Design, Manufacturing, and Testing of a Metallic Fuselage Panel Incorporating New Alloys and Environmentally Friendly Technologies

Within the ecoTECH project, a new fuselage section was designed based on an existing business jet panel, aiming to incorporate innovative technologies and environmentally friendly approaches. The Metallic Fuselage Panel Demonstrator developed in this project integrates the most promising technologies previously developed in the areas of manufacturing methods, including mechanical milling and friction stir welding, as well as surface treatments such as sol-gel and thin film sulphuric acid anodizing, along with a Chrome-free primer applied on a new Al-Cu-Li alloy structure. Two types of full-scale testing were performed to mature the newly developed technologies and assess the performance of the demonstrator. The first was a Static Full-Scale Test Demonstrator, designed and manufactured to undergo static full-scale testing. This testing evaluated the structural integrity and performance of the panel under various load conditions, representative of an operational aircraft, including compression, shear, pressure, tension, and combinations of these forces. The second type of testing conducted concerned endurance. Similar to the static test demonstrator, this demonstrator was subjected to fatigue to assess its durability and long-term performance by simulating representative flight loading spectrum of a business jet aircraft and providing valuable insights into the panel’s ability to withstand prolonged operational conditions. The successful completion of these phases in the ecoTECH project represents a significant milestone, demonstrating the effective integration of innovative manufacturing methods and environmentally friendly surface treatments for new aluminum alloys in the development of innovative environmentally friendly technologies. The project’s outcomes contribute to the advancement of sustainable and efficient technologies in the aerospace industry, providing a foundation for the future development of aircraft structures with improved performance, durability, and reduced environmental impact.


Introduction
Operational considerations and societal concerns are creating a growing demand for aircraft with reduced fuel consumption, noise, emission of pollutants and maintenance costs.From operational point of view, materials used in manufacturing airframe components need to become lighter, cheaper and more eco-compliant for disposal.From environmental and societal point of view, it will be necessary to reduce the aviation footprint, through aircraft performance improvements (drag, weight and versatility) and an eco-friendly life cycle including a significant increase in recyclability as well as optimized material streams.In addition, ACARE [1] targets and REACH [2] regulations should be also fulfilled.In pursuit of the aforementioned objectives, a comprehensive endeavor was undertaken within the scope of the Clean Sky2 core project, ecoTECH, aimed at cultivating environmentally benign technologies targeting the eradication of hazardous substances, such as hexavalent Chromium.Additionally, endeavors were made to diminish the weight of airframe structures through the utilization of lighter alloys like Al-Cu-Li, opting for welding over riveting, and implementing novel designs tailored to capitalize on the enhanced mechanical properties of contemporary materials.
The principal aim of the present study was to advance the most promising manufacturing technologies arising from the metallic materials technological stream of the ecoTECH initiative, with the ultimate goal of facilitating their integration into industrial processes.To this end, the design, manufacturing, and evaluation of full-scale demonstrators were executed to represent an upper fuselage section of a business jet.The design was predicated upon an existing fuselage panel from IAI.Three types of full-scale demonstrators manufactured: 1.
A demonstrator for full-scale static tests 2.
A demonstrator for the full-scale endurance test 3.
A demonstrator only for exhibition reasons.The static and the endurance test demonstrators are the same, whereas the demonstrator for exhibition reasons includes a central frame fabricated by the additive manufacturing technology of Wire Arc Additive Manufacturing (WAAM) through the CfP IAWAS.
The implementation of the work performed in three phases: which started in 2020, was ongoing in 2021 and 2022 and ended in July 2023, including the following tasks:  Demonstrator definition: 1.1.2020-30.11.2021 (preliminary design and detailed design of the metallic fuselage panel demonstrator.Also, the preliminary design of necessary jigs, fixtures and auxiliary elements for the welding and other manufacturing activities performed in this WP). Demonstrator manufacturing: 1.12.2021-30.03.2023 (final design and manufacturing of necessary jigs, fixtures and auxiliary elements for the manufacturing of the demonstrators, followed by the manufacturing of panels for testing and exhibition purposes). Demonstrator testing 1.1.2021-30.07.2023 (Static and dynamic full scale testing through the CfP DEMONSTRATE Apart of the work performed by the ecoTECH participants (HAI, IAI, LTK and AKZO), parallel developments implemented by the partners of ecoTECH sub-projects:  in CfP IAWAS, a central frame manufactured by WAAM technology using new Aluminium wire solutions developed in the same project.This demonstrator part was integrated in the demonstrator for exhibition reasons. VULCAN demonstrator part manufactured and selectively stripped using an up-scaled robotic system.This demonstrator part is a separate item of the demonstrator for exhibition reasons. The CfP ReINTEGRA achieved the recycling without dismantling and sorting of welded demonstrator parts manufactured using different types of Al-Cu-Li alloys and surface treatments developed in ecoTECH. The ground testing of the demonstrators performed through the CfP DEMONSTRATE.

Technologies
The technologies applied in the manufacturing of the metallic demonstrators encompass processes related to machining and forming, assembly of components (specifically Friction Stir Welding (FSW), surface treatment for basic anti-corrosion protection, and the application of a Chromium-free Primer onto cutting-edge Al-Cu-Li alloys.These technologies are detailed in the table provided below.( The development of the technologies followed the building block approach from coupon level to elements and to demonstrators as shown in Figure 2.

Concept
The chosen design was founded upon the rear upper fuselage panel of a business jet manufactured by IAI, as illustrated in Figure 1.Furthermore, the strength criteria guiding this design were furnished by the strength department of IAI.
 Frames are not allowed to buckle up to Limit Loads  Skins are not allowed to buckle up to 40% of Limit Load  No permanent buckling up to Ultimate Loads  Skins: where the pocketing applied by mechanical milling and rolling of skins by IAI (Figure 3)  Stringers, cleats and frames: where the forming of all stringers, cleats and frames, as well as all heat treat (including tempering of Skins) performed by HAI (Figure 4)  Joining: where the Stringer-skin and Frame-skin welded using Friction Stir Welding (FSW) by (Figure 5). Cr-free surface treatments (Sol-Gel and TFSAA) and primer and top coat application by HAI (Figure 14 and Figure 15) where the Cr-free primer developed and provided by AKZONOBEL    IOP Publishing doi:10.1088/1742-6596/2716/1/0120377 and top-coats, is anticipated to curtail maintenance operations and prolong the lifecycle of structures, thereby minimizing material wastage. End-of-Life (EoL) Process Benefits: An energy-efficient EoL methodology has been devised, streamlining the disassembly and recycling of reinforced fuselage panels due to the use of base components within the same alloy family (Al-Cu-Li). Additive Manufacturing Advantages: These encompass reduced material usage and waste, particularly when compared to fully machined components (improved buy-to-fly ratio).Additionally, resource efficiency is enhanced in both production and use phases, as manufacturing processes and products can be reengineered for additive manufacturing. Laser Shock Selective Stripping Benefits: This process eliminates worker exposure to hazardous substances, resulting in reduced hazardous material disposal costs, nearly eliminating water waste, and minimizing pre-strip preparation and post-strip rinsing.

Full scale testing of the demonstrators
In order to advance the maturation of the developed technologies, comprehensive full-scale testing of two demonstrators was conducted in strict adherence to a test plan encompassing both static and endurance loading conditions, designed to closely mirror the actual loads experienced by the reference panel of IAI's business jet.
To facilitate this assessment, and specifically onset of buckling, a Digital Image Correlation (DIC) system was deployed, enabling the real-time monitoring of static testing.Cameras were strategically positioned atop the tool, directed downwards towards the specimen surface via structural beams.The DIC images provided a comprehensive representation of displacements relative to loading throughout the mechanical testing process.Preceding this, Finite Element (FE) analysis was executed to simulate the anticipated mechanical response of the panels under the predefined loading conditions.
For both static and endurance testing, the DIC images are very similar to the FE models.Indicative comparable images of experimental and modelling work are given in Figure 8 for the static compression loading and Figure 9 for the endurance testing.

Conclusions
The work performed for the design, the manufacturing and testing of the metallic demonstrator incorporating the most promising technologies of the metallic technological stream of the ecoTECH project can be summarized as follows:  New environmentally friendly surface treatments and a Cr-free primer developed, evaluated and up-scaled to industrial level  A fullscale metallic fuselage demonstrator incorporating many environmentally friendly technologies has been designed based on an existing reference panel.Four demonstrators were manufactured.1 for adjustments, 2 for testing + 1 for exhibition  Robotic FSW has been demonstrated to be a feasible technology for advanced manufacturing of fuselage structures using Al-Cu-Li alloys. A full scale static test performed and the results are comparable with the models predictions  A full scale endurance test performed proving the high damage tolerance properties of new Al-Cu-Li alloys. Data Collection and LCI analysis was performed for the involved technologies  CfP IAWAS: An innovative demonstrator part was developed manufactured by WAAM technology using Al-Cu-Li alloys as base and new filler material (first time). CfP VULCAN: Selective stripping performed successfully at demonstrator parts using a robotic system  CfP ReINTEGRA: Recyclability process without dismantling and sorting alloys was shown to be feasible for most of the structures manufactured using Al-Cu-Li alloy combinations. All technologies applied on the demonstrator level matured successfully to a TRL close to industrialization.

Figure 1 -
Figure 1 -From the initial design to the manufacturing of demonstrators

Figure 3 :
Figure 3: Manufacturing of skins by IAI.

Figure 4 :Figure 5 :
Figure 4: Manufacturing of stringers, cleats and frames by HAI

Figure 6 :
Figure 6: Surface treatments, primer and top-coat application, by HAI

Figure 8 .
Figure 8. Out-of-plane displacement distributions of test 5 at ultimate load, at 235 kN in the experiment (a), and 203 kN in the FE analysis (b)

Table 1 :
Technologies considered for the development of metallic demonstrators. the technologies applied on the demonstrators as indicated in Table1, the technologies developed in the relevant CfP-s are shown in Table2.These technologies are demonstrated as standalone demonstrator parts or as integrated parts in the demonstrator for exhibition purposes.

Table 2 :
Technologies from CfP-s considered for the development of metallic demonstrators.