usually by about 2 degree."/>
Figure 1.35 Friction stir butt welding: (a) tool (b) photo of tool; (c) butt welding; (d) tilting tool forward, usually by about 2°. Tight clamping of the workpiece (not shown) is required during welding.
To keep the workpiece from moving during welding, the workpiece must be clamped down very tightly and supported firmly at the bottom by an anvil. To initiate welding, the rotating tool is plunged into the workpiece until the shoulder is slightly below and rubs the workpiece surface. Friction heat softens the material around the tool. After a brief dwell time, the rotating tool is traversed along the joint. Welding is achieved by plastic flow of frictionally heated material from ahead of the pin to behind it. At the end of the weld, the rotating tool is withdrawn, leaving behind a small crater (or keyhole). The tool is typically tilted forward about 2° during welding. This allows the tool to move forward without shaving the surface of the workpiece and to compress the stirred material under the trailing portion of the shoulder.
Figure 1.36 shows the transverse cross‐section of the stir zone and its surroundings. In the stir zone, which is also called the nugget, very fine grains form by dynamic recrystallization. In the thermally affected zone, which is equivalent to the HAZ of an arc weld, no deformation of grains occurs. Between the stir zone and the HAZ, a thermomechanically affected zone may exist, where grains are twisted by the action of tool rotation without dynamic recrystallization.
Figure 1.36 Stir zone (dynamically recrystallized zone) and HAZ (thermally affected zone) in a friction stir weld. A thermomechanically affected zone may exist between the two, where grains are only twisted but not dynamically recrystallized into very fine grains.
FSW is also suitable for making a lap weld, as illustrated in Figure 1.37. The pin needs to be longer than the thickness of the upper sheet in order to penetrate into the lower sheet to stir it and weld it to the upper sheet. When the process is used to make a spot weld, it is called friction stir spot welding (FSSW). As illustrated in Figure 1.37, in FSSW the rotating tool dwells at one spot after the penetration depth is reached. In friction stir lap welding, however, the rotating tool travels after reaching the desired penetration depth.
Figure 1.37 Friction stir lap or spot welding: (a) plunging rotating tool into upper sheet; (b) dwelling (in spot welding) or traversing (in lap welding) rotating tool after pin tip penetrating into lower sheet and shoulder penetrating upper sheet slightly; (c) retracting rotating tool.
FSW is ideal for welding Al alloys. Joint designs for FSW include butt, lap, T‐, fillet, and corner joints. Mg alloys and Cu alloys are also very weldable by FSW. FSW of steels is much more difficult because the tools need to be very hard and durable. Tools such as W, W─Re, and pyrolytic BN have been used for FSW of steels.
The major advantages of FSW are as follows: (i) solid‐state welding, allowing joining of materials difficult to fusion weld (e.g. 2000 and 7000 series aluminum alloys, which often crack in arc welding); (ii) low distortion; and (iii) excellent mechanical properties due to fine grains in the stir zone. There are some limitations, however: (i) tool wear is a problem in FSW of steels, stainless steels, and titanium alloys; (ii) rigidly clamping of the workpieces is required; (iii) a backing anvil is required; and (iv) a keyhole is left at the end of the weld.
1.6.2 Friction Welding
Friction welding is a solid‐state joining process that joins metals by heating two surfaces through mechanically induced rubbing motion and forcing them against each other. Figure 1.38 shows rotary friction welding (RFW). First, one member of the workpiece is rotated and the other is stationary. When the appropriate rotational speed is reached, the stationary member is forced against the rotating one to cause friction heating to soften the materials at the faying surfaces. This is the rubbing or friction stage, during which the welding heat is developed. After that, the force is increased significantly to squeeze out the softened materials and their oxide films as flash. This is the upsetting or forging stage, during which weld is further heated, the softened materials and their oxide films are squeezed out as flash to allows fresh, oxide‐free materials to bond together. Under the combined action of heating and forging, a hot plasticized layer is formed between the two workpieces in which material mixing occurs. Excessive upsetting results in too big a flash while insufficient upsetting can cause poor bonding.
Figure 1.38 Schematic illustration of rotary friction welding: (a) first piece (axisymmetric) rotating; (b) second piece brought into contact with first piece to cause friction heating; (c) second piece forced against first piece to squeeze materials and oxide films out of the joint area as flash; (d) rotation stopped.
There are two variations of RFW. In direct‐drive (or continuous‐drive) friction welding, the rotating member of the workpiece is directly attached to a motor drive unit to rotate at a constant speed while rubbing against the nonrotating member. The motor is stopped after a predetermined heating time or until a preset axial shortening (called upsetting) occurs. The drive is then disengaged to allow the rotating member to be braked to a stop. In inertia friction welding, on the other hand, the rotating member of the workpiece is connected to a flywheel driven by a motor. After the flywheel is accelerated to the predetermined speed, it is disengaged from the motor. The nonrotating is then pushed against the rotating one until the rotation stops.
There are at least four advantages of RFW:
1 The HAZ is very narrow because the friction is localized at the interface.
2 Dissimilar metals can be welded because there is no melting to cause massive brittle intermetallic compounds to form.
3 Welded joints produced by this process are often as strong as the base metal.
4 The power requirement is low.
There are three disadvantages to friction welding:
1 In general, at least one member of the workpiece must have an axis of symmetry and be capable of being rotated about that axis.
2 Careful preparation and alignment of the members is required to ensure uniform rubbing and heating, particularly with large diameters.
3 Equipment
