Resistance Welding of Thermoplastic Composites: Process and Performance

Abstract

Compared to thermoset composites, thermoplastic composites are drawing more and more attention by aircraft industries not only due to their excellent material properties but also due to their potentials to reduce cycle time and structure cost by using low-cost manufacturing technologies such as welding. Resistance welding has been regarded as one of the most promising welding techniques owing to the low energy consumption, simplicity of welding operation and capability for scaling up. Previous researches on resistance welding of thermoplastic composites are mainly focused on understanding the welding mechanisms and characterizing the welding qualities. The research work was aimed to gain deeper knowledge of resistance welding, in both micro-level and macro-level. In micro-level study, the specific properties that influence the welding quality were discussed, such as the surface properties of adherends, the weld line properties and the welding induced voids. In macro-level study, the emphasis was put on improving the current welding process, for both static resistance welding and continuous resistance welding. Micro-level study: 1. Resistance welding of woven fabric reinforced thermoplastic composites was investigated. Both the type of the majority fibres (warp yarns or weft yarns) and the apparent orientation of the majority fibres on the adherend surface were found to influence the failure mode and lap shear strength. Fibre sizing was found to be crucial for a good fibre-matrix adhesion, and therefore it was crucial for a good weld. 2. Other than fibre de-compaction, the residual volatiles inside the adherends was found to be a main cause of the voids in the joints for a welding process performed under a moderate welding pressure. Non-uniform void distribution was observed inside the joints, with void concentrations near the middle of weld overlap. The voids could be reduced by using pre-dried adherends or using a higher welding pressure. 3. A thinner weldline, usually obtained by using a thinner heating element, was found to be preferable to a thicker one. The relatively weaker welding quality near the edges of the joints was found to be a limitation of weld performance, but this could be improved by tailoring heat generation at the weld overlap or creating resin fillets near the edges. Micro-level study: 1. The possibility of using displacement measurement data for process monitoring and processing window definition was investigated, and it showed ability to detect voids generation and resin squeeze flow during welding and to construct processing windows. 2. The process of continuous resistance welding was analysed, and a model was developed to simulate the heat generation and heat transfer during welding.