Friction Stir Welding: Automotive Processes and Developments

TWI Ltd

Paper presented at 2006 SAE World Congress, 3-7 April 2006, Cobo Center, Detroit, Michigan, USA.

Abstract

Friction stir welding (FSW) is now extensively used in industry for joining and material processing applications. The (FSW) technology has gained increasing interest and importance since its invention at TWI almost 14 years ago. Thebasic principle and the continuing development of the FSW technology are described, and recent applications in the automotive industries are reviewed. The paper will introduce dual-rotation friction stir welding, whereby the shoulderrotates at a slower speed than the probe, effectively lowering the welding temperature when compared with welds made by conventional rotary friction stir welding. The lower temperatures produced in dual-rotation friction stir weldshave been shown to produce better mechanical properties than conventional friction stir welds and are believed to also improve the corrosion properties of the weld.

Introduction

With increasing international competition and the need to reduce the weight of cars, this paper reviews first the industrial uptake and some automotive applications of the conventional rotary friction stir welding (FSW) process andthen introduces new variants of the FSW technology. Friction Stir Welding (FSW) was invented and patented in 1991 by TWI and has since then been developed to a stage where it is being applied in production. Currently 120 organisationshold non-exclusive licences to use the process. Most of them are industrial companies, and they have filed more than 1300 patent applications related to FSW.

The basic principle of conventional rotary friction stir welding (FSW) and the main terms that define the process characteristics are shown in Fig.1.

Fig.1. Basic principle of conventional rotary friction stir welding

FSW is conducted below the melting point by pressing a rotating tool into the joint line. The wear-resistant FSW tool has a profiled probe and a shoulder with a larger diameter than that of the probe. The probe length is similar tothe required weld depth. The tool is traversed along the joint line, while the shoulder is pressed onto the surface of the workpiece, to provide consolidation of the plasticised workpiece material.

Automotive applications of FSW

The subsequent examples of automotive applications for FSW are taken from the public domain and represent the growing use of the technology. Acknowledgements are made to the source and it is noted that these examples arerepresentative but not exhaustive.

In 1998, TWI started a study on aluminium tailored blanks for door panels ( Fig.2) and demonstrated new concepts on FSW drive shafts and space frames in a confidential group sponsored project involving BMW, DaimlerChrysler, EWI, Ford, General Motors, Rover, Tower Automotive and Volvo.

Fig.2. FSW tailor welded blank produced from 6000 series aluminium in 1998

TWI, BMW, Land Rover

As a consequence of the encouraging results of this project, FSW and its variant Friction Stir Spot Welding (FSSW) are now being used in the series production of aluminium automotive components at several locations worldwide:

Ford in Detroit (USA) uses a friction stir welded centre tunnel for the Ford GT sports car ( Fig.3). The centre tunnel is a structural part that increases the rigidity of the chassis and is also used as a vapour tight fuel tank, ( Fig.4). The location of the tank provides good weight distribution and crashworthiness. The mechanical components, including the fuel pumps, level sensors and vapour control valves are first mounted on a steel rail. Then, asingle-piece tank is blow-moulded around the rail. This 'ship-in-a-bottle' design concept maximizes the fuel volume and reduces the number of connections to the fuel system.

Fig.3. The friction stir welded aluminium centre tunnel of the Ford GT houses the fuel tank (Courtesy Ford)

Fig.4. Friction stir welding of the centre tunnel of the Ford GT (Courtesy Tower Automotive and Ford)

Tower Automotive in Grand Rapids (Michigan, USA) produces aluminium suspension links for Lincoln Town Cars designated as stretched limousines. These have heavy-duty rear axles installed, while the rest of the rear suspension remainsunchanged. The suspension link is made from two identical extrusions, friction stir welded simultaneously with two spindles from both sides ( Fig.5). This provides excellent fatigue properties.

Fig.5. Friction stir welded suspension links for Lincoln stretched limousines

(Courtesy Tower Automotive)

Sapa in Finspång (Sweden) uses a purpose built FSW machine with two welding heads for welding hollow aluminium extrusions from both sides simultaneously, to produce foldable rear seats of the Volvo V70 station wagon. Themachine has a carousel-type loading and unloading station and is automatically loaded by an articulated arm robot ( Fig.6).

Fig.6. FSW production of Volvo rear car seats

(Courtesy Sapa)

Mazda in Hiroshima (Japan) uses friction stir spot welding for the rear doors and bonnet of the Mazda RX-8 ( Fig.7). The bonnet of this sports car has an impact-absorbing structure aimed at enhancing pedestrian protection. They use this process to avoid spatter and to reduce the energy consumption significantly in comparison toresistance spot welding.

Fig.7. Friction stir spot welding of rear doors for the Mazda RX-8

(Courtesy Mazda)

Showa Denko in Oyama City (Japan) joins extruded end-pieces to 20-30 mm diameter tubes for the manufacture of suspension arms. The rubber of the end-pieces of the suspension arms can be vulcanised prior to welding due to the lowheat input of the new assembly method ( Fig.8).

Fig.8. FSW suspension struts

(Courtesy Showa Denko)

Simmons Wheels in Alexandria (Australia) developed a new method of producing a wheel rim from rolled aluminium 6061-O sheet. From this they form a cylinder with a longitudinal friction stir weld. After cutting this into rim sectionsthey spin form it into the desired rim profile and finally subject this part to heat treatment to the required T6 temper. The company is now supporting UT Alloy Works in Guandong (China) during FSW production ramp-up of light alloywheels ( Fig.9).

Fig.9. Aftermarket three-piece wheel made from friction stir welded and spinformed aluminium cylinders

(Courtesy Simmons Wheels and UT Alloy Works)

A new technique of joining two parts of a car wheel ( Fig.10) has been invented, in which cast or forged centre parts are friction stir welded to rims that are made from wrought alloys. This concept is now being industrialised by DanStir in Copenhagen (Denmark). This reduces thewheel weight by 20-25% providing a leading Norwegian wheel supplier with a business advantage over its competitors.

Fig.10. FSW of a casting to a spin formed wheel rim

(Courtesy Hydro)

Riftec in Geesthacht (Germany) provides subcontract production and engineering consultancy, e.g. during the installation of FSW robots in automotive manufacturing lines. One of their automotive related projects concerned theproduction of welded test specimens for study of the Berlin-based company Inpro ( Fig.11).

Fig.11. Robotic FSW of automotive parts

(Courtesy Riftec)

Friction Stir Link in Waukesha (Wisconsin, USA) is a service supplier focussing on the automotive industry. It provides FSW process development, technology transfer, moderate-volume production and friction stir welding systemintegration services ( Fig.12).

Fig.12. CNC controlled FSSW gun on an articulated arm robot
( Courtesy Friction Stir Link)

Sapa produced a friction stir welded prototype engine cradle recently. The cradle is the result of a lightweight study to reduce the weight in the front end of the vehicle. The weight of this substructure is 16kg, as compared to23kg for the steel version.

This assembly uses various semi-fabricated products and joining methods. The side members are hydro formed aluminium extrusions. The front cross member is a straight extruded member. The rear cross member is built up of a sand castpart and a plate joined to the casting by FSW. The FSW operation was executed in three dimensions. A cost analysis showed the concept to be competitive to other concepts within the framework put forward by the customer ( Fig.13).

Fig.13. Prototype FSW engine cradle
(Courtesy Sapa)

Dual-rotation Friction Stir Welding

The systematic development of Friction stir welding (FSW) has led to a number of variants of the technology. The following describes preliminary studies being carried out on dual-rotation friction stir welding, its effect onlowering welding temperature and minimising the thermal softening of the weld region of certain heat-treatable aluminium alloys.

A dual-rotation FSW variant is being investigated at TWI, whereby, the probe and shoulder rotate separately. The dual-rotation FSW variant provides for a differential in speed and/or direction between the independently rotatingprobe and the rotating surrounding shoulder as shown in Figure 14.

Fig.14. Principle of dual-rotation friction stir welding with rotation of the probe and shoulder in the same direction

The apparatus can enable a range of different rotational speeds to be pre-selected or varied automatically by in-process control to suit the desired welding conditions.

In conventional rotary FSW, the relative velocity of the tool increases from zero at the probe centre to maximum velocity at the outer diameter of the shoulder. The dual-rotation technique can significantly modify the velocitygradient between the probe centre and the shoulder diameter. This technique provides a differential in rotation speed and the option for rotation in opposite directions. For example the shoulder rotational speed can be infinitelyvaried from approximately 30% less than the probe rotational speed down to almost zero rotational speed while rotating in the same direction or about 25% less than the probe rotational speed down to almost zero rotational speed whenthe shoulder is rotated in the opposite direction.

This dual-rotation technique effectively allows for a high probe rotational speed without a corresponding increase in shoulder peripheral velocity. This technique can provide for a more optimised rotational speed for both probe andshoulder.

Dependent on the material and process conditions used, over-heating or melting along the 'near shoulder side' of the weld surface of certain friction stir welds can occur. Melting can lead to fusion related defects along the 'nearshoulder side' weld surface. The dual-rotation technique can be used to reduce the shoulder rotational speed as appropriate and, therefore, help reduce any tendency towards over-heating or melting, while maintaining a higher rotationalspeed for the probe.

Figure 15 shows the appearance of the weld surface that is formed beneath the tool shoulder after dual-rotation stir welding.

Fig.15. Surface appearance of dual-rotation stir weld made in 16 mm thick 5083-H111 aluminium alloy at a welding speed of 3 mm/sec (180 mm/min), using 584 rev/min for the probe and 219 rev/min for the shoulder

Owing to the relatively low temperature reached, with solid-phase welding techniques such as FSW, the problems of solidification and liquation cracking when fusion welding certain materials, can be significantly reduced. However,the thermal cycle produced in FSW is sufficient to modify the original alloy temper in certain heat-treatable materials (e.g. 2xxx and 7xxx series aluminium alloys) producing a reduction in both the mechanical and corrosion propertiesacross the weld.

One advantage of dual-rotation FSW is that it reduces the peak temperature reached during the weld thermal cycle. Figure 16 shows a comparison of thermal profiles produced by conventional rotary and dual-rotation friction stir welds made in AA7050-T7451 using similar probes and process conditions. For a given travel speed of 5.25 mm/sec(315 mm/min), a difference of approximately 66°C in the maximum temperature of the HAZ region close to the probe (5 mm from the weld centre line) is shown.

Fig.16. Thermal profiles of conventional rotary friction stir welds and dual-rotation friction stir welds made in 6.35 mm AA7050-T7451, using the same probe geometry and a travel speed of 5.25 mm/secs (315 mm/min). The probe rotation speed was 394 rev/min and 388 rev/min for conventional rotary and dual-rotation stir welding techniques respectively

The lower temperatures reached in the dual rotary weld reduce the change in mechanical properties produced during friction stir welding. After two months natural ageing ( Figures 17 & 18), the dual-rotation friction stir weld shows higher hardness values in the stirred zone, thermo mechanical affect zone (TMAZ) and heat affected zone (HAZ) compared to the conventional friction stir weld.This indicates that the lower temperatures produced by the dual-rotation technique reduced thermal softening resulting in an increase in weld hardness.

Fig.17. Hardness traverses as a function of depth through the cross section of a conventional friction stir weld made in 6.35 mm AA7050-T7451, using a travel speed of 5.25 mm/sec (315 mm/min) and a probe rotation speed of 394 rev/min

Fig.18. Hardness traverses as a function of depth through the cross section of a dual-rotary friction stir weld made in 6.35 mm AA7050-T7451, using the same probe geometry used in the conventional friction stir weld ( Figure 17), a travel speed of 5.25 mm/sec (315 mm/min), and a probe rotation speed of 388 rev/min and a shoulder rotational speed of 145 rev/min

The HAZ of conventional friction stir welds in both 2xxx and 7xxx series aluminium alloys has been shown to be the region most susceptible to localised corrosive attack. ^[4] Figure 19 shows a comparison of the extent of corrosion in specimens from conventional and dual-rotation friction stir welds that were exposed to the same test. Both welds were made in 6.35 mm AA7050-T7451, using similarprobes and process conditions.

Fig.19. Photomacrograph of the top surface of a) conventional friction stir weld b) dual-rotation friction stir weld. After two months natural ageing the 'near shoulder side' of the weld surface was removed and the surface prepared to a ¼ micron finish before being immersed in a 0.1M NaCl aerated solution at ambient temperature for 7 days. Both welds were made in 6.35 mm AA7050-T7451 using the same probe geometry and a travel speed of 9.2 mm/secs (552 mm/min). The probe rotation speed was 394 rev/min and 388 rev/min for conventional rotary and dual-rotation stir welding techniques respectively. A shoulder rotational speed of 145 rev/min was used for dual-rotation

In the conventional friction stir weld the high temperature HAZ is shiny due to severe localised attack that has occurred in this region, therefore cathodically protecting the surrounding areas in the HAZ. In the dual-rotationfriction stir weld there is no shiny region evident in the HAZ suggesting the degree of localised attack occurring.

Conclusion

This paper provides examples of the growing use of friction stir welding technology for welding and spot welding applications. Further developments of the technology are likely to increase the types of applications that can bejoined by FSW.

Results are shown for the dual-rotation technique that can significantly modify the velocity gradient between the probe centre and the shoulder diameter. These trials confirm that use of slower shoulder rotational speed lowers theHAZ temperature during the welding operation. This effectively reduces thermal softening in the HAZ region. TWI is continuing studies to develop and perfect the dual-rotation friction stir welding and material processing technique.

Work will continue at TWI to investigate the use of dual-rotation on spot, butt, and lap welds. In addition, trials will be undertaken to achieve improvements in traverse rate and investigate tool tilt angle. Further work will alsobe undertaken to study the use of the contra-rotation variant.

Acknowledgements

Acknowledgements are made for the support and contributions provided by C S Wiesner, I M Norris, P Woollin, C Goodfellow, and E R Watts.

References

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