1. Sal College of Engineering
Class :- Mechanical
Subject:- MSM ( )
Topic:- Solidification of metals & alloys
Prepared by:
1. Malik Varma
2. Jaymin vasoya
3. Priyansh Shah
4. Pooja patel
5. Dhairya Patel
6. Raj Shah
7. Krishna Sheth
8. Chetan Patel
Submetted to :- Asst Proff Divyesh Jaisar

2. Introduction Solidification :-
The transformation of metals/alloys from the liquid state to the solid crystalline state is called Solidification or Crystallization.

3. Conditions for crystallization to occur
Solidification proceeds under the conditions in which system is converted to a more stable thermodynamic state with lesser free energy, or thermodynamic potential E; i.e. when the free energy of the crystal is less than that of the liquid phase.
Free energy or thermodynamic potential is the portion of the total energy that is reversibly changed upon changes in temperature or is available as work.
If H = Total energy of the system,
T = Absolute temperature,
S = Entropy,
Then, for a small change in volume during the transformation, E = H – TS.

4. The variation of free energy of liquid and solid states with respect to the temperature is shown in diagram

5. Cooling curves , showing the solidification of pure metals at various cooling rates are shown in diagram.
At very low rates (γ1), the degree of supercooling is small (ΔT1) & solidification proceeds at a temperature near to equilibrium one.

6. FORMATION OF NUCLIE & GRAIN STRUCTURE
The Process of Solidification begins with the formation of crystalline nuclei of Embryos so called “ Centre of -Crystallization”, & proceeds with their Growth.
The stable crystalline nuclei , capable to Grow further , appear in entire liquid Volume of a metal when it is supper - Cooled below Ts, such crystallization Nuclei are said to be “Crystal”.
As a result of this process , the growing Crystals , having a regular geometric shape Initially ,take on an irregular shape after Solidifying . They are called “Crystallites” Or “grains” with irregular grain boundaries.
The structure obtained in metals or alloy By such a process of crystallization is Known as “ grain structure”.

8. The Formation Of Stable Nuclie in Liquid Metals
The two main satges of solidification that occurs for metals or alloys are +
(1)Nucleation & (2) Growth.
Two main mechanisms of nucleation of solid particles in liquid metal.
(1)Homogeneous nucleation & (2) Hetrogeneous nucleation

9. Homogeneous Nucleation
This is consider as it is the simplest case of nucleation Homogeneous nucleation in liquid metal occurs when the metal itself provides the atoms to form nuclei.
Considering the case of pure metal solidifying under the given condition, when a pure liquid metal is cooled below its equilibrium freezing (solidification) temperature Ts to a sufficient degree ∆T
A number of homogeneous nuclei are created by slow moving atoms bonding together.

10. Energies involved in Homogeneous Nucleation :
The Gibbs frees energy ∆G is a direct measure of the tendency for the homogeneous nucleation to occur.
The energies, which make up this gibbs free energy & are responsible for the solidification process, are of two types.
(1)Volume (Bulk) Free energy &
(2)Surface free energy.
Volume free energy:
When a pure liquid metal is cooled below its equilibrium solidification temperature, the driving energy for the liquid to solid transformation is the difference in the volume free energy ∆Gv of the liquid and that of the solid.

11. Surface Free energy
There exists an opposing energy to the formation of embryos and nuclei ; i.e the energy required to form the surface of these particles
The energy needed to create a surface for this spherical particles ∆Gs, is equal to 4γ , where γ Is specific energy of the particle and 4 is the surface area of a spherical particle.

12. If ∆Gv is the change in the free energy between the solid and liquid for unit volume of metal, then free energy change for a spherical nucleus of radius r is π ∆Gv because the volume of sphere is π.

13. ** Total Energy of nucleation :
The total free energy associated with the formation of an embryo or nucleus, which is the sum of the volume free energy and surface free energy changes.
An equation form of the total free energy change for the formation of a spherical embryo or nucleus of radius r formed during freezing of pure metal is,
∆Gt = π ∆Gv + 4γ
Where,
∆Gt = Total free energy change
r = Radius of embryo or nucleus
∆Gt = Volume free energy change
γ = Specific surface free energy

14. In case of pure metal if the solid particles being formed have radii less than the critical radius r*, the energy of the system will be lowered if they redissolve.
Thus, small embryos can redissolve in the liquid metal.
If the solid particles have radii grater than r*, the energy of the system will be lowered when these [articles (nuclei) grow into larger particles or crystals .
When radius ‘r’ reaches the critical radius r*, has its maximum value of ∆G*.
Thus, any solid partical of at least the critical size (r*) can grow into crystal due to lowering of its energy state by its growth.

15. ** Critical radius v/s Super cooling:
The grater the degree super cooling ∆T below the equilibrium melting temperature of the metal, the grater the change in volume free energy ∆Gv, but the change in the free energy due to surface energy ∆Gs does not change much with temperature.
Thus, the critical nucleus size is determined mainly by ∆Gv. Near the freezing temperature , the critical nucleus size must be infinite since ∆T approaches zero.
As the amount of super cooling increases , the critical nucleus size decreases.
The critical- sized nucleus is related to the amount of super cooling by the following relation:

16. r* = 2 γ Tm / ∆ Hf ∆T
Where,
r* = Critical radius of the nucleus
γ = Specific surface free energy
∆ Hf = Latent heat of fusion
∆T = Amount of supercooling at which nucleus is formed
Tm = Temperature of melting or freezing
This type of structure is desirable as it gives strength to a metal/ alloy being solidified.

17. Heterogeneous nucleation
It is the nucleation that occurs in a liquid on the surfaces of its container, insoluble impurities or other structural material which lower the critical free energy required to form a stable nucleus.
During industrial casting operation large amount of undercooling is not obtained, hence nucleation must be heterogeneous and not homogeneous one because heterogeneous nucleation requires small amount of total free energy.
For heterogeneous nucleation to take place, the solid nucleating agent must be wetted by the liquid metal. These agents are also called “PREFERRED SITES” for this type of nucleation.

18. Also, the liquid solidify easily on the nucleating agent
Also, the liquid solidify easily on the nucleating agent. For these to occur, a low contact angle between the solid metal and the nucleating agent must be formed.
Heterogeneous nucleation take place on the nucleating agent because surface energy is lower to form a stable nucleus resulting into lower total free energy change and smaller critical size nucleus.

19. Inoculation
It is mostly desired that the metal or alloy to be used in application where strength is the major criterion, must have fine grain structures. Such a structure can be obtained by adding certain impurities or admixtures to the melt during solidification, the process being known as “INOCULATION”.
When added to a liquid metal or alloy in very small amounts, these inoculants do not change the chemical composition of the metal or alloy.
But, they are capable of producing fine grain structures, which in turn improve the mechanical properties of the metals or alloys, e.g. in case of magnesium alloys, the grain size is reduced from 0.2 or 0.3 mm to 0.01 or 0.02 mm.

20. Growth of Nucleus
Once a stable nucleus is formed as discussed above, the second phase of solidification, called growth starts.
In general, growth may be defined as increase in the size of previously formed nucleus. The previously formed nucleus is mostly two dimensional in nature, which then grow by addition of new atoms diffusing towards it & getting attached to the same by strong metallic bonding layer by layer.
Once, these newly added atoms complete this second layer, another layer of atoms is laid on this two dimensional nucleus formed in similar fashion, and this process continues in that growth direction till the liquid is available & assessable.

21. during the growth phase of solidification the atoms get transferred to the locations required for bonding by diffusion which follows the Arrhenius law.
Rate= C * exp(-Q/RT)
Where,
C= Constant for the process of diffusion
Q= activation energy required for the diffusion to occur, J/mol
R= gas constant= 8.314, J/mol-K
T= absolute temperature at which diffusion takes place,
This rate decides the rate of growth.
Thus, in general, both rates of nucleation (RN) & growth (RG) depends upon the degree of supercooling.

22. Types /Manners of Growth for Metals & Alloys:
Once solid nuclei have formed ,growth occurs as atoms get attached to the solid surface formed.
In pure metals , the nature of growth of solid during solidification depends on how heat is removed from the solid-liquid system.

23. Planner growth:
Considering a well-inoculated liquid (metal),cooling slowly, under equilibrium condition.
The temperature of the liquid metal is greater than the freezing temperature and the temperature of the solid formed is at or below the freezing temperature.
The latent heat of fusion must be removed by conduction from solid-liquid interface through the solid to the surrounding for solidification to continue.
Any small protuberance which begins to grow on the interface is surrounded by liquid metal above the freezing temperature.

24. Temperature profiles in solid , liquid & at interface required for planar growth *
The growth of the protuberance then stops until the reminder of the interface catches up. This growth mechanism, known as planar growth, occurs by the movement of a smooth solid-liquid interface into the liquid.

25. Dendritic Growth:
When nucleaion is poor , the liquid undercools to a temperature below the freezing temperature before the solid forms.
Under this conditions, a small solid protuberance called a dendrite, which forms at the interface is encouraged to grow.

26. These grown dendrites called primary arms
These grown dendrites called primary arms. The secondary and tertiary dendrites arms can also form on the primary arms to speed up the evolution of the latent heat shows the etched microstructure of an aluminum alloy showing dendritic structure.
Dnedritic growth continues until the undercooled liquid warms to the freezing temperature.

27. Solidification Time
The rate which growth of the solids occurs during solidification depends on the cooling rate, or rate of heat extraction.
Almost always , a shorter solidification time produces a finer grain size & a stronger casting.
The solidification time also affects the size of the dendrites that grow. Normally dendrite size is characterized by measuring the distance between the secondary dendrite arms.

28. Solidification of an Ingot/Casting
Molten metals are poured into moulds & permitted to solidify to obtain an ingot or a casting.
When the mold produces a finished shape (part/component) it is called ‘a casting’. A casting usually can be put to service with minimum or no machining.
In other cases, the mold may produce a simple shape which is called “ an ingot”. The ingot requires a very high plastic deformation or machining to be converted into a finished product.
In both, a casting or an ingot, the macrostructure which is produced upon solidification can contain three major zones/parts. These three zones are shown in figure as per the solidification and explained next.

30. Zones
**Chill Zone :-
This zone is a narrow band (portion) of randomly oriented grains at the surface of the casting near the mold walls.
The metal at the mold walls cools first to or below the freezing temperature as it comes in direct contact with them first.
The mold wall also provides many surface at which heterogeneous nucleation may occur. Therefore, a large number of grains begin to nucleate & grow.

31. ** Columnar zone
The columnar zone contains elongated grains oriented in a particular crystallographic direction.
As the heat is removed from the casting by the mold material, the grains in the chill zone begin to grow in the direction opposite to the heay flow, or from the coldest towards the hottest areas of the casting.
Usually, it means that grains grow in a direction perpendicular to mold walls.
The grains in the columnar zone have < > directions parallel to one another, giving the anisotropic properties to this zone.

32. ** Central Equiaxed Zone :-
In most cases, a pure metal continues to grow in a columnar manner until all of the liquid has solidified.
But in alloys & in special circumstances in pure metals, an equiaxed zone is formed in the center of the casting or ingot.
The equiaxed zone contain new, randomly oriented grains in the center of the casting or ingot.
They are produced mainly due to
(1) Low pouring temperature
(2) Presence of alloying elements
(3) Grain refining
(4) Addition of inculating agents

33. SOLIDIFICATION DEFECTS
Shrinkage:
Almost all materials are denser in the solid state than in the liquid state. During solidification, the materials contract, or shrink, by about 2% to 7%.
If the shrinkage is unidirectional, only one dimension of the solid casting would be smaller than the dimensions of the mold.
=>Remedy: The mold then can be made oversized by appropriate amount in order to compensate for the shrinkage.

34. Cavity: In most situations, when the solidification begins at all surfaces of the casting and shrinkage occurs in the bulk of the casting . it is termed as ‘cavity’. This produces a defective casting.
Remedy: To control the cavity shrinkage, a riser or an extra reservoir of liquid metal is placed adjacent & connected to casting.

35. Pipe: During solidification, if one surface (usually top) solidifies more slowly than the others the shrinkage defects is called ‘pipe’. This also produces a defective casting.
Remedy: To control the pipe (shrinkage), a riser or an extra reservoir of liquid metal is placed adjacent & connected to casting.

36. Interdendritic shrinkage:
It is found when extensive dendrites growth occurs. Liquid metal may be unable to flow from a riser through the fine dendritic network to the solidifying metal.
Consequently, small shrinkage pores are produced throughout the casting.
This defect also called ‘microshrinkage’ or ‘shrinkage porosity’ is difficult to prevent by the use of risers.
Remedy: Fast cooling rates may reduce problems with interdendritic shrinkage; the dendrites may be shorter, permitting liquid to flow through the dendritic network to the solidifying solid interface. In addition, any shrinkage that remains may be finer & more uniformly distributed.

37. GAS POROSITY
Many metals dissolve a large amount of gas when they are liquid; e.g. aluminum dissolves hydrogen. Of course, when it solidifies, the solid aluminum retains only small fraction of hydrogen in its structure.
The excess of dissolved hydrogen forms bubbles that may be trapped in the solid metal during solidification, producing gas porosity.
The porosity may be spread uniformly throughout the casting or may be trapped between dendrite arms.

38. The amount of gas that can be dissolved in the molten metal is given by Sievert’s law as given below:
Percent of gas = K √pgas (1)
Where,
Pgas = partial pressure of the gas in contact with the metal
K = a constant which increases for a particular metal-gas system with increasing temperature
**Remedy**
=> Gas porosity can be minimized in casting by keeping the liquid temperature low, by adding materials to the liquid to continue with the gas and form a solid, or by assuring that the partial pressure of the gas remains low.

39. Deep shrinkage pipe or Cavity:
=> When oxygen gets dissolved in liquid steel during steel- making process, it combines with carbon which is an alloying element, and carbon monoxide [CO] gas bubbles get trapped in the steel casting.
Remedy:
The dissolved oxygen can be completely eliminated if aluminum is added before start of solidification. The aluminum combines with oxygen, producing solid alumina (Al2O3).In addition to eliminating gas porosity, the tiny Al2O3 inclusions prevent the grain growth by pinning grain boundaries.

40. Method to control the grain structure
To produce the castings with isotropic properties and improved strength by grain size strengthening, the solidification of casting should be controlled in a way to produce a large number of small equiaxed grains.
Also, to improve strength of casting and to refine microshrinkage & gas porosity, the dendrites should be as small as possible. This too requires the control of solidification.

41. Method to control the grain structure during solidification
(1)Introduction: By using appropriate inoculating or grain refining agents a wide spread nucleation can be produced during solidification that result in fine grain structure.
(2)Rapid solidification: By encouraging rapid solidification, a very small spacing of secondary dendrite arms may be achieved. The rate of solidification for any given metal can be influenced by the size of casting, the mold material and the casting process.
Thick casting solidifies slowly then thin casting.
Mold materials having a high density, thermal conductivity & heat capacity produce more rapid solidification.

42. (3)Directional solidification: In many applications, a small equiaxed grain structure in the casting is not desirable. Castings used for blades and vanes in turbine are such applications.
In directional solidification technique, the mold is heated from one end and cooled from the other, producing a columnar microstructure with all of the grain boundaries running in the longitudinal direction of the part.
In such solid, there are no grain boundaries in the transverse direction.
Better creep & fatigue resistance are obtained using the DS technique.

43. (4)Single crystal technique: In this technique, only one columnar grain becomes able to grow to the main body of the casting due to helical connection as shown in fig.
The single crystal casting has no grain boundaries at all but has its crystallographic planes directions in an optimum orientation.
Solidification of single crystal is also required for producing silicon semi-conductor wafers, from which electrinic devices are manufactured.

44. Solidification of alloys
Alloys is a material formed by mixing two or more metals or a metal with metalloid, the addition of certain elements called alloying elements improves strength & corrosion resistance.
Alloys, unlike metals, solidify over a range of temp.
The constant temp. heat evolution in case of pure metals is not followed by the alloys but depending upon the type of alloy being solidified, the range of temp. varies considerably.
Such behavior of an alloy can be understood by Gibb’s phase rule which is discussed in the next chapter.
The key requirement for the solidification of alloys is that the solutes must get redistributed by diffusion in the metal/liquid. This process take time.

45. By taking time, the solidification of alloys is controlled by temp
By taking time, the solidification of alloys is controlled by temp. and concentration gradient that prevail during the process of solidification.
In such cases the crystal formed initially & at a later stage will have difference in their composition which can be made uniform by a suitable process.
Solidification of alloys involves a phenomenon called ‘Constitutional supercooling’ besides the ‘Thermal supercooling’.
The Constitutional supercooling means composition changes due to diffusion of solutes along with changes in temp.