AU2019351410A1

AU2019351410A1 - Device and method for preparing ultrafine spherical metal powder using drop-by-drop centrifugal atomization method

Info

Publication number: AU2019351410A1
Application number: AU2019351410A
Authority: AU
Inventors: Zhaofeng BAI; Qing Chang; Wei Dong; Guofeng Han; Yang HAN; Guobin Li; Yao Meng; Zhiyong Qin; Zhiqiang REN; Jing Shi; Yu Sun; Tao TENG; Wenyu Wang; Xiaoming Wang; Yanyang WANG; Fumin XU; Yang Zhao; Sheng ZHU
Original assignee: Dalian University of Technology
Current assignee: Dalian University of Technology
Priority date: 2018-09-25
Filing date: 2019-09-25
Publication date: 2021-05-20
Also published as: CN109014227A; WO2020063625A1

Abstract

A device and method for preparing an ultrafine spherical metal powder by using a drop-by-drop centrifugal atomization method. The device comprises a housing (20), a crucible (2) provided in the housing (20), and a powder collection area. A turntable (8) provided in the powder collection area is an inlaid structure. A material with poor thermal conductivity is selected as a base portion of the turntable (8). A metal material that has a wetting angle of less than 90° with a droplet is selected and inlaid into a main portion as the atomizing plane of the turntable (8). A ventilation hole (24) is provided in the turntable (8). The method for preparing an ultrafine spherical metal powder by means of the drop-by-drop centrifugal atomization method mainly combines the two methods of a uniform droplet ejection method and a centrifugal atomization method, and surmounts a traditional metal splitting mode such that molten metal exhibits fibrous splitting, so that an ultrafine spherical metal powder that has a narrow particle size distribution range, high sphericity, good fluidity, excellent spreadability, a uniform and controllable size, and no satellite droplets may be efficiently prepared, and is suitable for industrial production.

Description

C21007AUO APPARATUS AND METHOD FOR PREPARING ULTRAFINE SPHERICAL METAL POWDER BY MEANS OF ONE-BY-ONE DROPLETS CENTRIFUGAL ATOMIZATION PROCESS Technical Field The present disclosure belongs to the technical field for preparing ultrafine spherical particles, specifically relates to an apparatus and a method for preparing ultrafine spherical metal powder by means of drop-by-drop centrifugal atomization process. Background Art Metal additive manufacturing technology has been widely used in energy sources, military and other fields because of its wide range of molding and its ability to process various parts with complex shapes. As the raw material for molding, the quality of the spherical metal powder has great influence on that of the final products. The requirements of additive manufacturing technology for metal powder include the performances such as narrow particle size distribution, low oxygen content, high sphericity, average particle size less than 50 tm, and satellite droplets free. However, at present, the quality of metal powder in China's market is lower, which has a big gap with foreign technical level. The powder in the market cannot meet the needs of additive technology, which seriously limits the development of additive technology in our country. At present, the main method for preparing spherical metal powder is atomization method, including gas atomization method, water atomization method, centrifugal atomization method,

rotating electrode atomization method, etc. Although the atomization method has a very high efficiency, the size dispersity of the prepared powder is large, and powder that meets a particle size requirement can be obtained only through multiple screenings, which greatly reduces the production efficiency, especially when the size is strictly required. Satellite droplets are easily produced by using the atomization method, which makes the surface of the powder adhere to the satellite droplet, thereby reducing the flowability and spreadability of the powder. Moreover, it is easy to be incorporated with impurities in the production process, which cannot meet the requirements of the powder for 3D printing. Therefore, how to prepare metal powder with narrow particle size distribution, controllable, high sphericity and satellite droplets free has become a big problem to be solved.

Summary of the Invention According to the above mentioned technical problems of poor sphericity, spreadability

C21007AUO and flowability in the process of preparing metal powder for 3D printing, the present disclosure provides an apparatus and a method for preparing ultrafine spherical metal powder by means of drop-by-drop centrifugal atomization process. The present disclosure combines the uniform droplet spray method and the centrifugal atomization method, at the same time, the structure of the turnplate is designed and an induction heating coil is added to perform induction heating on a surface of the disc plate, thereby the metal liquid breaks through the traditional split mode of molten metal, and implements the fibrous split mode which can be implemented only when the atomizing medium is aqueous solution or organic solution. Though this mode, the ultrafine refinement of metal powder can be prepared and a great leap can be made in particle size control. Spherical metal powder with high sphericity, good flowability and spreadability, satellite droplets free and a very high fine powder yield that meets the requirements of 3D printing may be prepared. The technical solutions adopted by the present disclosure are as follows: An apparatus for preparing ultrafine spherical metal powder by means of drop-by-drop centrifugal atomization process, including a housing, a crucible and a powder collection area arranged in the housing. The powder collection area is arranged at the bottom of the housing and the crucible is arranged above the powder collection area. The crucible is provided with a thermocouple inside and a heating tape outside. The crucible is provided with a nozzle with small holes at the bottom. The crucible is provided with an oscillation generator connected with a piezoelectric ceramic arranged on the top of the housing. A plate electrode is arranged right below the crucible. The housing is provided with a crucible air inlet extending into the crucible. The housing is also provided with a diffusion pump and a mechanical pump. The housing is also provided with a cavity air inlet and a cavity exhaust valve. The powder collection area includes a collection tray arranged at the bottom of the housing, and a turnplate arranged above the collection tray and connected with a motor for atomizing metal droplets. The turnplate includes a base, an atomization plane and an air hole. The base is a structure of "T-shaped" longitudinal section constituted of an upper receiving portion and a lower support portion. The upper surface of the receiving portion is provided with a circular groove with a certain radius and coaxial with the center of the receiving portion. The base is made of a material with a thermal conductivity less than 20W/m/k. The atomization plane is a disc structure, matching with the circular groove and in

C21007AUO interference fitting with the circular groove. The atomization plane is made of a material with wetting angle less than 900 to the atomized metal droplet. The air hole is through arranged passing through the receiving portion and the support portion. The upper end of the air hole is in contact with the lower end of the atomization plane, and the lower end of the air hole communicates with the outside world. An induction heating coil is also arranged outside the turnplate. The volume of the housing should be large enough to make the centrifugally broken droplets fly onto the collection tray at the bottom, so as to ensure that the droplets will not solidify on the inner wall of the housing. The area of the collection tray should be large enough to collect powder. Preferably, the height of the support portion of the base should not be too high, which should be smaller than the height of the receiving portion. The upper end face of the atomization plane protrudes from the upper end face of the receiving portion with a protrusion height ranging from 0.1 mm to 0.5 mm. The protrusion height should meet the condition that the dispersed metal droplets directly fly into the cavity and fall into the collection tray without touching the base. The base is made of a material with thermal conductivity less than 20W/m/k, such as zirconia ceramic, silica glass or stainless steel. The upper end face of the air hole is less than or equal to the lower end face of the atomization plane. The air hole is provided to pump the gas in the gap of the turnplate more cleanly during vacuumizing, so that the turnplate is safer when rotating at a high speed. Therefore, the larger the contact area between the upper end face of the air hole and the lower end face of the atomization plane, the better the stability of the atomization plane when vacuuming. Further, a wetting angle between the material of the crucible and the melt in the crucible is greater than 90°. Further, an aperture of the small hole of the nozzle ranges from 0.02 mm to 2.0 mm. Further, a voltage of the plate electrode ranges from 100 V to 400 V. The induction heating coil is connected with a frequency converter and a stabilized voltage supply arranged outside the housing. The heating thickness of the induction heating coil ranges from 5 mm to 20 mm, and a voltage control of the stabilized voltage supply ranges from 0 v to 50 V. Further, a rotational speed of the tumplate ranges from 10000 rpm to 50000 rpm. Further, the piezoelectric ceramic, the oscillation generator, the crucible, the nozzle, the plate electrode the turnplate, and the induction heating coil are located coaxially from top to bottom of the apparatus. The present disclosure also discloses a method for preparing ultrafine spherical metal

C21007AUO powder by means of drop-by-drop centrifugal atomization process, including the following steps:

Si. charging: charging the metal material into the crucible arranged in the upper portion

of the housing, and manually adjusting, in the height direction, a distance between the

s induction heating coil and the turnplate to a preset distance, then sealing the housing.

S2. vacuumizing: vacuumizing the crucible and the housing by using the mechanical

pump and the diffusion pump, and filling the crucible and the housing with a high-purity inert

shielding gas, to make the pressure inside the housing reach a preset value.

S3. heating the crucible: setting the heating parameters of the heating tape according to a

melting point of the metal material to-be-heated, monitoring the temperature inside the

crucible in real time by the thermocouple arranged in the crucible, and maintaining the

temperature after the metal material is completely melted.

S4. induction heating: enabling the turnplate to rotate at a preset high speed by using the

motor, and heating the upper surface of the turnplate rotating at the high speed to a

temperature higher than a melting point of the metal material by using the induction heating

coil.

S5. making the powder: introducing a high-purity inert shielding gas into the crucible by

using the crucible air inlet arranged on the housing and extending into the crucible, to form a

positive pressure difference between the inside and the outside of the crucible; then inputting

a pulse signal with a certain wave mode to the piezoelectric ceramic, so that the oscillation

generator generating a certain frequency of oscillation; and then, setting the voltage of the

plate electrode to form an electric field of a preset strength.

Because of existence of the pressure difference between the inside and the outside of the

crucible, the metal flows out through the nozzle to form a columnar metal flow. At this time

the columnar metal flow breaks into a series of small metal droplets under a certain frequency

of oscillation. In the falling process of the metal droplets and under the effect of electric field,

the metal droplets repel each other due to the surface effect of electric charge to avoid the

repolymerization of metal droplets.

The metal droplets land freely on the turnplate rotating at a high speed. The metal

droplets first drop in the center of the turnplate. Because the centrifugal force is small at this

C21007AUO time, the droplets will not disperse immediately, but spread in a circle on the turnplate. When

the droplets spread in a certain range and the centrifugal force is large enough, the spread

metal disperse on the turnplate to the edge of the turnplate in a fiber line shape under the

action of centrifugal force, and finally split into tiny droplets to fly out. The tiny droplets

solidify without a container in the falling process to form the metal powder and fall onto a

collection tray.

S6. collecting the powder: collecting the metal powder by the collection tray arranged at

the bottom of the housing.

Further, an added amount of the charged metal material ranges from 1/4 to 3/4 of a

capacity of the crucible.

Further, the position of the induction heating coil is manually adjusted to be 1 mm to 2

mm higher than the turnplate.

Further, the high-purity inert shielding gas is argon or helium gas, which is filled into the

housing to make the pressure in the housing reach 0.1 MPa. A holding time is 15 minutes to

20 minutes after the metal material completely melted.

Further, an induction heating voltage of the induction heating coil ranges from 0 to 50V,

and an induction heating time ranges from 5 to 15 minutes.

Further, a pressure difference between the crucible and the housing ranges from 0 to 200

kPa.

Compared with the prior art, the present disclosure has the following advantages:

The present disclosure designs an apparatus combining a uniform droplet spray method

and a centrifugal atomization method to prepare ultrafine spherical metal powder by metal

droplets in a fibrous split mode. A melted metal material in the crucible will be sprayed

through the nozzle at the bottom of the crucible, under the action of the pressure difference

and the oscillation generator, to form small droplets. In the falling process, the small droplets

will land on the turnplate rotating at a high speed, without aggregate under the action of

electric field. Due to the effect of induction heating, the uniform droplets will be still in a

molten state when they reach the upper surface of the turnplate. Because the droplet metal

and the material of the upper surface of the turnplate have good wettability and under the

action of centrifugal force, the uniform droplets will spread out in a fibrous shape on the

C21007AUO turnplate, and split into tiny droplets at the edge of the turnplate to fly out, then freely fall and

solidify to form metal powder. The particle size of metal particles produced by the uniform

droplet spray method is controllable, but the production of single-orifice preparing particles is

not enough to meet the increasing demand. Through innovation, by combining the uniform

droplet spray method and centrifugal atomization method, designing the structure of the

turnplate, the material with good wettability to the metal is selected as the atomization

surface and the induction heating device is added, the present disclosure realizes the fibrous

split mode of the molten metal, which effectively reduces the diameter of the atomized

powder and greatly improve the productivity of the metal powder. Therefore, the metal

powder obtained by the combination of the two methods has fine particle size, narrow

particle size distribution interval, high sphericity, controllable particle size distribution,

consistent thermal history, high yield of fine powder, meeting the requirements of industrial

production.

The method of the present disclosure is highly controllable, which is shown in the

following aspects: A heating temperature of the crucible can be accurately controlled by using

the heating tape. A pressure difference between the crucible and the housing can be controlled

by introducing an inert gas into the crucible and the housing. The size of the uniform droplets

may be controlled by the size of the nozzle with small holes at the bottom of the crucible. The

plate electrode can control the electric filed. The induction heating coil can control the

temperature of the surface of the turnplate and the rotational speed of the turnplate is

controllable, which can control a fibrous split effect of the molten metal, thereby further

controll the particle size distribution of the metal particles. The process parameters can be

adjusted and controlled to obtain spherical metal powder meeting different requirements of

particle sizes and distribution, and the production efficiency is high.

The present disclosure can efficiently prepare metal powder required by 3D printing by

means of the fibrous splitting of molten metal. The prepared powder has controllable particle

size, small particle size, narrow particle size distribution interval, high sphericity, satellite

droplets free, good flowability and spreadability, consistent thermal history, high production

efficiency, low production cost. The present disclosure can be used for industrial production.

C21007AUO BRIEF DESCRIPTION OF THE DRAWINGS

To describe the technical solutions in the embodiments of the present disclosure or in the

prior art more clearly, the following briefly describes the accompanying drawings required

for describing the embodiments or the prior art. Apparently, the accompanying drawings in

the following description show some embodiments of the present disclosure, and a person of

ordinary skill in the art may still derive other accompanying drawings from these

accompanying drawings without creative efforts.

Fig. 1 is a structural schematic diagram of the apparatus in the present disclosure.

Fig. 2 is a structural schematic diagram of the turnplate in the present disclosure.

Fig. 3 is a comparison diagram between a surface of the turnplate in the present

disclosure after an experiment and that of an original turnplate after an experiment; wherein,

(a) is a surface of the tumplate with fibrous splitting, and (b) is a surface of the turnplate in

the prior art.

In the figures: 1. piezoelectric ceramic; 2. crucible; 3. oscillation generator; 4. crucible

air inlet; 5. melt; 6. heating tape; 7. plate electrode; 8. turnplate; 9. metal powder; 10.

collection tray; 11. motor; 12. induction heating coil; 13. metal droplet; 14. nozzle; 15. cavity

air inlet; 16. mechanical pump; 17. diffusion pump; 18. cavity exhaust value; 19.

thermocouple; 20. housing; 21. receiving portion; 22. support portion; 23. atomization plane;

24. air hole.

DESCRIPTION OF THE EMBODIMENTS

It should be noted that, in the case of no conflicts, the embodiments and the features in

the embodiments of the present disclosure can be combined mutually. The present disclosure

will be described in detail below with reference to the accompanying drawings and the

embodiments.

To make the objectives, technical solutions, and advantages of the present disclosure

clearer, the following clearly and completely describes the technical solutions in the

embodiments of the present disclosure with reference to the accompanying drawings in the

embodiments of the present disclosure. Apparently, the described embodiments are merely

some rather than all of the embodiments. The following description of at least one exemplary

C21007AUO embodiment is actually only illustrative, and in no way serves as any limitation on the present

invention and its application or use. Based on the embodiments of the present disclosure, all

the other embodiments obtained by those of ordinary skill in the art without inventive effort

are within the protection scope of the present disclosure.

It should be noted that the terms used herein are only intended to describe specific

embodiments and are not intended to limit the exemplary embodiments of the present

disclosure. As used herein, unless indicated obviously in the context, a singular form is

intended to include a plural form. Furthermore, it should be further understood that the terms

"include" and/or "comprise" used in this specification specify the presence of features, steps,

operations, devices, components and/or of combinations thereof. Unless specifically stated otherwise, the relative arrangement of components and steps, numerical expressions, and numerical values set forth in these embodiments do not limit the scope of the present disclosure. In addition, it should be clear that, for ease of description, sizes of the various components shown in the accompanying drawings are not drawn according to actual proportional relationships. Technologies, methods, and devices known to those of ordinary skill in the relevant fields may not be discussed in detail, but where appropriate, the technologies, methods, and devices should be considered as a part of the authorization specification. In all the examples shown and discussed herein, any specific value should be interpreted as merely being exemplary rather than limiting. Therefore, other examples of the exemplary embodiment may have different values. It should be noted that similar reference signs and letters represent similar items in the accompanying drawings below. Therefore, once an item is defined in one accompanying drawing, the item does not need to be further discussed in a subsequent accompanying drawing. In the description of the present disclosure, it should be noted that orientations or position relationships indicated by orientation terms "front, rear, upper, lower, left, and right", "transverse, vertical, perpendicular, and horizontal", "top and bottom", and the like are usually based on orientations or position relationships shown in the accompanying drawings, and these terms are only used to facilitate description of the present disclosure and simplification of the description. In the absence of description to the contrary, these orientation terms do not indicate or imply that the apparatus or element referred to must have a specific orientation or be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on the protection scope of the present disclosure:

C21007AUO orientation words "inner and outer" refer to the inside and outside relative to the contour of each component. For ease of description, spatially relative terms such as "on", "over", "on the upper surface", and "above" can be used here to describe a spatial positional relationship between one device or feature and another device or feature shown in the figures. It should be understood that the spatially relative terms are intended to include different orientations in use or operation other than the orientation of the device described in the figure. For example, if the device in the figure is inverted, the device described as "above another device or structure" or "on another device or structure" is then be positioned as being "below another device or structure" or "beneath a device or structure". Therefore, the exemplary term "above" can include both orientations "above" and "below". The device can also be positioned in other different ways (rotating by 90 degrees or in another orientation), and the spatially relative description used herein is explained accordingly. In addition, it should be noted that using terms such as "first" and "second" to define components is only for the convenience of distinguishing the corresponding components. Unless otherwise stated, the foregoing words have no special meaning and therefore cannot be understood as a limitation on the protection scope of the present disclosure. As shown in Fig. 1, the present disclosure provides an apparatus for preparing ultrafine spherical metal powder by means of drop-by-drop centrifugal atomization process, including a housing 20, a crucible 2 and a powder collection area arranged in the housing 20. The powder collection area is arranged at the bottom of the housing 20 and the crucible 2 is arranged above the powder collection area. The crucible 2 is provided with a thermocouple 19 inside and a heating tape 6 outside. The crucible 2 is provided at the bottom with a nozzle 14 with a plurality of small holes. The wetting angle between the material of the crucible 2 and the melt 5 arranged in the crucible is greater than 90. The aperture of the small hole of the nozzle 14 ranges from 0.02 mm to 2.0 mm. The crucible 2 is provided inside with an oscillation generator 3 connected with a piezoelectric ceramic 1 arranged on the top of the housing. A plate electrode 7, with a voltage range of 100V to 400V, is arranged right below the crucible. The housing 20 is provided with a crucible air inlet 4 extending into the crucible 2, and is also provided with a diffusion pump 17, a mechanical pump 16, a cavity air inlet 15 and a cavity exhaust valve 18. The powder collection area includes a collection tray 10 arranged at the bottom of the housing 20, and a turnplate 8 arranged above the collection tray 10 and connected with a

C21007AUO motor 11 for atomizing metal droplets. As shown in Fig. 2, the turnplate 8 includes a base, an atomization plane 23 and an air hole 24. The base is a structure of a "T-shaped" longitudinal section constituted of an upper receiving portion 21 and a lower support portion 22. The upper surface of the receiving portion 21 is provided with a circular groove with a certain radius and coaxial with the center of the receiving portion. The base is made of a material with a thermal conductivity less than 20W/m/k. The atomization plane 23 is a disc structure, matching with the circular groove and in interference fitting with the circular groove. The atomization plane 23 is made of a material with a wetting angle less than 900 to with an atomized metal droplet 13. The air hole 24 is arranged passing through the receiving portion 21 and the support portion 22. The upper end of the air hole 24 is in contact with the lower end of the atomization plane 23, and the lower end of the air hole 24 communicates with the outside world. An induction heating coil 12 is also arranged outside the turnplate 8. The rotational speed of the turnplate 8 ranges from 10000 rpm to 50000 rpm. The induction heating coin 12 is connected with a frequency converter and a stabilized voltage supply arranged outside the housing 20. The heating thickness of the induction heating coil 12 ranges from 5 mm to 20 mm, and the voltage control of the stabilized voltage supply ranges from is 0 to 50 V. The piezoelectric ceramic 1, the oscillation generator 3, the crucible 2, the nozzle 14, the plate electrode 7, the tumplate 8 and the induction heating coil 12 are located coaxially from top to bottom of the apparatus. The purpose is that the droplets can evenly drop on the center of the turnplate, which is conducive to spread. The volume of the housing 20 should be large enough make the centrifugally broken droplets fly onto the collection tray at the bottom, so as to ensure that the droplets will not solidify on the inner wall of the housing 20. The area of the collection tray 10 should be large enough to collect powder. During operating, the mechanical pump 16 and the diffusion pump 17 are used to vacuumize the housing 20 and the crucible 2. The crucible 2 is provided at the bottom with a nozzle 14 with small holes. The heating tape 6 is used to heat the metal materials to-be-prepared in the crucible 2. A high-purity inert shielding gas, such as helium gas and argon gas, is introduced into the crucible 2 and the housing 20 through the crucible air inlet4 and the cavity air inlet 15, to maintain a certain positive pressure difference between the

C21007AUO crucible 2 and the housing 20. And then, the piezoelectric ceramic 1 is input pulse signals with a certain wave mode to make the oscillation generator 3 generate a certain frequency. Finally, the voltage of the plate electrode7 is set to form an appropriate electric field. Because of the existence of the pressure difference between inside and outside of the crucible 2, the

metal flows out through the nozzle 14 in a columnar metal flow. At this time the columnar metal flow breaks into a series of small metal droplets 13 under a certain frequency of oscillation. In the falling process of metal droplets 13, under the effect of electric field, the metal droplets 13 repel each other, due to the surface effect of electric charge, to avoid the repolymerization of metal droplets 13. The metal droplets 13 land freely on the tumplate 8 rotating at a high speed, which first drop in the center of the turnplate 8. Because the centrifugal force is small at this time, the metal droplets 13 will not disperse immediately, but spread in a circle on the tumplate 8. When the droplets spread in a certain range and the centrifugal force is large enough, the spread metal disperse on the turnplate 8 to the edge of the turnplate 8 in a fiber line shape under the action of centrifugal force, and finally split into tiny droplets to fly out. The tiny droplets solidify without a container in the falling process to form the metal powder 9 and fall onto the collection tray 10. The present disclosure also discloses a method for preparing ultrafine spherical metal powder by means of drop-by-drop centrifugal atomization process, including the following steps: Si. charging: charging the metal material into the upper crucible 2 arranged in the housing 20, and manually adjusting, in the height direction, a distance between the induction heating coil 12 and the tumplate 8 to a preset distance, then sealing the housing 20; an added amount of the charged metal material accounts for 1/4 to 3/4 of a capacity of the crucible. S2. vacuumizing: vacuumizing the crucible 2 and the housing 20 by using the mechanical pump 16 and the diffusion pump 17, and filling the crucible and the housing 20 with a high-purity inert shielding gas, to make the pressure inside the housing 20 reach a preset value. S3. heating the crucible: setting heating parameters of the heating tape 6 according to a melting point of the metal material to-be-heated, monitoring the temperature inside the crucible 2 in real time by the thermocouple 19 arranged in the crucible 2, and maintaining the temperature after the metal is completely melted.

S4. induction heating: with a rotational speed preset, enabling the tumplate 8 to rotate at

a high speed by using the motor 11, and heating the upper surface of the turnplate 8 rotating

C21007AUO at the high speed to a temperature higher than a melting point of the metal material by using

the induction heating coil 12; wherein an induction heating voltage of the induction heating

coil 12 ranges from 0 to 50 V, and an induction heating time ranges from 5 minutes tol5

minutes. S5. making the powder: introducing a high-purity inert shielding gas into the crucible 2 by using the crucible air inlet 4 arranged on the housing 20 and extending into the crucible 2, to form a positive pressure difference between the inside and the outside of the crucible 2; then inputting a pulse signal with a certain wave mode to the piezoelectric ceramic 1, so that the oscillation generator 3 produces a certain frequency of oscillation; and then, setting the voltage of the plate electrode 7 to form an electric field of a preset intensity. Because of the existence of the pressure difference between the inside and the outside of the crucible 2, the metal flows out through the nozzle 14 to form a columnar metal flow. At this time the columnar metal flow breaks into a series of small metal droplets 13 under a certain frequency of oscillation. In the falling process of metal droplets 13, under the effect of electric field, the metal droplets 13 repel each other, due to the surface effect of electric charge, to avoid the repolymerization of metal droplets 13. The metal droplets 13 land freely on the turnplate 8 rotating at a high speed, which first drop in the center of the turnplate 8. Because the centrifugal force is small at this time, the droplets will not disperse immediately, but spread in a circle on the turnplate 8. When the droplets spread in a certain range and the centrifugal force is large enough, the spread metal disperse on the turnplate 8 to the edge of the turnplate 8 in a fiber line shape under the action of centrifugal force, and finally split into tiny droplets to fly out. The tiny droplets solidify without a container in the falling process to form the metal powder 9 and fall onto a collection tray 10. S6. collecting the particles: collecting the metal powder by the collection tray 10 arranged at the bottom of the housing. Embodiment 1 A batch preparation of Sn63Pb37 alloy spherical powder is as follows: The raw material of Sn63Pb37 is charged to the crucible 2 after ultrasonic vibration cleaning, and the added amount of the Sn63Pb37 is up to 3/4 of the capacity of the crucible 2. The heating tape 6 is installed on the crucible 2, and the thermocouple is inserted inside the crucible 2. The selected tumplate 8 is installed on the motor 11. The induction heating coil 12 is installed around the turnplate 8 and is 1mm higher than the turnplate 8, and then the

C21007AUO housing 20 is sealed. The housing 20 and the crucible 2 is pumped to a low vacuum below 5 Pa by using the mechanical pump 16, and then the housing 20 and the crucible 2 are pumped to a high vacuum of 0.001 Pa by using the diffusion pump 17. A high-purity inert shielding gas of argon gas is introduced into the housing and the crucible through the crucible air inlet 4 and the cavity air inlet 15 to make the pressure inside the housing 20 and crucible 2 reach 0.1 MPa. The crucible 2 is heated by the heating tape 6 to 300 °C with a heating speed of 15 °C/min, and the temperature is kept for 10 minutes, so that all the metal materials in the crucible 2 are melted into the melt 5. The rotational speed of the tumplate 8 is 24000 r/min by using the motor 11. The induction heating voltage of the induction heating coil 12 is set at 21 V, the induction heating current is set at 8 A, and the induction heating time is set at 10 minutes. The surface of turnplate 8 rotating at a high speed is heated to a temperature above the melting point of the metal material of 183°C. The voltage of the plate electrode is set at 300 V. The high-purity inert shielding gas of argon gas is introduced into the crucible 2 through the crucible air inlet 4, to make a positive differential pressure of 50 kPa between the crucible 2 and the housing 20. A pulse signal of trapezoidal wave with frequency 1MHZ is input to the piezoelectric ceramic 1 to make the piezoelectric ceramic 1 oscillate up and down. The oscillation transmits to the melt 5 in the area near the nozzle 14 by the oscillation generator 3 connected with the piezoelectric ceramic 1, so that the melt 5 is sprayed through the nozzle 14 with small holes to form uniform metal droplets 13. The uniform metal droplets 13 land freely on the turnplate 8 rotating at high speed. The uniform metal droplets 13 in the molten state spread in a fibrous shape on the turnplate 8 under the centrifugal force to split into tiny droplets to fly out. The tiny droplets solidify without a container in the falling process to form the metal powder 9 and fall onto the collection tray 10. The collection tray can be a ring-shaped disk or disk. After the preparation is completed, stop inputting the pulse signal of trapezoidal wave to the piezoelectric ceramic 1, that is, stop spraying the droplets. Stop the motor 11 rotating at a high speed, thereby the tumplate 8 stops rotating. Close the heating tape 6 and the induction heating coil 12. The metal powder 9 is removed from the collection tray 10 after the temperature decreased to room temperature. At last, the cavity air inlet 15 and the crucible air inlet 4 are closed, and the crucible 2 and the housing 20 are pumped to a low vacuum below 5 Pa by using the mechanical pump 16, so as to make the apparatus in a vacuum state when

C21007AUO stopped. As shown in Fig. 3, (b) is an atomization plate obtained after atomization in the prior art. Because the wettability between the materials of the atomization plate and the prepared metal powder is too small and the temperature of the turnplate during the atomization process is too low, resulting that the metal liquid splits in a film shape and there's a thick solidified liquid film on the atomization surface. The surface of the liquid film is too rough to atomize the subsequent metal droplets well, thereby affecting atomization effect and atomization efficiency seriously. Fig. 3(a) is an atomization surface obtained by using the method in the present disclosure. It can be seen that the atomization mode is transformed into an obvious fibrous split mode, which greatly improves the fineness and production efficiency of the metal powder. At last, it should be stated that the above various embodiments are only used to illustrate the technical solutions of the present invention without limitation; and despite reference to the aforementioned embodiments to make a detailed description of the present invention, those of ordinary skilled in the art should understand: the described technical solutions in above various embodiments may be modified or the part of or all technical features may be equivalently substituted; while these modifications or substitutions do not make the essence of their corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

C21007AUO Claims 1. An apparatus for preparing ultrafine spherical metal powder by means of drop-by-drop centrifugal atomization process, comprising a housing (20), a crucible (2) and a powder collection area arranged in the housing (20); the powder collection area is arranged at the bottom of the housing (20) and the crucible (2) is arranged above the powder collection area; the crucible (2) is provided with a thermocouple (19) inside and a heating tape (6) outside; the crucible (2) is provided at the bottom with a nozzle (14) with small holes; the crucible (2) is provided inside with an oscillation generator (3) connected with a piezoelectric ceramic (1) arranged on the top of the housing; a plate electrode (7) is arranged right below the crucible; the housing (20) is provided with a crucible air inlet (4) extending into the crucible (2), and is also provided with a diffusion pump (17), a mechanical pump (16), a cavity air inlet (15) and a cavity exhaust valve (18); the powder collection area comprises a collection tray (10) arranged at the bottom of the housing (20), and a turnplate (8) arranged above the collection tray (10) and connected with a motor (11) for atomizing metal droplets; wherein: the turnplate (8) comprises a base, an atomization plane (23) and an air hole (24); the base is a structure of a "T-shaped" longitudinal section constituted of an upper receiving portion (21) and a lower support portion (22); the upper surface of the receiving portion (21) is provided with a circular groove with a certain radius and coaxial with the center of the receiving portion; wherein the base is made of a material with a thermal conductivity less than 20W/m/k; the atomization plane (23) is a disc structure, matching with the circular groove and in interference fitting with the circular groove; wherein the atomization plane (23) is made of a material with a wetting angle less than 900 to an atomized metal droplet (13); the air hole (24) is arranged passing through the receiving portion (21) and the support portion (22); the upper end face of the air hole (24) is in contact with the lower end face of the atomization plane (23), and the lower end of the air hole (24) communicates with the outside world; and an induction heating coil (12) is also arranged outside the tumplate (8).
2. The apparatus for preparing ultrafine spherical metal powder by means of drop-by-drop centrifugal atomization process according to claim 1, wherein a wetting angle

C21007AUO between a material of the crucible (2) and a melt (5) in the crucible is greater than 90.
3. The apparatus for preparing ultrafine spherical metal powder by means of drop-by-drop centrifugal atomization process according to claim 1, wherein an aperture of the small hole of the nozzle (14) ranges from 0.02 mm to 2.0 mm.
4. The apparatus for preparing ultrafine spherical metal powder by means of drop-by-drop centrifugal atomization process according to claim 1, wherein a voltage of the plate electrode (7) ranges from 100 V to 400 V; the induction heating coil (12) is connected with a frequency converter and a stabilized voltage supply arranged outside the housing (20); a heating thickness of the induction heating coil (12) ranges from 5 mm to 20 mm, and a voltage control of the stabilized voltage supply ranges from 0 v to 50 V.
5. The apparatus for preparing ultrafine spherical metal powder by means of drop-by-drop centrifugal atomization process according to claim 1, wherein a rotational speed of the turnplate (8) ranges from 10000 rpm to 50000 rpm.
6. The apparatus for preparing ultrafine spherical metal powder by means of drop-by-drop centrifugal atomization process according to claim 1, wherein the piezoelectric ceramic (1), the oscillation generator (3), the crucible (2), the nozzle (14), the plate electrode (7), the turnplate (8), and the induction heating coil (12) are located coaxially from top to bottom of the apparatus.
7. A method for preparing ultrafine spherical metal powder by means of drop-by-drop centrifugal atomization process according to any one of claims 1 to 6, comprising the following steps: Si. charging: charging the metal material into the crucible (2) arranged in the upper portion of the housing (20), and manually adjusting, in height direction, a distance between the induction heating coil (12) and the turnplate (8) to a preset distance, then sealing the housing (20); S2. vacuumizing: vacuumizing the crucible (2) and the housing (20) by using the mechanical pump (16) and the diffusion pump (17), and filling the crucible (2) and the housing (20) with a high-purity inert shielding gas, to make the pressure inside the housing (20) reach a preset value;

S3. heating the crucible: setting heating parameters of the heating tape (6) according to a

melting point of the metal material to-be-heated, monitoring the temperature inside the

crucible (2) in real time by the thermocouple (19) arranged in the crucible (2), and

maintaining the temperature after the metal material is completely melted;

C21007AUO S4. induction heating: enabling the turnplate (8) to rotate at a preset high speed by using

the motor (11), and heating the upper surface of the turnplate (8) rotating at the high speed to

a temperature higher than a melting point of the metal material by using the induction heating

coil (12);

S5. making the powder: introducing a high-purity inert shielding gas into the crucible (2)

by using the crucible air inlet (4) arranged on the housing (20) and extending into the crucible

(2), to form a positive pressure difference between the inside and the outside of the crucible

(2); then inputting a pulse signal with a certain wave mode to the piezoelectric ceramic (1), so

that the oscillation generator (3) generating a certain frequency of oscillation; and then,

setting the voltage of the plate electrode (7) to form an electric field of a preset strength;

wherein,

during the making process, because of the existence of the pressure difference between

inside and outside of the crucible (2), the molten metal flows out from the nozzle (14) to form

a columnar metal flow; at this time the columnar mental flow breaks into a series of small

metal droplets (13) under a certain frequency of oscillation; in the falling process of the metal

droplets and under the effect of electric field, the metal droplets (13) repel each other due to

the surface effect of electric charge to avoid the repolymerization of metal droplets (13);

the metal droplets (13) land freely on the tumplate (8) rotating at a high speed; the metal

droplets (13) first drop in the center of the tumplate (8), because a centrifugal force is small at

this time, the droplets will not disperse immediately, but spread in a circle on the turnplate(8);

when the droplets spread in a certain range and the centrifugal force is large enough, the

spread metal disperses on the turnplate (8) to an edge in a fiber line shape under the action of

centrifugal force, and finally split into tiny droplets to fly out; the tiny droplets solidify

without a container in the falling process to form the metal powder and fall onto a collection

tray (10); and

S6. collecting the powder: collecting the metal powder by the collection tray (10)

arranged at the bottom of the housing.
8. The method for preparing ultrafine spherical metal powder by means of drop-by-drop

centrifugal atomization process according to claim 7, wherein an added amount of the

charged metal material ranges from 1/4 to 3/4 of a capacity of the crucible (2).

C21007AUO
9. The method for preparing ultrafine spherical metal powder by means of drop-by-drop

centrifugal atomization process according to claim 7, wherein the high-purity inert shielding

gas is argon or helium gas, which is filled into the housing (20) to make the pressure in the

housing reach .1MPa; a holding time is 15 to 20 minutes after the metal material is

completely melted.

10. The method for preparing ultrafine spherical metal powder by means of

drop-by-drop centrifugal atomization process according to claim 7, wherein an induction

heating voltage of the induction heating coil (12) ranges from 0 to 50V, and an induction

heating time ranges from 5 tol5 minutes.

C21007AU0

Drawings

Fig.1

23

21

24 22 Fig.2

C21007AU0

（a）（b）

Fig.3