Densification Level Research Articles

Highly dense spinel ceramics with completely suppressed grain growth prepared via SPS with NaF as a sintering additive

For the preparation of transparent spinel ceramics it is common practice to use LiF as a sintering additive to achieve transparency. However, it is well known that in this case the grain size exhibits a significant increase compared to pure spinel ceramics, which can lead to a deterioration in mechanical properties. The findings of this paper indicate that when NaF is used as a sintering additive for the preparation of spinel ceramics via spark plasma sintering (SPS) translucent materials with a high level of densification can be obtained without observable grain growth. It is shown that the grain size after SPS at 1500 °C with 1 h dwell is essentially the same as the primary particle size of the spinel powder, whereas pure spinel ceramics prepared via SPS under the same conditions exhibit grain growth by approximately a factor 5.

Experimental and theoretical study on enhanced electrical properties of nickel-substituted La0.5Sr0.5CoO3−δ ceramics

The effect of Ni substitution on the thermal behavior, crystal structure, densification, and electrical properties of La0.5Sr0.5Co1–yNiyO3-δ (y = 0.00–0.08) (LSCN) ceramics was discussed based on experimental measurements and theoretical calculations to search for a ruthenium–free and lead–free conductive oxide for thick film resistors. Ceramics were synthesized by the solid–state reaction, and calculations were performed with first–principle density functional theory (DFT). Results showed that the replacement of Ni ion to Co ion could help decrease the densification temperature and enhance the densification level and improve the conductivity of LSCN. Theoretical calculations, including the crystal structure, bond population, total energy, and density of states (DOS), supported the experimental results well. The maximum conductivity of 3155 S/cm was achieved as y = 0.04 was sintered at 1200 °C, and the peak temperature coefficient of resistance (TCR) of 2405.7 ppm/°C occurred at y = 0.06.

Densification, mechanical behaviors, and machining characteristics of 316L stainless steel in hybrid additive/subtractive manufacturing

Directed energy deposition (DED) is capable of manufacturing high-density and complex metal components, whereas traditional machining methods are limited in these regards. Nevertheless, the fabrication of parts with DED that have accurate dimensions and geometries as well as acceptable surface states is a concern, as these factors are inferior to those of traditionally machined components. Therefore, the application of parts fabricated with DED is restricted. Thus, the hybrid additive/subtractive manufacturing (HASM) method has been exploited to comprehensively utilize the advantages of both. In this paper, the HASM method was used to fabricate bulk parts with four different scanning strategies (alternating the orientation of subsequent layers by 0°, 45°, and 90° and island scanning are referred to as an X-scan, a Rot-scan, an XY-scan, and an Island-scan, respectively) followed by subtractive milling to gain a smooth surface with a determined thickness for the next scanning and deposition period until the parts were completely finished. The influence of the scanning strategy on the densification level and mechanical behavior of the specimens fabricated with HASM was studied. The results show that the specimens fabricated with the 7 × 7 mm2 island scan with short scan vector lengths showed a higher densification than the specimens fabricated using long scan vector lengths (the three methods mentioned above). Finally, to obtain a fine surface finish, the influence of the feed per tooth (fz) on the surface quality of a 316L stainless steel during the milling process that occurs during the HASM process was also investigated. The result shows a fz of 0.25 mm yielding the minimum surface roughness value among the samples observed herein, which implies that the surface quality was better than that of the other studied conditions.

Effect of cobalt-doping on the dielectric properties and densification temperature of Li2MgSiO4 ceramic: Calculation and experiment

The first principle calculation was conducted to predict the effect of Co2+ ion substitution on the crystal structure, electron density, formation energy and vibration properties of Li2MgSiO4 ceramic. All the samples were synthesised through the solid-state reaction method. Variations in sintering and dielectric properties were analysed through X-ray diffraction, scanning electron microscopy, network analysis, Raman spectrometry, thermomechanical analysis and simultaneous thermal analysis. Bond length, bond population and electron distribution in a MgO4 tetrahedron were modified due to the substitution of Mg2+ ion by Co2+ ion. The densification temperature was lowered from 1250 °C to 1050 °C with the addition of 5% (molar ratio) Co2+ ions. The sintering window was put forward slightly, and the vibration property was changed. The densification level and microwave dielectric properties were improved accordingly at following conditions: εr = 5.75, Q × f = 26,500 GHz at 16 GHz, τf = −9.4 ppm/°C and relative density = 95.2%. The dielectric and sintering properties of Li2MgSiO4 ceramic could be enhanced with the substitution of Mg2+ ion by Co2+ ion.

Investigation of Decarburization Behaviour during the Sintering of Metal Injection Moulded 420 Stainless Steel

The decarburization behaviour during the sintering of metal-injection-moulded 420 stainless steel was investigated. Particular attention has been paid to the decarburization mechanisms and carbon distribution of parts sintered at different temperatures. The results indicate that the loss of carbon content occurs mainly through the reduction of surface oxides of powder particles or reaction with the atmosphere. Depending on the densification level, the sintering decarburization is separated into two stages. Below 1200 °C, decarburization occurs at the powder particle surface. The surface oxides react with the amorphous carbon or residual organics to pose the metal surface. At this time, the carbon content distribution of the sintered body is similar to that of the as-debound sample. The reduction of surface oxides promotes sintering. At 1200 °C, the densification speed is higher in the centre region of the sample, where the interconnected pores close and the second stage of decarburization takes place. At temperatures above 1200 °C, carbon atoms in the inner layers must migrate to the surface to react with the atmosphere. The decarburization speed is reduced and the carbon content distribution of the as-sintered part is similar to that of the dense decarburized part.

Effect of ceramic particle size on densification behavior, microstructure formation, and performance of TiB2-reinforced Al-based composites prepared by selective laser melting

Abstract

Improved microwave dielectric properties and sintering behavior of LiZnPO4 ceramic by Ni2+-ion doping based on first-principle calculation and experiment

Theoretical calculation and experimental verification were performed to investigate the effect of Ni2+-ion substitution on the microstructure, sintering behavior, and microwave dielectric properties of LiZnPO4 ceramic. The crystal structure, electron density, formation energy, and vibration mode of the doped system were discussed on the basis of density functional theory. Samples were synthesized using the traditional solid-state reaction method and characterized by X-ray diffraction, scanning electron microscopy, energy-dispersive spectroscopy, network analysis, differentia-thermal and thermo-mechanical analyses, and Raman spectrometry. The substitution of Ni2+ ions to Zn2+ ions could modify the bond length, bond population, and electron distribution of ZnO4 tetrahedron. The vibration, microstructure, and sintering properties changed accordingly. The densification level and microwave dielectric properties improved (εr=5.57, Q×f = 63,951 GHz at 15 GHz, τf = -79.5 ppm/°C, relative density = 97.27%) when 4% (molar ratio) Zn2+ ions were replaced by Ni2+ ions sintered at 825 °C. The Ni2+-ion substitution could enhance the dielectric and sintering properties of LiZnPO4 ceramic.

Synthesis of powder metallurgy Ti-3Fe-2Al alloy using high-frequency induction heating

A powder metallurgy approach was applied for the synthesis of an [Formula: see text] Ti-2Al-3Fe alloy. Blends of the elemental Ti, Al and Fe powders were compacted and subsequently sintered. High-frequency induction heating (HFIH) instead of conventional high-vacuum furnace heating was used for the sintering, due to its high efficiency. The effect of temperature on the level of densification, residual porosity and mechanical properties was studied. Electron dispersive spectrum analysis was used to study the dissolution and homogenization of the alloying elements. The results showed that a short induction sintering (IS) cycle in the range of 10–15 min is sufficient to achieve significant powder consolidation, evident by the increase of the density and mechanical properties. The residual porosity diminishes with the increase of the sintering temperature. Full dissolution of the alloying powders is completed after sintering at temperatures above those of [Formula: see text]- to [Formula: see text]-phase transformation.

Elementary plastic events in a Zachariasen glass under shear and pressure

In this paper, we investigate the response of a two-dimensional silica model glass under shear and pressure. The glass network is generated based on statistical data extracted from real two-dimensional silica. We present an amended dual bond-switching algorithm, which enables us to control both the level of heterogeneity as well as physically meaningful ring neighborhood statistics. The two-dimensionality of the model glass allows us to directly track all atoms during deformation and overcome the imaging problem of bulk silica glass. Firstly, we subject different sets of pressure-free samples to athermal quasistatic simple shear deformation. We observe a serrated flow for all levels of heterogeneity, where the stress drop statistics is described by a power law. Using the divergence theorem, it is shown that the outward flow of the local rearrangements associated with the stress drops is always higher than the inward flow, resulting in density fluctuations and nano-voids, the latter of which are detected using the shortest path criterion. However, the difference in the out-to-inward flow decreases with increasing heterogeneity. Secondly, we subject a set of one level of heterogeneity to various levels of compression, followed by athermal quasistatic shear deformation. After comparing the stress–strain results to those in bulk silica, we show that the statistics of the occurring stress drops of the densified material follow a power law. We identify an initial decrease of the out-to-inward flow with increasing pressure until a level of 10% densification followed by a slight increase up to a densification level of 15%.

Additive/subtractive hybrid manufacturing of 316L stainless steel powder: Densification, microhardness and residual stress

316L stainless steel specimens were manufactured by additive/subtractive hybrid manufacturing (ASHM). The densification level, mi-crostructure, microhardness and residual stress characterization in the different zones of the part were investigated. The density was determined by the Archimedes method, and the density measurement was divided into three regions, namely, the bottom, middle and top zones. The results show that the middle zone has a much smoother melting surface and that a relative density of nearly 100 % was achieved for the part in this study. The hardness profiles at room temperature, along the width and height directions of the cross-section of the top and bottom zones, were also studied. The residual stress was evaluated by X-ray diffraction (XRD) for the 316L SS specimen fabricated by the ASHM process, and it was compared with a specimen manufactured by the additive laser directed energy deposition (DED) process. The results show that the top and bottom zones exhibit tensile stress, and compressive stress occurs in the middle area. Moreover, the residual stress of ASHM shows a slightly smaller trend than that of the simplex DED due to the stress relaxation of the subsequent subtractive milling. These results may offer guidance for ASHM-fabricated 316L parts.

Effect of V Substitution on the Electrical Properties of La0.5Sr0.5Co1−xVxO3 (x = 0.00–0.10) Ceramics

In this paper, the microstructures, resistivity as well as temperature coefficient of resistance (TCR) of La0.5Sr0.5Co1−xVxO3 (x = 0.00–0.10) ceramics were studied. These ceramic samples were prepared through conventional solid-state reaction method. Results showed that the substitution of Co ions with V ions improved the densification level of ceramic samples and changed their resistive characteristics accordingly. A uniform microstructure of La0.5Sr0.5 Co1−xVxO3 ceramic was realized at x = 0.05 when sintered at 1100°C. The resistivity of ceramics increased slightly as the V content varied in the range of 0.00–0.10. The TCR value of all tested samples was maintained between +1304.35 ppm/°C and +2958.26 ppm/°C. A low resistivity value of 0.38 mΩ cm and the minimum TCR value of +1304.35 ppm/°C were achieved as x = 0.05 in La0.5Sr0.5Co1−xVxO3. It was found that the substitution of Co ions with V ions facilitated lowering the TCR value without changing the resistivity too much. Thus, these ceramics are promising materials for conductive oxide used in thick-film resistors.

In-situ Cu-doped MnCo-spinel coatings for solid oxide cell interconnects processed by electrophoretic deposition

Abstract The Cu doping of the Mn–Co spinel is obtained “in-situ” by electrophoretic co-deposition of CuO and Mn 1.5 Co 1.5 O 4 powders and subsequent two-step reactive sintering. Cu-doped Mn 1.5 Co 1.5 O 4 coatings on Crofer22APU processed by electrophoretic co-deposition method are tested in terms of long term oxidation resistance and area specific resistance tests up to 3600 h. The introduction of Cu in the spinel lead to higher level of densification of coatings for all the considered aging periods at 800 °C and stabilizes the cubic phase of the Mn 1.5 Co 1.5 O 4 spinel. Corrosion rate of the Cu-doped Mn 1.5 Co 1.5 O 4 coated Crofer22APU is ∼10x lower than for the uncoated Crofer22APU. The stabilization of the cubic phase due to Cu doping, which reduces the extent of the tetragonal-cubic phase transition and limits possible thermal stresses due to mismatch of coefficients of thermal expansion or volume changes, is reviewed and discussed by means of electrical conductivity measurements together with diffraction patterns and elemental analyses. These novel electrophoretic co-deposited Cu-doped MnCo spinel coatings represent an innovative approach to obtain coatings with higher density and have future applications in the view of reaching lower rates of Cr evaporation form the steel.

Ti3SiC2-Cf composites by spark plasma sintering: Processing, microstructure and thermo-mechanical properties

MAX phases, and particularly Ti3SiC2, are interesting for high temperature applications. The addition of carbon fibers can be used to reduce the density and to modify the properties of the matrix. This work presents the densification and characterization of Ti3SiC2 based composites with short carbon fibers using a fast and simple fabrication approach: dry mixing and densification by Spark Plasma Sintering. Good densification level was obtained below 1400 °C even with a high amount of fibers. The reaction of the fibers with the matrix is limited thanks to the fast processing time and depends on the amount of fibers in the composite. Bending strength at room temperature, between 437 and 120 MPa, is in the range of conventional CMCs with short fibers and according to the resistance of the matrix and the presence of residual porosity. Thermo-mechanical properties of the composites up to 1500 °C are also presented.

Densification Behaviour and Wear Performance of Brass-Based Composite Reinforced with SiO2 Nanoparticles: Switchable Water Pump Clutch Application

The friction disc and the impeller ring of a switchable water pump (SWP), having a similar principle to that of the wet clutch of an automobile, but it also exhibits a relatively low pressure, a high velocity and temperature. However, SWP possesses an inadequate friction coefficient that has a high tendency to result in the relative slip and severe wear of the counterparts. To acquire such a friction material with an improved friction coefficient and wear resistance to satisfy the new operating condition of SWPs, brass-based composites reinforced by SiO2 nanoparticles were successfully fabricated by the powder sintering in this study, and the influence of the mass fraction of SiO2 nanoparticles on the friction and wear behaviour of the composites was also discussed. The results show that the composite with an appropriate mass friction of SiO2 nanoparticles up to 5 wt% exhibited a homogeneous microstructure. Meanwhile, the friction coefficient was raised to 0.23 with a wear volume reduced to 7.3 × 10− 5 mm3, and the predominant wear mechanism changed from adhesive wear to abrasive wear, due to its enhancement of the densification level and the Vickers micro-hardness.

Energy Management Framework for 5G Ultra-Dense Networks Using Graph Theory

The next-generation 5G networks are being developed with high promised capabilities. Beyond just multitudes faster data speed, 5G is expected to serve billions of connected devices and the Internet of Things (IoT), with the right trade-offs between speed, latency, and energy at an affordable cost. 5G radio networks will strongly depend on using ultra-dense integrated Small Cells (SCs) beside the Macro Cells (MCs). This kind of Ultra-Dense Networks (UDN) consisting of a large number of MCs and SCs will significantly increase network energy demands. A practical method to control energy consumption is by dynamically controlling power-saving modes in radio networks. In this paper, we propose a novel cooperative energy management framework for 5G UDN using graph theory. The 5G network is first modeled as a graph, then graph theory methods are exploited to determine the order of nodes at which power-off/on procedure is applied. We also show that significant power savings are achievable by considering only a subset of network nodes and thus reduce traffic migration and control plane signaling. We evaluated the proposed algorithm at different network densification levels and several load factors including two real-life networks. We also present the convergence of the proposed algorithm and the robustness of networks optimized using it. We also show that power savings up to 25% at full load and 65% during off-peak can be achieved using the proposed algorithm. These power savings increase further if no constraints are imposed on traffic migration and control signaling.

Characterization of Powder Metallurgy High-Entropy Alloys Prepared by Spark Plasma Sintering

In the present work, the High-Entropy Alloys Al0.2Co1.5CrFeNi1.5Ti, Al1.3Co1.4Cr2.0FeNi4.0Ti4.0 and Al6.0Co1.2Cr2.5FeNi3.5Ti6.0 were produced by Mechanical Alloying and subsequent Spark Plasma Sintering processes to obtain properly densified bulks. The characterization of the materials was accomplished through X-Ray Diffraction, Scanning Electron Microscopy, microhardness and nanoindentation tests to identify and analyze the acquired microstructures’ features, phases formed, morphology and size of the grains and its average hardness. The results indicate that it was possible to obtain alloys presenting high values of hardness and multi-phase microstructures. The effect of the multiple phases on the microstructures was discussed in terms of its influence on the mechanical properties. A satisfying densification level of the materials was achieved with the selected parameters.

Neutron Diffraction and Neutron Radiography Investigation into Powder Sintering of Ti/Al and TiH2/Al Compacts

Sintering behaviors of two types of powder compact, i.e., Ti/Al and TiH2/Al, under vacuum were studied using in situ neutron diffraction, in situ thermal dilatometric measurement, and ex situ neutron radiography. Their densification, microstructure, hydrogen concentration, and phase transformation features were compared. The results indicate that the predominant phase identified in the samples sintered from both compacts is γ-TiAl phase with minor Ti3Al phase. Dehydrogenation of TiH2 plays a decisive role in the sintering behavior and the resultant microstructure. In comparison with the Ti/Al compact, dehydrogenation occurring in the TiH2/Al compact leads to a higher level of densification and faster sintering kinetics.

Densification behavior, microstructure evolution and fretting wear performance of in-situ hybrid strengthened Ti-based composite by laser powder-bed fusion

The in-situ TiB and TiC hybrid strengthened Ti-based composites were produced through laser powder-bed fusion (LPBF) of the B4C/Ti composite powder system. The influence of scan speed on the densification response, microstructural evolution, Vickers micro-hardness and fretting wear performance of LPBF-ed B4C/Ti composite parts has been evaluated. It revealed that the densification level of composite part was generally reduced as successively increased the scan speed from 600 mm/s to 1800 mm/s, while the microstructure was inversely refined with an initial enhancement of 459.3 HV0.2 and a subsequent reduction of 361.2 HV0.2. As an appropriate scan speed of 1000 mm/s was used, the composite part exhibited a relatively low friction coefficient of 0.23, and was predominated by the fatigue wear mechanism with a low wear rate of 0.85 × 10−13 m3/m, due to its high densification level and refined strengthen effects of hybrid reinforcements. However, as the scan speed increased to 1800 mm/s, the friction coefficient was measured with apparent fluctuations throughout the sliding process, and the abrasive wear accompanied with a severely adhesive wear was dominated with a high wear rate of 6.88 × 10−13 m3/m, ascribing to its further low densification level and Vickers micro-hardness.

Effect of surface treatment on the dispersion of nano zirconia particles in non-aqueous suspensions for stereolithography

Stereolithography (SLA) requires the creation of stable and well dispersed ceramic suspensions with high solid loading in order to achieve dense ceramic parts. Five different dispersants used were stearic acid (SA), oleic acid (OA), Disperbyk (BYK), coupling agent KH560 and variquat CC 42 NS (CC). The results indicated that BYK, KH560 and OA are all capable of creating low viscosity, well dispersed suspensions with nanozirconia particles; of these 3 wt% BYK was the best option. 3 wt% BYK treated suspensions displayed shear thinning behaviour and had a viscosity of 1680 mPa s at a shear rate of 18.6 s−1. A zirconia part was fabricated using the optimized suspension and its production followed the Beer-Lambert law. After sintering, a significant shrinkage was measured (21.9% in X–Y plane and 28.9% in Z directions), the SEM image showed the sintered part offered a high level of densification.

Laser Powder Deposition Parametric Optimization and Property Development for Ti-6Al-4V Alloy

In this study, Ti6Al4V alloy was produced via laser powder deposition (LPD). To obtain Ti6Al4V alloy with maximum density, LPD parameters for preparing Ti6Al4V samples were optimized using the Taguchi method. Results were analyzed on the basis of the signal-to-noise (S/N) ratios and analyses of variance. A high energy density should be used to achieve higher levels of densification. The optimal combination of parameters for density was a scanning speed of 550 mm/min, laser power of 160 W, powder feeding rate of 0.99 g/min, and shield gas flow of 8 L/min. An almost fully dense Ti6Al4V sample was prepared using the optimized LPD process, and the relative density was greater than 99%. In addition, the microstructure and properties of Ti6Al4V samples prepared by optimized LPD process were investigated. The microstructure investigation revealed that the LPD-prepared Ti6Al4V sample was predominantly composed of fine acicular α phase and lath-type α phase. Tensile and microhardness tests indicated that the LPD sample had higher mechanical properties than the traditional cast Ti6Al4V alloy because of the acicular martensitic phase and smaller grain size.