SOLIDIFICATION OF CASTING: Nucleation, Grain Growth, Chvorinov’s Rule, Solidification time, Types of Solidification, Progressive Solidification and Directional Solidification

SOLIDIFICATION OF CASTING 

Solidifications of casting and its control for obtaining sound casting is the most important problem for foundry men.


Soundness of casting may be defined as the degree of true metallic continuity.

Casting is called sound if the volumetric shrinkage (due to change from molten metal to solid) is compensated by liquid metal provided by risers.

The mechanism of solidification has to be understood properly to prevent defects due to shrinkage of metal.

Solidification starts as soon as the molten metal is poured in the mould.


Process of solidification affects

(i) grain size, shape and orientation

(ii) distribution of alloying element

(iii) crystal structure and its imperfections.

Volumetric shrinkage/contraction occurs in three stages. So contraction is of three types.

(i) Liquid contraction/shrinkage 

– It occurs when the metal is in liquid state (pouring temperature to starting of solidification).

(ii) Solidification contraction / shrinkage 

– It occurs when molten metal changes from liquid to solid, (when metal losses its latent heat).


(iii) Solid contraction / shrinkage 

– It occurs when metal is solid.

– Occurs after solidification

– Does not affect shrinkage defect.

Solidification occurs due to nucleation of very small grains called crystals.

Grain growth occurs when solidification continues.

The solidification may be (i) directional or (ii) discrete which is dependent upon nucleation and grain growth.

Nucleation

It is the starting point of phase transformation.

In this tiny new phase called nuclei grow in the molten metal. This nuclei grow into solid by further deposition of atoms.


Nucleation is of two types. They are:

(i) Homogeneous nucleation occurs in homogeneous materials as pure metals.

(ii) Heterogeneous nucleation – occurs at surfaces, imperfections.

Grain Growth

It occurs after nucleation

It determines the final crystallographic structure of solid.

Grain growth may be defined as increase of nucleus in size.

Nuclei grow by addition of atoms.

Total free energy is reduced due to grain growth.

Metal in molten condition has high energy.

Energy is lost as the molten metal cools to form crystals.


Heat loss is more rapid near mould walls than any other place. So nuclei are formed here first.

If no nuclei is in impurity form, then it becomes difficult to start crystallization.

In this condition, the melt undercools and nuclei/seed crystal forms.

The growth of crystal occurs in dendritic manner.

Dendrites grow by forming small arms on original branches of individual dendrites. Slow cooling produces long dendrites and fast cooling produces short dendrites.

Dendrites result in grains. So slow cooling produces large grain structure and fast cooling produces small grain structure.

When solidification proceeds, more and more arms grow on existing dendrite and more and more dendrites form till the whole melt is crystallized.

Dendritic growth
Dendritic growth


Unit cell and dendutic growh
Unit cell and dendutic growh

Dendrites grow until they come in contact the neighboring dendrites and produce grain boundaries. After sometimes dendrite arms become thick and produce solid crystal or grain and there remains no indication of dendritic growth.


Chvorinov’s Rule

It is observed that, when mould wall is plain and flat, the thickness of the skin formed is proportional to the square root of time.

Chvorinov’s Rule

Total time taken for solidification of casting can be expressed by Chvorinov’s rule which is proportional to the square of the ratio of volume and surface area.

Total time taken for solidification of casting can be expressed by Chvorinov’s rule which is proportional to the square of the ratio of volume and surface area

This rule helps in determining the solidification time of casting. However, it is not very accurate, since it does not take into account the solidification contraction or shrinkage.


This method is generally used for calculating proper riser size for short frezing-range alloys such as steel and pure metals.

Types of Solidification

Progressive and

Directional

Progressive Solidification

It may be defined as the growth of practically solid and partially liquid zone from the outside to the interior of the casting.

Solidfication from a flat mould wall.
Solidfication from a flat mould wall.

Directional Solidification

When the gating system design is utilized to control this progressive solidification is such a way that no portion of the casting is isolated from liquid metal feeding channels during complete solidification cycle, it is called as directional solidification.

It may be defined as solidification which is forced to occur in a particular direction.


Directional solidification in castings starts and progress from the thinnest section towards the heaviest section.

Effective directional solidification starts from those parts of the feeder heads (riser system). It proceeds through casting to the feeder heads. Feeder heads are designed to solidify last so that liquid metal can be supplied throughout freezing process.


Progressive and Directional solidfication
Progressive and Directional solidfication




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