Mechanical Response of 3d Printed Functionally Graded Foam

Abstract

In additive manufacturing, fused filament fabrication (FFF) based three-dimensional printing (3DP) is one of the most popular rapid processing technologies. The key benefit of 3DP is the ability to build integrated, complex, and tailored components. Increasing the wide variety of materials that can be processed using this process helps increase the flexibility toward part generation. This made the current work focus on developing a glass micro balloon (GMB) reinforced high density polyethylene (HDPE) based syntactic foam filament. Reinforcing the hollow fillers helps in developing the filament for weight sensitive applications. Nevertheless, processing these fillers with improper process parameters and random volume fractions results in filler failure and agglomeration defects. Hence, taking the quality measuring parameters like filament ductility, roughness, ease of process-ability, and defects like agglomeration, filler percentage is maintained in the range of 20-60 volume %. Syntactic foam filaments of 20, 40, and 60 volume percentage GMB filler are extruded with proper circularity and uniform diameter. Part quality mainly depends on the selected manufacturing method and its process parameters. Hence, after filament development, this work's primary objective was to optimize 3D printing parameters to develop a defectless part. An outcome of the number of pilot studies helps identify possible defects in 3D printing and overcome strategies. Finally, printing parameters like speed, nozzle temperature, bed temperature, infill percentage, raster angle, layer height, etc., are finalized for processing syntactic foam filaments through an FFF 3D printer. Using these optimized parameters initially plain H, H20, H40, and H60 beams that are 3D printed. Sandwich and functionally graded beams have many advantages compared to plain beams. The current work trail has developed functionally graded foams, and all configurations of foams like SH 20-60, FGF- 1, 2, 3, and FGSF- 1, 2, 3 are successfully 3D printed using the filament replacement method. Extensive scanning electron microscopic analysis was performed to study the interface bonding between the foam layers, filler sustainability, and filler matrix interface. Results showed that the layers of similar and dissimilar compositions are properly fused by forming a seamless bonding. There is no observable filler failure, but an improper filler matrix interface was observed, which creates porosity in a sample. These voids help in enhancing the weight reduction potential. 3D printed samples are subjected to micro- CT scan to observe the porosity distribution. In this experiment, there was no observable porosity in HDPE layers, whereas some porosity was observed in the foams, and it was quite minimal in H20 and comparatively increased in H60. This porosity estimation is essential. So five samples of each composition are experimentally tested for density measurement, and theoretical density was calculated using a rule of mixtures. The theoretical and experimental density difference is represented as the void percentage. Results showed that the density of the foam increased with an increase in filler percentage and void percentage shows a similar trend for filler. Among graded foams and their respective sandwiches, the void percentage varied in the 4-7% range. The present material is aimed at weight-sensitive applications where the weight saving potential (WSP) plays a crucial role. This WSP increased with an increase in the filler, and it is higher for H60, and in graded foams and their respective sandwiches, it is higher for FGF-2 and FGSF-2. The percentage of WSP for FGF and FGSFs varied in the range of 8-14%. These graded foams are developed for weight sensitive structural and naval applications, so the current work response of these 3D printed foams under various loads and loading conditions was explored. These developed foams are most prone to thrust forces, so the behavior of these foams under compressive loading was studied. It is noted that the compressive modulus increases with the filler content. The graded foams' specific properties exhibited superior response compared to neat HDPE. Among functionally graded foams (FGFs) and functionally graded sandwich foams, FGF-2 (H20-H40-H60) and FGSF-2 (H-H20-H40-H60-H) showed the highest modulus and yield strength. FGF and FGSFs exhibited better energy absorption compared to plain foams. FGF and FGSFs exhibited better energy absorption than foams and are 8 to 19 % more than pure HDPE. All functionally graded foams exhibited a sacrificial failure mechanism. Due to the higher compressive forces, hollow GMB failure was observed in the tested sample. The response of FGF and FGSFs toward transverse loading was studied by performing three-point bending experiment. The test was conducted at crosshead displacement velocities of 2.54 and 3.41 mm/min for FGF and FGSFs. Experimental results of the flexural test showed that graded sandwiches exhibited better strength than the graded core alone. Among all the functionally graded foams (FGFs), FGF-2 exhibited a better specific modulus, and the modulus of FGF-2 enhanced by 33.83% compared to pure HDPE. FEA results showed unsymmetrical stress distribution along the thickness of the sample. A comparative study of experimental and numerical results showed a slight deviation. The better specific properties of the developed graded foams help to create their preference for weight-sensitive structural applications. The behavior of the FGF and FGSFs subjected to axial compressive load and their natural frequency under zero and non-zero loading conditions was studied through buckling and free vibration analysis. The buckling load of these 3D printed beams was estimated from experimentally acquired load-deflection data using the double tangent method (DTM), modified Budiansky criteria (MBC), and vibration correlation technique (VCT). Results showed that critical buckling load increased with an increase in hollow GMB percentage. Among all FGFs, FGF-2 exhibited the highest buckling load. Compared to pure HDPE buckling strength of FGF-1, FGF-2, and FGF-3 calculated using DTM and MBC methods are increased by 39, 78.4, 47 %, 44.68, 87.23, and 53.19%, respectively. Mechanical stability of the 3D printed graded cores increased post sandwiching them with HDPE skin. All FGSFs outperformed their respective cores in terms of buckling load. FGSFs exhibited a similar trend in the core sample. There is no observable delamination between the 3D printed layers and the skin and core interface, even after increasing the load beyond the sample critical buckling load. Natural frequency is one of the crucial parameters of the beam and is evaluated by performing a free vibration test. Results showed that at mode-1, the natural frequency of all FGF and FGSF beams decreases with increasing load up to critical buckling load, and a further increase in load increases the natural frequency. The natural frequency of the beam increases with an increase in filler percentage. Corresponding to all modes, among all FGF and FGSFs, FGSF-2 exhibited higher natural frequency. The damping factor of the beam increases with an increase in load up to the critical buckling point; further, an increase in load results in a decrease in the damping factor. In practical applications of 3D printed beams, they are subjected to non-uniform heating conditions. This necessitates the current work to study the response of these 3D printed plain, graded, and their respective sandwiches towards non-uniform heating. This non-uniform heating was created in an experimental setup by varying the heating position of the IR heater. In case 1 sample was heated at one end. In case- 2 sample was heated at the center of the sample, and in case-3, both ends of the sample are subjected to thermal load. Results showed that the thermal stability of the beams enhanced with an increase in GMB percentage. This thermal stability was further enhanced by varying the GMB volume percentage along the thickness direction and sandwiching it with HDPE skin. All 3D printed samples exhibited maximum deflection in case 2 and minimum deflection in case 1. Comparative results concluded that the beams' thermal stability could be enhanced by grading the material property along the thickness direction and sandwiching it.