This project focused on the development of a viable method of additively manufacturing carbon-fiber-reinforced composites. Using an adaptation of direct ink writing (DIW) three-dimensional (3D)-printing technology, we created a latent thermoset resin that is the first example of a class of additively manufactured carbon-fiber-reinforced composites (AMCFRCs). We developed a means of printing these high-performance composites that enables the fiber component of the resin-and-carbon-fiber fluid to be aligned in three dimensions by means of controlled microextrusion and subsequently cured into complex geometries. Subsequent characterization of the composite systems indicated that we achieved a high order of fiber alignment within the composite microstructure that enables these materials to outperform equivalent carbon-fiber and polymer composites. Furthermore, these AMCFRC systems exhibit highly orthotropic mechanical and electrical responses as a direct result of the alignment of carbon-fiber bundles at the microscale. We predict that our work will lead to the design of truly tailorable carbon-fiber/polymer hybrid materials that display locally programmable complex electrical, thermal, and mechanical responses.
High-performance carbon fiber/epoxy (CF/Epoxy) composites are a potentially transformative materials solution for a range of applications (including aerospace and defense) because they exhibit mechanical properties approaching that of steel at a fraction of the density. These lightweight materials consist of high-aspect-ratio carbon fibers (Donnet and Bansal 1998; Burchell 1999; Hermanutz and Buchmeiser 2012; Huang 2009; Minus and Kumar 2005) that reinforce a polymer matrix (i.e., a highly crosslinked aromatic thermoset resin) to enhance mechanical, electrical, and thermal characteristics (Chung 1994). They have found wide application in the aerospace (Williams 2007), automotive (Holbery and Houston 2006), construction (Bakis et al. 2002), and energy-storage (Razaq et al. 2012) industries due to their high strength-to-weight ratio and potential multifunctionality. However, the development and application of these materials is limited by the process by which they are manufactured. High-performance, continuous-filament CF/Epoxy composites are produced by a costly, labor-intensive winding-and-hand-layup process that severely limits the control over the final product's micro- and mesostructures, and ultimately the performance, reliability, and repeatability of the product and process. Additive manufacturing (AM) and 3D-printing technologies offer the potential to fully automate the production of CF composites and enable better control over the fiber placement, orientation, and microstructure. This results in improved performance, reliability, and scalability, as well as reductions in cost.
Examples of the conventional fabrication of high-performance continuous-fiber composites include hand lay-up filament-winding processes and vacuum-assisted resin-transfer molding. Due to the limited control over the composite’s microstructure, these methods may be costly and unreliable, viable for only a limited subset of fiber-pattern and component geometries, and lack manufacturing repeatability. In contrast, short- or chopped-fiber composites have become practical low-cost alternatives with improved physicochemical properties. In comparison to the wound composites, short carbon-fiber composites can be easily fabricated by extrusion compounding or injection-molding processes. It is noteworthy that the mechanical properties of these composites largely depend on the fiber length, orientation, and distribution, as well as fiber-to-matrix interfacial adhesion. During the manufacturing process, fiber breakage occurs due to the fiber-to-fiber and fiber-to-polymer interactions, thus attenuating the mechanical properties of the final composite part. Similarly, directionality of fiber orientation is also affected by the aforementioned factors. Therefore, the processing method, matrix material, and fiber loading will determine the ultimate performance of both continuous-fiber and short-fiber composites, and can be further altered by tuning their mesostructure and geometric design.
The MarkForged 3D system (MarkForged 2017) is a commercially available example of a continuous-filament carbon-fiber-composite printing system that enables a high degree of control over fiber placement and greater versatility of product output in the carbon-fiber-composite production process. However, the technology developed by MarkForged utilizes a thermoplastic polymer resin and relies on melt-flow processing of a thermoplastic-coated carbon-fiber filament. This ultimately limits the strength and mechanical and thermal performance of the composite material to something significantly below that of current aerospace-grade thermoset resin carbon-fiber systems. More importantly, the MarkForged 3D technology does not allow 3D printing of CF/Epoxy continuous-filament composites.
There are, in fact, no commercial means of additively manufacturing continuous-fiber or short-carbon-fiber epoxy composites using direct ink writing (DIW) or any other AM or 3D printing process. This is partly a result of (1) the limitations of current processing technologies, which do not allow accurate spatial orientation of the carbon fibers within the resin matrix, and (2) the limitations of the commercial resin-based systems that do not have either the dynamic-curing response or the rheological properties required to enable spatial orientation and constraint of a carbon fiber during a printing process, which is required for true 3D printing of a CF/epoxy composite. Any method of additively manufacturing continuous-fiber or short-carbon-fiber epoxy composites using DIW is limited by the inability of current epoxy-resin systems to use DIW or any other method of constructing a self-supporting shape of any complexity that can mechanically constrain the fibers on a reasonable manufacturing time frame (minutes/seconds).
It is clear from the extant commercial, industrial, and academic state of the art that there is a gap in the field of carbon-fiber composites between high performance and manufacturability. High-performance wound or continuous CF composites have impressive physical properties compared to those of metals and bulk polymers. However, that improvement comes at a cost in terms of manufacturing complexity and other constraints. We identified a near-term performance gap between filament-wound CF/Epoxy materials and thermoplastic AMCFRCs that may be filled by materials created using AM thermoset carbon-fiber technology. Long-term technological developments in this area may lead to further enhancements of properties over the current best filament-wound systems through optimization and control of composite mesostructures via design-optimization-enabled AM techniques.
Our approach was to develop a novel DIW printing platform consisting of a delivery system, print head, and resin chemistry that will allow the printing of complex 3D structures with controlled fiber alignments at ultimate volume fractions in the region of 40–60 volume-percent carbon fiber utilizing either a continuous filament or shear-aligned slurry of discontinuous fibers in a resin matrix. Our approach was fundamentally different from those of previous efforts (MarkForged 2017; Tekinalp et al. 2014; Hofstätter et al. 2017) by virtue of our proposed use of high glass-transition temperature (Tg) aromatic thermoset polymers as the resin component of the manufacturing process. Whereas the majority of current AM approaches for polymers require the use of thermoprocessable plastic resins (e.g., acrylonitrile butadiene styrene, polylactic acid, and polystyrene) that have poor mechanical properties, our approach was to develop a high-performance, mechanically robust thermoset material that is compatible with AM technology. Furthermore, we proposed to make use of the inherent control afforded by DIW to both align fibers within a filamentary core (thus increasing the directional strength of the material) and demonstrate the ability to print highly ordered microstructures with rational internal architecture. Such structures would be designed based on computational optimization methodologies that we aimed to develop and apply to the problem of optimal placement of a fiber layout within a 3D volume to optimize strength and stiffness, while minimizing weight and compliance.
Central to our approach has been the development of a suitable high-Tg thermoset resin ink suitable for carrying discrete carbon-fiber dispersions with a rheological property set compatible with a DIW AM process. We based our ink on a bisphenol-F epoxy resin oligomer (BPFE) system that we modified with both colloidal silica and dispersed high-aspect-ratio, discrete carbon fibers. This carbon-fiber-loaded ink has been shown to be extrudable through nozzles with minimum exit diameters of 250 µm at room temperature with rapid physical solidification into a filamentary extrudate. It displays the rheological characteristics necessary for a processable DIW substrate. Our thermoset resin system employs latent thermal-cure catalysis, which (through the generation of a strong Lewis acid above approximately 70 °C) can efficiently ring open oxriane groups and therefore initiate a thermally activated self-crosslinking homopolymerization reaction between the epoxide functionalities of the BPFE resin. Controlled homopolymerization (i.e., polymerization of a single type of monomer to form a homopolymer) eliminates the necessity to combine the reactive polymer components immediately prior to or during the printing process. In addition, our resin-ink system requires only a moderate post-curing stage to achieve full network density (80 °C for 12 hours) at low catalyst loadings (0.1 Wt%). Furthermore, due to its advantageous temperature and reactivity profile (that enables long resin lifetimes at ambient temperatures, while maintaining a rapid curing response at elevated temperatures), formulated inks may be printed on a flexible timescale yet rapidly cured when required for the formation of complex structures.
The ability of a given polymer-ink formulation to be writable using a DIW process is largely dependent on the static and flow rheology of the formulation. The rheological properties of the CF/epoxy ink used in the manufacture of our AMCFRC materials are therefore a consequence of the physical multiphase structure of the ink we developed. Our rheological characterization of the CF/epoxy ink formulation indicated that the BPFE resin is modified to behave as a thixotropic, non-Newtonian fluid (i.e., becomes a fluid when agitated but is solid or semisolid when allowed to stand) by the addition of a low-volume fraction of high-surface-area silica nanopartices. The shear-rate-dependent response of this silica-modified resin has been shown to behave as a classical Carreau fluid (i.e., a Newtonian fluid where viscosity depends upon the shear rate).
Gaining an understanding beyond simple bulk rheology of the fluid dynamics of the extrusion process with this complex, multiphase system is critical to the further development and implementation of such DIW methods for the manufacture of AMCFRC materials. We used computational modeling to better understand flow behavior and fiber alignment during the manufacturing process. Using a numerical model developed for this project, we were able to simulate (at high resolution) short-fiber flow at volume fractions up to 20 percent of the total ink volume (Figure 1).
The mechanical properties of our 3D-printed AMCFRC systems were assessed for both tensile and compressive strength as a function of carbon-fiber volume fraction and fiber length; they were also compared with both in-house and commercial unfilled BPFE resins and random chopped carbon-fiber-filled pressed parts. Figure 2 depicts the results of compressive-strength testing on a series of fabricated structures.
Design optimization was another important aspect of this project. We made significant advancements in the development and application of design-optimization strategies for printing AMCFRC structures with rationally optimized internal structures. The general strategy that we developed within this project is shown in Figure 3.
Our integrated simulation, design, manufacturing, and characterization approach for AMCFRCs allowed us to manufacture not only test parts and demonstration pieces within this project, but also examples of functional components for various applications. One such example is the production of a series of AMCFRC sabots (i.e., devices used to keep subcaliber projectiles, such as undersized cannon munitions, in the center of the gun barrel when fired) for developmental high-explosive projectile assembly that we designed, printed, and tested within three weeks (Figure 4).
The assembly consisted of 10 individually printed rings and was designed, tested, and printed in less than one month. It represents the first large-scale application of AMCFRC technology. The sabots were tested at Lawrence Livermore National Laboratory's High Explosives Applications Facility in a series of powered gun shots. In each case, the sabots performed well.
We have developed the fundamental basis for a disruptive and potentially transformative AM technology based on fiber-filled thermoset composite materials. This represents a significant contribution to the Laboratory's mission to develop advanced AM technologies. Furthermore, the reduction to practice of the technology we have developed for printing low-density, low-outgassing, high-bulk-modulus AMCFRC materials in complex shapes will have immediate and long-term potential for application for stockpile stewardship materials replacement.
This project advanced the scientific and technological development of microextrusion 3D-printing techniques for the additive manufacture of high-performance, high-aspect-ratio carbon-fiber-filled thermoset composite materials. This development was enabled by the use of versatile new homopolymerized epoxide-based resin chemistries and the large degree of microstructural control afforded by DIW technology. We demonstrated the utility of modified DIW techniques for efficient shear alignment of carbon fibers during a 3D-printing process; through computational modeling and simulation we may now predict and optimize this process to maximize the degree of fiber alignment within an extruded filament. Additionally, we have clearly demonstrated that, with the microstructural control and degree of alignment afforded by DIW technology, it is possible to fabricate AM composites that outperform conventional engineering polymers and metals.
Further investment in 3D-printing technology is required to continue to develop this technology. Specifically, to increase fiber volume fraction, the advanced nozzle concepts outlined in our patent (but not reduced to practice) must be realized. Nozzle/print stage parallelization and the further development of real-time curing technologies would enable the manufacture of large-scale, highly complex parts with mechanical properties in excess of those of high-grade aluminum alloys. Such technology development would most efficiently occur with the input of and collaboration with an external industrial partner.
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