Friction Stir Welding for Manufacture of Aluminium Ferries

E Dave Nicholas, Technology Manager
Paul M. Burling, Principal Composite Engineer

TWI Ltd

Paper presented at the 4th International Forum on Aluminium Ships, New Orleans, 10-11 May 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.

The low temperatures generated during friction stir welding permit joining of thin aluminium skins of honeycomb or sandwich panels, avoiding delamination of skins and core. The core of these panels can for example be made from vacuum brazed tubes, from thermoplastic foam or from Barrikade®, an innovative fire resistant composite consisting of vermiculite particles in an inorganic binder. ^[3]

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.

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 (Figure 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 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 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 45 organisations hold non-exclusive licences to use the process.^[4] 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 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 1.2mm to 75mm (Figure 2).

A series of specially profiled FSW tools has been designed and tested.^[5,6,7] 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 (Figure 3; Reference^[8] ). 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^[9,10] 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 Figure 1. Outside the nugget there is a thermomechanically affected zone, which has been plastically deformed and shows some areas of partial recrystallisation.^[11] 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 (Figures 4 & 5). These panels are made from friction stir welded aluminium extrusions.^[12] 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 of high-speed ferries

Pre-fabricated wide aluminium panels for high-speed ferryboats can be produced by friction stir welding and are commercially available.^[14] 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.^[15,16] After welding the panels can be rolled for road transport, as they are stiff only in the longitudinal direction. When they can be transported by ship, they can be stacked on top of each other.

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 (Figure 6). 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.^[17,18] 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 (Figure 7). 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 (Figures 8 & 9). 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. Several welds were also assessed by ultrasonic test methods.

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.^[19,20] 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 let air being expelled during the deformation. The mould was filled with water, before detonating the explosion (Figure 10). 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 (Figure 11).

The research and development programme resulted in an innovative and patented^[22] prototype vessel, which combines the attributes of a fast ferry with those of a semi-submersible reef viewing vessel (Figures 12 & 13). 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 (Figure 14) and sea water resistant panels (Figure 15). 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. ^[23]

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,^[24] Gränges Technology in Finspång, Sweden^[13], and Hydro Aluminium in Håvik, Norway.^[25] 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).

Fatigue tests on friction stir welds made from 6mm thick 5083-O and 2014-T6 have been conducted.^[24] 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.^[26] 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.^[27,28,29] A suggestion has been made to machine chamfers on the bottom edge of the workpieces^[30] or to grind a groove into the backing bar in order to avoid root defects.^[31] Engraving of the backing bar enables the user to imprint information about the manufacturer and the FSW machine at the bottom of the weld.^[32] For filling gaps between the workpieces a slight thickness increase in the joint area seems advantageous^[33] or a lateral reciprocating motion perpendicular to the weld direction could be investigated. ^[34]

5. FSW Machines

Up to 16m long SuperStir^TM machines have been designed, built, and commissioned by Esab in Laxå, Sweden. One of them has been installed at Marine Aluminium (Figure 16). 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.

Another Esab SuperStir^TM machine has been installed at SAPA and will be used 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.

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 (Figure 17). It can run linear and circumferential welds on specimens with 3.0m 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. Production of fire resistant aluminium sandwich panels

6.1. Conventional insulation materials

Fire protection and thermal insulation barriers in the form of panels are of critical importance in the marine industry for the protection of people and equipment. Materials used in this industry sector must conform to strict rules and regulations, which define the performance requirements that must be achieved. Typical regulations include the International Maritime Organisation (IMO), Safety of Life at Sea (SOLAS) and High Speed Craft Construction (HSC). These regulations provide rules for: -

• The use of non-combustible materials as means of escape routes or for structural fire protection.
• Materials which prevent fire spread.
• Materials which do not emit toxic fumes when subjected to fire situations.
• Maximum allowable front face temperatures as a result of rear face heating.

In an effort to meet these requirements, the marine industry currently employs a range of materials as core and skin materials for sandwich panel construction - including mineral wool, balsa wood, foam and clays, steel, glass reinforced polymers (GRP) or phenolic skins and intumescents. Although commonly used throughout this sector, these materials possess disadvantages.

Mineral wool is primarily used as a core for sandwich panels, but the material itself has poor structural properties and, due to its fibrous nature, it is difficult to bond to panel skins. Furthermore, mineral wool evoked some health and safety concerns, which may result in legislation to minimise its use. Balsa wood, primarily end-grain balsa, does not burn; instead it chars but emits smoke and fumes. In addition, balsa wood delaminates when it absorbs water causing it to swell. Hardwood can overcome many of these problems, but its high density and cost prove restrictive. Environmental concerns exist in respect to both materials due to the cutting down of tropical trees. Alternative core materials include polyurethane foams, blown glass and ceramic 'foams'. However, the high cost and brittle nature of glasses and ceramic foams prevent their wide spread adoption. Polyurethane foams possess health and safety concerns during manufacture and when exposed to fires. An alternative solution is to use fire resistant skin materials, such as steel, GRP and phenolics. Steel is of high density and does not offer the thermal barrier due to its high thermal conductivity, whereas phenolics emit fumes when burning or charring.

There is a need, therefore, for a low cost, lightweight material that can be used as a core material for panels which is resistant to fire, has a low thermal conductivity and does not emit toxic fumes. TWI has recently developed a material, known as Barrikade^®, which is capable of satisfying this need.

6.2. Barrikade^®

Barrikade^® is a low density, fire resistant inorganic material, consisting of vermiculite particles and a blended silicate binder (in principle, any inorganic binder could be used;^[3]). It is fire resisting and non-combustible, and when heated produces negligible emission of toxic fumes. It is ideal for use as a core material in a range of applications such as bulkheads, firewalls and fire doors - since it is lightweight (200-300kg/m³) and offers thermal insulation (Figure 18). Trials have shown that control of processing variables enables the material to be produced in a range of thicknesses and densities and, once cured, Barrikade^® can be easily cut, routed and shaped with hand held tools (similar to those used for woodworking). Furthermore, it has been shown that Barrikade^® can adhere to a range of materials including wood, steel and aluminium, while a high temperature adhesive is available for applications requiring greater adhesion. Trials on a 20mm thick panel to evaluate its thermal properties have demonstrated that the temperature on one surface did not exceed 170°C after 70 minutes when the rear face had been exposed to a flame in excess of 1000°C (Figure 19). This suggests that the material has a potential to meet SOLAS A60 regulations.

Temperature control during curing enables Barrikade^® to be produced in densities of 150-350kg/m³, and thicknesses of 3-200mm. Thermal and mechanical properties can also be influenced (Figure 20). Typically, 250kg/m³ panels provide a flexural strength of 0.40MPa and a compressive strength of 0.70MPa, which compares favourably with competitive materials (e.g. ceramic foam shows 0.31MPa and 0.10MPa respectively). A flexural strength of 6.0MPa could be achieved by laminating Barrikade^® to 1.6mm thick aluminium skins. Adhesion to skins (such as wood, aluminium, steel, and composites) can be achieved using the silicate-based binder, although other adhesive materials can be used to match application requirements (Figure 21).

The major advantages which Barrikade^® has over competitors are its low density, low health and safety concerns and low cost (typically £15/m² (25mm thick), compared with £30/m² for Balsa wood, £32/m² for ceramic fibreboard and £35/m² for foamed ceramics). A typical comparison of Barrikade^® with common core materials is illustrated in Table 2.

6.3. Typical applications for Barrikade^®

The shipbuilding industry is considering the use of Barrikade^® in the manufacture of fire resistant walls, partitions and doors for high speed ferries, cruise ships and for offshore accommodation modules. Low temperature insulation of freezer compartments of fishing boats, or heat-shielding of ammunition storage decks on military vessels are other potential applications of this new insulation material. Experiments are being conducted to demonstrate how to join sandwich panels, consisting of a core between aluminium skins (Figure 22). Friction stir welding of the skins and adhesive joining are currently the most promising joining technologies for joining these panels. It has been proposed to encapsulate sensitive control modules, computers or electronics, and to produce samples of safety deposit boxes to demonstrate the technical and commercial and technical benefits (Figure 23). Further research work is necessary to optimise the production of Barrikade^® panels and to determine suitable joining processes for industrial application.

7. Research needs of the shipbuilding industry

7.1. Friction stir welding of transport structures

The next stage of utilisation of friction stir welding 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 (Figures 24 & 29). 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 take advantage of these developments, which will lead to improved manufacturing procedures.

7.2. 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.

7.3. Friction stir welding of steels

To develop FSW for joining of steel, experiments have to be conducted to establish the best material for the FSW tool (Figures 26 & 27). 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.

7.4. 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.

8. Conclusions

• Friction stir welding is in the shipbuilding industry successfully being exploited to produce prefabricated aluminium panels from 6000 and 5000 series extrusions and large honeycomb panels with aluminium skins.
• 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 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 and for joining of fire resistant sandwich panels.

9. 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.

10. References

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Friction stir welding has been invented and patented by TWI

Stephan W Kallee

Stephan Kallee is a Senior Project Leader at TWI (The Welding Institute) in Cambridge, United Kingdom. He focuses on business development in Europe and the USA 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.

E Dave Nicholas

Dave Nicholas is Technology Manager for Friction and Forge Welding Processes and Business Development Manager for the Electron Beam Welding Technology Group at TWI. His interests centre on solid phase welding with the focus on friction welding. Several welding technologies were developed by his team from initial laboratory curiosity to their industrial exploitation, such as friction surfacing, linear friction welding and, since 1991, friction stir welding.

Paul M Burling

Paul Burling is Principal Composite Engineer in the Technology Group for Advanced Materials, Processes and Microtechnology at TWI. He gained his know-how about adhesives and composites in industry and works on collaborative research and development projects for international customers. Recently, he developed fire resistant composites and evaluated joining technologies for sandwich panels. He concentrates now on business development for the aerospace, construction and heavy engineering sectors with a special interest in shipbuilding structures.