HELACS project is conceived as a solution for future aircraft dismantling issues derived from introduction of innovative Carbon Fibre Reinforce Polymer (CFRP) material. This these materials are fully introduced in industries such as the automotive industry due to their interesting mechanical properties combined with their low density. Furthermore, the possibilities of these materials allow them to replicate joining processes used in conventional metallic materials, such as riveting and welding. Nevertheless, their recycling and reuse represents an operational and environmental challenge.
Regarding these problems, HELACS is looking for innovative dismantling solutions, capable of separating these new CFRP parts quickly, safely and with a high level of automation. Moreover, this project scope reaches CFRP materials that are not even in use for aeronautical industry, previewing their forthcoming implementation. In this respect, one of the main technologies developed by the project, is resistance welding bonding and debonding.
As mentioned in previous articles in this project, resistance welding process is an assembly process available for thermoplastic materials (which falls under the family of CFRP materials). This proceeding, which is expected to be one of the main assembly methods for thermoplastic materials in the future, is based on the placement of a metallic mesh between the parts to weld. Once the parts are placed, and electric current is applied to the mesh, generating a high temperature area due to Joule effect. This area reaches the thermoplastic glass transition temperature (Tg), that, combined with an applied pressure, creates a joined area when cold down.
HELACS project has attempt a large amount of resistance welding test, focus not only on the assembly, but also in the reversibility of the process, this is, the debonding resistance. A specific tooling was developed to apply the both the aforementioned pressure and electric current to the welding area. This join present significant advantages over bolted and riveted joints, creating a bonding surface, rather than punctual bonding areas, which are weakened by the necessary drilling. The followed image shows 2 thermoplastic plates, welded using the metallic mesh. Should be highlighted the welding process not only affects the CFRP plate on the welded face. The opposite face of the plate shows an affected zone of the thermoplastic.
On the other hand, HELACS proved the reversibility of this process. Once the weld is completed, the application of a second electric current, equal to the welding one, will bring the material back to its Tg. This process, linked to a force in the direction opposite to the welding direction, would create the optimum conditions to perform the debonding operation. As it can be observed at the image, the weld area is affected by the process. Further tests should be accomplished in other to optimize the process, ensuring the maintenance of the material properties after it.
The conclusions drawn from the experiments carried out are very positive with regard to the joining and dismantling processes of thermoplastic parts by means of resistance welding. This process, at the time of its integration in the aeronautical industry, could allow the dismantling of complete assemblies and their subsequent welding without the need for intermediate processes, allowing an improvement in terms of logistics and process times.