Field-activated Sintering Research Articles

Effect of sintering duration on microstructure and properties of Inconel 718 superalloy prepared by electric field-activated sintering

Effect of sintering duration on microstructure and properties of Inconel 718 superalloy prepared by electric field-activated sintering

Effect of sintering temperature on microstructure and mechanical properties of Inconel 718 superalloy prepared by micro-FAST

Micro-forming combined with field-activated sintering technology, named micro-FAST, is a novel method to efficiently fabricate metallic or nonmetallic materials with excellent mechanical properties and fine microstructure. In this research, the Inconel 718 superalloy powders after ball milling were sintered via micro-FAST at three temperature conditions of 950 °C, 1000 °C and 1050 °C, respectively. The microstructural evolution and macroscopical mechanical properties of the sintered samples were researched. The results showed that the local Joule heat generated by the alternating electric current significantly promotes the formation and growth of the sintering necks, intensifies the volume diffusion of the substance, and improves the densification of the samples. In addition, the relative density of the sintered samples is all greater than 98%. During the micro-FAST, low sintering temperature promotes the grain refinement of the samples, and high sintering temperature enhances the grain boundaries migration. The average grain size of the sintered samples increased from 2.03 μm at 950 °C to 17.8 μm at 1050 °C, and the proportion of twins rises from 36.6% to 47%. Furthermore, the proportion of high-angle grain boundaries (HAGBs) is greater than 94% under the three sintering temperatures. The yield strength and hardness of the sintered sample are 554 MPa, and 397 Hv at 950 °C, respectively, presenting the most desirable mechanical properties. Compared with mechanical properties of the sample sintered by spark plasma sintering (SPS), the samples achieved by micro-FAST present superior strength and plasticity.

The analysis of thermoelectric powder compaction mechanisms within field-activated sintering of skutterudites and Heusler alloys

The analysis of the shrinkage rate of powders, based on the power-law creep model of a porous body, was carried out in this paper to calculate the compaction parameters of CoSb3-based skutterudites and Fe2VAl-based Heusler alloys within field-activated sintering. It was indicated that this method, which had already been used for metal and ceramic powders, is applicable for thermoelectric powders. The values of strain rate sensitivity were obtained, and the corresponding powder compaction mechanisms have been defined. The main creep mechanism for skutterudites was found to be a dislocation climb, that later was replaced by grain boundary sliding, and the last sintering stage was associated with diffusional creep. The main creep mechanism for Heusler alloys was grain boundary sliding, later replaced by diffusional creep. Keywords: field-assisted sintering, numerical simulation, powders compaction, skutterudites, Heusler alloys, thermoelectrics.

Анализ механизмов уплотнения термоэлектрических порошков скуттерудитов и сплавов Гейслера в процессе активированного полем спекания

The analysis of the shrinkage rate of powders, based on the power-law creep model of a porous body, was carried out in this paper to calculate the compaction parameters of CoSb3-based skutterudites and Fe2VAl-based Heusler alloys within field-activated sintering. It was indicated that this method, which had already been used for metal and ceramic powders, is applicable for thermoelectric powders. The values of strain rate sensitivity were obtained, and the corresponding powder compaction mechanisms have been defined. The main creep mechanism for skutterudites was found to be a dislocation climb, that later was replaced by grain boundary sliding, and the last sintering stage was associated with diffusional creep. The main creep mechanism for Heusler alloys was grain boundary sliding, later replaced by diffusional creep.

Investigation of the Mechanisms of Compaction of CoSb3 Skutterudite Nanopowders under Field-Activated Sintering

We calculated the compaction parameters of CoSb3 thermoelectric powders under field-activated sintering. The values of the creep activation energy, sensitivity to the strain rate, and the complex parameter of the material characterizing the state are obtained. The compaction of the powders is due to the mechanisms of grain sliding in the sample. We performed the computer simulation of the sintering process by the finite element method. The change in the dimensions of the sample is calculated using the creep deformation model. The simulation results correspond to the experimental data.

Direct evidence for surface cleaning mechanism during field-activated sintering

The cleaning of surface oxides to achieve direct joining of metal powders is one of the most important subjects in field-activated sintering technology (FAST). We reveal a surface cleaning mechanism during the FAST process of Cu components by using transmission electron microscopy (TEM) and scanning transmission electron microscope (STEM) equipped with energy-dispersive X-ray spectroscopy technique. The oxide layer is removed from the surface of Cu powders and squeezed to the triple junctions owing to the viscous flow, which leads to the direct joining on most boundaries. A schematic model is proposed to elucidate the surface cleaning process.

Simulation of the field-activated sintering of thermoelectric materials

The field-activated sintering of segmented thermoelement branches is considered within a timedependent computer model. Modified asymmetric tooling with an electrical insulating layer is proposed. This tooling facilitates the formation of a temperature difference as high as several hundreds of kelvins in the sample. The influence of the insulating-layer thickness on the axial and radial temperature differences in the samples is analyzed. The possibility of reducing the radial temperature gradient is demonstrated.

Forming of Miniature Components from Powders by Combining Field-activated Sintering and Micro-Forming

Abstract Micro-FAST is a process concept which scales down conventional FAST to the micro-scale process (dealing with miniature an micro-sized components) and it combines the sintering process with a micro-forming process to enable shaping components under coupled multi-fields actions and hence, to achieve high-density, near-net-shaped components with high efficiency. The main techniques developed for overcoming the barriers for the applications of FAST at the miniature/micro-scales include: (i). Directly pressing/forming loose powders in the die without using binders; (ii). Combining heating and shaping to enable complex shapes/features; and (iii). Dedicatedly controlling fusion bonding and material’s plastic flow to enable high-quality forming. Forming from powders without using binders significantly shortened the process cycle, which also led to high-purity of the parts formed; Combining forming and sintering has led to high-density components produced as well as achieving complex-shaped components; Large current density (e.g., >100~400 KA/cm -2 ) enables very high heating rates and using small volumes of materials results in high cooling rates, leading to much shorter forming/sintering cycles which enables consolidation of micro/nanocomposites into bulk-sized components while also preserving their micro/nanostructures.

Temperature Distribution in Metallic Powder Particles During Initial Stage of Field‐Activated Sintering

The temperature distribution in copper and martensitic steel spheres has been investigated for the initial stage of field‐activated sintering (FAST)/spark plasma sintering (SPS) using capacitor discharges (CD) with applied voltages from one to 15 V as model experiments. At first, the evolution of the contact resistance between the spheres has been studied. The results show the reduction in the contact resistance after discharge with increasing electrical load, yet no significant dependence on the length or number of the discharge pulses. Thereby the initial resistance is only decreased distinctly if at least a certain minimal voltage was applied. Subsequently, the melting of thin coatings of different metals on copper spheres has been studied and the occurrence of molten phase and its melting point were assigned to the corresponding discharge current. Extrapolation from the currents necessary to melt the coating layers in the CD experiments to lower values typical for FAST was used to estimate the contact overtemperature in the latter case. Resulting values for copper range from 0.05 K for normal heating with 100 K/min to 5 K for maximum current output.

The performance of Mg2Si-based thermoelectric materials prepared from MgH2

In this paper, thermoelectric materials of pure Mg2Si, Mg2Si1-xSnx and 1% Bi-doped Mg2Si were prepared by a low-temperature solid-state reaction using MgH2, Sn and Si, followed by a field-activated and pressure-assisted sintering process for consolidation. The reaction between silicon, tin and magnesium hydride, instead of pure magnesium, minimises the oxidation and volatilisation of magnesium. After consolidation, the prepared thermoelectric materials, which had a grain size of 500 nm, were evaluated for their thermoelectric properties. Furthermore, the electrical and thermal conductivities and the Seebeck coefficient were measured over the temperature range of 300–750 K. A sample of 1% Bi-doped Mg2Si showed a maximum figure of merit, ZT, of 0·53 at 750 K. Additionally, the Mg2Si1-xSnx samples were seen to possess an extra low thermal conductivity. The results are discussed in comparison with previously reported values and a prediction of modelling studies.

Simulation of heat distribution in particles during micro field-activated sintering by considering electrical concentration and contact resistance

Finite element simulations have been conducted to investigate the heat distribution between the particles from contact resistance and current density during the Micro-forming Fields Activated Sintering Technology (Micro-FAST). Results of effect for the arrangement modes of powder particles (Series connection and parallel connection) showed that the electro-heat focusing could be attributed to the remarkable difference of contact resistance between the particles.

Densification behavior of copper powder during the coupled multi-physics fields-activated microforming

External electric field-activated sintering techniques have been widely investigated and applied for the forming of large-sized components. These techniques are, however, rarely utilized for the manufacture of miniature and microsized components. In this paper, a novel, coupled forming, and sintering method is reported, which has been used for the fabrication of microcomponents, wherein the loose powder is loaded directly into the die, followed by simultaneous electrical forming and electric sintering (named coupled multi-physics-fields activation). In the study, the gears with the module of 0.2 and the pitch diameter of 1.6 mm were formed from copper powder. The coupled multi-fields activations were enabled using a Gleeble-1500D thermal simulation machine. Sintered samples with a relative density of 97.20 % have been fabricated at a sintering temperature of 700 °C, heating rate of 50 °C/s, forming pressure of 100 MPa, while these parameters were applied cyclically. The study showed that the axial reduction of the samples increased rapidly with the increase of temperature during the sintering, while the external pressure was maintained. Based on the experimental observations, it can be concluded that the deformation of the particles resulted in an increase in, and then subsequent disappearance of, the interface areas among the particles, which feature plays a key role in the densification of the copper powder.

Field-activated sintering of blended elemental γ-TiAl powder compacts: Porosity analysis and growth kinetics of Al3Ti

Various powder compacts with targeted compositions of TiAl and Ti3Al were consolidated by field-activated sintering, specifically spark plasma sintering (SPS) at various temperatures, to examine phase growth under a superimposed electric field. While compacts with a composition of TiAl showed significant porosity after sintering, Ti3Al compacts showed no porosity. The porosity is attributed principally to volume changes associated with the formation of Al3Ti and the Kirkendall effect. The kinetics underlying Al3Ti growth during SPS are also explained.

Effect of Electric Current on the Performance of Bi1.2Sb4.8Te9 Thermoelectric Material Prepared by a Field-Activated Pressure-Assisted Sintering Process

Field-activated pressure-assisted sintering (FAPAS) was applied to sinter Bi1.2Sb4.8Te9 thermoelectric materials under different conditions, including no-current sintering (NCS), low-density current sintering (LCS), and high-density current sintering (HCS). The effect of the current density on the final thermoelectric performance of the products was investigated. Applying a higher-density electric current and shorter dwell time can improve the thermoelectric performance of the sample by increasing its electric conductivity and decreasing its thermal conductivity. The maximum figure of merit ZT values of the NCS, LCS, and HCS samples were 0.46, 0.48, and 0.57, respectively. Therefore, applying a high-density electric current in the sintering process may be an effective way to obtain Bi1.2Sb4.8Te9 thermoelectric material with high ZT value.

Discrete Finite-Element Simulation of Thermoelectric Phenomena in Spark Plasma Sintering

Realistic microstructures of compacted powders formed by spark plasma sintering or field-activated sintering technology were modeled using the discrete finite-element method. Two key thermoelectric characteristics were studied: (1) the effect of the electric current pattern, i.e., direct current (DC) and pulsed current, on temperature distributions in the compacted powders, and (2) the effect of compaction modes, i.e., isostatic compaction and uniaxial compaction, on conductivity. Simulations showed that, for the same electric power input, pulsed current offered faster heating and more uniform temperature distribution in the compact than did DC. Additionally, using uniaxial compaction, the effective conductivity of the compact in the compaction direction was higher than in the transverse direction, by as much as 20%. Experimental measurements confirmed the existence of anisotropy of conductivity in the compact.

Microstructure evolution and wear properties of bulk MoSi 2 fabricated by field activated sintering

Field activated sintering (FAS) was employed to fabricate dense bulk MoSi 2 from Mo and Si powders. During the FAS process, Si was melted first, and then Mo was dissolved leading to a precipitation of MoSi 2. A eutectic reaction of MoSi 2–Mo 5Si 3 resulted from a combined effect of Mo–Si exothermic reactions and field induced Joule heating. With hyper-stoichiometric amount of Si (Mo/Si = 1:2.06), the FAS MoSi 2 is free of Mo 5Si 3 phase. Subsequently, an un-lubricated sliding wear test of MoSi 2 against carbon steel was conducted. Both normal load and temperature have crucial points related to a transition from mild to severe wear. Sliding speed and relative density have a negative effect on wear rate. In addition, at room temperature the wear is with a protective layer composed of oxide and transferred substance, whereas poor oxidation resistance leads to spalling of the layer and brittle fracture at an elevated temperature up to 500 °C.

Structural and magnetic properties of FeCr2S4 spinel prepared by field-activated sintering and conventional solid-state synthesis

We report on the fast preparation of the ternary magnetic semiconductor spinel FeCr2S4 using the Field-Activated Sintering Technique (FAST). The structural (X-ray) and magnetic (Squid) characterization demonstrates a similarity of the properties of the FAST crystals and those prepared by conventional solid-state synthesis. Residual structural disorder in FAST samples has a pronounced influence on the ferrimagnetic–paramagnetic transition and the orbital ordering at low temperatures.

Quasicrystal phase formation in Al–Cu–Fe nanopowders during field-activated sintering (FAST)

The sequence of solid-state phase transformations in high-energy ball-milled Al–Cu–Fe alloy powders upon constant-rate heating were examined by in situ synchrotron radiation diffraction and thermal analysis (DSC/DTA) methods. The as-milled Al–Cu–Fe nanopowders were consolidated into disk-shape pellets using field-activated spark-plasma sintering (FAST/SPS). The chemical homogeneity and microstructure of the sintered bodies were investigated using scanning electron microscopy (SEM/EDX). The effect of electric field application on the formation of quasicrystalline phases from mechanically activated nanopowders during sintering by FAST/SPS is discussed.

High-energy ball-milling synthesis and densification of Fe–Co alloy nanopowders by field-activated sintering (FAST)

Nanocrystalline Fe–Co soft magnetic alloy powders also containing approximately 4% Cr and 1% V were prepared by high-energy mechanical alloying in liquid media (hexane). Bulk nanostructured magnets of 40 mm diameter were obtained by field-assisted sintering (FAST) also known as spark-plasma sintering (SPS). We used a FAST furnace type HPD25-1 (FCT Systeme, Rauenstein, Germany). The FAST method was shown to successfully assist in decoupling densification from microstructural coarsening, mainly due to the limited exposure of the alloy nanopowders to high temperatures during their sintering into nanostructured bulk compacts. Results of chemical and microstructure analysis by SEM/EDX are presented to evidence the achievement of uniform microstructures and absence of composition gradients across sintered bulk specimens in spite of their large size.

Modelling of the temperature distribution during field assisted sintering

The evolution of the current density and temperature distribution in the punch-die-sample set-up during field activated sintering (FAST), also known as spark plasma sintering or pulsed electric current sintering, was modelled by finite element calculations supported by in situ measured electrical and thermal input data. The thermal and electrical resistances induced by the contacts of the different constituent parts of the die-punch-sample set-up were assessed experimentally by comparing the in situ thermal and electrical response of three different graphite set-ups with increasing complexity during FAST heating, allowing for the differentiation of the influence of horizontal as well as vertical contact resistances. In the present investigation, graphite paper was used in all contacts to ensure good electrical and thermal contact conduction. The measured pulsed direct current (DC) input was converted into an equivalent constant DC current for finite element (FE) method calculations. This allowed the theoretical determination of the current and power needs in order to realize a preset temperature profile. Using the developed FE code, case studies with an electrical conductor (TiN) and an electrical insulator (ZrO 2) were performed. In the case of a TiN sample, the radial temperature gradient in the sample was much larger compared to the temperature gradient in an electrically insulating ZrO 2 sample. However, independent of the sample’s electrical properties, the proposed temperature measurement design allowed a very accurate temperature control since the temperature difference between the centre of the sample and the controlling pyrometer was always below 5 °C.