Study and design of process parameters for additive manufacturing of PEEK and C-PEEK parts for aeronautical applications

The growing need for more sustainable solutions has led research in the aeronautical and aerospace fields to the production of increasingly lighter and more efficient aircraft. For lightening purposes, advanced composite materials made of carbon fibers and epoxy resins were developed. Nevertheless, the use of thermoplastics as a valid alternative to thermosets, has the advantage of being easily processable, repairable and recyclable. The most interesting thermoplastic materials for the aerospace industry are the so-called “ technopolymers”, such as polyariletherketones (PAEK) family, which combine high mechanical, thermal and chemical resistance with very low weight. One of the most sustainable ways to process these materials are Additive Manufacturing technologies (AM), and in particular the Fused Filament Fabrication (FFF) technique. The present work is an in-depth study on the optimization of FFF process parameters and possible strategies aimed at resolving the technological limitations for the printing of Polyether-ether ketone (PEEK) and Carbon-PEEK parts making them usable for lightening purposes in the aeronautical sector. In the first part of the study, the optimal process conditions for the 3D printing of 100% dense PEEK parts are found by characterizing both mechanical and structural properties of printed coupons. It was discovered that the mechanical performances are strongly dependent on PEEK degree of crystallinity. Therefore, by varying the process temperatures and the printing speed, it is possible to control the crystallization, customizing the mechanical properties and possible applications. Subsequently, the research activity was focused on the enhancement of the interlayer adhesion in 3D printed parts by means of atmospheric plasma superficial treatments. The study was carried out on both Polycarbonate (PC) and PEEK because they have very similar features and processing issues. The choice of treatment parameters was made by studying the plasma-induced improvement in wettability through several analyses. The interlayer adhesion of both untreated and treated PC samples was verified by mechanical tests, and an improvement of about 30% in strength was achieved. The last part was dedicated on the study of 3D printed Gyroid structures made of Carbon PEEK for both the optimization of the compressive strength-to-weight ratio and the enhancement of mechanical isotropy. The specimens were printed by varying the infill density, and the results were compared to the Gibson-Ashby model, finding a compressive behavior similar to bending-dominated lattice structures. The strength-to-weight ratio was optimized for a density of 70% allowing a saving of 30% of material and time and it was found that isotropy in compression is possible for a density of 25%. It was studied also the response of gyroid lattice structures in the impact absorption when used as core pattern in sandwich-like panels. The absorbed impact energy increased as the infill density increased while the gyroid was not damaged but was detached from the impacted skin. Non-destructive inspection (NDI) highlighted that the extent of the detachment increased with the impact energy. However, further investigation for the optimization and characterization of sandwich panels is necessary. The results presented in this thesis work pave the way towards a turning point in the engineering and manufacturing of molds, tools and end-use products for aeronautics.