these conditions a small solid
protuberance called a dendrite which forms at
the interface is encouraged to grow.
As the solid dendrites grow the latent heat of
fusion is conducted into the undercooled liquid
Raising the temperature of the liquid
towards freezing temperature.
Dendritic growth continues until the
undercooled liquid warms to the freezing
temperature.
Any remaining liquid then solidifies by planar
growth.
These are thermal dendrites different from
dendrites in alloys

16. GROWTH
Planar growth Dendritic growth

17. SOLIDIFICATION DEFECTS
 Shrinkage :
Most materials contract or shrink during solidification and cooling.
Shrinkage is the result of:
Contraction of the liquid as it cools prior to its solidification
Contraction during phase change from a liquid to solid
Contraction of the solid as it continues to cool to ambient temperature.
Shrinkage can sometimes cause cracking to occur in component as it solidifies.
Gas porosity :
Many metals dissolves a large quantity of gas when they are liquid(Sieverts Law).
However when metal solidify they retain only a small part of the gas. But these
form bubbles trapped in the solid metal producing gas porosity.

18. DESIGN CONSIDERATIONS IN CASTING PROCESS
To minimize the damaging effects of shrinkage RISER is used in casting process.
Riser is a reservoir of molten metal to compensate the shrinkage.
Thus molten metal is continually available from risers to prevent shrinkage voids .
it is desirable for regions of the casting most distant from the liquid metal supply to
freeze first and for solidification to progress from these remote regions toward the
riser.
Directional solidification is obtained by using chills.
Also to minimize shrinkage problems with final cast product an allowance is given
for the size of the mould during pattern design.

19. CONCLUSIONS:
Principles involved in the solidification of metals have been only discussed.
The solidification of alloys are much more complex and involves solute
partitioning studies.
Based on the solidification studies design considerations were implemented in
the casting process.

20. REFERENCES
[1]. Cambridge University e-lectures :Part IB Materials Science & Metallurgy
H. K. D. H. Bhadeshia Course A, Metals and Alloys ,lecture 6:solidification
[2].en.wikipedia.org