Компьютерное моделирование и экспериментальные исследования пластиковых регулярных конструкций полученных аддитивными технологиями

Additive technologies of 3D printing methods by fused deposition modelling (FDM) have a number of advantages over other traditional processing methods for manufacturing composites: powder metallurgy, fabric and woven prepregs, investment casting, etc. They enable obtaining complex spatial structures with unprecedented geometry and high performance in accordance with a computer model. Their use makes it possible to form so-called lattice structures formed by repeating regular geometric cells in space (stochastic and regular) – arranged by 3D printing, this architecture gives polymer, metal, or ceramic materials a unique combination of properties that cannot be achieved by traditional methods: increased values of specific strength, specific surface area, heat transfer coefficient, elastic modulus, etc. As a result, we have extended prospects for application in various fields: manufacture of geometrically-complex parts for mechanical engineering, aerospace industry, biomedicine, oil and gas industry, etc.

The first stage of research was to analyze the state of the problem, and promising directions and designs of regular structures for subsequent manufacture of composite materials. Preliminary numerical calculations (Cura, Prusa, and Solidworks Simulation software products) to determine the stress-strain state of various types of structures are necessary to reduce the number of field experimental studies. The images of the models, the topology of the samples, and the assessment of the stresses arising in the samples’ volumes are shown in Fig. 1.  The numerical analysis of the strength of various types of structures according to the model showed that the longitudinal construction of layers is preferable to the transversal (35%), and the honeycomb structure provides strength higher than the gyroid one (75%).

Figure 1 - Types of structures:

a – gyroid, transversal along the normal to the construction axis; b – gyroid, longitudinal along the construction axis; c – honeycomb, longitudinal along the axis of construction; d – honeycomb, transversal along the normal of the axis of construction

DOI:10.60797/IRJ.2024.148.152.1

A numerical assessment of the strength properties was carried out on samples of ten different types of structures made of plastics – acrylonitrile butadiene styrene (ABS) and polylactide (PLA). Using the FDM technology and a Hercules printer, our team made 40x40x40 mm samples with cell sizes from 1 mm to 5 mm (large honeycomb) for compression testing. An experimental analysis of the strength properties was carried out on the samples of ten different types of structures made of ABS and PLA plastics.

The compression tests of the samples were carried out in accordance with ISO 604:2002 on a Tinius Olsen 100ST universal testing machine, which enables to measure deformations with a video extensometer (Fig. 2a). The specific strength was calculated taking into account the fact that the density of the samples when filled with plastic material for 20% of the total volume is 0,25 g/cm3 for the honeycomb, and 0,21 g/cm3 for the gyroid.

The Tinius Olsen H25KT universal testing machine was used to determine the bending strength (Fig. 2b). The VT6 titanium samples with an aluminium or bronze binder had dimensions and shape in accordance with ISO 3325:1996: length: 50 mm; width: 10,0 ± 0,1 mm; height: 5,0 ± 0,10 mm. Rockwell hardness was measured on a TP50-14 on a C scale with a load of 1500 N subject to ISO 6508-86. Determination of the density and calculation of the porosity of the samples was carried out according to the standard method of hydrostatic weighing on an EX205 analytical balance by Mettler Toledo with a hydrostatic weighing kit. The study of the morphology of the initial powders, microstructure and elemental composition of the obtained samples was carried out by the methods of secondary electrons and energy dispersive microanalysis using a JEOL JSM-7001F scanning electron microscope equipped with an INCA Penta FETx3 X-ray spectrometer by Oxford Instruments. The microstructure of the samples was studied on polished surfaces using a JEOL JSM-7500FA scanning electron microscope.

Figure 2 - Testing equipment:

a – for compression tests; b – for bending strength

DOI:10.60797/IRJ.2024.148.152.2

It is worth noting that the deformation processes and the destruction pattern of most samples with structures of different topologies occur in accordance with the forecasts based on the model and classical concepts of destruction of materials, for example, along the direction of the shear planes (Fig. 3). This indicates a fairly high quality of 3D-print construction of the regular structures.

Figure 3 - Stages of destruction of the walls of the honeycomb under loading

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The corresponding experimental deformation diagrams in the load-displacementl and stress-deformation coordinates for some samples of honeycomb and gyroid structures with different topologies are shown in Fig. 4 and 5.

Figure 4 - Measurement results of sample 3

Note: PLA2 honeycomb, longitudinal along the growth axis

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Figure 5 - Sample 8 – ABS4 gyroid, longitudinal along the growth axis

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A more visual representation of the measurement results and differences in the behaviour of the samples with different topological honeycomb and gyroid structures and materials under load is shown in Fig. 6. The analysis of the obtained curves and their nonmonotonic nature indicates the staging of fracture in most of the samples.

Figure 6 - Deformation patterns during compression:

a – for all samples; b – for samples with the best strength values

DOI:10.60797/IRJ.2024.148.152.6

Obviously, according to the diagrams in Fig. 6a), the most preferred material for the manufacture of frame structures is PLA, which provides a higher level of strength properties (curves 1, 3, 5, 7, 9) for all types of structures in contrast to the ABS samples (curves 2, 4, 6, 8, 10). Comparison of the samples of different topologies shows the advantages of the honeycomb structures with their location parallel to the loading axis (samples 3, 4 in Fig. 6a and Fig. 6b). Regardless of the material, reliable load strengths for the PLA and ABS samples lie in the range from 10 to 25 kN, which is the main governing factor. For the samples with other topology – a gyroid and a transverse honeycomb (samples 1 and 5 in Fig. 6b) made of PLA – the operability zone is from 5 to 10 kN. The curves for the remaining samples have insignificant differences, the maximum load strength lies in the region under 5 kN. But their distinctive feature is the pronounced cyclicity (discreteness) of the destruction processes, and numerous peaks, which is explained by the layered structure. In

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, the authors observed similar effects on the gyroid and Schwartz primitive structures, and explained them by the fact that each of the peaks-extrema corresponds to the destruction of one of the layers of cells lying in a plane perpendicular to the loading axis. At the same time, they considered the first peak on the deformation curve, which corresponds to the transition of the sample to the region of plastic deformation, as the strength of the samples. For the structures studied in this work, however, this approach is not entirely correct since each of the numerous peaks on the curves, in our opinion, corresponds to the gradual folding of the walls in the adjacent (most stressed) layers of the sample following the mechanism of elastic-plastic deformations (irreversible upon reaching peak stresses on the curves at which a load drop is observed). A further increase in the load and the appearance of extrema on the curves refers to the next stage of the deformation processes of other successive layers of the structure along the compression axis. At the same time, honeycomb structure (curves 3 and 4 – Fig. 6) have a more monotonous nature, similar to the behaviour of plastic materials under load, which is their main advantage. Thus, the results of processing all experimental data (table 1) indicate that the most preferred structure is the longitudinal honeycomb, and the most preferred material is PLA (sample 3), which provide the best combination of elastic and strength properties. Besides, the experimental results show a satisfactory agreement with the model prediction.

That is why, we used computer models of particularly these porous regular structures with the best strength indicators to manufacture TI6AL4V titanium alloy samples by additive technologies (SLM) and then infiltrate them with a melt of a more fusible aluminium bronze as a binder. The results of studies of the microstructure of Ti6Al4V+aluminium bronze composite materials are given in Fig. 7. Application of the phase contrast method revealed the presence of two main phases: Ti6Al4V (Fig. 7c) – spectrum 2; and aluminium bronze (Fig. 7c) – spectrum 1. The results of scanning electron microscopy of the microstructure of the regular honeycomb samples impregnated with an aluminium bronze melt (composite material), and determination of relative density (98% of the theoretical density) indicate a fairly high efficiency of filling the honeycomb voids and the elimination of residual porosity (Fig. 7b). Images of the microstructure and chemical analysis of phase components indicate a fairly high uniformity of the structure and efficiency of infiltration (Fig. 7d) – spectrum 1-3); density and residual porosity (less than 1%) confirm this fact. A very interesting effect, which was found during the study of the microstructure of the low-temperature-melting phase, is the formation of an acicular structure with marked elongated crystallized grains – “whiskers” with a cross-sectional size of 0,1 μm (Fig. 6d). The appearance of this reinforced fragmentary-nanostructured phase in a composite material gives cause to assume the possibility of dispersion strengthening of aluminium bronze and, as a result, of the composite as a whole. The fracture surface of the samples after bending tests has a quasi-plastic pattern, which can be explained by the dominant influence of the properties of the infiltrated material (aluminium bronze). Similar structures are formed during ordinary sintering of hard alloy composites obtained from bimodal powder mixtures of WC-Co-Al(Al2O3)

Figure 7 - Microstructure of Ti6Al4V+aluminium bronze composite materials:

a – scheme of infiltration; b – surface of the composite material infiltrated with aluminium alloy; c – images of the microstructure with selected areas of chemical analysis and their composition; d – acicular structure by volume of the binder with selected areas of chemical analysis and their composition

Note: c — x500; d — x3500

DOI:10.60797/IRJ.2024.148.152.8

In addition to increasing the density, the level of strength properties of honeycomb structures made of titanium alloy also increases, since aluminium bronze provides a redistribution of stresses that occur in the titanium frame under load. Measurements of the bending strength of the Ti6Al4V-bronze samples obtained using the infiltration technology showed an up to 25% increase in strength in comparison to the “green model” (table 2). Bending strength values vary in the range from 1440 to 1560 MPa, Young’s modulus varies from 49500 to 54000 MPa depending on the composition of the composite material, and SLM modes – radiation power and beam movement speed.

In the process of complex numerical and experimental studies, our team found the patterns of deformation processes and destruction of various types of regular structures 3D-printed of plastics in accordance with the computer model and the most preferred materials and honeycomb structures from the point of view of strength properties. The results of preliminary tests on plastic prototypes were used in the development of compositions and manufacturing technologies for metal composite materials based on Ti6Al4V titanium alloy powders. The study of the strength properties of Ti6Al4V structures obtained by 3D printing in combination with infiltration with aluminium bronze confirmed the effectiveness of the adopted design and technological solutions – an increase in bending strength by 20-25% compared to the base Ti6Al4V, and up to 1,5 times increase in the hardness values. Electron microscopy confirmed the homogeneity of the phase distribution in the composite structure, and the absence of residual porosity.