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Friction Stir Welding

As mentioned earlier, FSW was the parent technology invented by Thomas et al. in 1991. In the last ten years, this technology has taken off in a number of applications due to a number of benefits (Table 1). Apart from these advantages, it can be considered as enabling technology for joining of high strength aluminum alloys which are classified as unweldable by fusion welding techniques. The joint efficiency of 75-96% has been reported for 7XXX and 2XXX alloys (Table 2). The research and development activities in FSW have reached a critical level and TWI organizes one international conference every year from 1999 and a number of other conferences are organized every year by various materials related societies around the world. The developments in FSW have been periodically reviewed in the last few years and therefore only limited information is presented here. The readers will find the conference proceedings from the TWI, ASM and TMS particularly informative.


Table 1. Benefits of Friction Stir Welding
Metallurgical Benefits Environmental Benefits Energy Benefits

· Solid phase process

· Low distortion of workpiece

· Good dimensional stability and repeatability

· High joint strength

· No loss of alloying elements

· Excellent metallurgical properties in the joint area

· Fine microstructure

· Absence of cracking

· Replace multiple parts joined by fasteners

·No shielding gas required

· No surface cleaning required

· Eliminated grinding wastes

· Eliminates solvents required for degreasing

· Consumable materials saving, such as rugs, wire or any other gases

· Improved materials use (e.g., joining different thicknesses) allows reduction in weight

· Only 2.5% of the energy needed for a laser weld

· Decreased fuel consumption in light weight aircraft, automotive and ship applications

Table 2. Friction stir weld joint efficiency for various aluminum alloys.
Alloy Base metal UTS (MPa) Friction stir weld UTS (MPa) Joint efficiency (%)

2024-T351 (5mm)

2024-T3 (4mm)

2519-T87 (25.4mm)

7050-T7451 (6.4mm)

7075-T7351

7075-T651 (6.4mm)

483-493

478

480

545-558

472.3

622

410-434

425-441

379

427-441

455.1

468

83-90

89-90

79

77-81

96

75

The material flow and microstructural development during FSW depend on the tool design and process parameters. In the beginning, the tool was quite simple with a cylindrical pin and shoulder (Figure 3a). With the simple tool design material flow and mixing was limited and in turn the welding speeds were relatively low. The tool design has progressively become more complicated and its design features have evolved to move and mix material more efficiently. Figure 3 shows a few pin and shoulder designs. With these design innovations from TWI, the welding speeds of more than 1 m/min are currently being commercially practiced for low strength aluminum alloys.