Page 260 - 04. Subyek Engineering Materials - Manufacturing, Engineering and Technology SI 6th Edition - Serope Kalpakjian, Stephen Schmid (2009)

As an example of the grain structure that

which are at ambient temperature or typically are
section of a box-shaped mold. At the mold walls, it

Section 10.2
239
Solidification of Metals
This
is an important consideration, because
Chill zone
shrinkage can lead to microcracking and associ-
Columnar zone
Equiaxed structure
Equiaxed zone
ated porosity, which can in turn compromise the
mechanical properties of the casting.
develops in a casting, Fig. 10.2a shows a cross
have favorable orientation grow preferentially §'apgi'§}¢ tvgtgsq

much cooler than the molten metal, the metal
cools rapidly and produces a solidified skin, or
5...*==-si 603%
shell, of fine equiaxed grains. The grains generally
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grow in a direction opposite to that of the heat
-e
transfer out through the mold. Those grains that ‘OJ a#
%'=-‘S-‘Maw "|;|§§i'
grains that have substantially different orienta- \g-in? 22 '»m=~'~e *vga-'8-
and are called columnar grains (Fig. 10.3). Those
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,
ri>¢\.-"‘¥'~
il#
wlau lg
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tions are blocked from further growth. As the .9-I -v »¢» will: 4~'¢'=l;y
wi-4 »""~=s
OW!! Q
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driving force of the heat transfer is reduced away 'li-‘S f*l¢'¢"*"J~¢ #dl my
s-
from the mold walls, the grains become equiaxed i1"__i ~¢I~|l%1{‘¢n¢* %v=0»$l}'&\
&'i|\u¢,,l:s&‘l
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and coarse. This process of grain development is Q5 '|.‘q| 1|
known as homogenous nucleation, meaning that °-'allr-»~‘
the grains (crystals) grow upon themselves, start- »
ing at the mold wall.
<a> <b> <c>
l0.2.2 Alloys FIGURE I0.2 Schematic illustration of three cast structures of
metals solidified in a square mold: (a) pure metals; (b) solid-solution
Solidification in alloys begins when the tempera-
alloys; and (c) structure obtained by using nucleating agents.
ture drops below the liquidus, TL, and is com-
Source: After GW Form, ].F. Wallace, ].L. Walker, and A. Cibula.
plete when it reaches the solidus, TS (Fig. 10.4).
Within this temperature range, the alloy is in a
mushy or pasty state consisting of columnar den-
drites (from the Greek dendron, meaning “akin to,” T Columnar
and drys, meaning “tree”). Note the presence of liq-
ZOl'l€
uid metal between the dendrite arms. Dendrites have
three-dimensional arms and branches (secondary
arms), which eventually interlock, as can be seen in
Fig. 10.5. The study of dendritic structures (although
complex) is important, because such structures con- Mold
tribute to detrimental factors, such as compositional
variations, segregation, and microporosity within a
cast part. FIGURE |0.3 Development of a preferred texture at a cool
The width of the mushy zone (where both liquid mold wall. Note that only favorably oriented grains grow
away from the surface of the mold.
and solid phases are present) is an important factor
during solidification. This zone is described in terms of
a temperature difference, known as the freezing range:
Freezing range = TL - TS. (10.1)
lt can be seen in Fig. 10.4 that pure metals have a freezing range that approaches
zero and that the solidification front moves as a plane without forming a mushy
zone. Eutectics (Section 4.3) solidify in a similar manner, with an essentially
plane front. The type of structure developed after solidification depends on the
composition of the eutectic. In alloys with a nearly symmetrical phase diagram, the
structure is generally lamellar, with two or more solid phases present, depending on