Field Assisted Sintering Technology Research Articles

Influence of grain size on the electrochemical performance of Li7‑3xLa3Zr2AlxO12 solid electrolyte

Contemporary Li‐ion batteries are facing substantial challenges like safety and limited energy density. The development of all‐solid‐state battery cells mitigates safety hazards and allows the use of Li‐metal anodes increasing energy density. Garnet‐type solid electrolytes can be vital to achieving an all‐solid‐state cell and an understanding of the influence of its microstructure on the electrochemical performance is crucial for material and cell design. In this work the influence of grain size on the Li‐ion conductivity of Li7‐3xLa3Zr2AlxO12 (x = 0.22) is presented. The synthesis and processing procedure allows changing the ceramic grain size, while maintaining the same synthesis parameters, eliminating influences of the synthesis on grain boundary composition. Field assisted sintering technology is a powerful method to obtain dense, fine‐grained ceramics with an optimal grain size of 2‐3 µm, where the conductivity is double that of the counterpart (0.7 µm). A total Li‐ion conductivity of 0.43 mS cm‑1 and an activation energy of 0.36 eV were achieved. The oxide‐based all‐solid‐state battery cell combining the garnet‐type electrolyte, a Li‐metal anode and a thin‐film LiCoO2 cathode was assembled and cycled at room temperature for 90 hours. This represents a proof of concept, for the application of oxide‐based electrolytes at ambient temperatures.

Study of an iron phosphate soft magnetic composite and its stability at high temperatures

Most studies on soft magnetic composites (SMC) have focused on traditional compaction. However, this consolidation method leads to low densities and therefore, poor magnetic properties. For this reason, this work focuses on the study of an iron phosphate (Fe3(PO4)2) coating to develop SMCs consolidated through a novel field-assisted sintering technology (FAST) and compares it with traditional compaction. A direct relationship is demonstrated between the thickness of the coating obtained and the amount of reagent added to the synthesis as well as the inverse relationship between the thickness of the coating and the powder/acetone ratio. By contrast, thermal analysis shows that the desired phase Fe3(PO4)2 is stable up to 900 °C. Between 900 and 960 °C, iron phosphate decomposes into Fe–P and SiO2. At higher temperatures, the decomposition process is completed, and the Fe–P phase reacts with Fe–3Si particles, resulting in an FeSiP matrix with a non-continuous layer of surrounding SiO2. The highest compact density and static magnetic properties are obtained for the SMC prepared with 2 wt% H3PO4 and a powder/acetone ratio of 2.5 g/mL consolidated by FAST, providing a density of 6.59 g/cm3, a magnetic saturation of 1.752 T and a coercivity of 313.4 A/m. In addition, this SMC results in the highest permeability in the entire audio-frequency range (40 up to 100 kHz) as well as high electrical resistivity (3.18·105 μΩ cm) and the lowest power losses up to 10–20 kHz (31.7 mW/cm3 at B = 50 mT and f = 10 kHz). However, cold-press consolidation leads to higher operating frequencies, reaching a permeability of 24 up to 1 MHz, and the lowest power losses for frequencies greater than 10–20 kHz (414 mW/cm3 at 100 kHz and B = 50 mT) due to the reduction of eddy currents obtained by the higher electrical resistivity and lower number of coating defects.

Annealing effect on deuterium retention in W-Cr-Y alloy

Deuterium (D) trapping in the W-11.4Cr-0.6Y alloy manufactured by field assisted sintering technology (FAST) and pre-annealed at temperatures of 1050, 1250, 1473 K was studied by in-situ thermal desorption spectroscopy (TDS). D incorporation in W-11.4Cr-0.6Y alloy was carried out by irradiation with deuterium ions (670 eV/D+) to a fluence of 5 × 1019 D/m2. The change in Cr concentration in the bulk of the W-Cr-Y alloy was studied using energy dispersive spectroscopy (EDS). The elemental composition on the surface of alloy samples before and after annealing was investigated using low energy ion scattering spectroscopy (LEIS). The Cr concentration increases on the surface of alloy sample during annealing at 1000 and 1050 K due to intensive Cr segregation, while it decreases at temperatures of 1250 K and above by dominant Cr sublimation. Pre-annealing of W-Cr-Y alloy at 1050 K leads to a decrease in the D trapping in the alloy which is connected with an increase in the Cr concentration on the surface. At high annealing temperatures of 1250 K and above, the appearance of new phases and decomposition-induced grain refinement leads to new types of defects with high binding energy for D.

Effect of the heating rate and Y2O3 coating on the microstructure of Wf/Y2O3/W composites via field assisted sintering technology

Field assisted sintering technology (FAST) is one of the potential methods to fabricate tungsten fiber reinforced tungsten (Wf/W) composites. The microstructure and mechanical properties of Wf/W composites are closely related to the sintering parameters. In the present work, the Wf/W composites with a Y2O3 coating on the fiber (Wf/Y2O3/W) were fabricated via a FAST process and the microstructure was characterized. The influence of the heating rate and Y2O3 coating on the microstructure of Wf/Y2O3/W composites was discussed in detail. It is observed that the Y2O3 coating can get damaged and move into the adjacent matrix. A higher heating rate (100 ℃/min) causes more serious damage on the Y2O3 coating which could be attributed to the higher electric current intensity. The samples produced with a lower heating rate (50 ℃/min) or with a thicker Y2O3 coating (3 µm) show an interface with better continuity. In addition, the integrity of the Y2O3 coating could by protected by an additional W-coating produced by chemical vapor deposition (CVD).

Assessing the mechanical properties of out-of-specification additive manufacturing Ti-6Al-4V powder recycled through field-assisted sintering technology (FAST)

A gas atomised Ti-6Al-4V powder, classified as out-of-specification for additive manufacturing (AM), was consolidated via Field-Assisted Sintering Technology (FAST). Fully dense 250 mm diameter discs with lamellar or bimodal microstructures were produced by FAST processing either above or below the β-transus temperature. Static and dynamic mechanical properties were assessed by testing full-size specimens in the ‘as-FAST’ condition. Material from both processing conditions exceeded the ASTM Ti-6Al-4V powder metallurgy requirements for yield/tensile strength and elongation. Furthermore, material from the edge of the disc processed below the β-transus temperature meets ASTM requirements for wrought Ti-6Al-4V. Fatigue performance also compared favourably with conventionally processed Ti-6Al-4V. This work establishes that surplus AM powders can be successfully recycled via the one-step FAST process and provides confidence that this ASTM-grade material can be used in a range of applications under both static and dynamic loading, which will improve the sustainability credentials of the AM sector.

Powder production, FAST processing and properties of a Nb-silicide based alloy for high temperature aerospace applications

A Nb-silicide based alloy with nominal composition Nb–18Ti–22Si–6Mo-1.5Cr–2Sn-1Hf (at. %), designed for high temperature aerospace applications, was produced via a powder metallurgy (PM) route. The raw elements were arc melted, crushed, and milled to powder, then consolidated using Field Assisted Sintering Technology (FAST). The compressive creep of the alloy was evaluated using electro-thermal mechanical testing (ETMT). The study demonstrated the production of larger 60 mm diameter samples, with potential for further scale up. The microstructure of the FAST alloy, which is comprised of bcc Nbss and tetragonal αNb5Si3 was more homogenous compared with the cast alloy, with some interstitial contamination that occurred during powder production. The FAST alloy had lower density than state of the art Ni-based superalloys and refractory metal complex concentrated alloys (RCCAs) and high entropy alloys (RHEAs), and its yield strength and specific yield strength was higher than those of the latter metallic Ultra high temperature materials (UHTMs) and comparable to those of Nb-silicide based alloys with B addition. The stress exponent n in compressive creep was in the range 1.7–2.6, similar to that of binary Nb–10Si and Nb–16Si alloys and its creep rate at 1200 °C and 100 MPa was similar to that of the MASC alloy (Nb–25Ti–16Si-8Hf-2Al–2Cr (at.%)). Like the latter, the creep of the FAST alloy did not meet the creep goal.

Channelling electric current during the field-assisted sintering technique (FAST) to control microstructural evolution in Ti-6Al-4V

Perhaps the most defining feature of field-assisted sintering technology (FAST) is the application of an electric current, in addition to the uniaxial pressure, to create resistive heating in and around the sample region. However, with a few exceptions, most research takes this as an unchangeable part of the process. Here, this current flow has been directed to specific regions within the toolset, using boron nitride as electrically insulating material. This caused the heating to occur in differing regions within the Ti-6Al-4V sample and mould over four insulating configurations, with the shift in current density resulting in an extreme disparity in the final microstructures. The samples were imaged and analysed with deep learning in MIPAR, alongside comparisons with finite element analysis (FEA) models for 20 s and 5 min dwell times, to provide the technique with predictive capabilities for grain size and microstructure. The results gathered imply significant potential for this concept to improve the flexibility of FAST, and reduce negative effects such as undesirable temperature profiles in size scaling sintering for industry.

A Machinability Assessment of the Novel Application of Field-Assisted Sintering Technology to Diffusion Bond (FAST-DB) and Functionally Grade Dissimilar Nickel-Based Superalloys

This work presents an alternative processing route to the conventional powder HIP—forge route for Nickel-based superalloys. Demonstrating how the field-assisted sintering technology (FAST) process can be exploited to successfully diffusion bond or functionally grade two or more Nickel-based superalloys from powder feedstock. The robustness of the process has been further demonstrated by the successful bonding of one alloy in powder form to another in the solid form. Chemical and microstructural analysis of the diffusion bond between the alloys is characterised, in both cases, with a short diffusion zone—in agreement with thermodynamic model predictions. A gradual transition in microhardness across the bond region was measured in all samples. A machinability assessment was also carried out through a simple face turning operation. Analysis of the cutting forces and machined surface shows signs of a directionality when machining across the bond region between two alloys, indicating that care must be taken when machining multi-alloy FAST-DB components.

CIP-FAST: assessing the production of complex geometry titanium components from powders by combining Cold Isostatic Pressing (CIP) and Field Assisted Sintering Technology (FAST)

ABSTRACT A novel, two-step, solid-state method to produce complex geometry titanium parts was investigated by combining Cold Isostatic Pressing (CIP) with Field Assisted Sintering Technology (FAST). Hydride-dehydride powders of commercially pure titanium and Ti-6Al-4V were CIP’ed into shaped compacts using silicone moulds, then further consolidated using FAST, with ZrO2 powder as a secondary pressing media. The final parts retained the complex features from the CIP moulds but were compressed in the pressing axis. Densities >99% were achieved, with optimised FAST processing parameters required for the different alloys. High hardness and fine equiaxed microstructures were observed at the edges of the parts, suggesting oxygen transfer from the ZrO2 pressing media had occurred, with more investigation needed to better understand and prevent this. Despite this, the CIP-FAST process route has been demonstrated to be a fast, low-cost and material-efficient option to produce a wide variety of complex titanium parts.

Microstructure and mechanical properties of Wf/W composites influenced by Y2O3 coating

Tungsten fiber reinforced tungsten (Wf/W) composite is one of the candidates to improve the toughness of tungsten materials. Powder metallurgy is a potential method to produce Wf/W composites, but it may cause the recrystallization and grain growth of the reinforcing fibers during the high temperature sintering process. In the present work, a layer of Y2O3 is coated on the fiber surface by magnetron sputtering to protect the fibers and prevent recrystallization and abnormal growth of grains. Wf/W composites with and without Y2O3 coating were fabricated by a field assisted sintering technology (FAST) process. Microstructure and mechanical properties were characterized. The influence of the Y2O3 coating on the properties of the fiber was discussed in detail. The Y2O3 coating can effectively prevent recrystallization and abnormal grain growth of fibers during the sintering process. The Wf/W composites with Y2O3 coating shows higher strength and pseudo-ductile behavior.

Mechanical properties of molybdenum containing MoO2 and HfO2 processed by field assisted sintering technology (FAST): Characterization and modeling

Molybdenum and its alloys are of interest for applications with extreme thermomechanical requirements such as nuclear energy systems, electronics, aerospace vehicles, and hypersonic vehicles. In the present study, pure molybdenum and samples with added hafnium carbide (HfC) grain refiners were produced using field assisted sintering technology (FAST). The molybdenum and HfC reacted with oxygen to produce MoO2 and HfO2, and increased HfC content from 1 wt% to 5 wt% decreased grain size while the microhardness correspondingly increased. Room temperature three-point bending tests were conducted, and finite element modeling was used to define HfC-dependent bilinear material models. The presence of oxygen most severely affected pure molybdenum, which exhibited little strength and limited ductility, whereas for samples with added HfC, HfO2 was present, resulting in increased toughness hypothesized to be due to microcrack toughening. The samples with 1 wt% added HfC had the greatest energy absorption capability.

Residual Stress Distributions in Dissimilar Titanium Alloy Diffusion Bonds Produced From Powder Using Field-Assisted Sintering Technology (FAST-DB)

The conventional approach when engineering components manufactured from titanium is to design the thermomechanical processing to develop an optimal microstructure in a single alloy. However, this conventional approach can lead to unnecessary over-engineering of components, particularly when only a specific subcomponent region is under demanding service stresses and environments. One approach being developed to join multiple alloys in a single component and enhance engineering performance and efficiency is FAST-DB—whereby multiple alloys in powder form are diffusion bonded (DB) using field-assisted sintering technology (FAST). But the joining of multiple alloys using conventional welding and joining techniques can generate high residual stress in the bond region that can affect the mechanical performance of the components. In this study, the residual stress distribution across dissimilar titanium alloy diffusion bonds, processed from powder using FAST, were measured using X-Ray diffraction and the Contour method. The measurements show low residual stress in the bulk material processed with FAST as well as in the diffusion bond region. In addition, FAST-DB preforms subsequently hot forged into different near-net shapes were also analyzed to understand how the residual stress in the bond region is affected by a subsequent processing. Overall, no sharp transitions in residual stress was observed between the dissimilar alloys. This study reinforces confidence in the solid-state FAST process for manufacturing next generation components from multiple titanium alloy powders.

Application of U-FAST Technology in Sintering of Submicron WC-Co Carbides

This article presents the microstructure, hardness, fracture toughness coefficient KIC and phase composition of submicron WC-4Co carbides. The carbides were sintered using the innovative U-FAST (Upgraded Field Assisted Sintering Technology) method, from mixtures of WC-Co powders with an average WC grain size of 0.4 µm and 0.8 µm. The obtained sinters were characterized by a relative density above 99% of the theoretical density. The hardness of the obtained composites was above 2000 HV30, while the KIC coefficient was about 8 MPa m1/2.

Deuterium retention in W-Cr-Y alloy: Impact of the manufacturing method and helium presence

The retention of deuterium in tungsten-chromium-yttrium alloys, W-11.4Cr-0.6Y and W-10Cr-0.5Y, was investigated by means of in-situ thermal desorption spectroscopy (TDS). The first alloy was manufactured by field-assisted sintering technology (FAST), while W-10Cr-0.5Y alloy was produced by hot isostatic pressing followed by heat treatment (HIP+HT). Both alloys were irradiated with D3+ ions (670 eV/D) and a fluence of 1021 D/m2 at temperatures of 300, 600, 900 K. In the case of W-11.4Cr-0.6Y alloy, deuterium retention was investigated during sequential irradiation with helium (3 keV) and deuterium (670 eV/D) ions at room temperature with fluences of 1019–7 × 1022 He/m2 and 1021 D/m2, respectively.The material structure has a great influence on deuterium retention, as evidenced by the significantly increased deuterium trapping in W-11.4Cr-0.6Y alloy at both room and elevated irradiation temperatures due to the presence of high-energy trapping centers compared to W-10Cr-0.5Y alloy. In both alloys, the observed amount of deuterium released during TDS was higher than in pure tungsten.The general trend of helium influence on the deuterium retention in W-11.4Cr-0.6Y alloy is very similar to bare tungsten. Namely, deuterium retention increased until helium implantation reached a fluence of 1021He/m2, and then sharply decreased when the fluence exceeded this value.

Brazing of Ti-coated SiC using a CoFeCrNiCu high entropy alloy filler via electric field-assisted sintering

Electric field-assisted sintering technology (FAST) was used to join SiC ceramic by using a CoFeCrNiCu high-entropy alloy filler. A thin Ti film was deposited on SiC surface by physical vapor deposition to reduce the residual thermal stress in the joint. The microstructure, bending strength, and bonding mechanism were systematically investigated. It was found that a discontinuous TiC layer could be formed at the interface and the interfacial reaction products changed with different brazing temperatures. A joint with a bending strength of 72 MPa was achieved at 1200 °C, which is 73% higher than that of the SiC brazed without Ti coating. This work provides a new insight in the interfacial design of ceramic brazing with high mechanical properties.

Processing and Properties of Sintered W/Steel Composites for the First Wall of Future Fusion Reactor

Functionally graded tungsten/steel composites are attractive to be used as an interlayer to join tungsten (W) and steel for the first wall of future fusion reactor to reduce the thermally induced stresses arising from the different coefficient of thermal expansion (CTE) of W and steel. W/steel composites, with three W contents: 25, 50 and 75 vol% W, will serve as individual sublayers of this functionally graded material. Therefore, the present work exploits an emerging sintering technique, field-assisted sintering technology, to produce these composites. Firstly, a systematic parameter study was conducted aiming to reduce the residual porosity to a minimum while keeping the formation of intermetallic phases at the W/steel interface at a low level. The optimized composites 25, 50 and 75 vol% W achieved a relative density of 99%, 99% and 96%, respectively. Secondly, mechanical tests at elevated temperatures reveal that these composites are ductile above 300 °C, which is the minimum operating temperature of the first wall. Lastly, the measured CTE, specific heat capacity and thermal conductivity were consistent with the theoretically expected values.

Solid-state processing of surplus aluminium alloy powders through a combination of field-assisted sintering technology and hot rolling

ABSTRACT Metal additive manufacturing techniques typically operate using powders with limited particle size ranges, but atomisation processes produce significant amounts of particles outside these ranges, resulting in an accumulation of out-of-size specification metal powders without a clear use case. Field-assisted sintering technology (FAST) can provide an alternative, solid-state processing route to consolidate these powders into billets for subsequent processing, or directly into near-net shape components. In this study, surplus powders of A20X, an aerospace approved aluminium alloy developed by Aluminium Materials Technologies (ECKART GmbH), were processed using FAST and subsequently hot rolled to produce sheet material. Tensile properties were similar to hot rolled conventional cast material and comparable to additively manufactured product. This indicates that FAST is an effective option for converting surplus metal powders into useful products, while improving sustainability in the additive supply chain.

Influence of processing on the microstructural evolution and multiscale hardness in titanium carbonitrides (TiCN) produced via field assisted sintering technology

Titanium carbonitride (TiCN) is an advanced, high-performance hard ceramic of great commercial importance that has been widely developed and employed. Nonetheless, it has only been in recent years that binderless titanium carbonitride bulk ceramics have been successfully fabricated using field-assisted sintering technology (FAST). However, the underlying structure-processing-property-performance relationships have yet to be fully evaluated, especially concerning indentation hardness of these materials across a broad range of loads and deformation length scales. In this work we aim to address these fundamental relationships and characterize the multiscale hardness phenomena in detail. It was found that the effects of soak temperature and time directly impacted the sintered microstructure and were reflected in the observed mechanical properties over various loads. Valuable insight into the load-dependence of hardness distributions, sensitivity/correlation with elastoplastic parameters, and multiscale parameterization were developed using micro-/nanoindentation. Particularly, the load-dependent hardness sensitivity and resolvability demonstrate a fundamental tradeoff with respect to the manifested mechanical response influenced by the presence of underlying heterogeneities. These new insights relating the interplay of compositional/microstructural evolution with FAST processing parameters and multiscale hardness are an important step in advancing next-generation hard ceramics.

A parametric study of the microstructural evolution of an advanced Ni-based superalloy powder when consolidated using Field Assisted Sintering Technology, compared to a hot isostatic pressing benchmark

The use of Field Assisted Sintering Technology (FAST) is examined for the consolidation of an advanced Ni-based superalloy powder feedstock. FAST processed material was directly compared to a benchmark material prepared via hot isostatic pressing (HIP) where it was found that density, hardness values and microstructures were comparable. FAST enables the retainment of the prior particle grain morphology when using sintering dwell times ranging from 10 to 240 min. The application of dwell temperatures above the γ′ solvus of the alloy resulted in significant grain growth. Measured densities reveal that the applied load and dwell time used during sintering have minimal effect on the final density of the consolidated material. The crystallographic texture was also shown to be isotropic in FAST consolidated material. This study demonstrates that FAST is potentially a viable complementary and/or alternative processing route for consolidating Ni-based superalloy powders.

Interface and mechanical properties of the single-layer long fiber reinforced Wf/W composites fabricated via field assisted sintering technology

Tungsten fiber reinforced tungsten composites (Wf/W) show a pseudo-ductile behavior because of extrinsic toughening mechanisms such as interface de-bonding, pull-out and plastic deformation of the fiber. In the present work, single-layer long fiber Wf/W composites with and without yttrium oxide (Y2O3) interface were fabricated by a field assisted sintering technology (FAST) process. The microstructure and mechanical properties of the prepared Wf/W composites were characterized. The fracture behavior and toughening mechanisms were analyzed in detail combining the results of experiments and finite element modelling. Wf/W composites with Y2O3 interface (weak interfacial strength) show a typical pseudo-ductile fracture behavior and a higher flexural strength than the composites without Y2O3 interface (strong interfacial strength). The fracture energy dissipation is mainly driven by plastic deformation of the fibers, but interface de-bonding is a necessary factor to ensure any extrinsic toughening mechanisms.