The first time people hear about friction stir welding (FSW) they usually state: “How does it work?” After a brief introduction describing the process their next reaction is always disbelief: “And that works?” Without seeing the very unspectacular process themselves – looking, touching, feeling – they cannot believe the extraordinarily good properties described to them... A very impressive way to demonstrate these properties is to give them a bending sample not yet bent and let them perform the 180° bending test themselves... This is a very effective way of getting rid of any remaining doubts... Watching the process is always impressive, although as said earlier, it is quite unspectacular...
All one sees is a rotating tool pushing through material... Usually there are no fumes, spattering, red glowing material and the noise is also quite low, making it difficult to believe that such a simple process can produce such good joints (it is similar to inspecting a vacuum cleaner which is not loud enough when it is used - People will not believe that its suction performance is sufficient)... So for FSW there is a great need to demonstrate and show the process to the world in order to have a widespread introduction into many industry sectors... The demand for increasing energy density of fusion welding is a major driving force for a number of important innovations and developments in fusion welding... For solid state welding, the thermomechanical principle of friction welding had actually laid an important base for the later invention of FSW...
The Welding Institute (TWI) in the UK had for years engaged in various R&D and industrial activities on friction welding and surfacing... Wayne Thomas and his colleagues in TWI had long worked on and developed a number of variants of friction welding... In particular, they developed friction extrusion, friction hydropillar processing and third-body friction joining processes... Over the long period of working on and developing these materials processes, the group in TWI observed and studied a number of important phenomena and accumulated an in-depth working knowledge of those processes...
These include the highly plasticized third-body effect and the transportation phenomena of the plasticized material, the adiabatic heating during deformation, and the relationship between the torque and rotation speed when a sufficient amount of the plasticized material is present during processing... With the in-depth understanding and working knowledge of the various friction-based material processes, a welding technique with an effective transportation mechanism for the plasticized material suited for a wide range of geometries of structures to be welded was never far from the minds of Wayne Thomas and his colleagues...
One day in 1991, in a flash of inspiration, Wayne Thomas realized that with the use of a rotational probe of a harder material than the workpieces, the workpiece material could be plasticized and an effective transportation mechanism could be provided for the plasticized material to join the workpieces together... This eventual moment of realization, after a long period of gestation, marked the discovery of FSW as we know it nowadays... Based on this discovery, the engineering restriction is quite low for applying the simple friction stir action provided by the probe for welding to a very large range of structures/parts and a wide range of weld geometry...
Welding without gross melting is highly significant, as welds can then be readily made free of solidification-related porosities and cracking and with low distortion...
Furthermore, there is no need to use protective gases, at least for aluminium alloys, no arc-related emissions and no fumes... Thus FSW is an environmentally friendly process... Furthermore, no filler material is needed during FSW... This gives a further advantage that formation of unwanted phases in the weld microstructure due to the mixing of the filler metal and parent metal, which often differ to a certain extent in alloying, can be avoided... The industrial significance of FSW, due to its distinctive advantages, was immediately realized...
Wayne Thomas, representing TWI filed the first patent for FSW in 1991... To date it is with aluminium alloys that FSW is most successfully applied. The reason for the predominant use of FSW on aluminium alloys is a combination of process simplicity in principle and the wide use of aluminium alloys in many major industries... It is especially the case where some aluminium alloys are difficult to fusion weld as, for example, is clearly evident in FSW application made by Boeing for making the Delta 2 rocket tanks...
When the variable polarity plasma arc welding process was used, the defect rate was over 90%... FSW allowed them to dramatically reduce their defect rate to nearly zero!!! Maximum temperature during FSW can reach just below the solidus of the workpiece alloy... For most aluminium alloys, it is significantly less than 660∞C... Thus, H13 tool steel or high-speed tool steel, which is quite inexpensive, is a satisfactory tool material... Therefore, FSW of aluminium alloys is relatively straightforward, although FS engineering, particularly for components and structures of high geometry complexity, can be quite challenging...
In principle, FSW could be applied for welding of all solid metallic materials... The practical restriction is primarily the integrity issue of the tool during FSW of high temperature materials... For example, during FSW of steels, the local operating temperature generated by both friction and deformation needs to be at 1100–1200°C so that the workpiece material is sufficiently plasticized for stirring and welding... Such high operating temperatures and the necessary forces acting on the tool during FSW create an
extraordinary demand on the mechanical properties of the tool material... Forces and torque during FSW require the structures/parts to be welded quite rigidly clamped and the tool rigidly positioned... The precision of positioning also needs to be high... Thus, from an engineering point of view, it is reasonable to suggest that the application of FSW is more suited to fabrication environments... As it has turned out, during the last 15 years, FSW has actually enabled many manufacturing processes to become significantly more efficient or enabled new and efficient manufacturing routes to be developed...
The first FSW industrial application was a good example of FSW not only being a welding technology but also an enabling technology... After an initial intensive industrial FSW development program (TWI and Hydro Aluminium) specifically for aluminium shipbuilding, FSW was successfully applied in 1995 to weld aluminium extrusions into large panels with minimum distortion for shipbuilding... The use of these large prefabricated panels without the large amount of fit up work associated with the use of fusion welding and resulted in a significantly more cost-effective route of building fishing boats and high-speed ferries... Since then, design and building of various types and sizes of sea vessels
using prefabricated panels by FSW has become a normal and cost-effective industry practice...
Not long after the first major application of FSW, Hitachi in Japan started the intensive development of applying the technology... One of the major applications has resulted from the use of FSW for welding long aluminium extrusions... The best example of this is the manufacturing of modern traincar bodies by FSW of long aluminium extrusions into double or single skin structures... The use of FSW with low distortion eliminates post-weld straightening and filling... Because of this feature of welding long extrusion with very low distortion, by 2000, Hitachi had incorporated FSW in their cost-effective modular design and manufacturing of high integrity high-speed train-car bodies...
However, the biggest interest in FSW has actually been from the aerospace industry, starting FSW development work in the early-mid 1990s on structures such as the Delta rocket fuel tank (Boeing), aircraft structures (Airbus) and external fuel tank barrel section (NASA )... These and their related organizations, through the development and application of FSW to many aerospace structures over the years, have made a huge forward step in FSW... In late 1990s, the newly formed Eclipse Aviation designed a manufacturing route of making small jets incorporating the intensive use of FSW in the major assembling of Eclipse 500 jets... They have eliminated 70% of their rivets on the aircraft by welding mainly overlap connections in 2024, 7075 or dissimilar 2024 to 7075 alloys... By doing so they have been able to accomplish a production rate of four aircraft per day, which will then lead to significant, reduced lead times for their customers...
It is worthwhile to point out that it is quite logical for the aerospace industry to have a strong interest in applying FSW technology... Traditionally, riveting is the dominant joining method in aircraft manufacturing, as the bulk of the high strength aircraft grade aluminium alloys are susceptible to weld defects associated with fusion welding... Since it is a solid state welding process and thus there is no solidification-associated porosity and cracking, all the traditionally unweldable aluminium alloys have become weldable using FSW...
Underlying PatentsThe original patent for friction stir welding was filed by The Welding Institute (TWI) in Cambridge, UK... It was initially filed in the United Kingdom in December 1991 and was granted as EP 0 615 480... There are two major claims in the patent. Claim 1 states: “A method of joining workpieces defining a joint region there between, the method comprising carrying out the following steps without causing relative bodily movement between the workpieces... Causing a probe of material harder than the workpiece material to enter the joint region and opposed portions of the workpieces on either side of the joint region... While causing relative cyclic movement between the probe and the workpieces whereby frictional heat is generated to cause the opposed portions to take up a plasticized condition... Then removing the probe, and allowing the plasticized portions to solidify and join the workpieces together...”
In Claim 2, the movement of the probe along the joint line is described... Further to this, many more items are already identified in this original patent such as the materials,
interrupted or spot welding.. Crack repair welding, tool configurations regarding motions (rotation, oscillation, reciprocation), adjustable and self-reacting as well as heating of tools... TWI’s interest has not been to keep this technology only for themselves, but they have made great efforts to spread this technology into the world engineering community by issuing FSW licences to a wide range of organizations including end users, equipment suppliers, academia and R&D institutes... These organizations have further developed the process and its applicability... This becomes obvious when analyzing the high number of patent filings... Especially the end users have used the process of filing patents in order to protect the use of FSW in their applications, which makes up about 90% of the filed patents...
Not many processes have ever reached as many patents as FSW has reached in only 23 years... This process of FSW has very quickly found its way into industries... For new end users, of course, an important question to ask is – "Am I infringing on a patent, if I use it on my application?" – But normally the answer should be no, as most of the use of FSW is included in the basic patent of TWI to which a licence is accessible and therefore further applications should not be prevented... Thoughts?
Respectfully,
Henry
