Solid-State Welding Processes
The American Welding Society lists nine processes
and four subprocesses in the solid-state welding
group. Refer to Figure 2-6. Solid-state welding (SSW)
may be defined as a group of welding processes that
produce a fusion weld by application of pressure at a
welding temperature below the melting temperature
of the base metal and filler metal. Solid-state welding
may be done cold, warm, or hot, but never above the
melting temperature of the base or filler metal. The
following sections describe some of the more common
solid-state welding processes.
Friction Welding (FRW)
When two objects are rubbed together, they gen-
erate heat. This principle is the basis of friction
welding (FRW). Figure 29-10 shows the steps in a fric-
tion weld. One part is held stationary, while the other
part is held in a chuck and rotated rapidly. The parts
are pressed tightly together. Friction heats the two
parts to their welding temperature. The welding tem-
perature is lower than the solidus of either part, but
high enough to allow fusion under pressure. When
the welding temperature is reached, the rotation is
stopped. The parts are then suddenly forced together
under heavy pressure. After the heavy pressure is
applied, the parts are held firmly until they cool. The
finished weld is strong, with complete fusion over
the entire joining surface. As shown in Figure 29-10,
the weld is upset (enlarged) where the parts meet.
Friction welds are often produced in less than
15 seconds. Friction welding has been used successfully
to join some dissimilar (unlike) metals that normally
cannot be welded by other processes.
Friction Stir Welding (FSW)
Friction stir welding (FSW) is similar to friction
welding except a rotating tool, not a rotating part,
creates the heat. This welding process was developed
at TWI, Cambridge UK in 1991.
A special rotating tool, firmly pressing on the
parts to be welded develops heat. The rotating tool
has a wide shoulder and a probe or pin that pene-
trates the parts along the weld joint. Friction created
by the rapidly rotating tool softens the base metals
and the base metals are mixed together. Full penetra-
tion welds can be made. Figure 29-11 shows a draw-
ing of the process and a completed butt weld.
Ultrasonic Welding (USW)
Sound waves can cause vibrations in objects. For
example, a loud stereo can make the walls vibrate. In
ultrasonic welding (USW), a very high-pitched sound is
used to vibrate the surfaces of the metals to be welded.
Ultrasonic welding has many advantages. Since
there is literally no heat, there is no metal distortion.
Fluxes and filler metal are not needed. Very thin metals
can be joined easily. Ultrasonic welds are normally
small welds like resistance spot welds. Seam welds
can also be made.
Welding Technology Fundamentals
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Figure 29-9. Underwater welding electrode holder. Note that
the holder is well insulated, with no bare metal areas exposed.
(Broco, Inc.)
Figure 29-10. The steps in making a friction weld on two pieces
of 1″ (25.4 mm) diameter carbon steel. The rod at the left
is spun at a high speed. The rod at the right is then forced against
it. Friction creates enough heat to produce a strong weld.