Induction heating is a fast method to heat metals or other electrically conductive material. It is also effective to weld fiber reinforced thermoplastic composites.
The principle of induction heating is the application of an alternating voltage to a conductive coil which results in an alternating current. This current induces a magnetic field. When a conductive and magnetically susceptible material is in the proximity of the coil, eddy currents are induced with a frequency matched to the frequency of the magnetic field. The material is then heated due to resistive losses of the eddy currents. Eddy currents can only be induced if there is a closed conductive path. A closed-loop circuit is present in the case of carbon fiber woven fabrics or cross plies. When no closed circuit is present in the composite (for example short carbon fibre composites) or the composite is not conductive at all (e.g. glass fiber composites) it is necessary to put a susceptor at the welding interface. A susceptor absorbs electromagnetic energy and convert it into heat. It is a conductive material that is used to transfer heat to another piece of metal or non-conductive material. It transfers then the heat to the target by conduction or radiation. Thermoplastic composites can be welded with and without a susceptor.
There are already several papers and articles showing that induction welding is an effective technology to weld thermoplastic composites. In Helacs, it is investigated if the induction technology is also interesting for dismantling composites from end-of-life aircrafts. This means that the welding device should be preferably mobile because it is difficult to install a robotic system which is often used for welding at the dismantling place.
Therefore, a portable DHI-15 PKW Inverter induction heater with working frequency of 25-60 kHz, and a capacity of 1.5 kW was bought from the Welding Company.
Firstly, it was investigated if the welding device was capable of heating carbon composites. Three carbon fiber composites were selected to check if induction heating is feasible.
Two types of woven carbon fiber composites which are carbon/PA and carbon/PEEK composites, and a short carbon fiber PEEK composite were selected for the welding trials. In a first step, it was investigated if these composites can be heated by induction. The induction coil was put for 15s on the surface of the composite and afterwards the temperature was measured with an IR camera.
As expected, heating was possible for the woven composites, but not for the short fiber composite as the measured temperature was only 25°C while the temperature for the PA composite was around 62°C and 70°C for the PEEK woven composite.
In a next step, the two woven composites were used for welding and welding was done without using a susceptor. Welded was possible in less than 1 minute and the composite was not damaged. Welding of the PEEK composite was more problematic and took more time (+/- 2 minutes) because PEEK has a higher melting point. Besides the longer welding time, some damage of the surface was observed indicating that not all the heat is concentrated at the welding interface.
To do the debonding, a weight was put on the lower part of the welded composites and the induction coil was placed on the surface of the composite on top.
After 30s the PA composites were debonded and the composites were not damaged.
On the other hand, about 1 minute was necessary to debond the PEEK composites and the surface was damaged. So, a repairing step is required to reuse the composite or the debonded part can go to recycling like for example sending to pyrolysis to recover the carbon fiber.
In order to avoid surface damage, it was investigated if the addition of a susceptor at the interface would be a solution. A copper mesh from CThru Metals, Inc. was placed at the interface, and this resulted in a more efficient welding and no surface damage was observed.
Debonding was also feasible and resulted in no damaged separated composite plates. However, the susceptor was damaged due to overheating. To prevent this, temperature control is recommended, but this was not possible with our induction heater.
Ultrasonic and induction heating are interesting technologies to debond welded thermoplastic composites. Ultrasonic heating is more recommended for resin rich composites. However, during debonding it is likely that the composite will be damaged and that a repairing step is necessary to reuse the composite part. Debonding by induction heating is recommend for composite with a high fiber volume fraction and textile reinforcements that can induce eddy currents (e.g., carbon fabrics). A point of attention is that composite damage can occur when the heat is not sufficiently concentrated at the joint. If this is the case, it is advised to add a susceptor at the welding interface not only to facilitate the welding, but also the debonding process.