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Linear Friction Welding of Carbon Fibre Reinforced Plastic

TWI Core Research Project 1123/2020

For the purpose of this project, continuous fibre carbon reinforced PEEK (Polyetheretherketone) thermoplastic composite (CFRTPC) was joined using linear friction welding (LFW). Thanks to a PEEK sacrificial interlayer, the strength of the joints produced matched, without any surface pre-treatment, the typical strength of an adhesively bonded joint.

Overview

The rising production rates of the aerospace industry are most likely to be met using thermoplastic composites which, unlike thermoset composites, do not suffer from long cure schedules. Thermoplastic composites can also be joined by thermal welding processes.

LFW is a rapidly developing, solid-phase metals joining process. It is a key technology for critical aero-engine components and is now being qualified for use on aero structures. LFW would typically degrade continuous fibres at the weld interface during welding (Taylor and Jones, 1989) and its application to continuous CFRTPCs has not been achieved.

Adhesively bonded carbon/PEEK laminates have demonstrated joint strengths of up to 28MPa but only after the application of appropriate surface treatment.

Objectives

  • Demonstrate welding CFRTPC by LFW
  • Mitigate fibre degradation during LFW by adding an unreinforced PEEK interlayer
  • Evaluate the strength the CFRTPC joint made on LFW equipment designed for welding metallic parts
  • Achieve a minimum joint strength of 25MPa without any surface pre-treatment

Solution

Adding an unreinforced PEEK interlayer at the weld location was found to be critical in increasing join strength from 17MPa (Taylor and Jones, 1989).

Linear friction welding of pure (unreinforced) PEEK material was first trialled to establish suitable welding parameters.

Then a CFRTPC laminate was manufactured to suit these parameters and a small series of three joints was produced. Figure 1 shows a fully bonded interlayer.

 

Conclusion

The most successful parameters for the CFRTPC joint were 50Hz, ±1.0mm, 40MPa. The PEEK interlayer showed adequate heat generation with non-degraded polymer forged out of the joint. Visual assessment from PEEK welding suggests that applying LFW to CFRTPCs using current metal welding equipment at TWI has the potential to effectively produce welded joints. The welded coupons achieved an apparent lap shear strength of 25.4MPa without any surface preparation.

 

This project was funded by TWI’s Core Research Programme.

Figure 1. Sacrificial PEEK interlayer between two carbon/PEEK laminates. Yellow lines denote weld location
Figure 1. Sacrificial PEEK interlayer between two carbon/PEEK laminates. Yellow lines denote weld location
Avatar Dr Chris Worrall Technology Fellow and Consultant – Polymer Composite Technologies

Chris is an expert in composite materials and a Consultant and Technology Fellow at TWI. His 35 year career covers: joining, mechanical behaviour, fire testing, finite element analysis, electromagnetic surface waves and metamaterials. Chris started his career in the oil and gas industry participating in the Marinetech North West project, and also spent 10 years working in Japan in the automotive, aerospace and energy sectors. He has presented guest lectures at Cambridge, Liverpool and Surrey University, and holds the position of ‘Engineer in Residence’ at Liverpool university. Recently, Chris has been supporting the Non-metallic Innovation Centre (NIC) in providing non-metallic solutions for the oil and gas industry.

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