Stage Of Sintering Research Articles

Microstructure development and strengthening behaviour in hot-extruded Ti-Mo alloys with exceptional strength-ductility balance

A series of strong and ductile Ti-Mo alloys containing 2.5, 5, 7.5 and 10 wt% Mo was experimentally investigated by a two-stage blended-elemental powder metallurgy process. This process consisted of a spark plasma sintering stage for powder-consolidation and in-situ-homogenisation, and a hot extrusion stage for densification and microstructure enhancement. Microstructures in the extruded alloys changed from α dominant to β dominant with molybdenum addition. Simultaneously, a grain refinement effect was observed due to suppressed α precipitation and increase β retention. A significant strengthening effect was observed with molybdenum addition. In comparison with similarly processed commercially pure titanium, Ti-10Mo exhibited a yield strength improvement of 290 % to 1382.8 MPa, and an ultimate tensile strength improvement of 239 % to 1496.5 MPa. Meanwhile, Ti-5Mo exhibited a yield strength improvement of 211 % (1007.9 MPa) without any substantial reduction in ductility, resulting in an exceptional tensile toughness of 361.3 MJ.m−3. A quantitative analysis of strengthening mechanisms reveals considerable strengthening effects from solid solution strengthening, dislocation strengthening, and grain refinement effects. These factors arise from the novel multi-modal microstructures formed by thermomechanical processing at super-transus temperatures. The newly achieved balance of strength and ductility appears to be unrivalled amongst all previously reported binary Ti-Mo alloys.

Effect of Adding W Nanopowder to Inhibit Sintering of Ni Nanopowder for MLCC

Multilayer ceramic capacitors (MLCCs) are essential fundamental components in electronic devices, enabling miniaturization and high capacitance for energy storage and filtering. A key focus in MLCC research is enhancing capacitance density, for which improving the connectivity of Ni internal electrodes is imperative. The primary cause of reduced electrode connectivity in MLCCs is the sintering shrinkage mismatch between Ni internal electrodes and dielectric materials. In this study, we propose a method to mitigate the sintering shrinkage mismatch between the Ni internal electrodes and dielectric materials by incorporating W nanopowder into Ni to retard the sintering shrinkage of the Ni nanopowder. The degree of sintering retardation due to the increase in W content was quantified through measurements of shrinkage rate, density, and porosity, confirming the retarded sintering of Ni. Particularly, microstructural analysis and phase analysis were conducted to elucidate the role of W during the dissolution and sintering processes. Furthermore, the activation energy of each sintering stage was determined, with an analysis of the sintering retardation mechanism induced by W. Additionally, similar sheet resistance values before and after the addition of W were obtained through electrical resistance measurements, and results suggested that the incorporation of W effectively retards the sintering of Ni while enabling its function as an internal electrode in MLCCs.

Structural defect and activated alumina of spheric α-Al2O3 improving alumina ceramics density from the industrial coarse gibbsite

The quality of alumina ceramics generally depends on alumina powder. High-density alumina ceramics were prepared from industrial coarse gibbsite. Near-spheric α-Al2O3 with particles smaller than 500 nm was produced via the synergy of B and F from NH4BF4. The near-spheric α-Al2O3 calcined at low temperature generated alumina ceramics with high density and low porosity. Meanwhile, the wet pretreatment yielded smaller particles with structural distortion and high microstrain and aluminum hydroxide, notably increasing the relative density of alumina ceramics. 99.18% relative density, 0.017% porosity, and 27.54 GPa Vickers hardness of alumina ceramics were achieved at 1650°C for 2 h without additives and pressed sintering. In addition, the early stage of sintering of alumina green body followed particle boundary diffusion with reduced activation energy in the sintering process. The close packing of the spheric α-Al2O3, high microstrain of α-Al2O3, structural defects with considerable AlO4 units calcined at low temperature, structural distortion in the alumina, and activated alumina on the α-Al2O3 surface collectively promoted the densification of alumina ceramics, achieving remarkable hardness, high density, and tiny porosity.

In-situ ash sintering process analysis during the co-combustion of the coal gasification fine slag and corn straw

The co-combustion of coal gasification fine slag (CGFS) and biomass is proposed to enhance the poor combustion characteristic of CGFS and alleviate the ash-related issues of biomass. In this study, the sintering mechanism of CGFS ash (FA) and corn straw ash (SA) was revealed by in-situ image analysis and thermodynamic modelling. The sintering process of FA includes initial sintering stage (S1), holding stage (S2), and primary melting stage (S3). The S1 is initiated by the flow of the low-viscosity slag, whereas the subsequent S2 and S3 stages are predominantly governed by the crystallization and then the melting of anorthite with increasing temperatures, respectively. The AAEMs in SA loosens the structure of FA, thereby facilitating the onset of S1 at a low temperature. Concurrently, the anorthite is gradually converted to diopside, which possesses a lower melting point, and subsequently into sanidine, which exhibits a higher melting point. This transformation extends the temperature range across S1 and S2, leading to an initial decrease followed by an increase of ash fusion temperatures with the SA addition. The dry bottom boiler suits for the co-combustion of CGFS and corn straw and the boiler exit temperature is suggested to be higher than 968 °C.

Характер матрикса — индикатор температурных условий формирования зювитов (на примере обломочных импактитов Карского метеоритного кратера)

The paper proposes a new approach for the study of suevites formation features — the analysis of temperature conditions of clastic impactites formation based on the analysis of local matrix sintering using materials science methods. Structural and textural features and substance composition of the suevite matrix of the bottom and ejecta facies of the Kara astrobleme are studied. The petrochemical composition of the clastic impactite matrix reflects the composition of the target rocks, it can be complicated with the suevites post-impact altering. Quartz and albite clusters in the matrix have signs of different sintering stages, indicating a strong variety in local temperature conditions of the suevites formation. In this regard, it makes no sense to conclude about a certain temperature of rock formation as a whole. During lithification the suevites of aerodynamic facies the local temperature could reach 1200 °C. The bottom facies had relatively higher temperature formation where the maximum temperatures locally reached 1700 °C. The sintering nature of the suevite matrix is an indicator of the specifics of the formation conditions and can be used to diagnose facies of detrital impactites and to clarify the parameters and scenario of impact events.

A modified pore evolution particle model applied to CaCO3 decomposition and CaO sintering during calcium looping CO2 cycles

The calcium looping of calcium-based carbonation/calcination cyclic reaction is an efficient decarbonization technology that can be applied to post-combustion carbon capture, solar thermal storage, and enhanced methane reforming hydrogen. The pore structure generated during the calcination stage greatly affects adsorption performance. Hence, the change of pore structure is crucial during the calcination. Existing models related to pore structure still need to be further improved regarding pore formation mechanisms. In this paper, a modified pore evolution model is proposed. The modified pore evolution model contains the kinetic equation, the CO2 diffusion equation, and the vacancy aggregates equation. The modified model considers the possibility of spontaneous pore formation in the mechanism, which can better fit the experimental results and provide higher simulation accuracy. The model reveals the diffusion of CO2 within the particles during decomposition, and spatiotemporal evolutionary properties of pore parameters. The pore change trend is the most obvious when the initial pores are mainly 3 nm mesopores. The pore evolution is relatively stable when the initial pores are mainly 25 nm mesopores. Besides, the particles experience significant pore migration during the sintering stage when the temperature reaches 1273 K. At 1073 K, the maximum pore size occurring during pore evolution did not exceed 40 nm. The model estimates a maximum pore size of more than 60 nm at 1273 K. The modified model can aid in a deeper comprehension of how the pore structure changes throughout the calcination process.

Influence of stabilized zirconium dioxide and high hydrostatic pressure on the kinetics of sintering nanopowders of metastable aluminum oxide

The paper presents an analysis of the effect of processing powders with high hydrostatic pressure (HHP) (300, 500, 700 MPa) and stabilized zirconium dioxide (ZrO2 + 3 mol.% Y2O3, (YSZ)) doping on the compaction of metastable nanopowders mixture with γ+θ-Al2O3 + n% YSZ (n = 0, 1, 5, 10, 15 wt%) composition during sintering. The behavior of nanopowders at the initial stage of sintering was studied by dilatometry at a constant heating rate of 5 °C/min, in temperature range from 20 °C to 1500 °C. It has been determined that sintering of compacts occurs in two stages: compaction of γ + θ-Al2O3 (Ⅰ-st stage metastable phases), compaction of the stable α-Al2O3 (ⅠⅠ-nd stage), accompanied by inhibition between them. An “effect of mutual protection against crystallization” of powder mixtures was determined using the X-ray diffraction method. This is characteristic of systems obtained by co-precipitation. A connection between the above mentioned inhibition and an increase in the YSZ additive was found. That, also, correlates to the effect of “mutual protection against crystallization” of Al2O3 – YSZ powder mixtures. A study of the mechanisms and activation energies of γ+θ-Al2O3 + n% YSZ systems’ (n = 0, 1, 5, 10, 15 wt%) sintering depending on the YSZ additive amount and the HHP value showed that a dopant and pressure increase leads to activation energy decrease of the initial sintering stage, and the prevailing sintering mechanism is volumetric diffusion in corundum grains. Also, the work shows that for γ+θ-Al2O3 + ≥5 wt% YSZ systems, the optimal compaction pressure is HHP ≥500 MPa, which corresponds to the transition of the sintering mechanism from grain-boundary diffusion to volumetric diffusion.

FeSiBCCr bulk metallic glasses with excellent soft magnetic properties prepared using spark plasma sintering technology

Fe-based amorphous alloys have received much attention in the fields of electronics and electrical due to their excellent soft magnetic properties at high frequency. However, the controllable preparation of large-sized Fe-based bulk metallic glasses (BMGs) still faces numerous challenges. In this study, FeSiBCCr amorphous powders prepared by gas-water combined atomization were used as raw materials, and FeSiBCCr BMGs were prepared through spark plasma sintering (SPS) at different pressures. The microstructure, phase and magnetic properties of the compacts were systematically characterized using SEM, XRD, TEM, DSC, and VSM. The results showed that when the sintering pressure was 50 MPa, the slow densification process dominated by particle rearrangement at the initial densification process made the internal contact resistance of the compacts larger. This led to macroscopic inhomogeneous physical fields and severe local overheating during the later stages of sintering, causing the precipitation of α-(Fe, Si) and Fe3B phases. Although the material exhibited a relatively high saturation magnetization (Bs) of 1.08 ± 0.01 T, the low density and amorphous fraction resulted in a high coercivity (Hc) of 1060.8 ± 37.8 A·m-1. When the sintering pressure was increased to 250 MPa, the degree of particle densification significantly improved, and the temperature inhomogeneity was markedly suppressed due to reduced internal resistance. Thus, the prepared FeSiBCCr BMGs exhibited an extremely high density of 6.66 ± 0.06 g·cm-3 and excellent soft magnetic properties with Bs of 1.23 ± 0.01 T and Hc of 18.8 ± 1.8 A·m-1. This indicated that the FeSiBCCr BMGs prepared in this study exhibited broad application prospects in efficient electromagnetic conversion and advanced electronic devices.

Effect of laser speed on neck formation in SS316L powder joining for LPBF

The objective of this study is to comprehend how changes in laser speed affect the behaviour and features of neck formation in SS316L during powder bed fusion. This study examines the impact of various laser speeds on the neck formation in the commonly used SS316L. Uniweld Laser 3000 was used for laser joining. A constant laser power of 15 W and laser scan speed of 25–150 mm/s were implemented, and powder joining within a single track was observed. Results show that neck size decreases with increasing laser scanning speed and no particle joining occurs at 150 mm/s scan speed, as it is considered too fast for sufficient heat absorption and melting. As laser speed decreases to 125 mm/s, neck starts to form because the particles absorb more heat, which facilitates melting and bonding. Surface diffusion occurs early at this stage because it requires only a small amount of energy to break atomic bonds on the surfaces of particles compared to those in the internal part. This process involves atoms moving along the surfaces of the particles, facilitating the initial stages of sintering by reducing surface energy and causing neck formation between particles. At 125 mm/s, the particle shape remains spherical, and the average neck size is 49 μm. At 100 μm, the powder average neck size is 62 μm. Moreover, the powder completely merges at 50 mm/s and 25 mm/s. The neck formed between particles considerably improves the product's mechanical strength by strengthening bonds between particles. Material density increases with neck quality, and stable neck growth can reduce the porosity of the final parts. Ensuring thermal stability in the laser powder bed fusion allows consistent heat to particles, ensuring even temperature in materials and ultimately facilitating stable neck growth. Thermal stability enables atoms in particles to migrate efficiently and prevents defects, such as bubbling, which increases porosity in printed parts. Therefore, a slow scanning speed allows particles to absorb more heat for melting, increasing the neck size of particles. This knowledge is useful for optimizing laser processing parameters in various industrial applications.

Full Density Powder Metallurgical Cold Work Tool Steel through Nitrogen Sintering and Capsule-Free Hot Isostatic Pressing

Vanadis 4E (V4E) is a powder metallurgical cold work tool steel predominantly used in application with demand for wear resistance, high hardness, and toughness. It is of interest to have a processing route that enables full density starting from clean gas-atomized powder allowing component shaping capabilities. This study presents a process involving freeze granulation of powder to facilitate compaction by means of cold isostatic pressing, followed by sintering to allow for capsule-free hot isostatic pressing (HIP) and subsequent heat treatments of fully densified specimens. The sintering stage has been studied in particular, and it is shown how sintering in pure nitrogen at 1150 °C results in predominantly closed porosity, while sintering at 1200 °C gives near full density. Microstructural investigation shows that vanadium-rich carbonitride (MX) is formed as a result of the nitrogen uptake during sintering, with coarser appearance for the higher temperature. Nearly complete densification, approximately 7.80 ± 0.01 g/cm3, was achieved after sintering at 1200 °C, and after sintering at 1150 °C, followed by capsule-free HIP, hardening, and tempering. Irrespective of processing once the MX is formed, the nitrogen is locked into this phase and the austenite is stabilised, which means any tempering tends to result in a mixture of austenite and tempered martensite, the former being predominate during the sequential tempering, whereas martensite formation during cooling from austenitization temperatures becomes limited.

Rapid densification of nanocrystalline zirconia: Pressureless versus pressure-assisted spark plasma sintering

Spark plasma sintering (SPS) is the most straightforward way to rapidly sinter nanoceramics, but the applied pressure prevents sintering of additively manufactured ceramics. Therefore, fast firing techniques such as “pressureless” SPS and ultra-fast high-temperature sintering, based on intense thermal radiation, are gaining interest. Here we compare pressure/current-assisted and pressureless SPS techniques for the rapid heating (∼300 °C/min, 5 min) of nanocrystalline zirconia with high sintering activity. The applied pressure and current indeed contributed to the lowestr temperatures needed for full densification of nanocrystalline zirconia, retaining very fine grain size, but also induced tetragonal phase transformations in the final sintering stages. When the radiative heat transfer was “decoupled” (pressureless SPS), a pronounced temperature difference between graphite crucible wall and simulated specimen temperature along with non-steady-state conditions during dwell were observed. Nevertheless, high heating rates facilitated fine and dense microstructures even in the absence of pressure/current.

Few-layered graphene/Al-Cu alloy matrix composites: Mechanical, tribological and corrosion properties

Nano-sized graphene incorporated Al-5.5 wt% Cu alloy matrix composites were produced via powder metallurgy. Nano-sized graphene powders obtained via the arc-discharge method were incorporated into Al and Cu powders in varying amounts (0–5 wt%) by applying a high-energy ball milling (HEBM) process for different durations. The consolidated composites were prepared by the succeeding uni-axial pre-compaction and pressureless sintering stages. The as-blended and mechanically alloyed (MAed) powders and bulk composites were comparatively characterized in terms of their physical, thermal, microstructural, tribological, mechanical, and corrosion properties depending on the graphene amount and the duration of mechanical alloying (MA). After 4 h of MA, the starting as-blended powders presented as coarse and discrete particles gained a refined and homogenized microstructure with more equiaxed dimensions. Bulk FLG/Al-5.5Cu (FLG in amounts of 0, 0.5, 1, 2, and 5 wt%) composites are ever-increasing hardness values with rising graphene as 109, 101, 128, 238, and 263 HV, respectively. The compressive strength improved gradually to 495 MPa using graphene up to 1 wt%, though contrary to the high hardness values, a drastic decline was observed by 5 wt% graphene incorporation. Besides, the wear resistance of composites outperformed that of the Al-5.5Cu matrix by incorporating this amount of reinforcement together without a deterioration in the corrosion resistance.

W–CeO2 Core–Shell Powders and Macroscopic Migration of the Shell via Viscous Flow during the Initial Sintering Stage

To retard the mutual contact of W grains to inhibit their growth, in this study, CeO2·2H2O was first coated on the surface of pure W (undoped) particles by a weight percentage of 4% using a wet chemical method to prepare CeO2·2H2O-doped W-based (doped) powders, with W particles as the core and CeO2·2H2O as the shell (W–CeO2·2H2O core–shell structure), without hydrogen reduction treatment. The undoped and doped powders were subsequently sintered using a spark plasma sintering (SPS) apparatus to fabricate bulk materials. The macroscopic migration of the CeO2 shell in the core–shell W–CeO2 system via viscous flow during the initial sintering stage was studied through simulations and experiments. The results showed that a core–shell structure with W particles as the core and CeO2·2H2O as the shell was successfully prepared. The doped powder contained approximately 3.97% CeO2, consistent with the designed content of 4%. The shell materials migrated among the selected four sintered powders, filling the pores and contributing to the improvement in the relative density of the sintered bulk.

Evolution of electrical resistance of graphite foils during spark plasma sintering

The progression of the electrical resistance of graphite foils during spark plasma sintering process (SPS) was investigated at constant temperature and pressure. The study applied various set-ups of the SPS device, and the electrical data used for the evaluation of electrical resistance (heating power and current) was obtained from the SPS apparatus in real-time. The contact resistance and resistance due to graphite foil/s was evaluated by subtracting the resistance of the single punch set-up from the set-up of two punches in direct contact and the set-ups with various graphite foils. The results showed that during the initial stages of sintering, set-up resistance increases with time and that, overall, set-up resistance increases with number of graphite foils. Both contact resistance and resistance due to graphite foils was found to decrease with sintering time. In contrast to previous conceptions, the electrical resistance of graphite foils changes in response to sintering conditions during the SPS process.

Sintering Mechanism of Pure B4C Ceramic Prepared by Hot Oscillating Pressing and with Excellent Mechanical Properties

Pure boron carbide (B4C) ceramics are sintered by hot oscillating pressing at temperatures of 1700–1900 °C for different soaking times. The densification mechanism, grain growth kinetics, and mechanical properties obtained under these sintering conditions are systematically investigated. It is found that the fully densified B4C ceramic is obtained at 1900 °C within 120 min, possessing an excellent combination of hardness (≈38.5 GPa) and fracture toughness (4.8 MPa m1/2). The densification mechanisms during sintering are as follows: viscous flow at the initial stage (relative density D ranged from 55% to 65%) under 1700 °C; lattice diffusion/grain boundary diffusion at the sintering stage (62% < < 75%) of 1800 and 1850 °C; grain boundary sliding by dislocation motion at the late stage (65% < < 82%) of 1700 °C and the sintering stage (75% < D < 90%) at 1800–1900 °C; and dislocation gliding and stacking faults at the final stage (90% < D < 95%) occurring at 1850–1900 °C. The dislocation motion exhibits a critical impact on the densification process of the hot oscillating–pressed B4C ceramic. The grain growth kinetics at 1900 °C is controlled by grain boundary diffusion.

Manufacturing and performances of MnZn ferrite cores with thin walls prepared by paste material extrusion 3D printing

The 3D printing technique using material extrusion of a paste filled with ferrite powder (Mn,Zn)Fe2O4 and organic additives has been successfully improved to produce sintered samples for studying their magnetic and structural properties. The rheological behaviour of the pastes and the printing parameters have been optimised to obtain parts with regular geometries, in particular by minimising sagging effect during filament deposition. Printing is followed by a debinding and sintering stage, for which the temperature and oxygen partial pressure conditions were adjusted. These conditions enable the densification of the (Mn,Zn)Fe2O4 material in the spinel structure without parasitic phases, with a low grain size dispersion and a relative density of 91 %. The core loss has been measured on the 3D printed samples to be 144 mW/cm3 at a frequency of 500 kHz and an induction of 50 mT. This performance is comparable to the nominal value of a part produced by the conventional process. This work paves the way for an advantageous manufacturing method in terms of accessible geometries, while maintaining satisfactory magnetic characteristics.

Densification mechanism and microstructural evolution of Si3N4 composites with CeO2 as additives during spark plasma sintering

The densification mechanism of CeO2 doped Si3N4 ceramics during spark plasma sintering (SPS) was investigated. The sintering process was observed to undergo significant densification at the intermediate stage of sintering (1435 °C–1816 °C), and the density and grain size of CeO2 doped Si3N4 ceramic were found to be greater than those of pure Si3N4 ceramic under the same sintering conditions. To determine the densification mechanism, a creep model was utilized to determine the densification mechanism which could be interpreted based on the values of the stress exponent (n) and the apparent activation energy (Q d). The results revealed that the Q d of pure Si3N4 (381.15 kJ mol−1) is higher than that of Si3N4-CeO2 powder (265.61 kJ mol−1). Moreover, the stress exponent n of Si3N4-CeO2 powder (1.12–1.33) was higher than that of pure Si3N4 powder (1.00–1.18). This can be attributed to the fact that the addition of CeO2 resulted in the formation of a liquid phase at the grain boundary which leading to a shift in the controlling mechanism from grain boundary diffusion to grain boundary sliding.

Finite element simulation for Si3N4 heating and sintering

The fabrication of large silicon nitride ceramic components is intricate and demands expertise in gas pressure liquid phase sintering (GPS-LPS). The forefront technology of finite element (FEM) thermo-mechanical modeling plays a key role in sizing aerospace components that necessitate high precision and mechanical strength. A comprehensive sintering simulation should encompass not only densification and grain growth models but also consider the heating environment, accounting for gas convection, conduction, and surface-to-surface radiation. Moreover, silicon nitride sintering simulation addresses the intricate behavior, including the swelling phenomenon during the transitions from intermediate to final stage sintering. The challenge of significant slumping in hollow parts, resulting in costly rejection, is a primary concern for industrial companies when dealing with low-viscosity liquid phase sintering. Validation of the model is achieved through the computation of simple cylindrical cases, while a meticulous comparison between simulations and sintering experiments on complex hollow bodies and deflection bars facilitates precise adjustment of the shear viscosity theoretical parameter derived from the Skorohod-Olevsky continuum theory of sintering. The findings from these simple cases guide the simulation of a complex geometry representative of typical aerospace components, determining the ideal sintering configuration to mitigate slumping issues.

In situ observation of the multistep process of cold sintering

AbstractA compact cold sintering stage was constructed to densify ceramic samples in capillary tubes for the purpose of conducting in situ experiments designed to elucidate fundamental cold sintering mechanisms. The stage was used to successfully densify samples composed of ZnO/CH₃COOH mixtures under a range of pressures (25–175 MPa) over a modest temperature range (room to 150°C) in capillary tubes (0.75 mm inner diameter). The progression of the cold sintering process of ZnO was monitored by both piston displacement and a camera. These measurements allowed the strain rate and fluid extraction during densification to be recorded. The results of these measurements are compared to existing models of cold sintering and pressure solution creep, and direct correlations are observed between abrupt changes to the densification rate during constant heating rate experiments and the observation of solvent extraction from the powder compact.

Nano-borosilicate glass-silica ceramic material jetting technology for additive manufacturing of multilayer LTCC substrates

Although one of the simplest methods, various must be resolved before industrializing material jetting technology to fabricate low-temperature co-fired ceramics (LTCC). This research examines related glass-ceramic ink configurations and multi-material jet-sintering. Firstly, we present a method for obtaining a nanoscale borosilicate glass powder via radio frequency plasma spheronization, and its surface is grafted with acrylate molecules to create a jettable nanosol. By fusing this configured glass nanosol with silica nanosol in the typical glass-ceramic LTCC combination (4:6) and adding suitable photosensitizing elements, it can be configured into light-curable LTCC inks. 3D printing of multilayer structures using a multi-material jetting device provides a three-layer structure of ceramic layer - conductive layer - ceramic layer, with the ceramic layer cured by ultraviolet light and the conductor pattern scanned and surface-dried by a laser to obtain high-precision delivery of the green part. The sintering effect between different materials is intact and synchronized shrinkage is achieved. Conductivity tests during the sintering stage yielded high conductivity values up to 1.43 × 107 S/m, which further provides the microscopic morphology of the two embedded wires, and the complete and continuous results demonstrate the effectiveness of multilayer wire fabrication. The nanopowder, sol-gel configuration and material jetting under multiple energy loading proposed in this paper significantly support the LTCC additive manufacturing route.