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Engineering Materials
Chapter 3
1
INTRODUCTION
Within the last couple of decades, very rapid development of engineering materials has
taken place, resulting in a huge number of commercially available materials with a
wide spectrum of properties.
The engineer's choice of material is based not only on the physical, chemical, and
mechanical properties but also on the technological properties, which describe the
suitability of a material for a particular manufacturing process.
IMPORTANT MATERIAL PROPERTIES IN MANUFACTURING
• It is very difficult to state exactly which properties or, more correctly, which
combination of properties a material intended for a given process must possess. But
it is often possible to identify certain dominating properties or characteristics which
any material must have for it to be processed by a given process or process group.
• To evaluate these technological properties, many specialized test methods have
been developed which describe suitability of a material for the particular process or
group of processes.
• Forming from the Liquid Material State: It includes the following phases:
Phase 1: melting Phase 2: forming (creation of shape) Phase 3: solidification
(stabilization of shape)
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INTRODUCTION
•
Forming from the liquid state requires primarily that the material can be melted,
and that furnace to do this is available. This depends on the range of melting points
and the requirements of the furnace equipment in producing a complete melt. These
requirements depend mainly on the chemical composition of the material. If the
melt can be produced, the next question is the availability of a suitable mold or die
material for an appropriate solidification.
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Forming from the Solid Material State
Forming from the solid material state can be carried out by mass-conserving processes, massreducing processes, or joining processes.
•
Mass-Conserving Processes
In the forming of metals, the primary basic process is mechanical plastic deformation. The
suitability of a material to undergo plastic deformation is determined primarily by its ductility
(measured by the reduction of area in the tensile test). The amount of plastic deformation
necessary to produce the desired component depends on the chosen surface creation principle
and the intended increase in shape information. In other words, the ductility of a given material
decides the surface creation principle and the information increase obtainable without fracture.
•
•
Stress-strain curves are the most important information source when evaluating the suitability
of a material to undergo plastic deformation. The strain at instability, the percent elongation,
and the reduction of area are the most important characteristics.
For most forming processes, there is a good correlation between the reduction of area and the
"formability" of the material. The stress-strain curves also reveal the stresses necessary to
produce the desired deformation. The stresses and strains and the resulting forces, work, and
energy are important for tool or die design and for the choice of process machinery.
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Forming from the Solid Material State
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As mentioned previously, the conditions under which a given process is carried out can
influence "formability" to a great extent. The important parameters are state of stress, strain
rate, and temperature. Concerning the state of stress, it can be stated that forming under
compressive stresses is generally easier than under tensile stresses, since the tendencies
toward instability and tensile fracture are suppressed.
Furthermore, a superimposed hydrostatic pressure increases formability (ductility) and is
utilized in certain processes. In most processes the state of stress varies throughout the
deformation zone; therefore, it can sometimes be difficult to identify the limiting state of
stress. As seen in Fig. 2.5, the strain rate also influences the ductility of a metal.
Increased strain rate leads to decreased ductility and an increase in the stresses required to
produce a certain deformation. The most commonly utilized industrial processes are carried
out at room temperature; consequently, the strain rate does not create problems. However, for
those processes that are carried out at elevated temperatures, the effects of strain rate must be
taken into consideration (see Fig. 2.5). High temperatures can result in a material
with a constant flow stress (yield stress) which is independent of the strain. In this state the
material is able to undergo very large deformations, as the temperature is above the
recrystallization temperature, where new strain-free grains are produced continuously and
almost instantly.
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Classification of some Engineering Materials
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