Since the casting conditions have not changed, the mold constant B is
unchanged. The V/A ratio of the new casting is
If x is the required thickness for our redesigned casting, then
Therefore, x = 1.68 in.
This thickness provides the required solidification time, while reducing the
overall weight of the casting by more than 15%.
Solidification begins at the surface, where heat is dissipated into the
surrounding mold material. The rate of solidification of a casting can be
described by how rapidly the thickness d of the solidified skin grows:
(9-5)
where t is the time after pouring, ksolidification is a constant for a given casting
material and mold, and c1 is a constant related to the pouring temperature
Effect on Structure and Properties
The solidification time affects the size of the dendrites. Normally, dendrite
size is characterized by measuring the distance between the secondary
dendrite arms (Figure 9-5). The secondary dendrite arm spacing (SDAS)
is reduced when the casting freezes more rapidly. The finer, more extensive
dendritic network serves as a more efficient conductor of the latent heat to
the undercooled liquid. The SDAS is related to the solidification time by
(9-6)
where m and k are constants depending on the composition of the metal.
This relationship is shown in Figure 9-6 for several alloys. Small secondary
dendrite arm spacing s are associated with higher strengths and improved
ductility (Figure 9-7). Rapid solidification processing is used to produce
exceptionally fine secondary dendrite arm spacing s; a common method is
to produce very fine liquid droplets that freeze into solid particles. This
process is known as spray atomization. The tiny droplets freeze at a rate of
about 104°C/s, producing powder particles that range from5–100 ?m. This
cooling rate is not rapid enough to form a metallic glass, but does produce
a fine dendritic structure. By c
المادة المعروضة اعلاه هي مدخل الى المحاضرة المرفوعة بواسطة استاذ(ة) المادة . وقد تبدو لك غير متكاملة . حيث يضع استاذ المادة في بعض الاحيان فقط الجزء الاول من المحاضرة من اجل الاطلاع على ما ستقوم بتحميله لاحقا . في نظام التعليم الالكتروني نوفر هذه الخدمة لكي نبقيك على اطلاع حول محتوى الملف الذي ستقوم بتحميله .
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