Lars Moser

• Induction welding is a technique for joining of thermoplastic composites. An alternating electromagnetic field is used for contact-free and fast heating of the parts to be welded. In case of a suitable reinforcement structure heat generation occurs directly in the laminate with complete heating in thickness direction in the vicinity of the coil. The resulting temperature field is influenced by the distance to the induction coil with decreasing temperature for increasing distance. Consequently, the surface facing the inductor yields the highest, the opposite surface the lowest temperature. The temperature field described significantly complicates the welding process. Due to complete heating the laminate has to be loaded with pressure in order to prevent delamination, which requires the usage of complex and expensive welding tools. Additionally, the temperature difference between the inductor and the opposite side may be greater than the processing window, which is determined by the properties of the matrix polymer. The induction welding process is influenced by numerous parameters. Due to complexity process development is mainly based on experimental studies. The investigation of parameter influences and interactions is cumbersome and the measurement of quality relevant parameters, especially in the bondline, is difficult. Process simulation can reduce the effort of parameter studies and contribute to further analysis of the induction welding process. The objective of this work is the development of a process variant of induction welding preventing complete heating of the laminate in thickness direction. For optimal welding the bondline has to reach the welding temperature whereas the other domains should remain below the melting temperature of the matrix polymer. For control of the temperature distribution localized cooling by an impinging jet of compressed air was implemented. The effect was assessed by static heating experiments with carbon fiber reinforced polyetheretherketone (CF/PEEK) and polyphenylenesulfide (CF/PPS). The application of localized cooling could influence the temperature distribution in thickness direction of the laminate, according to the specifications of the welding process. The temperature maximum was shifted from the inductor to the opposite side. This enables heating of the laminate to welding temperature in the bondline and concurrently preventing melting and effects connected to this on the outer surface. Inductive heating and the process variant with localized cooling were implemented in three-dimensional finite-element process models. For that purpose, the finiteelement- software Comsol Multiphysics 4.1 was used for the development of fully coupled electromagnetic-thermal models which have been validated experimentally. A sensitivity analysis for determination of different processing parameters of inductive heating was conducted. The coil current, field frequency, and heat capacity were identified as significant parameters. The cooling effect of the impinging jets was estimated by appropriate convection coefficients. For transfer of the developed process variant to the continuous induction welding process, a process model was created. It represents a single overlap joint with continuous feed. With the help of process modeling a parameter set for welding of CF/PEEK was determined and used for joining of specimens. In doing so, the desired temperature field was achieved and melting of the outer layers could be prevented.