The Institute of Composite Structures and Adaptive Systems, the Institute of Structures and Design, the Institute of Materials Research and the Center for Lightweight Production Technology (ZLP) – at its locations in Stade and Augsburg – of the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) have teamed up to develop automated processes for the production of components made of high-performance fibre metal laminates. Funded within the framework of the German Aviation Research Programme (LuFo), the ProfiRumpf project by the German Federal Ministry for Economic Affairs and Energy (BMWi) has, for the first time, succeeded in the production of double-curved fuselage segments from aluminium fibreglass laminate in assured quality using coordinated robot technology.
Fibre metal laminate (FML) consists of thin layers of fibreglass-reinforced plastic and aluminium and is a particularly durable material that is, for example, already used in the fuselage of the A380. By utilising FML, an aircraft can have far thinner walls and is therefore lighter. In addition, when using automated production techniques, aircraft components made of FML are more economical than components made of carbon fibre reinforced plastic (CFRP). The components must be produced in a highly automated environment to make FML economically viable and efficient. The DLR engineers have now developed an integrated solution for this purpose. "We have developed and tested automated and quality-assured processes for the production of large-surface fuselage sections made of FML. Not only did we achieve the required output rates for future aircraft models, we also ensured highly precise, reproducible quality," says chief project manager Hakan Uçan from the DLR Institute of Composite Structures and Adaptive Systems.
Multiple steps – one process
The production process for a fuselage shell made of FML consists of many individual steps. One of them is to build the layers. The individual layers made of aluminium foil, fibreglass and an adhesive film are placed automatically by robots. The positional accuracy of the deposited pre-cut aluminium segments is ensured by a laser-based procedure. When laying the fibreglass, a laser sensor also checks for unwanted gaps between the individual fibre sheets or whether material defects are present.
The component is then hardened in an autoclave – in this case in the BALU® research autoclave at DLR. Special sensors enable extremely precise monitoring of the hardening process to ensure that the construction components are produced in outstanding quality. "It was DLR's task to turn all the individual steps into one automated process", explains Dorothea Nieberl from the Center for Lightweight-Production-Technology (ZLP) in Augsburg. "An uninterrupted production chain is essential to ensuring high process reliability and constant product quality. We were able to validate the points in the production process at which integrated testing is necessary, possible, and ultimately cost-efficient."
Joining forces to achieve a common goal
Research facilities for automatic production of fibre composite components have been operated at ZLP in Stade and Augsburg since 2009. They are so flexible that they can be modified for the production of components made of different material combinations. For the first time, the ZLP in Augsburg now demonstrated the laying of curved aluminium foils in reliable quality using a multi-robot system, while ZLP Stade used the GroFi® system to premiere the placing of fibreglass on aluminium in combination with automated tape laying (ATL) and automated fibre placement (AFP) technology. A virtual twin of the Stade facilities exists, allowing for the optimal process to be determined through a simulation before actual production begins. Moreover, the developed software enables automated, simultaneous and downstream quality testing of all components. The basic research performed at the institutes yielded additional synergy aspects as well. A biaxial test conducted at the Institute of Materials Research in Cologne identified the behaviour of FML with rivet holes when exposed to realistic loads. Ultimately, the findings can help to further exploit the limits of the material capabilities and hence save weight.
Not only does the insight acquired in the project represent a ground-breaking advancement in the production of FML components, it can also be applied to the production process for many other composite materials such as CFRP. Hakan Uçan explains why DLR is predestined to continue the research into this field as follows: "Our research facilities allow us to manufacture large-surface structural components in an automated environment. They are ideal for full-scale testing. We are therefore able to offer our industrial partners the flexible opportunity to develop a technology to a high level of maturity, so that it can be transferred to industrial production without requiring significant investments for further development."
Among others, the research and industrial partners involved in the project included the Fraunhofer Society, Airbus, its suppliers Premium Aerotec and Fokker, as well as Stelia Aerospace.