Friction Weld, Spin
Weld, Linear Friction Weld
a. General.
(1) Friction welding is a solid state
welding process which produces coalescence of materials by the heat obtained
from mechanically-induced sliding motion between rubbing surfaces. The work
parts are held together under pressure. This process usually involves the
rotating of one part against another to generate frictional heat at the
junction. When a suitable high temperature has keen reached, rotational notion
ceases. Additional pressure is applied and coalescence occurs.
(2) There are two variations of the
friction welding process. They are described below.
(a) In the original process, one part
is held stationary and the other part is rotated by a motor which maintains an
essentially constant rotational speed. The two parts are brought in contact
under pressure for a specified period of time with a specific pressure.
Rotating power is disengaged from the rotating piece and the pressure is
increased. When the rotating piece stops, the weld is completed. This process
can be accurately controlled when speed, pressure, and time are closely
regulated.
(b) The other variation is inertia
welding. A flywheel is revolved by a motor until a preset speed is reached. It,
in turn, rotates one of the pieces to be welded. The motor is disengaged from
the flywheel and the other part to be welded is brought in contact under
pressure with the rotating piece. During the predetermined time during which
the rotational speed of the part is reduced, the flywheel is brought to an
immediate stop. Additional pressure is provided to complete the weld.
(c) Both methods utilize frictional
heat and produce welds of similar quality. Slightly better control is claimed
with the original process. The two methods are similar, offer the same welding
advantages.
b.
Advantages.
(1) Friction
welding can produce high quality welds in a short cycle time.
(2) No filler
metal is required and flux is not used.
(3) The process
is capable of welding most of the common metals. It can also be used to join
many combinations of dissimilar metals. Friction welding requires relatively
expensive apparatus similar to a machine tool.
c. Process
Principles.
(1) There are
three important factors involved in making a friction weld:
(a) The
rotational speed which is related to the material to be welded and the diameter
of the weld at the interface.
(b) The
pressure between the two parts to be welded. Pressure changes during the weld
sequence. At the start, pressure is very low, but is increased to create the
frictional heat. When the rotation is stopped, pressure is rapidly increased so
forging takes place immediately before or after rotation is stopped.
(c) The welding
time is related to the shape and the type of metal and the surface area. It is
normally a matter of a few seconds. The actual operation of the machine is
automatic. It is controlled by a sequence controller, which can be set
according to the weld schedule established for the parts to be joined.
(2) Normally
for friction welding, one of the parts to be welded is round in cross section.
This is not an absolute necessity. Visual inspection of weld quality can be
based on the flash, which occurs around the outside perimeter of the weld. This
flash will usually extend beyond the outside diameter of the parts and will
curl around back toward the part but will have the joint extending beyond the
outside diameter of the part.
(a) If the
flash sticks out relatively straight from the joint, it indicates that the
welding time was was too short, the pressure was too low, or the speed too
high. These joints may crack.
(b) If the
flash curls too far back on the outside diameter, it indicates that the time
was too long and the pressure was too high.
(c) Between
these extremes is the correct flash shape. The flash is normally removed after
welding.
