Page 258 - Manufacturing Engineering and Technology - Kalpakjian, Serope : Schmid, Steven R.

Fundamentals of

Metal Casting

l0.I Introduction 237
° First used about six thousand years ago, casting continues to be an important l0.2 Solidification of
manufacturing process for producing very small, as well as very large and Metals 238
complex, parts. l0.3 Fluid Flow 243
l0.4 Fluidity of Molten
° The first topic discussed is solidification of molten metals, including the differ- Metal 245
ences between solidification of pure metals and alloys, as well as concepts such I0.5 HeatTransfer 247
as grain nucleation and freezing range. I0.6 Defects 249
° Fluid flow in casting is then described, with Bernoulli’s and the continuity EXAMPLES:
equations being applied to establish a framework for analyzing molten metal l0.l Solidification Times for
flow through a mold. Various Shapes 248
l0.2 Casting of Aluminum
° The concept and importance of turbulence versus laminar flow is introduced. Automotive Pistons 252
° Heat transfer and shrinkage of castings are also discussed, including Chvorinov’s
rule for solidification time.
° The chapter ends with a description of the causes of porosity in castings and
common methods of reducing them to improve cast-metal properties.

l0.l Introduction

The casting process basically involves (a) pouring molten metal into a mold patterned
after the part to be manufactured, (b) allowing it to solidify, and (c) removing the part
from the mold. As with all other manufacturing processes, an understanding of the
underlying science is essential for the production of good-quality, economical castings
and for establishing proper techniques for mold design and casting practice.
Important considerations in casting operations are as follows:

° Flow of the molten metal into the mold cavity
° Solidification and cooling of the metal in the mold
° Influence of the type of mold material.

This chapter describes relationships among the many factors involved in cast-
ing. The flow of molten metal into the mold cavity first is discussed in terms of
mold design and fluid-flow characteristics. Solidification and cooling of metals in
the mold are affected by several factors, including the metallurgical and thermal
properties of the metal. The type of mold also has an important influence, because
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