Friction and forge welding processes for the automotive industry
Stephan Kallee and Dave Nicholas, TWI
Presented at International Body Engineering Conference, Detroit, USA, 28-30 September 1999 (Paper No. 99-IBECC-13)
A multitude of new friction welding processes is available for the automotive industry and many are being commercially exploited. Rotary friction welding is widely used in the manufacture of round parts such as steering columns and drive shafts. The first publications about the use of friction stir welding in car body engineering have been released during the last IBEC conferences [1,2,3,4], and new studies on friction stud welding and linear friction welding are now being proposed. This paper lists some of these processes and their applications in this industrial sector.
Conventional friction welding processes have been well accepted by the automotive industry, and are now widely used to produce safety relevant parts such as suspension parts, steering columns and driveshafts ( Fig 1). Several newer processes are currently under investigation by automotive companies and their tier 1 suppliers.
Fig 1. Rotary friction welded car parts (From top: two suspension rods, a steering column and a driveshaft produced by Blacks Equipment) 
Friction welding processes are forge welding processes in which the heat for joining metallic workpieces is generated by a relative motion of two components while a load is applied. They join workpieces in the solid phase without reaching the melting point of the materials. This allows many materials which cannot be joined by fusion welding processes to be welded. The weld region is normally narrow and shows a refined microstructure  . Using solid phase processes leads to high reproducibility and high productivity.
Rotary friction welding
Rotary friction welding can be performed in three ways - continuous drive, inertia friction welding or a combination of the two energy variants. In all these variants, friction welds are made by holding a rotating component in contact with a non-rotating component while under an axial load. Having achieved a suitable plasticised condition at the weld interface, the rotating part is stopped or allowed to come to rest, while the pressure is either maintained or increased to consolidate the joint.
The process is suitable for many dissimilar metal combinations. Metals with different microstructures as well as with differing thermal and mechanical properties can often be joined  . One of the inherent features of friction welding is the efficient utilisation of the thermal energy developed. The automotive manufacturers like the process because of the short cycle times, its excellent reproducibility and simple on-line monitoring of the welding parameters.
Fig 2. Rotary friction welding
For continuous drive friction welding the rotation speed of the component is generated during the friction phase by a continuously driven motor ( Fig 2). After friction heating has caused the pre-set burn-off, the relative motion is stopped and a higher forging force is applied.
Fig 3. Inertia friction welding
In inertia friction welding one of the components is connected to a flywheel, which is declutched from its drive when the right speed is reached ( Fig 3). On contact with the workpieces, friction at the weld interface acts as both a heat source and a brake.
The automotive industry has installed a large number of rotary friction welding machines for producing parts where at least one component has rotational symmetry. These are for example parts of the drive chain such as rear axles, drive shafts and gears which have high requirements on alignment and concentricity. Bimetallic engine exhaust valves, gear box components and airbag inflators are other typical automotive parts welded with this process ( Fig 4). Even aluminium alloy wheel rims are now being friction welded in mass production  . Often more than one weld can be carried out simultaneously, e.g. for joining round tubes to square hollow sections in the manufacture of vehicle structural assemblies  .
Fig 4. Rotary friction welded valves and a gear box fork 
Welding studs to plates
In friction stud welding a round stud is rotated and pressed against a sheet or plate ( Fig 5). The process can be used for joining dissimilar materials, e.g. joining a steel earthing pin to an aluminium car body. For welding small diameter studs to thin sheets, small portable friction stud welding machines are available, which can be operated either attached to a robot or hand held. Extensive studies are necessary to evaluate whether applying high rotation speeds can reduce the forging forces  .
Fig 5. Friction stud welding
In some cases, a mechanical interlock between the workpieces is requested, when joining dissimilar materials. The friction plunge welding process fulfils this requirement ( Fig 6). From the harder material a pin with a recessed area and a containment shoulder has to be machined. This pin is then rotated until the plasticised material of the softer workpieces is forged into this recess by the forces generated by the shoulder. The process offers significant technological benefits for extremely safety-relevant parts such as bimetallic towing hitches and drawbar couplings.
Fig 6. Friction plunge welding
The friction stud welding process can only be applied, if the workpieces have different forging temperatures. In cases of similar hardness, the use of an interlayer with a relatively low melting point can be considered. The interlayer is softened and is extruded between the two components being joined. The presence of re-entrant features promotes a good mechanical lock, even when a true metallurgical bond is not achieved ( Fig 7). The process is known as third body friction joining.
Fig 7. Third body friction joining
Linear and orbital friction welding
In 1990, Europe's first linear friction welding machine for joining metals was installed and commissioned at Abington ( Fig 8)  . The machine uses a fully balanced reciprocating mechanism to move one component across the face of the second rigidly clamped component and can provide frequencies up to 75 Hz, with amplitudes of up to ±3mm.
Fig 8. Electro mechanical linear friction welding machine for joining metals
Linear friction welding ( Fig 9) is suitable for joining metals with non-round cross-sections, and complex parts with a number of weld sites. Concepts have been developed to reduce the cost of linear friction welding machines, and the first automotive applications are expected very soon, such as the production of brake disks, wheel rims and engine parts.
Fig 9. Linear friction welding
Linear friction welding can be used for a large variety of alloys and no shielding gas is required even for those reacting with atmospheric gases, such as titanium ( Fig 10). The process has been proposed for the manufacture of wheel rims, where a generally planar piece of metal material is placed into a coil position with two ends in abutment  . The abutting ends are then joined by generating a linear reciprocating motion.
Fig 10. Linear friction welding of titanium in air
Fig 11. Orbital friction welding
Non round parts can also be joined by applying an orbital relative motion, as done in orbital friction welding. This motion can either be generated by moving one part with an orbital motion and to balance it by compensation weights, or alternatively the requested relative motion can be generated by rotating both workpieces with the same rotation speed in the same direction with the axes off set ( Fig 11). At the end of the friction phase, the relative motion will be stopped by moving the workpieces to the same rotation axis.
The potential advantage of using non-rotary motions is due to the achievement of the same relative speed over the full joining area compared to rotary friction welding, where the relative speed is zero in the workpiece centre and at a maximum at the circumference.
Friction taper stitch welding
A new friction welding method has been developed for the repair of cracks. It is known as friction taper stitch welding ( Fig 12). In this process a tapered hole is drilled to remove part of the crack. A tapered plug is then welded into this hole and the process is repeated with a series of overlapping holes until the whole crack is repaired  . This process could also be used to repair the defect at the end of welds, for example the end hole of friction stir welds or the frozen weld pool of keyhole processes (laser and EB).
Fig 12. Friction taper stitch welding
Friction hydro pillar processing
Friction hydro pillar processing (FHPP) can be used to fill holes by rotating a consumable bar in a hole, which has a slightly larger diameter than the bar. The rotating rod is pressed downwards with an axial load applied. One end of the rotating rod contacts the bottom of the hole and friction heats the rotating rod and the walls of the hole. During FHPP, the consumable rod is fully softened across the bore of the hole. This generates a plasticised layer, and conditions result, which let this layer climb to the top of the hole, i.e. through the thickness of the workpiece. Subsequently, the consumable material is transformed to a fine-grained microstructure. [13,14] . If tapered holes and consumables are used, the downward forces can be increased as the process continues. Additionally the increasing diameter of the tapered consumable guarantees that it is not sheared off before being plasticised.
The process is still in the development stage, but it has been successfully demonstrated for the repair of steel and aluminium structures. Manufacturers of armoured military vehicles and producers of bridge laying trucks are considering the application of this new process. It could also be used for the production of metal matrix composites, e.g. for automotive brake disks. Its effectiveness has been proven on reprocessing of cast Ni-Al bronze and Cupro-Nickel, where extremely fine microstructures can be achieved. A European Patent Specification has been granted on friction hydro pillar processing .
Friction stir welding
Friction stir welding (FSW) uses a non-consumable rotating tool, which moves along the joint between two components to produce high quality butt or lap welds ( Fig 13). The FSW tool is made with a profiled probe, which is contained in a shoulder with a larger diameter than that of the probe. For butt joining the length of the probe approximates the thickness of the workpiece. The probe is traversed through the joint line while the shoulder is in intimate contact with the top surface of the workpiece to avoid expelling softened material.
Fig 13. Friction stir welding
The FSW tools are manufactured from a wear resistant material with good static and dynamic properties at elevated temperature. They are made in a manner that permits over 1000m of weld to be produced in 5mm thick aluminium extrusions without changing the tool. The workpieces have to be clamped onto a backing bar and secured against the vertical, longitudinal and lateral forces, which will try to lift and push them apart. Development trials have established that a gap of up to 10% of the sheet thickness can be tolerated before weld quality is impaired.
The following components have been produced or are being considered to be produced by FSW:
- Tail lifts for lorries 
- Wheels [17,18]
- Tailor welded blanks (TWBs) 
- Suspension arms 
- Hollow panels [20,21,22,23]
- Single sheet panels with stiffeners [24,25,26,27]
- Fuel tanks ( Fig 14) [28,29,30,31,32,33]
- Hem joints and sandwich profiles [2,34,35]
- Explosively formed sheets [36,37]
Fig 14. Prototype cylindrical fuel tank with two longitudinal butt welds and four circumferential lap welds
The process can be applied to many joint designs. Butt and lap welds can be made even from materials with dissimilar thickness. Annular or circumferential joints can be produced by rotating the workpiece underneath the FSW machine, and CNC machines or robots are used for non-linear and three-dimensional joint lines. The development of new tongue and groove joints and swivel joints has been proposed to the transport industry for compensating angular and lateral tolerances ( Fig 15). A Group Sponsored Project is currently experimentally assessing them at TWI  . Dissimilar aluminium alloys can be joined (e.g. 5xxx to 6xxx series), and welding castings or extrusions to sheets is possible. Magnesium can be joined to either other magnesium alloys or to aluminium.
Fig 15. Variable position joint with two friction stir welds to seal the crevice
FSW was invented and patented  in 1991 at TWI in Cambridge, United Kingdom, and has been developed  to a stage where it is applied in production. Currently 42 organisations hold non-exclusive licences to use the process. Since the invention of FSW, a large number of application based patents have been filed in several countries.
By traversing a substrate underneath a rotating consumable, deposits can be laid onto that substrate, as the consumable tries to friction weld itself to the substrate. This process is being commercially applied in the manufacture cutting edges of blades ( Fig 16).
Fig 16. Commercial friction surfacing machine 
In the automotive industry friction surfacing could be used to apply wear resistant layers onto brake disks or drums ( Fig 17).
Fig 17. Friction surfacing of a cylindrical part
Magnetically impelled arc butt welding
Magnetically impelled arc butt welding (MIAB welding) is a forge welding technique, in which heat is generated prior to forging by an arc, which rapidly moves around the circumference between tubular work pieces. Although melting occurs at the ends of the workpieces, this process produces a microstructure similar to that of solid phase joints, as all molten material will be expelled into the flash, when the tubes are forged together. MIAB welding is also known as rotating arc, MBL or Magnetarc TM welding.
MIAB welding operates with extremely short weld times. It produces some flash but normally only little spatter. Only tubular workpieces can be joined but the process can be used for tubes with non-round shapes (e.g. square or hexagonal tubes). MIAB welding is currently only suitable for a maximum wall thickness of approximately 3mm. For thicker wall thickness material the arc fails to heat the full width of the joining faces, and so a successful weld cannot be achieved.
In general the MIAB process works as follows: To start the process, a DC voltage is applied to the workpieces, which are kept initially at a small distance. The tubes are then squeezed together and moved apart to strike the arc. Aradial magnetic field is applied to the joint area ( Fig 18). This field can either be produced by permanent magnets or by applying a direct current (DC) to coils into which pole pieces are inserted. The magnetic field affects the current in the arc, which is struck between the ends of the workpieces. The arc is initially accelerated in the tangential direction and follows then the circumference at a constant speed. After sufficient arc rotating time, when sufficient material has been melted, the tubes are forged together, immediately after switching off the electric current. The forging force is applied until the flash has been consolidated.
Fig 18. Magnetically impelled arc butt welding during arc rotation (left) and forging (right)
Although the original MIAB patents and publications date from the 1940s, significant exploitation has only really occurred since 1970  . The process is commercially mainly applied to joining mild steel tubes and/or cast iron parts. The automotive industry uses it for example to produce axles, benefiting from the very short cycle times.
Little progress has been made with aluminium and stainless steel alloys. Historically, the problems encountered when welding such materials have been ascribed to inadequate power sources, unsteady arc rotation, low rates of axial displacement and a lack of reproducibility. Commercial machine producers saw also a conflict in the relatively long gas purging time compared to the short welding cycle. It is now believed that improvements in power source technology, control systems, magnetic coil designs and gas shielding will provide opportunities for significant improvements in the process when welding these materials. TWI owns and operates a vertical MIAB machine which has been built especially for welding non-ferrous tubes ( Fig 19) and is currently developing welding procedures for these materials.
Fig 19. TWI's vertical MIAB welding machine for non-ferrous tubes
It is important to remember that all friction welding of metals takes place in the solid phase below the melting point of the materials to be joined. The benefits yielded therefore include: refined microstructures, little distortion, no porosity and the ability to join dissimilar or difficult to fusion weld materials. The following conclusions can be drawn from surveying mass-produced and prototype parts that are made by friction welding in the automotive sector:
- Rotary friction welding is well accepted by the automotive industry, especially for producing safety relevant parts such as steering columns, drive shafts and air bag inflators.
- Linear friction welding, which is already applied by the aero-engine manufacturers, offers significant benefits for the automotive industry, especially if the equipment cost could be reduced.
- A large number of pre-production prototype parts has been produced by friction stir welding and it is expected that the first mass production will start very soon. Typical workpieces are wheels, tailor welded blanks, engine cradles and suspension parts.
- Further R & D work is necessary to apply the newer friction welding processes in the production of automotive components.
Stephan Kallee, Dave Nicholas, TWI
FHPP: Friction hydro pillar processing
FSW: Friction stir welding
MIAB: Magnetically impelled arc butt
MMC: Metal matrix composite
TWB: Tailor welded blank
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