Application of friction stir welding in the shipbuilding industry

Stephan Kallee, TWI, Cambridge, UK

Presented at Lightweight Construction - Latest Developments
The Royal Institution of Naval Architects , London, 24 & 25 Feb 2000

Summary

In friction stir welding (FSW), which has been invented and patented by TWI ^[1,2] , a wear resistant rotating tool is used to join sheet and plate materials such as aluminium, copper and lead. In laboratory experiments, magnesium, zinc, titanium 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 are attributed to the low heat input.

In the shipbuilding industry three Scandinavian companies and several Japanese companies now use the process for the production of large aluminium panels, which are made from aluminium extrusions. Research and commercial FSW machines are now available and include complete installations to weld up to 16m lengths.

Author's biography

Stephan Kallee is a Senior Project Leader at TWI (The Welding Institute) in Cambridge. He focuses on business development in Europe and manages large projects, e.g. on linear friction welding and the application of friction stir welding in the automotive industry. Recently he conducted on-site consulting at shipyards in Australia and Europe and developed design guidelines for transportable friction stir welding machines.

Nomenclature

FSW	Friction stir welding
HERF	High energy rate forming

1. Introduction

Friction stir welding (FSW) uses a non-consumable rotating tool, which moves along the joint between two components to produce high quality butt or lap welds. The FSW tool is made with a profiled pin, which is contained in a shoulder with a larger diameter than that of the pin ( Fig.1). For butt joining the length of the pin approximates to the thickness of the workpiece. The pin is traversed through the joint line while the shoulder is in intimate contact with the top surface of the workpiece to avoid expelling softened material.

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 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 the vertical, longitudinal and lateral forces, which will try to lift 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 impaired.

FSW was invented and patented ^[1,2] in 1991 at TWI in Cambridge and has been developed to a stage where it is applied in production. Currently 42 organisations hold non-exclusive licences to use the process. Most of them are industrial companies, and several of them exploit the process in commercial production in Scandinavia, USA, Japan and Australia.

2. Materials

FSW is, as rotary friction welding, a solid phase process, which operates below the melting point of the workpiece material. It can weld all aluminium alloys, including those such as aluminium-lithium alloys that cannot normally be joined by conventional fusion 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 1.2mm to 75mm ( Fig.2).

A series of specially profiled FSW tools has been designed and tested [3,4,5]. The tool related know-how about welding parameters has been developed at TWI to serve the 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.3) ^[6] . The onion ring like structure of the nugget is typical of high quality stir welds in which no porosity or internal voids are detectable.

The process can also be applied to copper, titanium, magnesium, zinc and lead. Even pilot trials on steel sheets and plates [7,8] are showing considerable success. Preliminary trials have also yielded encouraging results when FSW was used to join aluminium based metal matrix composites (MMCs), and when the process was applied to dissimilar materials such as cast magnesium alloy to extruded aluminium alloy.

In macrosections of good quality welds in aluminium alloys a well-developed nugget is visible at the centre of the weld, as schematically shown in Fig.1. Outside the nugget there is a thermomechanically affected zone, which has been plastically deformed and shows some areas of partial recrystallisation ^[9] . The overall shape of the nugget is very 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 4 & 5). These panels are made from friction stir welded aluminium extrusions ^[10] . The minimal distortion and high reproducibility make FSW both technically and economically a very attractive method to produce these stiff panels.

3.2 Panels for deck and wall construction

Pre-fabricated wide aluminium panels for high-speed ferryboats can be produced by friction stir welding and are commercially available ( Figs 6 & 7) ^[12] . 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 ^[13] .

After welding the panels can be rolled for road transport, as they are stiff only in the longitudinal direction ( Fig.8 ). When they can be transported by ship, they can be stacked on top of each other ( Fig.9).

3.3 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 ( Fig.10). The transportable FSW machine was commissioned in close co-operation with TWI engineers and laboratory experiments were undertaken with curved sheets on a mock-up jig. These were conducted to test and improve the machine, to determine a tolerance envelope and to ensure reproducibility.

The machine was then disassembled and transported to the Research Foundation Institute (RFI) in Cairns for use under site conditions of a low-tech shipyard [15]. It was possible to lift the machine and align it above the joint line by two operators without using a crane. In the shipyard further trials were carried out in the actual jig on samples ( Fig.11). It was found that a re-design of the weld lines was necessary to keep the tool at a constant angle to the sheet surface. Further welding samples were produced and evaluated, and then it was decided to start the production of the bow section of a prototype ocean viewer vessel.

Six friction stir welds were made in the bow section of the prototype ship using used 5mm thick DNV approved aluminium alloy 5083 H321 ( Figs 12 & 13 ). Determination of the optimum weld line, cutting, clamping and tacking of the aluminium sheets and alignment of the FSW machine was complicated under site conditions. The set-up was relatively time-consuming compared with the welding time. Welding speeds of approximately 35mm/min were achieved. The welding speed was considerably slower than under laboratory conditions, where up to 90mm/min were achieved during this project. In addition to the inclined welds of the curved surface bow section, a number of successful flat surface FSW joints were placed in the foredeck structure of the bow.

The surface of all welds was assessed visually and documented by photographs. The width of the weld should be constant. A reduction of weld width directly after the start is a sign that the tool shoulder was not in sufficient contact with the workpiece surface and therefore internal voids are assumed to be present. Acceptable welds have a smoothly rippled and consistent surface. If a line (parallel to the weld centreline but approximately 3-5mm offset) is visible on the surface, this can only be caused by an internal or surface-breaking void. The amount of expelled material should be constant over the weld length and should not exceed the amount needed for good welds.

The friction stir welded sheets of the bow section were deformed after welding by high energy rate forming (HERF) with the intention to achieve the flared shape of the structure [16,17]. Explosive plates were fixed onto the aluminium sheets, which were positioned in a mould. The mould was made from 19 x 19mm mild steel bars with 3mm gaps between them to allow air to escape during the deformation. The mould was filled with water, before detonating the explosion ( Fig.14). Unfortunately, the amount of explosives chosen was too high at the initial forming trial, which resulted in a non-perfect shape of the bow section. The friction stir welds permitted the assembled plates to deform uniformly with absolutely no evidence of localised deformation near the weld regions. Subsequent radiographic inspections of the welds showed the weld to be free of defects. Comparison trials were then conducted with MIG welded sheets using a much smaller amount of explosives. These sheets were finally installed in the prototype ocean viewer vessel ( Fig.15)

The research and development programme resulted in an innovative and patented [19] prototype vessel, which combines the attributes of a fast ferry with those of a semi-submersible reef viewing vessel ( Figs 16 & 17). This vessel was developed for the rapidly growing international tourist trade at the Australian Great Barrier Reef and other international marine parks, where high-speed access to the distant offshore reefs is needed. At the same time it is required to manoeuvre safely, at low speed in close proximity to fragile coral structures in order to facilitate underwater viewing.

The prototype ocean viewer vessel has a 24m long aluminium planing monohull. It provides more than 25kt speed for the transit to the offshore reefs, ensuring a one hour transit maximum and hence two turnaround reef excursions per day. The vessel stops at the reef and hydraulically lowers an aluminium and glass viewing pod from the hull, with room for 25 of the total 112 passengers carried. In this mode the vessel idles at 1-2kt in close proximity to the reef shelf, giving excellent viewing for the tourists while they sit in or move through the viewing pod. After some two hours of viewing, by which time all guests have enjoyed the underwater experience, the pod is retracted and the vessel speeds back to shore.

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. Indeed there is currently an expansion moratorium on Australia's Great Barrier Reef because of the undesirable impact of further permanently moored pontoon structures.

3.4 Honeycomb panels and corrosion resistant panels

New FSW applications are now being reported from Japan, where the process is used to produce honeycomb panels ( Fig.18) and sea water resistant panels ( Fig 19). The latter are made from five 250mm wide 5000 series aluminium extrusions to make a panel with the size of 1250 x 5000mm. They are used for ship cabin walls because of the good flatness of the root and weld underside ^[20] .

4. Weld quality

The weld nugget strength in the as-welded condition can be in excess of that in the heat affected zone. In the case of annealed materials, tensile tests usually fail in the unaffected 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 thermal cycle, in particular by reducing the annealing and overageing effects in the thermomechanically affected zone, where the lowest hardness and strength are found after welding. For optimum properties, it would seem that, for the latter, a heat treatment after welding is the best choice, although it is recognised that this will not be a practical solution for many applications.

Typical tensile properties of friction stir welded 5000, 6000 and 7000 series alloys are given in Table 1. The studies have been conducted by TWI ^[21] , Gränges Technology ^[11] in Finspång, Sweden, and Hydro Aluminium ^[22] 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 1. Typical mechanical properties of friction stir welded aluminium specimens (numbers in brackets have been calculated from the referenced publication).

Material	0.2% Proof strengthMPa	Tensile strengthMPa	Elongation%	Welding factor UTS FSW / UTS Parent
5083-O Parent [21]	148	298	23.5	N/A
5083-O FSWed [21]	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 [11]	286	301	10.4	N/A
6082-T6 FSWed [11]	160	254	4.85	(0.83)
6082-T6 FSWed and aged [11]	274	300	6.4	(1.00)
6082-T4 Parent [11]	149	260	22.9	N/A
6082-T4 FSWed [11]	138	244	18.8	(0.93)
6082-T4 FSWed and aged [11]	285	310	9.9	(1.19)
7108-T79 Parent [22]	295	370	14	N/A
7108-T79 FSWed [22]	210	320	12	(0.86)
7108-T79 FSWed naturally aged [22]	245	350	11	(0.95)

Fatigue tests on friction stir welds made from 6mm thick 5083-O and 2014-T6 have been conducted [21]. The fatigue performance of friction stir butt welds in alloy 5083-O was comparable to that of the parent material when tested using a stress ratio of R=0.1. 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 recommendations for fusion welded joints ^[23] . Analysis of the available fatigue data has shown that the performance of friction stir welds is comparable with that of fusion processes, and in most cases substantially better results, with low scatter, can be obtained.

The outstanding fatigue results can only be achieved if the root of butt welds is fully bonded. As known from other welding processes, it is also essential in FSW to avoid root flaws. If the pin is too short for the actual material thickness, the workpieces are only forged together without stirring up the oxide layers. These flaws are 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 ^[24,25,26] . A suggestion has been made to machine chamfers on the bottom edge of the workpieces ^[27] or to grind a groove into the backing bar ^[28] in order to avoid root defects. Engraving of the backing bar enables the user to imprint information about the manufacturer and the FSW machine at the bottom of the weld ^[29] . For filling gaps between the workpieces a slight thickness increase in the joint area seems advantageous ^[30] or a lateral reciprocating motion perpendicular to the weld direction could be investigated ^[31] .

5. FSW machines

Up to 16m long SuperStir machines have been designed, built, and commissioned by Esab in Laxå, Sweden. One of them has been installed at Marine Aluminium ( Fig.20). 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 of the machine in Haugesund.

TWI owns and operates several FSW machines to weld a wide range of workpieces. The biggest laboratory machine was built to accommodate large sheets and structures ( Fig.21). It can run linear and circumferential welds on specimens with 3.4m length x 4.0m width and 1.15m height or diameter with welding speeds of up to 1.7m/min. The modular construction enables it to be enlarged for specimens with even greater dimensions.

6. Research needs of the shipbuilding industry

6.1 Friction stir welding of transport structures

The next stage of utilisation of this new welding technique is to take advantage of its other characteristics, which offer the possibility of a radical redesigning of the welded joints currently used in aluminium fabrications. It will be necessary to assess the use of FSW in a range of innovative joint designs that can simplify the methodologies adopted in the welding of structures, and bring even greater cost efficiencies to such fabrications ( Figs 22 & 23). A current 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, will be the first to be able to take advantage of these developments, which will lead to improved manufacturing procedures.

6.2 Mechanical and corrosion properties of friction stir welds in aluminium alloys

Naval architects, surveyors and classification societies are now requesting design data and predictions of the in-service behaviour of friction stir welds. Fracture, fatigue and corrosion design data have to be generated, and a better understanding of failure mechanisms is necessary. R & D work is currently concerned mainly with butt welds in 6-10mm thick alloy plate typical of the 2000, 5000, 6000 and 7000 series. This will enable the shipbuilding industry to reduce the material thickness of aluminium sheets and to develop acceptance criteria for friction stir welded marine structures.

6.3 Friction stir welding of steels

To develop FSW for welding of steel, experiments have to be conducted to establish the best material for the FSW tool (Figs 24 & 25). Once a suitable material has been identified, work on developing tool designs and optimised procedures for a number of steels can commence. This work should also examine the mechanical and metallurgical properties of the welds and provide data from which potential users can realistically estimate the costs of using the process in production. The suitability of the process for production will be investigated at TWI in the near future by making a number of prototype components by FSW.

6.4 Friction stir welding of titanium and its alloys

Preliminary investigations have established that the FSW technique can be applied to titanium alloys, and this offers the potential for a rapid, cost effective and technically simple method of making high quality welds in titanium alloys. A current TWI project aims to develop the process to a point where it can be considered for industrial use. Initial studies have been on Ti-6Al-4V, but it is intended to investigate other alloys in due course. Although the main interest in these alloys stems from the aerospace industry, producers of oil pipelines and offshore platforms would like to use friction stir welded titanium for applications where extreme corrosion resistance is required.

7. Conclusions

Friction stir welding is in the shipbuilding industry successfully being exploited to produce prefabricated aluminium panels from 6000 and 5000 series extrusions. Research work has demonstrated that the process could also be applied to curved sheets of a bow section of an ocean viewer vessel by using a transportable FSW machine. Further research and development work is necessary to experimentally assess new joint designs for naval structures, to establish mechanical and corrosion data, to determine acceptance criteria and to develop procedures for welding steel and titanium.

8. Acknowledgements

The research programme on the ocean viewer vessel was undertaken by the Research Foundation Institute in Cairns, Australia. The programme was financially supported by the Australian Government's International Science and Technology Incentive Scheme. The transportable FSW machine was designed and manufactured at the Department of Mechanical Engineering of the University of Adelaide, which is a core partner of the Australian Co-operative Research Centre for Materials Welding and Joining.

9. References

Friction stir welding has been invented and patented by TWI.

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