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A new approach to thermoplastic composite induction welding

By Chris Worrall

TWI has invented 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. The method does not employ any additional susceptor implant, instead exploiting the electrical conductivity of the carbon fibres in the composite parts being joined. Using a particular combination of parameters and properties, TWI’s technique has the advantage that the heat generated is 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.

Thermosets vs thermoplastics

One of the limitations of composites is the joining of pre-fabricated parts. Thermoplastic composites (TPCs) lend themselves better to joining, but there is always difficulty in ensuring that only the interface is heated without a foreign material left at the bond line. Recent years have seen a renewed interest in TPCs. The predicted production rates for single-aisle aircraft exceed the current economic production capabilities of thermoset composites. Compared to thermosets, TPCs can offer shorter processing times. Thermoset composites must go through a lengthy chemical cure process; TPCs can be processed more rapidly by thermo-forming. This makes TPCs an attractive option, but as yet there has not been a significant uptake for structural applications.

 

One reason behind this has been joining; TPCs can be joined by fusion bonding (welding), a process that heats the interface under pressure to a temperature above the glass transition temperature, Tg (for amorphous polymers), or the melting point, Tm (for semi-crystalline polymers), forming a welded joint on cooling.

In induction welding, heat is generated through Joule losses from the induced eddy currents. The magnetic field is produced by an alternating electric current passed through a conducting coil. Implants are frequently made of metal, although they can be non-metallic materials such as carbon fibres, or even the carbon fibres in the parent composite materials.

Objective

TWI’s aim was to demonstrate a novel induction welding technique that provides improved control through focused heating of thermoplastic composites.

Figure 1 Thermal image of induction heating with insulating gauze (top) and without (bottom)
Figure 1 Thermal image of induction heating with insulating gauze (top) and without (bottom)

Proving the technique

To demonstrate the focused heating effect, two  laminates were manufactured, each containing carbon/PEEK (APC-2) plies at 0/90 orientation, 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 (Figure 1) 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 has therefore been focused at the interface between the two laminates.

To find out more about this research and possible applications, email contactus@twi.co.uk

Figure 2 Temperature peak is focused at the weld interface
Figure 2 Temperature peak is focused at the weld interface
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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