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Apr 2, 2014 - density of the consolidated zirconium powder alloy coatings on ceramic and metallic ... Methods of electric pulse consolidation of powder.
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DOI: 10.1002/adem.201400106

Structure of Zirconium Alloy Powder Coatings Processed by High Voltage Electric Discharge Consolidation** By Evgeny G. Grigoryev, Lyuba Yu. Lebedeva, Oleg L. Khasanov and Eugene A. Olevsky*

Zirconium alloy powders (Zr þ 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. 1. Introduction Methods of electric pulse consolidation of powder materials are related to high-performance techniques of sintering, which are presently intensively developing. Their main principles consist in the joint action of a powerful electrical discharge and of mechanical pressure on the processed material. The main advantage of these methods is in the opportunity to closely control within wide ranges, the powder processing parameters like amplitude, duration, and

[*] E. A. Olevsky, E. G. Grigoryev, L. Yu. Lebedeva Key Laboratory for Electromagnetic Field Assisted Materials Processing, Moscow Engineering Physics Institute, Moscow, Russian Federation O. L. Khasanov, E. A. Olevsky National Research Tomsk Polytechnic University, Tomsk, Russian Federation E. A. Olevsky Powder Technology Laboratory, San Diego State University, San Diego, CA, USA E-mail: [email protected] [**] The support of the Ministry of Science and Education of Russian Federation (grant 11.G34.31.0051) is gratefully appreciated. The support of the San Diego State University researcher by the US Department of Energy, Materials Sciences Division, under Award No. DE-SC0008581 is gratefully acknowledged. The support of the National Research Tomsk Polytechnic University is gratefully acknowledged. 792

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form of the pulses of the electric current; the magnitude of these ranges is determined by the capacity of the experimental equipment. Another benefit is these methods’ capability to obtain ceramic and metallic powder specimens, such as freestanding components or powder coatings, of high quality at low prime cost. The additional undeniable advantage of these techniques is the short or ultra-short process duration resulting in the increased productivity and, frequently, in the dramatically improved structure of the manufactured components. The rising interest towards electric pulse consolidation methods of powder materials is reflected in the ever-growing number of the related scientific publications.[1–10] Methods of electro-consolidation can be sub-divided into two main groups[1]: (i) the ones using low- and moderatevoltage current sources (voltages of the order of tens of volts and electric current of the order of thousands of amperes), the known techniques, such as spark-plasma sintering and resistance sintering belong to this group; and (ii) the ones using high-voltage power supplies (high voltage capacitor discharge through a powder sample, the amplitude of the voltage—tens of kilovolts, the amplitude of the current pulse— hundreds of thousands of amperes)—the related techniques include electric discharge sintering (or high voltage electric discharge consolidation—HVEDC). The distinctive features of HVEDC methods are: high rate of heating, low integral temperature of consolidation, ultra-short duration of the process of consolidation. Full advantages of the methods of HVEDC can be realized under optimal process parameters. Intense electro-thermal effects occurring inside a

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E. G. Grigoryev et al./Structure of Zirconium Alloy Powder Coatings …

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powder ceramic or metallic material can lead to the formation of the inhomogeneous structure of the treated specimen, to the instability of the consolidation process, and even to the destruction of the resulting products and of the industrial equipment. Optimal parameters of the high voltage electric discharge consolidation strongly depend on the initial properties of consolidated powders, in particular, on the conductivity of ceramic or metal powders and alloys under applied mechanical pressure. The conductivity of ceramic or metal powders is directly dependent on the size distribution of powder particles. Thereby, the analysis of the influence of the particle size and particle shape of a powder on the HVEDC process outcomes is of considerable importance. In Ref.,[11] the effect of particle size and pressure on the densification mechanisms in spark plasma sintering was studied. In the present study, we investigate the density of the twodimensional (coatings) and three-dimensional samples obtained by high-voltage electric discharge consolidation, depending on the different shapes of powder particles (for zirconium alloy Zr þ 1% Nb powders deposited on a ceramic or metallic substrates or processed as a free-standing powder component) and depending on the amplitude of the pulse electric current. Alloy Zr þ 1% Nb can be produced by various methods. For example, Ref.[12] describes the method of double vacuum arc remelting, which provides fewer impurities than when using a single melting technique. It has been shown that the chemical composition, quality of surface and properties of the alloy obtained by the double vacuum arc remelting were superior to the properties of the alloy produced by electron beam melting.[13] The choice within the present study of zirconium alloy powders as of the investigated processed materials is related to the usefulness and variety of these materials systems’ applications. Due to the high corrosion-resistant properties, functional zirconium-based coatings on ceramic substrates can be used in a variety of applications including neurosurgery and electronics. In small-volume, nonporous state zirconium is one of the most corrosion-resisting metals despite its very high chemical activity in powdery state. Zirconium high corrosion resistance is the result of the protective action of thin but dense and chemically resistant oxide film. Zirconium can be applied as a coating for operating elements on ceramics-based appliances and fittings. Zirconium weakly absorbs slow neutrons, which favors its application in the field of nuclear technology, especially in the construction of nuclear reactors.

Fig. 1. Schematic diagram of the measurement of electrical conductivity of powders.

Fig. 2. Spherical atomized powder particles of alloy Zr þ 1% Nb.

and 3), and (iii) the schematic diagram of the high voltage electric discharge consolidation apparatus (Figure 4). Figure 1 shows the schematics of the apparatus for measuring the electrical conductivity of powders. A powder sample (3) (mass of sample 1–3 g) is compressed between steel

2. Experimental Section The electrical conductivity of a powder compact depends on the properties of surface films surrounding powder particles, on the externally applied pressure, and on the pulse electric current parameters. This section provides (i) the schematic diagram of the powder electrical conductivity measuring apparatus (Figure 1), (ii) the characteristics of the powder particles (Figure 2 ADVANCED ENGINEERING MATERIALS 2014, 16, No. 6

Fig. 3. Flake atomized powder particles of alloy Zr þ 1% Nb.

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Fig. 4. Schematic diagram of high voltage electric discharge consolidation apparatus.

punches (2, 6) (punch diameter is 1 cm). The pressure is produced by the hydraulic press, and is measured by the dynamometer (1). A fiberglass die (4) has been utilized. Shrinkage of the powder sample was determined by the displacement sensor (5) with an accuracy of 10 microns. Density of samples was determined by mass conservation. The electric current source (7) has created a constant electric current (1–100 mA) through the punches and the powder sample. Electronic voltmeters (9, 10) determine voltage potential in between the punches, and the standard ohmic resistance (8). The electric conductivity of the powder was determined by the measured electric current and voltage (with the relative error of 0.2%). We investigated the electric conductivity of the industrial atomized powder (alloy Zr þ 1% Nb) with particles of spherical (Figure2) and flake (Figure 3) shapes. The amount of the generated Joule heat and the temperature of the consolidated sample depend on the conductivity of the ceramic or metallic powder material. Hence, the optimal parameters are different for the high voltage electric discharge consolidation of powders of spherical and of flake shape. The schematic of a high voltage electric discharge consolidation system is shown in Figure 4. The HVEDC apparatus consists basically of: a charging unit (1); a bank of capacitors (2); trigatron switch (3); and an electrical discharge ignition system (4). The capacitor bank consists of thirty 200 mF capacitors that can store up to 6 kV. HVEDC uses the pulse current generated from the capacitor bank to quickly heat a powder backfill column subjected to constant pressure during the process. Powder column (8) was a circular crosssection rod of 13.7 mm diameter and of 80 mm length. Before processing, the powder (8) (which may include a single phase material or a layered substrate-coating structure) is poured into an electrically non-conducting ceramic die (which in special cases can be utilized as a future ceramic substrate). In the technological pressure unit (6) the ceramic die is connected at the bottom end and at the top end with molybdenum electrode-punches, and an external pressure of up to 400 MPa is applied to the powder by pneumatic press (5). A high voltage capacitor bank is discharged through the powder 794

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column. The discharge current is measured by a toroidal Rogowsky coil (7) bent around the powder column. The HVEDC method uses the passage of the pulse electric current to provide the resistive heating of the powder by the Joule effect. The Joule heating occurs at the interparticle contacts to instantaneously weld powder particles, resulting in powder densification. The achieved powder compact density as a result of HVEDC process depends on the applied external pneumatic pressure, the magnitude and the waveform of the pulse current that depends on RLC— parameters of the electric discharge circuit. Density measurements after the high voltage electric discharge consolidation process were performed using the Archimedes principle in distilled water. The surfaces and cross-sections of the powder compacts consolidated by HVEDC process were polished for microstructural observation by optical microscopy. 3. Experimental Results It was found that the electric conductivity and shrinkage increase monotonically during 1–2 min after the application of pressure. We have applied pressure for 2 min before the measurements of conductivity. Experimental data of conductivity of the powders of spherical and flake forms, depending on the applied pressure, are shown in Figure 5. These dependences (curves 1, 2) indicate that for the pressure of up to 60 MPa, particles of spherical and flake powders deform elastically. Figure 5 shows that the dependence of the conductivity of flake powder is changed at a

Fig. 5. The dependence of the electrical conductivity of the industrial atomized powder (alloy Zr þ 1% Nb) on pressure (1—flake form of particles, 2—spherical shape of particles).

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Fig. 6. The effect of the magnitude of the current pulse on the resulting density of the consolidated samples after HVEDC at different applied pressures for flake particles.

pressure of 60 MPa. At pressures above 60 MPa, flake particles begin to deform plastically and break down (curve 1). The dependence of the conductivity of the spherical powder (curve 2) does not have such a feature, and increases monotonically for the pressures of 0–350 MPa (Figure 5). In the pressure range of 250–350 MPa, the conductivities of the spherical and flake powders are approximately equal to each other (Figure 5). Figures 6 and 7 show the effect of the magnitude of the current pulse on the resulting density of the consolidated samples after HVEDC conducted under different applied pressures (P). Experimental dependence for flake particles shows a roughly parabolic trend, with a maximum for optimal current densities (Figure 6). The density of the consolidated samples is increased to a maximum value depending on the applied pressure with an increase in the amplitude of the current pulse. With a further increase in the amplitude of the current pulse, density of the consolidated samples is significantly reduced. The maximum density of the samples corresponds to about 180 kA cm2 for applied pressures of 165 and 190 MPa (curves 1 and 2 in Figure 6). The maximum density of the samples corresponds to about 280 kA cm2 for the applied pressure of 307 MPa (curve 4 in Figure 6). The different positions of the maxima of the density of the consolidated samples (curves 1, 2, and 4) indicates a significant influence of the friction at the surface of the die on the process of powder compaction. Experimental dependences of the final density of the consolidated samples (spherical powder of zirconium alloy with 1% Nb) on the amplitude of the current pulse are shown in Figure 7 for different applied pressures. The density of the consolidated samples in the range of current densities from 260 kA cm2 to 340 kA cm2 is constant for applied pressures of 250–350 MPa. When the pulse current densities increase above 340 kA cm2, the density of the consolidated samples decreases sharply (curves 1, 2, 3, Figure 7). In this case, the process of compaction of the powder sample at HVEDC becomes unstable with a possible ADVANCED ENGINEERING MATERIALS 2014, 16, No. 6

Fig. 7. The effect of the magnitude of the current pulse on the final density of the consolidated samples after HVEDC under different applied pressures for spherical particles.

blowout of consolidated material from the HVEDC tooling. The reason for the instability of the powder compaction process during HVEDC was considered in Ref.[14] It was found that the instability in the process of compaction was due to the cumulative nature of the collapse of inter-particle pores. High-density samples of the alloy (Zr þ 1% Nb) were obtained by HVEDC with optimal parameters (the applied pressure and the amplitude of the current pulse density) for powder particles of spherical shape and flake shape.

Fig. 8. The microstructure of the consolidated sample for flake particles.

Fig. 9. The microstructure of the consolidated sample for spherical particles.

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E. G. Grigoryev et al./Structure of Zirconium Alloy Powder Coatings … We examined experimental dependence of the electric conductivity of the powders of spherical shape and flake shape on the applied pressure. The influence of the current pulse amplitude on the final density of the consolidated zirconium powder alloy coatings and free-standing samples was investigated 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

Fig. 10. Consolidated by HVEDC coating and substrate.

Earlier obtained research results indicate that the most important factors, which determine the success of the HVEDC process are the current density amplitude in the powder specimen and the applied pressure [14]. We examined the effect of the amplitude of the current pulse on the density of the consolidated samples of Zr þ 1% Nb powder. Figures 8 and 9 show the typical microstructure of high-density samples of alloy Zr þ 1% Nb. Figure 8 shows the microstructure of the consolidated sample for flake particles and Figure 9 shows the microstructure of the consolidated sample for spherical particles. Microstructures of consolidated material shown in Figure 8 and 9 demonstrate the preservation of the initial microstructure of powder particles after the high voltage electric discharge consolidation. It should be noted that the different particle morphology required different pressure levels for obtaining the same levels of the densification. Figure 10 shows the microstructure of a consolidated by HVEDC coating on a substrate. This microstructure is typical for the produced interfaces between conductive (zirconium alloy) coatings and non-conductive (alumina) or conductive (stainless steel) substrates. Thereby, the carried out research indicates that this technology can be successfully applied for producing metal coatings on both ceramic and metallic substrates. The damage-free interface implies high adhesive strength of the obtained structures. 4. Conclusions Zirconium alloy powders (Zr þ 1% Nb) of spherical and flake forms were consolidated by HVEDC for the fabrication of coatings on ceramic and metallic substrates as well as of free-standing powder components. The amount of the generated Joule heat and the temperature of the consolidated sample depend on the conductivity of the powder material.

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consolidation. The conducted research indicates that this technology can be successfully applied for producing metal coatings on both ceramic and metallic substrates. The damage-free interface implies high adhesive strength of the obtained structures. Received: March 1, 2014 Final Version: March 4, 2014 Published online: April 2, 2014

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