Field-assisted Sintering Research Articles

Research on densification mechanism of Inconel 718 alloy miniature gear by electric field-assisted sintering

The densification mechanism of the electric field-assisted sintering (E-FAST) for Inconel 718 alloy miniature gears was studied using the Bernard-Granger model. Under the conditions of sintering temperature of 950 °C, 1000 °C and 1050 °C and dwelling time of 240 s, 300 s and 360 s, the Inconel 718 powders were subjected to E-FAST using the Gleeble thermal simulator, and the densification rate was up to 10−3 s−1. With the increase of the sintering temperature, the peak of the densification rate of the sintered sample was enlarged, and the time for the peak to appear became shorter. The densification of Inconel 718 alloy miniature gears was mainly concentrated in the dwelling stage. When the dwelling time reached 300 s, the densification rates of the samples under different temperature conditions were similar, indicating that the gear samples were close to full density at this time. Simulation and sintering test results show that the densification stages of the gear samples can be divided into low sintering stress stage with stress index n = 0.4 and high sintering stress stage with stress index n = 3.5. The apparent activation energy of the densification changes with the variation of the sintering stress stage.

Rapid Sinterability of Mechanically Alloyed MgAl/MgO Composite with the Field-Assisted Sintering Technique

Rapid Sinterability of Mechanically Alloyed MgAl/MgO Composite with the Field-Assisted Sintering Technique

A universal all-solid synthesis for high throughput production of halide perovskite

Halide perovskites show ubiquitous presences in growing fields at both fundamental and applied levels. Discovery, investigation, and application of innovative perovskites are heavily dependent on the synthetic methodology in terms of time-/yield-/effort-/energy- efficiency. Conventional wet chemistry method provides the easiness for growing thin film samples, but represents as an inefficient way for bulk crystal synthesis. To overcome these, here we report a universal solid state-based route for synthesizing high-quality perovskites, by means of simultaneously applying both electric and mechanical stress fields during the synthesis, i.e., the electrical and mechanical field-assisted sintering technique. We employ various perovskite compositions and arbitrary geometric designs for demonstration in this report, and establish such synthetic route with uniqueness of ultrahigh yield, fast processing and solvent-free nature, along with bulk products of exceptional quality approaching to single crystals. We exemplify the applications of the as-synthesized perovskites in photodetection and thermoelectric as well as other potentials to open extra chapters for future technical development.

A novel approach to fabricate load-bearing Ti6Al4V-Barium titanate piezoelectric bone scaffolds by coupling electron beam melting and field-assisted sintering

A critical-size bone defect in load-bearing areas is a challenging clinical problem in orthopaedic surgery. Titanium alloy (Ti6Al4V) scaffolds have advantages because of their biomechanical stability but lack electrical activity, which hinders their further use. This work is focused on the fabrication of Ti6Al4V-Barium Titanate (BaTiO3) bulk composite scaffolds to combine the biomechanical stability of Ti6Al4V with electrical activity through BaTiO3. For the first time, a hollow cylindrical Ti6Al4V is additively manufactured by electron beam melting and combined with piezoelectric BaTiO3 powder for joint processing in field-assisted sintering. Scanning electron microscope images on the interface of the Ti6Al4V-BaTiO3 composite scaffold showed that after sintering, the Ti6Al4V lattice structure bounded with BaTiO3 matrix without its major deformation. The Ti6Al4V-BaTiO3 scaffold had average piezoelectric constants of (0.63 ± 0.12) pC/N directly after sintering due to partial dipole alignment of the BaTiO3 tetragonal phase, which increased to (4.92 ± 0.75) pC/N after a successful corona poling. Moreover, the nanoindentation values of Ti6Al4V exhibited an average hardness and Young’s modulus of (5.9 ± 0.9) GPa and (130 ± 14) GPa, and BaTiO3 showed (4.0 ± 0.6) GPa and (106 ± 10) GPa, respectively. It reveals that the Ti6Al4V is the harder and stiffer part in the Ti6Al4V-BaTiO3 composite scaffold. Such a scaffold has the potential to treat critical-size bone defects in load-bearing areas and guide tissue regeneration by physical stimulation.

High-throughput characterization of elastic moduli of Ti-Nb-Zr-O biomedical alloys fabricated by field-assisted sintering technique

Compositionally graded samples of Ti-xNb-7Zr-yO alloys (11 ≤ x ≤ 35, 0.49 ≤ y ≤1.01 wt%) were prepared from elemental powders and TiO2 and compacted by the field-assisted sintering technique. By this approach, four layered samples covering together twenty four different chemical compositions were prepared. It allowed efficient sampling of alloys for potential biomedical use. Elastic constants of each layer (the shear modulus G and the Young’s modulus E) were determined using resonant ultrasound spectroscopy. The results show that in the considered compositional space, elastic constants exhibit a dependence on the volume fraction of β-stabilizing niobium and interstitial oxygen. Minimum elastic modulus was achieved for the compositions close to the stability limit between parent β matrix and martensitic α″ phase, while maximum elastic constants were caused by the presence of isothermal ω particles.

Assessing the effect of compaction pressure on the mechanical properties of polytetrafluoroethylene elaborated by field assisted sintering technique

We study the effect of compaction pressure on the mechanical properties of polytetrafluoroethylene (PTFE) synthesized by the field-assisted sintering technique (FAST). PTFE is a commonly polymer use material with various technological applications in the automotive, aerospace, and chemical industries. While the manufacture of PTFE is problematic due to its high molecular weight and hence a very high melt viscosity, FAST is an adequate process which can fix this problem by offering rapid densification (a few minutes). In this context, we explore the mechanical behavior of sintered PTFE produced with different compaction pressures, all other operating parameters being held constant. The stress-strain curves of PTFE investigated by uniaxial tension tests show a significant dependence of the compaction pressure on the elastic and plastic mechanical characteristics. An attempt is made to correlate the applied pressure during the sintering process to microstructural changes according to the de Gennes creep model. For plastic deformation, the stress-strain curves are compared to the predictive Hollomon's and Haward and Thackray's models and discussed in physical terms. We conclude that a better mechanical behavior of sintered PTFE materials can be achieved by using an optimum value of compaction pressure which is not necessary found above the yield flow. • PTFE samples produced by the field-assisted sintering technique are characterized. • Stress-strain curves show significant dependence on compaction pressure. • Hollomon's and HT analysis allow us to characterize the strain hardening behavior. • Optimal properties correspond to sintered samples at pressure less than 5 MPa.

Electric field-assisted in situ fabrication of carbon/zirconia nanocomposites with tunable conductivity for electromagnetic interference shielding applications

Carbon/ceramic nanocomposites have been considered as ideal candidates for electromagnetic interference shielding (EMI) applications in harsh environments. However, the conventional fabrication methods of carbon/ceramic composites are often complicated, costly and time-consuming. Herein, by applying a DC electric field during hot pressing sintering, we realized the direct growth of graphene-like carbon nanosheets (GCNs) in zirconia ceramics during sintering, achieving in situ fabrication of GCNs/zirconia nanocomposites with both high EMI shielding effectiveness (>40 dB) and mechanical strength (>1000 MPa) in one step. In particular, we found that applying a DC electric field could significantly enhance the carburization during sintering, and GCNs are in situ formed at zirconia grain boundaries. When the initial electric field is constant, the amount of in situ formed GCNs could be controlled by simply adjusting the current density, resulting in the tunable electrical conductivity. These findings will provide new opportunities for both experimental and theoretical studies on carbon/ceramic EMI shielding materials and the DC electric field-assisted sintering technique.

In-Process Fingerprints of Dissimilar Titanium Alloy Diffusion Bonded Layers from Hole Drilling Force Data

The manufacture of components that have specific material properties in subcomponent regions is highly desired in many sectors. However, it is challenging to achieve via conventional ingot-wrought and joining processing routes. Recently, diffusion bonding titanium alloy powder using field assisted sintering technology (FAST) has demonstrated that multi-material billets can be manufactured. Such billets still need to be machined into final net shaped components. The machinability and machining strategy of such components needs to be better understood if manufacturing of multi-material components is to be economically viable. This is the first study where drilling machinability of FAST diffusion bonded titanium alloys has been investigated. Location indexed force and torque feedback in-process fingerprinting is utilised during the drilling of multi-material titanium alloy billets. The in-process fingerprinting enabled rapid identification of the types and layering order of alloys within the FAST billets. In addition to force feedback, the hardness, hole surface topography and subsurface microstructure were characterised. Although hardness was found to contribute to variation in bond to bond machinability, results highlighted how alloy chemistry and bond composition are intrinsic to the machining directionality and significantly influence the machined surface quality and process stability. The work demonstrates that machining strategy of multi-material drilling needs to be tailored with respect to direction and diffusion bonded alloy pairing to avoid undesirable surface and subsurface damage at bond locations.

Ultrafast low-temperature near-seamless joining of Cf/SiC using a sacrificial Pr3Si2C2 filler via electric current field-assisted sintering technique

A novel layered structure material, Pr3Si2C2, was synthesized at a low temperature of 850 °C using a molten salt approach for the first time, and subsequently used as the joining filler for carbon fibers reinforced SiC composites (Cf/SiC). A robust near-seamless Cf/SiC joint was successfully obtained at 1509 °C (Ti) for 30 s, while an ultrafast heating rate of 6000 °C/min was applied via electric field-assisted sintering technology. The near-seamless joining process was attributed to the newly precipitated SiC grains, which were densified well with the Cf/SiC matrix by liquid-assisted sintering. The liquid phase was in-situ formed by the eutectic reaction between Pr3Si2C2 and SiC. The shear strength of the near-seamless joint obtained at 1509 °C for 30 s was 17.6 ± 3.0 MPa. The failure occurred in the Cf/SiC matrix. The formation of near-seamless Cf/SiC joints dismisses the issues related to thermal mismatch between Cf/SiC matrices and traditional joining fillers.

Ti-Mo-xTiC composites manufactured by U-FAST reactive sintering

The paper presents the characteristics of Ti-Mo-xTiC composites manufactured under experimentally selected conditions using the upgraded field-assisted sintering technique (U-FAST). Mixtures of microstructural titanium powders and nc-Ti0.9Mo0.1C/C carbide powders protected from oxidation by a carbon shell were subjected to sintering. The powders with nc-Ti0.9Mo0.1C/C contents of 10 and 20 wt% were used. The content of carbon forming the carbon shell was approximately 3 or 40 wt%. Composites with near full density were reinforced with titanium carbides in a Ti-Mo matrix. The composites with the highest content of reinforcing phase are characterized by the highest values of hardness, Young's modulus and wear resistance. Although the hardness of these composites is similar to that of ceramics, their nature is not brittle. Despite the high value of the Young's modulus, the addition of hard particles of the reinforcing phase to the titanium matrix significantly increases the values of the H/E ratio of the composites compared to the reference samples of cp-Ti and Ti6Al4V. A comparison of the research results for the composites with the highest share of titanium carbides showed that lowering the sintering temperature from 1300 to 1150 °C resulted in the inhibition of grain growth, a reduction in composite heterogeneity, composite roughness and hardness as well as a rise in the Young's modulus. An increase in the sintering temperature from 1150 to 1300 °C contributed to the higher high angle grain boundaries (HAGB) content.

Near-seamless joining of Cf/SiC composites using Y3Si2C2 via electric field-assisted sintering technique

A novel Y3Si2C2 material was synthesized at a relatively low temperature (900 °C) using a molten salt method for the first time, and subsequently used as the joining material for carbon fiber reinforced SiC (Cf/SiC) composites. The sound near-seamless joints with no obvious remaining interlayer were obtained at 1600 °C using an electric field-assisted sintering technique (FAST). During joining, a liquid phase was formed by the eutectic reaction among Y3Si2C2, γ(Y—C) phase, and SiC, followed by the precipitation of SiC particles. The presence of the liquid promoted the sintering of newly formed SiC particles, leading to their complete consolidation with the Cf/SiC matrix. On the other hand, the excess of the liquid was pushed away from the joining area under the effect of a uniaxial pressure of 30 MPa, leading to the formation of the near-seamless joints. The highest shear strength (τ) of 17.2±2.9 MPa was obtained after being joined at 1600 °C for 10 min. The failure of the joints occurred in the Cf/SiC matrix, indicating that the interface was stronger than that of the Cf/SiC matrix. The formation of a near-seamless joint minimizes the mismatch of thermal expansion coefficients and also irradiation-induced swelling, suggesting that the proposed joining strategy can be potentially applied to SiC-based ceramic matrix composites (CMCs) for extreme environmental applications.

Polymer-Garnet-Based Composite Cathodes for Solid-State Li Batteries

All-solid-state lithium batteries (ASSLBs) can potentially outperform conventional Li-ion batteries with liquid or polymer electrolyte. One example for solid electrolytes is the garnet-type oxide Li7La3Zr2O12 (LLZO). LLZO has a wide electrochemical window, stability vs. lithium, and good ionic conductivity at room temperature. The cathode in ASSLBs is manufactured from a cathode active material (CAM), such as LiCoO2 (LCO). The efficiency of Li-ion storage can be improved by the use of a composite cathode consisting from a CAM and an ion-conducting ceramic, e.g. LCO/LLZO. In such a composite cathode, LLZO delivers Li-ions through the whole bulk enhancing the volumetric loading of LCO. In this work the addition of polymer electrolyte into LCO/LLZO composite cathode was proposed, aiming at further increase of cell performance due to facilitation of CAM usage, similar to the approach of manufacturing of polymer-ceramic electrolytes. They are fabricated mostly by tape casting of slurry with polymer matrix, ceramic filler and a solvent. An alternative technology includes free sintering of tape-casted LCO/LLZO porous network and subsequent infiltration by liquid or polymer electrolyte. Free sintering of LCO/LLZO composite requires relatively high temperature and/or long sintering time. This results in loss of volatile Li with decrease in electrochemical performance.In the present work the LCO/LLZO composite cathode was manufactured in a powder-based process by Field-Assisted Sintering Technique also known as Spark Plasma Sintering (FAST/SPS). Fast heating (100°C/min and higher) and application of mechanical pressure during FAST/SPS enable reduction of sintering temperature and processing time needed for fabrication of nearly-fully-dense composite.[1] Thereby, Li evaporation and grain growth can be significantly reduced. This technology was used in our previous work for fabrication of half-cells with dense LLZO electrolyte and dense LCO/LLZO composite cathode. However, the appearance of side phase after sintering at low pressure and a residual porosity was observed. The reason for that was partial reduction of oxides by carbon originated from graphite foil in FAST/SPS setup. In the presented work, the graphite foil was replaced by carbon-free mica foil. This measure enabled FAST/SPS sintering of porous LCO/LLZO network without side phase formation. The obtained porous skeleton was infiltrated with polymer electrolyte to fabricate a polymer-ceramic composite cathode. The cathode was assembled with an anodic half-cell consisting of dense FAST/SPS-sintered LLZO electrolyte and attached indium (In) foil used as anode. The ASSLB with polymer-ceramic composite cathode showed significantly lower interfacial impedance and remarkably higher area-specific storage capacity as compared to the similar ASSLBs with pure ceramic (porous or dense) composite cathodes. Thus, the functionality and the advanced storage capacity of the proposed polymer-ceramic cathode and related ASSLB architecture were demonstrated.[2]

Development and comparison of field assisted sintering techniques to densify CeO2 ceramics

Cerium dioxide (CeO2) was densified by conventional and field assisted sintering techniques in order to examine the effect of a range of sintering parameters on the resultant pellet microstructure, namely: temperature, hold time, atmosphere, electric field strength and polarity. CS at 1400 °C for 2 h in static air atmospheres provided the highest densities and grain sizes compared with an argon atmosphere, due to the retention of near stoichiometry maintaining sintering kinetics. SPS produced dense ceramics at similar sintering temperatures (1300 °C), but with greatly reduced sintering hold (5 min) and cycle times. However, microstructural inhomogeneity arose from the direct current polarity which led to oxygen ion diffusion toward the positive electrode. FS was performed with an alternating current electric field and produces samples of comparable density at sintering temperatures of ~1100 °C for a hold time of around 1 h with no inhomogeneity due to the alternating current employed.

A new MgB2 bulk ring fabrication technique for use in magnetic shielding or bench-top NMR systems

We report a new methodology in bulk MgB2 ring production for use in small-scale magnetic shielding or bench-top nuclear magnetic resonance systems. This process is a modified field-assisted sintering technique (mFAST) which enables direct formation of the rings without the need for machining or additives into the precursor powder. The shielding and trapped field capabilities of three mFAST MgB2 rings were determined using zero-field- and field-cooled magnetic experiments. Individual bulks trap magnetic fields up to 1.24 T at 20 K comparable to the highest published data for a ring sample. It is anticipated that for many applications, multiple rings will be stacked to form the required experimental structure. We find, for the three ring stack, a trapped field of 2.04 T and a maximum shielded field of 1.74 T at 20 K. The major factor limiting performance at low temperatures are flux jumps which cause rapid loss of the trapped field or shielding capability. Preliminary studies of magnetic field ramp rate dependence on flux jumps were conducted illustrating that even at very slow ramp rates (0.007 T min−1) they remain a significant issue. Despite this concern, we conclude that mFAST represents an exciting new fabrication methodology for bulk MgB2 rings.

The influence of powder characteristics on densification behavior and microstructure evolution of W-Cr-Zr alloy consolidated by field-assisted sintering technology

In the present study, the influence of powder characteristics on the densification behavior and microstructure evolution of the W-Cr-Zr alloy during consolidation was investigated. The W-Cr-Zr alloy powders were prepared through mechanical alloying in different modes, i.e. continuous mechanical alloying (CMA) and intermittently mechanical alloying (IMA). It can be found that the IMA powders have a higher grain boundary density and homogenous microstructure, which exhibits a better sintering performance than the CMA powders. The IMA powders are capable of full densification and homogenization after sintering at low temperature (1395 °C), with the grain size approaching 284 nm. Through the characterization of antioxidant properties, it was confirmed that the alloy prepared using IMA powders has an excellent anti-oxidation performance.

Improved additive manufacturing of silicon carbide parts via pressureless electric field‐assisted sintering

High solids loading silicon carbide (SiC)-based aqueous slurries containing only .5 wt. % organic additives were utilized to create specimens of various geometries via an extrusion-based additive manufacturing (AM) technique. Pressureless electric field-assisted sintering was performed to densify each specimen without deformation. The combination of these techniques produced parts with >98% relative density despite containing only 5 wt.% oxide sintering additives. After sintering, specimens contained only the α-SiC and yttrium aluminum perovskite phases. This suggests the evolution of a nonequilibrium yttrium aluminate phase, as well as transformation from β-SiC to α-SiC. The fabrication method presented in this work has advantages over other AM techniques commonly used with SiC, because it does not require significant organic additives nor additional postprocessing steps such as chemical vapor infiltration or polymer impregnation and pyrolysis.

Progress in aluminium and magnesium matrix composites obtained by spark plasma, microwave and induction sintering

ABSTRACT Aluminium and magnesium matrix composites are attractive lightweight materials with strength surpassing that of the corresponding matrix metals. Recent years have seen extensive research aimed at finding ways to control the structure of metal matrix composites and obtaining materials with new sets of properties. This review aims to show that field-assisted sintering is a promising approach for the powder metallurgy processing of aluminium and magnesium matrix composites. The review focuses on the achievements in the compositional and microstructural design and properties of aluminium and magnesium matrix composites obtained by spark plasma sintering, microwave sintering and induction sintering. Issues related to the process scale-up in the field-assisted sintering technologies are also addressed.

MulTi-FAST: A Machinability Assessment of Functionally Graded Titanium Billets Produced from Multiple Alloy Powders.

Technological developments in the area of functionally graded multi-material manufacture are poised to disrupt the aerospace industry, providing the means for step-change improvements in performance through tailored component design. However, the challenges faced during the downstream processing, i.e., machining of such functionally graded multi-materials are unclear. In this study, the challenges involved when face-turning billets consisting of multiple alloys are assessed. To achieve this, a cylindrical billet consisting of Ti-64, Ti-6242, Ti-5553 and Beta C alloys was manufactured from powder feedstock using field-assisted sintering technique (FAST) and termed MulTi-FAST billets. A detailed study of the structural integrity during machining at the diffusion bond interfaces of multiple titanium alloy bond pairings in the MulTi-FAST billet was conducted. The machining forces were measured during face-turning to investigate the impact and behaviour of different alloy pairings during a continuous machining operation. The results showed the significant differences in force machining response, surface topography and the type of surface damage was dependent on the direction the titanium alloy graded pairings were machined in. In terms of subsurface microstructural damage, regardless of the machining direction, no critical damage was found in the vicinity of the bonded alloys. The findings provide an insight into the deformation characteristics and challenges faced in the machining of functionally graded components with multiple titanium alloys.

Heat Treatment Influencing Porosity and Tensile Properties of Field Assisted Sintered AlSi7Mg0.6.

In this study, an attempt was made to improve the mechanical properties and in particular the strength of a precipitation-hardenable aluminum alloy while still maintaining high ductility. For this purpose, AlSi7Mg0.6 (A357) powder with an average particle diameter of d50 = 40 µm was consolidated using field assisted sintering technique (FAST), and two material conditions were compared: an as-sintered and an underaging heat treated condition (T61). Mechanical properties were determined using tensile tests and hardness measurements. In addition, the microstructure was investigated by optical microscopy. Further, porosity and density were analyzed after the different heat treatments. By the underaging heat treatment, the surface hardness was increased by 100% and the yield strength was increased by 80% compared to the as-sintered material. However, the elongation to failure dropped to one third of that of the as-sintered material. Presumably, this effect was a result of an increased porosity due to the heat treatment. It is assumed that the observed pores were generated by artefacts from the FAST process used to manufacture the samples. The internal gas pressure and equilibrium diffusion supported by heat treatment temperature, and the reduction in surface energy caused by coalescent micropores, led to the enlargement of previously undetectable inhomogeneities in the as-sintered material that resulted in pores in the heat-treated sintered alloy.

Time-dependent investigation of a mechanochemical synthesis of bismuth telluride-based materials and their structural and thermoelectric properties.

Here, we report on the time dependence of a synthesis procedure for generation of both n- and p-type bismuth telluride-based materials. To initiate the reaction, the starting materials were first mechanical pre-reacted. The Rietveld refinements of X-ray diffraction (XRD) data collected after different milling times demonstrate that Bi2Te3 was formed after only 10 min, and longer milling times do not alter the composition. To complete the phase formation, the powders were treated by field-assisted sintering and heat treatment afterwards. The effect of this fast procedure on the structural and thermoelectric properties was investigated. Samples were obtained with relative densities above 99%. A clear preferred orientation of the crystallites in the samples is evidenced by Rietveld refinements of XRD data. The thermoelectric characteristics demonstrate a good performance despite the short milling time. Further, it was demonstrated for this fast synthesis that the physical transport properties can be varied with well-known n- and p-type dopants like CHI3 or Pb. For these non-optimized materials, a ZT value of 0.7 (n-type) and 0.9 (p-type) between 400 and 450 K was achieved. The long-term stability is demonstrated by repeated measurements up to 523 K showing no significant alteration of the thermoelectric performance.