- Linear Friction Welding of Carbon Fibre Reinforced Plastic
A 2020 core research project investigated the use of linear friction welding (LFW) to join continuous fibre carbon reinforced PEEK (Polyetheretherketone) thermoplastic composite (CFRTPC) as an alternative to adhesive bonding.
Thermoplastic composites allow for fast production rates and can be joined using thermal welding processes, while LFW was a rapidly developing, solid-phase metals joining process and a key technology for critical aero-engine components being qualified for use on aero structures.
The aim of this project was to demonstrate welding CFRTPC by LFW, mitigating fibre degradation during the process by adding an unreinforced PEEK interlayer, before evaluating the joint strength to achieve a minimum of 25MPa without any surface pre-treatment.
By testing different parameters, our project team was able to demonstrate that current metal welding equipment at TWI has the potential to effectively produce welded joints with an apparent lap shear strength of 25.4MPa without any surface preparation by applying LFW to CFRTPCs (Figure 1).
- A New Approach to Thermoplastic Composite Induction Welding
Another invention at TWI was a new method for induction heating composites containing carbon fibres. The heat generated using this technique is sufficient to be used for adhesive curing or fusion bonding. It works by exploiting the electrical conductivity of the carbon fibres in the composite parts being joined. Using a particular combination of parameters and properties, this technique has the advantage of the heat generated being concentrated around the joint interface, rather than close to the top surface of the composite, where the proximity of the induction coil is greatest. This avoids the problem of having to remove excess heat from the surface of the composite to avoid thermal damage, and provides greater control over the process.
This work focused on thermoplastic composites (TPCs), with two laminates manufactured containing carbon/PEEK (APC-2) plies at 0/90 orientation. They were manufactured using TWI’s novel technique, so that when placed together, heating would be concentrated at the interface between the two laminates. The two laminates were then offset by 45 degrees to allow measurement of the surface temperature of the lower laminate using a thermal image camera. A 1kW induction power supply was used to apply an alternating electromagnetic field through a solenoid coil at a frequency of 165kHz for a period of 60 seconds.
Results of the temperature rise (Figures 2 and 3) show that despite being closer to the source of the electromagnetic radiation, the top laminate experienced a much smaller temperature rise compared to the bottom laminate. The heating was therefore focused at the interface between the two laminates.
- Plasma Treatment for Bonding of Aerospace Components
Outside of welding, we have also investigated developments to adhesive bonding of composites with a research project to explore the potential of using plasma treatment on the surface of composite aerospace components before they are adhesively bonded. This work aimed to improve the repair and lifetime extension of airframes through the use of adhesively bonded composite patches that have undergone plasma treatment to the surface (Figure 4) as an alternative energetic technique. This method sidesteps many of the disadvantages of the existing pre-treatments, such as debris, environmental issues and health and safety concerns. The plasma surface treatment was compared to both abrasion and grit blasting surface treatment techniques. The study considered treatment distance, dwell time, the number of passes required and the gas flow rate. Around 200 joints were manufactured and tested to identify the best treatment conditions.
- Patent Granted for Novel Composite Pipe Joining Technology
The oil and gas industry has also adopted composites for their pipes with our work leading to the granting of a number of patents for novel joining technologies for composite pipes. Working under the umbrella of the Non-Metallic Innovation Centre (NIC), TWI invented a novel, corrosion-free, reinforced composite pipe connection system, marking another milestone in the NIC’s commitment to advancing composite joining solutions. The joining concept originated from a 2019 project that led to the successful development of a composite welding process as well as drawing on our 40-year heritage of induction welding thermoplastic composites.
This new joining process was designed to improve the reliability, precision, and sustainability of composite pipe joining, combining the advantages of two different welding. This milestone added to the portfolio of intellectual property developed by TWI, supporting our Industrial Members in protecting their research and development.
- New Approach to Joining Dissimilar Materials
TWI has also focused on processes to join dissimilar materials, with the development of a polymer coated material (PCM) welding technique, that has successfully joined carbon fibre thermoplastic composites to aluminium alloy and alumina ceramic as well as joining aluminium to alumina ceramic.
The process involves the surface pretreatment of one component with a thermoplastic coating, after which a conventional plastics welding technique can be applied to join the plastic coated component to either a thermoplastic component or to a second coated component (Figure 5). The process is simple in application and does not involve any curing time. Lap shear strengths for the joints also compare very favourably with those achieved using adhesives.
- Metal to Composite Joining Breaches a New Frontier
A different type of surface treatment was used to join fibre reinforced polymers to metals so that industry could gain the polymer benefits of high strength and low weight. With this solution, the joint is produced by allowing the polyester resin and glass fibre composite to bond to a prepared metal surface. In some circumstances a further adhesive layer may be called for, and additional standard surface preparations like etching and priming have also been used to great effect. By using this method, the joint fails at a much higher load and absorbs far more energy before failure, than a conventional joint of identical dimensions.
- Ceramic Matrix Composites – Joining to Metals
Ceramic matrix composites, such as SiC/SiC and C/SiC, are lightweight, hard, wear resistant and stable in oxidising environments to temperatures as high as 1600°C. Because of their fibre reinforcement, they have improved mechanical properties, failing in a gradual rather than catastrophic mode. They are particularly suitable for applications in high temperature burner environments where they can outperform more conventional superalloys.
Many high temperature applications using ceramic matrix composites require them to be joined to other materials, including metals. Brazing is a highly effective joining technique for many ceramic-ceramic and ceramic-metal joint systems. However, differences in coefficients of thermal expansion require specialised approaches to accommodate the mechanical stresses introduced by joining. This can be achieved, for example by flexible or ductile interlayers, or by careful selection of interlayers expansion matched to the ceramic. Additionally, non-melting materials, such as ceramic particles, can be dispersed in the braze to adjust its thermal expansion coefficient or ductility (Figure 6). Design of such joints, using techniques such as finite element modelling, is also of critical importance to minimise stresses.
- Laser Riveting for Composite-to-Metal Joining
Our experts investigated the use of laser riveting to join composites to metals. The aim of the project was to design and produce composite / metal assemblies with an interface region that acts as a ‘transition zone’ tailored to each application and geometry. This would allow industries such as automotive and aerospace to take advantage of the high specific strength and stiffness, energy absorption, high bearing load resistance, as well as the corrosion and fatigue resistance, and typically lower costs offered by composites. The use of laser riveting was researched to avoid the drawbacks associated with traditional mechanical fastening (additional weight) and adhesives (durability issues).
The concept of the work was to join a composite to a metal using a metal rivet, improving the fundamental empirical understanding of key variables and their influence on a joint’s properties. The solution was also designed to allow for higher productivity. Figure 7 shows the technical concept of the laser riveting solution we developed.
- D-JOINTS Collaborative Composites Joining Project Begins
The D-JOINTS project also looked at the topic of composite to metal joining, except this time with the additional need to deliver lightning strike protection for aircraft. Joints were designed using a dedicated sizing tool with key design parameters identified and integrated in the tool. The material properties for the design of each joint were determined with the use of a database before the joints were integrated into a composite nose part. Manufactured demonstrators were then tested for lightning strike protection properties.
- Ultimate Project to Develop Dissimilar Joining for Industry
The Ultimate project also investigated solutions for dissimilar materials joining through the development of a fusion welding solution to join dissimilar metals and also metals to composites. Initially developed at laboratory scale, the solution was then validated through relevant case studies indicated by the end users. The overall objectives were to develop laser welding solutions for joining composite to metal parts in an overlap joint configuration. This approach can be exploited over different joining configurations (Figure 8-10). The Ultimate Project also included the integration and validation of intelligent fixturing and on-line quality assurance (QA), capable of performing real-time monitoring of weld depths during processing.
- High Volume Joining of Composites to Metal for Automotive
Another project that looked into composite to metal joining was the Lightjoin project on behalf of automotive industry organisations seeking a solution for a high-volume manufacturing environment (Figures 11-12). The project scope sought to find a solution for inserting carbon fibre floors into Nissan vehicles. Two techniques were investigated for the project – blind riveting using a cold curing polyurethane adhesive and using a steel element before direct resistance spot welding. Our experts also investigated carbon fibre sheet, high strength steel and aluminium for high volume production. In addition, a joining design software tool was developed to allow engineers to select any combination of materials (material type, sheets thickness and orientation of the joint stack), before generating data on the feasibility of the materials for joining in a production line, including the predicted cost of the production process and valuable joint performance data.
- Thermoplastics On Doors Project Update
Another composites-based project, ‘Thermoplastics on Doors’ (TOD), sought to use thermoplastic composites to reduce costs and save weight in aeroplane structures. The overall aim of the project was to demonstrate and validate the manufacturing process for thermoplastic door components, including an induction welding assembly process, additive manufacturing, additional metallic parts for the door mechanism and metallic and thermoset parts for the surrounding structure of an aircraft’s passenger and service door.
- TCTool: Developing Large-Scale Thermoplastic Aerostructures
Also using thermoplastics for aerospace, but on a much larger scale was the TCTool project. This project saw our experts deliver one of the largest thermoplastic aerostructures in the world. Working alongside a group of project partners, we helped develop innovative tooling, end effectors and industrialisation for the welding and assembly of thermoplastic fuselage components. This involved new approaches to tooling and automation to lower or remove fixed tooling costs that could account for over one-third of non-recurring aircraft assembly costs.
As well as reducing costs, the use of thermoplastic composites in aircraft aids light-weighting as being able to weld them together removes the need for thousands of weight-increasing fasteners.
The 8 metre by 4 metre multi-functional fuselage demonstrator (MFFD) includes over 400 thermoplastic fibre-reinforced parts, thousands of spot welds and hundreds of metres of continuous welds, demonstrating the feasibility of high-rate production, while also delivering a promised 10% reduction in fuselage weight and a 20% reduction in recurring costs (Figure 13).
- TWI-Led PLEIADES Project
Another project addressing the use of advanced composites, the PLEIADES project, aimed to address the need for advanced composite materials for the aerospace industry. The materials needed to be lightweight and able to withstand harsh environments, improving performance and delivering cost savings.
This project brought together a range of expertise and technologies including the formulation and characterisation of new composite materials, automation of induction welding processes for composites through integrated sensing, disassembly of composites joints, healing, and maintenance schedules. In addition, the project team worked to develop passive PIC based multi sensors and a unified QA-SHM methodology, as well as extensive modelling for induction welding and the development of material, healing, damage propagation, and de-icing models. It was expected that, by making full use of the new technologies to promote sustainability and circularity, cost savings of at least 30%-40% can be achieved.
- Ultrasonic/Radiographic NDT of Polyethylene Fusion Joints
Of course, the integrity of composite joins need to be assessed to ensure they are suitable for their desired purpose. As such, TWI conducted a core research project to develop ultrasonic and radiographic testing procedures for polyethylene pipes joined by hot plate butt fusion. The procedures were developed on pipes with diameters greater than 180mm and thicknesses greater than 12mm, although the underlying techniques can be used on testpieces other than pipes, and on a range of different polyethylene types and thicknesses.
The absorption characteristics of polyethylene to radiation and ultrasonic energy can vary significantly from one polyethylene type to another. However, some types can be grouped together for the purpose of exposure charts, image quality indicators (IQls) and ultrasonic calibration blocks. A range of lQls were manufactured from polyethylene and compared, including ASME plaques, British Standard step wedges and wire types. Prototype radiographic procedures were found to be more reliable in detecting and sizing flaws of interest (i.e. inclusions, voids and cold welds) than the prototype ultrasonic procedures.
- SWAK Project: Determining Aerospace Composite Bond Quality
Also investigating NDT for composites and adhesively bonded structures in the aerospace industry was the collaborative SWAK (‘Sealed Without A Kiss’) project. Taking a focus on kiss bonding, our experts investigated different non-destructive testing (NDT) technologies and models for determining the quality of bonds in aerospace composites.
While adhesive joints reduce weight, and thereby fuel consumption and emissions, they can also be susceptible to manufacturing defects and environmental degradation. Kissing bond defects, also called zero-volume dis-bonds between adhesive and adherend, appear to show solid-to-solid contact, but there will be no tensile strength or volume at the interface between the adhesive and the adherend. Because the bond appears to have been made, these defects are difficult to locate using NDT techniques. Of course, these defective bonds can be dangerous as they compromise the joint strength.
The project team tested a range of NDT techniques to locate discontinuities in kissing bonds, including computed tomography, thermography, high frequency C scans, ultrasonic phased array inspections, laser shock testing, and guided wave NDT. Each of these techniques was evaluated for its effectiveness in locating kissing bond defects in a range of assemblies. The project team also investigated inline process inspection and mechanical testing during the manufacturing process.
The project deliverables led to the production of documents demonstrating the range of testing and results as well as instructions on how to prepare, test and analyse samples and results to successfully determine if a kissing bond defect is present.