Approaching total joining technology for carbon fibre reinforced thermoplastics

TWI Bulletin, January/February 1992

After gaining a degree in physics, Roger Wise joined the Electron Beam Department at TWI in 1986. He has co-ordinated research on miniaturisation of existing high-power electron beam gun column technology and has also worked on finite element analysis of components for the 150kW non-vacuum electron beam system currently being developed as part of the EUREKA initiative. Roger is now a Senior Research Physicist with the Plastics Joining Department. He is working on the development of prototype welding equipment, and joining carbon fibre reinforced thermoplastic composites.

Within the range of commercially available advanced materials is a family called thermoplastic composites. Roger Wise describes some of the work at TWI which has focused on joining technology for continuous carbon fibre reinforced thermoplastic composite materials.

Thermoplastic composites comprise a thermoplastic matrix, for example polyetheretherketone (PEEK), reinforced with fibres typically of glass, aramid or carbon.

Sometimes the fibres are very short, perhaps a few millimetres, and sometimes they are continuous i.e. up to many metres. The fabrication techniques applicable to these materials are to some extent governed by the fibre length, for example it is possible to injection mould components using short fibre reinforced composite material whereas this technique would be inappropriate for a continuous fibre material.

These latter materials may be purchased in the form of unidirectional preimpregnated tape (prepreg) which comprises continuous fibres all lying in the same orientation and bonded together by the matrix material. Consolidated composite sheet may then be manufactured by placing many layers of this prepreg tape in a mould and using a processing schedule of heat and pressure.

Alternatively, some materials are available in the form of a drapeable fabric or cloth. In this case complex shapes may be held together by stitching using filament made from the matrix thermoplastic, and the component may be consolidated under the action of heat and pressure in an oven or autoclave.

The term Total joining technology has been used to describe the enabling knowledge for joining carbon fibre thermoplastic composites to themselves, to metals and to ceramics.

Joining processes

Investigations into joining thermoplastic composites to themselves have concentrated on the material APC-2 from ICI. ^[1] This material, which comprises carbon fibres in a matrix of PEEK, was one of the first commercially available of its type and possesses excellent fracture toughness, damage tolerance and chemical resistance. Over the last five years, TWI has run a Group Sponsored Project addressing the problem of joining APC-2 so that this and other carbon fibre reinforced thermoplastic composites may achieve optimum exploitation in industry.

In the early stages of the work, several joining techniques were studied including hot plate, vibration, ultrasonic, resistive implant and induction welding, and adhesive bonding.

The results were assessed on the basis of single lap shear tests for each method of joining. Each process was appraised by measuring weld strength from these mechanical tests and by assessing the suitability of the process for use in production in the aerospace industry. The three processes selected under these criteria were hot plate, resistive implant and induction welding.

Hot plate welding

This technique involves a heated plate being clamped between the surfaces to be joined until they soften. The plate is then withdrawn and the surfaces brought together under controlled pressure for a specific period. ^[2] Work conducted on hot plate welding of APC-2 indicated that welds can be achieved with a lap shear strength up to 50 N/mm ².

Resistive implant welding

Resistive implant welding is based on the principle of trapping a conducting, usually metallic, implant between the two parts to be joined and then heating the insert by resistive heating. The heat causes the surrounding plastic material to melt and the weld is effected by subsequent cooling under pressure. The conductive implant remains within the joint and as such affects the final strength of the weld. To avoid use of a metallic implant and introduction of foreign material into the thermoplastic joint, prepreg carbon fibre tape, specifically unidirectional fibres set in PEEK, can be used as a compatible implant material.

The resistively heated implant technique has been developed to a stage at which welds of up to 127 X 25mm can be achieved with a lap shear strength of up to 50 N/mm ².

Induction welding

Induction welding of carbon fibre reinforced composite materials involves generation of induced eddy currents in the material. A workcoil provides the energy input to the material and this coil converts energy from a high frequency power supply into a dynamic magnetic field. The shape of the workcoil and the geometry of the composite component being welded determine the pattern of eddy currents generated by the magnetic field in the material.

By an approach including empirical and computer modelling techniques it is possible to optimise the workcoil design to achieve maximum eddy current generation in the vicinity of the weld.

Induction welding can use the amorphous bonding technique ^[3] which involves making a joint by melting a layer of the amorphous polymer PEI at the weld line. PEI has a melting temperature of approximately 270°C whereas PEEK has a melting temperature of 345°C and it is this difference which allows a joint to be made by melting the PEI while leaving the PEEK composite still fully consolidated.

Figure 1 shows a section through such a bonded joint illustrating the laminae of unidirectional fibres within the quasi-isotropic composite and the layer of amorphous PEI between the two pieces of bonded APC-2.

Induction welds using the amorphous bonding technique have achieved lap shear strengths of 40 N/mm ².

Recent work

For the three processes under consideration, induction welding presents the best opportunity for exploitation in a production environment. It is possible to imagine an NC machine positioning a work coil over a joint and, with adequate control, manufacturing reproducible welds of high integrity. With this ultimate goal in mind, work recently completed on welding APC-2 has concentrated on induction welding using the amorphous bonding technique.

In particular, a number of demonstration components have been welded, including tube to tube joints and joints between stringers and variable thickness curved panels up to a metre long. Successful welds have been made on quasi-isotropic APC-2 flat material from 2 to 13mm in thickness using this technique. Figure 2 shows three stiffeners joined to a flat APC-2 demonstrator, 2mm thick, using induction welding.

Joining to metals and ceramics

Work started in January 1991 to investigate the feasibility of joining thermoplastic composite materials to aluminium alloy (grade L113). The technique used in this operation is called polymer coated material (PCM) joining and enabled a maximum single lap shear strength of 30 N/mm ² to be achieved for a joint between APC-2 and L113. Figure 3 shows a small demonstration joint made between an L113 L shaped stiffener and a 2mm thick APC-2 consolidated plaque.

There are a number of technical difficulties to be addressed before this technique can be integrated into production. For example, the materials possess different coefficients of thermal expansion, so there are likely to be residual stresses within welded joints ( Fig.4). In addition, the optimum welding condition is likely to be dependent on the geometries of the components being welded since heat will be conducted away through metallic components relatively rapidly.

The PCM welding technique has been extended to include joining of thermoplastic composite to alumina ( Fig.5). Mechanical test results have yet to be quantified for this material combination but qualitative assessment has indicated that this application merits a much more thorough investigation.

Summary

Work at TWI over the last five years has addressed the need to join carbon fibre reinforced thermoplastic composites and has embraced a number of welding processes applied to one such material, ICI's APC-2. Of the processes initially studied, induction welding appears to offer the greatest potential for exploitation on the production lines of the aerospace industry. A number of APC-2 demonstration structures have been welded using the amorphous bonding technique.

A feasibility study into joining carbon reinforced composites to aluminium alloy and to alumina has also been carried out. From this study a new technique called PCM welding has been developed which offers great potential for further development. Industrial Members interested in this work are invited to contact Roger Wise at Abington.

References