Continuous induction welding provides enormous potential to increase the industrial application of thermoplastic carbon fiber reinforced polymer composites (CFRPC). Due to the rapid and non-contact heat input, this welding process is suitable to efficiently join complex and large-area structures made of thermoplastic CFRPC. However, the complex heat distribution that occurs in the thickness direction of the laminate as a result of induction heating, has represented a challenge for the process engineering. The maximum temperature typically occurs on the laminate surface facing the inductor due to the decrease in magnetic field strength with increasing distance from the inductor. For an ideal welding process however, it would be desirable if this temperature maximum were located in the joining zone.
Within the frame of a project, funded by German Research Foundation (Deutsche Forschungsgemeinschaft – DFG), Leibniz-Institut für Verbundwerkstoffe GmbH (IVW) in Kaiserslautern is intensively researching the fundamentals of inductive heating of CFRPC. The aim of this work is to optimize the process of continuous induction welding of CFRPC organic sheets. Based on the results gained so far, two independent approaches have already been developed, which make it possible to shift the temperature maximum to the joining zone.
The first approach involves optimizing the laminate structure of the organic sheets for the induction heating process (Figure 1). In the region of the laminate, where the joining zone is supposed to be, the laminate structure is layered finely. The difference in fiber orientation between adjacent layers should be 90° in this area. This results in low electrical resistances in the laminate which maximizes the current strength of the induced eddy currents. In the laminate section close to the inductor, on the other hand, a layup consisting of thick individual layers with only slight differences in fiber orientation between adjacent layers. Thus, the intensity of the eddy currents in the laminate region close to the inductor is minimized due to high electrical resistances. Since the dissipated heat depends quadratically on the current strength of the induced eddy currents, the temperature maximum is consequently present in the region of the joining zone.
The second approach is used for welding a single overlap configuration. By means of this approach, the maximum temperature is generated in the plane of the joining zone without using additional materials or without adapting the laminate structure of the organic sheets (Figure 2). For this purpose a novel method for continuous induction welding of thermoplastic CFRP is used, which was developed at IVW and was applied for a patent. By using a special inductor geometry and by a special positioning of the eddy currents flow, heating in the joining surface section facing the inductor can be prevented. There is only a heating in the bottom adherend. Since the magnetic field strength on the bottom adherend is at a maximum on the laminate surface facing the inductor, a temperature maximum occurs there. Hence, even thick-walled laminates of thermoplastic CFRP can be joined by means of induction welding. This method also provides great potential to significantly increase the welding speed compared to the conventional continuous induction welding process. For both process variants, the aim is to achieve bonding strengths at autoclave level The necessary process design will be optimized in further work at IVW.
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