welding has been used by the automotive industry for decades in the
manufacture of a range of components. The process is attractive for
· The friction heating is generated locally, so there is no widespread softening of the assembly
· The weld is formed across the entire cross-sectional area of the interface in a single shot process
· The technique is capable of joining dissimilar materials
process is completed in a few seconds with very high reproducibility -
an essential requirement for a mass production industry.
a new variant from the same stable of friction processes, known as
‘friction stir welding’, is finding increasing use in the fabrication
of automotive components, even though its processing speed is not yet
How does Friction Welding Work?
friction welding is carried out by moving one component relative to the
other along a common interface, while applying a compressive force
across the joint. The friction heating generated at the interface
softens both components, and when they become plasticised the interface
material is extruded out of the edges of the joint so that clean
material from each component is left along the original interface. The
relative motion is then stopped, and a higher final compressive force
may be applied before the joint is allowed to cool. The key to friction
welding is that no molten material is generated, the weld being formed
in the solid state.
Rotary Friction Welding
friction welding, in which one component is rotated against the other,
is the most commonly used of the processes, and many carbon steel
vehicle axles and sub-axles are assembled in this way. The process is
also used to fabricate suspension rods, steering columns, gear box
forks and driveshafts, as well as engine valves, in which the ability
to join dissimilar materials means that the valve stem and head can be
made of materials suited to their different duty cycles in service,
Figure 1. Friction welded parts including valves and a gearbox fork.
Linear Friction Welding
friction welding, (so named because the relative motion is linear
across the interface, rather than rotary), is already used to join
blades onto discs in the aeroengine industry. Lower cost linear
friction welding machines are now being developed for automotive
applications, such as the fabrication of brake discs, wheel rims and
Friction Stir Welding
stir welding also produces a plasticised region of material, but in a
different manner. A non-consumable rotating tool is pushed into the
materials to be welded and then the central pin, or probe, followed by
the shoulder, is brought into contact with the two parts to be joined,
figure 2. The rotation of the tool heats up and plasticises the
materials it is in contact with and, as the tool moves along the joint
line, material from the front of the tool is swept around this
plasticised annulus to the rear, so eliminating the interface.
Figure 2. Schematic of the friction stir welding process.
Advantages of Friction Stir Welding
process was patented in December 1991 and the first applications have
been in aluminium fabrications. The weld quality is excellent, with
none of the porosity that can arise in fusion welding, and the
mechanical properties are at least as good as the best achievable by
fusion welding. The process is environmentally friendly, because no
fumes or spatter are generated, and there is no arc glare or reflected
laser beams with which to contend.
major advantage is that, by avoiding the creation of a molten pool
which shrinks significantly on resolidification, the distortion after
welding and the residual stresses are low. With regard to joint fit up,
the process can accommodate a gap of up to 10% of the material
thickness without impairing the quality of the resulting weld. As far
as the rate of processing is concerned, for materials of 2mm thickness
welding speeds of up to 2m.min-1 can be achieved, and for 5mm thickness up to 0.75m.min-1. Recent tool developments are confidently expected to improve on these figures.
stir welding has been used to weld all wrought aluminium alloys, across
the 2xxx, 5xxx, 6xxx and 7xxx series of alloys, some of which are
bordering on being classed as virtually unweldable by fusion welding
techniques. The process can also weld dissimilar aluminium alloys,
whereas fusion welding may result in the alloying elements from the
different alloys interacting to form deleterious intermetallics through
precipitation during solidification from the molten weld pool.
stir welding can also make hybrid components by joining dissimilar
materials such as aluminium and magnesium alloys. The thicknesses of
6082-T6 that have so far been weld have ranged from 1.2mm to 50mm in a
single pass, to more than 75mm when welding from both sides. Welds have
also been made in pressure die cast aluminium material without any
problems from pockets of entrapped high pressure gas, which would
violently disrupt a molten weld pool encountering them.
original application for friction stir welding was the welding of long
lengths of material in the aerospace, shipbuilding and railway
industries. Examples include large fuel tanks and other containers for
space launch vehicles, cargo decks for high-speed ferries, and roofs
for railway carriages.
the automotive sector the drive to build more fuel efficient vehicles
has led to the increased use of aluminium in an effort save weight,
which also improves recyclability when the vehicles a scrapped.
Friction stir welding is being use increasingly to replace fusion
welding techniques when alumnium alloys are involve the main advantage
being low distortion and the ability to weld awkward materials material
combination The process has already been applied to the manufacture of
tail light panels by Marine Aluminium Aanensen in Norway, and SAPA in
Sweden have recently taken delivery of a new friction stir welding
machine from ESAB for the production of automotive parts. However, the
technique has not yet been adopted in production for the fabrication of
aluminium tanks for bulk road transport of liquids and powders.
regard to smaller components, automotive suspension arms have been made
in Japan by Showa Aluminium and Tokai Rubber, applying the formerly
large-scale process to components with diameters as small as 20-30mm.
Wheel assemblies using two aluminium alloys have been made by Hydro
Aluminium in Norway, figure 3, in which the butt or lap welds can be
fabricated in wrought and/or cast materials.
Figure 3. A friction stir welded aluminium wheel.
tailor welded blanks, in which dissimilar thicknesses of material are
welded together, are also being made by friction stir welding, for
example in the inner door panels of aluminium cars. The AO Smith
Corporation in the USA has developed a prototype aluminium friction
stir welded engine cradle, while in the same country Tower Automotive
has produced the Simulform connector for the fabrication of chassis
members and space frames, figure 4. Friction stir welding has also been
used to weld lightweight panels made of plastic foam sandwiched between
two sheets of aluminium, for which any fusion welding technique would
encounter serious problems because of the much higher temperatures
involved. Foamed aluminium itself has been friction stir welded too.
Other current applications under review include the bodies and floors
of coaches and buses, military bridge-laying vehicles (and
bridges/pontoons), and waste skips.
Figure 4. A friction stir welded aluminium connector.
Applications for the Future
has been the interest in friction stir welding, which was patented no
so long ago, that considerable effort is being made in transferring the
technological benefits from aluminium and magnesium to higher
temperature materials such as copper, titanium and steels. TWI has two
projects in conjunction with several industrial users to develop the
friction stir welding of titanium and steels.
project on the friction stir welding of aluminium alloys involving
industrial firms has also recently started. This is assessing
innovative weld joint geometries for friction stir welding aluminium
extrusions and wrought alloys together in transport structures. This
will take the process beyond its current use of mainly simple butt and
lap joint configurations and make it a much more flexible fabrication
process. Other firms who are interested in the benefits of adopting
this new technology can join these consortia.
the new millennium there is no doubt that the automotive sector will
find an increasing number of uses for this process as its
cost-effectiveness and ability to weld dissimilar material combinations
with minimal distortion is more widely appreciated.
message for lightweight vehicle design engineers is that they can also
specify alloys in welded components and structures that they had
previously shied away from using because of their inherent fusion