Friction welding is a solid-state welding process that generates heat
through mechanical friction between workpieces in relative motion to one
another, with the addition of a lateral force called "upset" to plastically
displace and fuse the materials. Technically, because no melt occurs,
friction welding is not actually a welding process in the traditional
sense, but a forging technique. However, due to the similarities between these
techniques and traditional welding, the term has become common. Friction welding
is used with metals and thermoplastics in a wide variety of aviation and
automotive applications. Benefits
The combination of fast joining times, and direct heat input at the weld
interface, yields relatively small heat-affected zones. Friction welding
techniques are generally melt-free, which avoids grain growth in engineered
materials, such as high-strength heat-treated steels. Another advantage
is that the motion tends to "clean" the surface between the materials being
welded, which means they can be joined with less preparation. During the
welding process, depending on the method being used, small pieces of the plastic
or metal will be forced out of the working mass. It is believed that the
flash carries away debris and dirt. Another advantage of friction welding is
that it allows dissimilar materials to be joined. This is particularly useful
in aerospace, where it is used to join lightweight aluminum stock to
high-strength steels. Normally the wide difference in melting points of the two
materials would make it impossible to weld using traditional techniques, and
would require some sort of mechanical connection. Friction welding provides a
"full strength" bond with no additional weight. Other common uses for these
sorts of bi-metal joins is in the nuclear industry, where copper-steel
joints are common in the reactor cooling systems; and in the transport of
cryogenic fluids, where friction welding has been used to join aluminum alloys to
stainless steels and high-nickel-alloy materials for cryogenic-fluid piping and
containment vessels. Friction welding is also used with
thermoplastics, which act in a fashion analogous to metals under heat and
pressure. The heat and pressure used on these materials is much lower than
metals, but the technique can be used to join metals to plastics with the metal
interface being machined. For instance, the technique can be used to join
eyeglass frames to the pins in their hinges. The lower energies and pressures
used allows for a wider variety of techniques to be used.
History Friction welding was first developed in
the Soviet Union, with first experiments taking place in 1956. The American
companies Caterpillar, Rockwell International, and American
Manufacturing Foundry all developed machines for this process. Patents were
also issued throughout Europe and the former Soviet Union. The most extensive
historical records are kept with the American Welding Society.
Metal techniques = Spin welding =
Spin welding systems consist of two chucks for holding the materials to be
welded, one of which is fixed and the other rotating. Before welding one of
the work pieces is attached to the rotating chuck along with a flywheel of
a given weight. The piece is then spun up to a high rate of rotation to store
the required energy in the flywheel. Once spinning at the proper speed, the
motor is removed and the pieces forced together under pressure. The force is
kept on the pieces after the spinning stops to allow the weld to "set". This
technique is also known as inertia welding, rotational welding or inertial
friction welding. = Linear friction welding =
Linear friction welding is similar to spin welding except that the moving
chuck oscillates laterally instead of spinning. The speeds are much lower in
general, which requires the pieces to be kept under pressure at all times. This
also requires the parts to have a high shear strength. Linear friction welding
requires more complex machinery than spin welding, but has the advantage that
parts of any shape can be joined, as opposed to parts with a circular meeting
point. Another advantage is that in most instances quality of joint is better
than that obtained using rotating technique.
= Friction surfacing = Friction surfacing is a process derived
from friction welding where a coating material is applied to a substrate. A
rod composed of the coating material is rotated under pressure, generating a
plasticised layer in the rod at the interface with the substrate. By moving
a substrate across the face of the rotating rod a plasticised layer is
deposited between 0.2–2.5 millimetres thick depending on mechtrode diameter
and coating material. Thermoplastic techniques
= Linear vibration welding = In linear vibration welding the
materials are placed in contact and put under pressure. An external vibration
force is then applied to slip the pieces relative to each other, perpendicular to
the pressure being applied. The parts are vibrated through a relatively small
displacement known as the amplitude, typically between 1.0 and 1.8 mm, for a
frequency of vibration of 200 Hz, or 2–4 mm at 100 Hz, in the plane of the joint.
This technique is widely used in the automotive industry, among others. A
minor modification is angular friction welding, which vibrates the materials by
torquing them through a small angle. = Orbital friction welding =
Orbital friction welding is similar to spin welding, but uses a more complex
machine to produce an orbital motion in which the moving part rotates in a small
circle, much smaller than the size of the joint as a whole.
Seizure resistance Friction welding may unintentionally
occur at sliding surfaces like bearings. This happens in particular if the
lubricating oil film between sliding surfaces becomes thinner than the
surface roughness, which may be the case for low speed, low temperature, oil
starvation, excessive clearance, low viscosity of the oil, high roughness of
the surfaces. The seizure resistance is the ability of
a material to resist friction welding. It is a fundamental property of bearing
surfaces and in general of sliding surfaces under load.
See also Friction stir welding
References External links
Forge Tech, Inc. Friction Forge Bonding Friction welding at TWI