Page 268 - Manufacturing Engineering and Technology - Kalpakjian, Serope : Schmid, Steven R.

Section 10.5 Heat Transfer 2

10.5 Heat Transfer

The heat transfer during the complete cycle (from pouring, to solidification, and to
cooling to room temperature) is another important consideration in metal casting.
Heat flow at different locations in the system is a complex phenomenon and depends
on several factors relating to the material cast and the mold and process parameters.
For instance, in casting thin sections, the metal flow rates must be high enough to
avoid premature chilling and solidification. On the other hand, the flow rate must
not be so high as to cause excessive turbulence-with its detrimental effects on the
casting process.
A typical temperature distribution at the mold liquid-metal interface is
shown in Fig. 10.10. Heat from the liquid metal is given off through the mold wall
and to the surrounding air. The temperature drop at the air-mold and mold-metal
interfaces is caused by the presence of boundary layers and imperfect contact at
these interfaces. The shape of the curve depends on the
thermal properties of the molten metal and the mold.

l0.5.l Solidification Time <- Air -><- Mold -><- Soli d Liquid-»

During the early stages of solidification, a thin skin be-
gins to form at the relatively cool mold walls, and as time
passes, the thickness of the skin increases (Fig. 10.11).
CD
With flat mold walls, this thickness is proportional to the § |\/|e|fi,-,Q
<5
square root of time. Thus, doubling the time will make 5 polnt
the skin \/E = 1.41 times of 41% thicker. 3
The solidification time is a function of the volume |9
of a casting and its surface area (Chi/orinoz/’s rule):
Solidification time = C *& " IM at metal-mold
V
rf
1
Surface area , (10.7) 'me ace
where C is a constant that reflects (a) the mold material, temperatureFl! AT fit m0|d'3lV
Room
(b) the metal properties (including latent heat), and (c) the 'menace
temperature. The parameter n has a value between 1.5 Distance
and 2, but usually is taken as 2. Thus, a large solid sphere
will solidify and cool to ambient temperature at a much FIGURE |0.l0 Temperature distribution at the interface
slower rate than will a smaller solid sphere. The reason of the mold wall and the liquid metal during the
for this is that the volume of a sphere is proportional to solidification of metals in casting.
the cube of its diameter, and the surface area is propor-
tional to the square of its diameter. Similarly, it can 1.
AB
be shown that molten metal in a cube-shaped mold will
solidify faster than in a spherical mold of the same volume _|,, . _ _ _
(see Example 10. 1).
The effects of mold geometry and elapsed time on _ _
=;
skin thickness and shape are shown in Fig. 10.11. As
illustrated, the unsolidified molten metal has been
5 s 1 min 2 min 6 min
poured from the mold at different time intervals ranging
from 5 seconds to 6 minutes. Note that (as expected) the
FIGURE I0.l I Solidified skin on a steel casting. The
skin thickness increases with elapsed time, and the skin
remaining molten metal is poured out at the times indicated
is thinner at internal angles (location A in the figure)
in the figure. Hollow ornamental and decorative objects are
than at external angles (location B). The latter condition made by a process called slush casting, which is based on this
is caused by slower cooling at internal angles than at principle. Source: After H.F. Taylor, J. Wulff, and M.C.
external angles. Flemings.