soldification engineering materials 06

7/31/2019 Soldification Engineering Materials 06

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Solidification

The solidification of metallic, polymeric, and ceramic materials is

an important casting process, and its study plays an important

role for manufacturer, because of its effect on the properties of

the materials involved.


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Technological Significance

The ability to use heat to produce, melt, and cast metals such as

copper, bronze, and steel is regarded as an important hallmark in the

development of mankind. The use of fire for reducing naturally

occurring ores into metals and alloys led to the production of useful

tools and other products. Today, thousands of years later, solidification

is still considered one of the most important manufacturing processes.

Several million pounds of steel, aluminum alloys, copper, and zinc are

being produced through the casting process.


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Cont..

The solidification process is also used to manufacture specific

components (e.g., aluminum alloys for automotive wheels). Industry

also uses the solidification process as a primary processing step to

produce metallic slabs or ingots (a simple, and often large casting that

later is processed into useful shapes). The ingots or slabs are then hot

and cold worked through secondary processing steps into more useful

shapes (i.e., sheets, wires, rods, plates, etc.). Solidification also isapplied when joining metallic materials using techniques such as

welding, brazing, and soldering.


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Solidification of metals

Solidification of metals and alloy is an important industrial process

science most metals are melted and then cast into a semifinished or

finished shape.

In general solidification of metal or alloy can be divided into the

following steps:

1. Formation of stable nuclei

2. Growth of nuclei into crystals & formation of grains.


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Nucleation Formation

In the context of solidification, the term nucleation refers to the

formation of the first nanocrystallites from molten material.

Formation of nuclei in liquid metals:

a. Homogenous Nucleation ( Metal itself provides the atoms needed to form a nuclei)

b. Heterogeneous Nucleation ( Nucleating agent is used )


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Growth Mechanisms

Once the solid nuclei of a phase form (in a liquid or another solid

phase), growth begins to occur as more atoms become attached to the

solid surface. In the solidification process, two types of heat must be

removed: the specific heat of the liquid and the latent heat of fusion.

The specific heat is the heat required to change the temperature of a

unit weight of the material by one degree. The specific heat must be

removed first, either by radiation into the surrounding atmosphere orby conduction into the surrounding mold, until the liquid cools to its

freezing temperature. This is simply a cooling of the liquid from one

temperature to a temperature at which nucleation begins.


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Contt..

We know that to melt a solid we need to supply heat. Therefore, when

solid crystals form from a liquid, heat is generated! This type of heat is

called the latent heat of fusion (Hf). The latent heat of fusion must

be removed from the solid-liquid interface before solidification is

completed. The manner in which we remove the latent heat of fusion

determines the materials growth mechanism and final structure of a

casting.


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Planar Growth

When a well-inoculated liquid (i.e., a liquid containing nucleating

agents) cools under equilibrium conditions, there is no need for

undercooling since heterogeneous nucleation can occur. Therefore, the

temperature of the liquid ahead of the solidification front (i.e., solid-

liquid interface) is greater than the freezing temperature. The

temperature of the solid is at or below the freezing temperature.

During solidification, the latent heat of fusion is removed by

conduction from the solid liquid interface.


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Contt..

Any small protuberance that begins to grow on the interface is

surrounded by liquid above the freezing temperature (Figure 9-3). The

growth of the protuberance then stops until the remainder of the

interface catches up. This growth mechanism, known as planar

growth, occurs by the movement of a smooth solid-liquid interface

into the liquid.


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Dendritic GrowthWhen the liquid is not inoculated and the nucleation is poor, the liquid

has to be undercooled before the solid forms (Figure ). Under these

conditions, a small solid protuberance called a dendrite, which forms

at the interface, is encouraged to grow since the liquid ahead of the

solidification front is undercooled. The word dendrite comes from the

Greek word dendron that means tree. As the solid dendrite grows, the

latent heat of fusion is conducted into the undercooled liquid, raising

the temperature of the liquid toward the freezing temperature.

Secondary and tertiary dendrite arms can also form on the primary

stalks to speed the evolution of the latent heat.


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Contt..

Dendritic growth continues until the undercooled liquid warms to the

freezing temperature. Any remaining liquid then solidifies by planar

growth. The difference between planar and dendritic growth arises

because of the different sinks for the latent heat of fusion. The

container or mold must absorb the heat in planar growth, but the

undercooled liquid absorbs the heat in dendritic growth. In pure

metals, dendritic growth normally represents only a small fraction of

the total growth and is given by

soldification engineering materials 06

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