Dissimilar materials....novel solutions to joining the unjoinable
TWI Bulletin, January - February 2008
A sideways approach to the challenge of joining previously considered unlikely-to-join combinations of materials
Anita Buxton obtained an honours degree in Materials Technology from the University of Surrey and was subsequently awarded a PhD on 'Interfacial failure phenomena in carbon/epoxy composites' from the University of Sydney. She joined TWI in 2000 having sailed a two person yacht from Australia to England. As a Senior Project Leader in the Electron Beam Section, Anita leads projects for individual clients and consortia across a broad range of industry sectors, particularly those involving the industrial application of Surfi-Sculpt technology. She is co-ordinator for TWI's dissimilar materials joining activities.
Ian Norris originally joined the Laser Department at TWI in 1985 after obtaining a BMet from Sheffield University and a PhD on alternative welding techniques for pipeline steels. Following five years as a Project Leader, he left for a period in industry before returning to the Consultancy Services Group in 1999. He became Manager of the Manufacturing Support Group in 2000 and is now the Technology Group Manager for Electron Beam, Friction and Forge.
A wide range of materials is available from which today's engineers can manufacture increasingly complex products. Each material has distinct properties making it eminently suitable for use under particular conditions or for particular components, but it is rare that any material will satisfy all the requirements for an entire product. As Anita Buxton and Ian Norris discuss, a recent review at TWI indicated that the need to join dissimilar materials is common to many industry sectors as customer demand is for products of enhanced performance, increased quality and reduced cost. This drives designers to devise individual components from different materials and then join them together to make multi material finished articles. Although the trend is a generic one spanning all industry sectors, the joining solutions tend to be highly application or even product specific.
Mechanical fasteners have been used for this purpose for many years, as have adhesives. The drive for lightweighting, increasingly stringent operating conditions and the development of composite materials has dictated that alternative joining solutions have had to be developed. However, this is not as easy as it sounds. Whilst it is relatively simple to find a welding process that will join similar metals together, welding dissimilar metals may not be straightforward. If the two metals to be joined have widely different melting temperatures, the temperature required to melt the higher temperature alloy may be sufficient to degrade the other material. Also, the combination of some metals or alloys can result in the generation of undesirable intermetallic compounds that give rise to poor as-welded joint properties.
When it comes to joining metals to non metals or different non metals to each other, the widely different material characteristics often present real challenges which cannot be overcome using conventional welding techniques that rely on material melting and fusion. In this case, novel approaches are required and much effort is spent in developing, refining, demonstrating and qualifying such techniques for production use.
This article highlights a number of new and novel technologies that have been successfully applied at TWI to specific, often challenging joining tasks in a range of materials for particular industry sectors.
Joining textile to wood within the furniture industry
The furniture industry uses an enormous number of staples in applications involving the covering of furniture with upholstery covers. Not only is the stapling process time consuming and labour intensive, it is a common cause of repetitive strain injuries amongst factory workers and the joints are not aesthetically appealing. Wouldn't it be good if the fabric could simply be welded to the wooden base? Exactly this was realised within a DTI funded project, Furnitureweld, led by FIRA, the Furniture Industry Research Association along with project partners, TWI, Silentnight Beds, Knightsbridge Furniture Productions, Laser Lines and Motoman Robotics.
The objective of Furnitureweld, was to assess the Clearweld ® process for the production of textile seams within the furniture industry. The Clearweld process, which has been patented by TWI and commercialised by the Gentex Corporation, offers a solution for laser-welding thermoplastic components, including textiles. A 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.
An infrared laser beam is then directed along the seam line. The beam passes through the fabric, heats the 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 joint that has a greater flexibility and softer feel than can be achieved with other welding processes, hence its suitability for textile applications. The outer texture of the fabric is also retained.
One of the more challenging demonstrator items within the Furnitureweld project involved the welding of fabric to wood laminate ( Fig.1) to cover the divan drawers of a bed base ( Fig.2). Not only did the drawers pass an industry standard testing protocol, but the visual appearance was thought to be better than the stapled version.
Fig.1. Fabric being welded to wood laminate using the Clearweld ® process
Fig.2. A complete bed with laser welded components
Joining metal to bone for orthopædic implants
It has been well documented that the UK is supporting an increasingly ageing population. Combined with the trend for patients to undergo orthopædic implant operations at an earlier age, this has led to a requirement for implants with a longer service life. TWI is working with Symmetry Medical to develop a new generation of metallic orthopædic implants using a novel EB materials processing technology patented by TWI. It's called Surfi-Sculpt TM.
The Surfi-Sculpt process uses an electron beam to generate a variety of complex surface structures which may enhance the performance of orthopædic implants. Using a sophisticated beam deflection system, material is melted and manipulated on a substrate surface in a controlled manner to create features with dimensions from tens of microns to several millimetres. The process is flexible enough to generate bespoke features, which would be impossible to produce using any other processing route. Surfi-Sculpt is a clean process, performed under vacuum and is fast, taking just seconds to process a square centimetre.
Surfi-Sculpt has the potential to offer benefits to orthopædic implants on a number of levels. First, robust high aspect ratio features can be produced on the surface that would penetrate the bone, providing fixation of the implant during the post operative six-week period before bone growth stabilises the implant. Second, finer scale textures may provide osseointegration properties that would enhance long-term performance. Thirdly, interconnected pores may improve the vascularity of the new bone and help to generate an implant with a stiffness more closely tailored to the surrounding bone. Early designs of Surfi-Sculpt surfaces for orthopædic implants are shown in Figure 3.
Fig.3. Early designs of Surfi-Sculpt ® surfaces for orthopædic implants
Joining metal to fibre reinforced polymers using Comeld TM
The performance benefits of fibre reinforced polymer (FRP) composite materials in applications in industry sectors such as aerospace, marine, motorsport, military and specialist construction are well recognised and are already exploited in a variety of established production components and structures. A factor that limits further exploitation is the practical difficulty of producing high strength, high integrity joints between FRP composite parts and metal parts to allow large, multi-material structures to be produced.
Comeld is a hybrid, composite to metal, joining process that involves pre-treatment of a metal surface using the Surfi-Sculpt technique. An array of protrusions is formed on the surface of the metallic substrate which penetrates the layers of fibre within the composite, resulting in a highly energy absorbing joint. Figure 4 shows a cross-section through a Comeld joint. Tests have demonstrated that Comeld joints have the capacity to absorb in excess of twice as much energy before failure than control joints ( Fig.5). Comeld joints have also been shown to prevent sudden bond-line failure from occurring, giving a more progressive and hence detectable failure mode.
Fig.4. A Comeld TM joint between carbon/epoxy composite and titanium
Fig.5. A load/displacement curve demonstrating the capacity of Comeld joints to absorb in excess of twice as much energy before failure than control joints
Comeld offers the potential to create high performance joints between composites and metal and is currently under development for a range of industrial applications.
Joining dissimilar metals using friction techniques
Rotary friction welding has long been established as a technique with the capability to produce high integrity joints in some dissimilar material combinations that are difficult or impossible to join by fusion welding processes. The process produces a solid state weld ( ie no significant melting occurs), so differences in material melting points can easily be accommodated. Rotary friction welding is used routinely to join dissimilar steel combinations, dissimilar Al alloys, pure Al to steels,copper and stainless steel and even aluminium to ceramic materials.
With the advent of friction stir welding, new opportunities have arisen and the technique has been applied to welding aluminium to copper and even aluminium to steel. In this process the rotating tool is offset by a fixed distance from a butt weld joint line to give a degree of control of the heating cycle and the material mixing at the joint interface. The process is also used routinely to produce overlap welds between dissimilar Al alloys ( Fig.6). An example lies in joining 7000 series alloy stringers to 2000 series aluminium alloy skins in the production of the body and wings of the Eclipse 500 business jet.
Fig.6. A photograph of a section through a friction stir weld between aluminium alloys 7075 and 2024
An extension of the principle of friction stir welding has led to the development of a novel technique for joining materials with markedly different softening temperatures. In the technique called Stir-lock TM, the harder material is perforated and the softer material is then extruded through the perforations under the action of a simple friction stir welding tool.
With appropriate joint design and choice of processing parameters, material extruded through the perforations can bond with underlying material to produce a mechanical interlock between two or more plates. Figure 7 shows a possible application for steel to aluminium joining in a T-joint configuration.
Fig.7. The Stir-lock TM technique could be applied to a T-joint between steel and aluminium
A further example of this approach in which a joint between stainless steel mesh and Al plates has been produced by extruding the Al through the holes in the mesh is shown in Figure 8.
Fig.8. A joint between stainless steel mesh and aluminium plate made using the Stir-lock TM technique
Making parts with graded composition
Laser direct metal deposition (DMD) is a technique that is used to generate shaped deposits of closely controlled composition by highly accurate melting of powder using a focused laser beam. Examples of complex structures made usingthis technique are shown in Figure 9. The powder is fed from hoppers and through nozzles to the focus position of the beam and it is possible to vary powder composition during the deposition process to produce structures with graded composition andproperties.
Fig.9. Complex structures made using laser direct metal deposition
In principle, components can be deposited in which composition changes continuously from one material to another. The technique is in the early stages of exploitation but the potential exists to make components with a graded transition between dissimilar materials that would be difficult or impossible to join by other techniques. Figure 10 shows an example of a graded component with Ti-6Al-4V at one end and Ti-6242 at the other produced using the DMD process at TWI.
Fig.10. A component with a graded transition between Ti-6Al-4V at one end and Ti-6242 at the other end, produced using the direct metal deposition process
Joining dissimilar metals by percussive arc welding
Percussive arc welding is a process offering many advantages in the joining of dissimilar materials within the microelectronics industry. However, it is not widely understood or controllable and hence the industrial uptake of the process has been limited. TWI has recently conducted a study to address these issues.
The process is a forge welding technique in which a voltage is applied between the two parts to be welded, resulting in the striking of an arc. The contact area of the two parts is melted by the arc and the application of pressure results in the formation of a weld. Conventional percussive arc power supplies provide the arc current using a capacitor discharge technique. This form of open loop technique results in an uncontrollable current output amplitude and time. A prototype system developed at TWI uses an arc welding power supply used in TIG or MIG processes. The power unit makes use of a pilot supply and an initiation supply to stabilise the current, before the arc is supplied by a current controlled power supply. This allows the amount of current to be set by the user. An extended pulse length allows the current level to be maintained using a feedback loop, enabling longer weld times than with conventional percussive arc welding. High speed filming of the process resulted in an enhanced understanding of the process allowing it to be optimised for various material combinations and wire end shapes.
Copper stranded wires have been successfully welded to stainless steel 308, a combination commonly found in activators such as those used to initiate airbags, and useful electronic connectors have been formed between gold alloy and stainless steel 316 ( Fig.11). The increased repeatability of the welds compared to those made with standard percussive arc welding equipment is an important factor for reliable production processes. With further work percussive arc welding may offer a solution to the joining of Nitinol to stainless steel, a challenge faced in the manufacture of many medical devices.
Fig.11. Percussive arc welds between a) 0.8mm-diameter 308L stainless steel to Au alloy wire and b) 1mm-diameter 308 stainless steel wire to stranded copper wire
Summary
Today's engineers have many materials available to them to allow full optimisation of the design and performance of components and assemblies. With the diversity of materials comes a requirement to join them and this often involves the adoption of new and innovative techniques. This article has presented a selection of processes that have been developed at TWI, but is far from exhaustive. New joining requirements are highlighted by our Member Companies every dayand TWI strives to apply or, if necessary, develop suitable techniques to meet those needs.
If you have a need to join dissimilar materials for a specific application or product please contact the authors.
