Doina Frunzaverde

This study examines how manufacturer-specific additive formulations used to obtain nominally identical black PLA filaments influence the thermal, mechanical, and tribological performance of FDM-printed parts. Five commercial filaments were analyzed under identical processing conditions using thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), tensile testing, pin-on-disc measurements, and stereomicroscopy. The filaments exhibited substantial compositional variability, with total additive contents ranging from 2.08 wt.% to 27.82 wt.%. One filament (M5) contained a significant fraction of inorganic fillers, confirmed by SEM/EDX as Ca-, Na- and Mg-based oxides and silicates, identifying it as a PLA-based composite despite being marketed as standard PLA. These differences strongly affected thermal behavior (Tg, Tcc, Tm) and translated directly into the performance of the printed parts. Ultimate tensile strength varied by 88.91% across all filaments (19.38–36.61 MPa), but only by 13% among the four conventional PLA filaments (M1–M4). Tribological performance differed markedly: mean coefficients of friction ranged from 0.246 (M3) to 0.368 (M2), a spread of approximately 50%, with wear-track morphologies reflecting the frictional response. Overall, the results show that PLA filaments cannot be treated as interchangeable materials. Greater transparency and standardized reporting of filament composition are needed to ensure reproducibility and support informed material selection in FDM applications.

This study examines the effect of elevated printing speeds (100–600 mm/s) on the dimensional accuracy and tensile strength of PLA components fabricated via fused deposition modeling (FDM). To isolate the influence of printing speed, all other parameters were kept constant, and two filament variants—natural (unpigmented) and black PLA—were analyzed. ISO 527-2 type 1A specimens were produced and tested for dimensional deviations and ultimate tensile strength (UTS). The results indicate that printing speed has a marked impact on both geometric precision and mechanical performance. The optimal speed of 300 mm/s provided the best compromise between dimensional accuracy and tensile strength for both filaments. At speeds below 300 mm/s, under-extrusion caused weak layer bonding and air gaps, while speeds above 300 mm/s led to over-extrusion and structural defects due to thermal stress and rapid cooling. Black PLA yielded better dimensional accuracy at higher speeds, with cross-sectional deviations between 2.76% and 5.33%, while natural PLA showed larger deviations of up to 8.63%. However, natural PLA exhibited superior tensile strength, reaching up to 46.59 MPa, with black PLA showing up to 13.16% lower UTS values. The findings emphasize the importance of speed tuning and material selection for achieving high-quality, reliable, and efficient FDM prints.

The mechanical properties of 3D-printed parts produced by material extrusion (MEX) have been extensively investigated in recent years. However, there are no standards defining the test conditions and specimens' geometry, specifically for the parts obtained by this technology. The dogbone specimens, as defined by ISO 527-2 and ASTM D638, are mainly used for tensile tests of 3D printed polymers. This study presents a comparative analysis of four types of dogbone specimens made from polylactic acid (PLA) by material extrusion. Experimental results showed only slight variations (up to 2,34%) in the ultimate tensile strength (UTS) across the different types of specimens. On the other hand, they revealed the occurrence of failure at the boundary of the gauged section of the specimens, caused by a stress concentration in this area (as shown by numerical simulations) combined with irregularities and voids generated by the 3D printing strategy.

To enhance the operational life of hydraulic machinery, protective coatings against wear, particularly cavitation erosion, and corrosion might be applied on the surfaces of components. The experiments conducted in this study aimed to assess the suitability of 80/20 NiCrBSi/WC-Co composite coatings for this purpose. A coating of NiCrBSi self-fluxing alloy, which served as the reference material, was deposited alongside a NiCrBSi coating reinforced with 20% WC-Co, both applied by flame spraying onto X3CrNiMo13-4 substrates, the martensitic stainless steel type frequently utilized in turbine blade manufacturing. The improved density of the coatings and adhesion to the substrate was achieved by remelting with an oxyacetylene flame. The cavitation and corrosion performance of both the reference and composite coating were evaluated through cavitation tests and electrochemical measurements conducted in the laboratory. The results demonstrate that the addition of 20% WC-Co significantly enhances the cavitation resistance of the composite material, as evidenced by the reduction to 3.76 times of the cumulative erosion (CE), while the stabilization rate remained at half the value observed for the reference self-fluxing alloy coating. Conversely, the addition of WC-Co into the NiCrBSi coating resulted in a slight decrease in the corrosion resistance of the self-fluxing alloy. Nevertheless, the corrosion rate of the composite coating (124.80 µm/year) did not significantly exceed the upper limit for excellent corrosion resistance (100 µm/year).

Dimensional accuracy and mechanical properties of components printed by fused deposition modeling (FDM) are influenced by several process parameters. In this paper, the authors targeted the effect of the printing scenario and the PLA (polylactic acid) color on parts’ quality. Three scenarios were analyzed: individually printing, simultaneously printing of three, respective five specimens of natural (transparent), red, grey, and black PLA. The temperature variations of successive deposited layers were recorded for the black PLA. The dimensional accuracy of tensile specimens was evaluated, tensile tests were performed, and the results were correlated with the mesostructure of the prints. The effect of the independent variables on the measured parameters was analyzed by ANOVA. The experiments revealed differences for the same printing scenario regarding cross-section area (up to 5.71%) and tensile strength (up to 10.45%) determined by the material color. The number of specimens printed simultaneously and the position of the specimens on the build plate were found to influence too, but less than the color. Thus, increasing from one to five the number of specimens printed at a time altered both the dimensional accuracy (up to 3.93% increase of the cross-section area) and the tensile strength (up to 3.63% reduction).

Among the FDM process variables, one of the less addressed in previous research is the filament color. Moreover, if not explicitly targeted, the filament color is usually not even mentioned. Aiming to point out if, and to what extent, the color of the PLA filaments influences the dimensional precision and the mechanical strength of FDM prints, the authors of the present research carried out experiments on tensile specimens. The variable parameters were the layer height (0.05 mm, 0.10 mm, 0.15 mm, 0.20 mm) and the material color (natural, black, red, grey). The experimental results clearly showed that the filament color is an influential factor for the dimensional accuracy as well as for the tensile strength of the FDM printed PLA parts. Moreover, the two way ANOVA test performed revealed that the strongest effect on the tensile strength was exerted by the PLA color (η² = 97.3%), followed by the layer height (η² = 85.5%) and the interaction between the PLA color and the layer height (η² = 80.0%). Under the same printing conditions, the best dimensional accuracy was ensured by the black PLA (0.17% width deviations, respectively 5.48% height deviations), whilst the grey PLA showed the highest ultimate tensile strength values (between 57.10 MPa and 59.82 MPa).

Additive manufacturing technologies enable the production of components with lightweight cores, by means of infills with various patterns and densities. Together with reduced mass and material consumption, infill geometries must ensure that strength and stiffness conditions are fulfilled. For the proper correlation of the infill type with the loading case of the part, the mechanical behavior of the infill along all three principal axes of inertia has to be known. In this paper, the behavior in symmetric and asymmetric bending of three infill geometries, commonly used in 3D printing processes (honeycomb, grid and triangles) is analyzed. The variations of deflections as a function of force orientation are presented, showing that honeycomb and triangular structures exhibit similar behaviors along the Y and Z principal axes of inertia. Furthermore, the displacements obtained for the three types of structures are compared, in relation to the consumed volume of material. The larger displacements of the grid structure compared to the honeycomb and triangular structures are highlighted.

The printing variable least addressed in previous research aiming to reveal the effect of the FFF process parameters on the printed PLA part’s quality and properties is the filament color. Moreover, the color of the PLA, as well as its manufacturer, are rarely mentioned when the experimental conditions for the printing of the samples are described, although current existing data reveal that their influence on the final characteristics of the print should not be neglected. In order to point out the importance of this influential parameter, a natural and a black-colored PLA filament, produced by the same manufacturer, were selected. The dimensional accuracy, tensile strength, and friction properties of the samples were analyzed and compared for printing temperatures ranging from 200 °C up to 240 °C. The experimental results clearly showed different characteristics depending on the polymer color of samples printed under the same conditions. Therefore, the optimization of the FFF process parameters for the 3D-printing of PLA should always start with the proper selection of the type of the PLA material, regarding both its color and the fabricant.

Flame-sprayed NiCrBSi/WC-12Co composite coatings were deposited in different ratios on the surface of stainless steel. Oxyacetylene flame remelting treatment was applied to surfaces for refinement of the morphology of the layers and improvement of the coating/substrate adhesion. The performance of the coated specimens to cavitation erosion and electrochemical corrosion was evaluated by an ultrasonic vibratory method and, respectively, by polarization measurements. The microstructure was investigated by means of scanning electron microscopy (SEM) combined with energy dispersive X-ray analysis (EDX). The obtained results demonstrated that the addition of 15 wt.% WC-12Co to the self-fluxing alloy improves the resistance to cavitation erosion (the terminal erosion rate (Vs) decreased with 15% related to that of the NiCrBSi coating) without influencing the good corrosion resistance in NaCl solution. However, a further increase in WC-Co content led to a deterioration of these coating properties (the Vs has doubled related to that of the NiCrBSi coating). Moreover, the corrosion behavior of the latter composite coating was negatively influenced, a fact confirmed by increased values for the corrosion current density (i_corr). Based on the achieved experimental results, one may summarize that NiCrBSi/WC-Co composite coatings are able to increase the life cycle of expensive, high-performance components exposed to severe cavitation conditions.

Polylactic acid (PLA) is produced from renewable materials, has a low melting temperature and has a low carbon footprint. These advantages have led to the extensive use of polylactic acid in additive manufacturing, particularly by fused filament fabrication (FFF). PLA parts that are 3D printed for industrial applications require stable mechanical properties and predictability regarding their dependence on the process parameters. Therefore, the development of the FFF process has been continuously accompanied by the development of software packages that generate CNC codes for the printers. A large number of user-controllable process parameters have been introduced in these software packages. In this respect, a lot of articles in the specialized literature address the issue of the influence of the process parameters on the mechanical properties of 3D-printed specimens. A systematic review of the research targeting the influence of process parameters on the mechanical properties of PLA specimens additively manufactured by fused filament fabrication was carried out by the authors of this paper. Six process parameters (layer thickness, printing speed, printing temperature, build plate temperature, build orientation and raster angle) were followed. The mechanical behavior was evaluated by tensile, compressive and bending properties.