RU2133173C1 - Process of production of powder with microcrystalline structure - Google Patents

Abstract

FIELD: preparation of powders with increased anticorrosion properties. SUBSTANCE: process includes continuous feed of metal wire and gas into zone of burning of electric arc, injection of formed drops of metal into liquid coolant and subsequent removal of powder. Mixture of air and hydrocarbon substance at temperature exceeding that of metal boiling is used in the capacity of gas, solution of glycerin or ethylamine in distilled water is employed as coolant. Fast hardening of metal particles dusted by air plasma leads to formation of microcrystalline structure with high concentration of defects. EFFECT: increased anticorrosion properties. 4 dwg, 2 tbl

Description

 The invention relates to a technology for the production of metal powders with a microcrystalline structure for powder metallurgy, galvanic cells, etc.

 A known method of producing metal powders, in which an arc is created between the electrodes that melts the ends of the electrodes, melt droplets fall on a cooled rotating disk, which scatters droplets due to centrifugal force (see, for example, PCT 88/01919, MKI B 22 F 9 / ten).

 The disadvantage of this method is that for cooling the particles an additional device is used in the form of a disk with a drive and a disk cooling system, a special chamber for collecting powder is needed, in an explosion-proof version due to the pyrophoricity of many fine powders.

 The closest to this invention in technical essence and the achieved result is a method of producing a metal powder with a microcrystalline structure by melting a wire continuously fed into the combustion zone of an electric arc and evacuating the drops of melt into a liquid cooler (see, for example, German application N 3233402, MKI B 22 F 9/14, 1980.

 However, in this method only one wire of the atomized material is used, which is the electrode. A water-cooled rotating disk will be used as the second electrode. A direct current arc burns between the wire and the disk, and the particles are discharged by a magnetic field specially created in the spray zone. The operations are carried out in an inert gas atmosphere, in a vacuum or in a bath with a liquid.

 One of the disadvantages of the prototype is the presence of a passive, non-consumable electrode disk, not involved in the process of powder formation, since it serves only to maintain arc burning. Another disadvantage is the fundamental impossibility of stable operation of the system with a rotating electrode on alternating current. Therefore, the prototype requires a special type of power source with a steeply falling (“bayonet”) characteristic (as for any freely burning arc). These shortcomings liquefy process performance and unnecessarily increase installation complexity.

 The powder obtained in this way does not have high anticorrosive properties, since the powder cooling process is not intensive enough.

 The aim of the present invention is to obtain a powder with a microcrystalline structure having enhanced anti-corrosion properties.

 This goal is achieved due to the fact that in the method of producing a metal powder with a microcrystalline structure, which includes the continuous supply of a metal wire and gas to the combustion zone of an electric arc, the introduction of metal droplets into a liquid cooler and subsequent extraction of the powder, combustion products are used as gas, for example , a stoichiometric mixture of air and a hydrocarbon substance at a temperature exceeding the boiling point of the metal, and when receiving zinc powder as the cooler use a solution of glycerol or ethylamine in distilled water, before extraction, the powder is separated into fractions in the cooler by centrifugal segregation, and the powder is extracted by evaporation of the cooler.

 An analysis of the technical and patent literature showed that despite the presence of a large number of works on obtaining materials with a microcrystalline structure (Doklady AN SSSR, 1982, vol. 267, N 3, pp. 619-623, accepted application of the United Kingdom National classification C7X N 1587895, 1978, and others), the full set of sequential actions described above was not found, which allows us to conclude that the proposed technical solution meets the criterion of "Novelty."

In FIG. 1 shows a structural diagram of a device for implementing this method;
In FIG. 2, 3 are diagrams of coaxial type plasmatrons in which the atomized metal is fed through a central electrode;
In FIG. 4 - installation diagram with a two-wire electric arc metallization apparatus;
A device for producing metal powder contains installed on the basis of: a continuous wire feed unit 1, a device for melting wire 2, a unit 3 for evacuating melt drops into which air is supplied from unit 4, and hydrocarbon gas from unit 5. A mixture of hydrocarbon gas from node 3 sprays the metal into a bath 6 with a liquid, which is used as a solution of ethylamines in distilled water. Granules of powder are separated into fractions in a separator of 7 fractions. After separation into fractions, the powder is dried in the powder drying unit 8, to which inert gas is supplied from the source 9. The device is equipped with a power source 10 and control devices 11 and control 12.

 A device for melting wire can be made in the form of a unit for melting 2 wires or in the form of plasmatrons with the melting of one rod by a coaxial stream of low-temperature plasma.

 The plasma torch (Fig. 2) consists of a hollow copper cooled electrode 13, mounted inside it along the axis of the copper cooled electrode 14, having an axial through channel 15 for supplying the molten wire material, a solenoid 16, worn on the electrode body 13, an exhaust replaceable shaped cooled nozzle 17 , node 18 for introducing a mixture of air with a hydrocarbon gas, which is a plasma-forming gas, containing an end rear cover-insulator 19 with openings 20 for supplying a plasma-forming gas, a spraying unit 2 dies.

The plasma torch works as follows. Water is supplied to the cooling systems of the electrodes 13, 14 and the nozzle 17. Then, through the holes 20 in the lid-insulator 19, gas is supplied into the cavity of the electrode 13. An arc discharge is excited in the annular gap between the electrodes 13 and 14. As a result of the interaction of the intrinsic magnetic field of the arc and the axial field of the solenoid 16, the arc sets in motion, rotating in the gap in the plane normal to the plasma torch axis. When gas flows through the zone of rotation of the arc, its temperature rises to 4000 - 4500 K. Since the arc is a poorly blown body, it swirls the gas flow during its movement. In order to avoid this effect (leading to peripheral separation of material particles), the openings 20 in the cover-insulator 19 are made so that the gas entering the plasma torch chamber acquires a swirl in the direction opposite to the direction of arc rotation. As a result, the gas passing through the arc zone completely loses the swirl along the axis of the plasma torch and enters the output nozzle 17. Through the axial channel 15 in the electrode 14, the block 1 feeds material in the form of a wire, which is heated, melted by the plasma and heated to a temperature close to the temperature boiling. The melt is sprayed with a mixture of air with hydrocarbon gas into a bath with a solution of ethylamines, providing a cooling rate of droplets of 10 ⁵ - 10 ⁷ deg / s.

Another embodiment of a plasma torch for producing powder is shown in FIG. 3. The plasma torch contains a solenoid 21, an external cooled electrode 22, an internal electrode 23, an insulating sleeve 24, through the channel 25 of which a sprayed material is supplied to the chamber 26, and a replaceable nozzle 27, consisting of an input narrowing section 28 and an output expanding 29,
The plasma torch works as follows. Plasma-forming gas (a mixture of air with a hydrocarbon gas) and coolant are fed into the plasmatron.

 An electric discharge is excited in the annular gap between the electrodes 22 and 23. Through the channel 25 in the insulating sleeve 24, a wire rod is fed into the chamber 26.

 In the cross section of the transition from the tapering cone to the expanding boundary layer, which is formed on the nozzle walls upstream, it breaks down and the passage section for gas turns out to be equal to the nozzle section. As a result, the gas flow through the nozzle increases. The plasma torch allows to obtain powders from bar and wire materials with a diameter of up to 20 mm, which makes it possible to use rolled ferrous and non-ferrous metals. The life of one pair of electrodes when working on compressed air directly from the compressor with a pressure of up to 0.6 MPa at a current of 500-700 A is approximately 150 hours.

 Melt and spraying can be carried out in a two-wire electric arc metallization apparatus. The installation diagram is shown in figure 4.

 The installation comprises a frame 27, a frame 28, wire cassettes 29, an electric arc metallizer 30, a block of tops 31, a powder receiver 32, a trolley 33 and a latch 34. Electrometallizers 30 are mounted with the possibility of vertical movement to adjust the distance to the free surface of the liquid in the powder receiver 32. Tops 31 are truncated cones with holes in the center and holes in the conical part for the gas outlet, offset relative to each other in a checkerboard pattern. The number of tops, the gap between them, the location in the receiver is determined experimentally for each powder. The installation of a separate unit includes a shielding gas generator 35, which consists of a fan, a gas burner, a combustion chamber and a shut-off and control valve system to adjust the coefficient of excess air and the consumption of combustion products.

 The implementation of the method is illustrated by the example of producing zinc powder in a two-wire electric arc metallization apparatus operating on alternating current and copper powder on the plasma torch shown in FIG. 4.

 An example implementation of the method.

The method is carried out in the following sequence:
1. The powder receiver is filled with an aqueous solution of ethylamine. The type of ethylamine and the concentration of the aqueous solution is determined previously, based on the heat capacity of the solution and the chemical activity of the resulting powder.

 2. The electric arc metallizer is mounted on the frame so that its nozzle faces down to the free surface of the liquid. The axis of the nozzle coincides with the axis of the powder receiver.

 3. The sprayed wire is cleaned of dirt and preservation grease, wound on cassettes and installed on the mounting bracket. The wire from both cassettes is threaded into an electric arc metallizer so that their ends are connected.

 4. Turn on the generator of the protective environment, set the flow rates of hydrocarbon gas and air to provide the required composition of the combustion products.

 5. Turn on the compressor and connect its inlet to the outlet line of the shielding gas generator.

 6. Open the gas supply from the compressor to the electric arc metallizer and set its flow rate equal to a predetermined value for each material and the required dispersion of the resulting powder.

 7. Turn on the voltage supply to the electric arc metallizer from a network transformer of alternating current of industrial frequency.

 8. Turn on the electric arc metallizer in a pre-selected mode for current and voltage and spray the wire into the powder receiver.

 9. The powder, together with the liquid, is dispersed into the necessary fractions.

 10. The desired fraction is removed from the liquid and dried in an inert gas atmosphere.

11. After drying, the powder is fractionally packed in an airtight container. Thus, when implementing the present method, in contrast to the known methods, the cooling rate necessary for the formation of microcrystalline powder structures is 10 ⁵ - 10 ⁷ deg / s, as a result of which the metal particles solidify before their crystallization occurs, due to the fact that the boiling metal sprayed with a high-speed gas stream into a liquid at room temperature; this provides the process with increased efficiency and ease of implementation. While in existing devices the same cooling rate is achieved by cooling from the melting point to cryogenic temperature.

 Thus, this method of producing a metal powder allows the manufacture of powders with enhanced anti-corrosion properties, i.e. the objective of the invention is achieved. In this way, microcrystalline zinc, nickel, copper, aluminum powders can be obtained.

 The obtained zinc powder was tested for corrosion resistance with a basic particle fraction of 63 μm. The shape of the powder particles is mainly spherical, metallographic analysis of etched sections of powder particles on a tool microscope with a 200-fold increase showed that the microparticles consist of microcrystalline grains 0.7-1.1 μm in size with the inclusion of needle structures 0.03 μm in diameter and 0.7-0.9 microns in length, as well as dendritic structures with branching up to 0.12-0.17 microns. The presence of three structures orders that the tested zinc powder was a mixture of microcrystalline and amorphized zinc. An electron microscope study of zinc powder samples at a magnification of 8500 showed dislocation of lattice defects along the grain edges and up to 50% of zinc lattice defects in needle and dendritic structures.

Tests of three types of microcrystalline zinc powder were carried out in corrosion gauges. The following types were tested:
- produced in a shielding gas medium with a controlled oxidation state and drying under argon atmosphere at room temperature by spraying in a 2% solution of monoethylamine in distilled water at a cooling rate of 10 ⁶ - 10 ⁷ deg / s;
- made by spraying with air in a 1% solution of monoethylamine in distilled water with a dielectric constant of 80 in a metal corrosion-resistant container at a powder cooling rate of 10 ⁷ - 10 ⁸ deg / s. With this method, the basic substance was obtained 89%;
- made by spraying in bidistilled water with a 5% solution of diethylamine with a dielectric constant of 81 in a glass container at a powder cooling rate of 10 ⁷ - 10 ⁸ deg / s. With this method, the basic substance was obtained 93%.

 As a control sample, a mixture of zinc powders PTs6 and PTs7, made in accordance with the existing production technology, was taken.

Tests of the first type of zinc powder showed the following results:
- corrosion rate of amorphous zinc powder
K m A.ts. = 0.056 ml H ₂ / g • day
- corrosion rate of the control mixture of zinc powders
To m counter. = 0.12 ml of H ₂ / g • day
With the VNIIT method, the corrosion rate of zinc at a Celsius temperature of 10 to 30 degrees should not exceed:
for PC-6 0.1 ml. H ₂ / g • day
for PC-7 0.3 ml. H ₂ / g • day
Thus, it follows that the zinc powder obtained by the described method has higher anti-corrosion properties than known zinc powders.

 The active zinc mass of the negative electrode without mercury was made from the first type of zinc powder. The active mass is a microdispersed structure with good thickening to a penetration of 200-230 units. For the test, two batches of A343 alkaline chemical current sources were manufactured using the active mass of pasteurized zinc without mercury and two batches of control sources of A343 type were selected at a time. Observations were made of the electrical parameters of the manufactured chemical current sources (IT) A343 for 10 months. The observation results are presented in table 1 (see the end of the description).

Tests of the second type of zinc powder showed the following results:
- corrosion rate of microcrystalline zinc powder sprayed in a metal container with a solution, wet powder
K m a.ts.m.d. = 0.33 ml H ₂ / g • day
- corrosion rate of microcrystalline zinc powder sprayed in a metal container with a solution, then before testing dried in an oven at a temperature of 105 degrees Celsius and aged for three days in air
Km A.C.M.D. = 0.09 ml of H ₂ / g • day
- the corrosion rate of microcrystalline zinc powder sprayed in a metal container with a solution, then before testing dried in a heating cabinet at a temperature in Celsius of 105 degrees and without exposure seasoned in corrosion gauges
K m a.ts.m.d. = 0.05 ml of H ₂ / g • day
Tests of the third type of zinc powder showed the following results:
- corrosion rate of microcrystalline zinc powder sprayed in a glass container with a solution, wet powder
Km A.C.S.d. = 0.24 ml of H ₂ / g • day
- the corrosion rate of microcrystalline zinc powder sprayed in a glass container with a solution, then before testing dried in an oven at a temperature of 105 degrees Celsius and aged for three days in air
K m A.ts.s.d. = 0.12 ml of H ₂ / g • day
- the corrosion rate of microcrystalline zinc powder sprayed in a glass container with a solution, then before testing dried in a heating cabinet at a temperature in Celsius of 105 degrees and without exposure seasoned in corrosion gauges
K m A.ts.s.d. = 0.05 ml of H ₂ / g • day
The active powders of the negative electrode were made from zinc powders of the second and third types with a corrosion resistance of 0.05 ml of H ₂ / g • day, moreover, from each type separately. The masses had a penetration of 190-220 with a microdispersion composition and thickened quickly. After that, alkaline HIT A332 of three elements in each variant were collected and three control elements made in the standard way were selected. The test results are given in table. 2 (see the end of the description).

 The above results allow us to draw the following conclusions.

 1. The corrosion rate of microcrystalline zinc powder is two times lower than the corrosion rate of a mixture of zinc powders PC-6 and PC-7, as well as in comparison with each of them separately.

 2. The corrosion rate determined in laboratory corrosion gauges was 2.5–4 times lower than that of standard powders.

 3. The experiments performed show the possibility of controlling the corrosion rate of microcrystalline zinc powder and allow its use in HITs without mercury or with its replacement with another inhibitor.

 4. Collected experimental alkaline power sources with the proposed zinc powder, especially the first type, show the real possibility of using such a powder in HITs, which will improve the environmental situation at the enterprises. At the same time, this will have a great economic effect, taking into account the costs of mercury, treatment facilities and waste disposal.

 The powder of the first type had an oxidation state of 0.1% - 0.3%, and the coefficient of excess air in the mixture was 1.1. The powder of the second type had an oxidation state of 4.8% - 5.2%. The powder of the third type had an oxidation state of 2.3% - 2.7%, and the coefficient of excess air in the mixture was 1.4 - 1.7.

The temperature of the sprayed metal particles was monitored at the ends of the wires by the method of small monochromatization, as well as by the ISP-51 spectrograph. In all cases, the temperature was corresponding to 900 ± 10 ^o C, which corresponds to the boiling point.

Copper powder was obtained from rods with a diameter of 8 mm from copper MZ. X-ray phase analysis of copper powder particles was carried out on a DRON-2 X-ray diffractometer using copper radiation with a nickel filter. In all samples, a high-temperature phase of Cu ₂ O (copper oxide) was detected in an amount less than 5% by volume. In the starting material, a M3 copper bar, it is not. It appears in powder particles due to the interaction of copper particles with atmospheric oxygen.

 A characteristic feature of the microcrystalline structure are crystals near the interface at which rapid cooling occurs. Such a boundary is the outer surface of the particles. As follows from the analysis of thin sections, the columnar orientation of particle grains gradually weakens with distance from the particle surface. It is assumed that the transition from columnar to randomly oriented grain morphology is the result of a significant decrease in the cooling rate and, consequently, the grain growth rate, which, in turn, is a consequence of the release of heat of crystallization. The formation of a metastable phase, which is the result of the rapid solidification of molten particles, should to some extent depend on the topography of the surface of the liquid and on how the particles are located on it upon impact with the liquid.

 Analysis of grain sizes showed that in the columnar structure zone they do not exceed 1-2 microns. As it turned out, there are common features of the process of solidification of particles. All microstructures observed in the upper layers of the obtained particles are characterized by elongated columnar grains. Two morphological systems of grains are distinguishable. A significant number of twins is noticeable both in small and larger oblong grains. Omnidirectional (chaotic) orientation can arise in the process of rapid grain growth due to recrystallization and twinning along the {1 1 1} planes. In addition, the high density of dislocations in copper leads to recrystallization and the appearance of many small cracks.

 In general, many microstructural analogies are found between copper hardened from a liquid state by the usual method (for example, on a copper cooled disk) and copper obtained by plasma spraying on a created installation.

 It should be noted that in very extensive zones of thin sections, the structure of grain morphology could not be revealed.

 This fact, combined with the data of X-ray phase analysis, also indicates the microcrystalline structure of the material of the obtained powder particles.

 Thus, the rapid solidification of metal particles sprayed by air plasma leads to the formation of a microcrystalline structure with a high concentration of defects.

Claims (4)

 1. A method of producing a metal powder with a microcrystalline structure, comprising continuously supplying a metal wire and gas to the combustion zone of an electric arc, introducing the formed metal droplets into a liquid cooler and then extracting the powder, characterized in that the products of combustion are a mixture of air and a hydrocarbon substance at a temperature exceeding the boiling point of the metal.  2. The method according to p. 1, characterized in that the gas used is the combustion products of a stoichiometric mixture of air and a hydrocarbon substance.  3. The method according to claim 1 or 2, characterized in that upon receipt of the zinc powder, a solution of glycerol or ethylamine in distilled water is used as a cooler.  4. The method according to any one of claims 1 to 3, characterized in that before extraction, the powder is separated into fractions in the cooler by centrifugal segregation, and the extraction of the powder is carried out by evaporation of the cooler.

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