Deliverables

Based on the results of the previous model, critical modes of the structure and their influence on performances under operational stress have been identified and they will be addressed aiming at safety enhancement. It follows the development of the joint local model, aimed at topology optimization, lightweighting (-51% with respect to riveted joints) and mechanical performances improvement (tensile mechanical strength equal to 36 MPa). Cohesive zone modeling method has been used to define the best metal pin geometry for the interface zone, based on the status of the contact zone when the joint is subject to forced displacement and manufacturability of the pin. The second set of simulations is focused on CFRP and is based on modeling different layup combinations using different failure criteria (max stress, max strain, Puck, Tsai-Wu).

Failure mechanisms of the joints have been studied from the point of view of the single metal pin penetrating the CFRP. Two different FEM modeling approaches have been considered: the first one is based on the homogenization of the fibers present in the matrix, while the second one takes into account the single fiber tows topology. Both no-separation and metal-CFRP debonding behaviors have been applied to the model, to catch different results such as fiber failure, debonding or pin failure. According to these simulations, damage starts initiating both in the resin-rich zone and the pin base before debonding. Data provided by these models will be extremely useful for the prediction of multi-material joints failure.

The introduction of a lattice substrate has been proposed to compensate for the thermal expansion mismatch between aluminum and CFRP, that could lead to damage initiation due manufacturing-induced residual tensions. First of all, thermal expansion properties of lattices have been investigated using both FEM and dilatometry. Once defined the lattice type to be employed for the substrate, a model that includes the presence of CFRP and lattice in contact while being exposed to autoclave temperature has been developed. Tests have also been conducted on lattice-CFRP samples for validation.
Despite achieving a good expansion ratio, the application of a substrate has been excluded due to excessive deformation induced by residual tensions.

Volumetric energy density (VED) parameters for the laser beam powder bed fusion (LB-PBF) process have been optimized, aiming at the maximization of the density and quality of the specimen to be manufactured. A design of experiment (DOE) methodology has been adopted, where different process parameters (power, scanning speed, hatching, layer thickness, VED and more) combinations are investigated to establish the one that results in the best sample quality. These optimization phases have been crucial to achieving a relative density higher than 99,95%.

Once process parameters have been optimized for production, a set of different samples meant to be tested under different conditions have been produced via AM. Based on the results obtained in terms of mechanical properties and improved relative density, the most suitable heath treatment has been selected for the AM parts (HT22). Tests necessary for the mechanical characterization of the AlSi10Mg alloy are of different nature and will evaluate both the static and dynamic properties: evaluation of density and microstructure, static tensile tests, static compression tests and fatigue tests.

After the final characterization of the AM parts, both regarding the mechanical properties and the microstructural optimization to achieve the best quality possible, metal inserts meant to be joined with CFRP are finally produced. Four different sets of metal inserts with different metal pin geometries have been produced and once jointed with CFRP, will be tested under static load, after that the best pin geometry will be tested for fatigue life assessment. These tests will be essential to define the efficacy of MIMOSA technology adhesion and the strength of the joint.

The first step towards the final MIMOSA use case demonstrator manufacturing is the printing of the full-scale metal fittings of the vertical stabilizer. With the same procedure previously used for the metal inserts, the fittings will be joined with CFRP and tested under realistic conditions representing the operational loads. The components have been printed in one single printing job, and the design is optimized for the LB-PBF process. Representative samples are tested together with the fitting in order to measure the relative density and mechanical performances of the components present in the printing chamber.

In order to understand the best procedure for the application of the anti-corrosion treatment to be applied on the joint, firstly temperatures reached on the application area need to be estimated and measured. A thermal FEM model has been developed in Ansys, to simulate the plasma spray flux investing the aluminum substrate. Temperature fringes obtained from the simulations are then compared to measurements made on a real substrate exposed to the plasma spray, so that the model can be adjusted and calibrated to achieve a more accurate estimation.

Several considerations can be made on the applicability of surface coating on AlSi10Mg specimens. First of all, the hirtisation pre-treating of the surface has been defined, aiming at the optimization of the coating application process. Once chemical composition of the coating is established, on the basis of the desired effects, the coating itself needs to be characterized. Different behaviors of the coated surface are interest of studies: layer adhesion, wettability, adhesion to cured resin, mechanical properties of the layer, tribology and wear, media resistance and corrosion resistance.

Coating parameters initially defined in the previous deliverable are meant to be applied to a flat AlSi10Mg surface, while the situation is different when coating needs to be applied to more geometrically complex items. Parameters of the application process have been redefined, based on the new requirements met. Coating has to be applied on anchors of the joint metal part: therefore, once applied, properties of the coating have been validated again. Given the necessity to coat test samples as well, aiming at mechanical validation of metal coated parts, multiple axes application has been enabled. The result of this characterization process is the successful application of the coating on the demonstrator parts.

Before starting the high-volume production for both testing campaigns and demonstrator parts, the autoclave curing process parameters need to be optimized, with the objective of maximizing the mechanical properties and the general quality of the manufactured parts. A challenge of this process is that optimization must be valid for both CFRP and CFRP-metal parts. Different manufacturing parameters are subject to variations until the tested mechanical properties of the produced samples are satisfactory and meet high-level standards. Such parameters include bond type, materials type (aeronautic prepreg has been employed), mold, laminate plan, storage temperature and autoclave pressure and temperature profiles.

A set of 80 samples for static tensile tests and 75 samples for fatigue tests entirely composed of CFRP has been manufactured with the aim of characterizing the single material, apart from the metal inserts, and validating the optimization of the autoclave - curing process parameters. Samples have been designed and manufactured according to ASTM D3039 standard, typically employed for the static characterization of composite materials. Regarding fatigue tests, the prescribed standard is ASTM D3479.

After the production of the metal inserts in Task 3.3, the CFRP was applied on the 24 samples for static tests with 4 different anchor geometries and then on the 56 samples with the anchor geometry that proved better mechanical performances during static tensile tests. In this task the production process was also optimized in order to prevent the formation of porosities in the composite matrix and to guarantee the correct penetration of the metal anchors between the composite fibers during the stacking of the layers. Those specimens were fundamental to validate the mechanical characteristics of the final joint at a smaller scale.

Finally, the prototype fittings printed in Task 3.4 are joined with CFRP panels with the optimized stacking process and autoclave cycle, the custom tooling for this process was designed and produced to scale the manufacturing of the smaller samples to the manufacturing of bigger parts. On the fittings joined with this process will be performed the static tests to validate the final demonstrator of the project.

The qualification pathway has been defined throughout the whole MIMOSA joint production. Critical issues, methodologies applied and most relevant results achieved during the whole process have been identified and classified. Such activities are important from different points of view: first, they are necessary in the frame of the aeronautical certification achievement. Moreover, the formalization of such practices also serves as a knowledge transfer instrument, offering practical guidance for companies and organizations that need to undergo similar qualifications efforts.
Different standards have been used and their application integrated in a structured manner to generate a coherent qualification framework.

The first step of the recycling process of multi-material joints consists of the separation of the two different materials composing the structure. Before the actual separation occurs, the joints must be downsized to a smaller dimension, in order to maximize the recovered material. The separation process has been implemented on machines typically employed on the industrial scale. This is a significant milestone for the robustness and efficacy of the MIMOSA project, demonstrating how big-scale recycling implants can also be used for the multi-material joint. It has been estimated that up to 80-90% of the total joint weight can be recycled.

In order to validate the recycling process of the AlSi10Mg alloy and determine the optimal conditions for both the input material and the processing parameters, 20 green powder atomization batches have been produced. Laboratory-scale equipment, consisting of an ultrasonic atomizer, has been used. Pre-processing operations for the scrap material were implemented to ensure minimal contamination levels throughout the process, resulting in the production of high-quality metallic powder.
As an additional result, 51,48% in average of the feedstock material has been transformed into powder (with a peak value of 75,53%), in order to demonstrate the feasibility of a circular production cycle establishment, where both raw material consumption and scrap generation are significantly reduced.

Following the demonstration of the feasibility of converting metal scrap into metallic powder suitable for use in additive manufacturing, the work now aims at gathering deeper insights into the atomization process of metallic material separated from polymer-based composites. This work can be considered as a preliminary task for the upscaling of the recycling process to the industrial level.
At this stage, the resultant green powder has been obtained from the early separation of AlSi10Mg and CFRP (once constituting the MIMOSA joint) and later recycling of the metal parts.

Characterization activities have been conducted on powders generated from the recycling process of AlSi10Mg material (green powders). Resulting properties are compared to those of commercial benchmark and to the specific requirements set by the MIMOSA project.
The recycling of the powders from the multi-material joints doesn’t affect sensibly the mechanical properties or the final density of the recycled alloy, as well as the final properties of recycled alloys treated with solution annealing and artificial aging. It has to be specified though that the contamination from iron increases the formation of intermetallic β-Al5FeSi compounds, which could negatively affect fatigue behavior.

Fillers made with thermoplastic particles mixed with chopped fibers can be produced starting from the recycling process of the CFRP, previously separated from the multi-material joint. These fillers are widely used to enhance mechanical and thermal properties of resins and to generate new composites in turn. In the first place, larger CFRP parts are subject to hammer milling, aiming at generating smaller elements. Secondly, aiming at obtaining even smaller debris, a second blade-refining milling is performed. These smaller dust-like particles are later sieved, in order to avoid cross-contamination and air pollution. Final step of the process is the employment of the densiometric table, used to perform a separation of the particles based on their density. Final evaluations guarantee that 90% of the initial weight has been recovered.

Technical and environmental feasibility of a fully optimized CFRP recycling process, aligning with the objectives of a circular economy and sustainable industrial innovation, has been demonstrated. The process starts with the separation of the two materials, followed by different steps of size reduction, that makes use of different physics principles (air turbulence, centrifugal force and inter-particle impacts).
Also, economic viability of the proposed recycling approach has been analyzed, ensuring that the process not only meets technical performance standards but also represents a competitive and sustainable business opportunity.

Fatigue tests were conducted on LB-PBF AlSi10Mg specimens with different surface finishes to account for the as-printed rough finish of the anchors and the machined finish of the back of the prototype, at different scales to study the behavior of the small anchors inside the joint compared to the larger bulk aluminum part and with the introduction of machined notches to simulate the stress concentrations that occur at the base of the anchors during the loading of the part. These tests are necessary in the aviation industry due to the rigid safety standards and the impossibility of direct inspection of the metal pins inside the joints.

A fatigue testing campaign was conducted on CFRP specimens, with the same material and optimized process and layup as the final joint composite part. Those tests are useful to better understand the fracture modes and how the layup influences the fatigue life of the component. In addition, those tests can give precious information about how the composite parts interacts with the metal anchors during the service life of the component and the numerous cycles of loading and unloading that it will encounter.

Once mechanical properties of single materials (AlSi10Mg and CFRP) have been assessed, static and fatigue tests have been performed on joints designed with the MIMOSA technology.
Static tests have been useful in detecting both the maximum stress that these components are able to bear before failure and the relative failure mode under static conditions. Different pin geometries have been tested, in order to both validate FEM models and to define the best solution. Fatigue tests helped defining the S-N curve of the joints and explore how cracks propagate through the materials constituting the part.
A weakening factor (with respect to traditional joints) lower than 0,5, with a coefficient of variation minor than 8%, has been achieved.

Static tests have been finally conducted on the MIMOSA prototype, which has been described in D5.4. The test aims at reproducing the pin-bearing shear conditions that the joint is subjected to, though the application of a tensile load. Digital image correlation has been employed to detect the entire strain field of the component throughout the whole loading process.
Final results are promising, considering that the maximum load reached is equal to 259 kN, with the failure occurring on the metal part of the joint and no though-the-thickness delamination. Only post-test c-scans were able to detect a limited and localized adhesion degradation. A good agreement with results previously obtained with FEM has been achieved.

Financial and sustainability performances during the production of the prototype have been assessed, as such features represent key concepts in shaping both the academic and industry fields. This technological, economical and sustainability feasibility study can set the foregrounds for the adaptation of this new technology for aircrafts and provide insights on new businesses not only limited to the aviation sector.
Manufacturing hours, energy and GWP (Global Warming Potential) per MIMOSA joint are reduced with respect to traditional joints.