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A porous materials production with an electric discharge sintering
Int. Journal of Refractory Metals and Hard Materials 59 (2016) 67–77
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Int. Journal of Refractory Metals and Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
A porous materials production with an electric discharge sintering Minko Dzmitry a,b,⁎, Belyavin Klimenty a a b
Belarusian National Technical University, 65, Nezavisimost Avenue, 220013 Minsk, Belarus National Research Nuclear University MEPhI, 31, Kashirskoe highway, 115409 Moscow, Russia
a r t i c l e
i n f o
Article history: Received 2 January 2016 Accepted 24 May 2016 Available online 26 May 2016 Keywords: Refractory metal Powder High-voltage discharge Porous material
a b s t r a c t A short review of a porous materials production from the powders of titanium, niobium and tantalum with a high-voltage discharge current is presented. The experimental dependences of bending strength, porosity, specific electrical resistance, radial and axial shrinkage from the sizes of particles of the examined powders and from the parameters of the electric discharge are given. The maximum correlations of the diameters and the length of the experimental samples of porous powders are stated. The examples of perspective applications of produced porous materials in the manufactured articles of electronics and medicine are shown. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction A direct electric current passing through the powder is the most simple and economical method of producing powder materials. The first patent on using a direct electric current to heat the powder of the hard alloy during hot pressing was obtained in 1933 [1]. In 1944 for the first time the use of alternating current of industrial frequency with a mechanical pressure sintering powders of copper, brass, bronze and aluminum was proposed [2]. In 1955 for the first time the use of equipment for spot welding capacitor powders by sintering under external pressure was described [3]. A wide range of the electrical parameters of the impact on the powder causes a large number of these methods collectively called «an electric current sintering» [4]. The existing methods for the direct influence of the electric current have a number of features allowing them to be used successfully for powder materials of different densities. Common to all of these methods is that the consolidation of the particles takes place in a closed matrix compression molding and discharges DC or a pulsed electric current generating heat passing through the powder. As a result of the electric current effect, there is an intensive mass transfer in the solid phase at the contact areas between the neighboring particles of the powder. Furthermore, the surface of the powder particles in the contact zone can be melted which is accompanied by a more intensive mass transfer. The result is a rapid consolidation of the particles throughout the volume of the powder. Depending on the magnitude of the compacting pressure and the range of values of the electrical
⁎ Corresponding author at: Belarusian National Technical University, 65, Nezavisimost Avenue, 220013 Minsk, Belarus. E-mail address: [email protected] (M. Dzmitry).
http://dx.doi.org/10.1016/j.ijrmhm.2016.05.015 0263-4368/© 2016 Elsevier Ltd. All rights reserved.
parameters (the nature, the amplitude and the duration of the flow of the electric current), the consolidation process may proceed in different ways. This widely varies the structure and properties of the resulting powder materials. Methods of consolidating the powder with an electric current can in many cases dispense with the usage of protective gas or vacuum, and combine molding and sintering of powder performs in a single operation. One of the significant trends of the powder metallurgy is a porous material production, the efficiency and the field of application of which are defined by their pore structure capable of passing liquids and gases through itself [5]. The most propagation is due to the application of porous materials as filters used for the separation of gases and liquids from foreign admixtures, for the transportation of liquids in pore channels under the effect of capillary forces in capillary pumps, heat pipes wicks, evaporators, condensers, as fire blocks and noise suppressers and in many other cases. The porous materials are produced from the powders of metals, oxide and nitride ceramics, glasses, polymers and other materials. Each of these materials has a unique set of characteristics and properties and can be used in different fields of engineering. The porous materials produced from the powders of refractory metals – titanium, niobium, tantalum, molybdenum, tungsten and their alloys – have the unique properties of refractoriness, hardness, corrosive stableness, wear resistance and many specific characteristics. The production of porous materials from the refractory metal powders includes some traditional technological operations of the powder metallurgy: a preparation of the initial powder including dissemination and mixing with pore-forming substances; a forming of intermediates as a rule, under pressure applied; a sintering and an additional
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Fig. 1. A scheme of the experimental setup 1 — step-up transformer; 2 — rectifier; 3 — capacitive energy storage; 4 — limiting resistor; 5 — ignitrons arrestor; 6 — powder; 7 — dielectric matrix; 8 — upper electrode-punch; 9 — bottom electrode-punch. Table 1 Technical characteristics of the unit “Impulse-BM”. The name of the characteristic
The value
A maximum energy of the accumulator, kJ A maximum capacity of the accumulator, μF A range of voltage, kV A maximum intrinsic frequency of a discharge, kHz A maximum effort of pressing, kN
32 1800 1…5.9 20 5
processing of sintered intermediates (under pressure, mechanical, chemical-thermal processing and others) [6,7]. As for the powders of refractory metals especially with a spherical shape of particles it is typical to have a low compression rate and formability, the inclination to exfoliation during compression. The external surface of compressing has a closed porosity due to a friction against the walls of a press-form. The addition of pore-forming and connecting substances brings foreign admixtures and significantly complicates a technological process of the porous material production. Thus, a development of technologies considering and excluding the above mentioned limitations is very urgent.
The production of porous materials with an electric discharge sintering from the powders of refractory metals is considered as very perspective for the development of such technologies. An electric discharge sintering (EDS) is one of the electric current sintering methods. The investigations of the possibility of using the powerful current pulses arising from the discharge of a high-voltage capacitor for powder sintering for the first time began in the late 1970s and in early 1980s [8,9]. The EDS essentially differs from the other methods of consolidating the direct influence of the electric current with a more powerful heat over a shorter time interval (10−4 ‐ 10−3 s). The EDS major advantage is the achievement of very high temperatures (103–104 K) of contacts between the powder particles while maintaining low temperatures within the particles themselves [10]. An important feature of EDS is a manifestation of the pinch effect [11], which occurs when the powder is passed through high-current transient electrical discharges. The resulting radial force from its own electromagnetic field reduces the size of the resulting product (shrinkage) in the direction perpendicular to the transmitted current. The theoretical aspects of a contact formation at EDS and the assessment resulting from the pinch effect of the radial pressure are considered in detail in the papers [12,13].
Table 2 Characteristics of the starting powders. Powder
Particle size, μm
Pycnometric density, kg/m3
The specific surface area, m2/kg
Micro-hardness, MPa
Bulk density, kg/m3
Tap density, kg/m3
Form factor, FF
Fluidity, s
Titan
160–200 315–400 40–63 10–63 10–40 5–40 3–30
4490 4510 8570 8570 8570 16,600 16,600
8.14 4.91 25.0 42.5 55.5 45.3 70.0
206.2 181.4 167.2 155.2 127.6 184.4 210.7
2710 2670 3700 3510 3010 7520 6840
2920 2820 3920 3704 3175 7730 7080
0.98 0.96 0.60 0.60 0.65 0.65 0.63
27 32 23 21 20 22 24
Niobium
Tantalum
Fig. 2. Topogramms of powder particles: a — titanium, b — niobium, c — tantalum.
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Fig. 3. The deformation of the spherical titanium powder particles under pressure in the EDS process.
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Fig. 5. A local melting of titanium powder samples in the current flow.
2. The study When passing through the powder of a powerful high-voltage discharge the oxide films on the surface of the particles are destroyed and the batch is pressed into the preform with a sufficient strength. The axial seal is done by means of charge electrodes punches, and a radial one is done by a magnetic field generated after passing a highvoltage discharge. The consolidation of particles occurs because of local melting of the contacts between the particles to form interparticle necks [8]. The degree of compaction of the charge depends on its properties and power parameters of the process. Due to the fact that during EDS the operations of pressing and sintering of powders are combined, the necessity of applying connecting and pore-forming substances disappears. The practical and economic advantage of EDS is the ease of extraction of the produced intermediates from a matrix [11], which may be manufactured from a cheap material and to be used over and over again. An open porosity of the surface of intermediates remains. It was found experimentally that by using a high-voltage electrical discharge there can be consolidated a majority of powders of pure metals, including refractory, metal alloy powders, amorphous powders and nanostructural materials, batch incorporating nanoparticles as well as a mixture of conductive and nonconductive components [4]. The aim of this work is an experimental study of the conformity of the EDS refractory metal powders and the definition of the main parameters of the technological process of the porous material production.
2.1. Materials used and experimental methods To carry out the studies, spherical powders of commercially pure titanium produced by the plasma dispersion were specially made. The selection of such powders is firstly due to the ease of the investigations of the contacts between the particles research during sintering and, secondly, due to the fact that the spherical powders in comparison with non-spherical ones provide the production of porous materials with more uniform and stable structure characteristics. Besides titanium powders, the powders of tantalum and niobium were selected as a material for investigations. Usually, their traditional technologies of sintering require the application of temperatures ~ 2000 °С and the expensive vacuum equipment [6,7]. The powders selected by the sizes of particles for the studies were distributed into fractions. To analyze the distribution of titanium particles within the fractions a laboratory vibrosieve “Analisette-3” was used. The investigations of the content of the tantalum and niobium powder fractions were carried out using a photo sedimentograph “Аnalisette-20” (Fritsch, Germany). A pycnometric density of powder particles was defined at the vacuum pycnometer. A specific surface of the powders was defined with a BET method at the analyzer of a specific surface “Accusorb 2100D” (Micromeritics, USA). A microhardness of powder particles was defined at a digital micro hardness gage “Micromet” (Bueher Met, Germany).
Fig. 4. The dependence of the resistivity of the titanium powders (a), the tantalum powders and niobium powders (b) on the compacting pressure P0 1 — titanium 160–200 μm; 2 — titanium 315–400 μm; 3 — tantalum 3–30 μm; 4 — tantalum 5–40 μm; 5 — niobium 10–40 μm; 6 — niobium 10–63 μm; 7 — niobium 40–63 μm.
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Fig. 6. The dependence of the bending strength of samples of powders of titanium (a), tantalum (b), niobium (c) with different particle sizes on the compression pressure P at constant values of a specific energy W and a discharge duration τ 1 — titanium 160–200 μm; 2 — titanium 315–400 μm; 3 — tantalum 3–30 μm; 4 — tantalum 5–40 μm; 5 — niobium 10–40 μm; 6 — niobium 10–63 μm; 7 — niobium 40–63 μm.
A poured density and a fluidity of the powders were defined using a calibrated funnel and with the method of Scott volumometer, the tap density as per ISO 3953-85. A microstructure of the powders and porous samples were studied with a metallographic microscope “Polivar” (Reichert, Austria). A form and a morphology of the particles surface were studied with the electronic scanning microscope “CamScan” (Oxford, Great Britain). A form of the powder particles (FF) was defined at a programmed complex of processing and analyzing of images “AUTOSCAN” (Spectroscopic systems, Belarus). To define a main structure and physicomechanic properties of porous materials the experimental samples of a cylindrical form with a diameter 6 mm and the length 12 mm were manufactured from the studied powders. To produce the experimental samples, a unit «Impulse-BM» was used (a company «Potok», Russia) with a capacitive energy storage system (a battery of condensers). To realize EDS, a powder was put into a matrix made of a quartz glass between two electrodepuncheons and an external force of pressing was applied to them to produce an initial electric contact between the particles (Fig. 1).
The process of EDS was performed in the open air with a constant atmospheric pressure. Technical parameters of the unit are presented in Table 1. A porosity of the experimental samples was defined with a hydrostatic weighing method. The investigations of the distribution of porosity, the pore sizes and the contact sizes between the powder particles in different sections of the samples were carried out at a programmed complex for processing and analysis of images “AUTOSCAN” (Spectroscopic systems, Belarus). The tests of the experimental samples durability for bending were carried out at a universal test unit 1195 (Instron, Great Britain) using a special accessory. To measure the electrical resistance of the test samples, a bridge of a constant current Р333 was used with a special accessory for their fixation. 2.2. Results and discussion The results of the studies of the basic properties of these powders are shown in Table 2.
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Fig. 7. The dependence of the porosity of samples of powders of titanium (a), tantalum (b), niobium (c) with different particle sizes on the compression pressure P at the constant values of a specific energy W and a discharge duration τ 1 — titanium 160–200 μm; 2 — titanium 315–400 μm; 3 — tantalum 3–30 μm; 4 — tantalum 5–40 μm; 5 — niobium 10–40 μm; 6 — niobium 10–63 μm; 7 — niobium 40–63 μm.
It is shown that the powder particles of niobium and tantalum have an irregular shape and have a much more developed surface as compared with the almost perfectly spherical powders of titanium (Fig. 2). The EDS process is characterized by a series of parameters that can be divided into mechanical and electrical ones. The main electrical parameters of EDS are a powder electrical resistance energy, a duration and a number of electric current pulses. The mechanical parameter of EDS is a compression pressure. When applying the pressure, a normal and a tangential load appears in particles contacts, which leads to a contact compression, a shear and a fracture of the surface layers of the particles. The result is a violation of surface adsorption layers and oxide films. During the process applying pressure, the number, the size and the quality of the contacts change, causing a change in the electrical resistivity of the powder. The resistivity value largely determines the kinetics of subsequent EDS, as well as a sintering conditions repeatability and stability properties of the resulting products. Besides the compacting pressure P0, which determines the initial electrical resistivity of the powder and when it is applied before the transmission of the electric discharge, a value of the pressing pressure P, later applied to the powder during and after the passage of the electric
discharge, has a large influence on the physical-mechanical properties of the articles. In this case there are three modes of application of pressure pressing P: 1) P = 0 (after pre-pressing of a powder a movable upper electrode punch is secured by a catch); 2) P = const (is not changed during the process EDS); 3) P N P0. By changing the final compression pressure P, we can regulate in a wide range the obtained properties of the products. The use of the first mode of application of the pressure compression produces samples with the highest porosity but a small strength. In the second application mode of the pressing pressure in comparison with the first one, the porosity of the samples is decreased as the strength is increased. This mode is mostly preferred for the preparation of the porous articles and is the most technologically advanced mode due to constant pressure throughout the process EDS. The third mode is a more preferable for production of the high density products, because in this case the metal under the influence of heating becomes plastic for a short period of time and is deformed easily, filling the interparticle space (Fig. 3). The features of this EDS mode application are not
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Fig. 8. The dependence of the resistivity of the sample titanium (a), tantalum (b), niobium (c) powders with a different particle size on the compression pressure P at constant values of a specific energy W and a discharge duration τ 1 — titanium 160–200 μm; 2 — titanium 315–400 μm; 3 — tantalum 3–30 μm; 4 — tantalum 5–40 μm; 5 — niobium 10–40 μm; 6 — niobium 10–63 μm; 7 — niobium 40–63 μm.
considered, as they are typical for high density products. Taking into consideration the above mentioned information, all the experiments concerning the production of porous materials were carried out with the second mode of pressure application. The amount of energy released in the powder under an electric current largely depends on its initial resistivity which in turn exerts pressure P0 applied to the powder to pass an electrical discharge. Therefore, the study of the powder resistivity dependence compaction pressure P0 is significant for understanding the flow of the EDS process. Under the influence of the applied pressure, the resistivity of powder initially falls sharply because of the formation of new interparticle contacts and increasing their area, as well as the partial destruction of the oxide film (Fig. 4). As the pressure is increased, the electrical resistivity of the powder is decreased less rapidly and tends to a constant value. In this area a drop of the resistivity is mainly due to an increase of the contacts. In this area the drop of the specific resistance is caused by the growth of the contact value. A powder with a smaller particle size has a larger resistivity due to its higher degree of oxidation in the initial state. Typically, the latter is associated with the technology of a powder production. The research allowed determining the effect of the initial value of the resistivity of the powder, defined by a compaction pressure P0 on the
process of EDS and the physicomechanical properties of the samples. It was found that in the area of a resistivity a sharp drop under pressure P0 b 10 MPa, passing an electric discharge, leads to the formation of the current channel and the melting of the powder in place of the current flow (Fig. 5). This is due to a high resistivity of the interparticle contacts powder (more than 10−1 Ω m) and as a result the increase of heat in the contact zones takes place there. At a pressure corresponding to the transition part of the curve (P0 = 10–20 MPa) there is a reduction of the electrical resistivity of powder to the values of 10− 1–10− 3 Ω m, the most optimal one for the production of porous materials. A further increase of pressure (P0 N 20 MPa) leads to a significant reduction of the powder electrical resistivity because of increasing the interparticle contact area and the energy, released when discharging, becomes insufficient for the powder sintering. The dependences of the ultimate bending strength and porosity of the test samples from the powders of niobium, tantalum and titanium with different particle sizes produced as a result of EDS from the compression pressure Р were studied with constant values of a specific energy W and the duration of a discharge τ (Figs. 6 and 7). From these curves it is seen that the greatest strength of the samples with minimum porosity values was obtained in the pressure range of
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Fig. 9. The dependence of the axial shrinkage of powders of titanium (a), tantalum (b), niobium (c) samples with a different particle size on the compression pressure P at constant values of a specific energy W and a discharge duration τ 1 — titanium 160–200 μm; 2 — titanium 315–400 μm; 3 — tantalum 3–30 μm; 4 — tantalum 5–40 μm; 5 — niobium 10–40 μm; 6 — niobium 10–63 μm; 7 — niobium 40–63 μm.
10–20 MPa, which corresponds to the transition area on the resistivity curve (Fig. 4). While studying the process of EDS it was found that the value of the electrical resistivity of the experimental samples is a convenient parameter to control other characteristics as it is clearly correlated with the quality of the contacts between the particles. The dependence of the electrical resistivity of samples on the pressing pressure P for all the investigated powders has a similar appearance (Fig. 8). This dependence can be divided into two sections. In the first segment a sharp drop in resistivity of the samples to a minimum value is observed due to the destruction of the oxide films and the formation of good metal contacts between the particles of the powder. The increase of the electrical resistivity of samples in the second region can be explained by a sharp reduction of the powder resistivity pressing at pressures more than 20 MPa, and as a result there is no sufficient heat of the process EDS. In the pressure range of 10–20 MPa for all the investigated powders, the value of the samples resistivity is minimal, leading to the conclusion of the optimality of this application mode of pressure at EDS. Under the influence of a compaction pressure P, the powder shrinkage in the direction of compression force takes place during the EDS. The shrinkage depends on the particle size of the powder and is increased
with decreasing of their size. With an increase of compaction pressure (Fig. 9), the axial shrinkage of the powder reaches a maximum value under pressure of 10–20 MPa and then begins to decrease. This is explained, as in the previous cases, by a decrease of the electrical resistivity of powder and a reduced heat during the electric discharge. As it was mentioned above, besides the external axial pressing pressure applied to the powder with via-electrodes punches during EDS, the radial pressure from the own magnetic field effects the powder due to the pinch effect. This pressure helps to seal the powder particles in a direction perpendicular to the flowing direction of the electric current and allows the sintered product to be removed from the mold. The external axial compression pressure and the pressure from its own magnetic field are interrelated and significantly affect the process of EDS. It was found that for small values of the external pressure (10 MPa) the pinch effect is dominating. The radial contraction of experimental samples in this case can reach 4–6% (Fig. 10). However, the radial shrinkage is observed only in the middle part of the samples, while in the portions adjacent to the electrode punches due to friction on the powder surface, the diameter of the samples is not changed. With an increase of the external pressure to 10–20 MPa the radial shrinkage of the samples is decreased to 1–1.5% which is
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Fig. 10. The dependence of the radial shrinkage of the samples of titanium (a), tantalum (b), niobium (c) powders of different particle sizes on the compression pressure P at constant values of a specific energy W and a discharge duration τ 1 — titanium 160–200 μm; 2 — titanium 315–400 μm; 3 — tantalum 3–30 μm; 4 — tantalum 5–40 μm; 5 — niobium 10–40 μm; 6 — niobium 10–63 μm; 7 — niobium 40–63 μm.
sufficient for their easy removal from the mold. It should be noted that in this case it is a porous sample (a porosity 30–50%) obtained at a relatively small axial compaction pressures (up to 20 MPa). The increase of the external pressure compression leads to a decrease in the role of the pinch effect and the disappearance of the radial shrinkage. The most important parameter of the EDS process, which has the strongest influence on its course and the properties of the resulting product is the energy of the electric discharge. Therefore, of a great importance is the study of the discharge energy dependence on the size of the product in order to establish a parameter independent of the scale factor parameter, which can be used for a further research. It was established that the increase of the transverse section and the length of the sample are proportional to the increase of the discharge energy necessary for its sintering (Fig. 11). This is because the enlargement of transverse section of the sample increases the number of wirings between electrode punches, which leads to the reduction of the electric current flowing in each chain. Therefore, for a normal
process of EDS the discharge energy should be increased. The linear dependence of discharge energy on the length of sintered sample is explained by a proportional powder resistivity increase, which also leads to a growth of the energy necessary for the powder sintering. The experiments showed that the length of the sample at a constant cross-sectional area at EDS can not exceed certain defined values for each powder. This is because with increasing a cross-sectional area, the probability of a local breakdown of the sample path is increased with the least electrical resistance. With increasing the length of the sample, these limits of a cross-sectional area are also increased. In this regard it was impossible to obtain the test specimens of the niobium and tantalum powders with a cross-sectional area of 60–80 mm2 (a diameter 9–10 mm) and 10 mm of length, but for the samples with the same cross-sectional area and with a twice larger length a breakdown was not observed. It was impossible to conduct more research related to a dependence of the limits of the sample sectional area on the sample length due to the limited power of the experimental facility.
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Fig. 11. The dependence of the discharge energy on the diameter of the samples with the length 10 mm (а) and the length of the samples with the diameter 6 mm (b) at a compression pressure Р = 15 MPa 1 — titanium 160–200 μm; 2 — titanium 315–400 μm; 3 — tantalum 3–30 μm; 4 — tantalum 5–40 μm; 5 — niobium 10–40 μm; 6 — niobium 10–63 μm; 7 — niobium 40–63 μm.
With the EDS samples of the same diameter but with a different length, a melting powder in the middle of the matrix is sometimes observed. Studies have shown that there is a limit length to a diameter ratio, at which the sample is baked uniformly throughout. For the examined tantalum and niobium powders, this ratio does not exceed 10, for spherical powders of titanium - 20. Melting of the powder with increasing length of the sample is explained as due to the uneven distribution of its density because of the particles friction against the matrix walls. When applying the external axial pressure to the electrodes punches, a powder density is more than in the zones adjacent to the electrode core and less in the middle of the sample. Therefore, in the middle of the sample, wherein the electrical resistivity is larger, there is a greater heat and the powder is melted. The attempts to obtain samples of niobium and tantalum powders with a diameter of 6 mm and a length of 60–80 mm were unsuccessful. However, the samples of the same diameter up to 120 mm may be prepared from a spherical titanium powder, which has a uniform density distribution and stacking. The EDS study of the process showed that the discharge energy required to produce samples of the same cross-sectional area but with a different length is increased in a direct proportion to the length of the sample. This same pattern was observed for EDS'ed samples having the same length but a different cross-sectional area. It can be concluded that the discharge energy required for EDS is directly proportional to the geometric volume of the sintered samples, and the specific energy
value, equal to the ratio of energy discharge to the volume of the sintered sample, is an important parameter to specify a powder particle size and a chemical composition. This parameter is constant over a range of values, within which the formation of a uniform porous structure of the sample takes place. Therefore, it is important to determine the upper and lower limits of the specific energy, at the achievement of which there is a loss of quality of the sintered sample. The study of the brittle fracture fractogramms of the experimental models suggests that when the EDS technology is installed, the shape of the particles of the investigated powders remains unchanged. Since the study of the contact zones of powders with an irregular shape of the surface (niobium and tantalum) is a difficult problem, the study of the formation of inter-particle contacts was conducted on s spherical titanium powder. It was found that titanium powder particles after EDS retain a spherical shape. The diameters of the formed contact necks after EDS are 0.1–0.2 of the diameter of the particles, and this enables to produce a porosity of the examined samples not less than 36–37% (Fig. 12). The conducted studies allowed stating the properties of the porous materials with the help of EDS at optimal modes (Table 3). The obtained data were used during the development of the technologies of the porous articles production for different applications. 3. The development and EDS process of porous materials The results of the studies allowed to develop and to suggest a technology of producing niobium and tantalum surface-porous anodes Table 3 The properties of porous materials obtained with EDS. Powder
Particle size, μm
Bending strength, MPa
Porosity, %
Resistivity, ×10−5, Ω m
Axial shrinkage, %
Radial shrinkage, %
Titan
160–200 315–400 40–63 10–63 10–40 5–40 3–30
30–33 35–37 35–38 30–33 22–26 45–50 25–30
37–38 36–37 39–40 40–41 42–43 41–42 43–44
1.15–1.25 1.05–1.10 0.95–1.10 1.10–1.15 1.15–1.20 1.15–1.20 1.30–1.40
8–9 5–6 5–6 9–10 10–11 8–9 10–11
1–1.5 1–2 1–2 1–2 1–2 1–2 1–2
Niobium
Fig. 12. A fractogramm of a fracture of the porous material of the spherical titanium powder.
Tantalum
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Fig. 13. Niobium volumetric-porous anodes of capacitors (а) and the structures of volumetric-porous anodes produced under a traditional technology of sintering in vacuum (b) and under a designed technology EDS (c).
Fig. 14. The structure of the porous material of spherical particles of titanium (a) and a bone tissue germinated into it (b).
of the oxide-semiconducting condensers based on EDS. The porous materials based on tantalum and niobium are used for the production of an oxide semiconductor capacitor. The modern technology of oxide-semiconductor capacitors is not perfect due to the process of obtaining their basic elements – a space-porous anode. For larger capacities together with small dimensions of a capacitor, a porous body currently obtained by sintering of a pre-compacted powder in vacuum [14] is used as an anode. One of the disadvantages of the process is the uneven distribution of the porosity in the volume of the anodes and a reduced porosity at the surface. This reduces the capacitance of the capacitors. The research allowed developing the technology of niobium and tantalum porous anode body-oxide-semiconductor capacitors based on EDS. The comparison of the structure of porous materials obtained by a traditional technology and EDS (Fig. 13), developed by the authors, shows that the new technology could improve the uniformity of the porosity distribution. The nascent EDS pinch effect can easily press out a volume-porous anode from the matrix without altering its surface layer, thereby obtaining a material with an open porosity and a high specific surface (30–80% higher than that of anodes produced by currently used technologies).
The titanium products are widely used in medicine due to their good biocompatibility of the material [15]. The use of porous titanium for the production of intraosseous implants of various designs opens broad prospects for the application of the EDS technology. A covering from spherical powder particles of titanium produced with EDS forms a dimensional porous structure on the surface of the titanium implant carrier in which the bone grows (Fig. 14), fixedly fixing it in the desired position. The designed technology of EDS of titanium powders by the authors can find a potential application. A porous titanium is widely used in the manufacture of dental implants, implants for the interbody fusion of the vertebrae, stents in various designs, in porous pin heads endocardial electrodes for electrical stimulation of the heart (Fig. 15). The application of these items in a practical medicine is allowed with the corresponding certificates after a set of tests. 4. Conclusions EDS is a unique technology of the consolidation of powders allowing to produce a wide spectrum of the materials to be used in many fields of engineering. This method has a potential to provide the important
Fig. 15. Medical devices made of porous titanium dental implants (a), an implant for interbody fusion of the vertebrae (b) and a porous head endocardial electrode (c).
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technological and economic gains specifying such its characteristics as a short duration of the process, a combination of molding and sintering in one operation, rejecting to use a protective atmosphere or vacuum in many cases, a high precision of producing the items of a given form. One of the specific features of the EDS method is a production of the materials with a high structural uniformity which is a guarantee of the uniformity of their physical properties. It should be noted that a list of consolidated with the EDS method powders is limited to the condition of their electric conduction due to the necessity of a direct passing of the electric current through the powder. An essential defect is the use of a high voltage equipment. But the present advantages are much more important than the noted defects. High temperatures arising from the electric discharge, a short duration of the EDS process, a combination of forming and sintering in one operation, allow to produce the materials from the powders on the basis of refractory metals, of the chemically active metals and alloys which are inclined to a rapid oxidation, of the poorly formable spherical powders. It should be noted in particular about the prospectivity of the EDS application to produce the materials with special properties such as a metallic glass, gradient materials, composites from dissimilar materials – hard alloys and steel, complex mechanically alloyed materials, metals with inclusions from superhard materials and into rising of the hard materials properties due to the formation of a micro and nanocrystal structure. It becomes possible due to a short period of sintering and as a result a complete consolidation takes place at minimal changes of a microstructure of the material. On the basis of the above it is possible to make a conclusion that the EDS method is a very perspective one and it must be further developed on a new element and material basis.
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Contents lists available at ScienceDirect
Int. Journal of Refractory Metals and Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
A porous materials production with an electric discharge sintering Minko Dzmitry a,b,⁎, Belyavin Klimenty a a b
Belarusian National Technical University, 65, Nezavisimost Avenue, 220013 Minsk, Belarus National Research Nuclear University MEPhI, 31, Kashirskoe highway, 115409 Moscow, Russia
a r t i c l e
i n f o
Article history: Received 2 January 2016 Accepted 24 May 2016 Available online 26 May 2016 Keywords: Refractory metal Powder High-voltage discharge Porous material
a b s t r a c t A short review of a porous materials production from the powders of titanium, niobium and tantalum with a high-voltage discharge current is presented. The experimental dependences of bending strength, porosity, specific electrical resistance, radial and axial shrinkage from the sizes of particles of the examined powders and from the parameters of the electric discharge are given. The maximum correlations of the diameters and the length of the experimental samples of porous powders are stated. The examples of perspective applications of produced porous materials in the manufactured articles of electronics and medicine are shown. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction A direct electric current passing through the powder is the most simple and economical method of producing powder materials. The first patent on using a direct electric current to heat the powder of the hard alloy during hot pressing was obtained in 1933 [1]. In 1944 for the first time the use of alternating current of industrial frequency with a mechanical pressure sintering powders of copper, brass, bronze and aluminum was proposed [2]. In 1955 for the first time the use of equipment for spot welding capacitor powders by sintering under external pressure was described [3]. A wide range of the electrical parameters of the impact on the powder causes a large number of these methods collectively called «an electric current sintering» [4]. The existing methods for the direct influence of the electric current have a number of features allowing them to be used successfully for powder materials of different densities. Common to all of these methods is that the consolidation of the particles takes place in a closed matrix compression molding and discharges DC or a pulsed electric current generating heat passing through the powder. As a result of the electric current effect, there is an intensive mass transfer in the solid phase at the contact areas between the neighboring particles of the powder. Furthermore, the surface of the powder particles in the contact zone can be melted which is accompanied by a more intensive mass transfer. The result is a rapid consolidation of the particles throughout the volume of the powder. Depending on the magnitude of the compacting pressure and the range of values of the electrical
⁎ Corresponding author at: Belarusian National Technical University, 65, Nezavisimost Avenue, 220013 Minsk, Belarus. E-mail address: [email protected] (M. Dzmitry).
http://dx.doi.org/10.1016/j.ijrmhm.2016.05.015 0263-4368/© 2016 Elsevier Ltd. All rights reserved.
parameters (the nature, the amplitude and the duration of the flow of the electric current), the consolidation process may proceed in different ways. This widely varies the structure and properties of the resulting powder materials. Methods of consolidating the powder with an electric current can in many cases dispense with the usage of protective gas or vacuum, and combine molding and sintering of powder performs in a single operation. One of the significant trends of the powder metallurgy is a porous material production, the efficiency and the field of application of which are defined by their pore structure capable of passing liquids and gases through itself [5]. The most propagation is due to the application of porous materials as filters used for the separation of gases and liquids from foreign admixtures, for the transportation of liquids in pore channels under the effect of capillary forces in capillary pumps, heat pipes wicks, evaporators, condensers, as fire blocks and noise suppressers and in many other cases. The porous materials are produced from the powders of metals, oxide and nitride ceramics, glasses, polymers and other materials. Each of these materials has a unique set of characteristics and properties and can be used in different fields of engineering. The porous materials produced from the powders of refractory metals – titanium, niobium, tantalum, molybdenum, tungsten and their alloys – have the unique properties of refractoriness, hardness, corrosive stableness, wear resistance and many specific characteristics. The production of porous materials from the refractory metal powders includes some traditional technological operations of the powder metallurgy: a preparation of the initial powder including dissemination and mixing with pore-forming substances; a forming of intermediates as a rule, under pressure applied; a sintering and an additional
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Fig. 1. A scheme of the experimental setup 1 — step-up transformer; 2 — rectifier; 3 — capacitive energy storage; 4 — limiting resistor; 5 — ignitrons arrestor; 6 — powder; 7 — dielectric matrix; 8 — upper electrode-punch; 9 — bottom electrode-punch. Table 1 Technical characteristics of the unit “Impulse-BM”. The name of the characteristic
The value
A maximum energy of the accumulator, kJ A maximum capacity of the accumulator, μF A range of voltage, kV A maximum intrinsic frequency of a discharge, kHz A maximum effort of pressing, kN
32 1800 1…5.9 20 5
processing of sintered intermediates (under pressure, mechanical, chemical-thermal processing and others) [6,7]. As for the powders of refractory metals especially with a spherical shape of particles it is typical to have a low compression rate and formability, the inclination to exfoliation during compression. The external surface of compressing has a closed porosity due to a friction against the walls of a press-form. The addition of pore-forming and connecting substances brings foreign admixtures and significantly complicates a technological process of the porous material production. Thus, a development of technologies considering and excluding the above mentioned limitations is very urgent.
The production of porous materials with an electric discharge sintering from the powders of refractory metals is considered as very perspective for the development of such technologies. An electric discharge sintering (EDS) is one of the electric current sintering methods. The investigations of the possibility of using the powerful current pulses arising from the discharge of a high-voltage capacitor for powder sintering for the first time began in the late 1970s and in early 1980s [8,9]. The EDS essentially differs from the other methods of consolidating the direct influence of the electric current with a more powerful heat over a shorter time interval (10−4 ‐ 10−3 s). The EDS major advantage is the achievement of very high temperatures (103–104 K) of contacts between the powder particles while maintaining low temperatures within the particles themselves [10]. An important feature of EDS is a manifestation of the pinch effect [11], which occurs when the powder is passed through high-current transient electrical discharges. The resulting radial force from its own electromagnetic field reduces the size of the resulting product (shrinkage) in the direction perpendicular to the transmitted current. The theoretical aspects of a contact formation at EDS and the assessment resulting from the pinch effect of the radial pressure are considered in detail in the papers [12,13].
Table 2 Characteristics of the starting powders. Powder
Particle size, μm
Pycnometric density, kg/m3
The specific surface area, m2/kg
Micro-hardness, MPa
Bulk density, kg/m3
Tap density, kg/m3
Form factor, FF
Fluidity, s
Titan
160–200 315–400 40–63 10–63 10–40 5–40 3–30
4490 4510 8570 8570 8570 16,600 16,600
8.14 4.91 25.0 42.5 55.5 45.3 70.0
206.2 181.4 167.2 155.2 127.6 184.4 210.7
2710 2670 3700 3510 3010 7520 6840
2920 2820 3920 3704 3175 7730 7080
0.98 0.96 0.60 0.60 0.65 0.65 0.63
27 32 23 21 20 22 24
Niobium
Tantalum
Fig. 2. Topogramms of powder particles: a — titanium, b — niobium, c — tantalum.
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Fig. 3. The deformation of the spherical titanium powder particles under pressure in the EDS process.
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Fig. 5. A local melting of titanium powder samples in the current flow.
2. The study When passing through the powder of a powerful high-voltage discharge the oxide films on the surface of the particles are destroyed and the batch is pressed into the preform with a sufficient strength. The axial seal is done by means of charge electrodes punches, and a radial one is done by a magnetic field generated after passing a highvoltage discharge. The consolidation of particles occurs because of local melting of the contacts between the particles to form interparticle necks [8]. The degree of compaction of the charge depends on its properties and power parameters of the process. Due to the fact that during EDS the operations of pressing and sintering of powders are combined, the necessity of applying connecting and pore-forming substances disappears. The practical and economic advantage of EDS is the ease of extraction of the produced intermediates from a matrix [11], which may be manufactured from a cheap material and to be used over and over again. An open porosity of the surface of intermediates remains. It was found experimentally that by using a high-voltage electrical discharge there can be consolidated a majority of powders of pure metals, including refractory, metal alloy powders, amorphous powders and nanostructural materials, batch incorporating nanoparticles as well as a mixture of conductive and nonconductive components [4]. The aim of this work is an experimental study of the conformity of the EDS refractory metal powders and the definition of the main parameters of the technological process of the porous material production.
2.1. Materials used and experimental methods To carry out the studies, spherical powders of commercially pure titanium produced by the plasma dispersion were specially made. The selection of such powders is firstly due to the ease of the investigations of the contacts between the particles research during sintering and, secondly, due to the fact that the spherical powders in comparison with non-spherical ones provide the production of porous materials with more uniform and stable structure characteristics. Besides titanium powders, the powders of tantalum and niobium were selected as a material for investigations. Usually, their traditional technologies of sintering require the application of temperatures ~ 2000 °С and the expensive vacuum equipment [6,7]. The powders selected by the sizes of particles for the studies were distributed into fractions. To analyze the distribution of titanium particles within the fractions a laboratory vibrosieve “Analisette-3” was used. The investigations of the content of the tantalum and niobium powder fractions were carried out using a photo sedimentograph “Аnalisette-20” (Fritsch, Germany). A pycnometric density of powder particles was defined at the vacuum pycnometer. A specific surface of the powders was defined with a BET method at the analyzer of a specific surface “Accusorb 2100D” (Micromeritics, USA). A microhardness of powder particles was defined at a digital micro hardness gage “Micromet” (Bueher Met, Germany).
Fig. 4. The dependence of the resistivity of the titanium powders (a), the tantalum powders and niobium powders (b) on the compacting pressure P0 1 — titanium 160–200 μm; 2 — titanium 315–400 μm; 3 — tantalum 3–30 μm; 4 — tantalum 5–40 μm; 5 — niobium 10–40 μm; 6 — niobium 10–63 μm; 7 — niobium 40–63 μm.
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Fig. 6. The dependence of the bending strength of samples of powders of titanium (a), tantalum (b), niobium (c) with different particle sizes on the compression pressure P at constant values of a specific energy W and a discharge duration τ 1 — titanium 160–200 μm; 2 — titanium 315–400 μm; 3 — tantalum 3–30 μm; 4 — tantalum 5–40 μm; 5 — niobium 10–40 μm; 6 — niobium 10–63 μm; 7 — niobium 40–63 μm.
A poured density and a fluidity of the powders were defined using a calibrated funnel and with the method of Scott volumometer, the tap density as per ISO 3953-85. A microstructure of the powders and porous samples were studied with a metallographic microscope “Polivar” (Reichert, Austria). A form and a morphology of the particles surface were studied with the electronic scanning microscope “CamScan” (Oxford, Great Britain). A form of the powder particles (FF) was defined at a programmed complex of processing and analyzing of images “AUTOSCAN” (Spectroscopic systems, Belarus). To define a main structure and physicomechanic properties of porous materials the experimental samples of a cylindrical form with a diameter 6 mm and the length 12 mm were manufactured from the studied powders. To produce the experimental samples, a unit «Impulse-BM» was used (a company «Potok», Russia) with a capacitive energy storage system (a battery of condensers). To realize EDS, a powder was put into a matrix made of a quartz glass between two electrodepuncheons and an external force of pressing was applied to them to produce an initial electric contact between the particles (Fig. 1).
The process of EDS was performed in the open air with a constant atmospheric pressure. Technical parameters of the unit are presented in Table 1. A porosity of the experimental samples was defined with a hydrostatic weighing method. The investigations of the distribution of porosity, the pore sizes and the contact sizes between the powder particles in different sections of the samples were carried out at a programmed complex for processing and analysis of images “AUTOSCAN” (Spectroscopic systems, Belarus). The tests of the experimental samples durability for bending were carried out at a universal test unit 1195 (Instron, Great Britain) using a special accessory. To measure the electrical resistance of the test samples, a bridge of a constant current Р333 was used with a special accessory for their fixation. 2.2. Results and discussion The results of the studies of the basic properties of these powders are shown in Table 2.
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Fig. 7. The dependence of the porosity of samples of powders of titanium (a), tantalum (b), niobium (c) with different particle sizes on the compression pressure P at the constant values of a specific energy W and a discharge duration τ 1 — titanium 160–200 μm; 2 — titanium 315–400 μm; 3 — tantalum 3–30 μm; 4 — tantalum 5–40 μm; 5 — niobium 10–40 μm; 6 — niobium 10–63 μm; 7 — niobium 40–63 μm.
It is shown that the powder particles of niobium and tantalum have an irregular shape and have a much more developed surface as compared with the almost perfectly spherical powders of titanium (Fig. 2). The EDS process is characterized by a series of parameters that can be divided into mechanical and electrical ones. The main electrical parameters of EDS are a powder electrical resistance energy, a duration and a number of electric current pulses. The mechanical parameter of EDS is a compression pressure. When applying the pressure, a normal and a tangential load appears in particles contacts, which leads to a contact compression, a shear and a fracture of the surface layers of the particles. The result is a violation of surface adsorption layers and oxide films. During the process applying pressure, the number, the size and the quality of the contacts change, causing a change in the electrical resistivity of the powder. The resistivity value largely determines the kinetics of subsequent EDS, as well as a sintering conditions repeatability and stability properties of the resulting products. Besides the compacting pressure P0, which determines the initial electrical resistivity of the powder and when it is applied before the transmission of the electric discharge, a value of the pressing pressure P, later applied to the powder during and after the passage of the electric
discharge, has a large influence on the physical-mechanical properties of the articles. In this case there are three modes of application of pressure pressing P: 1) P = 0 (after pre-pressing of a powder a movable upper electrode punch is secured by a catch); 2) P = const (is not changed during the process EDS); 3) P N P0. By changing the final compression pressure P, we can regulate in a wide range the obtained properties of the products. The use of the first mode of application of the pressure compression produces samples with the highest porosity but a small strength. In the second application mode of the pressing pressure in comparison with the first one, the porosity of the samples is decreased as the strength is increased. This mode is mostly preferred for the preparation of the porous articles and is the most technologically advanced mode due to constant pressure throughout the process EDS. The third mode is a more preferable for production of the high density products, because in this case the metal under the influence of heating becomes plastic for a short period of time and is deformed easily, filling the interparticle space (Fig. 3). The features of this EDS mode application are not
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Fig. 8. The dependence of the resistivity of the sample titanium (a), tantalum (b), niobium (c) powders with a different particle size on the compression pressure P at constant values of a specific energy W and a discharge duration τ 1 — titanium 160–200 μm; 2 — titanium 315–400 μm; 3 — tantalum 3–30 μm; 4 — tantalum 5–40 μm; 5 — niobium 10–40 μm; 6 — niobium 10–63 μm; 7 — niobium 40–63 μm.
considered, as they are typical for high density products. Taking into consideration the above mentioned information, all the experiments concerning the production of porous materials were carried out with the second mode of pressure application. The amount of energy released in the powder under an electric current largely depends on its initial resistivity which in turn exerts pressure P0 applied to the powder to pass an electrical discharge. Therefore, the study of the powder resistivity dependence compaction pressure P0 is significant for understanding the flow of the EDS process. Under the influence of the applied pressure, the resistivity of powder initially falls sharply because of the formation of new interparticle contacts and increasing their area, as well as the partial destruction of the oxide film (Fig. 4). As the pressure is increased, the electrical resistivity of the powder is decreased less rapidly and tends to a constant value. In this area a drop of the resistivity is mainly due to an increase of the contacts. In this area the drop of the specific resistance is caused by the growth of the contact value. A powder with a smaller particle size has a larger resistivity due to its higher degree of oxidation in the initial state. Typically, the latter is associated with the technology of a powder production. The research allowed determining the effect of the initial value of the resistivity of the powder, defined by a compaction pressure P0 on the
process of EDS and the physicomechanical properties of the samples. It was found that in the area of a resistivity a sharp drop under pressure P0 b 10 MPa, passing an electric discharge, leads to the formation of the current channel and the melting of the powder in place of the current flow (Fig. 5). This is due to a high resistivity of the interparticle contacts powder (more than 10−1 Ω m) and as a result the increase of heat in the contact zones takes place there. At a pressure corresponding to the transition part of the curve (P0 = 10–20 MPa) there is a reduction of the electrical resistivity of powder to the values of 10− 1–10− 3 Ω m, the most optimal one for the production of porous materials. A further increase of pressure (P0 N 20 MPa) leads to a significant reduction of the powder electrical resistivity because of increasing the interparticle contact area and the energy, released when discharging, becomes insufficient for the powder sintering. The dependences of the ultimate bending strength and porosity of the test samples from the powders of niobium, tantalum and titanium with different particle sizes produced as a result of EDS from the compression pressure Р were studied with constant values of a specific energy W and the duration of a discharge τ (Figs. 6 and 7). From these curves it is seen that the greatest strength of the samples with minimum porosity values was obtained in the pressure range of
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Fig. 9. The dependence of the axial shrinkage of powders of titanium (a), tantalum (b), niobium (c) samples with a different particle size on the compression pressure P at constant values of a specific energy W and a discharge duration τ 1 — titanium 160–200 μm; 2 — titanium 315–400 μm; 3 — tantalum 3–30 μm; 4 — tantalum 5–40 μm; 5 — niobium 10–40 μm; 6 — niobium 10–63 μm; 7 — niobium 40–63 μm.
10–20 MPa, which corresponds to the transition area on the resistivity curve (Fig. 4). While studying the process of EDS it was found that the value of the electrical resistivity of the experimental samples is a convenient parameter to control other characteristics as it is clearly correlated with the quality of the contacts between the particles. The dependence of the electrical resistivity of samples on the pressing pressure P for all the investigated powders has a similar appearance (Fig. 8). This dependence can be divided into two sections. In the first segment a sharp drop in resistivity of the samples to a minimum value is observed due to the destruction of the oxide films and the formation of good metal contacts between the particles of the powder. The increase of the electrical resistivity of samples in the second region can be explained by a sharp reduction of the powder resistivity pressing at pressures more than 20 MPa, and as a result there is no sufficient heat of the process EDS. In the pressure range of 10–20 MPa for all the investigated powders, the value of the samples resistivity is minimal, leading to the conclusion of the optimality of this application mode of pressure at EDS. Under the influence of a compaction pressure P, the powder shrinkage in the direction of compression force takes place during the EDS. The shrinkage depends on the particle size of the powder and is increased
with decreasing of their size. With an increase of compaction pressure (Fig. 9), the axial shrinkage of the powder reaches a maximum value under pressure of 10–20 MPa and then begins to decrease. This is explained, as in the previous cases, by a decrease of the electrical resistivity of powder and a reduced heat during the electric discharge. As it was mentioned above, besides the external axial pressing pressure applied to the powder with via-electrodes punches during EDS, the radial pressure from the own magnetic field effects the powder due to the pinch effect. This pressure helps to seal the powder particles in a direction perpendicular to the flowing direction of the electric current and allows the sintered product to be removed from the mold. The external axial compression pressure and the pressure from its own magnetic field are interrelated and significantly affect the process of EDS. It was found that for small values of the external pressure (10 MPa) the pinch effect is dominating. The radial contraction of experimental samples in this case can reach 4–6% (Fig. 10). However, the radial shrinkage is observed only in the middle part of the samples, while in the portions adjacent to the electrode punches due to friction on the powder surface, the diameter of the samples is not changed. With an increase of the external pressure to 10–20 MPa the radial shrinkage of the samples is decreased to 1–1.5% which is
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Fig. 10. The dependence of the radial shrinkage of the samples of titanium (a), tantalum (b), niobium (c) powders of different particle sizes on the compression pressure P at constant values of a specific energy W and a discharge duration τ 1 — titanium 160–200 μm; 2 — titanium 315–400 μm; 3 — tantalum 3–30 μm; 4 — tantalum 5–40 μm; 5 — niobium 10–40 μm; 6 — niobium 10–63 μm; 7 — niobium 40–63 μm.
sufficient for their easy removal from the mold. It should be noted that in this case it is a porous sample (a porosity 30–50%) obtained at a relatively small axial compaction pressures (up to 20 MPa). The increase of the external pressure compression leads to a decrease in the role of the pinch effect and the disappearance of the radial shrinkage. The most important parameter of the EDS process, which has the strongest influence on its course and the properties of the resulting product is the energy of the electric discharge. Therefore, of a great importance is the study of the discharge energy dependence on the size of the product in order to establish a parameter independent of the scale factor parameter, which can be used for a further research. It was established that the increase of the transverse section and the length of the sample are proportional to the increase of the discharge energy necessary for its sintering (Fig. 11). This is because the enlargement of transverse section of the sample increases the number of wirings between electrode punches, which leads to the reduction of the electric current flowing in each chain. Therefore, for a normal
process of EDS the discharge energy should be increased. The linear dependence of discharge energy on the length of sintered sample is explained by a proportional powder resistivity increase, which also leads to a growth of the energy necessary for the powder sintering. The experiments showed that the length of the sample at a constant cross-sectional area at EDS can not exceed certain defined values for each powder. This is because with increasing a cross-sectional area, the probability of a local breakdown of the sample path is increased with the least electrical resistance. With increasing the length of the sample, these limits of a cross-sectional area are also increased. In this regard it was impossible to obtain the test specimens of the niobium and tantalum powders with a cross-sectional area of 60–80 mm2 (a diameter 9–10 mm) and 10 mm of length, but for the samples with the same cross-sectional area and with a twice larger length a breakdown was not observed. It was impossible to conduct more research related to a dependence of the limits of the sample sectional area on the sample length due to the limited power of the experimental facility.
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Fig. 11. The dependence of the discharge energy on the diameter of the samples with the length 10 mm (а) and the length of the samples with the diameter 6 mm (b) at a compression pressure Р = 15 MPa 1 — titanium 160–200 μm; 2 — titanium 315–400 μm; 3 — tantalum 3–30 μm; 4 — tantalum 5–40 μm; 5 — niobium 10–40 μm; 6 — niobium 10–63 μm; 7 — niobium 40–63 μm.
With the EDS samples of the same diameter but with a different length, a melting powder in the middle of the matrix is sometimes observed. Studies have shown that there is a limit length to a diameter ratio, at which the sample is baked uniformly throughout. For the examined tantalum and niobium powders, this ratio does not exceed 10, for spherical powders of titanium - 20. Melting of the powder with increasing length of the sample is explained as due to the uneven distribution of its density because of the particles friction against the matrix walls. When applying the external axial pressure to the electrodes punches, a powder density is more than in the zones adjacent to the electrode core and less in the middle of the sample. Therefore, in the middle of the sample, wherein the electrical resistivity is larger, there is a greater heat and the powder is melted. The attempts to obtain samples of niobium and tantalum powders with a diameter of 6 mm and a length of 60–80 mm were unsuccessful. However, the samples of the same diameter up to 120 mm may be prepared from a spherical titanium powder, which has a uniform density distribution and stacking. The EDS study of the process showed that the discharge energy required to produce samples of the same cross-sectional area but with a different length is increased in a direct proportion to the length of the sample. This same pattern was observed for EDS'ed samples having the same length but a different cross-sectional area. It can be concluded that the discharge energy required for EDS is directly proportional to the geometric volume of the sintered samples, and the specific energy
value, equal to the ratio of energy discharge to the volume of the sintered sample, is an important parameter to specify a powder particle size and a chemical composition. This parameter is constant over a range of values, within which the formation of a uniform porous structure of the sample takes place. Therefore, it is important to determine the upper and lower limits of the specific energy, at the achievement of which there is a loss of quality of the sintered sample. The study of the brittle fracture fractogramms of the experimental models suggests that when the EDS technology is installed, the shape of the particles of the investigated powders remains unchanged. Since the study of the contact zones of powders with an irregular shape of the surface (niobium and tantalum) is a difficult problem, the study of the formation of inter-particle contacts was conducted on s spherical titanium powder. It was found that titanium powder particles after EDS retain a spherical shape. The diameters of the formed contact necks after EDS are 0.1–0.2 of the diameter of the particles, and this enables to produce a porosity of the examined samples not less than 36–37% (Fig. 12). The conducted studies allowed stating the properties of the porous materials with the help of EDS at optimal modes (Table 3). The obtained data were used during the development of the technologies of the porous articles production for different applications. 3. The development and EDS process of porous materials The results of the studies allowed to develop and to suggest a technology of producing niobium and tantalum surface-porous anodes Table 3 The properties of porous materials obtained with EDS. Powder
Particle size, μm
Bending strength, MPa
Porosity, %
Resistivity, ×10−5, Ω m
Axial shrinkage, %
Radial shrinkage, %
Titan
160–200 315–400 40–63 10–63 10–40 5–40 3–30
30–33 35–37 35–38 30–33 22–26 45–50 25–30
37–38 36–37 39–40 40–41 42–43 41–42 43–44
1.15–1.25 1.05–1.10 0.95–1.10 1.10–1.15 1.15–1.20 1.15–1.20 1.30–1.40
8–9 5–6 5–6 9–10 10–11 8–9 10–11
1–1.5 1–2 1–2 1–2 1–2 1–2 1–2
Niobium
Fig. 12. A fractogramm of a fracture of the porous material of the spherical titanium powder.
Tantalum
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Fig. 13. Niobium volumetric-porous anodes of capacitors (а) and the structures of volumetric-porous anodes produced under a traditional technology of sintering in vacuum (b) and under a designed technology EDS (c).
Fig. 14. The structure of the porous material of spherical particles of titanium (a) and a bone tissue germinated into it (b).
of the oxide-semiconducting condensers based on EDS. The porous materials based on tantalum and niobium are used for the production of an oxide semiconductor capacitor. The modern technology of oxide-semiconductor capacitors is not perfect due to the process of obtaining their basic elements – a space-porous anode. For larger capacities together with small dimensions of a capacitor, a porous body currently obtained by sintering of a pre-compacted powder in vacuum [14] is used as an anode. One of the disadvantages of the process is the uneven distribution of the porosity in the volume of the anodes and a reduced porosity at the surface. This reduces the capacitance of the capacitors. The research allowed developing the technology of niobium and tantalum porous anode body-oxide-semiconductor capacitors based on EDS. The comparison of the structure of porous materials obtained by a traditional technology and EDS (Fig. 13), developed by the authors, shows that the new technology could improve the uniformity of the porosity distribution. The nascent EDS pinch effect can easily press out a volume-porous anode from the matrix without altering its surface layer, thereby obtaining a material with an open porosity and a high specific surface (30–80% higher than that of anodes produced by currently used technologies).
The titanium products are widely used in medicine due to their good biocompatibility of the material [15]. The use of porous titanium for the production of intraosseous implants of various designs opens broad prospects for the application of the EDS technology. A covering from spherical powder particles of titanium produced with EDS forms a dimensional porous structure on the surface of the titanium implant carrier in which the bone grows (Fig. 14), fixedly fixing it in the desired position. The designed technology of EDS of titanium powders by the authors can find a potential application. A porous titanium is widely used in the manufacture of dental implants, implants for the interbody fusion of the vertebrae, stents in various designs, in porous pin heads endocardial electrodes for electrical stimulation of the heart (Fig. 15). The application of these items in a practical medicine is allowed with the corresponding certificates after a set of tests. 4. Conclusions EDS is a unique technology of the consolidation of powders allowing to produce a wide spectrum of the materials to be used in many fields of engineering. This method has a potential to provide the important
Fig. 15. Medical devices made of porous titanium dental implants (a), an implant for interbody fusion of the vertebrae (b) and a porous head endocardial electrode (c).
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technological and economic gains specifying such its characteristics as a short duration of the process, a combination of molding and sintering in one operation, rejecting to use a protective atmosphere or vacuum in many cases, a high precision of producing the items of a given form. One of the specific features of the EDS method is a production of the materials with a high structural uniformity which is a guarantee of the uniformity of their physical properties. It should be noted that a list of consolidated with the EDS method powders is limited to the condition of their electric conduction due to the necessity of a direct passing of the electric current through the powder. An essential defect is the use of a high voltage equipment. But the present advantages are much more important than the noted defects. High temperatures arising from the electric discharge, a short duration of the EDS process, a combination of forming and sintering in one operation, allow to produce the materials from the powders on the basis of refractory metals, of the chemically active metals and alloys which are inclined to a rapid oxidation, of the poorly formable spherical powders. It should be noted in particular about the prospectivity of the EDS application to produce the materials with special properties such as a metallic glass, gradient materials, composites from dissimilar materials – hard alloys and steel, complex mechanically alloyed materials, metals with inclusions from superhard materials and into rising of the hard materials properties due to the formation of a micro and nanocrystal structure. It becomes possible due to a short period of sintering and as a result a complete consolidation takes place at minimal changes of a microstructure of the material. On the basis of the above it is possible to make a conclusion that the EDS method is a very perspective one and it must be further developed on a new element and material basis.
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