Multi-material Fittings with Carbon Fiber Reinforcement from AM Process: Design and Characterization

Multi-material Fittings with Carbon Fiber Reinforcement from AM Process: Design and Characterization Giorgio De Pasquale(B)and Ferdinando Ursi Smart Structures and Systems Lab, Department of Mechanical and Aerospace Engineering, Politecnico di Torino, 10129 Torino, Italy {giorgio.depasquale,ferdinando.ursi}@polito.it Abstract. The incorporation of carbon fiber reinforcement into polymer components manufactured via Fused Deposition Modeling (FDM) or Fused Filament Fabrication (FFF) processes presents an opportunity to substitute traditional metal parts, yielding substantial resource savings. To address the outstanding concerns pertaining to mechanical strength, functional adequacy, and reliability, this study endeavors to redesign mechanical fittings tailored for industrial applications. Specifically, it focuses on exploiting polymeric composites processed through additive manufacturing (AM) as replacements for aluminum materials. Keywords: Additive Manufacturing ·Design for Additive ·Composite · Experimental Mechanics ·FEM 1 Introduction Additive manufacturing (AM) processes facilitate the fabrication of components characterized by intricate geometric complexity. Polymer matrix composites (PMCs) represent the most appropriate category of composites capable of merging the advantages derived from AM’s innovative processes and materials [1]. The realm of composite AM is progressively garnering attention to the extent that it is engendering a novel category of materials termed “composites 2.0” [2]. The least demanding form of reinforcement integrable into AM processes is fiber reinforcement; however, it has been demonstrated that continuous fibers confer superior strengthening effects compared to short fibers [3]. According to Hu et al. [3], the most established AM technologies capable of processing continuous fiber composites include Fused Deposition Modeling (FDM), also called Fused Filament Fabrication (FFF) and Selective Laser Sintering (SLS). Nevertheless, emerging process technologies such as Light-curing Additive Manufacturing (Lc-AM), Robot-assisted Additive Manufacturing (Ra-AM), and Green Manufacturing (GM) are also noteworthy. Due to its facile, rapid, cost-effective, and open-source framework, FDM can be regarded as the most prevalent AM process. However, the low manufacturing volume and extended printing time constitute the primary barriers to the © The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 F. Concli et al. (Eds.): ISIEA 2024, LNNS 1124, pp. 183–197, 2024. https://doi.org/10.1007/978-3-031-70462-8_18 184 G. De Pasquale and F. Ursi seamless integration of this technology into manufacturing chains [4]. FDM operates on the principle of extruding a thermoplastic material through a nozzle, which is then deposited onto a build platform following a predetermined path. Typically, the polymers utilized in this process are thermoplastics characterized by a low melting temperature. It is noteworthy that the extrusion temperature is directly proportional to both mechanical and thermal resistance; hence, polymers that are easy to extrude often exhibit inferior mechanical properties. Numerous investigations have demonstrated that Continuous Fiber Reinforced FDM (CFR-FDM), a technology embedding continuous fibers within 3D-printed components, can augment the tensile strength or tensile modulus. In certain instances, the mechanical properties achieved surpass even those of lightweight alloys [5–7]. Consequently, CFRFDM finds extensive applications across aerospace, structural, automotive, and medical sectors [8]. Several case studies have illustrated that in specific scenarios, CFR-FDM processes can substitute aluminum components manufactured through conventional methods, thereby achieving time or material savings while preserving component functionality. Azarov et al. [9] harnessed the distinct mechanical properties exhibited by components manufactured via CFR-FDM in practical applications such as the fabrication of a drone frame, resulting in a significant reduction in structural weight. Luzi et al. [10] undertook the redesign of a pushing mechanism for a packaging machine through topological optimization, leading to material conservation and a reduction in the maximum torque tolerated by the machine. Several case studies showcasing the application of such technology are available in the literature. For instance, J. Smith et al. [11] optimized and fabricated a suspension plate using nylon reinforced with continuous long fibers. Additionally, Junk et al. [12] demonstrated the effectiveness of redesigning for AM in achieving weight and cost reductions for a metal automotive component, providing a comprehensive mechanical and economic evaluation of the part. However, given that this technology is still in its nascent developmental stage, it is accompanied by several limitations. As highlighted by Ding et al. [13], components manufactured via FDM experience reheating at the boundaries of the preceding layer, resulting in anisotropic behavior and microstructural alterations at the layer interfaces. Owing to the intrinsic characteristics of the process, the cross-section of the final part exhibits high porosity and numerous interfaces between adjacent layers, adjacent filaments, and between the fiber and the resin. All these interfaces serve as nucleation sites that contribute to failure [2]. The presence of voids and pores predisposes the component to failure through debonding and delamination of layers, underscoring the critical and challenging nature of achieving fiber impregnation. Indeed, Fan et al. [14] noted this issue and attempted to mitigate anisotropy by employing impregnated fibers. Furthermore, according to Hou et al. [15], the utilization of fibers impregnated with polymers introduces pores into the structure, with a corresponding correlation between the volume fraction of fibers and the pore content being observed. The CFR-FDM represents a recent technological advancement, with experimental characterization of fabricated parts typically focusing on tensile and bending tests. These analyses often investigate the influence of parameters such as infill types, build Multi-material Fittings with Carbon Fiber Reinforcement 185 orientations, temperature variations, material selection, and print resolution. However, there remains a notable gap in research concerning the dynamic response of CFR-FDM components [16]. Notably, Díaz-Rodríguez et al. [4] have compiled various studies pertaining to the mechanical properties of CFR-FDM, encompassing tension-compression, bending, in-plane shear, indentation, microscopy, fatigue, creep, and impact tests. Yet, limited attention has been given to the influence of fiber shape and size, factors that may introduce significant variability in results. From a cost-effectiveness viewpoint, AM holds promise for generating savings, particularly concerning mass reduction, design flexibility, inventory and storage minimization, and waste material mitigation [17–21]. These conclusions were provided in a comparative case study [12], wherein the economic implications of Computerized Numeric Control (CNC) were contrasted with CFR-FDM in the production of motorcycle components. This paper investigates the mechanical performance of fittings designed for industrial automotive chassis production utilizing CFR-FDM. The study details the design methodology applied for shape customization and topology optimization. Furthermore, mechanical characterization of the final parts is conducted to compare their properties with those of traditional metal counterparts under various loading conditions. This study endeavors to contribute to the existing literature by enriching the understanding of compressive testing while presenting a comprehensive case study on the application of CFR-FDM technology, thereby elucidating its primary advantages. 2 Design and Modeling 2.1 Components Description The components under investigation pertain to mechanical fittings utilized in the automotive sector to facilitate the welding process of chassis structures. The targeted production stage involves assembling various discrete parts previously manufactured. External frames are employed to maintain the proper positioning of individual chassis components, which are then secured in place using numerous small fittings (Fig. 1). These fittings serve to align the edges of the parts with the requisite precision for welding. The paramount requirement for these fittings lies in their strength and shape, which must ensure the accurate positioning of the parts during welding operations. Traditional fittings are typically crafted from aluminum using machining equipment. Their shape, quantity, and placement vary depending on the chassis model. Consequently, the production of these fittings generates significant waste material and necessitates lengthy machining processes. Moreover, the multitude of distinct geometries renders traditional manufacturing methods inefficient and inflexible. Hence, exploring alternative solutions holds the potential for substantial economic benefits. In this study, we replaced the metal fittings with composite counterparts produced using the CFR-FDM process, featuring customized shapes. The redesign process entails accommodating varying numbers of fittings. Additionally, considering the intricate nature of chassis designs, the fittings are tailored to specific geometries corresponding to the areas they support, contingent upon the prototype model of the vehicle. 186 G. De Pasquale and F. Ursi Fig. 1. Chassis welding process step with external frame holding the component by means of several fittings placed around. Organization of the Customized Shapes The design of a new fitting is often time consuming and thus, given the numerous potential clamping configurations to be fabricated, the redesign strategy aims to identify a limited number of recurring shape groups to which all other fittings can be categorized. The adopted strategy is the production of a unique 3D parametric model, incorporating user-defined variables, is developed for these select typologies and provided to the printing operators. This approach can reduce the time consumed during the design activity regardless the customization grade of the component and is particularly recommended in combination with the 3D printing process because it can significatively impact the total time of production due to the importance of the design step. The geometric attributes defining the shape of the fittings include: •The primary shape of the fitting, selectable among straight (Fig. 2a), L-shaped with one contact (Fig. 2b), and L-shaped with two contacts (Fig. 2c). •The shape of the contact, i.e., the portion of the fitting interfacing with the chassis, identified in eight possible variants, some of which are illustrated in Fig. 3. •The orientation of the holes accommodating the fastening screws, which can be either lateral (Fig. 4a) or vertical (Fig. 4b). Altogether, 48 distinct fitting shapes can be delineated using the customization process described. It is important to note that while fittings may share the same basic shape, they may vary in size. Thus, the 3D parametric model also incorporates a dimensional scaling function. Although this approach facilitates rapid geometry editing, it necessitates that all topological optimizations converge toward a common reticular structure to streamline the design of the parametric Computer-Aided Design (CAD). The output processed by the slicer software will consist of parametrically drawn geometries, tailored to each specific case. FEM-Based Shape Optimization The load applied to each fitting during operations, about 2000 N, is quantified using a load cell. A FEM (finite element method) model is established for the fitting, applying an Multi-material Fittings with Carbon Fiber Reinforcement 187 Fig. 2. Main shape of the fitting variants: straight (a), L-shaped with one contact (b) and L-shaped with two contacts (c). Fig. 3. Shape of the contact point between the fitting and the chassis: some of the eight variants. Fig. 4. Orientation of the holes hosting the fastening screws: lateral (a) and vertical (b). external load, perpendicular to the application surfaces colored in Fig. 2and Fig. 3, equivalent to the force acting on the surface in contact with the chassis. Geometric constraints are imposed on the fitting holes, encompassing their position, spacing, and diameter, as well as on the surface interfacing with the chassis, which includes its extent and spatial orientation. A safety factor of two is incorporated to amplify the load for safety considerations. Given the complexity of determining the actual pressure distribution on the contact surface, a uniform load distribution is assumed. Numerical simulations are performed using Ansys 2023 software. The material properties predefined in the software for aluminum alloy are adopted for the simulations (Young’s modulus E=70 GPa, Poisson’s coefficient ν=0.33, yield stress σy=280 MPa, ultimate stress σu=310 MPa). For the fitting group denoted as “L-shaped with one contact,” simulation results are presented in Fig. 5. The maximum displacement is approximately 0.128 mm, with a maximum stress of 105 MPa and a static safety factor of 1.957. A preliminary survey regarding the more recurrent shapes resulting from the topology optimizations regarding a given main shape is performed. The optimization objectives entail minimizing stress and maximizing stiffness, while simultaneously reducing mass to decrease the time and cost associated with the AM process. The fitting shape is updated and verified with a FEM analysis prior to the subsequent AM process. As done for the original shape, Geometric constraints imposed on the optimized geometry are the same adopted for the original shape. This approach ensures that the shape-optimized fittings 188 G. De Pasquale and F. Ursi Fig. 5. FEM analysis of the fitting (L-shaped with one contact) before the optimization: deformation (a), safety factor (b) and Von Mises equivalent stress (c). can seamlessly replace the existing fittings without affecting the supporting frames. The FEM analysis of the optimized shape is performed adopting the same aluminum alloy adopted for the original component so to assess the effective loss of weight and stiffness exclusively due to the shape change. The outcome of the described process, applied for the “L-shaped with one contact” group, is depicted in Fig. 6. The maximum displacement measures 0.047 mm (-63.3%), with the maximum stress reduced to 41.6 MPa (-60.4%), yielding a safety factor of 4.933 (+60.4%). This optimization process is reiterated for the other two groups corresponding to the primary shapes of the fittings. Fig. 6. FEM analysis of the fitting (L-shaped with one contact) after the topology optimization: deformation (a), safety factor (b) and Von Mises equivalent stress (c). 3 Fabrication Process 3.1 Parametric CAD Geometry In accordance with the results of the topological optimization and the FEM analysis, a parametric CAD model was developed using SolidWorks software. The optimized shape is incorporated into the model, enabling local dimensions to vary with respect to each Multi-material Fittings with Carbon Fiber Reinforcement 189 individual fitting to be produced. In the Fig. 7, the parametric model accommodating all the “L-shaped with one contact” fittings is illustrated, also marking the definition of important features like the hole spacing, the thickness of the reticular structures and the angle or distance of the contact surface respect to a certain reference point. This approach allows operators to significantly reduce the time required for fitting redesign tasks (e.g., for accommodating new chassis models) by starting from an already optimized shape that is adaptable for customization. Fig. 7. View of the parametric CAD file with interactive quotes able to reproduce all the fixture variants in the group “L-shaped with one contact”. 3.2 FDM Process with Integrated Carbon Fibers A fitting prototype is printed using the Markforged X7 system. The matrix material utilized is Onyx, a proprietary polyamide-based material reinforced with chopped carbon fibers. Long continuous carbon fibers (CCF) serve as the reinforcement material. The Markforged X7 system is equipped with the slicer software Markforged EIGER, which enables the design of the deposition pattern of continuous fibers and offers various deposition strategies. The user interface of the software is depicted in Fig. 8, illustrating the deposition pattern of fibers in the central layers. The same fitting prototype is printed with different volume fractions of carbon fibers, ranging from 15% to 35%. The percentage of carbon fibers within the volume significantly influences the final mechanical properties, a factor explored in the subsequent experimental characterization. Two configurations of CCF layout are suitable for fabricating the fitting geometry: “center deposition”, where fibers are positioned along a single plane at the mid-thickness, and “sandwich deposition”, where fibers are also distributed in the top and bottom layers, as illustrated in Fig. 9a. These two layout strategies are examined in the subsequent experimental characterization, as a function of the fibers’ volume fraction. The resulting sample is depicted in Fig. 9b. 190 G. De Pasquale and F. Ursi Fig. 8. User interface of the slicer software Markforged EIGER with fibers layout in the central layers of the fitting. Fig. 9. Visual description of the “sandwich deposition” layout with the CCF placed at midthickness and at top and bottom layers (a) and final sample after the printing process (b). 4 Experimental Characterization 4.1 Weight Measurement The weight of the samples is measured to quantify the mass reduction achieved through the redesign of the fittings. Weighing is conducted using the Mettler PM34-K Delta Range electronic balance. The weight results are presented in Table 1. 4.2 Loading Test Setup The mechanical tests are designed to replicate the compressive load experienced by the fitting during chassis holding operations. The objective of these tests is to assess the stiffness of the manufactured components and to investigate the influence of the CCF layout and volume fraction on the force-displacement relationship. Compression testing is conducted using the INSTRON 8801 hydraulic testing system. Due to the unique shape of the fitting samples, a specialized clamping system is required for installation on the testing machine, as depicted in Fig. 10a. These clamps are custom-designed and fabricated from steel to ensure compressive stiffness significantly Multi-material Fittings with Carbon Fiber Reinforcement 191 exceeding that of the fittings. The clamping system is mounted on the upper moving crosshead of the machine, as illustrated in Fig. 10b. Control and data processing are provided through the Bluehill Universal software, which allows for presetting and recording of load and displacement measurements. The tests are conducted at a controlled strain rate of 1 mm/min. To ensure statistical significance, five tests are performed for each CCF volume fraction utilizing the “central deposition” layout. Additionally, one test is conducted for each volume fraction employing the “sandwich deposition” layout. Fig. 10. Clamping system for the compression test of fitting samples (a), compression test setup on the INSTRON 8801 machine (b) and close-up to the sample before the compression test (c). 5 Results and Discussions 5.1 Results The measured force-displacement curves and the estimated elastic stiffness curves are reported in Fig. 11 for the fittings with “center deposition” layout and CCF volume fraction ranging from 15 to 35%. Similar results are reported in Fig. 12 for the “sandwich deposition” layout, with CCF volume fraction ranging from 15 to 35%. The compressive behavior shown by the two layouts discussed is afterward pointed in Fig. 13. The weights of the samples and the elastic stiffness values for the two fiber configurations are presented in Table 1. Additionally, Table 2displays the ultimate compressive loads for the CCF “sandwich distribution,” which exhibits higher mechanical strength.