Experiment (1)
Effect of volume and surface
area
on the total solidification
time of metals & microstructure
Introduction
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The amount of heat that must be removed from a casting to cause it to solidify
is directly proportional to the amount of superheating and the amount of
metal in the casting, or the casting volume.
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Conversely, the ability to remove heat from a casting is directly related
to the amount of exposed surface area through which the heat can be extracted
and the insulating value of the mold.
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Chvorinov’s rule: states that the total solidification time can be computed
by:
Ts = B(V/A)n where
n = 1.5 to 2.0
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The total solidification time is the time from pouring to the completion
of solidification; V is the volume of the casting; A is the surface area;
and B is the mold constant, which depends on the characteristics of the
metal being cast (its density, heat capacity, and heat of fusion), the
mold material (its density, thermal conductivity, and heat capacity), the
mold thickness, and the amount of superheat.
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A thermal arrest is the plateau in the cooling curve that occurs during
the solidification of a material with a fixed melting point. While the
material is at this temperature, the heat being removed from the mold generated
during the solidification process and does not require a decrease in the
material’s temperature.
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Chvorinov’s rule can be used to ensure that the casting will solidify before
the riser. This is necessary if the liquid within the riser is to effectively
feed the casting to compensate for solidification shrinkage.
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Test specimens can be cast to determine B for a given mold material, metal,
and condition of casting. This value can then be used to compute the solidification
times for other castings made under the same conditions. Since a riser
and a casting are both within the same mold and fill with the same metal
under the same conditions.
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Different cooling rates and solidification times can produce substantial
variation in the resulting structure and properties. For instance, die
casting, which uses metal molds, has faster cooling and produces higher-strength
castings than sand casting, which uses a more insulating mold material.
The various types of sands can produce different cooling rates. Sands with
high moisture contents extract heat faster than sand with low moisture.
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If an alloy is used that does not have a distinct melting point, the difference
between the liquidus and solidus temperatures is known as the freezing
range.
Procedures
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Clean the steel mold then spray the mold with graphite to facilitate removal
of the cast structure.
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Pour the molten metal into the mold, for example; Aluminum.
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The mold consists of three different cavities with geometrical shapes and
dimensions and hence different V/A ratios in order to get different solidification
times.
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Measure the cooling temperatures for each structure at equal intervals
of time using thermocouples.
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Plot the cooling curve for each structure (temperature vs. time) and indicate
the major casting features related to solidification.
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Calculate the total solidification time for each cast.
Maintained by: Eng.
Moataz M. Attallah
© The American University in
Cairo