W M Thomas, E D Nicholas, E R Watts, and D G Staines
Paper presented at The 8 th International Conference on Aluminium Alloys, 2 nd to 5 th July 2002, Cambridge, UK
The friction processes include rotary friction welding, friction surfacing and friction stir welding (FSW). The principles involved are described.
Friction stir welding (FSW) has had a revolutionary effect in certain welding applications, so that now FSW is the first choice joining method for the fabrication of the aluminium alloy external fuel tanks for aerospace and for other extreme applications. This paper describes FSW tool evolution, and future applications.
The friction welding processes are solid phase techniques, which have advantages over certain conventional fusion processes that have limited metallurgical and mechanical properties and suffer from greater distortion.
Rotary friction welding
The earliest and most common friction welding technique is rotary, see Fig.1(a)
. The parts to be joined are subjected to dry friction during the initial contact, but quite rapidly, as a result of microscopic local seizures and subsequent rupture, a 'third-body' layer of finite thickness is formed. This transient 'third-body', effectively provides a quasi-hydrodynamic bearing. [1-3]
Fig.1. Friction welding variants
Two variants of the rotary friction welding process have been developed. These are known as conventional 'continuous drive friction welding' and 'stored energy friction welding'. The most widely adopted is inertia friction welding.
The essential difference between the two rotary friction welding methods is that, while under a constant or increasing axial load, continuous drive welding is carried out at a constant rotational speed (that may be changed to higher or lower constant rotational speeds at different stages of the weld cycle), while inertia welding starts at a relatively high rotational speed and progressively reduces to zero. The concept underlying inertia friction welding is that a predetermined amount of kinetic energy can be stored in a flywheel and converted into heat at the weld interface. Other non-rotary friction welding techniques are known, these include orbital, linear, and angular reciprocating friction welding.
Friction surfacing uses a consumable in the form of a solid round bar, or tube, and is now established as a surfacing technique. [4,5]
The basic principle of the friction surfacing process is illustrated in Fig.1b
. The deposit, a product of a hot forging action, is inherently homogeneous and of good mechanical strength. The interface region usually remains intact, even after resisting loads equal to the ultimate tensile strength of the weaker material. Fig.2
, shows a 6082 aluminium friction surfaced deposit.
Fig.2. Aluminium alloy 6082 friction-surfaced deposit on 6082 substrate with side bend tested specimen
Friction stir welding
Friction stir welding (FSW), Fig.1c
, was invented in 1991 and developed at TWI 
but is still regarded as a new process. FSW is a technique which allows aluminium, lead, magnesium, titanium steel and copper to be welded, continuously, with a non-consumable tool. [7-8]
The technique brings the benefits of solid-phase friction welding to certain materials regarded as difficult to weld by fusion processes. Fig.3
, shows details of the macrostructural features within a 25mm thick 6082-T6 condition friction stir welded test weld.
Fig.3. Macrostructural features in the weld region of an FSW weld produced in 25mm thick 6082-T6 aluminium alloy using a Whorl TM tool, at a weld travel rate of 4mm/sec (240mm/min)
Early in the development of FSW, it was realised that the form of the welding tool was critical in achieving sound welds with good mechanical properties. In general terms, the tools shown in Fig.4 comprise a shoulder and a probe.
Fig.4. Triflute family of probe variants for friction stir welding a) MX Triflute TM for butt welding b) Flared-Triflute TM with tip profile for lap welding
The shoulder compresses the surface of the workpiece and contains the plasticised weld region. Heat is generated on the surface by friction between the rotating shoulder and the workpiece surface and, when welding thin sheets, this is the main source of heat.
As the workpiece thickness increases, more heat must be supplied by friction between the rotating probe and the workpiece. Also, the main function of the probe is to ensure sufficient working of the material at the weld line and to control the flow of the material around the tool to form a satisfactory weld. The probe generally has a flat or re-entrant fluted shape profile.
Preferably, the probe has an odd number of equally spaced flats or flutes to maintain maximum cross-section opposite to any re-entrant feature. It should also be noted that all change in section are well radiused in order to reduce stress concentration to minimize the tendency of tool fracture during welding. In essence, the core of the probe is tapered to maintain approximately a uniform stress distribution owing to torsion and the forward thrust.
For butt welding, the probe is frustum shaped (see Fig.4a). Typically, the probe features for butt welding incorporates three helical flats or flutes and a coarse helical ridge (usually a coarse thread) around the lands of the probe. These flat or re-entrant features reduce the probe volume and provide a suitable swept volume to static volume ratio. The greater the volume ratio the greater the path for material flow and the more efficient the probe becomes. In addition, these re-entrant features help break up and disperse the surface oxides, within the joint region.
For lap welding a probe has been developed to provide a wider region and also to help avoid problems associated with upper plate thinning at the lap weld interface. The weld interface of a butt weld is essentially perpendicular to the workpiece plate surface and penetration through-the-thickness to the bottom of the plate is usually the main requisite. For lap welds the breadth of the weld interface and shape of the notch at the edge of the weld is of fundamental importance, especially for those applications that are subject to fatigue.  Figure 4b shows a Triflute flared probe, with straight flutes designed for lap welds. The core remains as a taper frustum but the flute lands are flared out at an inverted angle so as to increase the tip diamater. Moreover, a tip profile has been included in the shape of a three pronged whisk.  These features collectively increase the differential between the swept volume and the static volume of the probe further improving the flow path around and underneath the probe. The whisk type tip profile further provides and improved mixing action for oxide fragmentation and dispersal at the weld interface. A lap joint made with a Flared-Triflute TM probe is shown in Fig.5. In this example the width of the weld region measures 190% of the plate thickness and shows little upper plate thinning at the weld interface.
Fig.5. Macrosection showing a wide weld nugget produced by using a Flared-Triflute TM probe. Lap weld produced in 6mm thick 5083-0 condition aluminium alloy, at a weld travel rate of 4mm/sec (240mm/min)
Application of FSW
FSW is now increasingly used industrially for the welding of aluminium products for aerospace, rail rolling stock, automotive and marine applications.
With regard to the future, investigations at TWI are continuing to explore and develop a number of techniques for welding extrusions and hollow sections. The friction stir welding of certain thin walled hollow sections requires internal mandrel support, e.g. moving anvil, hydraulic actuated or scissor action support Fig.6a, b & c, illustrates some design concepts being studied at TWI
Fig.6a) Collapsible Mandrel support
Fig.6c) Hydraulic Bladder/Anvil support
Fig.6. Extrusion & hollow section supports for FSW
Invariably manufacturers want to fabricate their products as fast as possible. Increasing the welding speed or, as with certain applications, the use of purpose designed multi-headed friction stir welding machines, can increase productivity and profitability. Figure 7 illustrates a design concept for an opposed rotation multi-head friction stir welding machine. Such opposed tool rotation balances side force asymmetry and enable a reduction in the reactive torque normally necessary to clamp plates together. A roller clamping system would be used in conjunction with the multi-head technique.
Fig.7. Multi-head friction stir welding machine
This paper has described some of the friction welding techniques, which are currently in varying stages of technical development. These offer potential technical and commercial advantage for the aluminium industries. Some of these solid-phase techniques are able to achieve what was previously considered impossible by conventional welding and technology.
The authors wish to thank K I Johnson, I Norris, S W Kallee, P D Sketchley, N L Horrex, P Evans and S M Norris.
- Boldyrev R N and Voinov V P: Possible Reasons for the formation of Extremum of Torque During heating in Friction Welding Weld Prod. No. 1, (1980), p. 10-12.
- Godet M: The third-body approach: A mechanical view of wear Wear, Vol. 100, (1984), p. 437-452.
- Singer Irwin L: How Third-Body Process Affects Friction & Wear MRS Bulletin (1998), p. 6.
- Klopstock H and Neelands R A: An improved method of joining or welding metals British Patent Specification 572789, (1941).
- Nicholas E D and Thomas W M: Metal deposition by friction welding Welding Journal, (1986), p. 17-27.
- Thomas W M, Nicholas E D, Needham J C, Murch M G, Temple-Smith P and Dawes C J (TWI): Improvements relating to friction welding. European Patent Specification EP 0 615 480 B1. l2.espacenet.com/dips/viewer?PN=EP0615480
- Kallee S W, Nicholas E D and Thomas W M: Industrialisation of friction stir welding for aerospace structures Structures and technologies - challenges for future launchers third European Conference, (2001), Strasbourg, France.
- Cederquist L and Reynolds A P: Factors affecting the properties of friction stir welded aluminium lap joints Welding research supplement, (December 2001), pp 281-287.
- Boon T, Thomas W M and Temple-Smith P: Friction welding apparatus and method Patent No. US 6,325,273 B1 (priority date December 6, 1996)