Page 113 - 04. Subyek Engineering Materials - Manufacturing, Engineering and Technology SI 6th Edition - Serope Kalpakjian, Stephen Schmid (2009)

Chapter 3 Physical Properties of Materials

8
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Temperature, °C

FIGURE 3.3 Specific strength (tensile strengthfdensity) for a variety of materials as a function
of temperature. Note the useful temperature range for these materials and the high values for
composite materials. MMC-metal-matrix composite; FRP-fiber-reinforced plastic.

3.3 Melting Point

The melting point of a metal depends on the energy required to separate its atoms.
As Table 3.1 shows, the melting temperature of a metal alloy can have a wide range,
depending on its composition, and is unlike that of a pure metal, which has a defi-
nite melting point. The temperature range within which a component or structure is
designed to function is an important consideration in the selection of materials.
Plastics, for example, have the lowest useful temperature range, while graphite and
refractory-metal alloys have the highest useful range.
The melting point of a metal has a number of indirect effects on manufacturing
operations. Because the recrystallization temperature of a metal is related to its melt-
ing point (Section 1.7), operations such as annealing and heat treating (Chapter 4),
and hot-working (Part III), require a knowledge of the melting points of the materi-
als involved. These considerations are also important in the selection of tool and die
materials. Melting point also plays a major role in the selection of the equipment
and the melting practice employed in casting operations (Part Il). The higher the
melting point of the material, the more difficult the operation becomes. In the
electrical-discharge machining process (Section 27.5), the melting points of metals
are related to the rate of material removal and of electrode wear.

3.4 Specific Heat

A material’s specific heat is the energy required to raise the temperature of a unit
mass by 1 degree. Alloying elements have a relatively minor effect on the specific heat
of metals. The temperature rise in a workpiece, resulting from forming or machining
operations (Parts III and l\L respectively), is a function of the work done and of the
specific heat of the workpiece material (Section 2.12). An excessive temperature rise