Papers by Keyword: Porosity

Abstract: The paper presents a new concept of a thermally sprayed composite coating obtained by mixing NiCrAl powder with chromium carbide Cr₃C₂ in an amount of approximately 30 wt.%. The aim of the research was to obtain a material combining the advantages of a metallic matrix and a ceramic phase, with increased resistance to wear and erosion. The plasma spraying (APS) process was carried out on a carbon steel substrate with variable technological parameters: arc current intensity (300/500/700 A) and hydrogen flow (4/8/12 NLPM), while maintaining the other conditions constant.The thickness, porosity, microstructure, chemical composition (using the EDS method), hardness, erosion resistance, and tribological wear of the coatings were evaluated. The results showed that the greatest thickness (approx. 150 µm) and lowest porosity (below 3 vol. %) were obtained at the maximum process parameters – 700 A and 12 NLPM. In turn, the thinnest and most irregular coating (approx. 70 µm) was obtained at the lowest hydrogen flow (4 NLPM), which was due to insufficient melting of the powder particles.Increasing the current intensity and hydrogen flow had a beneficial effect on all analyzed coating properties – especially hardness (up to 273.7 HV0.2), erosion resistance (the smallest mass loss of 0.007 g), and tribological wear resistance (the smallest volume loss of 2.925 mm³). A decrease in any of the parameters resulted in a deterioration of the layer properties. The optimal mechanical and structural properties of the NiCrAl + Cr₃C₂ composite coating were achieved at the maximum plasma spraying parameters: a power current of 700 A and a hydrogen flow rate of 12 NLPM.

Abstract: The microstructural characteristics of externally solidified crystals (ESCs) and porosities in a non-heat-treated high-pressure die-cast AlSi9MnVZr alloy are investigated under two distinct process conditions: one with the application of lower intensification pressure and the other with higher intensification pressure. Optical microscopy (OM), scanning electron microscopy (SEM), and computed tomography (CT) are employed to analyze the ESCs and porosity distribution. The alloy's microstructure primarily consists of primary α-Al, ESCs, Al-Si eutectic, and iron-rich phases. ESCs nucleate in the shot sleeve, while α-Al forms within the die cavity. When the lower intensification pressure is applied, larger dendritic ESCs are observed, along with significant gas porosity, shrinkage pores, and numerous smaller dispersed pores, resulting in a high porosity fraction. Conversely, the application of higher intensification pressure results in a notable refinement in ESCs morphology, with a significant reduction in their diameter and area fraction. Additionally, the size and fraction of porosity decrease substantially, indicating a marked improvement in casting quality.

Abstract: Sugarcane bagasse ash (SCBA) is a by-product of the ethanol and sugar industry. SCBA is generally used as fertilizer or dumped in landfill, which has led to increasing environmental problems. In the recent years, SCBA has been investigated in the field of construction materials due to its pozzolanic character. This research aims at examining some physical and mechanical properties of mortars with partial replacement of sugarcane bagasse ash from sugar cane refineries. In the present case, the cement substitution was made with SCBA at 0%, 15%, and 30% of the binder (Cement + ash). The physical and mechanical testing of the mortar was carried out to determine the effect of ash addition on porosity, density, flexural and compressive strengths of the mortar. In general, the findings revealed that the mechanical and physical behavior of the mortar mixtures improved over time because of pozzolanic effects. On one hand, the physical changes were relatively restricted and do not show a well-established trend. On the other hand, reduction of mechanical strength was observed with the addition of SCBA, and with a 15% cement replacement percentage, it is possible to obtain a material with favorable physical and mechanical properties.

Abstract: Flax fibre–reinforced polymers (FFRPs) are attractive for lightweight structures due to their low density, favourable specific stiffness and partially bio-based origin. However, their performance is strongly affected by manufacturing route, porosity and moisture uptake, which are typically more critical than for conventional glass or carbon fibre composites. This study compares flax, glass, carbon and carbon–flax hybrid epoxy laminates produced by vacuum infusion, hand lay-up and autoclave prepreg processing. Fibre volume fraction and void content are determined from density measurements, optical microscopy and X-ray computed tomography. Tensile and flexural properties are measured according to EN ISO 527-4 and EN ISO 14125 in the dry state and after accelerated conditioning at 35 ± 2 °C and 100 % relative humidity. Infused unidirectional flax laminates reach a tensile strength of about 259 MPa and a specific tensile strength of approximately 0.21 MPa·m³·kg⁻¹, comparable to glass laminates. Moisture exposure increases thickness by 11.8–13.9 % for infused flax laminates and about 2.3 % for an infused carbon–flax hybrid laminate and leads to strength reductions up to roughly 30 % in flax-rich laminates, while autoclave-processed laminates show only minor losses. The results provide process-dependent design guidelines for FFRPs in moisture-exposed, weight-sensitive components.

Abstract: Using Sol-Gel and impregnation processes, the GaNFe₂O₃ and GaNFe₂O₃-PPY nanocomposites were synthesized with varying concentrations of PPY 3%, 10%, and 30% by weight. Using the Sci Finder software could not trace any report in the literature for this synthesized Ga_(2x+2)NFe_2(49-x)O₃-PPY nanocomposites. The prepared gallium nitride ferrite and gallium nitride ferrite-Polypyrrole samples were subjected to structural analysis using X-ray diffraction. The X-Ray diffraction characterization confirmed Nano state formation. From the XRD spectra the dislocation density, average crystallite size, number of unit cells, and porosity were calculated and analyzed . It has been observed that with increment of unit cells and dopant concentration there is a decrease of dislocation density of gallium nitride ferrite. When the concentration of PPY is increased in gallium nitride ferrite-Polypyrrole nano composites, the dislocation density increases and the number of unit cells decreases. The porosity is increased as the concentration of PPY is increased from 3%,10% to 30% when compared to GaNFe₂O₃ nano ferrites.

Abstract: Fused deposition modeling (FDM) has several advantages, including design freedom, part customization, and ease of realizing complex geometries. However, there exist some challenges with the process; these include but are not limited to porosity, anisotropy, roughness, and material compatibility. This study is focussed on the additive manufacturing of polymer composites (short carbon fiber reinforced polyamide 6) through the process of FDM. Such 3D-printed parts are very lightweight and possess superior mechanical properties, which makes them a potential candidate for applications where a high strength-to-weight ratio is desired. The combination of FDM parameters, namely nozzle temperature, layer height, and flow rate, are studied in this work. The effect of variation in these parameters on the porosity and flexural strength is recorded following the Taguchi design of experiments. In calculating porosity, the weight difference between the printed part and the CAD part is used. For the flexural test, the standard three-point bending test is performed. The optimal combination of parametric settings is observed to be the same for minimum porosity and maximum flexural strength. Moreover, the flow rate is identified as a significant parameter for FDM printing of the composite material under study. The prints obtained at a raster angle 0˚/90˚ and on-edge orientation are observed to have better flexural strength than the prints at a raster angle ±45˚ and flat orientation.

Abstract: The need for porous materials is rapidly increasing in different applications like electrolysis, direct vaporization, and fluid transport by capillary lift. Additional to a certain porosity between 30 and 60 vol.%, the surface quality, mechanical stability and easy machinability come more and more into focus. Further requirements are raw materials that are low cost and available for mass production and the possibility to select from different materials like titanium, nickel, iron and their alloys for the chosen manufacturing route. In this work, the authors show a powder metallurgical approach on sintered titanium sponge to create a porous structure that meets the above-mentioned requirements. The sintering combines a fast hot pressing by Spark Plasma Sintering with a stop-controlled densification. With this setup it is possible to sinter a titanium sponge to sheets or discs. The sintered titanium sponge was densified to a porosity of 25 and 53 vol.% without any addition of organic or other sintering agents and creating a material that is close as possible to the purity of the initial sponge powder. The shape of the top and bottom of the sintered titanium sponge adapts the surface of the used punches during sintering and can be flat or structured. The plates or sheets can be easily machined to the desired shape using water jet cutting, wire erosion or drilling or milling. Preliminary tests were carried out on porous structures for capillary lift and direct heating elements for vaporisation.

Abstract: Nowadays the construction of palm oil industry mills is advancing rapidly and the next one is Refined Palm Oil (RPO). Refined Palm Oil (RPO) is one of the derivatives of palm oil processing. One option to use structures in the construction of palm oil mills is to use concrete. Plant parts that have concrete components often suffer damage, cracks and reduced concrete strength.This study is intended to examine the mechanical properties of concrete including compressive strength, flexural strength and porosity in concrete in the RPO environment, examining changes in the weight of coarse and fine aggregates immersed in RPO. This study used concrete experiments in RPO baths with 3 combinations, 54 samples and 2 types of concrete plan life, then analyzed in the laboratory. The parameters measured are changes in aggregate weight, porosity, bending strength and compressive strength of concrete. The results showed that the higher the percentage of RPO immersed in concrete, the lower the compressive strength, bending strength and porosity of the concrete. Fine aggregate undergoes weight change when immersed in RPO for 28 and 56 days, while coarse aggregate undergoes no weight change

Abstract: Porous Titanium (Ti) is one of the leading biomedical materials with high biocompatibility, durability, high stability, and non-toxic to the human body. In this study, highly porous Ti have been fabricated by spark plasma sintering process using NaCl as a pore former (70% wt.), and then, NaCl was dissolved in water. All the samples were sintered at 625 °C and held there for 10 min under applied pressure ranging from 20 to 50 MPa with heating rate of 100 °C/min in vacuum. The results suggested that NaCl is a proficient porogen, the porosity of all samples was in the range of 66.95 - 68.8%. When the pressing pressure increased from 20 - 50 MPa, the porosity of the samples decreased but the size of the pores increased. The pore size concentrated in the range of 300 - 350 µm. This implies that the compression pressure plays a crucial role in influencing both the porosity and pore size of the titanium material produced using the SPS technique. The compressive strength is 18.42 - 25.23 MPa, and the elastic modulus is 0.35 - 1.05 GPa, which matches the strength and the modulus of elasticity of biological implants.

Abstract: This work presents the finite element modelling of porosity in super alloys coatings, developed with cobalt-base/chromium/molybdenum/silicon metallic powders, which were thermally sprayed on oil & gas steel pipeline substrates, with the aim to protect the steel against H₂S and CO₂ corrosive environments. Therefore, in the developed finite element models, a small level of porosity, identified and analysed on the cross-section of the developed coatings, was incorporated in the developed models in order to perform a more realistic analysis of the structural response of the coating with some level of porosity by the local damage modelling technique. The porosity was incorporated in the developed finite element models with the micromechanical Gurson-Tvergaard-Needleman damage model, consequently the damage model parameters of Gurson-Tvergaard-Needleman model were calibrated against the true stress-strain material curve of the coating. The damage model was applied only on the finite elements subjected to higher bending loads. The values of and damage parameters are in the range of those published in the literature, for different type of steels, however value was lower, showing that for super alloy coatings, is quite lower than for steels. For the case of the initial and critical void volume fraction, the best calibrate values are higher compared to steels values reported in the literature. The relative density was similar compared to data published in the literature. Once the damage model parameters were properly calibrated, the modelling was employed to evaluate the stresses and strain states in the coating/substrate structure and in coating-substrates interface. The developed models were able to properly simulate the hardening material response of the coating with good agreement with material data. The results showed that Gurson-Tvergaard-Needleman damage modelling technique was able to model porosity damage in cobalt-base/chromium/molybdenum/silicon hard coatings, since numerical results agree well with true stress-strain material curve of coating material.