CASTING SOLIDIFICATION PROCESS

1. Lecture - 2
Solidification of
Castings

2. Topics to be covered
 Introduction to solidification
 Concept of solidification on casting
 Solidification of pure metals
 Solidification of alloys
 Nucleation
 Growth

3. 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

4. • 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.

5. 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.

6. • 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

7. 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.

8. Solidification curve for metals

9. 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.

10. 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.

11. 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

12. Solidification curve for alloys

13. 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

14. Phase Diagram

15. • 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.

16.  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.