Friction stir welding of aluminium ships
Fred Delany, Stephan W Kallee and Mike J Russell
TWI China, Baliqiaobei Chaoyang District, P O Box 863, 100024 Beijing, P.R. China
Tel: +86 (0)10 8570 3255, firstname.lastname@example.org
Paper presented at 2007 International Forum on Welding Technologies in the Shipping Industry (IFWT) Held in conjunction with the Beijing Essen Welding and Cutting Fair in Shanghai, 16-19 June 2007
In friction stir welding (FSW), which has been invented and patented by TWI (References  and  ), a wear resistant rotating tool is used to join sheet and plate materials such as aluminium, magnesium and copper. In laboratory experiments, zinc, lead, titanium, nickel and steel have been friction stir welded. The welds are made below the melting point in the solid phase. The excellent mechanical properties and low distortion produced by FSW are attributed to the low heat input and smooth profile of the weld.
The relatively low temperatures generated during friction stir welding permit joining of thin aluminium skins of honeycomb or sandwich panels, avoiding delamination of skins and core. Low temperature FSW also permits a number ofdissimilar material welds to be made.
Since the invention of friction stir welding at TWI in 1991, companies from all parts of the world have implemented the process, predominantly in the fabrication of aluminium components and panels. Trendsetters were the Scandinavian aluminium extruders, who were in 1995 the first to apply the process commercially for the manufacture of hollow aluminium deep-freeze panels, and for ship decks and bulkheads. Friction stir welded structures are now revolutionising the way in which high-speed ferries, hovercraft and cruise ships are built from prefabricated lightweight modules ( Fig 1&2).
In the shipbuilding industry several companies use the FSW process for the production of large aluminium panels, which are made from aluminium extrusions. Commercial FSW machines are now available and include complete installations to weld up to 16m lengths. Currently 171 organisations hold non-exclusive licences from TWI to use the process. Most of these licensees are industrial companies, who exploit the FSW process in commercial production in Japan, USA,China, Europe and Scandinavia.
Fig.1. Marine Aluminium's prefabricated FSW deck panels for 'The World' cruise ship
Fig.2. Fosen Mek's cruise ship 'The World' contains friction stir welded decks
Friction stir welding (FSW) uses a non-consumable rotating tool, which moves along a joint line between two components to produce high quality butt or lap welds. The FSW tool consists of a profiled pin, which is contained in ashoulder of larger diameter than that of the pin ( Fig.3). For butt welding, the length of the pin is similar to the thickness of the workpiece, so that the tool penetrates almost completely through the joint line. The pin is traversed through the joint while the shoulder isin intimate contact with the top surface of the workpiece to avoid expelling softened material. The process can be viewed as a method of in-situ extrusion around the rotating tool probe to produce a continuous solid-phase weld.
The FSW tools are manufactured from a wear resistant material with good static and dynamic properties at elevated temperature. They are made in a manner that typically permits up to 1000m of weld to be produced in 5mm thick aluminium extrusions without changing the tool. The workpieces have to be clamped onto a backing bar and secured against vertical, longitudinal and lateral forces, which will try to lift them and push them apart. Development trials have established that a gap of up to 10% of the sheet thickness can be tolerated before weld quality is seriously impaired.
Fig.3. Schematic showing friction stir welding principle a) Unaffected material b) Heat affected zone (HAZ) c) Thermomechanically affected zone (TMAZ) d) Weld nugget (Part of thermomechanically affected zone)
As with all friction processes, FSW is a solid phase welding method which operates below the melting point of the workpiece material. FSW can weld all aluminium alloys, including those such as aluminium-lithium alloys that cannot normally be joined by conventional fusion welding techniques. Dissimilar aluminium alloys can also be joined (e.g. 5000 to 6000 series or even 2000 to 7000 series). No shielding gas or filler is required for welding aluminium alloys.TWI has developed FSW for aluminium alloys in the thickness range of 0.8mm to 75mm ( Fig.4).
Fig.4. Double sided friction stir weld in 75mm thick aluminium plates produced at TWI
Specially profiled FSW tools have been designed and tested at TWI (Reference  ). The technology of FSW tool designs and welding parameters has been developed to serve industrial demands. The stirring effect of the tool is clearly visible in transverse macrosections if different types of materials have been welded such as extrusions to wrought sheets, or wrought aluminium sheets to cast aluminium ( Fig.5). The 'onion ring' like structure of the nugget is typical of high quality stir welds, in which no porosity or internal voids are detectable.
Fig.5. Transverse macrosection of 6mm thick wrought aluminium welded to cast aluminium
The FSW process can also be applied to copper, magnesium, zinc and lead. Laboratory experiments on steel, titanium and nickel sheets and plates are showing considerable success. Prelimi¬nary trials have also yielded encouraging results when FSW was used to join aluminium based metal matrix composites (MMCs), oxide dispersion strengthened (ODS) alloys, and when the process was applied to dissimilar materials, such as steel to aluminium joints.
In macrosections of good quality welds in aluminium alloys a well-developed nugget is visible at the centre of the weld, as shown schematically in Fig.3. Outside the nugget there is a thermomechanically affected zone, which has been plastically deformed and shows some areas of partial recrystallisation. The overall shape of the nugget is variable, depending on the alloy used and the actual process conditions. The diameter of the nugget is typically slightly greater than that of the pin, and significantly less than the shoulder diameter.
3. FSW applications the shipbuilding industry
3.1. Freezer panels
The first commercial application of friction stir welding concerned the manufacture of hollow aluminium panels for deep freezing of fish on fishing boats ( Figs 6 & 7). These panels are made from friction stir welded aluminium extrusions. The minimal distortion and high reproducibility achieved make FSW both technically and economically a very attractive method to produce these stiff panels.
Fig.6. Sapa FSW panel for pre-pressing of fish blocks
Fig.7. Joint design of Sapa's freezer panels (weld penetration 4.5mm, total weld length 16m)
3.2. Honeycomb panels and corrosion resistant panels
New FSW applications are now being reported in Japan, where the process is being used to produce honeycomb panels ( Fig.8) and sea water resistant panels ( Fig.9). The latter are made from five 250mm wide 5000 series aluminium extrusions joined by FSW to make a panel of 1250 x 5000mm. These panels are used for ship cabin walls because of the good flatness of the welded structure, and the excellent mechanical properties of the welds.
Fig.8. Friction stir welded honeycomb panel produced by Sumitomo Light Metal
Fig.9. Ship panel from AA5083-H112 extrusions by Sumitomo Light Metal
3.3. Panels for deck and wall construction of high-speed ferries
Pre-fabricated wide aluminium panels for high-speed ferry boats can be produced by friction stir welding and are commercially available. The panels are made by joining extrusions, which can be produced in standard size extrusion presses. Compared to fusion welding, the heat input is very low and this results in low distortion and reduced thermal stresses. 1700 panels with an overall weld length of 110km have been produced and delivered by Marine Aluminium in Haugesund (Norway) from 1996 to 1999. After welding the panels can be rolled for road transport, as they are stiff only in the longitudinal direction. When they are transported by ship, they can be stacked on top of each other.
Friction stir welded panels are now being used all over the world for high-speed ferries, oil rigs, hovercraft and cruise ships. Large prefabricated aluminium modules can be lifted by crane into ships, to save time during the final assembly. The total length of friction stir welds made by Sapa in Finspång (Sweden) up to date is probably more than 2,000km. The commercial use of structures produced by FSW has been supported by component approvals, based on an appropriate welding procedure specification (WPS), for each case.
Component approvals have been conducted by the following classification organisations:
Table 1. International surveying bodies and classification societies have approved FSW procedures and for products
- ABS - American Bureau of Shipping
- BV - Bureau Veritas
- DNV - The Norske Veritas
- GL - Germanischer Lloyd
- LR - Lloyd's Register of Shipping
- RINA - Registro Italiano Navale etc
3.4. Explosively formed hull of an ocean viewer vessel
In Australia, the Department of Mechanical Engineering at the University of Adelaide has developed a portable prototype FSW machine to manufacture a new type of ocean viewer vessel. The machine was transported to the Research Foundation Institute (RFI) in Cairns for use under site conditions at a relatively low-tech shipyard.
Six friction stir welds were made in the bow section of the prototype ship using used 5mm thick aluminium alloy 5083 H321 ( Figs 12 & 13). Welding speeds of approximately 35mm/min were achieved.
Fig.12. Portable FSW machine at the shipyard of the Research Foundation Institute in Cairns, Australia
Fig.13. Friction stir welds on the starboard side of the bow section prior to explosive forming at RFI
The friction stir welded sheets of the bow section were given their final three-dimensional shape after welding by high energy rate forming (HERF). Explosive plates were fixed onto the aluminium sheets, which were positioned in amould. The mould was filled with water, before detonating the explosive ( Fig.14). The resulting structure was then installed in the prototype ocean viewer vessel ( Fig.15).
Fig.14. Detonation of the explosive for high energy rate forming of the bow section
Fig.15. Explosively formed bow section of the ocean viewer vessel at RFI in Cairns
This development programme resulted in an innovative and patented prototype vessel, which combines the attributes of a fast ferry with those of a semi-submersible reef viewing vessel ( Figs 16 & 17).
The benefits of the new concept include operator flexibility to access different reefs depending on daily sea and wind conditions, so providing a more reliable service. This low impact and environmentally sustainable mode of reef viewing is increasingly being required by international authorities endeavouring to protect their sensitive marine parks ( Fig.18).
Fig.16. Side view of the 24m long RFI ocean viewer vessel with the viewing pod retracted into the hull
Fig.17. Plan view of the RFI ocean viewer showing the hydraulically operated viewing pod
Fig.18. Explosively formed ship 'The Boss' by RFI in Australia
4. Industrial use of FSW in the shipbuilding industry
The Tamano Works of Mitsui Engineering & Shipbuilding (MES) in Japan used FSW when they built a combined passenger and freight ship with a maximal speed of 42.8 knots. This ship was given the name 'Super Liner Ogasawara' and can transport up to 740 persons and 210t of freight ( Fig.19). She was successfully tested in up to 2m high waves.
The Nichols Brothers Boat Builders in Freeland (Washington, USA) use FSW aluminium panels for a 55 knots military ship of the X-Craft class, which has recently been named 'Sea Fighter' ( Fig.20).
Fig.19. Super Liner Ogasawara' with 42.8 knots max speed by MES
Fig.20. Bow of US Navy X-Craft 'Sea Fighter' by Nichols Bros
There are many types of aluminium alloy ships, ranging from 5m long rigid inflatable boats, 50m long military patrol boats to more than 200m long luxury cruise ships. Therefore, the shipbuilding industry needs aluminium panels ofdifferent shapes and sizes. In 2003, China FSW Center (CFSWT) in Beijing designed and fabricated its first FSW industrial product-line for a small company in Chang Zhou ( Fig.21). This PLC controlled equipment can weld 2600 x 1100mm panels from 6mm thick aluminium extrusions for use in various sectors of the transport industry.
Fig.21. The first industrial FSW production facility in China
China FSW Center designed and produced the first large FSW machine for wide ship-panels in China in 2006 after considering production, weight and transport aspects. This FSW machine ( Fig.22) is designed in separate modular parts. The main frame is separated from the hydraulic clamping system. The clamping forces, which are needed to keep the parts in position, do not influence its working precision. TheCNC system, which can be used to set and adjust the welding parameters, is also developed as an independent unit. The system provides two integrated control methods and can be used either in closed-loop position or closed-loop pressure control mode according to different parts and their structure. This machine can weld aluminium alloy sheets and extrusions from 2 to 6mm thickness. The largest size panel produced so far was 60m 2 (12m x 5m, Fig.23).
Fig.22. CFSWT's first 12m long FSW machine for wide Al panels
Fig.23. FSW of 5 x 12m large aluminium ship panels at CFSWT
The CFSWT machine can be used for batch production of wide stiffened panels, which are used in high-speed aluminium alloy ships. Since the introduction of FSW panels, a new era started for the Chinese aluminium shipbuilding industry. First of all, batch production of panels by FSW helps to resolve on-site welding problems significantly. This technology has also simplified the design of ships. And most importantly naval architects have now more options, when they design new structures regarding material selection:
- FSW can join extruded AA 6082 to corrosion resistant AA 5083.
- Forged or stamped parts can be welded to castings.
- Thin sheets can be welded to thick sheets or plates.
- MIG welds can cross over friction stir welds, e.g. when joining FSW panels to girders ( Fig.24).
Fig.24. Crossover of MIG and FSW
Batch production by FSW also reduces the welding workload in shipyards. Shipbuilding changes from manual fieldwork to standardised production lines. Production efficiency of shipbuilding is therefore greatly improved. And finally,the residual stresses of friction stir welded aluminium alloy panels are very low and distortion is very small. Parts of ships can therefore be assembled more accurately, and the precision of ship modules and the final shape of shipscan be significantly improved. Nowadays, the concept of using prefabricated FSW panels for shipbuilding is popular at shipyards in Dalian, Shanghai, Wuhan, Guangxi and Guangzhou. These wide panels have successfully been used in many shipbuilding projects, including ships designed and fabricated in China for export to Vietnam and Micronesia ( Fig.25).
Fig.25. FSW used on Chinese aluminium alloy ships for various export markets
The Type 022 Houbei Class is the Chinese People's Liberation Army Navy's new-generation stealth missile fast attack craft (FAC). The boat features a unique high-speed, wave-piercing catamaran hull with evident radar cross-section reduction design features ( Fig.26). A number of Chinese shipyards across the country have been involved in the construction of the boat and it has been reported that FSW aluminium alloy panels have been used to produce this very advanced navy vessel in China. The futuristic design of this vessel has been admired internationally. This military stealth catamaran is believed to be equipped with 4 anti-ship missiles, 12 surface-to-air missiles and a 30mm gun. It has great stealth capability, and can move very quickly in various sea conditions.
Fig.26. 'Type 022' new-generation stealth missile fast attack craft (FAC)
5. Weld quality
The FSW weld nugget strength, in the as-welded condition, can be in excess of that in the heat affected zone. In the case of many annealed materials, tensile tests usually fail in the un-affected material well away from the weld and heat affected zone. The weld properties of fully hardened (cold worked or heat treated) alloys can be improved by controlling the FSW thermal cycle, in particular by limiting the annealing and overageing effects in the thermomechanically affected zone, where the lowest hardness and strength are found. The flexibility and relative ease of control possible in FSW allows the properties of the welds to be tailored (to some extent) to suit the requiredapplication.
Typical tensile properties of friction stir welded 5000, 6000 and 7000 series alloys are given in Table 2. The studies have been conducted by TWI, Sapa in Finspång, Sweden, and Hydro Aluminium in Håvik, Norway. They show that for solution treated plus artificially aged 6082-T6 aluminium by post weld heat treatment a tensile strength similar to that of the parent material could be achieved, although the ductility was not fully restored. A further improvement was possible when weld specimens were made from solution treated and naturally aged 6082 base metal in the T4 condition and then after welding subjected to normal ageing. Natural ageing at room temperature led, in the recently developed 7108 aluminium alloy, to a similar effect which resulted in a tensile strength of 95% of that of the base material.
Table 2. Typical mechanical properties of friction stir welded aluminium specimens (numbers in brackets have been calculated from publications)
|Material ||0.2% Proof strength |
|Tensile strength |
|Welding factor |
UTS FSW /UTS Parent
|5083-O Parent ||148 ||298 ||23.5 ||N/A |
|5083-O FSWed ||141 ||298 ||23.0 ||(1.00) |
|5083-H321 Parent ||249 ||336 ||16.5 ||N/A |
|5083-H321 FSWed ||153 ||305 ||22.5 ||0.91 |
|6082-T6 Parent ||286 ||301 ||10.4 ||N/A |
|6082-T6 FSWed ||160 ||254 ||4.85 ||(0.83) |
|6082-T6 FSWed and aged ||274 ||300 ||6.4 ||(1.00) |
|6082-T4 Parent ||149 ||260 ||22.9 ||N/A |
|6082-T4 FSWed ||138 ||244 ||18.8 ||(0.93) |
|6082-T4 FSWed and aged ||285 ||310 ||9.9 ||(1.19) |
|7108-T79 Parent ||295 ||370 ||14 ||N/A |
|7108-T79 FSWed ||210 ||320 ||12 ||(0.86) |
|7108-T79 FSWed naturally aged ||245 ||350 ||11 ||(0.95) |
Fatigue tests on friction stir welds made from 6mm thick 5083-O and 2014-T6 have been conducted. The fatigue performance of friction stir butt welds in alloy 5083-O was comparable to that of the parent material. Despite the fact that the fatigue tested friction stir welds were produced by a single pass from one side, the results have substantially exceeded design recommenda¬tions for fusion welded joints. Analysis of the available fatigue data has shown that the performance of friction stir welds in Al alloys is commonly superior to that of fusion processes. FSW results also display a high degree of repeatability, with very low scatter in the data.
The outstanding fatigue results can only be achieved if the friction stir weld is fully bonded. As known from other welding processes, it is essential to avoid root flaws. If the FSW tool pin is too short for the actual material thickness, then the workpieces are only forged together without stirring up the oxide layers. These flaws can be difficult to detect by non-destructive testing. In cases of large variations in sheet thickness, it could be even necessary to have extendable pins, which can be adjusted dependent on the actual sheet thickness. For high quality FSW, it is important to apply effective tool technology and processing conditions, in combination with an appropriate process control system, and according to a proven welding procedure specification (WPS).
So far, no specific problems of corrosion of FSW weldments have been reported in aluminium alloys of interest to the shipping sector (5xxx and 6xxx series alloys). There are few data available in the public domain on corrosion ofFSW welds in 5xxx and 6xxx series alloys, e.g. the examination of welds in 5454 in O and H34 tempers. In both instances, the results were compared with TIG welds. No appreciable attack was noted in the friction stir welds, which performed as well as or better than TIG welds. Some susceptibility to stress corrosion cracking (SCC) in anodically polarised slow strain rate tests has been reported. However, the testing conditions in this study were very severe,and the investigators pointed out that SCC tests repeated on U-bend specimens did not reveal any susceptibility. TWI has performed corrosion tests on 5083-O (salt spray and alternate immersion SCC) and on 6082-T6 (salt spray), as partof a confidential group sponsored project. There is only one published study on a friction stir welded 6xxx series alloy (6061-T4). Tests in this study showed that the friction stir welds were less susceptible than TIG welded counterparts in intergranular corrosion tests.
6. FSW Machines
Up to 16m long FSW machines have been designed, built, and commissioned. The first commercially built machine was installed at Marine Aluminium ( Fig.27) in Haugesund (Norway). Surveying bodies such as Germanischer Lloyd, Det Norske Veritas and Registro Italiano Navale have given approval to the welding procedure for specific applications, after successful testing ofthis machine.
Fig.27. FSW machine at Marine Aluminium being used to weld shipbuilding panels
Fig.28. TWI's first modular FSW machine being used to weld large workpieces
Another FSW machine has been installed at Sapa for the production of large panels and heavy profiles with a welding length of up to 14.5 metres. This machine has three welding heads, which means that it is possible to weld from two sides of the panel at the same time, or to use two welding heads (positioned on the same side of the panel) starting at the centre of the workpiece and welding in opposite directions. Using this method, the productivity of the FSW installation is substantially increased.
7. Cost savings by implementing friction stir welding
The following comments on cost savings have been published by users of the FSW process and speak for themselves:
- Ole Midling of Hydro Aluminium reported that at shipyards using prefabricated FSW panels the 'improvement in the aluminium fabrication has resulted in 15% reduction in the man-hour per ton rate.'
- Stig Oma of Fjellstrand claimed 'a total fabrication cost saving of approximately 10% based on improved ship design, streamlined fabrication at the shipyard and by supply of prefabricated FSW panels and structures based on extruded profiles.' He said that using prefabricated FSW panels "has enabled the yard to reduce the production period for a 60m long aluminium catamaran hull from 10 to 6 months, which means a 40% increase in production capacity and turn-over at the yard.'
- Doug Waldron of The Boeing Company reported that 'the FSW specific design of Delta IV and Delta II (satellite launch rockets) achieved 60% cost saving, and reduced the manufacturing time from 23 to 6 days.'
- For 'Slipper', the US Army's new cargo interface pallet, 'FSW processing reduced the sandwich assembly cost, including raw materials, extruding, and welding, from 61% to only 19% of the total fabrication cost. The Air Force estimates the total cost savings attributed to FSW (for a projected buy of 140,000 Slippers) at $315 million.'
8. Research needs of the shipbuilding industry
8.1. Friction Stir Welding of transport structures
The next stage of utilisation of friction stir welding is to take advantage of its unique characteristics, which offer the possibility of a radical redesigning of the welded joints currently used in aluminium fabrications. It will be necessary to assess a range of innovative joint designs available through FSW, which can simplify the methodologies adopted in the welding of structures, and bring even greater cost efficiencies to such fabrications ( Figs 29 & 30). A TWI Group Sponsored Project was initially targeted at marine fabrications, but has aroused such interest in other areas of transport applications that it has been broadened to cover all areas at the outset. The 12 sponsors of this project, which started in January 1999, were the first to take advantage of these developments, which led to improved manufacturing procedures. Further development work on joint designs is now being conducted in confidential single client projects.
Fig.29. Variable position joint with second weld to seal the crevice
Fig.30. FSW of sheets with dissimilar thickness using a tilted FSW tool
8.2. Friction Stir Welding of steels
FSW is being developed for the joining of steel plates, with encouraging results to date ( Fig.31 & 32). Recent work has concentrated on developing tool designs and optimised procedures for a range of steels. The mechanical and metallurgical properties of the welds produced is being assessed, in order to provide data from which potential users can realistically estimate the costs of using the process in production. The suitability of FSW for production welding of steels is being demonstrated and developed by the production of a number of prototype components by FSW. Application of FSW to steel panels offers the potential for high quality solid phase welding, with good surface finish and low distortion.
Fig.31. Transverse section of 12mm thick 12% chromium alloy steel FSW weld made in two passes
Fig.32. Dissimilar joint of 12% Cr alloy steel to low carbon steel made in two passes
8.3. Friction Stir Welding of titanium and its alloys
Preliminary investigations have established that the FSW process can be applied to titanium alloys, and this offers the potential for a rapid, cost effective and highly repeatable method of making high quality welds in titanium alloys. TWI projects are underway to develop the process for industrial use. Initial studies concentrated mainly on Ti-6Al-4V, but good quality welds have also been produced in commercially pure Ti, and in a range of ß-alloys.Although the main interest in these alloys stems from the aerospace industry, producers of oil pipelines and offshore platforms may consider the use friction stir welded titanium for applications where extreme corrosion resistance is required.
8.4. Current and future developments in TWI's Core Research Programme
TWI's Core Research Programme (CRP) consists of a series of applied research projects, which underpin TWI's contract and consultancy services. Having a multi-million pound annual budget, this research programme aims to develop relevant knowledge and skills for transfer into industry in order to reduce manufacturing costs, encourage innovation, improve product quality and meet safety and reliability requirements. The CRP helps to ensure that Industrial members of TWI gain and maintain a competitive edge in the market place. Two projects in TWI's current core research programme focus on friction stir welding of aluminium alloys, as follows:
8.4.1. Friction based repair technologies
Friction based processes for repair are now being developed. Applications where friction repair techniques would be beneficial are being investigated in a CRP project on 'friction based repair techniques including the development ofportable friction stir welding'. In this project the technical requirements of portable equipment for making friction stir welds in thin aluminium sheets will be specified.
8.4.2. Fundamentals of Friction Welding
The relationship between spindle rotation speed, applied force, and material softening response in the generation of heat by friction is being investigated in a CRP project on 'fundamentals of friction welding and friction processing of materials'. This project has produced guidelines for effective rubbing velocities for FSW in a range of commonly used workpiece materials. The project has also investigated the friction stir processing (FSP) of a number of alloys. It has been demonstrated that the refined FSP microstructures of aluminium alloys can provide better formability during super plastic forming than those of the parent material.
8.1. Reports on pre-competitive research in TWI's Core Research Programme
Industrial members of TWI have access to 25 CRP reports on friction stir welding so far. For instance, the microstructures of arc and friction stir welds have been characterised in 'a study of arc and friction stir welding of two aluminium alloys containing a low level scandium addition'. The researchers compared solidification crack susceptibility of weld metal with and without scandium and determined the mechanical properties of TIG and friction stir welds in similar scandium and non-scandium containing alloys.
Macro and microstructural features of friction stir welds were examined in various materials, and a microstructural classification scheme for friction stir welds has been introduced as part of the CRP. The 'corrosion resistance offriction stir welds in aluminium alloys 2014A-T651 and 7075-T651' and 'fracture toughness of friction stir welds in 2014A, 7075 and 5083 aluminium alloys' has been determined experimentally.
Reports on 'flaws in aluminium alloy friction stir welds', and more specifically 'the significance of root flaws in friction stir welds' are available to industrial members of TWI. The former report describes microstructurally the types of flaws that can occur in aluminium alloy friction stir welds, when the welding conditions diverge from the established operating window. The latter report covers the effect of root flaws on the static and fatigue performance of friction stir welds made from one side.
In a study on 'forces in friction stir welding of aluminium alloys' a commercially available dynamometer measured the horizontal and vertical forces and the torque generated during FSW. These data were used to evaluate the effects of friction stir welding parameters and tool geometry on the forces and torques generated during friction stir welding of selected aluminium alloys.
Earlier TWI CRP reports covered the 'tool developments for FSW of 6mm thick aluminium alloys' describing FSW tools capable of operating with zero tilt or FSW bobbin tools that can contain the weld metal near to the tool pin and react the weld metal forging forces necessary for making sound welds. A prototype FSW tool for making lap joints that does not exhibit top sheet thinning or serious oxide related flaws in the weld nugget has also been developed.
9. New TWI office in China and technology centres in Wales and Yorkshire
The Chinese Friction Stir Welding Centre was founded in 2002, after an agreement was signed by TWI and Beijing FSW Technology Ltd, which is an offshoot of BAMTRI - Beijing Aeronautical Manufacturing Technology Research Institute. The Centre has made remarkable progress in the last five years, employing over 30 people and building a number of different machines for customers. TWI now has 22 FSW licensees in China, most of them as a direct result of the collaboration with BAMTRI. So far the following Chinese companies have been licensed to use the patented FSW process:
Table 3. TWI and BAMTRI have issued 22 licenses to Chinese companies to use the patented FSW process
- Beijing FSW Technology Co. Ltd
- Beijing Xinfeng Mechanism Factory
- Capital Aerospace Machinery Company
- Changzhou Railcar Propulsion Eng. R&Dmp;D Centre
- China Aerospace Science & Industry Nanjing Group
- East China Shipbuilding Institute
- Gansu Polytechnical University
- Harbin Institute of Technology
- Hongyang Machinery Factory
- Institute of Metal Research Chinese Academy of Science
- Jilin 3305 Machinery Works
- Lanzhou University
- Nanchang Institute of Aeronautical Technology
- Northwestern Polytechnical University
- Qing Hua University
- School of Industry Senior Mechanic Hunan
- Shanghai Aerospace Equipment Manufactory
- Tianjin University
- Tsinghua University
- University of Science & Technology Beijing
- UT Alloy Works Corporation
- Xi'an Gu Ben Technology Corporation
TWI recently opened an office in the Baliqiaobei Chaoyang District of Beijing. The role of TWI in China is to assist TWI experts in communicating with the Chinese market and to identify co-operation opportunities.
TWI has also opened new laboratories in Wales and Yorkshire, where projects aligned to the regional emphasis on advanced manufacturing are carried out complementary to the activities at the headquarters in Cambridge ( Fig.33). TWI Technology Centre Wales in Port Talbot focuses on the development and use of non-destructive testing technologies. Phased array ultrasonic systems are being developed there e.g. for assessing characteristic flawsin friction stir welds. These offer the possibility of performing inspections with ultrasonic beams of various angles and focal lengths using a single array of transducers.
TWI Technology Centre Yorkshire in Rotherham (near Sheffield) focuses on laser and friction stir welding. Two new FSW machines and a heavy-duty FSW robot have been commissioned for this laboratory: One FSW machine has a very accurate spindle to weld a range of steels and aluminium-based metal matrix composites. The accuracy of the spindle increases the lifetime of the brittle tools that are used for FSW of high-temperature materials. The other new FSW machine is a high-force machine that can friction stir weld up to 100mm thickness aluminium alloys in a single pass ( Fig.34). It has a twin head configuration allowing simultaneous welding from both sides. Eleven CNC programmable axes provide a true three-dimensional welding capability, to weld contoured and complex shapes. The machine also has a sophisticated data acquisition system for data logging, analysis and process control.
Fig.33. TWI's EuroStir ® machine with a welding area of 8 x 5m prototypes
Fig.34. High-force FSW machine at TWI Technology Centre Yorkshire to weld up to 100mm thick aluminium plates
- Friction Stir Welding is a remarkable new welding method that has rapidly grown into an important industrial process since its invention, by TWI, in 1991.
- The shipbuilding industry successfully exploits friction stir welding to produce prefabricated aluminium panels from 6000 and 5000 series extrusions and large honeycomb panels with aluminium skins.
- Welding procedure specification or component approvals have been issued by many classification societies including ABS, BV, DNV, GL, Lloyds and RINA.
- There are currently 171 licensees of FSW technology worldwide, and more than 1685 patent applications have been made covering applications and developments of the technology (check: www.twi.co.uk/frictionstirwelding).
- Several manufacturers throughout the world supply purpose built FSW machines (for a list of TWI member companies who manufacture friction welding systems please visit: www.twi.co.uk).
- Further research and development work is currently underway to assess new FSW joint designs, to establish mechanical and corrosion data, to determine acceptance criteria, and to develop procedures for the FSW of steel, titanium and other challenging materials.
- www.twi.co.uk/frictionstirwelding, Tel: +44 1223 899000