Induction Welding


Optimizations of the continuous induction welding process of CFK organosheets by using FEA

Continuous induction welding provides significant potential for manufacturing complex components made of carbon fiber-reinforced plastic (CFRP) in thermoplastic construction. Its efficient and non-contact intrinsic heating enables the welding of complexly designed joining zones and large components. A crucial factor in achieving high-quality weld joints is the distribution of heat and pressure in the joining zone. However, due to the multitude of physical influences on these parameters, relying solely on experimental methods for process optimization requires substantial resources. In order to minimize the experimental aspect of optimization processes, numerical models for continuous induction welding of CFRP organosheets have been developed within the scope of a project funded by the German Research Foundation (DFG).

To comprehensively simulate the induction welding process, three sequential finite element simulation models were developed (Figure 1). In conventional coupled multiphysics approaches, electromagnetic, thermal and mechanical calculations are performed alternately. The resulting computation time is further increased by the use of high-resolution meshes necessary for electromagnetic calculations, resulting in calculation times of several days even for small planar welding geometries (400 mm x 100 mm). The approach developed at Leibniz-Institut für Verbundwerkstoffe (IVW) requires only a single preceding electromagnetic calculation step, reducing the overall calculation time by more than 90%:

  1. In the first step, the static electromagnetic behavior of the joining zone geometry is calculated. For this purpose, an electromagnetic material model for fabric-reinforced CFRP organosheets has been developed. In this material model, the fabric layers are simplified and modeled as orthotropic unidirectional plies in a cross-ply configuration. This mesoscale modeling approach allows the influence of the ply stacking sequence on the qualitative manifestation of the heating pattern to be represented.
  2. In the second step, the calculated heating pattern is transferred as a volumetric heat source to a continuous thermal simulation of the induction welding process. For these calculations, homogenized laminate properties are used in combination with a coarse resolution grid. This allows a further reduction of the calculation time.
  3. In the third step, the temperature distribution in the thermally steady state region of the induction welding process is transferred to a finely meshed mechanical simulation of the consolidation roller and the joining partners. The highly temperature-dependent stiffness of the laminate in this model allows for investigating the influence of the roller position on the qualitative expression of the pressure distribution in the joining zone. Subsequent analytical calculations can then be used to assess the quality of the resulting weld joint.

The developed simulation methodology has been comprehensively validated by static heating experiments and welding trials. By simulating the pressure distribution, an optimized positioning of the consolidation roller could be derived. This optimization was successfully validated by welding experiments using microsections and tensile-shear tests. In further research, the developed simulation methodology will be applied to complex, application-specific welding geometries and used for a joining-zone specific design of the induction system. In addition to achieving a near-complete elimination of originally occurring deconsolidations, interlaminar shear strengths exceeding even the level of autoclave specimens were achieved (Figure 2).

Further informationen:

Thomas Hoffmann, M.Sc.
Leibniz-Institut für Verbundwerkstoffe GmbH
Press & Joining Technologies
Erwin-Schrödinger-Straße 58
67663 Kaiserslautern
Telephone: +49 631 2017-237

Schematic representation of the developed simulation methodology using the example of a tensile shear test specimen. Each of the three individual steps is realized via a separate FE model

By simulating the pressure distribution using the temperature field simulated in the continuous process simulation, an optimized distance of consolidation roll to inductor could be realized. In addition to the almost complete elimination of pores at all the speeds investigated, the interlaminar shear strength measured in tensile tests was also significantly increased and exceeded autoclave reference values

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