Pore Coalescence Research Articles

Study of Powder Gas Entrapment and Its Effects on Porosity in 17-4 PH Stainless Steel Parts Fabricated in Laser Powder Bed Fusion

Powder-entrapped gas, which can occur naturally in gas-atomized powder, can induce porosity in parts fabricated with powder-based metal additive manufacturing processes. This study utilized synchrotron-based x-ray computed tomography and an in situ high-speed imaging technique, dynamic x-ray radiography (DXR), to investigate the formation of powder-induced porosity using 17-4 PH stainless steel powders with a controlled size distribution and intentionally varied entrapped gas contents. While powder with a low entrapped gas content showed no net part porosity increase, the results showed a strong correlation between the porosity in the powder and the porosity in the builds made from powder with a high entrapped gas content relative to typical gas-atomized powder. A threshold value was developed to classify porosity induced by powder-entrapped gas based on pore morphology measured using computed tomography. Transfer and coalescence of pores during laser melting was observed directly with DXR.

The role of defects and characterisation of tensile behaviour of EBM Additive manufactured Ti-6Al-4V: An experimental study at elevated temperature

The use of the Electron Beam Melting (EBM) Additive Manufacturing (AM) process to fabricate parts for applications which require strength and reliability is limited. EBM parts suffer from manufacturing defects and poor surface finish, and its strength properties can be dependent on strain rates and temperature. In this paper, the effects of parameters such as the strain rate, temperature, surface finish and build orientation together with the role of defects on tensile properties of EBM Ti-6Al-4V alloy have been studied and discussed. Due to reduced material density and multiplication rate of dislocations, a significant decrease in material tensile strength but an increase in material ductility was observed for tests on machined specimens at higher temperatures. Surface nucleating micro-cracks (poor surface finish) and inherent internal defects caused a reduction in tensile strength and material ductility for the as-built specimens. Coalescence of small size spherical pores and the presence of large voids have a direct deleterious effect on the ductility of Ti-6Al-4V alloy. The orientations of voids, difference in thermal history resulting into different microstructures, presence of grain boundary α and alignment of prior columnar β grains with respect to the loading direction for vertical and horizontal built specimens are the reasons for observed anisotropy in material strength of EBM Ti-6Al-4V alloy. The interaction effects of different parameters are also discussed, and it is suggested that these parameters should be optimised, in addition to the process parameters, to make AM parts using the EBM process which can reliably and safely be used for load bearing applications.

An attempt to quantitatively and qualitatively differentiate the pore structure originating from air naturally present in hardened concretes and introduced by an air-entraining admixture

The new generation superplasitcizers affect air-entraining stability and, consequently, the air void characteristics in hardened concrete. As part of research 6 air entrained and 6 non air entrained concretes with a variable water-cement ratio liquefied, using modified polycarboxylates. The air void characteristics in hardened concretes were determined using the automatic RapidAir 457 system according to EN 480-11. Porosity structure parameters change with the w/c ratio and were described by mathematical relationships. Water-cement ratio does not affect the content of favourable pores from air entraining, however is responsible for entrapped air (above 300 µm) content. The most favourable pore structure was obtained in concrete, which has the optimal viscosity allowing to escape large pores during compaction and preventing coalescence of fine pores, as well as the superplasticizer content that does not cause additional air entraining.

Diffusion of CH4in amorphous solid water

Context.The diffusion of volatile species on amorphous solid water ice affects the chemistry on dust grains in the interstellar medium as well as the trapping of gases enriching planetary atmospheres or present in cometary material.Aims.The aim of the work is to provide diffusion coefficients of CH4on amorphous solid water (ASW) and to understand how they are affected by the ASW structure.Methods.Ice mixtures of H2O and CH4were grown in different conditions and the sublimation of CH4was monitored via infrared spectroscopy or via the mass loss of a cryogenic quartz crystal microbalance. Diffusion coefficients were obtained from the experimental data assuming the systems obey Fick’s law of diffusion. Monte Carlo simulations were used to model the different amorphous solid water ice structures investigated and were used to reproduce and interpret the experimental results.Results.Diffusion coefficients of methane on amorphous solid water have been measured to be between 10−12and 10−13cm2s−1for temperatures ranging between 42 K and 60 K. We show that diffusion can differ by one order of magnitude depending on the morphology of amorphous solid water. The porosity within water ice and the network created by pore coalescence enhance the diffusion of species within the pores. The diffusion rates derived experimentally cannot be used in our Monte Carlo simulations to reproduce the measurements.Conclusions.We conclude that Fick’s laws can be used to describe diffusion at the macroscopic scale, while Monte Carlo simulations describe the microscopic scale where trapping of species in the ices (and their movement) is considered.

Features of setting and strength gain of the foam concrete massif

The paper presents a study of the regularities of the processes occurring in the matrix and aerated concrete mass at the initial time after laying and later, during the strength gain. The colloidal properties that appear in the foam concrete mix begin at the stage of mixing the components containing binder and aggregate particles, and are activated by the mixer blades. In this case, the mixture is transformed by mass transfer of the matrix material onto the foam grid and a new spatial reorientation of the bubbles under conditions of physical and mechanical effects on the thin-film structure of the foam. In technology, it should be taken into account that the processes occurring in the foam concrete mixture can be attributed to two opposite actions occurring simultaneously. First, in a freshly laid foam concrete mixture, as in any thermodynamically unstable system, sedimentation and coalescence of air pores begins, which is especially manifested at forcedly high water-solid ratios. This can lead, if the technology is not followed, to complete stratification of the mixture. Secondly, in the cement slurry, the setting and further formation of the cement stone occurs. A necessary technological condition for product quality is compliance with the optimal water-solid ratio of the mixture and ensuring the beginning of the setting of the foam concrete mass before its stratification.It was also experimentally determined that an increase in the hardening duration of foamcement systems for more than 28 days is impractical, since it was revealed that an increase in the strength of foam concrete is similar to the same process in structural concrete. It is quite possible to achieve the strength required for foam concrete according to GOST 25485-89 within 28 days and even earlier due to the high technological discipline and the use of modern plasticizing additives.

The foaming mechanism of glass foams prepared from the mixture of Mn3O4, carbon and CRT panel glass

Cathode ray tube (CRT) panel glass is processed into glass foams by using carbon and Mn3O4 as foaming agents. We investigate the foaming ability and foaming mechanism of the mixture by varying the composition and the temperature. Carbon load of 5 wt% inhibits the sintering of the glass powder, while 2 wt% carbon promotes the coalescence of pores during foaming. Carbon load of ≤0.4 wt% is suitable for achieving low density foams (0.15–0.23 g cm-3) with closed pores (75–100%). Foaming is initiated by the reaction between carbon and Mn3O4 and this is the main source of melt expansion. Reduction of cations in the glass melt, including Mn3+ dissolved from Mn3O4, makes an important contribution to the melt foaming at >800 °C. The main gaseous product is CO2 (67–95 vol%). CO appears above 800 °C and the concentration increases with temperature.

Pore structure analysis of directionally solidified porous copper

Directionally solidified porous copper is considered as a potential candidate in the field of microchannel heat sinks. By the Bridgman-type directional solidification method, a porous copper ingot was fabricated. Evolution of the porosity, pore number density, average pore diameter and average interpore spacing at different ingot heights was investigated. The results show that with the increase of ingot height, the porosity firstly increases and then basically remains unchanged from the ignot height of 65 mm; the pore number density rapidly decreases at first, and the decreasing speed becomes slower when the ignot height higher than 85 mm; the average pore diameter increases and then remains unchanged from the ingot height of 85 mm; the average interpore spacing increases, and the increasing speed of average interpore spacing becomes slower with the increase of height to higher than 85 mm. In order to study the evolution of diameter and spatial distribution of pores, the distribution ranges of pore diameter, nearest-neighbor distance and radial cumulative pore number were analyzed. As the ingot height increases, the distribution ranges of pore diameter and nearest-neighbor distance firstly increase and then tend to be stable. There are no pore clusters and for long distance, the spatial distribution of pores is uniform at different ingot heights. Pore structure and 3D pore morphology of porous copper were observed with the help of light illumination and X-ray tomography. Pore nucleation, pore interruption, pore coalescence, diameter change of pores and lateral displacement of pores were found to exist in the pore structure.

Influence of Pore Characteristics on Anisotropic Mechanical Behavior of Laser Powder Bed Fusion–Manufactured Metal by Micromechanical Modeling

In recent times, additive manufacturing (AM) has proven to be an indispensable technique for processing complex 3D parts because of the versatility and ease of fabrication it offers. However, the generated microstructures show a high degree of complexity due to the complex solidification process of the melt pool. In this study, micromechanical modeling is applied to gain deeper insight into the influence of defects on plasticity and damage of 316L stainless steel specimens produced by a laser powder bed fusion (L‐PBF) process. With the statistical data obtained from microstructure characterization, the complex AM microstructures are modeled by a synthetic microstructure generation tool. A damage model in combination with an element deletion technique is implemented into a nonlocal crystal plasticity model to describe anisotropic mechanical behavior, including damage evolution. The element deletion technique is applied to effectively model the growth and coalescence of microstructural pores as described by a damage parameter. Numerical simulations show that the shape of the pores not only affects the yielding and hardening behavior but also influences the porosity evolution itself.

Formation of Vesicular Pores in Aggregates from the Eluvial Horizon of Albic Glossic Retisol during Freeze-Thaw Cycles

In this article, we discuss the contribution of capillary wetting and multiple freezing and thawing to the formation and evolution of vesicular micro– and mesoporosity in an aggregate of 3 mm in diameter from the eluvial horizon of soddy–podzolic soil (Albic Glossic Retisol (Loamic, Cutanic)). The main visible changes occur during capillary wetting and the first four cycles of freezing and thawing. Capillary wetting of the air-dry aggregate initiates the formation of vesicular micropores in the soil mass. The first freezing fixates the newly formed vesicles and contributes to the development of microschlieren in the aggregate. The first thawing causes the isolation and growth of individual vesicular pores. In the subsequent three freeze–thaw cycles, freezing fixates the vesicular pore space pattern, whereas thawing promotes the coalescence of small vesicular pores into larger vesicular pores. During the fifth and next cycles, many large vesicular pores leave the deformed aggregates aggregate, so that horizontally oriented cryogenic microschlieren tend to predominate in the frozen aggregate after 10–20 freeze–thaw cycles.

Study of silicon carbide dissociation into Fe and Fe C matrixes produced by die pressing and sintering

During the sintering process of self-lubricating steels, the dissociation of Silicon Carbide (SiC) in the iron matrix forms turbostratic graphite nodules which provide the lubricating properties of these materials. Previous works have explored the turbostratic nature of the graphite nodules and the influence of the SiC content in the microstructure and tribological properties of these materials, however, the thermodynamics and kinetics of SiC dissociation have not been fully explored yet. This study uses the versatility of die pressing to study the influence of particle sizes, sintering temperature and carbon content in the dissociation of SiC. The resulting materials were analysed by optical microscopy and scanning electron microscopy (SEM). The evolution of the size and morphology of pores and graphite nodules was investigated through image analysis software and simulations with the DICTRA® software were used to a better assessment of the kinetical aspects of SiC dissociation. Also, the thermodynamic influence of the particle size in the equilibria of the Fe–SiC system is discussed. Results indicate that SiC particle size rather than Fe/SiC size ratio governs the dissociation process, sintering temperature causes coalescence and rounding of pores as well as of graphite nodules. In addition, excessive activity of carbon in the system causes the precipitation of graphite flakes, which are not desired for technological applications.

Microstructure evolution and electromechanical properties of (K,Na) NbO3‐based thick films

ABSTRACTThis study investigated an unconventional method of electrophoretic deposition (EPD) for the processing of environmentally benign (K0.5Na0.5)0.99Sr0.005NbO3 (KNNSr) thick films on Pt/alumina substrate. EPD allows rapid, economical, and low‐waste processing of thick films and thus offers an integration advantage for electronics manufacturing. To understand the functional response of the KNNSr thick films, the effect of the sintering temperature and atmosphere on their structure, microstructure, and electromechanical properties was investigated. KNNSr thick films densify in constrained conditions in a very narrow temperature range only a few 10°C below the melting temperature of 1140°C. Up to 1100°C the relative density increases to 80%, upon further heating to 1110°C we observed only the grain growth and pore coalescence. The densification is not affected significantly by the atmosphere. The local domain structure of 25‐33 μm thick KNNSr films was similar, while the dielectric and electromechanical properties increased with the increasing sintering temperature. KNNSr thick film sintered at 1100°C has a thickness‐coupling factor kt of 0.4, comparable to that of bulk. The results reveal that the EPD enables the economic processing of high‐performance thick films on complex‐shape substrates that are difficult to fabricate using conventional thick‐film methods.

Correlation between pore structure, compressive strength and thermal conductivity of porous metakaolin geopolymer

This paper investigates the effect of mixing parameters (that are, alkali concentration, AA ratio, and MK/AA ratio) on the thermal conductivity of metakaolin geopolymers. The combination effect of foaming agent (H2O2) and surfactant (Tween 80) on the physical properties, compressive strength, and pore characteristic was also elucidated. Results showed that metakaolin geopolymer with maximum compressive strength of 33 MPa, bulk density of 1680 kg/m3, porosity of 18% and thermal conductivity of 0.40 W/mK were achieved with alkali concentration of 10 M, AA ratio of 1.0 and MK/AA ratio of 0.8. Gradation analysis demonstrated that AA ratio was the strength determining factor. Whilst, thermal conductivity was dependent on the MK/AA ratio. Adding H2O2 and surfactant produced geopolymer foam with acceptable compressive strength (0.4–6 MPa). The geopolymer foam had bulk density of 471–1212 kg/m3, porosity of 36–86% and thermal conductivity of 0.11–0.30 W/mK. Pore structure, size, and distribution were governed by H2O2 and surfactant dosages that have a great impact on the compressive strength. Narrower pore distribution and smaller pore diameter were achieved when both foaming agent and surfactant were used instead of foaming agent alone. The pore size and distribution varied to a greater extent with varying H2O2 contents. Surfactant illustrated distinct pore stabilizing effect at low H2O2 (<0.75 wt%) which diminished at high H2O2 content. In terms of thermal conductivity, even with increasing porosity at high H2O2 and surfactant content, the thermal conductivity did not show substantial reduction due to the interconnected pores as a result of pore coalescence.

Foaming of A1050 aluminum precursor by generated frictional heat during friction stir processing of steel plate

The foaming of an A1050 precursor was performed using only the frictional heat generated during the traversing of a friction stir processing (FSP) tool on a steel plate. In this study, the experimental conditions during FSP were optimized for relatively high melting point A1050 precursor. For tool traversing speeds of 10 mm/min and 20 mm/min, the A1050 precursor was successfully foamed, where the precursor gradually foamed during the traversing of the tool. However, too much heat was generated, which resulted in excess heat input, in the case of tool traversing speed of 10 mm/min, whereas there was less excess heat input in the case of tool traversing speed of 20 mm/min. Large pores were observed owing to the coalescence of pores, which may have induced the release of gas from the top surface of the Al foam during foaming, decreasing the porosity of the obtained Al foam. In the case of tool traversing speed of 30 mm/min, the latter half of the Al foam was sufficiently foamed and fine pores were observed, although the first half of the precursor was not foamed. Moreover, it was indicated that a uniform temperature distribution during FSP is necessary to obtain uniform pore structures, which may be achieved by gradually increasing the tool traversing speed. The use of a steel plate with lower thermal conductivity was effective for foaming the precursor, which may introduce too much heat into the precursor with less heat diffusion.

Fatigue damage, impact toughness and micromechanisms of fracture of spring steel after operation at low climatic temperatures

To solve urgent problems of ensuring the vehicles parts reliability under alternating loads, it is necessary to study the influence of operational conditions on the accumulation of fatigue damage in the material, leading to the appearance of micro- and macrocracks. The purpose of this work is to study the influence of fatigue damage formed in spring steel under operational impacts typical of the cryolithozone on the level of its resistance to brittle fracture. Methods of mathematical statistics and methods of metal research (metallography, fractography, microindentation, mechanical testing, etc.) were used in the work. Two main types of structural damage are identified, differing in scale level: microdamage, reflected by the microhardness parameter, and mesodamages in the form of pores of various diameters. New data on poorly studied fatigue damage accumulation processes under non-stationary loading and their influence on brittle fracture resistance were obtained. Mechanism of the effect of porosity on brittle fracture resistance is associated with the occurrence of deformation processes in the vicinity of pores, as well as during growth and coalescence of pores. The research results can be used to develop methods for obtaining and information processing on the structural damage of materials and to extend the service life of elastic elements of vehicles operating in the cryolithozone.

Фрактальная размерность поверхности разрушения пористого ZrO2 – MgO композита

Porous ZrO2–MgO composites were studied. The influence of the composition and sintering time on the grain size, porosity, and kinetics of pore size changes was investigated. The fractal dimension and surface roughness of the fracture surface were determined by the method of vertical cross sections by SEM images. The kinetics of change in the average size of micro- and macropores with an increase in sintering time indicates the pursuit of the system to establish a unimodal pore structure owing to volume shrinkage and coalescence of pores. It was shown that changes in the fractal dimension reflect the stages of solid-phase sintering of porous ZrO2–MgO composite and occur only at the second stage of solid-phase sintering. The maximum fractal dimension, equal to 1.5, is reached at the transition from the first to the second stage of solid-phase sintering and is observed when the ratio of large and small pores is 12.5, which indicates the formation of the most developed surface.

Characteristics of coke pore formation and its effect on solution loss degradation

Pore structure is an important factor that affects coke thermal properties. In order to reveal the formation regularity of coke pore structure, high temperature carbonization experiment of blending coals was carried out in 40 kg test coke oven. The pore structure of different regions and different orientations of the obtained coke was detected by microscope. The ratio of the average pore diameter of the coke to the average wall thickness was highly correlated with the square of porosity. The relationship shows that during coking, the development of the wall thickness is restricted by pore coalescence and coal expansion. The softened coal can form a thicker wall structure only after the pores have coalesced fully without excessive expansion. The four coke pore size distribution curves and pore-wall-thickness distribution curves reflect the quality change of the coal blending. Therefore, the adjustment of a coal blending scheme according to the pore structure distribution curve provides a promising, accurate coal blending method.

Phase field modeling of pore electromigration in anisotropic conducting polycrystals

Abstract A phase field model is developed to investigate the migration of pores driven by an electric field in polycrystalline materials with anisotropic electrical conductivity. Mass diffusion describing pore migration is coupled with charge conduction by solving microscopic Ohm’s law. The model accounts for grain structure dependent conductivity distribution which affects pore migration velocity and path. As an example, the model is applied to simulate pore migration in tin polycrystals. Significant effects of the grain orientation on pore migration is observed which are analyzed in terms of the orientation-dependent conductivity in individual grains and the mismatch of conductivity across grain boundaries. The effects of conductivity anisotropy on the interactions between pores and pore coalescence are also discussed.

In-situ 3D fracture propagation of short carbon fiber reinforced polymer composites

This paper explored the 3D progressive fracture process of a short carbon fiber reinforced polymer composite under uniaxial tensile loading via high-resolution in-situ micro X-ray computed tomography (μXCT). Microstructural features extracted from the μXCT images were analyzed with the Halpin-Tsai model, laminate analogy approach, and shear-lag model to calculate the mechanical properties and interpret the different fracture morphologies and progressions observed in the “skin-core-skin” structure. The in-situ μXCT scanning revealed a clear fracture progression in the skin layer, which was defined by four stages, i.e., nucleation of small pores at fiber ends, coalescence of pores, initiation of cracks, and propagation of cracks until fracture, whereas no significant fracture feature was found in the core layer. To analyze the difference in the fracture mechanisms in the skin and core layers, the maximum fiber normal stress and average interfacial shear stress were calculated from the microstructural features. The results were also confirmed by the 3D strain distribution quantified via a volumetric digital image correlation method. It was determined that the dominant fracture mechanism in the skin layer was fiber pull-out, rather than fiber breakage, as a consequence of pore formation at the fiber ends; its extent mainly depended on the fiber length and orientation angle. Fiber/matrix debonding was the main fracture mechanism in the core layer, which resulted from the propagation of cracks initiated from the skin layer.

In-situ X-ray tomography analysis of the evolution of pores during deformation of AlSi10Mg fabricated by selective laser melting

Abstract AlSi10Mg is an interesting structural material due to its high specific mechanical properties, and good weldability which makes it a good candidate for additive manufacturing by selective laser melting. In this paper, the evolution of pores is visualized and quantified in an AlSi10Mg fabricated by selective laser melting using in-situ tension coupled with X-ray computed tomography. Graphical models were produced in order to visualize the evolution of pores, and the volume fraction, density, and mean diameter of pores were determined at each stage of deformation. Significant coalescence of pores was found during post-uniform elongation. Dimples on the fracture surfaces indicate ductile fracture.

Phase-field study of grain growth in porous polycrystals

During sintering, a multitude of mechanisms act in different ways resulting in densification and coarsening. Since the material properties depend on the microstructure, the manufacturing of advanced ceramics requires a deep understanding of the sintering process. The present study focuses on the occurrence of grain growth during final stage sintering. In a porous microstructure, the pores exert a dragging force to grain boundary motion retarding the grain growth. However, as soon as the porosity becomes low enough, the grain growth exceeds the dragging force and the microstructure starts to coarsen. This interdependency of densification and grain growth is still not fully understood. The present study uses a 3D phase-field model to investigate grain growth in porous microstructures during final stage sintering. A model extension treats pore dynamics under consideration of pressure stability as well as pore coalescence. To account for the size effect of pores on its dragging force, a surface diffusion based mobility approache is incorporated. Large-scale 3D grain growth simulations are conducted to analyze the effect of different porosities and pore sizes with statistical relevance comparable to real microstructures. Depending on the porosity, two dominating effects on the reduction of grain growth rate are found. For low porosities, the growth rate depends on the number of pores whereas for higher porosities the pore size and their mobility dominate the process. The results show the need to consider the effects of the pore size and their distribution during final stage sintering.