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For the manufacture of large complex parts, joining is an important step in the process chain. The joining process is therefore subject to stiff requirements. Robotized induction welding is a highly flexible, automated joining process for dissimilar materials that allows joint debonding and continuous quality control.
(Published on November-December 2008 – JEC Magazine #45)
1- LARS MOSER, RESEARCHER, MANUFACTURING SCIENCE
2- PETER MITSCHANG, RESEARCH DIRECTOR, MANUFACTURING SCIENCE
3- ALOIS K. SCHLARB, MANAGING DIRECTOR, RESEARCH DIRECTOR, MATERIAL SCIENCE
INSTITUT FÜR VERBUNDWERKSTOFFE GMBH, KAISERSLAUTERN
Components for the aerospace or transportation industry are typical applications for fibre-reinforced thermoplastic polymer composites. These components often require the production of large, complex structures that are normally composed of substructures, often from dissimilar materials. The process chain includes a joining step such as mechanical joining, adhesive bonding, or welding.
In many cases, the production of composite structures is labour intensive and costs more than similar metal parts. Therefore, there is great demand for cost-effective production and automation. Robotized induction welding is a joining process that targets these needs. It has been adapted to industrial robots, allowing highly flexible automated production for both continuous or spot welds. Similar and dissimilar material combinations such as composites to metals can be welded. The process is capable of joining a variety of geometries, including three-dimensional curved structures. The joint is produced sequentially, allowing welding of different joint and part geometries without using component-specific tools. For repair purposes, the joint can be debonded, enabling substitution of components. In case of design alternations or product variations, reprogramming the control software and matching fixtures facilitate fast adaption of changes.
Induction welding utilizes inductive heating for melting the polymer matrix in the joining zone. The components to be welded are submitted to an alternating electromagnetic field. When there are electrically conductive loops in the component, e. g. with carbon-fibre reinforcements, eddy currents are induced, resulting in efficient, localized heating directly in the laminate. Glass-fibre reinforcements are electrically non-conductive and additional susceptors, e.g. metal grids, have to be applied to convert the magnetic energy into thermal energy. In the case of ferromagnetic materials, hysteresis effects contribute further to heating. Figure 1 shows the basic principle of inductive heating of composites.
The most relevant parameters for inductive heating are coupling distance, field frequency, coil current, and inductor geometry. Reinforcement properties such as geometry or the electrical properties of the welding susceptor also influence the heating behaviour.
Once the welding zone has been heated, the joint has to be consolidated under pressure and cooled, either actively using a cooling medium such as compressed air, or passively by convection. Up to now, the standard two-dimensional continuous induction welding process has been state of the art. In this process, the welding unit, consisting of induction coil and consolidation roller, is fixed and the joint is produced sequentially, making it necessary to translate the component during the welding process. Figure 2 shows the process conduct, the processing direction of the component, and a qualitative temperature gradient in the laminate.
The standard 2D induction welding process is limited to flat joints. A recent research project enhanced the standard process and adapted it to an industrial robot system. Thanks to this newly developed three-dimensional welding process, complex and curved parts can be joined with a high degree of automation and a quality control system. In contrast to the two-dimensional process, it is the components to be joined that are fixed, and the mobile welding unit is guided by a robotic system. Large and heavy components can thus be welded using a suitable handling system such as articulated or gantry robots. The surface temperature of the component is measured continuously for weld quality control, and the temperature profile is used to indicate quality problems, e. g. insufficient heating or gaps in the welding susceptor.
Figure 3 (a) shows the robotic welding system and Figure 3 (b) shows the main elements of the welding unit, that integrates the heating and consolidation steps. For continuous joints, rollers are used. In case of spot welds, these can be changed to stamp tools. The system can weld different joint configurations. If the components to be joined are supported by a surface, only the upper consolidation roller is used. When there is no direct support, a second roller is available as counterpart. These different consolidation modes allow welding in positions that are difficult to access. An example can be seen in Figure 4. The bumper support is joined from two thermoformed half shells, showing different seam types.
An example of induction-welded, aviation-certified material is presented in Figure 5. Here, the T-stiffener was joined from two thermoformed L-profiles. The cross-sectional morphology shows a homogeneous bond line of good quality.
Conclusion and outlook
Robotized induction welding of thermoplastic fibre-reinforced composites was successfully adapted to an industrial robot. The system is capable of welding different materials and geometries. A quality control system is integrated, allowing continuous quality evaluation and process documentation.
The compact design and the use of an automated handling system allow a reproducible production of complex geometries with curved seams that are difficult to weld with other technologies. The weld is produced sequentially by a generic tool, so that product variants can be produced by reprogramming the system without the need for re-tooling. This is advantageous for components with numerous variants and short product life cycles. Future work will include process modelling and the development of enhanced welding susceptors.
The authors would like to thank the partners within the project “Wirtschaftliches Fügen von Faser-Kunststoff-Verbundstrukturen mittels induktiver Erwärmung unter Einsatz flexibler Handhabungssysteme”: Bond- Laminates GmbH, Brilon, Jacob Composite GmbH, Wilhelmsdorf, KUKA Systems GmbH, Augsburg, Institute for Manufacturing Technology and Production Systems, University of Kaiserslautern, all from Germany. This work was supported by the Bundesministerium für Bildung und Forschung (BMBF) within the project “Forschung für die Produktion von morgen” (02PB2030) and is supervised by Forschungszentrum Karlsruhe (PTKA), Bereich Produktion und Fertigungstechnologien (PFT).