Fine Cell Size Research Articles

Melting mode driven tuning of local microstructure and hardness in laser powder bed fusion of 18Ni-300 maraging steel

Laser powder bed fusion (LPBF) of 18Ni-300 maraging steel has attracted abundant popularity in both academia and industry due to its superior mechanical properties and printability. Several studies showcased the optimization of process parameters based on relative density, but there is little exploration on tailoring the as-built microstructure. In this study, fully dense maraging steel samples are fabricated with three different melting modes, and they are examined for the cellular solidification microstructure, retained austenite, and the effect of cyclic reheating, the three characteristics that are unique to additively manufactured maraging steel. Microstructural discrepancies among the samples are traced back to the different cooling rate and residual heat, both of which are ultimately determined by the energy input. The findings suggest that a reasonably low energy input yields a microstructure containing fine cell size and little retained austenite, leading to higher hardness. Nanoindentation mapping reveals that the main bodies of all samples are strengthened around 13 %, likely because of the formation of precipitation zone due to four to five layers of subsequent cyclic reheating. This study demonstrates the important opportunity for microstructural tailoring, and consequently, engineering the mechanical properties of maraging steel via LPBF by controlling the solidification and subsequent cyclic reheating, which are both attributed to the printing parameters deployed.

Rheological behaviors and foaming behaviors of crosslinked ethylene–vinyl acetate with polyfunctional monomers

AbstractTo produce ethylene–vinyl acetate (EVA) foams with a higher foaming ratio and stable cell size, the efficiency of the crosslinking reaction is required to conduct simultaneously with the blowing reaction. Hence, the multifunctional monomer trimethylolpropane trimethacrylate (TMP) as a crosslinking promoter and a crosslinking agent dicumyl peroxide are used together as a crosslinking system to EVA foaming, when chemical blowing agent azodicarbonamide (ADC) is used. The TMP concentration of approximately 1.0 phr (per hundred resin) is most effective for balancing the expansion ratio and fine cell size. In the SEM diagram and expansion ratio results, it can be found that when the concentration of TMP ranges from 0 to 1.0 phr, the cell mean size is reduced from 360.3 to 140.5 μm and the expansion ratio is reduced from 13.6 to 9.1 at 170°C. The results of time‐sweep experiment of thermogravimetric analysis (TGA) show that the foaming time of ADC in the melt increases with the addition of TMP. Capillary rheometer and rotational rheometer tests show that the viscosity and the crosslinking efficiency of the melt are improved with the TMP addition. The curing time has been reduced from 2.93 to 2.22 min for 170°C.

Influence of sodium-montmorillonite nanoclay on flammability, thermal stability, morphology, and mechanical properties of rigid water-blown palm oil-based polyurethane foam

This paper presents the experimental works on rigid palm oil-based polyurethane (PU) foam reinforced with sodium-montmorillonite (Na-MMT). Filler loading was varied between 1 and 10 wt %., and the obtained foam was characterized for its combustibility, morphology, thermal stability, and mechanical response. Exfoliated clay microstructure was exhibited at lower Na-MMT loadings. Addition of nanoclay into the PU foam failed to impart any discernable improvement with regards to its flammability, believed due to stronger influence of low-functionality palm oil polyol used. Apparent improvement in thermal stability was observed at low clay amounts. Foam with finer cell size was obtained in the presence of Na-MMT, however only until a certain loading limit. Compressive strength generally increases with increasing clay content, but after 3 wt % the property deteriorated. Peculiarly, compressive strength rose again at 5 wt % and 6 wt % – postulated due to additional load-bearing effect of ‘integral skin’ – before plummeting back again beyond this value.

Numerical modelling of H2 storage with cushion gas of CO2 in subsurface porous media: Filter effects of CO2 solubility

The central objective of this study is to improve the understanding of flow behaviour during hydrogen (H2) storage in subsurface porous media, with a cushion gas of carbon dioxide (CO2). In this study, we investigate the interactions between various factors driving the flow behaviour, including the underlying permeability heterogeneity, viscous instability, and the balance between the viscous and gravity forces. In particular, we study the impact of CO2 solubility in water on the level of H2 purity. This effect is demonstrated for the first time in the context of H2 storage. We have performed a range of 2D vertical cross-sectional simulations at the decametre scale with a very fine cell size (0.1 m) to capture the flow behaviour in detail. This is done since it is at this scale that much of the mixing between injected and native fluids occurs in physical porous media. It is found that CO2 solubility may have different (positive and negative) impacts on the H2 recovery performance (i.e., on the purity of the produced H2), depending on the flow regimes in the system. In the viscous dominated regime, the less viscous H2 may infiltrate and bypass the cushion gas of CO2 during the period of H2 injection. This leads to a quick and dramatic reduction in the H2 purity when back producing H2 due to the co-production of the previously bypassed CO2. Interestingly, the impurity levels in the H2 are much less severe in the case when CO2 solubility in water is considered. This is because the bypassed CO2 will redissolve into the water surrounding the bypassed zones, which greatly retards the movement of CO2 towards the producer. In the gravity dominated scenario, H2 accumulates at the top of the model and displaces the underlying cushion gas in an almost piston-like fashion. Approximately 58% of H2 can be recovered at a purity level above 98% (combustion requirements by ISO) in this gravity-dominated case. However, when CO2 solubility is considered, the H2 recovery performance is slightly degraded. This is because the dissolved CO2 is also gradually vaporised during H2 injection, which leads to an expansion of mixing zone of CO2 and H2. This in turn reduces the period of high H2 purity level (>98%) during back-production.

Scaling analysis of hydrogen flow with carbon dioxide cushion gas in subsurface heterogeneous porous media

Subsurface hydrogen (H2) storage in geological formations is of growing interest for decarbonization. However, there is a knowledge gap in understanding the multiphase flow involved in this process, which can have a significant impact on the recovery performance of H2. Therefore, a full-compositional modeling study was conducted to analyze potential issues and to understand the fundamental hydrodynamic mechanisms of H2 storage. We performed a range of 2D vertical simulations at the decametre scale with a very fine cell size (0.1 m) to observe the detailed flow behaviour of H2 with carbon dioxide (CO2) as cushion gas in various flow regimes. Issues such as viscous instability, capillary bypassing, gas trapping and gravity segregation are analysed here. To generalize our calculations, we have validated and applied the scaling theory in the context of subsurface H2 storage. Since this study is focused on the hydrodynamic behaviour, three dimensionless groups, including aspect factor, capillary/viscous ratio and gravity/viscous ratio were identified to correlate recovery performance between various scales in a fixed heterogeneous system. It was found that H2 could infiltrate the cushion gas in the proximity of the injectors, meaning that CO2 is not displaced away from the injectors in a piston-like fashion. As a result, the purity of the back produced H2 is much degraded, particularly in a viscous-dominated scenario. On the other hand, the injected H2 mostly accumulates at the top forming a highly restricted mixing zone with CO2 in the gravity-dominated case. The recovery performance is therefore much improved in this case. Although the gas distribution can be significantly altered by capillary forces leading to bypassed zones, the recovery performance of H2 is hardly influenced. This is because the back-produced H2 recovery is not dependent on the sweep efficiency of the gas. H2 can be back produced following the same paths which were formed during injection.

Microstructure, mechanical and electrical characterizations of bimodal and nanocellular polypropylene/graphene nanoplatelet composite foams

In this work, linear and branched polypropylene (PP)/ graphene nanoplatelet (GnP) composite foams were prepared using supercritical CO2. The morphology of nanocomposite foams was imaged on SEM. Branched PP foams revealed bimodal cellular morphology, while linear PP exposed nanocellular structures. The effect of cell modality and adding GnP on the mechanical and electrical properties of nanocomposite foams were investigated. Addition of GnP to linear PP decreased cell density and made cell distribution uniform due to its functionality of crystal nucleating agent While in branched PP foams, it was seen by the dynamic observation that GnP acted as bubble nucleating agent, resulting in a fine cell size and increasing the foam density. Nanocellular foams exhibited better mechanical properties than bimodal but the limited expansion ratio. However, some bimodal foams presented comparable mechanical properties to non-foamed samples, in spite of high expansion ratio. Addition of graphene nanoplatelet to some extent improved the specific tensile modulus and strength of foams. The electrical conductivity of composites was reduced by foaming. However, this reduction was very significant in the case of bimodal foams.

Mechanical and Corrosion Properties of Additively Manufactured CoCrFeMnNi High Entropy Alloy

This study investigates the mechanical and corrosion properties of as-built and annealed equiatomic CoCrFeMnNi alloy produced by laser-based directed energy deposition (DED) Additive Manufacturing (AM). The high cooling rates of DED produced a single-phase, cellular microstructure with cells on the order of 4 μm in diameter and inter-cellular regions that were enriched in Mn and Ni. Annealing created a chemically homogeneous recrystallized microstructure with a high density of annealing twins. The average yield strength of the as-built condition was 424 MPa and exceeded the annealed condition (232 MPa), however; the strain hardening rate was lower for the as-built material stemming from higher dislocation density associated with DED parts and the fine cell size. In general, the yield strength, ultimate tensile strength, and elongation-to-failure for the as-built material exceeded values from previous studies that explored other AM techniques to produce the CoCrFeMnNi alloy. Ductile fracture occurred for all specimens with dimple initiation associated with nanoscale oxide inclusions. The breakdown potential (onset of pitting corrosion) was similar for the as-built and annealed conditions at 0.40 VAg/AgCl when immersed in 0.6 M NaCl. Pit morphology/propagation for the as-built condition exhibited preferential corrosion of inter-cellular Ni/Mn regions leading to a tortuous pit bottom and cover, while the annealed conditions pits resembled lacy pits similar to 304 L steel. A passive oxide film depleted in Cr cations with substantial incorporation of Mn cations is proposed as the primary mechanism for local corrosion susceptibility of the CoCrFeMnNi alloy.

A Novel 3-D TSS-FDTD for Distinguishing Underwater Targets at Low Frequency

A novel three-dimensional total-field scattered-field source finite-difference time-domain (TSS-FDTD) method is presented to analyze the propagation properties of low-frequency electromagnetic wave in underwater. The proposed method includes two-step FDTD calculations. The first calculation space is meshed with a fine cell size. The second is meshed with a coarse cell size. In addition, the ratio of a coarse cell size to a fine cell size is set to an arbitrary positive integer, such as $N=11$ in this letter. Numerical results prove the accuracy and validity of the proposed method. An analysis of the results shows that the run time spent in TSS-FDTD accounts for 0.35% of that in the conventional FDTD, and the consumed memory is 3.9% of that spent in the conventional FDTD.

APPLICATION OF UNDERWATER LOW FREQUENCY ELECTROMAGNETIC FIELDS DETECTION WITH TSS FDTD METHOD

Based on the conventional finite-difference time-domain (FDTD) method, a novel total- field scattered-field source FDTD method with a dual-meshed technique is presented to deal with the underwater detection problems in low frequency electromagnetic wave. A transformation surface connecting the coarse cell with the fine cell is implemented by applying a total-field scattered-field source (TSS) technique, which is carried out by two-step FDTD simulation. The ratio of a coarse cell size to a fine cell size can be set as an arbitrary integer, such as N = 10. Moreover, an analysis of numerical error illustrates that non-physical reflection fields from the TSS surface are avoided by introducing the TSS surface. We have derived, in detail, the update equations of fields on grids of the TSS surface. Three cases of dealing with different underwater electromagnetic problems are discussed. Numerical results show that by analyzing the magnitude and phase of scattered fields from obstacles underwater we can distinguish the category of the obstacles which belong to either a high resistivity body or a low resistivity body. Therefore, the proposed method provides us an effective tool for analyzing the electromagnetic response of materials underwater.

Formation of metastable phases and nanocomposite structures in rapidly solidified Al–Fe alloys

In the present work the structure and morphology of the phases of nanocomposites formed in rapidly solidified Al–Fe alloys were investigated in details using analytical transmission electron microscopy and X-ray diffraction. Nanoquasicrystalline phases, amorphous phase and intermetallics like Al 5Fe 2, Al 13F 4 coexisted with α-Al in nanocomposites of the melt spun alloys. It was seen that the Fe supersaturation in α-Al diminished with the increase in Fe content and wheel speed indicating the dominant role of the thermodynamic driving force in the precipitation of Fe-rich phases. Nanoquasicrystalline phases were observed for the first time in the dilute Al alloys like Al–2.5Fe and Al–5Fe as confirmed by high resolution TEM. High hardness (3.57 GPa) was measured in nanocomposite of Al–10Fe alloy, which was attributed to synergistic effect of solid solution strengthening due to high solute content (9.17 at.% Fe), dispersion strengthening by high volume fraction of nanoquasicrystalline phase; and Hall–Petch strengthening from finer cell size (20–30 nm) of α-Al matrix.

Effect of Fine Cell Size on Improvement of Bending Fracture Strength of Foams

Effect of Fine Cell Size on Improvement of Bending Fracture Strength of Foams

Dependence of Thermal Properties of Expandable Polystyrene Particle Foam on Cell Size and Density

An approach to improve the efficiency of the thermal insulation behavior of expandable polystyrene (EPS) particle foams by diminishing the thermal conductivity is the reduction of the radiation term of the thermal conductivity by an adjusted enlargement of the cell size of the particle foam. This correlation was investigated in detail by the determination of the dependences of cell size and thermal conductivity on the densities of the particle foam over a wider range using samples of expandable polystyrene particle foams showing conventional fine cell size as well as enlarged cell sizes. Based on the dependence of cell size on foam density of fine cell EPS foams, an equation is given also covering foams of larger cells. At the same mean diameter of the foam cells, the thermal conductivity of the EPS foam is increasing with a decrease in foam density in the whole range of diameters investigated from about 50—350 µm. At the same foam density, the thermal conductivity is in general independent of the mean cell diameter of the EPS foam at high foam densities, whereas at lower foam densities, the thermal conductivity is decreasing with increasing mean cell diameter of the foam, in a range of foam densities from about 10—35 g/L. Subsequently, a practical model to describe the dependence of thermal conductivity of expandable polystyrene particle foam on cell size and foam density is proposed and discussed.

Interaction fields based on incompatibility tensor in field theory of plasticity-Part II: Application-

The theoretical framework of the interaction fields for multiple scales based on field theory is applied to one-dimensional problem mimicking dislocation substructure sensitive intra-granular inhomogeneity evolution under fatigue of Cu-added steels. Three distinct scale levels corresponding respectively to the orders of (A)dislocation substructures, (B)grain size and (C)grain aggregates are set-up based on FE-RKPM (reproducing kernel particle method) based interpolated strain distribution to obtain the incompatibility term in the interaction field. Comparisons between analytical conditions with and without the interaction, and that among different cell size in the scale A are simulated. The effect of interaction field on the B-scale field evolution is extensively examined. Finer and larger fluctuation is demonstrated to be obtained by taking account of the field interactions. Finer cell size exhibits larger field fluctuation whereas the coarse cell size yields negligible interaction effects.

Manufacturing of Al–Mg–Si alloy foam using calcium carbonate as foaming agent

Aluminium foams can be manufactured by two main methods: casting and powder metallurgy. When the latter route is used, a foaming agent (usually TiH 2) is mixed with the aluminium or aluminium alloy powders, followed by powder mixture consolidation (usually hot extrusion) into a precursor and finally its foaming treatment. In this research, two calcium carbonate powders were used as foaming agents on an Al–Mg–Si (AA6061) alloy. Their different characteristics (particle size and chemical composition) modified the manufacturing process to achieve the final foam. AA6061 powders were then mixed with 10% calcium carbonate and, after cold isostatic pressing into green cylinders, hot extruded at different temperatures (475–545 °C). The foaming treatment was carried out in a furnace preheated to 750 °C using several heating times. The density changed from 2.03 to 2.10 g/cm 3 after cold isostatic pressing to 2.64–2.69 g/cm 3 in precursor materials obtained by hot extrusion. Foaming behaviour depends on the carbonate powder as well as the extrusion temperature. Thus, natural carbonate powder (white marble) produces a foam density close to 0.65 g/cm 3 after a shorter time than when chemical carbonate is used. The foam structure showed a low degree of aluminium draining, no wall cell cracks and a good fine cell size distribution. Compressive strength of 6.11 MPa and 1.8 kJ/m 3 of energy absorption were obtained on AA6061 foams with a density between 0.53 and 0.56 g/cm 3.

Metal Foam Production Process Using Hydro-Gel and its Improvement

Already, we are developing the process to produce stainless steel foam over 97% porosity using hydro-gel binder. However, this process is very sensitive process, and foaming condition is affected by the slight deference of heating temperature. Therefore, we tried to improve the process by changing the foaming agent and foaming conditions. By the improvement of the process, the foaming operation becomes stable and finer cell size stainless steel foam can be obtained.

Enhanced recovery in a particulate-reinforced aluminium composite

The microstructural development during cold rolling and static recovery of an Al alloy (AA 2014) and an AA 2014 particulate-reinforced metal matrix composite (PMMC) was investigated. The composite contained 20 vol.% of 15 μm diameter Al 2O 3 particulates. Both materials were rolled at room temperature and subsequently annealed to determine the effect of the reinforcement on static recovery. The addition of the ceramic reinforcement affected the matrix deformation microstructure by refining the cellular substructure near Al 2O 3 particles and generating shear bands and complex deformation zones in the vicinity of particle clusters. Following deformation, room temperature storage was sufficient to decrease the hardness of the composite, and annealing at 200 °C resulted in a more rapid rate of recovery compared with the alloy. This enhanced recovery was attributable to a larger driving force brought about by the finer cell size near the particles, but internal stress relaxation and interfacial diffusion may also be important factors, particularly at ambient temperature. The Al 2O 3 particles, particularly those in clusters, enhanced considerably the rate of subgrain growth of the matrix. Based on models of various stages of recovery, the enhanced rate of softening in the composite is not likely to be associated with a particular mechanism but involves a number of separate, but overlapping mechanisms.

Factors affecting the microstructure of a fine ceramic foam

Sintered cellular ceramics with varying degrees of reticulation were prepared using coarse and fine alumina powders and a 70/30 mixture thereof. The ceramic foams had fine cell size (about 150 μm diameter ) in each case. The method of preparation involved the free expansion of suspensions of these powders at 20 vol.% in a polyurethane foaming system. In order to modify the microstructure, the expansion was opposed using a die, and this reduced the void fraction and window size. In addition, directional rising was achieved by expanding the foam in a long cylindrical vessel and this modified the cell shape of the final foam. These foams have potential use in a range of applications including catalysis, but the main aim of the present work was to prepare preforms for metal matrix composites.

Development of foam shells for cryogenic laser fusion target

Spherical hollow polymer foam shells used as a sustainer of cryogenic fuel for the inertial fusion target have been developed. Foam shells which have 500–1000 μm diameter with a 30–100 μm uniform wall, 4 μm cell size and 40 mg/cm3 density, have been fabricated using a dual-nozzle droplet generator and a freeze-dry technique. Requirements to obtain a uniform thick wall and fine cell size are discussed.