Page 114 - Manufacturing Engineering and Technology - Kalpakjian, Serope : Schmid, Steven R.

Section 3 6 Thermal Expansion

in a workpiece can decrease product quality by adversely affecting its surface finish
and dimensional accuracy, can cause excessive tool and die wear, and can result in
undesirable metallurgical changes in the material.

3.5 Thermal Conductivity

Thermal conductivity indicates the rate at which heat flows within and through a
material. Metallically bonded materials (metals) generally have high thermal con-
ductivity, while ionically or covalently bonded materials (ceramics and plastics) have
poor conductivity (Table 3.2). Alloying elements can have a significant effect on the
thermal conductivity of alloys, as can be seen by comparing the metals with their
alloys in Table 3.1. In general, materials with high electrical conductivity also have
high thermal conductivity.
Thermal conductivity is an important consideration in many applications. For
example, high thermal conductivity is desirable in cooling fins, cutting tools, and
die-casting molds to extract heat. In contrast, materials with low thermal conductiv-
ity are used, for instance, in furnace linings, insulation, coffee cups, and handles for
pots and pans.

3.6 Thermal Expansion

The thermal expansion of materials can have several significant effects, particularly
the relative expansion or contraction of different materials in assemblies such as
electronic and computer components, glass-to-metal seals, struts on jet engines,
coatings on cutting tools (Section 22.5), and moving parts in machinery that require
certain clearances for proper functioning. The use of ceramic components in cast-
iron engines, for example, also requires consideration of their relative expansions.
Typical coefficients of thermal expansion are given in Table 3.1. (See also Im/ar
below.) Generally, the coefficient of thermal expansion is inversely proportional to
the melting point of the material. Alloying elements have a relatively minor effect on
the thermal expansion of metals.
Shrink #ts utilize thermal expansion and contraction. A shrink fit is a part,
often a tube or hub, that is to be installed over a shaft. The part is first heated and
then slipped over the shaft or spindle; when allowed to cool, the hub shrinks and the
assembly becomes an integral component.
Thermal expansion in conjunction with thermal conductivity plays the most
significant role in causing thermal stresses (due to temperature gradients), both in
manufactured components and in tools and dies, and molds for casting operations.
This consideration is particularly important in, for example, a forging operation
during which hot workpieces are repeatedly placed over a relatively cool die, sub-
jecting the die surfaces to thermal cycling. To reduce thermal stresses, a combination
of high thermal conductivity and low thermal expansion is desirable. Thermal
stresses can also be caused by anisotropy of thermal expansion; that is, the material
expands differently in different directions, a property generally observed in hexago-
nal close-packed metals, ceramics and composite materials.
Thermal expansion and contraction can lead to cracking, warping, or loosen-
ing of components during their service life, as well as cracking of ceramic parts and
in tools and dies made of relatively brittle materials. Thermal fatigue results from
thermal cycling and causes a number of surface cracks, especially in tools and dies