Hot gas welding of thermoplastic composite T-joints: Design and Development of the T-CAT2.0 Machine

The main goal of this EngD project is to design and assemble an instrumented lab-scale research set up, able to weld a skin to a stiffener in a butt-joint configuration, in a controlled manner. This set up serves as an alternative to the
more conventional joining methods like mechanical fastening and adhesive bonding. Furthermore, the production technique also proposes an alternative to the co-consolidated butt-joint of GKN Fokker by local heating and shaping
of the weld area instead of full co-consolidation of the structure. Three ingredients are necessary to join a laminate to a stiffener into a T-joint configuration using the proposed hot gas welding technique. Local pre-heating of the
laminates, extruding a molten filler material to the pre-heated laminates and cooling and finishing of the weld region. The system under design is meant to be used in research programs aiming at establishing a relation between the welding input parameters and mechanical performance of the T-joint.

A systems engineering approach is followed to structure the design process, starting from the customer wish. Integration of double sided welding is key for the new system under design. A start and stop function is developed, force control of the weld head towards the laminates is investigated and applied, the weld head will be tested and updated for aerospace grade thermoplastic materials and an active cooling device will be installed to control the temperature in the weld region. The weld head design of previous Thermoplastic Composite Automated T-joint (T-CAT) developments will be taken as baseline in this project.

The problem is decomposed and defined via the requirements, functions, functional solutions, concepts, and to be finalized by a 3D CAD design for T-CAT2.0. The main functions of the T-CAT2.0 are double sided welding,
integration of a start and stop function, pressurization of the weld region, stick-slip prevention, welding the T-joint, position the specimen and transporting the weld head.

Integration and verification of the components and subsystems is achieved by systematic verification experiments. The components are integrated into the subsystems and a new verification is performed, on subsystem level. The validated system experiment showed minor local design iterations are needed to complement the system under design.

The first welding sequences with Carbon/PPS with 15%wt short carbon fibers and Glass/PP laminates and a 40wt% short fiber glass filler show good interdiffusion between the filament and the laminates. Besides, repeatability experiments have been performed on Glass/PP laminates and filament material, showing consistent weld geometry over the length shown by cross-sectional micrography.

The main recommendations that follow from the design of T-CAT2.0 are to optimize the weld head, perform more welding sequences to identify the temperature cycle at more locations at the weld interface and to establish a relation between the welding process parameters and mechanical performance. For mechanical performance, pull-off experiments would be recommended.