S. W. Kallee, J. M. Kell, W. M. Thomas und C. S. Wiesner, TWI Ltd
Paper presented at DVS Annual Welding Conference 'Große Schweißtechnische Tagung', Essen, Germany, 12-14 September 2005
TWI (The Welding Institute, www.twi.co.uk) has developed and patented an array of new joining technologies applicable to the automotive and motorsport industry. TWI supports international vehicle manufacturers and their suppliersduring the development, implementation and industrial application of new and established welding processes.
With increasing international competition and the need to reduce the weight of cars, the automotive suppliers increasingly want to obtain licences for patented joining processes.
This paper reviews first the industrial uptake of the mature friction stir welding process and then introduces new concepts on laser and electron beam material processing, which are currently being developed.
2 Friction stir welding
2.1 Industrial applications of FSW
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 ( Fig.1). Currently 118 organisations hold 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.
Fig.1. Principle of the friction stir welding process invented at TWI (Courtesy TWI)
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 to the 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.
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 (Courtesy 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 world-wide:
Ford in Detroit (USA) uses a friction stir welded centre tunnel for the Ford GT sports car ( Fig.3).
Fig.3. The friction stir welded aluminium centre tunnel of the Ford GT houses the fuel tank (Courtesy Ford)
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 crash worthiness. The mechanical components, including the fuel pumps, level sensors and vapour control valves are first mounted on a steel rail. Then, a single-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.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 remains unchanged. 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. The machine 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 hood 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 to resistance 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 low heat 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 sections they 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)
2.2 Industrial development work on FSW
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 the wheel 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 the production 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 system integration 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 hydroformed aluminium extrusions. The front cross member is a straight extruded member. The rear cross member is built up of a sand cast part 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)
2.3 Laboratory R&Dmp;D Work on FSW
The Core Research Programme (CRP) of TWI consists of a series of applied research projects, which underpin TWI's highly confidential contract research services. Industrial members of TWI have access to 24 CRP reports on friction stir welding so far. Having a multi-million pound annual budget, this research programme aims to develop production relevant knowledge and skills for transfer into industry ( Fig.14).
Fig.14. Gantry type FSW machine with a vacuum clamping table used at TWI for FSW of 8 x 5m prototypes (Courtesy TWI)
Two projects in TWI's current Core Research Programme focus on friction stir welding, as follows: In the first CRP project, friction based processes for repair are being developed including the development of portable friction stir welding machines for thin sheets. In the second CRP project, the relationship between spindle rotation speed, applied force, and material softening response by friction heating is being investigated. This project will investigate in alater phase friction stir processing (FSP) of a number of alloys. It has already been demonstrated that the refined FSP microstructures of aluminium alloys provide better formability during superplastic forming than those of the parent material.
Now, a new group sponsored project is being launched at TWI, to establish tool designs, process parameters and tolerances for Friction Stir Spot Welding (FSSW) of a range of light alloys. The FSSW process ability to weld through coatings, adhesives and sealants will be explored, and data for the specification of welding machines and robots will be gathered.
Two new FSW machines are now being commissioned for TWI Technology Centre (Yorkshire) in Sheffield:
Smart Technology Group Ltd has been contracted to supply a high-force machine that can supply 15t welding force ( Fig.15). It has a twin head configuration allowing simultaneous welding from both sides. Eleven CNC programmable axes achieve 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 control.
Transformation Technologies Inc. has built a FSW machine with a very accurate spindle to weld a range of steels and aluminium-based metal matrix composites at this new laboratory. The accuracy of the spindle increases the lifetime of the brittle tools that are used for FSW of these and high melting temperature materials.
Fig.15. Smart Technology Group's high-force FSW machine for TWI Technology Centre (Yorkshire) to weld <150mm thick aluminium plates (Courtesy TWI)
3 Electron beam surface treatment for metal to composite joints
3.1 Surfi-Sculpt TM
TWI uses electron beams to reshape metal surfaces by growing protrusions that rise from the surface of the material. At the core of the process is the creation of the protrusions and holes. These are made by precise manipulation ofa high-intensity electron beam that is controlled by a programmable deflection system. As the electron beam is moved across the surface of a metal, it creates a small pool of molten metal in a track, or 'swipe' ( Fig.16). As the beam moves along the swipe, the material heats rapidly at the point of interaction, but cools as the beam moves away. While the rising surface tension behind the beam pulls the hotter, molten, material back tothe beginning of the swipe, the vapour pressure at the point of the beams action pushes the material backwards towards the beginning of the swipe. Material is displaced along the swipe in the direction opposite to the beam's travel. Such swipes may be repeated or overlapped several times, and each time more material is displaced from the bulk of the material to a common point. After several passes of the electron beam, a protrusion begins to grow and rises out of the swipe path. TWI has applied for a patent for this process and trade-marked it as Surfi-Sculpt TM .
Fig.16. Surfi-Sculpt TM method for electron beam surface modification (Courtesy TWI)
The height and width of each protrusion is defined by the electron beam size and the number and pattern of passes. While the physical limits of the process are being explored, protrusions in the order of 2 mm high and 0.2 mm wide are typical of those created within TWI's laboratories ( Fig.17). TWI has achieved success with Surfi-Sculpt TM in many metals, including aluminium, titanium and stainless steel. Surfi-Sculpt TM provides, for the first time, a tool that enables control and modification of materials properties at many different levels. Mechanical, electrical, magnetic, thermal, and other characteristics can be tailored precisely for specific applications, and this creates a multitude of exciting new possibilities.
Fig.17. TWI patented the Surfi-Sculpt TM process, e.g. to modify aluminium sheets (Courtesy TWI)
3.2 Comeld TM
TWI launched a group sponsored project, to examine the use of Surfi-Sculpt TM for joining composites to metals - a process called Comeld TM ( Fig.18).
Fig.18. TWI invented and patented Comeld TM for joining metals to composites (Courtesy TWI)
The ability of Surfi-Sculpt TM to form strong and reliable joints between metals and composite materials is the most developed application of this process, and the area most likely to be exploited first. In initial tests, planar cross-joint tensile Comeld TM specimens absorb more energy than conventional joints ( Fig.19). They exhibit failure in a consistent and predictable manner in the metal part of the sample rather than at the joint or in the composite.
The process will be used initially for producing aluminium to composite joints in prototypes representative to motorcycles and Formula 1 racing cars ( Table 1).
Fig.19. Load versus displacement traces for conventional and Comeld TM joints between stainless steel and glass reinforced polyester (Courtesy TWI)
Table 1: Suggestions for using Comeld TM in the car industry
|Glass to frames, e.g. windscreens to car bodies
||Comeld TM could be used for improving the adhesion between glass and a Surfi-Sculpt TM treated metal frame. The different thermal expansion of these materials requires an elastic interlayer, e.g. a fibre-reinforced adhesive.
|Ceramic brackets to glass
||For attaching rear mirrors to windscreens treat either the glass or the ceramic bracket or both by Surfi-Sculpt TM , to improve the joint strength, e.g. when using double-sided pressure sensitive tapes containing woven fibreglass fabrics.
|Crashworthy and soundproof sandwich panels
||The joint integrity between the layers of metal-plastic-metal sandwich sheets could be improved by surface texturing, to dampen vibrations and to increase the crashworthiness of lightweight stampings.
|Pressure sensitive tapes to metals
||Comeld TM could be used to increase the joint strength between pressure sensitive fibreglass tapes, decals or serial numbers and automotive components.
|Fibre-reinforced films to metal blanks
||Instead of spray-painting vehicles, coloured films are used, e.g. when partially or temporarily changing the colour of police vehicles or taxis. A more reliable bond between coloured film and painted blank could be achieved by micro-texturing the edges of the blanks.
|Metals to Fibre Reinforced hosepipes, rubber gaskets or tubeless tyres
||Silicone solid rubber and natural rubber are commonly reinforced with fibres to obtain a dimensionally-stable material for hoses, gaskets, bellows or expansion joints. When vulcanising metal fittings to rubber, Comeld TM could achieve a high-strength joints, e.g. for joining steel belted or radial tyres to metal rims.
|Fibre reinforced metal laminates (FRML)
||By texturing the metal foils, a better bond between the layers or for panel-to-panel joints could be achieved for glass fibre-reinforced aluminium panels (GLARE) or Aramid reinforced aluminium laminates.
|Asbestos replacement brake pads
||Comeld TM could be used for asbestos replacement when attaching eyelets or brackets to fibre reinforced brake pads.
|Fibre-reinforced plastic repair kits
||To build, modify or repair structures in wet environments, e.g. to repair corroded aluminium or steel panels or to join glass fibre pastes or resins to previously corroded metal structures
4 Laser welded sandwich panels with cores made from interlocking aluminium extrusions
Lightweight aluminium sandwich panels are extensively being used for trailers of heavy goods vehicles, where their stiffness and strength are of considerable importance. The honeycomb sandwich structures are usually bonded together using a high-performance adhesive. This is seen by many engineers to be the weakness in the construction, in particular when the sandwich structure is exposed to fire or water. Laser welded metal sandwich structures with extruded aluminium cores are now being developed ( Fig.20).
Fig.20. Male and female connections aluminium tubes with 30mm diameter and 2mm wall thickness in the core of TWI's Ex-Struct TM sandwich panels (Courtesy TWI)
In an experimental study, aluminium sandwich panels branded Ex-Struct TM were assembled from 2mm thick 5083 aluminium alloy sheets and 6060 aluminium alloy tubular extrusions. Interlocking connections were created on the external circumference of the tubes by the design of the extrusion die. The diameter of the tubes was 30mm and the wall thickness 2mm. The clearance between a female and male connection was closely controlled.
Laser spot welding and laser stake welding were used to demonstrate the feasibility of manufacturing sandwich panels and identify key factors affecting the weld quality and distortion. A 4047 (Al-13%Si) aluminium alloy filler wire with 1.2mm diameter reduced weld solidification cracking and to achieved a smooth top bead surface ( Fig.21).
Fig.21. Laser spot welding of an aluminium Ex-Struct TM sandwich panel in TWI's laboratory (Courtesy TWI)
A demonstrator Ex-Struct TM sandwich panel was fabricated using Nd:YAG laser spot welding. The spots were located at the mechanical joint between the tubes. Laser spot welding was conducted horizontally row by row from one side of the panel. The spot welds in each row were produced continuously by controlling the laser shutter open/close time at each joint. To avoid weld cracking, spot welding was conducted using a pulse pattern with a power ramp-down at the end of the pulse thus reducing the cooling rate. This sequence was then repeated for the other side of the panel.
5 Laser welding of plastics and textiles
The Clearweld ® process, which has been invented by TWI and commercialised by the Gentex Corporation, offers a solution for laser-welding thermoplastic components including textiles. A low visibility laser energy absorbing fluid is applied to one or both of the fabric surfaces, or to a polymer film, which is then inserted at the joint ( Fig.22). It absorbs infrared laser beams allowing an almost invisible weld to be produced between materials that are required to be clear or have a predetermined colour.
Fig.22. Principle of Clearweld ® laser welding using an infrared absorbing colourless medium (Courtesy TWI)
The process is especially suitable where the appearance of a product is important. The carbon black absorbing material commonly used in transmission laser welding is replaced by a colourless infrared absorbing medium, thus expanding the applicability of the technique to clear plastics ( Fig.23).
The laser beam passes through the plastic or fabric, heats the visually transparent absorber and generates a weld, which seals the interface. The use of an absorber restricts melting to the interface between materials, rather than through the full thickness. This results in a softer and more flexible joint compared with other welding processes hence its suitability for textile applications. The outer texture of the fabric is also retained.
The infrared absorbing medium is typically printed or painted onto one surface of the joint. Sometimes it is encompassed into the bulk plastic, or produced in the form of a film that can be inserted into the joint.
In the case of textile joining, positioning of the infrared absorbing medium at the joint restricts melting to the interface rather than through the full thickness of the joint as occurs in other welding methods for fabrics. Consequently, flexible seams are produced making the process suitable for the joining of fabrics for soft trim application in vehicles.
Fig.23. Transmission laser weld with carbon black laser absorbing material (top) and new colourless joint with Clearweld ® laser absorber (bottom) (Courtesy TWI)
Laser welding of textiles has found a unique application in airbag manufacture. Procedures for laser welding of textiles using Clearweld ® consumables have been developed. Airbags are often made using woven fabric made from nylon fibres and typically have stitched seams or are woven in one piece using specialised looms. Developments in automotive safety have led to the introduction of curtain airbags mounted in the sides of the seats and in the roof above the doors ( Fig.24).
Fig.24. Laser welded curtain air bag (Courtesy Autoliv)
These provide protection from side impacts and during multiple rollover events. However, rollover events in particular require the curtain bags to stay inflated for at least 10 seconds to offer effective protection. This places additional requirements on the sealing of the seams, and adds extra steps to the manufacturing process.
TWI has developed laser welding techniques for textiles which provide a rapid automated method for sealing a seam against gas or fluid leakage, whilst retaining the outward appearance, feel and flexibility of the fabric. Airbag demonstrator samples were prepared and tested. The results from these trials proved promising, with the welded seams showing leak versus pressure performance within the range of that achieved using conventionally sealed seams. The highly automated welding procedure offers the potential for cost savings by reducing the time and number of steps involved in airbag manufacture.
6 Heat resistant exhausts made from vermiculite particles in an inorganic binder
Barrikade ® is a low density, fire resistant inorganic material developed by TWI ( Fig.25). It consists of vermiculite particles and a blended silicate binder. It is 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 since it is lightweight (typically 200-300kg/m 3 ) and offers thermal insulation.
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.
Fig.25. Barrikade ® material for heat resistant components (Courtesy TWI)
The motorcycle industry is considering the use of Barrikade ® in the manufacture of heat resistant layers in exhaust systems ( Fig.26).
Fig.26. Barrikade ® motorcycle silencer (Courtesy TWI)
New ideas on joining technologies are contrived with an even greater emphasis on meeting current and future market drivers. As well as supporting developments in joining technology, TWI can now also assist in materials development, coating technologies, distortion prediction, modelling and the performance and reliability of automotive structures.
The industrial implementation of these new processes demonstrates the continuous need for further development of joining technologies, to maintain competitive in the international market place.
Internationale Automobilbaufirmen haben in enger Zusammenarbeit mit Forschungsinstituten und Schweißmaschinenherstellern mehrere moderne Fügeverfahren zur Serienreife gebracht. Der Bericht beginnt mit einem Rückblickauf die Industrialisierung des Reibrührschweißens im Karosserierohbau. Reibrührschweißen wurde am TWI (The Welding Institute) im Jahr 1991 erfunden und inzwischen so weit entwickelt, daß es in derSerienproduktion eingesetzt wird. Zur Zeit sind 118 Organisationen vom TWI für den Einsatz des Verfahrens lizenziert. Dies sind vor allem Industriefirmen, die ihrerseits bereits mehr als 1300 Patentanmeldungen eingereicht haben.Anschließend werden innovative Konzepte zur Elektronen- und Laserstrahlmaterialbearbeitung sowie dem Hybridfügen mit Form- und Stoffschluss vorgestellt. Zur Zeit werden Varianten dieser Fügeverfahren entwickelt, und inLaborstudien optimiert.