chapter 9: 9 - 1 · chapter 9: principles of solidification chapter 9: principles of solidification

© 2011 Cengage Learning Engineering. All Rights Reserved.9 - 1

Chapter 9:

Principles of Solidification

Chapter 9: Principles of Solidification

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Slide 2


Learning Objectives

1. Technological significance

2. Nucleation

3. Applications of controlled nucleation

4. Growth mechanisms

5. Solidification time and dendrite size

6. Cooling curves

7. Cast structure

8. Solidification defects

9. Casting processes for manufacturing components

10. Continuous casting and ingot casting


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Slide 3


Learning Objectives

11. Directional solidification [DS], single crystal growth, and epitaxial growth

12. Solidification of polymers and inorganic glasses

13. Joining of metallic materials


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Terminology for Mechanical Properties

Solidification process

Processing of materials involving solidification from the molten state

Primary processing Process involving casting of molten metals into ingots or semi-finished useful shapes such as slabs

Secondary processing

Processes such as rolling, extrusion, etc. used to process ingots or slabs and other semi-finished shapes


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Slide 5


Nucleation


Nucleation The formation of the first nanocrystallitesfrom molten material

Nuclei Small particles of solid that form from the liquid as atoms cluster together

Total change in energy (ΔG)

Embryo A tiny particle of solid that forms from the liquid as atoms cluster together

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Slide 6


Nucleation


Critical radius (r*) The minimum size that must be formed by atoms clustering together in the liquid before the solid particle is stable and begins to grow

Growth Refers to the formation of a stable solid as the liquid freezes

Undercooling (ΔT) The temperature to which the liquid metal must cool below the equilibrium freezing temperature before nucleation occurs

Superheat The difference between the pouring temperature and the freezing temperature

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Figure 9.1


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Slide 8


Nucleation


Homogeneous nucleation

Occurs when the undercooling becomes large enough to cause the formation of a stable nucleus.

whereHf latent heat of fusion

Tm equilibrium solidification temperature in kelvin

T = Tm- T undercooling when the liquid temperature is T

Heterogeneous nucleation Nucleation on preexisting surfaces.

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Slide 9


Figure 9.2


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Applications of Controlled Nucleation

Grain size strengthening

Process of intentionally introducing nucleating particles is called grain refinement or inoculation.

Chemicals added to molten metals to promote nucleation are known as grain refiners or inoculants.

Second-phase strengthening

Process of strengthening materials using ultra-fine precipitates. Also known as dispersion strengthening.

Solid-state phase transformation: A change in phase that occurs in the solid state.


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Slide 11


Applications of Controlled Nucleation

Glasses

Rapid solidification processing: Producing unique material structures by promoting unusually high cooling rates during solidification.

Photochromic glass: Glass that can change color or tint upon exposure to sunlight.

Glass-ceramics

Engineered materials that begin as amorphous glasses and end up as crystalline ceramics with an ultra-fine grain size.


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Slide 12


Figure 9.3


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Growth Mechanisms

Planar growth: A smooth solid-liquid interface grows with

little or no undercooling of the liquid.

Solidification front: Interface between a solid and liquid.


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Slide 14

© 2011 Cengage Learning Engineering. All Rights Reserved. 9 - 14

Figure 9.4


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Slide 15


Solidification Time and Dendrite Size


Chvorinov’s rule

where

V volume of the casting and represents the amount of heat that must be removed before freezing occurs

A surface area of the casting in contact with the mold and represents the surface from which heat can be transferred away from the casting

n constant (usually about 2)

B mold constant

Rate of solidification

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Figure 9.5


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Slide 17


Figure 9.8


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Slide 18


Figure 9.9


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Figure 9.10


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Slide 20


Cast Structure

Equiaxed zone

A region of randomly oriented grains in the center of a casting produced as a result of widespread nucleation.

Skin-forming alloys

Alloys with a microstructure that shows an outer skin of small grains

in the chill zone followed by dendrites.

Mushy-forming alloys

Alloys that produce castings with a macrostructure consisting

predominantly of equiaxed grains.

Superheat

The difference between the pouring temperature and the freezing temperature.


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Slide 21


Figure 9.11


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Figure 9.13


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Slide 23


Figure 9.14


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Slide 24


Figure 9.15


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Figure 9.16


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Slide 26


Figure 9.17


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Slide 27


Figure 9.18


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Figure 9.20


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Slide 29


Figure 9.21


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Slide 30


Figure 9.22


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Figure 9.23


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Slide 32


Figure 9.24


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Slide 33


Key Terms

Solidification

Primary processing

Secondary processing

Nucleation

Nuclei

Embryo

Critical radius

Growth

Undercooling

Superheat

Homogeneous nucleation

Heterogeneous nucleation

Grain refinement or inoculation

Inoculants

Dispersion strengthening or second-phase strengthening

Solid-state phase transformations

Rapid solidification processing

Photochromic glass

Glass-ceramics

Specific heat


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Key Terms

Latent heat of fusion

Solidification front

Planar growth

Dendrite

Chvorinov’s rule

Mold constant

Secondary dendrite arm spacing

Recalescence

Thermal arrest

Total solidification time

Local solidification time

Ingot structure

Chill zone

Columnar zone

Equiaxed zone

Skin-forming alloys

Mushy-forming alloys

Shrinkage

Cavities

Interdendritic shrinkage or microshrinkage or shrinkage porosity

Glass-ceramics

Gas porosity

Sievert’s law

Gas flushing


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Slide 35


Key Terms

Stainless steel

Argon oxygen decarburization

Sand casting

Investment casting

Lost wax process

Lost foam process

Permanent mold casting

Pressure die casting

Ingot casting

Continuous casting

Directional solidification

Epitaxial growth

Lamellar

Spherulite

Preform

Brazing

Soldering

Fusion welding

Fusion zone


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chapter 9: 9 - 1 · chapter 9: principles of solidification chapter 9: principles of solidification

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