The first patent proposing the use of friction as an efficient means of welding and processing materials was filed by Bevington in the late 19th century. Until then, friction had been regarded as a major inconvenience to engineers. In many situations it still is but today there are some 17 methods of using friction constructively to join, cut, clad and process materials.
- Rotary friction - widely used for joining bar, flange to tube etc
- Radial friction - used for pipe joining, shell banding
- Orbital friction - in which the relative motion of the parts is eccentric
- Linear and angular friction - as described below
- Friction stir welding - invented by Wayne Thomas at TWI and patented in 1991
- Friction seam welding - the original 1941 patent of Klopstock and Neelands
- Friction taper plug, hydropillar, stud and stitch welding - used for many applications especially repair of welds, castings and machine shop errors
- Friction plunge, third body friction welding, friction brazing, Tribserts TM - all processes used for joining dissimilar materials
- Friction extrusion, surfacing, transformation hardening - for reclamation and modification of material
Linear friction welding
Moving one part across the other through a small amplitude in a linear motion and at a suitable frequency extends friction joining to a wide range of complex profiles.
Although a UK patent was listed in 1969, no development appeared to take place until 1980 when TWI explored the benefits of the technology for rectangular cross section materials. Excellent welding was achieved in stainless steels and titanium alloys, with no gas shielding needed for titanium.
Following this success, TWI collaborated with industry partners who designed, constructed and installed at Abington the world's first dedicated linear friction welder. The process has had considerable subsequent success in aeroengine manufacture. It is ideally suited to batch production of specialised components in demanding materials whilst also being useful for general mass production of more straightforward materials.
A current international project involves eight organisations from four EU countries, part funded via the CRAFT programme, managed by TWI. The objective is to make the process more accessible, particularly to SMEs, by applying novel solutions to the machine design. The output will be the ability to build linear friction welding systems at greatly reduced capital cost.
More details can be obtained from Stephan Kallee at TWI. E-mail: firstname.lastname@example.org
Friction stir welding (FSW)
FSW was invented by Wayne Thomas at TWI and patented in 1991. The process involves a rotating tool traversing along the joint leaving a 'forged' weld in its wake.
For aluminium alloys, the process has been rapidly commercialised all over the world. Some 500km of welding has been done for road, rail and sea transport. The advantages of this process include reliability and reproducibility, no consumables, virtually no fume, minimal post weld distortion, the ability to weld alloys hitherto regarded as unweldable, excellent mechanical properties and no light, noise or spatter. The latter advantage recently won the TWI team an environment award.
Copper is also now being welded and TWI projects on welding titanium and steels are the next steps in extending the use of the friction stir process.
In 1992 when friction stir welding was very new, samples in 6mm thick 1200 and 8011 aluminium alloy were supplied to a potential user. Some time later, TWI received back some 0.5mm thick metal and a report. The company had rolled these samples down and it was not until there was over a 90% reduction that some signs of exfoliation became apparent. This was a one off exercise but some systematic research might achieve even more spectacular results.
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