ULTRASONIC ATOMIZATION METHOD

The invention relates to a device for the manufacturing of spherical metal powders by the ultrasonic atomization method.

Typically, spherical powders of metals and their alloys are produced by the gas atomization method. A liquid metal jet is formed in the upper section of the atomizer and subsequently broken down by a supersonic gas stream. The liquid metal is lifted up by the working gas and solidifies in flight, forming metal powder. Due to the high free path of the liquid droplet reaching up to several meters, atomizers require necessarily large commercial plants with working chambers up to several meters high. This solution is viable when large powder batches are considered, however, it often prevents the efficient and economically justified manufacturing of powder batches below 10 kg of the input material. Alternative solutions, such as rotating electrode atomization or plasma spheroidization do not allow for a small yield of useful powder fraction to be obtained or require the input material to be in powder form.

Another method for the manufacturing of high-quality spherical metal powders is the ultrasonic atomization method. In this method, material spraying takes place due to the standing wave instability in the liquid, occurring when the wave amplitude reaches a high enough value. As the viscous forces are overcome, an emission of a single droplet is emitted in every wave anti-node and the process is repeated after subsequent regaining of instability. The method is commonly used in low-temperature applications, such as the atomization of solutions of water of organic solvents, or in the atomization of fusible metals, tin solders in particular. Atomization of alloys in higher temperatures, i.e. above the aluminum melting point is typically excessively difficult due to the changes in sonotrode calibration and cavitation damage. The currently known methods for ultrasonic atomization of metals and their alloys can be divided into two groups.

In the first group of methods, the material is poured over the sonotrode from a crucible - such a setup has been described in DE1558356 and successfully implemented for production. The technology, however, is only viable for the processing of alloys with the melting temperature below 700°C. The main problem is the short sonotrode working life in higher temperatures and transducer overheating. Cooling is provided by the means of liquid flowing through a coil, water spray or an air blowing system. For the atomization to occur in a stable manner, it is required for the sonotrode to maintain good wettability in respect to the liquid metal and maintaining the temperature of the sonotrode tip above the melting point of the processed alloy. Due to the limit on the working temperature of the piezoelectric transducer, a high temperature gradient is created, which leads to the short working life of the sonotrode.

The second group consists of the methods concerning localized melting of the sonotrode material or of the material attached to the sonotrode. This allows for the spraying of materials with a higher melting point than in the first group. A laser, a plasma torch or an electron beam can be used as energy sources. An example of such an implementation is the US3275787 patent, where an electron beam is used, and the CN103433499, where a part of the sonotrode is melted using a plasma torch, or the method published in Ultrasonic vibration-assisted laser atomization of stainless steel, Habib A. et al., Powder Technology 2017, where a melting point with a laser as an energy source was used.

The liquid metal control and sonotrode wettability issues have been solved through the localized melting of the material, which however puts additional limits on the spraying cycle length. The first one is the change of the resonance frequency of the system due to loss of mass or changes in the geometry of the sprayed sonotrode. After a time, the wave amplitude in the location subjected to melting drops below a critical value, effectively leading the spraying process to a halt. The second limit is the heat accumulation in the sonotrode. During the heating, due to the loss of rigidity, the sonotrode effectively loses the capability to transmit vibrations or is irreparably damaged due to the lowering of the yield strength. The DE3032785 and CN105855558 applications present a solution to the abovementioned problems through using a wire fed with a high speed as a sonotrode, i.e. spraying of the material with a speed exceeding the speed with which the material softens. However, this solution requires for the sprayed material to have high mechanical strength and ductility, therefore being inapplicable for the spraying of brittle materials or those of low yield strength.

The purpose of the present invention is to present a device with no abovementioned constraints.

The core inventive advance of the invention is the sonotrode simultaneously acting as a radiator and the material wetted by the liquid metal. The combination of these three functions allows the process to be carried out in a continuous manner and with no significant limitations as for the form of the input material.

The subject of the present invention is a device for the manufacturing of spherical metal powders by the ultrasonic atomization method, 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 infusible sonotrode (2) made of a material with thermal conductivity above 100 W/mK and equipped with a melting tip (1), wherein the sonotrode is suitable to act also as a radiator, and wherein the melting tip is suitable to ensure good wettability by the liquid input material.

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

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

Preferably, the input material delivery system (6) is placed 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 threaded rod screwed into the sonotrode (2).

Preferably, the melting tip (1) is made of at least two different materials. Preferably, the sonotrode (2) has a length of half the longtitudal wave length in the sonotrode (2) material.

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

Preferably, the sonotrode (2) is adapted to cooling with the use of dielectric liquids, preferably distilled water or diethylene glycol.

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

Preferably, the device is equipped with a cyclone (13), powder drop bar (14), mechanical filter (15), circulation pump (16) with an efficiency in the range of 0,075 m3/s to 0,1138 m3/s with the suction pressure in the range of 100 mbar to 900 mbar with the processing chamber (12) volume in the range of 0,003 m3 to 0,140 m3.

Preferably, the ratio of the processing chamber (12) volume to the efficiency of the circulation pump is 0,2 to 4 1/s, and the processing chamber has a cylindrical shape with the diameter not exceeding 300 mm.

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

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

Preferably, the device is equipped with a vacuum pump connected to the processing chamber.

The subject of the present invention is also the method for the cleaning of the device according to the invention, characterized in that after the input material melting process is stopped, the cleaning medium is delivered through the dosing valve in order to fill the processing chamber until the sonotrode is covered in the medium entirely, subsequently starting the ultrasonic generator and continuing the process for no less than 30 s, with the subsequent removal of the cleaning medium along with the powder particles through the cleaning medium release.

Preferably, the cleaning medium is pumped through a particulate filter. Preferably, after the removal of the cleaning medium through the cleaning medium release, the processing chamber (12) is subjected to vacuum pumping to remove the cleaning medium vapors.

The device is equipped with a piezoelectric transducer connected by a wave guide to a cooled infusible sonotrode, made of a material with electrical conductivity above 100 W/mK, 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 source of mechanical energy used for material spraying, has a working frequency above 20 kHz, and is powered by the high voltage generator. The piezoelectric transducer is connected to the wave guide through a screw fastener, acting as a mounting to the device frame and increasing the transducer vibration amplitude. The wave guide is connected to the infusible sonotrode through a screw fastener. The infusible sonotrode is cooled inside the working chamber by a flowing cooling medium and at the same time acts as a radiator for the melting tip. The dual role of the sonotrode (transmission of vibrations and heat removal) requires materials with high thermal conductivity to be used, i.e. above 100 W/mK and hardness above 100 HV5. Seals preventing the leaking of the cooling medium and leaking into the processing chamber are located in zero-amplitude vibration nodes, i.e. at half- length of the wave guide and of the sonotrode. The cooling medium is air or diethylene glicol. The melting tip is connected to the sonotrode with a screw fastener or fixed through a diffusion bond. The melting system delivers thermal energy to the melting tip. Depending on the input material, the melting system may consist of an infusible tungsten electrode and a source powering arc discharge or a plasma torch, a source powering arc discharge and plasma gas blow system, or a lens and a laser power source. At the same time, the device is equipped with a system delivering the input material to the melting tip. Depending on the form of the input material, the system can be realized in the following ways: a known in the art wire feeder and a channel leading to the processing chamber, a vibrational feeder of material in the form of irregular granulate or powder, a mechanical pusher introducing the input material in the for of a rod to the melting tip. The working chamber is cooled as known in the art and is equipped with protective gas and input material channels, as well as channel serving as a connection with the vacuum pump. Preferably, the sonotrode is made of a CuCrZr or CuCrZr copper alloy or of sintered tungsten alloys. Such alloys assure thermal conductivity high enough for thermal stabilization of the system, also being characterized with high enough hardness.

During the operation of the device, the melting system, preferably a low voltage generator sustaining the arc discharge, melts the tip to which the input material is delivered in a continuous manner, the material preferably being in the form of a wire. At the same time, mechanical energy is transmitted from the piezoelectric transducer, through the wave guide and the infusible sonotrode to the melting tip, causing the input material to be sprayed and fall in the form of a powder. The powder is removed from the working chamber after the atomization cycle is completed, or it is removed by the pump. In the case of spraying of low- hardness or brittle materials, a transition layer is pad welded by the means of tip melting and the addition of the input material without the use of the piezoelectric transducer. Subsequently, after a layer of surplus material with a chemical composition corresponding with the input material is created, the transducer is activated and the atomization process described above is initiated. The high voltage generator is attuned to the changes in the resonance frequency of the system in order to compensate for the thermal drift of the system.

In a preferable realization of the method, more than one wire is supplied to the melting pool, with the supplied wires made from materials with different chemical compositions. This allows for multiple batches of powders with a chemical composition gradient to be quickly manufactured.

Taking into account the negligible powder particle dimensions and its quick cooling, the arc energy flow can be decomposed into two output constituents: the working chamber and the melting tip of the sonotrode. The processing chamber is cooled as known in the art. The heat from the melting tip is removed by the sonotrode, thereby taking a role of a radiator and cooled by air or diethylene glycol. It is critical for a material with a high thermal conductivity and hardness to be used, allowing for the permanent securing to the rest of the system with a screw fastener. The thermal energy limit can be circumvented if the melting system power is tuned to the heat removal capability of the sonotrode. Simultaneously, the mass of the sonotrode remains constant due to the replenishing of the sonotrode material mass loss through the addition of input material to the melting tip. The separation of the material delivery system and the melting tip allows for the spraying of materials with any mechanical properties and in any form.

The device according to the invention will now be described in more detail, basing on examples of implementation, with regard to technical drawings, where:

Fig. 1 presents a device for the production of spherical metal powders by the ultrasonic atomization method,

Fig. 2 presents the device along with the above-described system.

Example 1

An example of a preferable implementation of the device according to the invention is presented on fig. 1. The melting tip (1) is in direct contact with the sonotrode (2). Directly above the melting tip (1), the melting system is placed, consisting of an infusible electrode (3) and a generator (4) maintaining the arc discharge. The input material is fed in the form of a wire (5) through a feeder (6). The cooling medium flows in through an inlet (7) and flows out through an outlet (8), cooling the sonotrode (2) and the wave guide (9) connected to the piezoelectric transducer (10), powered by the high voltage generator (11). The system is enclosed in the working chamber (12).

Example la

A cylindrical sonotrode (2) with a length of 150 mm and diameter of 40 mm, made of the Ampcoloy 940 material with thermal conductivity of 204 W/mK was connected to a piezoelectric transducer (10) with a rated frequency of 20 kHz by a wave guide (9) with an amplification ratio of 2,5:1. The sonotrode (2) seal was placed in the wave node, 70 mm from the front plane of the sonotrode (2) and all the abovementioned parts were placed inside a working chamber (12) with controlled working gas composition. The cooling system was implemented outside the working chamber (12) through a compressed air system. The high voltage generator works in a loop, monitoring the resonance frequency of the system during heating. The melting tip (1) was made from AISI 308 steel and fixed to the sonotrode (2) by a screw fastener. The energy source was an electrical arc generated with an infusible tungsten electrode (3) with the following current parameters: amperage of 90 A with the voltage of 15 V. An AISI 308 steel wire with a diameter of 3,2 mm was fed to the melting pool, and the piezoelectric transducer (10) was subsequently activated. During the atomization process, particles with a mean diameter of 60 μιη were obtained.

Example lb

A cylindrical sonotrode (2) with a length of 150 mm and diameter of 40 mm, made of the Ampcoloy 940 material with thermal conductivity of 204 W/mK was connected to a piezoelectric transducer (10) with a rated frequency of 20 kHz by a wave guide (9) with an amplification ratio of 2,5:1. The sonotrode (2) seal was placed in the wave node, 70 mm from the front plane of the sonotrode (2) and all the abovementioned parts were placed inside a working chamber (12) with controlled working gas composition. The cooling system was implemented outside the working chamber (12) through a compressed air system. The high voltage generator works in a loop, monitoring the resonance frequency of the system during heating. The melting tip (1) was made from AISI 308 steel and fixed to the sonotrode (2) by a screw fastener. The energy source was an electrical arc generated with an infusible tungsten electrode (3) with the following current parameters: amperage of 90 A with the voltage of 15 V. An alloy with the chemical composition of Nd2Fel4B was fed in the form of irregular powder, graded below 300 μιη, and melted into the tip (1) of the sonotrode (2). After the addition of 2 g of the material, the piezoelectric transducer was activated and the output powder removed from the system. Subsequently, after the required working gas composition was obtained, the melting system (3, 4) was activated with the voltage of 15 V and amperage of 70 A. During the atomization process, particles with a mean diameter of 50 μιη were obtained.

Example 2

In the ultrasonic atomization device according to the invention, the material is not carried by the working gas and the time in which the particles remain inside the plasma is several times higher than in other plasma-assisted atomization methods. This leads to the evaporation of the smallest particles and the contamination of the working chamber by condensing vapors. The device according to the preferable realization of the invention allows to overcome such effects.

The device according to the preferable realization of the invention is presented on fig. 2 and includes a cooled sonotrode (2), working chamber (12), plasma torch (2), input material feeder (6), cyclone (13), powder drop bar (14), mechanical filter (15), circulation pump (16) and a nozzle guiding the gas flow (17). The cooled sonotrode supplies the melted material with mechanical energy through the plasma torch, atomizing the input material (5). The powder, along with the vapors of the evaporated material is lifted by the stream of the working gas in the processing chamber (12) and guided to the cyclone (13). In the cyclone, the powder is separated from the gas and drops into the drop bar (14), while the rest of the gas is purified from the dust particles by the mechanical filter (15), sucked by a circulation pump (16), preferably a side-channel exhauster, directed by the nozzle (17) and returned to the processing chamber (12). Quick exchange of the working gas is crucial for the efficient removal of the condensate, therefore the efficiency of the circulation pump (16) should be high enough to ensure the exchange of the working gas in the processing chamber (12) no less than once per 4 seconds. An additional requirement for maintaining the stability of the atomization process is to ensure gas flow near the processing chamber (12), hereby realized by the use of a flow-directing nozzle (17), placed at the inlet of the processing chamber (12).

Example 3

In another example of preferable realization of the device according to the invention, the device is equipped with a system delivering liquid cleaning medium and its release. In this case, the distance between the sonotrode and any wall of the processing chamber is not larger than 1 m. Inside the airtight processing chamber (12), there is placed the input material melting system, preferably using a plasma arc produced between the infusible electrode (3) and the surface of the sonotrode (2), preferably covered with a protective plate made of material identical to the input material. During the atomization process, a protective atmosphere is present in the working chamber (12), preferably in the form of a noble gas or low vacuum. The use of a liquid cleaning medium delivery system allows the double function of the sonotrode to be utilized - the atomization of the liquid metal and the cavitation of the liquid cleaning medium. This allows for the powder remains and contaminants to be removed, minimizing the required workload of the operator. The use of a liquid medium as a means of automatic cleaning ensures process safety by continuous isolation of the powder from atmospheric oxygen, which precludes the ignition and pyrophoric combustion of the powder, at the same time allowing the device to be readied for the atomization of another alloy in a considerably shorter time.

Preferably, the atomizer is equipped with a particulate filter and a cleaning medium pump. This allows for an increase in the powder yield, which is particularly important in the case of noble metals atomization.

Preferably, the atomizer is equipped with a vacuum pump connected to the working chamber. This allows for quick removal of excess cleaning medium through its evaporation.

The method according to the invention is based on the atomization of a metallic alloy in a protective atmosphere, with subsequent disengagement of the melting system and the ultrasound generator, then filling the processing chamber with the cleaning medium, preferably distilled water or isopropanol, in such a way that the sonotrode is submerged in the cleaning medium, then activating the ultrasound generator for no less than 30 s, then releasing the cleaning medium with the powder particles through evaporation. It is crucial to stop the atomization process during the cleaning cycle due to the risk of bringing the cooling medium to a boil and reaching a catastrophic increase of pressure in the system. Solvents such as distilled water and isopropanol allow for the safe removal of the unpassivated powder.

Preferably, the cleaning medium vapors are removed through the use of a vacuum pump. This serves to accelerate the removal of the cleaning medium and prevents the contamination of the powder in the next atomization cycle.

Preferably, the process of filling the working chamber with the cleaning medium, cleaning and subsequent removal of the medium is repeated multiple times.