Electric Discharge Consolidation Research Articles

SiAlON-TiN Ceramic Composites by Electric Current Assisted Sintering

Electric current assisted sintering of β-Si5AlON7-TiN ceramic composites from raw materials prepared by combustion synthesis was investigated. A high level of relative density (92% and higher) was achieved by using of two types of electric current assisted sintering technique: high voltage electric discharge consolidation, as well as spark plasma sintering. While only spark plasma sintering, it may be considered as promising technique for obtaining ceramic composites and items with high level of strength properties.

Current-Assisted Sintering of Combustion-Synthesized β-SiAlON Ceramics

Current-assisted sintering of combustion-synthesized β-Si5AlON7, h-BN, and TiN powders was explored in comparison with conventional sintering. High relative density of product (above 92%) was achieved by using high-voltage electric discharge consolidation (HVEDC) and spark plasma sintering (SPS). Better results were obtained in case of SPS processing.

The wave densification in high-voltage consolidation of powder materials

The densification of conductive powders by high-voltage electric discharge consolidation (HVEDC) is studied. HVEDC method includes a simultaneous exposure of a powder sample to mechanical pressure (50–500 MPa) and to a short (less than 300 ms) high-voltage (above 1 kV) electric discharge with the current pulse amplitude of a few hundred kA/cm2. It is important the right choice of the values of the applied pressure and the parameters of the current pulses (amplitude, and time-on). The densification kinetics of an industrial iron powder is analysed by ultrarapid video-monitoring under different amplitudes of the high current pulse and pressure. It was found that the high current pulse affects the powder before the movement of the punches–electrodes. The integral temperature of the sample has a maximum value at the beginning of the densification process. The compaction process lasts less than 16 ms for all the values of the parameters studied. The shortness of the densification process in comparison with the cooling of the sample provides a constant temperature throughout the entire compaction process. The consolidation process occurs at a constant speed of the punches until they stop. Values of the constant velocity of the punches vary in the experiments from 0.5 to 2 m/c. Under a constant pressure, the experimental results show an increase in both the densification rate and of the final density of the consolidated material for the increasing current pulse amplitude up to a certain value. For a current density exceeding a critical limit value, which depends on the properties of the powder and on the applied pressure, the loss of stability of the densification process and the formation of an inhomogeneous structure of the consolidated samples are observed. Based on the analysis of the obtained experimental results, a mathematical model of high compaction of a powder material under HVEDC is formulated. The constitutive equation of a consolidated material accounts for the plastic flow of the powder particles and for the collapse of the interparticle pores. The numerical simulation results revealed optimal values of the dimensionless parameters determining the HVEDC process.

Densification of zirconium nitride by spark plasma sintering and high voltage electric discharge consolidation: A comparative analysis

The densification behavior of zirconium nitride powder is investigated for various temperature and pressure conditions imposed by spark plasma sintering and high voltage electric discharge consolidation techniques. The crystal structure, chemical composition, porosity, and grain size of the powders and processed specimens are analyzed by X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. The outcomes of the two considered consolidation techniques are comparatively assessed. Vickers micro-hardness of the processed ZrN specimens is investigated, and hardness dependence on porosity is analyzed. The densification mechanism of ZrN consolidated by spark plasma sintering is revealed by applying the constitutive equation of the continuum theory of sintering, and it turns out to be a similar mechanism to hot pressing. Application of the sintering constitutive equations shows that the mechanism of ZrN densification by high voltage electric discharge consolidation method depends on the magnitude of the applied voltage.

Properties of UN Sintered by High Voltage Electric Discharge Consolidation

In the present work, the opportunity of the consolidation of uranium nitride tablets by high voltage electric discharge consolidation (HVEDC) is considered. It is shown that the consolidation by HVEDC allows the prevention of the expansion of uranium nitride powders and renders pellets with relative density of up to 97%. The thermal stability of the obtained samples has been investigated. The analysis of the microstructure of the processed samples indicates the retention of the initial powder structure

Structure of Zirconium Alloy Powder Coatings Processed by High Voltage Electric Discharge Consolidation

Zirconium alloy powders (Zr + 1% Nb) of spherical and flake forms have been consolidated by high voltage electric discharge processing. The consolidation enabled the fabrication of coatings on ceramic and metallic substrates as well as of freestanding powder components. The experimental dependence of the electric conductivity of the powders of spherical shape and flake shape on the applied pressure has been investigated. The influence of the current pulse amplitude on the final density of the consolidated zirconium powder alloy coatings on ceramic and metallic substrates and of the produced free‐standing powder samples has been analyzed for different applied pressures. The maximum amplitude of the electric current density above, which the consolidation process is unstable and has a nature of blowout has been determined. The conducted metallographic analysis showed the preservation of the microstructure of initial powder particles after the process of high voltage electric discharge consolidation.

Thermal processes during high-voltage electric discharge consolidation of powder materials

The results of a simulation of the thermal processes occurring within interparticle contacts of powder materials under high-voltage electric discharge consolidation are presented. The thermal processes in the contacts between powder particles are characterized by significant spatial inhomogeneity and time dependence. The determination of the behavior of the interparticle contact zones enables optimization of the pulse electrical current parameters. A good correlation between the modeling and experimental data for compacted iron and heat-resistant steel powders is demonstrated.

アモルファス ZrO2-20 mol%Al2O3 粉末のナノ結晶制御焼結

The instrumented pulse electric discharge consolidation method is used to provide a way of in process nanocrystalline control densification of the amorphous ZrO2-20 mol%Al2O3 powder as prepared by rotating-arm reaction ball milling. The cylindrical compact height (hf) of the amorphous ZrO2-20 mol%Al2O3 powder is found to be a dominant process variable; at 1 mm, it leads to the densification prior to major crystallization after a high relative density of roughly 0.86 at 800 K, and significant decreases down to 1284 K in temperature necessary to obtain the full densification under 100 MPa. The rapid densification for amorphous and nanocrystalline supersaturated cubic ZrO2-20 mol%Al2O3 is fairly well expressed by an Arrhenius-type equation of Newtonian viscous flow: ηp=ηpo exp (Q/kT) having a greatly decreasing apparent activation energy Q from 300 to 72 kJ mol-1 and the process viscosity ηp at 1200 K with decreasing hf from 14.6 to 1 mm. The Berkovich indentation testing permits us to derive a relatively low level of 4.4 GPa for the value of the yield stress at room temperature in the full-density nanocrystalline ZrO2-20 mol%Al2O3 sample having the Vickers hardness number of approximately 800 DPN.

In Process Nanocrystalline Control Consolidation of the Amorphous ZrO<SUB>2</SUB>-20 mol%Al<SUB>2</SUB>O<SUB>3</SUB> Powder

The instrumented pulse electric discharge consolidation method is used to provide a way of in process nanocrystalline control densification of the amorphous ZrO 2 -20mol%Al 2 O 3 powder as prepared by rotating-arm reaction ball milling. The cylindrical compact height (h f ) of the amorphous ZrO 2 -20mol%Al 2 O 3 powder is found to be a dominant process variable; at I mm, it leads to the densification prior to major crystallization after a high relative density of roughly 0.86 at 800 K, and significant decreases down to 1284 K in temperature necessary to obtain the full densification under 100MPa. The rapid densification for amorphous and nanocrystalline supersaturated cubic ZrO 2 -20 mol%Al 2 O 3 is fairly well expressed by an Arrhenius-type equation of Newtonian viscous flow: η p = η po exp(Q/kT) having greatly decreasing apparent activation energy Q from 300 to 72kJ mol -1 and process viscosity η p at 1200K with decreasing h f from 14.6 to 1 mm. The Berkovich indentation testing permits us to derive a relatively low level of 4.4GPa for the value of the yield stress at room temperature in the full-density nanocrystalline ZrO 2 -20 mol%Al 2 O 3 sample having the Vickers hardness number of approximately 800 DPN.

非平衡P/M法によるナノ結晶ハイドロキシアパタイトの固相創製

The solid state P/M processing that consists of rotating-arm reaction ball milling and pulse electric discharge consolidation provide a process control methodology to develop the hydroxyapatite (HA) with unique property inherent to the nanocrystalline (nc) structure. The mechanical alloying of the powder mixture is conducive to the solid state synthesis of HA{Ca10(PO4)6(OH)2} having the average crystallite size of 7 nm according to a reaction, 6CaHPO4•2H2O+4Ca(OH)2→nc-Ca10(PO4)6(OH)2+8H2O; this overall process is given by Johnson-Mehl-Avrami equation with the exponent of 1. The nc-Ca10(PO4)6(OH)2 powder compact can be consolidated at a full density; its rapid densification during heating is expressed by an Arrhenius-type equation of Newtonian viscous flow with the activation energy of 402 kJ•mol-1 under both pressure of 80 and 140 MPa. The average crystallite size of bulky Ca10(PO4)6(OH)2 increases from 10 to 50 nm with increasing temperature up to 1173 K. The high-speed superplastic forging is achieved having the strain rate of nearly 10-2 s-1 and the compressibility of 0.65 at a relatively low temperature of 1137 K, and characterized by the strain rate sensitivity exponent ranging from 0.7 to 1 and the activation energy of 75-113 kJ mol-1.

Mechanical Alloying and Powder Consolidation in SiC and Hydoxyapatite

The reaction ball milling can produce the solid state amorphization of covalent ceramic, SiC, including the mechanical alloying (MA) of the elemental crystalline powder mixture of Si and C and the mechanical grinding (MG) of the commercial β-SiC particle and MG of the nanocrystalline β-SiC powder as synthesized via ’high-energy’ MA. An increase in amorphous volume (X) by decreasing crystallite size (d) during MG is expressed by a relation of X= 1-{d/(d+∆)}3 with the intercrystal thickness (∆) of 1 nm. The rotating-arm reaction ball milling is used to synthesize nanocrystalline (nc) hydroxyapatite by MA of the powder mixture at 303 K, according to a reaction of 6CaHPO4･2H2O+4Ca(OH)2→nc-Ca10(PO4)6(OH)2+8H2O with JMA exponent of 1. By employing the pulse electric discharge consolidation, the amorphous SiC powder compact shows a rapid densification during Newtonian viscous flow as expressed by an Arrhenius relation with the activation energy of 495 kJ･mol-1, and then obtain the full densification at 2033 K under 100 MPa. The nanocrystalline hydroxyapatite powder with the crystallite size of 8 nm can be consolidated at full-density at 1023 K under 150 MPa, following a rapid shrinkage during superplastic flow from 900 K. The fracture toughness (KIC), as deduced from the indentation microfracture method, is the high level of 13 MPa･m0.5 for nanocrystalline SiC with 12 nm.

酸化物セラミックスの固相アモルファス化と放電焼結

Full density powder processing, which consists of reaction ball milling and the instrumented electric discharge consolidation method, is proposed for the development of bulky oxide ceramic materials with nanoscale grain. We find that mechanical alloying of the powder mixture, ZrO2-20 mass%Al2O3 can produce solid state amorphization, when the conditions for planetary reaction ball milling yield the optima. When electric discharge consolidation is combined with high-rate heating of the mechanically alloyed amorphous ceramic powder, we obtain fully dense ZrO2-20 mol%Al2O3 at 1347 K when a pressure of 100 MPa is applied. The densification in the supercooled liquid of ZrO2-20 mol%Al2O3 is fairly well expressed by an Arrhenius type equation of viscous flow: η=η0exp(H⁄kT). The process viscosity (η) is below 1012 Pa·s at the glass temperature and the activation energy (H) decreases from 337 to 184 kJ·mol−1 with increasing current applied to the graphite die. For cubic ZrO2-20 mol%Al2O3 with a monoclinic phase, synthesized via crystallization at 1366 K, the average grain size is estimated at 19.6 nm from X-ray line broadening. Vickers’ pyramidal hardness is 1051 DPN when the initiation of cracks at the corners of indentation is avoided.

放電焼結の基礎と応用 放電焼結の科学と新しい応用

This overview describes the fundamentals of the electric discharge consolidation which provide a route to the design and development of advanced materials. The proposed intelligent sintering consists of the pulse electric discharge system, combined with the process control methodologies and the microstruture control techniques. The process control and/or model is set up for the formation of the neck between spherical amorphous particles via feeding rectangular pulses and the full densification of amorphous ceramic compact under electric field. Furthermore, the electric field controlled technique can synthesize fully dense nanograined ceramics having the plastic strain rate of 3.10-2s-1 around 1350 K.

Powder processing and mechanical characteristics of fully dense nano-intermetallics

This article describes the basic idea that process control of electric discharge consolidation makes it possible to obtain fully dense high-strength nano-intermetallics. This powder processing provides sensors of pulse discharging and densification for making a process model and a control methodology. The discharging of rectangular pulses, monitored by voltage wave, is characterized by spark and arc, resulting in the neck formation between spherical amorphous particles. Then, densification of the amorphous Ti 50Al 50 powder under an electrical field is expressed by an Arrhenius-type equation of viscous flow: η p=η p0(I)exp{H(I)/kT}, where η p is process viscosity. A mechanism of bulk transport via enhanced viscous flow through artificial multiple necks accounts for rapid pore-free consolidation. For ductile nano-intermetallic Ti 50Al 50 synthesized via crystallization, the highest specific strength is 790 MPa Mg m −3 at the γ grain size of 17 nm; and the softening temperature is comparable to those of cast TiAl and Ni-base superalloy.