ALUMINUM - Materials for Civil and Construction Engineers - page 153

400

Ultimate strength

300

Yield strength

200

100

0

200 150 100 50 0 50

Temperature,

100 150 200 250

C

FIGURE 4.6 Tensile strength of aluminum at different temperatures.
(Courtesy of the Aluminum Association, 1987.)

Aluminum's coefficient of thermal expansion is 0.000023/°C (0.000013/°F),
about twice as large as that of steel and concrete. Thus, joints between alu-
minum and steel or concrete must be designed to accommodate the differential
movement.
Strengths of aluminum are considerably affected by temperature, as
shown in Figure 4.6. At temperatures above 150°C (300°F), tensile strengths
are reduced considerably. The temperature at which the reduction begins
and the extent of the reduction depends on the alloy. At temperatures below
room temperature, aluminum becomes stronger and tougher as the temper-
ature decreases.

4.4 Welding and Fastening

Aluminum pieces can be joined either by welding or by using fasteners.
Welding requires that the tough oxide coating on aluminum be broken and
kept from reforming during welding, so arc welding is generally performed
in the presence of an inert gas that shields the weld from oxygen in the at-
mosphere. The two common processes by which aluminum is welded are
gas metal arc welding, GMAW, and gas tungsten arc welding, GTAW. In the
GMAW process, the filler wire also serves as the electrode. GTAW uses a
tungsten electrode and a separate filler wire. Welding can alter the tem-
pering of the aluminum in the area of the weld. For example, the tensile
strength of 6061-T6 is 290 MPa (42 ksi), but the tensile strength of a weld
in this alloy is only about 165 MPa (24 ksi). For design purposes, it is as-
sumed the weld affects an area of 25 mm (1 in.) on each side of the weld.
In addition to welding, either bolts or rivets can join aluminum pieces.
Bolts can be either aluminum or steel. When steel bolts are used, they must