CN111315513A

CN111315513A - Device for producing spherical metal powder by means of ultrasonic atomization

Info

Publication number: CN111315513A
Application number: CN201880072782.6A
Authority: CN
Inventors: L·佐罗多斯基; R·罗洛维茨; J·罗兹彭多斯基; K·卡内卡; W·拉西斯; M·奥斯特里斯; K·卡钦斯基; B·提斯科; A·斯特罗兹; B·泽布罗夫斯卡
Original assignee: 3d Laboratory Co Ltd
Current assignee: 3d Laboratory Co Ltd
Priority date: 2017-11-09
Filing date: 2018-11-09
Publication date: 2020-06-19
Also published as: JP2021503044A; RU2020118274A3; RU2020118274A; KR102539861B1; WO2019092641A1; KR20200081444A; JP7228274B2; EP3638442A1

Abstract

The subject of the invention is a device for producing spherical metal powder by means of an ultrasonic atomization method, which is equipped with a melting system (3,4) and a feed material delivery system (6), a working chamber (12), a piezoelectric transducer (10) and a cooled refractory sonotrode (2), the sonotrode (2) being made of a material having a thermal conductivity higher than 100W/mK and being equipped with a melting tip (1), the sonotrode (2) acting as a heat sink, the melting tip (1) ensuring good wettability for the liquid feed material.

Description

Device for producing spherical metal powder by means of ultrasonic atomization

Technical Field

The invention relates to a device for producing spherical metal powder by means of an ultrasonic atomization method.

Background

Generally, spherical powders of metals and their alloys are produced by gas atomization. A liquid metal jet is formed in the upper part of the atomizer and subsequently broken up by the supersonic gas flow. The liquid metal is lifted by the working gas and solidifies in flight to form the metal powder. Due to the high free path of the droplets reaching several meters, the atomizer necessarily requires large commercial equipment with a working chamber reaching several meters high. This solution is feasible when large powder batches are considered, however, it is often not efficient and economically reasonable to manufacture powder batches of less than 10kg of input material. Alternative solutions such as rotary electrode atomization or plasma spheronization do not allow to obtain useful powder fractions with small yields or require the input material to be in powder form.

Another method for manufacturing high quality spherical metal powders is ultrasonic atomization. In this method, material sputtering occurs due to instability of the standing wave in the liquid, which occurs when the amplitude of the wave reaches a sufficiently high value. When the viscous forces are overcome, a single droplet shot will be emitted at the antinode of each wave, and the process repeated after the instability is subsequently regained. The method is generally used for low temperature applications such as the atomization of aqueous solutions of organic solvents, or the atomization of fusible metals, especially solder. Atomization of the alloy is often very difficult at higher temperatures, i.e. at temperatures above the melting point of aluminum, due to variations in sonotrode (sonotrode) calibration and cavitation (cavitation) damage.

Currently known ultrasonic atomization methods for metals and their alloys can be divided into two categories.

In a first type of method, the material is poured from the crucible over a sonotrode-an arrangement of this kind is described in DE1558356 and successfully used for production. However, this technique is only suitable for processing alloys with melting temperatures below 700 ℃. The main problem is the short working life of the sonotrode at higher temperatures and when the transducer is overheated. Cooling is provided by a liquid flowing through the coil, a water spray or a blower system. In order to atomize in a stable manner, it is necessary to maintain good wetting of the sonotrode with respect to the liquid metal and to keep the temperature of the sonotrode tip above the melting point of the alloy being treated. Since the operating temperature of the piezoelectric transducer is limited, high temperature gradients can occur, resulting in a short operating life of the sonotrode.

The second category of methods involves localized melting of the sonotrode material or material attached to the sonotrode. This allows the material to be sputtered at a higher melting point than in the first type of process. A laser, a plasma torch, or an electron beam may be used as the energy source. Examples of such embodiments are the patent US3275787 using an electron beam and CN103433499 using a plasma torch to melt a part of the sonotrode, or the method disclosed by Habib a et al in Ultrasonic vibration-assisted laser ionization station, Powder Technology 2017, wherein a melting point with a laser as energy source is used.

The liquid metal control and sonotrode wettability problems have been addressed by localized melting of the material, but this places additional limitations on the sputtering cycle length. The first limitation is the change in the resonant frequency of the system due to mass loss or geometry change of the sputtering sonotrode. After a period of time, the wave amplitude at the location where melting occurs drops below a critical value, which in effect causes the sputtering process to stop. A second limitation is the heat build-up in the superconductive rod. During heating, the sonotrode actually loses its ability to transmit vibrations, due to loss of rigidity, or is inevitably damaged, due to a reduction in yield strength.

The applications DE3032785 and CN105855558 propose to solve the above problems by using as sonotrode a wire fed at high speed, i.e. the material is sputtered at a speed exceeding the softening speed of the material. However, this solution requires that the sputtered material has high mechanical strength and ductility and is therefore not suitable for sputtering of brittle materials or materials with low yield strength.

Disclosure of Invention

The object of the present invention is to provide a device without the above mentioned constraints.

The core invention innovation of the invention is that the sonotrode acts as both a heat sink and a material wetted by the liquid metal. The combination of these three functions allows the treatment to be carried out in a continuous manner and there are no significant restrictions on the form of the input material.

The subject of the invention is a device for the production of spherical metal powder by ultrasonic atomization, equipped with a melting system (3,4) and a feed material delivery system (6), a working chamber (12), a piezoelectric transducer (10) and a cooled refractory sonotrode (2), which sonotrode (2) is made of a material with a thermal conductivity higher than 100W/mK and is equipped with a melting tip (1), wherein the sonotrode is suitable for use as a heat sink and wherein the melting tip is suitable for ensuring good wettability for liquid feed materials.

Preferably, the sonotrode is made of CuCrZr or a CuCrZr copper alloy or a sintered tungsten alloy.

Preferably, the input material delivery system (6) delivers material in the form of a wire (5).

Preferably, the input material delivery system (6) is located outside the working chamber (12).

Preferably, the melting tip (1) and the sonotrode (2) are diffusion-bonded, or the melting tip (1) is made in the form of a screw screwed into the sonotrode (2).

Preferably, the melting tip (1) is made of at least two different materials.

Preferably, the length of the sonotrode (2) is half the longitudinal wavelength of the material of the sonotrode (2).

Preferably, the initial resonance frequency of the melting tip (1) and the sonotrode (2) is 100-3000Hz higher than the working frequency.

Preferably, the sonotrode (2) is adapted to be cooled by using a dielectric liquid, preferably distilled water or diethylene glycol.

Preferably, the device is adapted to deliver at least two different materials to the melting tip (1).

Preferably, the device is equipped with a cyclone (13), a dust-reducing bar (14), a mechanical filter (15), a circulation pump (16), the efficiency of the circulation pump (16) being 0.075m³S to 0.1138m³A suction pressure in the range of 100mbar to 900mbar and a volume of the processing chamber (12) in the range of 0.003m³To 0.140m³Within the range of (1).

Preferably, the ratio of the volume of the treatment chamber (12) to the efficiency of the circulation pump is 0.2 to 41/s, and the treatment chamber has a cylindrical shape with a diameter not exceeding 300 mm.

Preferably, the device is equipped with a source of liquid cleaning medium, a liquid cleaning medium metering valve, and a cleaning medium releaser.

Preferably, the device is equipped with a pump for pumping the liquid cleaning medium and a particle filter.

Preferably, the apparatus is equipped with a vacuum pump connected to the process chamber.

The subject of the invention is also a method for cleaning the device according to the invention, characterized in that after stopping the input material melting process, the cleaning medium is fed through the metering valve to fill the process chamber until the sonotrode is completely covered with medium, whereafter the ultrasonic generator is activated and the process is continued for not less than 30s, whereafter the cleaning medium is removed together with the powder particles by means of the cleaning medium releaser.

Preferably, the cleaning medium is pumped through the particulate filter.

Preferably, after the cleaning medium is removed by the cleaning medium releaser, the process chamber (12) is evacuated to remove the cleaning medium vapor.

The device is provided with: a piezoelectric transducer connected by a waveguide to a cooled refractory sonotrode made of a material with an electrical conductivity higher than 100W/mK and equipped with a melting tip; an input material delivery system; a material melting system; a processing chamber; a high voltage generator; and a vacuum pump. The piezoelectric transducer is a mechanical energy source for sputtering of materials, operates at a frequency higher than 20kHz and is powered by a high voltage generator. The piezoelectric transducer is connected to the waveguide by a helical fastener that acts as a mount for the device frame and increases the vibration amplitude of the transducer. The waveguide is connected to the refractory sonotrode by a helical fastener. The refractory sonotrode is cooled in the working chamber by a flowing cooling medium and at the same time acts as a heat sink for the melting tip. The dual role of the sonotrode (transmission of vibrations and dissipation of heat) requires the use of materials with high thermal conductivity, i.e. higher than 100W/mK, and hardness higher than 100HV 5. At the zero amplitude vibration node, i.e. at half the length of the waveguide and the sonotrode, a seal is provided which prevents leakage of the cooling medium and into the process chamber. The cooling medium is air or diethylene glycol. The melt tip is attached to the sonotrode by a screw fastener or fixed by diffusion bonding. The melting system transfers thermal energy to the melting tip. Depending on the input materials, the melting system may include refractory tungsten electrodes and power supplies for arc discharge or plasma torches, power supplies for arc discharge and plasma gas blowing systems, or lens and laser power supplies. At the same time, the device is equipped with a system for delivering input material to the melting tip. Depending on the form of the input material, the system can be implemented as follows: wire feeders and channels to the process chamber, vibratory feeders for irregular granular or powder form of material, mechanical pushers to direct the input material in the rod to the melt tip, as known in the art. As is known in the art, the working chamber is cooled and equipped with channels for shielding gas and input material as well as channels for connection to a vacuum pump.

Preferably, the sonotrode is made of CuCrZr or a CuCrZr copper alloy or a sintered tungsten alloy. Such alloys ensure a sufficiently high thermal conductivity to thermally stabilize the system and also have a sufficiently high hardness.

During operation of the apparatus, the melting system, preferably a low voltage generator that maintains an arc discharge, melts the tip, and the input material is delivered to the tip in a continuous manner, preferably in wire form. At the same time, mechanical energy is transferred from the piezoelectric transducer to the melting tip via the waveguide and the refractory sonotrode, causing the input material to sputter and fall in powder form. After the atomization cycle is complete, the powder is removed from the working chamber or removed by a pump. In the case of sputtering low hardness or brittle materials, the transition layer is welded by tip melting and adding input material without using a piezoelectric transducer. Subsequently, after the formation of the remaining material layer having a chemical composition corresponding to the input material, the transducer is activated and the above described atomization process is initiated. The high voltage generator is adjusted according to the change of the system resonance frequency to compensate the thermal drift of the system.

In a preferred implementation of the method, more than one wire is supplied to the melt pool, wherein the supplied wires are made of materials having different chemical compositions. This allows for the rapid manufacture of multiple batches of powder with chemical composition gradients.

Considering the negligible powder particle size and its rapid cooling, the arc energy flow can be divided into two output components: the working chamber and the melt tip of the sonotrode. The process chamber is cooled as is known in the art. Heat from the melt tip is removed by the sonotrode, acting as a heat sink, and cooled by air or diethylene glycol. It is critical to use a material with high thermal conductivity and hardness so that it can be permanently secured to the rest of the system using a screw fastener. Thermal energy limitations can be avoided if the power of the melting system is adjusted to the heat removal capability of the sonotrode. At the same time, the mass of the sonotrode remains constant due to the loss of material mass that supplements the sonotrode by adding input material to the melting tip. The separation of the material delivery system and the melt tip allows sputtering of materials having any mechanical properties and in any form.

Drawings

The device according to the invention will now be described in more detail, according to an example of embodiment, with respect to the technical figures, in which:

figure 1 shows an apparatus for producing spherical metal powder by ultrasonic atomization,

figure 2 shows the device and the system described above.

Detailed Description

Example 1

Fig. 1 shows an example of a preferred embodiment of the device according to the invention. The melting tip (1) is in direct contact with the sonotrode (2). The melting system is positioned right above the melting tip (1), and consists of a refractory electrode (3) and a generator (4) for maintaining arc discharge. The input material is fed in the form of a strand (5) by a feeder (6). The cooling medium flows in through the inlet (7) and out through the outlet (8), cooling the sonotrode (2) and the waveguide (9) connected to the piezoelectric transducer (10), the piezoelectric transducer (10) being supplied with power by the high-voltage generator (11). The system is enclosed in a working chamber (12).

Example 1a

A cylindrical sound guide rod (2) which is made of an Ampcoley 940 material with the thermal conductivity of 204W/mK and has the length of 150mm and the diameter of 40mm passes through a magnification ratio of 2.5: the waveguide (9) of 1 is connected to a piezoelectric transducer (10) rated at 20 kHz. The sonotrode (2) seal was located at a node 70mm from the front face of the sonotrode (2), and all of the above components were located within a working chamber (12) with a controlled working gas composition. The cooling system is implemented outside the working chamber (12) by means of a compressed air system. The high voltage generator is cycled to monitor the resonant frequency of the system during heating. The melt tip (1) is made of AISI 308 steel and is secured to the sonotrode (2) by screw fasteners. The energy source is an electric arc generated by a refractory tungsten electrode (3), and the current parameters are as follows: the amperage was 90A at a voltage of 15V. AISI 308 steel wire with a diameter of 3.2mm is supplied to the melt pool, followed by activation of the piezoelectric transducer (10). During the atomization, particles with an average diameter of 60 μm were obtained.

Example 1b

A cylindrical sound guide rod (2) which is made of an Ampcoley 940 material with the thermal conductivity of 204W/mK and has the length of 150mm and the diameter of 40mm passes through a magnification ratio of 2.5: the waveguide (9) of 1 is connected to a piezoelectric transducer (10) rated at 20 kHz. The sonotrode (2) seal was located at a node 70mm from the front face of the sonotrode (2), and all of the above components were located within a working chamber (12) with a controlled working gas composition. The cooling system is implemented outside the working chamber (12) by means of a compressed air system. The high voltage generator is cycled to monitor the resonant frequency of the system during heating. The melt tip (1) is made of AISI 308 steel and is secured to the sonotrode (2) by screw fasteners. The energy source is an electric arc generated by a refractory tungsten electrode (3), and the current parameters are as follows: the amperage was 90A at a voltage of 15V. An alloy of chemical composition Nd2Fe14B is supplied in the form of irregular powder graded below 300 μm and melted in the tip (1) of the sonotrode (2). After 2g of this material was added, the piezoelectric transducer was activated and the output powder was removed from the system. Subsequently, after the desired working gas composition was obtained, the melting system was started at a voltage of 15V and an amperage of 70A (3, 4). During the atomization, particles with an average diameter of 50 μm were obtained.

Example 2

In the ultrasonic atomization apparatus according to the present invention, the material is not carried by the working gas, and the time during which the particles remain inside the plasma is several times longer than in other plasma-assisted atomization methods. This results in evaporation of the smallest particles and contamination of the working chamber by condensing the vapour.

The device according to the preferred embodiment of the invention allows to overcome this effect.

Fig. 2 shows an apparatus according to a preferred embodiment of the invention, comprising a cooled sonotrode (2), a working chamber (12), a plasma torch (2), an input material feeder (6), a cyclone (13), a powder dropping rod (14), a mechanical filter (15), a circulation pump (16) and a nozzle (17) to direct the gas flow. The cooled sonotrode provides mechanical energy to the molten material through the plasma torch, thereby atomizing the input material (5). The powder is lifted by the flow of working gas in the process chamber (12) together with the vapour of the vaporized material and is directed to a cyclone (13). In the cyclone, the powder is separated from the gas and falls into the dust-dropping bar (14), while the remaining gas is cleaned from the dust particles by means of a mechanical filter (15), sucked by a circulation pump (16), which circulation pump (16) is preferably a side channel exhauster, and then guided by a nozzle (17) and returned to the treatment chamber (12). The rapid exchange of the working gas is critical to the effective removal of condensate and therefore the efficiency of the circulation pump (16) should be high enough to ensure that the exchange of working gas in the process chamber (12) is not less than once every 4 seconds. Another requirement for maintaining the stability of the atomization process is to ensure an air flow in the vicinity of the process chamber (12), here by using a flow-directing nozzle (17) at the inlet of the process chamber (12).

Example 3

In another example of a preferred embodiment of the device according to the invention, the device is equipped with a system for delivering and releasing a liquid cleaning medium. In this case, the distance between the sonotrode and any wall of the process chamber is no more than 1 m. Inside the airtight treatment chamber (12) is placed a input material melting system, preferably using a plasma arc generated between the refractory electrode (3) and the surface of the sonotrode (2), the surface of the sonotrode (2) being preferably covered with a protective plate made of the same material as the input material. During the atomization process, a protective atmosphere, preferably in the form of an inert gas or a low vacuum, is present in the working chamber (12).

The use of a liquid cleaning medium delivery system allows for the dual function of using a sonotrode-atomization of the liquid metal and cavitation of the liquid cleaning medium. This results in the removal of powder residue and contaminants, thereby minimizing the amount of work required by the operator. The use of a liquid medium as a means of self-cleaning ensures process safety by continuously isolating the powder from atmospheric oxygen, which prevents ignition and pyrophoric combustion of the powder, while making the apparatus suitable for atomizing another alloy in a relatively short time.

Preferably, the atomizer is equipped with a particle filter and a cleaning medium pump. This leads to an increase in the yield of the powder, which is particularly important in the case of noble metal atomization.

Preferably, the atomizer is equipped with a vacuum pump connected to the working chamber. This allows excess cleaning medium to be quickly removed by evaporation thereof.

The method according to the invention is based on atomization of the metal alloy in a protective atmosphere, followed by separation of the melting system and the sonicator, followed by filling the treatment chamber with a cleaning medium, preferably distilled water or isopropanol, in such a way that the sonotrode is immersed in the cleaning medium, which is then activated for not less than 30s, and the cleaning medium is released together with the powder particles by evaporation. It is crucial to stop the atomization process during the cleaning cycle, since there is a risk that the cooling medium boils and the pressure in the system increases catastrophically. Solvents such as distilled water and isopropanol safely remove the unpassivated powder.

Preferably, the cleaning medium vapor is removed by using a vacuum pump. This serves to speed up the removal of the cleaning medium and prevent contamination of the powder in the next atomisation cycle.

Preferably, the process of filling the working chamber with cleaning medium, cleaning and subsequently removing the medium is repeated a number of times.

Claims

1. Device for the production of spherical metal powder by the ultrasonic atomization method, which device is equipped with a melting system (3,4) and an input material delivery system (6), a working chamber (12), a piezoelectric transducer (10) and a cooled refractory sonotrode (2), which sonotrode (2) is made of a material with a thermal conductivity higher than 100W/mK and is equipped with a melting tip (1), wherein the sonotrode is suitable for use as a heat sink, and wherein the melting tip is suitable for ensuring good wettability for liquid input materials.

2. The device of claim 1, wherein the sonotrode is made of a CuCrZr or a CuBe copper alloy or a sintered tungsten alloy.

3. The device according to claim 1 or 2, characterized in that the input material conveying system (6) is adapted to supply material in the form of a wire (5).

4. The apparatus according to any one of claims 1 to 3, wherein the input material delivery system (6) is located outside the process chamber (12).

5. The device according to any one of claims 1 to 4, wherein the melting tip (1) and the sonotrode (2) are diffusion bonded.

6. The device according to any one of claims 1 to 4, characterized in that the melting tip (1) is in the form of a screw threaded into the sonotrode (2).

7. The device according to any one of claims 1 to 6, characterized in that the melting tip (1) is made of at least two different materials.

8. The device according to any one of claims 1 to 7, wherein the length of the sonotrode (2) is half the longitudinal wavelength of the material of the sonotrode (2).

9. The device according to any one of claims 1 to 8, wherein the initial resonance frequency of the melting tip (1) and the sonotrode (2) is 100-3000Hz higher than the operating frequency.

10. The device according to any one of claims 1 to 9, wherein the sonotrode is adapted to be cooled by using a dielectric liquid, preferably distilled water or diethylene glycol.

11. The device according to any one of claims 1 to 10, characterized in that the device is adapted to deliver at least two different materials to the melting tip (1).

12. Device according to any of claims 1 to 11, characterized in that it is equipped with a cyclone (13), a dust-reducing bar (14), a mechanical filter (15), a circulation pump (16), the efficiency of the circulation pump (16) being 0.075m³S to 0.1138m³A suction pressure in the range of 100mbar to 900mbar and a volume of the processing chamber (12) in the range of 0.003m³To 0.140m³Within the range of (1).

13. The device according to claim 12, characterized in that the ratio of the volume of the treatment chamber (12) to the efficiency of the circulation pump (16) is 0.2 to 41/s and the treatment chamber has a cylindrical shape with a diameter not exceeding 300 mm.

14. The device according to any one of claims 1 to 13, characterized in that the device is equipped with a source of liquid cleaning medium, a liquid cleaning medium metering valve and a cleaning medium releaser.

15. The device according to claim 14, characterized in that the device is equipped with a pump for pumping the liquid cleaning medium and a particle filter.

16. The apparatus according to claim 14 or 15, characterized in that the apparatus is equipped with a vacuum pump connected to the process chamber.

17. A method for cleaning a device according to any one of claims 14 to 16, characterized in that after stopping the input material melting process, the cleaning medium is fed through the metering valve to fill the process chamber until the sonotrode is completely covered with medium, whereafter the sonicator is activated and the process is continued for not less than 30s, whereafter the cleaning medium is removed together with the powder particles by the cleaning medium releaser.

18. The method of claim 17, wherein the cleaning media is pumped through the particulate filter.

19. Method according to claim 17 or 18, characterized in that after removal of the cleaning medium by the cleaning medium releaser, the process chamber (12) is evacuated to remove cleaning medium vapours.