Lecture - 2

Solidification of Castings

Topics to be covered

Introduction to solidification

Concept of solidification on casting

Solidification of pure metals

Solidification of alloys

Nucleation

Growth

Introduction to Solidification

• Solidification mechanism is essential for preventing defects due to

shrinkage.

• As soon as the molten metal is poured in a sand mold, the process of

solidification starts.

• During solidification, cast forms develops cohesion and acquires structural

characteristics.

• The mode of solidification affects the properties of the casting acquires a

metallographic structure which is determined during solidification. The

metallographic structure consists of:

Grain size, shape and orientation

Distribution of alloying elements

Underlying crystal structure and its imperfections

• Volume shrinkage/volume contraction occurs during three stages:

Liquid contraction (shrinkage): liquid contraction occurs when the metal is

in liquid state.

Solidification contraction (shrinkage): solidification contraction occurs

during the change from liquid to solid

Solid contraction (shrinkage): solid contraction occurs when the metal is

solid; solid contraction occurs after solidification; solid contraction does

not influence shrinkage defects.

Concept of Solidification on Casting

• A metal in molten condition possesses high energy

• As the molten metal cools, it loses energy to form crystals

• Since heat loss is more rapid near the mold walls than any other place, the

first metal crystallites called ‘nuclei’ form here.

• Nuclei formed as above tend to grow at the second stage of solidification.

• The crystal growth occurs in a dendrite manner.

• Dendrite growth takes place by the evolution of small arms on the original

branches of individual dendrites:

Slow cooling makes the dendrites to grow long whereas fast cooling causes

short dendrite growth.

Since eventually dendrites become grains, slow cooling results in large

grain structure and fast cooling in small grain structure in the solidified

metal.

• As solidification proceeds, more and more arms grow on an existing

dendrite and also more and more dendrites form until the whole melt is

crystallized.

Fig.2.1 Figure showing formation of dendrites

Solidification of Pure Metals

Pure metals generally posses

Excellent thermal and electrical conductivity(e.g. Cu and Al).

Higher ductility, higher melting point, lower yield point and tensile

strength, and

Better corrosion resistance, as compared to alloys.

As metals posses high melting points, they exhibit certain difficulties in

casting,

Difficulties during pouring

Occurrence of several metal-mold reactions

Greater tendency toward cracking

Their mode of solidification, which may produce defective castings.

Above freezing point the metal is liquid and below freezing point, it is in solid.

Solidification curve for metals

From the above curve the following observations can be made:

Liquid metals cools from A to B

From B to C, the melt liberates latent heat of fusion; temperature remains

constant.

The liquid metal starts solidifying at B and it is partly solid at any point

between B and C and at C metal is purely solid.

From C to D, the solid metal cools and tends to reach room temperature.

The slopes of AB and CD depend upon the specific heats of liquid and

solid metals respectively.

If a pure metal cools rapidly or even otherwise when it is very pure and does not

contain impurity at all as nucleus to start crystallization, it may cool as per

Nucleation of solid does not start at point B (i.e., normal solidification

temperature) but it does so at B’, i.e., after the liquid metal has supercooled by

an amount Δt. This phenomenon is known as Supercooling or Undercooling.

Besides pure metals, supercooling may occur in alloys also e.g. grey cast iron.

Solidification of Alloys

Alloyed metals possess:

Higher tensile strengths

Better high temperature strengths

Better corrosion resistance

Improved machinability and workability

Lower melting points

Improved castability

Main types of alloys:

Solid solution alloys

Eutectic alloys

Peritectic alloys

Solidification curve for alloys

The above curve shows the cooling curve of a binary-solid solution alloy

From A to B, the alloy is in liquid state

Solidification starts at B and completes at point C.

Unlike pure metals, solidification occurs throughout the temperature

range(i.e., from Tb to Tc).

Latent heat of fusion is liberated gradually from B to C and it tends to

increase the time required for the solidification

Phase Diagram

• If two metals of a binary solid solution system are mixed in different

proportions and a cooling curve is constructed for each composition,

resulting diagram will be one which is known as PHASE DIAGRAM for

the alloy system.

• A phase diagram shows two different and distinct phases; one is liquid

metal solution and the other is solid solution.

• Within these two phases i.e., liquidus and solidus, the two phases – the

liquid and solid exist together.

• Liquidus is that line (a) above which the alloy is in liquid state, and

• Solidus is that line (a) below which the alloy is in solid state, and (b) where

solidification completes.

• If in a phase diagram, for each change of phase, adequate time is allowed

for the change to complete so that phase change takes place under

equilibrium conditions, the phase diagram will be known as Equilibrium

diagram.

Alloy solidification occurring under equilibrium conditions is known as

equilibrium solidification

Equilibrium conditions are not generally attained during the solidification

of castings because the diffusion involved may be extremely sluggish due

to fast cooling rate of castings.

Thus, most frequently castings solidify under non-equilibrium conditions

and the solidification process is known as non-equilibrium solidification.

Non – equilibrium solidification results in porous, columnar , cored

material which is usually of very inhomogeneous composition.