Innovative Induction Welding of Thermoplastic Composites

The recent years have seen a renewed interest in thermoplastic composites for high-end applications, especially aerospace, where their high performance is required and higher material cost is affordable when amortised over high volumes. Compared to thermosets, thermoplastic composites (TPCs) offer shorter processing times to meet rising production rates. This makes TPCs an attractive option for the aerospace industry, but as yet there has not been a significant uptake of TPCs for structural applications. One reason behind this has been joining. In aircraft, composite parts are traditionally joined by mechanical fastening. Unlike thermosets, TPCs can be welded; a process that does not cause damage by cutting holes for the fasteners, and offers advantages in terms of weight and speed. Since the introduction of materials such as APC-2 (Carbon fibre reinforced PEEK) in the 1980s many studies have been carried out on fusion bonding of TPCs.

 TWI was at this time also investigating joining possibilities of TPCs in several projects, but as a member based organisation, these projects were confidential and none of the results were published, until recently.  TWI has continued to investigate fusion bonding of TPCs and has recently invented a new method for welding thermoplastic composites such as Carbon fibre reinforced PEEK.  The technique does not employ a metallic implant but uses the conductivity of the carbon fibres themselves.  This is nothing new, but by using a particular combination of parameters and properties, TWI’s method has the advantage that the heat generated by the induced eddy currents 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 ensures that the power delivered by the coil is used to produce the joint and is not wasted. It can also be applied to very thick laminates as the increased power requirement to penetrate the laminate is concentrated at the interface and excess, unnecessary heat is not produced.