Page 323 - Manufacturing Engineering and Technology - Kalpakjian, Serope : Schmid, Steven R.

02 Chapter 12 Metal Casting: Design, Materials, and Economics

l2.2.4 Computer Modeling of Casting Processes
Because casting involves complex interactions among material and process variables,
a quantitative study of these interactions is essential to the proper design and produc-
tion of high-quality castings. Although in the past such studies have presented major
difficulties because of the large number of independent variables involved, rapid ad-
vances in computers and modeling techniques have led to important innovations in
modeling various aspects of casting-including fluid flow, heat transfer, and the mi-
crostructures developed during solidification-under various casting conditions.
Modeling of fluid flow in molds is based on Bernoulli’s and the continuity
equations (Section 10.3). A model predicts the behavior of the metal during pouring
into the gating system and its travel into the mold cavity, as well as the velocity and
pressure distributions in the system. Progress also is being made in the modeling of
heat transfer in casting. Modern software can couple fluid flow and heat transfer
and the effects of surface conditions, thermal properties of the materials involved,
and natural and forced convection on cooling. Note that the surface conditions vary
during solidification, as a layer of air develops between the casting and the mold
wall due to shrinkage. Similar studies are being conducted on modeling the develop-
ment of microstructures in casting. These studies encompass heat flow, temperature
gradients, nucleation and growth of crystals, formation of dendritic and equiaxed
structures, impingement of grains on each other, and movement of the liquid-solid
interface during solidification.
Such models now are capable of predicting, for example, the width of the
mushy zone (see Fig. 10.4) during solidification and the grain size in castings.
Similarly, the capability to calculate isotherms (lines of equal temperature) gives in-
sight into possible hot spots and the subsequent development of shrinkage cavities.
With the availability of user-friendly software and advances in computer-aided de-
sign and manufacturing, modeling techniques are becoming easier to implement.
The benefits are increased productivity, improved quality, easier planning and cost
estimating, and quicker response to design changes. Several commercial software
programs, such as Magmasoft, ProCast, Solidia, and AFSsolid, are now available
for modeling casting processes.

l2.3 Casting Alloys

The general properties and applications of ferrous and nonferrous metals and alloys
were presented in Chapters 5 and 6, respectively. This section describes the properties
and applications of cast metals and alloys; their properties and casting and manufac-
turing characteristics are summarized in Fig. 12.4 and Tables 12.2 through 12.5. In
addition to their casting characteristics, some other important considerations in Cast-
ing alloys are their machinability and weldability, as alloys typically are assembled
with other components to produce the entire part.
The most commonly used casting alloy (in tonnage) is gray iron, followed by
ductile iron, aluminum, zinc, lead, copper, malleable iron, and magnesium. Shipments
of castings in the United States are around 14 million metric tons per year.

l2.3.l Nonferrous Casting Alloys

Common nonferrous casting alloys are the following:
Aluminum-based Alloys. Aluminum alloys have a wide range of mechanical prop-
erties, mainly because of various hardening mechanisms and heat treatments that can