Unit 3

# Solidification: Solidification is the process where liquid metal transforms into solid upon cooling.

The basic solidification process occurs in two stages

1 Nucleation (formation of nuclei)

2 Growth of nuclei

1. Nucleation (formation of nuclei)

Fig: Formation of nuclei

2. Growth of nuclei

Fig: Growth of nuclei into crystals and grains

Types of nucleation:

(a) Homogeneous nucleation: It occurs spontaneously without the help of foreign particles.

(b) Heterogeneous nucleation: It occurs with the help of foreign particles (such as mould material,

impurities etc.)

Grain structure in castings:

At the surface heterogeneous nucleation takes place for few layers. These grains are known as

‘Equiaxed Grains’

Inside, absence of sand particles leads to homogeneously nucleated grains. Their orientation will be

from the surface to the center. These grains are known as the ‘Columnar Grains’

When a column forms side arms, it is known as a dendrite (dentrite grain structure) New nuclei resist

the growth of neighboring nuclei. Hence, an equiaxed grain structure is produced at the center.

Three Basic Types of Cast Structures

Columnar dendritic: Columnar dendritic means, the basic structure is like acolumn, but to that column

there will be side dendrites are there.

Equiaxed dendritic: Equiaxed dendritic means crystals have random orientation, they do not have

uniform orientation like the first one, but again though they have the random orientation still they have

the dendrites.

Equaxied nondendritic: Equiaxed nondendritic structure means crystals have random orientation and

they do not have dendrites, and this is an ideal cast structure.

Solidification of a pure metal:

Crystallization and freezing in a pure metal occurs at a constant temperature.

However, under cooling is required in the beginning to initiate the phenomenon of crystallization.

A crystal nucleated at the mould wall grows first along the mould surface until it comes in contact with

adjacent crystals that are also growing along the mould surface.

Growth of crystal along the mould wall is stopped at some stage and secondary arms of primary crystals

grow inside the liquid. The resultant tree-like structure is known as ‘dendritic structure’

# Fluidity of molten metal

Fluidity may be defined as an empirical measure of the distance a liquid metal can flow in a specific

channel before being stopped by solidification.

Physicists define fluidity as the reciprocal of the coefficient of its viscosity.

Metallurgists define it as the ability of a liquid metal or alloy to flow freely, and thus to feed a mould

cavity and produce the desired contour before freezing occurs.

Pure metals act with good fluidity.

Factors affecting fluidity

1. Factors related to melt

2. Factors related to casting parameters

Factors related to melt:

a) Freezing range of alloy: Fluidity is inversely proportional to the freezing temperature range.

b) Alloy composition: Eutectic composition has higher fluidity.

c) Inclusions: Insoluble particles can increase viscosity, reducing fluidity.

d) Surface tension: Decreases fluidity; often caused by oxide film.

e) Viscosity: Higher viscosity decreases fluidity.

Factors related to casting parameters:

a) Modulus: Fluidity length increases as the modulus (volume/surface area) of the casting

increases.

b) Section thickness: Larger thickness of section results in higher fluidity.

c) Heat transfer coefficient: A reduction in the rate of heat transfer will increase fluidity.

d) Superheating: The temperature increment above the melting point increases fluidity.

e) Mould temperature: Higher mould temperature increases fluidity.

f) Pouring rate: Lower pouring rates decrease fluidity because of larger cooling.

Methods for determining the fluidity:

1. Fluidity spiral tube method

2. Suction tube method

3. Long strips of different thickness

1. Fluidity spiral tube method

The Fluidity Index of the material is the length of the solidified metal in the spiral passage.

When the molten metal is being poured it will be flowing along this spiral passage.

If the fluidity is more it will be flowing more distance and if the fluidity is less it will be flowing to

a lesser distance.

Sometimes it may flow for 20 inches that is a lesser fluidity sometimes it may flow for 30 inches

means it is a higher fluidity.

The greater the length of the solidified metal greater is the fluidity.

2. Suction tube method

Suction tube method is a based on the application of the vacuum.So, here a simple tube is

immersed inside the moltenmetal. On the other side of the tube vacuum is applied, then

because of the application of the vacuum the molten metal will be flowing through this tube. If

the fluidity of the molten metal is very good it will be flowing to a longer distance and if the

fluidity is very less it will be flowing to a shorter distance. So, that is how the fluidity of the

molten metal can be measured using the suction tube method using the vacuum.

3. Long strips of different thicknesses

Long strips of different thicknesses method is similar to spiral tube method. In the case of the

spiral tube method a thin spiral wire is used, but here long sections of different thicknesses are

used. Now when the molten metal is being poured, molten metal will be flowing into runner

then it will be flowing through different sections and depending upon the fluidity of molten

metal it will be flowing into these different sections and stopping at a particular place.

# Gating and Risering

Gating System: It refers to all the sections through which the molten metal passes while

entering into the mould cavity.

Elements of Gating System:

1. Pouring cup

2. Sprue

3. Sprue well

4. Runner

5. Ingates(Gates)

6. Riser

Functions of Gating System:

Fill the mould cavity completely before freezing

Minimizing turbulence

Avoiding erosion

Removing inclusions

Regulate flow of molten metal

Types of Gating System

1. Pressurized Gating System: The total cross-sectional area gradually decreases.

2. Un- Pressurized Gating System: The total cross-sectional area gradually increases.

Comparison of Gating Systems:

Pressurized gating Un-pressurized gating

The total cross sectional area decreases towards the mould cavity.

The total cross sectional area increases towards the mould cavity.

More turbulence and chances of mould erosion Less turbulence.

Flow of liquid metal (volume) is almost equal from all ingates.

Flow of liquid metal(volume) is different from each ingate.

Complex and thin sections can be successfully cast. Complex and thin sections may not be successfully cast.

Design and Location of Ingates

Multiple ingates are often preferable for large castings.

A fillet should be used where an ingate meets a casting: produces less turbulence.

The minimum ingate length should be three to five times the ingate’s width, depending on the

metal being cast.

Curved ingates should be avoided, as far as possible.

Design of Riser

Primary function of a Riser: It acts as a reservoir of molten metal in the mould to compensate for

shrinkage during solidification.

Secondary function of a Riser:

It gives an indication that the cavity is full with the molten metal.

It also enables escape of hot gases during pouring of molten metal.

Why Design of Riser?

An undersized riser could lead to shrinkage defects and ultimately result in rejection of the

casting.

An oversized riser requires excess molten metal and results in excess power/fuel consumption

for melting.

Hence the size of the riser must be optimum.

Guidelines for Riser Design and Location:

The riser must not solidify before the casting.

The volume of riser(s) must be large enough to feed the entire shrinkage of the casting.

The pressure head from the riser should enable complete cavity filling.

Risers must be placed so that the liquid metal can be delivered to locations where it is most

needed.

Important Methods of Riser Design:

1. Caine’s method: Canie Suggested one have to find out two ratios before one can judge whether

a riser will give a sound casting or a defective casting. One ratio is riser volume by casting

volume one have to find and another is freezing ratio. Freezing ratio is casting surface area by

casting volume whole divided by riser surface area by riser volume.

Volume ratio (Y) = Riser volume/Casting volume

Freezing ratio (X) = Casting surface area/Casting volume

Riser surface area/Riser volume

X= (a/Y-b) + c

Here; a, b, c = constants, X= Freezing ratio, Y= Volume ratio

2. Modulus method:

Modulus of solidification: Modulus of solidification of casting (or riser) is defined as the ratio of

its volume and surface area.

Modulus of solidification = Volume/surface area

Modulus method is based on Chvorinov’s Rule.

Chvorinov’s Rule:

TST=Cm (V/A)n

where

TST= total solidification time

V = volume of the casting

A = surface are of casting

n = exponent usually taken as 2

Cm is a constant which depends upon mould material

What Chvorinov’s Rule Tells Us?

TST= Cm (V/A)n

A casting with a higher modulus (volume-to-surface area ratio) cools and solidifies more

slowly than the one with a lower modulus.

To feed molten metal to the casting. TST of the riser must be greater than TST of the

casting.

Since mould constants of riser and casting will be equal, design the riser to have a larger

modulus so that the main casting solidifies first.

Requirement of the riser to feed the casting

MR = 1.2 x MC

Where,

MR = Modulus of riser

MC = Modulus of casting

Moduli (M) of simple geometric shapes:

For Cube:

M= D/6

For Cylinder:

M= D/6

Sphere:

M= D/6

Assignment Problems:

Q1. Calculate the size of a cylindrical riser necessary to feed a steel slab casting of size 25 x 25 x 5 cm.

Height of the riser and its diameter are equal. Solve using Canie’s Method.

Q2. Determine the size of a riser for a casting of dimensions 25 x 25 x 5 cm, using modulus method.

Q3. During the casting of a certain alloy using a sand mould, it took 155 seconds for a cube-shaped

casting to solidify. The cube was 50 mm on each side. (a) Determine the value of the mould constant

(Cm) in Chvorinov’s Rule. (b) For the same alloy and mould, determine the total solidification time for a

cylindrical casting whose diameter is 30 mm and length is 50 mm.