By Ummen Sabu13ETMMO8M.Tech. Materials Engineering

School of Engineering Sciences and Technology UNIVERSITY OF HYDERABAD

SOLIDIFICATIONOF METALS

AND DESIGN CONSIDERATIONS IN CASTING

INTRODUCTION TO

CONTENTS

Introduction

Solidification

Driving force for solidification

Nucleation and growth

Solidification defects

Design considerations in casting process

References

INTRODUCTION

Solidification ?

solidification, is a phase transition in whicha liquid turns into a solid when its temperature islowered below its freezing point.

Why to study in detail ?

The solidification of metals and their alloys isimportant in various industrial process

Most of the components produced from metals areby casting process.

When a metal is welded a small portion of metalnear the weld melts and resolidifies.

SOLIDIFICATION

During solidification, the liquid changes in to solid as cooling proceeds.

The energy of liquid is less than that of the solid above the melting point. Hence liquid is stable above the melting point.

But below the melting point, the energy of liquid becomes more than that of the solid.

Hence below the melting point, the solid becomes more stable than the liquid.

Thermodynamically, both liquid and solid have equal energy at melting pointand therefore both are equally stable at melting point.

Freezing is almost always an exothermic process, meaning that as liquidchanges into solid, heat is released.

This heat must be continually removed from the freezing liquid or the freezingprocess will stop.

The energy released upon freezing is a latent heat and is the entropy part.

Some under-cooling will be essential for solidification.

Solidification occurs by two process : nucleation and growth.

SOLIDIFICATION

COOLING CURVES

A cooling curve is a graphical plot of the changes in temperature with time for a material over the entire temperature range through which it cools.

Total heat to be removed for solidification, Q = m ( SH + LH )

= m (Cp (Tm – Tf ) + L )

COOLING CURVE WITH UNDERCOOLING

DRIVING FORCE FOR SOLIDIFICATION

Solidification is undoubtedly the most important processing route for metals andalloys

For a pure metal at the fusion temperature Tf , ΔG = 0 so that

ΔGf = ΔHf − Tf ΔSf = 0 ( or ) ΔHf = Tf ΔSf

For any temperature other than Tf ,ΔG = ΔH − TΔS

~ ΔHf− TΔSf

= ΔSf (Tf− T) = ΔSf . ΔT

_

undercooling

The driving force is therefore proportional to the undercooling assuming that the latent heat and the entropy of fusion do not vary much with temperature.

NUCLEATION

The first step of metal solidification is the creation of tiny, stable, nuclei in the liquid metal.

Cooling the liquid below its equilibrium freezing temperature, or undercooling,provides the driving force for solidification.

Once a cluster reaches a critical size, it becomes a stable nucleus and continues to grow.

The mold walls and any solid particles present in the liquid make nucleation easier.

Cluster of atoms Embryo Nuclei Crystals Grainsr > r’

r < r’ r’ = critical radius

NUCLEATION

The volume free energy ΔGV – free energy difference between the liquid and solid

Δ GV = 4/3πr3ΔGv (- ve)

The surface energy ΔGs – the energy needed to create a surface for the spherical particles

ΔGs = 4πr2γ (+ ve)γ → specific surface energy of the particle

Total free energy Change, ΔGT = ΔGV + ΔGs

At low temperatures atoms form small cluster or groups.

Embryo’s formed may either form into stable nuclei or may re-dissolve in the liquid.

beyond the critical radius of the nuclei it will remain stable and growth occurs

NUCLEATION

Nucleation and formation of grains

TYPES OF NUCLEATION

Nucleation is of two types-

Homogeneous nucleation:

Homogeneous Nucleation – Formation of a critically sized solid from the liquid byclustering together of a large number of atoms at a high undercooling.

Heterogeneous Nucleation :

Formation of a critically sized solid from the liquid on an impurity surface.heterogeneous nucleation occurs in a liquid on the surface of its container,insoluble impurities and other structural materials that lower the critical freeenergy required to form a stable nucleus.

In practice, homogeneous nucleation rarely takes place and heterogeneousnucleation occurs either on the mould walls or on insoluble impurity particles.

TYPES OF NUCLEATION

GROWTH

Once solid nuclei form, growth occurs as atoms are attached to the solid surface.

Growth is the physical process by which a new phase increases in size. In the case of solidification, this refers to the formation of a stable solid particle as the liquid freezes.

Nature of growth of solid depends on how heat is removed from the system.

Sensible heat and latent heat is to be removed

The manner in which the latent heat is lost determines the growth mechanism

There are 2 growth mechanisms:

1. Planar growth

2. Dendritic growth

The differences in planar and dendritic growth arises because of the differences in sink for the latent heat.

GROWTH

Planar growth :

The heat is dissipated through the crystal, i.e.the growing crystal is colder than the melt.Here a solid bulge into the liquid would meltagain because the temperature in the bulge isabove Tm . Therefore, one obtains a stable flatsolidification front

Any small protuberance that begins to growon the interface is surrounded by liquid abovethe freezing temperature.

The growth of the protuberance stops untilthe remainder of the interface catches up withit.This planar growth occurs by the movement ofa smooth solid-liquid interface into the liquid.

GROWTH

Dendritic growth :

For a strongly undercooled melt the heat ofcrystallization can also be dissipated throughthe melt.Under these conditions a small solidprotuberance called a dendrite which forms atthe interface is encouraged to grow.As the solid dendrites grow the latent heat offusion is conducted into the undercooled liquidRaising the temperature of the liquidtowards freezing temperature.Dendritic growth continues until theundercooled liquid warms to the freezingtemperature.Any remaining liquid then solidifies by planargrowth.These are thermal dendrites different fromdendrites in alloys

GROWTH

Planar growth Dendritic growth

SOLIDIFICATION DEFECTS

Shrinkage :

Most materials contract or shrink during solidification and cooling.Shrinkage is the result of:

Contraction of the liquid as it cools prior to its solidificationContraction during phase change from a liquid to solidContraction of the solid as it continues to cool to ambient temperature.

Shrinkage can sometimes cause cracking to occur in component as it solidifies.

Gas porosity :

Many metals dissolves a large quantity of gas when they are liquid(Sieverts Law). However when metal solidify they retain only a small part of the gas. But these

form bubbles trapped in the solid metal producing gas porosity.

DESIGN CONSIDERATIONS IN CASTING PROCESS

To minimize the damaging effects of shrinkage RISER is used in casting process.

Riser is a reservoir of molten metal to compensate the shrinkage.

Thus molten metal is continually available from risers to prevent shrinkage voids .

it is desirable for regions of the casting most distant from the liquid metal supply to freeze first and for solidification to progress from these remote regions toward the riser.

Directional solidification is obtained by using chills.

Also to minimize shrinkage problems with final cast product an allowance is given for the size of the mould during pattern design.

CONCLUSIONS:

Principles involved in the solidification of metals have been only discussed.

The solidification of alloys are much more complex and involves solute partitioning studies.

Based on the solidification studies design considerations were implemented in the casting process.

REFERENCES

[1]. Cambridge University e-lectures :Part IB Materials Science & Metallurgy H. K. D. H. Bhadeshia Course A, Metals and Alloys ,lecture 6:solidification[2].en.wikipedia.org