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Effect of central gas velocity and plasma power on the spheroidizing copper powders of radio frequency plasma
Vacuum 174 (2020) 109195
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Effect of central gas velocity and plasma power on the spheroidizing copper powders of radio frequency plasma Bo Zeng a, Jun Wang a, *, Hong-yuan Fan a, Hong Chang b a b
School of Mechanical Engineering, Sichuan University, Chengdu, 610065, China Chengdu Qixing Vacuum Coating Technology Co. LTD, Chengdu, 610065, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Powder materials Copper Radio frequency plasma Spheroidization Processing parameters
To investigate the effect of central gas velocity and plasma power on the spheroidization of irregular copper powder, three radio frequency (RF) plasma torches were established. Relationships between the spheroidization rate and central gas velocity or plasma power were studied. The morphology, particle size distribution and phase composition of the powders before and after spheroidization were analyzed by metallographic microscope, laser particle size analyzer and X-ray diffractometer. The relationship between heat absorption of powder and plasma power is also explained by the theoretical calculation. Results show that the optimum parameters for the preparation of spherical copper powders are 1.6 m3/h central gas velocity and 8.1 kW plasma power. Under this condition, the percentage of spheroidization can reach 91%. After plasma treatment, the copper powders have regular shapes, smooth surfaces and no inner pores. Because the power of 8.1 kW is enough to melt the powders completely. Below 8.1 kW, the main factor affecting the spheroidizing rate is the plasma power. Exceed that, the stability of plasma jet is the main factor.
1. Introduction Copper has good electrical and thermal conductivity, wear resis tance, high plasticity and good workability. At the same time, it has strong corrosion resistance in the atmosphere and ocean. Therefore, copper has been widely used in aerospace, electronic information, transportation and energy fields [1–3]. There are many problems in traditional components manufacturing methods such as low material utilization and simple product structure [4]. The 3D printing technology has emerged as the times require, which provides a feasible route for the production of complex parts with low cost and high efficiency. The metal powders used for 3D printing requires high sphericity, good fluidity and no agglomeration, but the ordinary copper powder can not meet the requirements [5]. Plasma is often used as a special heat source for its extremely high energy. It has been widely applied in the field of obtaining coatings, such as plasma cladding [6] and plasma spraying [7]. Plasma Rotating Electrode Process (PREP) is main industrial metallurgy method pre paring superalloy powders [8]. Radio frequency (RF) plasma is one kind of thermal plasma which is driven by a radio frequency power supply. Thermal plasma is an excellent way to prepare spherical metal powders
with stable composition, high sphericity and good dispersion due to its advantages of high temperature, concentrated energy distribution, no electrode pollution and flexible control [9]. The heating mode of RF plasma torch is gas arc, with radiation and convection are its heat transfer ways. Therefore, the heating efficiency of RF plasma is very high, and the central temperature of the plasma jet can reach ten thousand degrees centigrade [10]. The arc temperature is sufficient for endothermic melting of all refractory metal particles when pass through the heating zone. The droplets condense into spheres under the action of surface tension, and leave the heating zone with a rapid cooling rate of 106 � C/s. It is the unique advantage of thermal plasma to provide a special temperature field to achieve rapid melting of particles and rapid cooling to obtain spherical powders with high sphericity and good fluidity. Researchers have successfully spheroidized W [11,12], Mo [13], Ti [14], Nb [15], Fe [16], Al2O3 [17] and other powders by using radio frequency plasma. Han et al. [12] used thermal plasma to trans form irregular feedstock tungsten powders into spherical powders, but the experimental parameters and their influence on the spheroidization were not mentioned. Sureshs et al. [17] and Li et al. [18] found it was possible to obtain spherical powders within a narrower size distribution from raw powders by thermal plasma, but they did not elaborate on the
* Corresponding author. E-mail address: [email protected] (J. Wang). https://doi.org/10.1016/j.vacuum.2020.109195 Received 17 November 2019; Received in revised form 14 January 2020; Accepted 14 January 2020 Available online 17 January 2020 0042-207X/© 2020 Elsevier Ltd. All rights reserved.
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Fig. 1. Picture of RF plasma equipment.
round cover plate of the reaction chamber. The hole is not perpendicular to the downward direction, but inclines at a certain angle (30� ) with the central axis of the cover plate. The three plasma generators are respec tively installed in the holes and fixed by flange connection. The jets of the three torches are converging in a mechanically fixed form to achieve energy concentration. Plasma power is mainly adjusted by changing the operating current. The change of central gas velocity will change the voltage between anode and cathode of plasma torch in a certain range, thus indirectly changing the power of plasma. In this experiment, the current and central gas velocity of the three plasma torches are uni formly adjusted to ensure their power is the same. The power mentioned in the article refers to the total power of three torches.
reason for the narrower size distribution. Zhu and co-workers research [19] reported that the internal porosity of metal particles were almost eliminated subsequently to the thermal plasma treatment by analyzing the density of the powder material, without explanation from the shape of the particles. Kumar et al. [16] found that decreasing the plasma gas velocity enhanced the degree of spheroidization of micro-size iron powders, but he did not explain how the gas velocity affected the spheroidization of particles. Chaturvedi et al. [20] found that the tem perature and velocity of the particles would increase with the increase of plasma power, but the relation between power and heat absorption of particles was not discussed. Most researches focused on the treatments of refractory metal pow der by plasma and the process parameters of the higher spheroidization rate, showing that plasma is an effective way to prepare spherical re fractory metal powders. Many reports ignored the reasons why the process parameters affect the experimental results, as well as the process of particle decalescence or melting. Based on the powder particle itself, this study explains the process of particle melting, solidification and crystallization from the micro point of view. In this paper, irregularly shaped reduced copper powders were treated by radio frequency plasma. The influence of the central gas velocity and plasma power on the spheroidizing effect of powders were studied. The process of elimi nating the pores and narrowing the distribution of powder particle size were discussed. The relationship between the spheroidizing effect of the powders and the plasma power was explained through calculation, so as that provide theoretical basis for the spheroidizing treatment of copper powders and other materials.
2.2. Spheroidization of copper powders Irregular reduced copper powder (99.5% pure, 400 to 500 mesh) provided by Chengdu Kelong Chemical Reagent Factory was used as the precursor. Argon (99.9% pure) provided by Chengdu Xuyuan Chemical Co. Ltd. was used as the central gas and powder-carrier gas. Powder feeder (FH-80, Shanghai Fahan Spraying Machinery Co. Ltd. Shanghai, China) was used as the conveying mechanism of raw powder. The original powder falls from the powder barrel and enters the rotary table. With the rotation of the rotary table, it reaches the powder outlet and leaves the powder feeder and enters the pipeline together with the powder-carrier gas (argon). Pipe’s tail is installed on the central axis of the cover plate of the reaction chamber. The nozzle of the pipe is aims at the confluence of the three torches. Powders is blown from the nozzle to the heating zone by argon. The speed of powder feeding into plasma torch can be adjusted by controlling the rotating speed of rotating disc and the velocity of powder-carrier gas. Previous powder feeding ex periments showed that the powder feeding rate was stable at 14.67 g/ min when the rotating speed of the rotating disc was 2 r/min and the powder-carrier gas velocity was 0.08 m3/h. In order to reduce the contamination to the sample by impurity gas, the air pressure in the reaction chamber was first pumped to 1 � 10 3Pa through the vacuum system, and then filled with argon until the internal pressure is 10Pa. Before transporting the raw powders into the high-temperature region, the reaction chamber was heated by the plasma jet for 5 min. When the temperature of the reaction chamber was stable, the raw copper pow ders were fed into the plasma flame through the feeding nozzle by argon gas flow. The powders absorbed of heat and melted in the high tem perature region of the flame, and the droplets shrunk into spherical shape under the action of surface tension. Then the droplets got out of the high temperature region and condensed rapidly to form spherical
2. Experimental 2.1. RF plasma setup The spheroidization of the copper powders was carried out in the RF plasma system and the equipment was jointly developed by School of Mechanical Engineering Sichuan University and Chengdu Qixing Vac uum Coating technology Co., Ltd. The experimental devices are shown in Fig. 1. The equipment mainly includes gas supply system (central gas and powder-carrier gas. Plasma is produced by the central gas and the powder-carrier gas is used to transport powder particles), plasma torch (internal water cooling), reaction chamber (double stainless steel structure, internal water cooling), power supply and powder feeder. At the top of the reaction chamber, as shown in Fig. 1, are three RF plasma torches whose jets gather together to form a heating zone. Three holes for fixing the plasma torch are arranged in the special position of the 2
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XZJ-2030B) and a high-resolution scanning electron microscopy (SEM, JSM-7500F), the latter which linked with an EDS X-Max Oxford spec trometer. The EDS photo of the samples was performed for the powder surface elements to be determined. The particle size distribution of the copper powders before and after plasma treatment was measured using laser particle size distribution instrument (MS 3000). For cross-sectional observations, the metallographic specimens were made as following process. The powder particles before and after spheroidization were fixed by resin, and then the samples after consol idation were polished. In order to facilitate observation, special corro sive solution was used to conduct metallographic corrosion on the polished surface for 15 s, and the composition of the corrosive solution was shown in Table 2.
Table 1 Experimental parameters for spheroidizing Cu powders. Process parameters
Numerical value 3
1
Central gas velocity/(m ⋅h ) Powder-carrier gas velocity/(m3⋅h 1) Feeding rate/(g⋅min 1) System pressure/kPa Plasma power/kW
1.0–2.0 0.08 14.67 0.01 5.7–11.7
Table 2 Composition of copper metallographic etchant. HCl
FeCl3
Distilled water
30 ml
10 g
120 ml
2.4. Statistics of the spheroidization rate The percentage of spherical powder particles in the sample after treated by plasma was counted from the OM images. If a particle with regular shape and the aspect ratio is greater than 0.85, we define it a spherical particle [21]. Each sample was randomly sampled and calcu lated five times. Finally, the mean percentage was used as the spher oidization rate of the sample. 3. Results 3.1. The transformation of powders morphology by spheroidization Fig. 2 shows the SEM images of original copper powders and those powders after spheroidization with different powers. It can be seen from Fig. 2(a) that the raw copper powder particles obtained by reduction method are in the shape of “wheat spike”, and the maximum size of “wheat spike” was close to 100 μm. Some particles were adhered to each other and agglomerate. Fig. 2(b) shows the sample with the highest spheroidization rate in this experiment. After the plasma treatment under certain parameters, it can be seen that the particles were trans formed into round and spherical ones with high sphericity and smooth surface, and were evenly dispersed without agglomeration. Fig. 2(c) shows the morphology of powder after treatment when the plasma power is 5.7 kW (lower than the optimal spheroidizing power of 8.1 kW in this experiment). It is easy to find that only a part of the particles at this time were transformed into spherical particles. Due to the low operating power, the total energy that can be absorbed by the powders was not enough to make all the particles completely melt, Therefore, some of the particles in Fig. 2(c) were boundary melting, and some of them were completely unmelted, as the arrow points to. The processing power corresponding to the sample in the Fig. (d) is 6.3 kW. It can be seen from that the increased of the plasma power improved the degree of heat absorption and melting of powder particles. Under the effect of surface tension, a small number of residual ellipsoidal droplets approach the spherical shape, and after cooling, the near-spherical particles are
Fig. 2. Surface topography of copper powders: (a) Raw powders, (b) Spheroi dized powders with plasma power 8.1 kW and central gas velocity 1.6 m3/h, (c) Spheroidized powders with power 5.7 kW and velocity 1.6 m3/h, (d) Spheroi dized powders with power 6.3 kW and velocity 1.6 m3/h.
powder particles, which eventually enter the powder collector. Detailed experimental parameters are listed in Table 1. 2.3. Characterization of the copper powders before and after spheroidization The phases composition of the powders were analyzed using an X-ray diffractometer (XRD, EMPYREAN) with the 2θ angle ranging from 30� to 80� at a scan rate of 0.2� /s. The morphology of the powders was observed by means of a forward optical metallographic microscope (OM,
Fig. 3. Cross-sectional morphology of copper pow der particle; (a) Raw powder, (b) Spheroidized powder with plasma power 8.1 kW and central gas velocity 1.6 m3/h.
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Fig. 5. The influence of velocity of central gas on spheroidization rate.
Fig. 4. Microscopic photographs of spheroidized copper powders at different central gas velocities: (a) 1.0 m3/h, (b) 1.2 m3/h, (c) 1.4 m3/h, (d) 1.6 m3/h, (e) 1.8 m3/h, (f) 2.0 m3/h.
obtained. Fig. 3 shows the cross-sectional morphology of powder particles in the two states. Fig. 3(a) is the section of a raw material powder particle. As the original particle was in the shape of “wheat spike”, there were gaps of 2–4 μm in width. During sample processing, the flow resin filled the gaps under pressure, and the morphology of the resin with copper was obtained after consolidation. The raw copper powder particles have irregular shape and more pores. Fig. 3(b) is the cross section of the powder particles shown in Fig. 2(b). The particles are regular in shape, high sphericity, and inner equiaxed grains is about 6–8 μm. This is consistent with the grain boundary morphology on the surface of par ticles in Fig. 2. In the spheroidization process, the particles entered the plasma flame, absorbing sufficient energy for integral melting. The droplets then left the high temperature zone and cooled rapidly. Because there was no impurity element and no constitutional supercooling, the spherical particles consisted of equiaxed crystal were finally obtained, with the pores were eliminated [18]. 3.2. Influence of velocity of central gas on spheroidization effect
Fig. 6. Microscope pictures of spheroidized copper powders at different plasma power: (a) 5.7 kW, (b) 6.3 kW, (c) 8.1 kW, (d) 9.0 kW, (e) 10.5 kW, (f)11.7 kW.
The gas used to trigger the plasma is called central gas. Central gas velocity is an important parameter in this spheroidization process. Low velocity will not trigger an arc of plasma but high velocity will blow out the plasma torch. Fig. 4 shows the graphs of spheroidized samples at different velocities within a reasonable range. Fig. 4(a) is a metallo graphic micrograph of spheroidized powders with the velocity of 1.0 m3/h in the central gas. As can be seen from the figure, only a few copper powder particles were spheroidized, and more of them only melted the sharp parts of the particles. Under this circumstance, the statistical spheroidization rate was only 38%. With the increase of gas velocity, the spheroidization rate increases gradually. When the velocity of central gas increases to 1.6 m3/h, more particles turn into spheroidization as Fig. 4(d), with the statistical spheroidization rate of 57%. However, as the gas flow velocity continues to increase, the proportion of regular particles in the field of vision
begins to decrease. When the gas velocity increases to 2.0 m3/h, the proportion of spherical particles significantly decreases with the statis tical data of less than 50%, as shown in Fig. 4(f). Fig. 5 shows the relationship between the velocity of central gas and the spheroidization rate obtained, according to Fig. 4. The spheroid ization rate increases first and then decreases with the increase of the velocity of the central gas, and the highest spheroidization rate is 57%. When the velocity of central gas exceeds 1.6 m3/h, the spheroidization effect begins to deteriorate. Central gas increases to 2.0 m3/h, and spheroidization rate decreases to 48%. With the increases of central gas velocity, plasma power and jet velocity increase simultaneously. When the velocity is 1–1.6 m3/h, the power gradually increases, which 4
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Fig. 9. XRD pattern of the untreated and spheroidized powders with power 8.1 kW and central gas velocity 1.6 m3/h.
Fig. 7. The relationship between plasma power and spheroidization rate.
spheroidization rate is 91%. As the power continues to increase, the proportion of “wheat spike” particles in the field of vision starts to rise, while the number of spheroidized particles decreases, the distribution of spheroidization rate is 81% and 71%, as shown in Fig. 6(e) and (f). With the increase of power, the heat absorption of powder increases, which promotes the spheroidization process [24]. But when the power exceeds a certain value, the interelectrode current is too large, the sta bility of jet flow is reduced, and the heat absorption and spheroidization of powder are inhibited. This will be detailed in the discussion section. 3.4. Particle size distribution, phase structure and element compositions of copper powders before and after treatment Fig. 8 shows the laser particle size distribution of untreated and spheroidized copper powders. The particle size of a large amount of untreated copper powder is between 20 and 70 μm. Compared with the original powders, the spheroidized particle size distribution range is more centralized [21], that is, the particle size is more uniform. This is because in the spheroidization process of copper powders, some fine powder particles fully absorb heat when passing through the high temperature zone, leading to evaporation, resulting in the reduction of the number of fine particles in the treated samples. At the same time, the shape of raw powder particles is irregular, and the spheroidization treatment makes the particles melt and shrink, the sharp parts disap pear, so that the large size particles become smaller, the particle size distribution of powders is more concentrated, the diameter of 20–40 μm particle proportion is relatively increased, so that the particle size dis tribution diagram of the wave peak increase. The phase of the powders before and after spheroidization was characterized by XRD, as shown in Fig. 9. The clear diffraction peak indicates that the powders is composed of pure copper. The untreated powders’ characteristics diffraction peak at 2θ of 43.25� , 50.39� and 74.07� respectively corresponding to pure copper (1 1 1), (2 0 0) and (2 2 0) crystal planes, which confirms face centered cubic Cu. The position of diffraction peak of copper powders after spheroidization has no de viation. Fig. 10(a) and (b) present the EDS spectra of the raw copper and the spheroidized copper powder, respectively. It could be observed that both raw and spheroidized powder mainly contained Cu and quite low weight percentages of O. After treatment, the change of oxygen weight percentage is less than 0.3%, which is below the detection limit of EDS. So we can come to a conclusion that there was almost no contamination to the Cu powder in the treatment process. The plasma flame did not contact with the electrodes, which eliminated additional sources of contamination. At the same time, argon acts as central gas and powdercarrier gas, providing a protective atmosphere and ensuring the high
Fig. 8. Laser particle size distribution of powders before and after spheroid ization with power 8.1 kW and central gas velocity 1.6 m3/h.
promotes heat absorption of particles. Meanwhile, the dispersion of the powder is gradually improved by argon flow, so the spheroidization rate increase. When the velocity exceeds 1.6 m3/h, the flight time of the powder particles in the plasma flame is shortened, and the total heat absorption is reduced, resulting in the particles are not easy to melt, thus the spherification rate is reduced. Combined with the analysis in liter ature [22,23], it can be seen that too much increase in the gas velocity will increase the fluctuation of plasma jet, make the high-temperature region unstable, affect the heat absorption and melting of powder par ticles, and thus reduce the spheroidization rate. Therefore, reasonable control of gas velocity is very important for the spheroidization of copper powders. 3.3. Influence of plasma power on spheroidization effect The operating power of the plasma torch represents the amount of energy in the high temperature zone. With the increase of power, the total energy increases, which contributes to the heat absorption and melting of powder particles and promotes the spheroidization process in a certain extent. Fig. 6 shows the microstructure topographies of spheroidized powders obtained by adjusting the plasma power when other parameters are invariable. The relationship between plasma power and spheroidization rate is demonstrated in Fig. 7. When the power does not exceed 8.1 kW, the change of spheroidization effect with power is shown as (a) to (c), which is consistent with Fig. 2. The highest 5
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Fig. 10. EDS analysis: (a) raw copper powders, (b) spheroidized copper powders with power 8.1 kW and central gas velocity 1.6 m3/h. Table 3 Physical properties of copper powders.
Table 4 Cooling water data at different power.
Theoretical density/ (kg⋅m 3)
Specific heat/ (J⋅kg 1⋅K 1)
Melting point/K
Room temperature/K
Latent heat/ (J⋅kg 1)
8930.00
390.00
1356.50
298.15
205000.00
purity of copper powders after treatment. 4. Discussion
T0 Þ þ Hm
�
(1)
In this formula, d is particle diameter (m), ρ is the theoretical density of material (kg⋅m 3), Cp is the specific heat of metal (J⋅kg 1⋅K 1), Tm is the melting point (K), T0 is room temperature (K), Hm is the latent heat of melting (J⋅kg 1). The physical properties of copper powders used in this experiment are shown in Table 3. Therefore, it can be calculated that a copper powder particle with a diameter of 3 � 10 5 m needs to absorb 7.8 � 10 5 J of heat for complete melting (SEM images of the processed powder showed that the particle diameter was mostly about 30 μm, so the particle diameter was selected as 30 μm in the calculation). In this experiment, the powders feeding rate is constant at 14.67 g/min. If all the particles can fully absorb heat and completely melt, they will absorb 9.06 kJ of heat in 1 min. Plasma power is the amount of heat produced by the plasma torches over a period of time, but only a fraction of that is absorbed by the powder, and its effective power is calculated by the following formula [20]. pE ¼ pin ⋅η
ΔT1/K
ΔT2/K
Q1 þ Q2/J
ƞ/%
5.7 6.3 8.1 9.0 10.5 11.7
7.4 8.3 9.5 11.1 11.6 12.1
10.0 11.0 14.6 16.0 19.2 21.7
334657 369734 475468 527999 616279 686349
2.15 2.19 2.17 2.22 2.18 2.23
exit temperatures, the power loss can be gotten in the process, then the powder heating efficiency of plasma torch can be calculated. ƞ is mainly decided by the equipment structure and plasma gas species. Plasma power acts on two aspects, one is absorbed by powder, the other is absorbed by cooling water. At different power, the relevant data of cooling water in the plasma torch and reaction chamber are listed in Table 4. ΔT1 is the temperature difference between inlet and outlet of cooling water in reaction chamber, ΔT2 is that in plasma torch. Q1 is the heat absorbed by the cooling water in the reaction chamber during 1 min, Q2 is that in plasma torch. Q1,Q2 and ƞ are calculated by the following formula.
The energy required for complete melting of powder particles can be calculated by the following formula [11]. � 1 1 Q ¼ πImd3 ρICpππ Q ¼ π d3 ρ Cp ðTm 6 6
Plasma power/kW
Q1 ¼ cΔT1 ρV1
(3)
Q2 ¼ cΔT2 ρV2
(4)
�
η¼ 1
Q1 þ Q2 Pin ⋅t
� (5)
� 100%
Table 5 Plasma power and the heat absorbed by powders.
(2)
PE is the effective power of plasma torch (kW), Pin is the total power of the plasma (kW), ƞ is the powder heating efficiency for plasma torch. By measuring the flow rate of the cooling water and it’s entrance and 6
Pin/kW
5.7
6.3
8.1
9.0
10.5
11.7
The amount of heat the powders can absorb in 1 min/J
7524
8316
10692
11880
13860
15444
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Fig. 11. Schematic diagram of copper powder spheroidization.
will be the next research direction.
Where c is the specific heat capacity of water (4.2⋅103J⋅kg 1⋅K 1), ρ is the density of water in 290.15 K, 0.1 MPa (999 kg m 3), V1 is the cooling water volume of reaction chamber in one minute (2.4 L), V2 is that of plasma torch (6.2 L). t is 1 min (60 s). Experiments show that the ƞ of equipment is about 2.2%. In other words, those can be used in powders endothermic melting power was only about 2.2% of the plasma power. The heat value that can be used for powders absorption at different plasma powers can be obtained by calculation, as shown in Table 5. When the power of the plasma sets at 5.7 kW, the total energy that can be absorbed by the powders is not enough to completely melt the powder particles. Therefore, some of the particles only melt the sharp parts of the surface to obtain the product particles with nearly ellipsoidal shape, as shown in Fig. 2(c). When the power increases to 6.3 kW, the heat absorbed by the powders increases, the melting degree increases, and the final powders collected is closer to the spheroidal, as shown in Fig. 2(d). When the power is 8.1 kW, the heat absorbed by the powders per minute (10.69 kJ) is more than the heat required for complete melting of this part of the powders (9.06 kJ). Therefore, all particles entering the heating zone can be almost completely melted. Under the action of surface tension, the droplet contracted into spheroidal and cooled to form a spherical particle, as shown in Fig. 2(b). With the increase of power, on the one hand, the total input energy increased to promote the heat absorption and melting of powders, improving the spheroidization rate. On the other hand, the current be tween the electrodes increases, and the plasma jet characteristics ejected from the discharge area change [25]. The accelerated flow velocity and reduced stability lead to shorter residence time of the raw material powders in the high temperature area, unstable endothermic process, increased chances of particle collision and adhesion, and ultimately lower spheroidization rate [26]. This explains that the heat in Fig. 6(c) met the requirement of complete melting of the powders, but the pow ders were not completely spheroidization. With other parameters un changed, about 8.1 kW is the optimal spheroidizing power of raw copper powders. The spheroidization process of copper powder particles is shown in Fig. 11. The particles of raw materials are in the shape of “wheat spike”, such as (1). After the particle absorbs the heat of the plasma torch, the outer part begins to melt, the corners gradually disappear, and the surface metal liquid begins to fill the inner pores, such as (2). After absorbing enough heat continuously, the particles melt completely, and the metal droplets contract into a sphericity under the action of surface tension [27], and the internal pores disappear completely, such as (3). After leaving the plasma jet, the temperature began to drop, and ho mogeneous nucleation core appeared in the pure metal droplet, such as (4). The uniform and rapid cooling makes the growth rate of crystal nucleus in all directions equal, and the spherical particles with equiaxed crystal composition are finally obtained, as shown in (5). At present, in the research of using plasma to spheroidize metal powders, most of the heat sources are single plasma torches, and this experiment is an attempt to combine multiple torches. Experiments show that three plasma torches can be combined to form a central heating zone at a small power. However, whether the combination can improve the total power at higher current remains to be further explored. The stability and influence factors of the combined plasma jet
5. Conclusions Copper powders with smooth surface, uniform size and good roundness can be prepared by radio-frequency plasma spheroidization. The spheroidization rate can reach 91%, and the internal pores of copper powders can be almost eliminated by spheroidization. The central gas velocity and plasma power are significant parameters in the spheroid ization process. Other conditions being the same, the spheroidization rate increases first and then decreases when either of these two pa rameters rises. The 8.1 kW plasma power is enough to completely melt the powders. Below this power, the main factor affecting the spheroid ization rate is the power of the plasma. Exceed that, the stability of plasma jet is the key factor that affecting the spheroidization rate. Declaration of competing interestCOI We declare that we have no conflict of interest. No conflict of interest exits in the submission of this manuscript, and manuscript is approved by all authors for publication. I would like to declare on behalf of my coauthors that the work described was original research that has not been published previously, and not under consideration for publication elsewhere, in whole or in part. All the authors listed have approved the manuscript that is enclosed. Acknowledgements The authors are very grateful to the grants provided by National Natural Science Foundation of China (Nos. 51471112 and 51611130204), Science and Technology Planning Project of Sichuan (No. 2016GZ0173). References [1] S. Hou, S.Y. Qi, D.A. Hutt, J.R. Tyrer, M. Mu, Z.X. Zhou, Three dimensional printed electronic devices realised by selective laser melting of copper/high-densitypolyethylene powder mixtures, J. Mater. Process. Technol. 254 (2018) 310. [2] Z.M. Zhang, P. Pan, X.W. Liu, Z.C. Yang, J. Wei, Z. Wei, 3D-copper oxide and copper oxide/few-layer graphene with screen printed nanosheet assembly for ultrasensitive non-enzymatic glucose sensing, Mater. Chem. Phys. 187 (2017) 28. [3] W.P. Liu, X.F. Yin, Recovery of copper from copper slag using a microbial fuel cell and characterization of its electrogenesis, Int. J. Miner. Metall. Mater. 24 (2017) 621. [4] M.J. Kim, M.A. Cruz, S.R. Ye, A.L. Gray, G.L. Smith, N. Lazarus, C.J. Walker, H. H. Sigmarsson, B.J. Wiley, One-step electrodeposition of copper on conductive 3D printed objects, Addit. Manuf. 27 (2019) 27. [5] H.P. Duan, X. Liu, X.Z. Ran, J. Li, D. Liu, Mechanical properties and microstructure of 3D-printed high Co–Ni secondary hardening steel fabricated by laser melting deposition, Int. J. Miner. Metall. Mater. 24 (9) (2017) 1027. [6] C. Brunner-Schwer, T. Petrat, B. Graf, et al., High speed-plasma-laser-cladding of thin wear resistance coatings: a process approach as a hybrid metal depositiontechnology[J], Vacuum 166 (2019) 123–126. [7] M.M. Verdian, Fabrication of FeAl(Cu) intermetallic coatings by plasma spraying of vacuum annealed powders, Vacuum 132 (2016) 5–9. [8] Y. Chen, J. Zhang, B. Wang, et al., Comparative study of IN600 superalloy produced by two powder metallurgy technologies: argon Atomizing and Plasma Rotating Electrode Process, Vacuum 156 (2018) 302–309. [9] Y.M. Wang, J.J. Hao, Y.W. Sheng, Spheroidization of Nd-Fe-B powders by RF induction plasma processing, Rare Metal Mater. Eng. 42 (9) (2013) 1810.
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[10] Z. Dai, Y.G. Cao, C.Y. Ma, F. Tang, J.T. Zhang, W.C. Wang, M.M. Du, J.Q. Long, X. Y. Yuan, Effect of main operating parameters on Al2O3 spheroidization by radio frequency plasma system, Rare Metal Mater. Eng. 46 (2) (2017) 333. [11] X.L. Jiang, M. Boulos, Induction plasma spheroidization of tungsten and molybdenum powders, Trans. Nonferrous Metals Soc. China 16 (2006) 13. [12] C. Han, H. Na, Y. Kim, H. Choi, In-situ synthesis of tungsten nanoparticle attached spherical tungsten micro-powder by inductively coupled thermal plasma process, Int. J. Refract. Metals Hard Mater. 53 (2015) 7. [13] B.Q. Li, H.C. Jin, F. Ding, L.Y. Bai, F.L. Yuan, Fabrication of homogeneous Mo-Cu composites using spherical molybdenum powders prepared by thermal plasma spheroidization process, Int. J. Refract. Metals Hard Mater. 73 (2018) 13. [14] Y.W. Sheng, Z.M. Guo, J.J. Hao, H.P. Shao, S.C. Wang, Preparation of microspherical titanium powder by RF plasma, Rare Metal Mater. Eng. 42 (6) (2013) 1291. [15] Q.J. Li, L. Zhang, D.B. Wei, S.B. Ren, X.H. Qu, Porous Nb-Ti based alloy produced from plasma spheroidized powder, Results Phys. 7 (2017) 1289. [16] S. Kumar, V. Selvarajan, Plasma spheroidization of iron powders in a nontransferred DC thermal plasma jet, Mater. Char. 59 (6) (2008) 781. [17] K. Suresh, V. Selvarajan, M. Vijay, Synthesis of nanophase alumina, and spheroidization of alumina particles, and phase transition studies through DC thermal plasma processing, Vacuum 82 (8) (2008) 814. [18] R. Li, M.L. Qin, H. Huang, C.C. Liu, Z. Chen, M. Huang, L. Zhang, X.H. Qu, Fabrication of fine-grained spherical tungsten powder by radio frequency (RF) inductively coupled plasma spheroidization combined with jet milling, Adv. Powder Technol. 28 (12) (2017) 3158.
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Effect of central gas velocity and plasma power on the spheroidizing copper powders of radio frequency plasma Bo Zeng a, Jun Wang a, *, Hong-yuan Fan a, Hong Chang b a b
School of Mechanical Engineering, Sichuan University, Chengdu, 610065, China Chengdu Qixing Vacuum Coating Technology Co. LTD, Chengdu, 610065, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Powder materials Copper Radio frequency plasma Spheroidization Processing parameters
To investigate the effect of central gas velocity and plasma power on the spheroidization of irregular copper powder, three radio frequency (RF) plasma torches were established. Relationships between the spheroidization rate and central gas velocity or plasma power were studied. The morphology, particle size distribution and phase composition of the powders before and after spheroidization were analyzed by metallographic microscope, laser particle size analyzer and X-ray diffractometer. The relationship between heat absorption of powder and plasma power is also explained by the theoretical calculation. Results show that the optimum parameters for the preparation of spherical copper powders are 1.6 m3/h central gas velocity and 8.1 kW plasma power. Under this condition, the percentage of spheroidization can reach 91%. After plasma treatment, the copper powders have regular shapes, smooth surfaces and no inner pores. Because the power of 8.1 kW is enough to melt the powders completely. Below 8.1 kW, the main factor affecting the spheroidizing rate is the plasma power. Exceed that, the stability of plasma jet is the main factor.
1. Introduction Copper has good electrical and thermal conductivity, wear resis tance, high plasticity and good workability. At the same time, it has strong corrosion resistance in the atmosphere and ocean. Therefore, copper has been widely used in aerospace, electronic information, transportation and energy fields [1–3]. There are many problems in traditional components manufacturing methods such as low material utilization and simple product structure [4]. The 3D printing technology has emerged as the times require, which provides a feasible route for the production of complex parts with low cost and high efficiency. The metal powders used for 3D printing requires high sphericity, good fluidity and no agglomeration, but the ordinary copper powder can not meet the requirements [5]. Plasma is often used as a special heat source for its extremely high energy. It has been widely applied in the field of obtaining coatings, such as plasma cladding [6] and plasma spraying [7]. Plasma Rotating Electrode Process (PREP) is main industrial metallurgy method pre paring superalloy powders [8]. Radio frequency (RF) plasma is one kind of thermal plasma which is driven by a radio frequency power supply. Thermal plasma is an excellent way to prepare spherical metal powders
with stable composition, high sphericity and good dispersion due to its advantages of high temperature, concentrated energy distribution, no electrode pollution and flexible control [9]. The heating mode of RF plasma torch is gas arc, with radiation and convection are its heat transfer ways. Therefore, the heating efficiency of RF plasma is very high, and the central temperature of the plasma jet can reach ten thousand degrees centigrade [10]. The arc temperature is sufficient for endothermic melting of all refractory metal particles when pass through the heating zone. The droplets condense into spheres under the action of surface tension, and leave the heating zone with a rapid cooling rate of 106 � C/s. It is the unique advantage of thermal plasma to provide a special temperature field to achieve rapid melting of particles and rapid cooling to obtain spherical powders with high sphericity and good fluidity. Researchers have successfully spheroidized W [11,12], Mo [13], Ti [14], Nb [15], Fe [16], Al2O3 [17] and other powders by using radio frequency plasma. Han et al. [12] used thermal plasma to trans form irregular feedstock tungsten powders into spherical powders, but the experimental parameters and their influence on the spheroidization were not mentioned. Sureshs et al. [17] and Li et al. [18] found it was possible to obtain spherical powders within a narrower size distribution from raw powders by thermal plasma, but they did not elaborate on the
* Corresponding author. E-mail address: [email protected] (J. Wang). https://doi.org/10.1016/j.vacuum.2020.109195 Received 17 November 2019; Received in revised form 14 January 2020; Accepted 14 January 2020 Available online 17 January 2020 0042-207X/© 2020 Elsevier Ltd. All rights reserved.
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Fig. 1. Picture of RF plasma equipment.
round cover plate of the reaction chamber. The hole is not perpendicular to the downward direction, but inclines at a certain angle (30� ) with the central axis of the cover plate. The three plasma generators are respec tively installed in the holes and fixed by flange connection. The jets of the three torches are converging in a mechanically fixed form to achieve energy concentration. Plasma power is mainly adjusted by changing the operating current. The change of central gas velocity will change the voltage between anode and cathode of plasma torch in a certain range, thus indirectly changing the power of plasma. In this experiment, the current and central gas velocity of the three plasma torches are uni formly adjusted to ensure their power is the same. The power mentioned in the article refers to the total power of three torches.
reason for the narrower size distribution. Zhu and co-workers research [19] reported that the internal porosity of metal particles were almost eliminated subsequently to the thermal plasma treatment by analyzing the density of the powder material, without explanation from the shape of the particles. Kumar et al. [16] found that decreasing the plasma gas velocity enhanced the degree of spheroidization of micro-size iron powders, but he did not explain how the gas velocity affected the spheroidization of particles. Chaturvedi et al. [20] found that the tem perature and velocity of the particles would increase with the increase of plasma power, but the relation between power and heat absorption of particles was not discussed. Most researches focused on the treatments of refractory metal pow der by plasma and the process parameters of the higher spheroidization rate, showing that plasma is an effective way to prepare spherical re fractory metal powders. Many reports ignored the reasons why the process parameters affect the experimental results, as well as the process of particle decalescence or melting. Based on the powder particle itself, this study explains the process of particle melting, solidification and crystallization from the micro point of view. In this paper, irregularly shaped reduced copper powders were treated by radio frequency plasma. The influence of the central gas velocity and plasma power on the spheroidizing effect of powders were studied. The process of elimi nating the pores and narrowing the distribution of powder particle size were discussed. The relationship between the spheroidizing effect of the powders and the plasma power was explained through calculation, so as that provide theoretical basis for the spheroidizing treatment of copper powders and other materials.
2.2. Spheroidization of copper powders Irregular reduced copper powder (99.5% pure, 400 to 500 mesh) provided by Chengdu Kelong Chemical Reagent Factory was used as the precursor. Argon (99.9% pure) provided by Chengdu Xuyuan Chemical Co. Ltd. was used as the central gas and powder-carrier gas. Powder feeder (FH-80, Shanghai Fahan Spraying Machinery Co. Ltd. Shanghai, China) was used as the conveying mechanism of raw powder. The original powder falls from the powder barrel and enters the rotary table. With the rotation of the rotary table, it reaches the powder outlet and leaves the powder feeder and enters the pipeline together with the powder-carrier gas (argon). Pipe’s tail is installed on the central axis of the cover plate of the reaction chamber. The nozzle of the pipe is aims at the confluence of the three torches. Powders is blown from the nozzle to the heating zone by argon. The speed of powder feeding into plasma torch can be adjusted by controlling the rotating speed of rotating disc and the velocity of powder-carrier gas. Previous powder feeding ex periments showed that the powder feeding rate was stable at 14.67 g/ min when the rotating speed of the rotating disc was 2 r/min and the powder-carrier gas velocity was 0.08 m3/h. In order to reduce the contamination to the sample by impurity gas, the air pressure in the reaction chamber was first pumped to 1 � 10 3Pa through the vacuum system, and then filled with argon until the internal pressure is 10Pa. Before transporting the raw powders into the high-temperature region, the reaction chamber was heated by the plasma jet for 5 min. When the temperature of the reaction chamber was stable, the raw copper pow ders were fed into the plasma flame through the feeding nozzle by argon gas flow. The powders absorbed of heat and melted in the high tem perature region of the flame, and the droplets shrunk into spherical shape under the action of surface tension. Then the droplets got out of the high temperature region and condensed rapidly to form spherical
2. Experimental 2.1. RF plasma setup The spheroidization of the copper powders was carried out in the RF plasma system and the equipment was jointly developed by School of Mechanical Engineering Sichuan University and Chengdu Qixing Vac uum Coating technology Co., Ltd. The experimental devices are shown in Fig. 1. The equipment mainly includes gas supply system (central gas and powder-carrier gas. Plasma is produced by the central gas and the powder-carrier gas is used to transport powder particles), plasma torch (internal water cooling), reaction chamber (double stainless steel structure, internal water cooling), power supply and powder feeder. At the top of the reaction chamber, as shown in Fig. 1, are three RF plasma torches whose jets gather together to form a heating zone. Three holes for fixing the plasma torch are arranged in the special position of the 2
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XZJ-2030B) and a high-resolution scanning electron microscopy (SEM, JSM-7500F), the latter which linked with an EDS X-Max Oxford spec trometer. The EDS photo of the samples was performed for the powder surface elements to be determined. The particle size distribution of the copper powders before and after plasma treatment was measured using laser particle size distribution instrument (MS 3000). For cross-sectional observations, the metallographic specimens were made as following process. The powder particles before and after spheroidization were fixed by resin, and then the samples after consol idation were polished. In order to facilitate observation, special corro sive solution was used to conduct metallographic corrosion on the polished surface for 15 s, and the composition of the corrosive solution was shown in Table 2.
Table 1 Experimental parameters for spheroidizing Cu powders. Process parameters
Numerical value 3
1
Central gas velocity/(m ⋅h ) Powder-carrier gas velocity/(m3⋅h 1) Feeding rate/(g⋅min 1) System pressure/kPa Plasma power/kW
1.0–2.0 0.08 14.67 0.01 5.7–11.7
Table 2 Composition of copper metallographic etchant. HCl
FeCl3
Distilled water
30 ml
10 g
120 ml
2.4. Statistics of the spheroidization rate The percentage of spherical powder particles in the sample after treated by plasma was counted from the OM images. If a particle with regular shape and the aspect ratio is greater than 0.85, we define it a spherical particle [21]. Each sample was randomly sampled and calcu lated five times. Finally, the mean percentage was used as the spher oidization rate of the sample. 3. Results 3.1. The transformation of powders morphology by spheroidization Fig. 2 shows the SEM images of original copper powders and those powders after spheroidization with different powers. It can be seen from Fig. 2(a) that the raw copper powder particles obtained by reduction method are in the shape of “wheat spike”, and the maximum size of “wheat spike” was close to 100 μm. Some particles were adhered to each other and agglomerate. Fig. 2(b) shows the sample with the highest spheroidization rate in this experiment. After the plasma treatment under certain parameters, it can be seen that the particles were trans formed into round and spherical ones with high sphericity and smooth surface, and were evenly dispersed without agglomeration. Fig. 2(c) shows the morphology of powder after treatment when the plasma power is 5.7 kW (lower than the optimal spheroidizing power of 8.1 kW in this experiment). It is easy to find that only a part of the particles at this time were transformed into spherical particles. Due to the low operating power, the total energy that can be absorbed by the powders was not enough to make all the particles completely melt, Therefore, some of the particles in Fig. 2(c) were boundary melting, and some of them were completely unmelted, as the arrow points to. The processing power corresponding to the sample in the Fig. (d) is 6.3 kW. It can be seen from that the increased of the plasma power improved the degree of heat absorption and melting of powder particles. Under the effect of surface tension, a small number of residual ellipsoidal droplets approach the spherical shape, and after cooling, the near-spherical particles are
Fig. 2. Surface topography of copper powders: (a) Raw powders, (b) Spheroi dized powders with plasma power 8.1 kW and central gas velocity 1.6 m3/h, (c) Spheroidized powders with power 5.7 kW and velocity 1.6 m3/h, (d) Spheroi dized powders with power 6.3 kW and velocity 1.6 m3/h.
powder particles, which eventually enter the powder collector. Detailed experimental parameters are listed in Table 1. 2.3. Characterization of the copper powders before and after spheroidization The phases composition of the powders were analyzed using an X-ray diffractometer (XRD, EMPYREAN) with the 2θ angle ranging from 30� to 80� at a scan rate of 0.2� /s. The morphology of the powders was observed by means of a forward optical metallographic microscope (OM,
Fig. 3. Cross-sectional morphology of copper pow der particle; (a) Raw powder, (b) Spheroidized powder with plasma power 8.1 kW and central gas velocity 1.6 m3/h.
3
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Fig. 5. The influence of velocity of central gas on spheroidization rate.
Fig. 4. Microscopic photographs of spheroidized copper powders at different central gas velocities: (a) 1.0 m3/h, (b) 1.2 m3/h, (c) 1.4 m3/h, (d) 1.6 m3/h, (e) 1.8 m3/h, (f) 2.0 m3/h.
obtained. Fig. 3 shows the cross-sectional morphology of powder particles in the two states. Fig. 3(a) is the section of a raw material powder particle. As the original particle was in the shape of “wheat spike”, there were gaps of 2–4 μm in width. During sample processing, the flow resin filled the gaps under pressure, and the morphology of the resin with copper was obtained after consolidation. The raw copper powder particles have irregular shape and more pores. Fig. 3(b) is the cross section of the powder particles shown in Fig. 2(b). The particles are regular in shape, high sphericity, and inner equiaxed grains is about 6–8 μm. This is consistent with the grain boundary morphology on the surface of par ticles in Fig. 2. In the spheroidization process, the particles entered the plasma flame, absorbing sufficient energy for integral melting. The droplets then left the high temperature zone and cooled rapidly. Because there was no impurity element and no constitutional supercooling, the spherical particles consisted of equiaxed crystal were finally obtained, with the pores were eliminated [18]. 3.2. Influence of velocity of central gas on spheroidization effect
Fig. 6. Microscope pictures of spheroidized copper powders at different plasma power: (a) 5.7 kW, (b) 6.3 kW, (c) 8.1 kW, (d) 9.0 kW, (e) 10.5 kW, (f)11.7 kW.
The gas used to trigger the plasma is called central gas. Central gas velocity is an important parameter in this spheroidization process. Low velocity will not trigger an arc of plasma but high velocity will blow out the plasma torch. Fig. 4 shows the graphs of spheroidized samples at different velocities within a reasonable range. Fig. 4(a) is a metallo graphic micrograph of spheroidized powders with the velocity of 1.0 m3/h in the central gas. As can be seen from the figure, only a few copper powder particles were spheroidized, and more of them only melted the sharp parts of the particles. Under this circumstance, the statistical spheroidization rate was only 38%. With the increase of gas velocity, the spheroidization rate increases gradually. When the velocity of central gas increases to 1.6 m3/h, more particles turn into spheroidization as Fig. 4(d), with the statistical spheroidization rate of 57%. However, as the gas flow velocity continues to increase, the proportion of regular particles in the field of vision
begins to decrease. When the gas velocity increases to 2.0 m3/h, the proportion of spherical particles significantly decreases with the statis tical data of less than 50%, as shown in Fig. 4(f). Fig. 5 shows the relationship between the velocity of central gas and the spheroidization rate obtained, according to Fig. 4. The spheroid ization rate increases first and then decreases with the increase of the velocity of the central gas, and the highest spheroidization rate is 57%. When the velocity of central gas exceeds 1.6 m3/h, the spheroidization effect begins to deteriorate. Central gas increases to 2.0 m3/h, and spheroidization rate decreases to 48%. With the increases of central gas velocity, plasma power and jet velocity increase simultaneously. When the velocity is 1–1.6 m3/h, the power gradually increases, which 4
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Fig. 9. XRD pattern of the untreated and spheroidized powders with power 8.1 kW and central gas velocity 1.6 m3/h.
Fig. 7. The relationship between plasma power and spheroidization rate.
spheroidization rate is 91%. As the power continues to increase, the proportion of “wheat spike” particles in the field of vision starts to rise, while the number of spheroidized particles decreases, the distribution of spheroidization rate is 81% and 71%, as shown in Fig. 6(e) and (f). With the increase of power, the heat absorption of powder increases, which promotes the spheroidization process [24]. But when the power exceeds a certain value, the interelectrode current is too large, the sta bility of jet flow is reduced, and the heat absorption and spheroidization of powder are inhibited. This will be detailed in the discussion section. 3.4. Particle size distribution, phase structure and element compositions of copper powders before and after treatment Fig. 8 shows the laser particle size distribution of untreated and spheroidized copper powders. The particle size of a large amount of untreated copper powder is between 20 and 70 μm. Compared with the original powders, the spheroidized particle size distribution range is more centralized [21], that is, the particle size is more uniform. This is because in the spheroidization process of copper powders, some fine powder particles fully absorb heat when passing through the high temperature zone, leading to evaporation, resulting in the reduction of the number of fine particles in the treated samples. At the same time, the shape of raw powder particles is irregular, and the spheroidization treatment makes the particles melt and shrink, the sharp parts disap pear, so that the large size particles become smaller, the particle size distribution of powders is more concentrated, the diameter of 20–40 μm particle proportion is relatively increased, so that the particle size dis tribution diagram of the wave peak increase. The phase of the powders before and after spheroidization was characterized by XRD, as shown in Fig. 9. The clear diffraction peak indicates that the powders is composed of pure copper. The untreated powders’ characteristics diffraction peak at 2θ of 43.25� , 50.39� and 74.07� respectively corresponding to pure copper (1 1 1), (2 0 0) and (2 2 0) crystal planes, which confirms face centered cubic Cu. The position of diffraction peak of copper powders after spheroidization has no de viation. Fig. 10(a) and (b) present the EDS spectra of the raw copper and the spheroidized copper powder, respectively. It could be observed that both raw and spheroidized powder mainly contained Cu and quite low weight percentages of O. After treatment, the change of oxygen weight percentage is less than 0.3%, which is below the detection limit of EDS. So we can come to a conclusion that there was almost no contamination to the Cu powder in the treatment process. The plasma flame did not contact with the electrodes, which eliminated additional sources of contamination. At the same time, argon acts as central gas and powdercarrier gas, providing a protective atmosphere and ensuring the high
Fig. 8. Laser particle size distribution of powders before and after spheroid ization with power 8.1 kW and central gas velocity 1.6 m3/h.
promotes heat absorption of particles. Meanwhile, the dispersion of the powder is gradually improved by argon flow, so the spheroidization rate increase. When the velocity exceeds 1.6 m3/h, the flight time of the powder particles in the plasma flame is shortened, and the total heat absorption is reduced, resulting in the particles are not easy to melt, thus the spherification rate is reduced. Combined with the analysis in liter ature [22,23], it can be seen that too much increase in the gas velocity will increase the fluctuation of plasma jet, make the high-temperature region unstable, affect the heat absorption and melting of powder par ticles, and thus reduce the spheroidization rate. Therefore, reasonable control of gas velocity is very important for the spheroidization of copper powders. 3.3. Influence of plasma power on spheroidization effect The operating power of the plasma torch represents the amount of energy in the high temperature zone. With the increase of power, the total energy increases, which contributes to the heat absorption and melting of powder particles and promotes the spheroidization process in a certain extent. Fig. 6 shows the microstructure topographies of spheroidized powders obtained by adjusting the plasma power when other parameters are invariable. The relationship between plasma power and spheroidization rate is demonstrated in Fig. 7. When the power does not exceed 8.1 kW, the change of spheroidization effect with power is shown as (a) to (c), which is consistent with Fig. 2. The highest 5
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Fig. 10. EDS analysis: (a) raw copper powders, (b) spheroidized copper powders with power 8.1 kW and central gas velocity 1.6 m3/h. Table 3 Physical properties of copper powders.
Table 4 Cooling water data at different power.
Theoretical density/ (kg⋅m 3)
Specific heat/ (J⋅kg 1⋅K 1)
Melting point/K
Room temperature/K
Latent heat/ (J⋅kg 1)
8930.00
390.00
1356.50
298.15
205000.00
purity of copper powders after treatment. 4. Discussion
T0 Þ þ Hm
�
(1)
In this formula, d is particle diameter (m), ρ is the theoretical density of material (kg⋅m 3), Cp is the specific heat of metal (J⋅kg 1⋅K 1), Tm is the melting point (K), T0 is room temperature (K), Hm is the latent heat of melting (J⋅kg 1). The physical properties of copper powders used in this experiment are shown in Table 3. Therefore, it can be calculated that a copper powder particle with a diameter of 3 � 10 5 m needs to absorb 7.8 � 10 5 J of heat for complete melting (SEM images of the processed powder showed that the particle diameter was mostly about 30 μm, so the particle diameter was selected as 30 μm in the calculation). In this experiment, the powders feeding rate is constant at 14.67 g/min. If all the particles can fully absorb heat and completely melt, they will absorb 9.06 kJ of heat in 1 min. Plasma power is the amount of heat produced by the plasma torches over a period of time, but only a fraction of that is absorbed by the powder, and its effective power is calculated by the following formula [20]. pE ¼ pin ⋅η
ΔT1/K
ΔT2/K
Q1 þ Q2/J
ƞ/%
5.7 6.3 8.1 9.0 10.5 11.7
7.4 8.3 9.5 11.1 11.6 12.1
10.0 11.0 14.6 16.0 19.2 21.7
334657 369734 475468 527999 616279 686349
2.15 2.19 2.17 2.22 2.18 2.23
exit temperatures, the power loss can be gotten in the process, then the powder heating efficiency of plasma torch can be calculated. ƞ is mainly decided by the equipment structure and plasma gas species. Plasma power acts on two aspects, one is absorbed by powder, the other is absorbed by cooling water. At different power, the relevant data of cooling water in the plasma torch and reaction chamber are listed in Table 4. ΔT1 is the temperature difference between inlet and outlet of cooling water in reaction chamber, ΔT2 is that in plasma torch. Q1 is the heat absorbed by the cooling water in the reaction chamber during 1 min, Q2 is that in plasma torch. Q1,Q2 and ƞ are calculated by the following formula.
The energy required for complete melting of powder particles can be calculated by the following formula [11]. � 1 1 Q ¼ πImd3 ρICpππ Q ¼ π d3 ρ Cp ðTm 6 6
Plasma power/kW
Q1 ¼ cΔT1 ρV1
(3)
Q2 ¼ cΔT2 ρV2
(4)
�
η¼ 1
Q1 þ Q2 Pin ⋅t
� (5)
� 100%
Table 5 Plasma power and the heat absorbed by powders.
(2)
PE is the effective power of plasma torch (kW), Pin is the total power of the plasma (kW), ƞ is the powder heating efficiency for plasma torch. By measuring the flow rate of the cooling water and it’s entrance and 6
Pin/kW
5.7
6.3
8.1
9.0
10.5
11.7
The amount of heat the powders can absorb in 1 min/J
7524
8316
10692
11880
13860
15444
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Vacuum 174 (2020) 109195
Fig. 11. Schematic diagram of copper powder spheroidization.
will be the next research direction.
Where c is the specific heat capacity of water (4.2⋅103J⋅kg 1⋅K 1), ρ is the density of water in 290.15 K, 0.1 MPa (999 kg m 3), V1 is the cooling water volume of reaction chamber in one minute (2.4 L), V2 is that of plasma torch (6.2 L). t is 1 min (60 s). Experiments show that the ƞ of equipment is about 2.2%. In other words, those can be used in powders endothermic melting power was only about 2.2% of the plasma power. The heat value that can be used for powders absorption at different plasma powers can be obtained by calculation, as shown in Table 5. When the power of the plasma sets at 5.7 kW, the total energy that can be absorbed by the powders is not enough to completely melt the powder particles. Therefore, some of the particles only melt the sharp parts of the surface to obtain the product particles with nearly ellipsoidal shape, as shown in Fig. 2(c). When the power increases to 6.3 kW, the heat absorbed by the powders increases, the melting degree increases, and the final powders collected is closer to the spheroidal, as shown in Fig. 2(d). When the power is 8.1 kW, the heat absorbed by the powders per minute (10.69 kJ) is more than the heat required for complete melting of this part of the powders (9.06 kJ). Therefore, all particles entering the heating zone can be almost completely melted. Under the action of surface tension, the droplet contracted into spheroidal and cooled to form a spherical particle, as shown in Fig. 2(b). With the increase of power, on the one hand, the total input energy increased to promote the heat absorption and melting of powders, improving the spheroidization rate. On the other hand, the current be tween the electrodes increases, and the plasma jet characteristics ejected from the discharge area change [25]. The accelerated flow velocity and reduced stability lead to shorter residence time of the raw material powders in the high temperature area, unstable endothermic process, increased chances of particle collision and adhesion, and ultimately lower spheroidization rate [26]. This explains that the heat in Fig. 6(c) met the requirement of complete melting of the powders, but the pow ders were not completely spheroidization. With other parameters un changed, about 8.1 kW is the optimal spheroidizing power of raw copper powders. The spheroidization process of copper powder particles is shown in Fig. 11. The particles of raw materials are in the shape of “wheat spike”, such as (1). After the particle absorbs the heat of the plasma torch, the outer part begins to melt, the corners gradually disappear, and the surface metal liquid begins to fill the inner pores, such as (2). After absorbing enough heat continuously, the particles melt completely, and the metal droplets contract into a sphericity under the action of surface tension [27], and the internal pores disappear completely, such as (3). After leaving the plasma jet, the temperature began to drop, and ho mogeneous nucleation core appeared in the pure metal droplet, such as (4). The uniform and rapid cooling makes the growth rate of crystal nucleus in all directions equal, and the spherical particles with equiaxed crystal composition are finally obtained, as shown in (5). At present, in the research of using plasma to spheroidize metal powders, most of the heat sources are single plasma torches, and this experiment is an attempt to combine multiple torches. Experiments show that three plasma torches can be combined to form a central heating zone at a small power. However, whether the combination can improve the total power at higher current remains to be further explored. The stability and influence factors of the combined plasma jet
5. Conclusions Copper powders with smooth surface, uniform size and good roundness can be prepared by radio-frequency plasma spheroidization. The spheroidization rate can reach 91%, and the internal pores of copper powders can be almost eliminated by spheroidization. The central gas velocity and plasma power are significant parameters in the spheroid ization process. Other conditions being the same, the spheroidization rate increases first and then decreases when either of these two pa rameters rises. The 8.1 kW plasma power is enough to completely melt the powders. Below this power, the main factor affecting the spheroid ization rate is the power of the plasma. Exceed that, the stability of plasma jet is the key factor that affecting the spheroidization rate. Declaration of competing interestCOI We declare that we have no conflict of interest. No conflict of interest exits in the submission of this manuscript, and manuscript is approved by all authors for publication. I would like to declare on behalf of my coauthors that the work described was original research that has not been published previously, and not under consideration for publication elsewhere, in whole or in part. All the authors listed have approved the manuscript that is enclosed. Acknowledgements The authors are very grateful to the grants provided by National Natural Science Foundation of China (Nos. 51471112 and 51611130204), Science and Technology Planning Project of Sichuan (No. 2016GZ0173). References [1] S. Hou, S.Y. Qi, D.A. Hutt, J.R. Tyrer, M. Mu, Z.X. Zhou, Three dimensional printed electronic devices realised by selective laser melting of copper/high-densitypolyethylene powder mixtures, J. Mater. Process. Technol. 254 (2018) 310. [2] Z.M. Zhang, P. Pan, X.W. Liu, Z.C. Yang, J. Wei, Z. Wei, 3D-copper oxide and copper oxide/few-layer graphene with screen printed nanosheet assembly for ultrasensitive non-enzymatic glucose sensing, Mater. Chem. Phys. 187 (2017) 28. [3] W.P. Liu, X.F. Yin, Recovery of copper from copper slag using a microbial fuel cell and characterization of its electrogenesis, Int. J. Miner. Metall. Mater. 24 (2017) 621. [4] M.J. Kim, M.A. Cruz, S.R. Ye, A.L. Gray, G.L. Smith, N. Lazarus, C.J. Walker, H. H. Sigmarsson, B.J. Wiley, One-step electrodeposition of copper on conductive 3D printed objects, Addit. Manuf. 27 (2019) 27. [5] H.P. Duan, X. Liu, X.Z. Ran, J. Li, D. Liu, Mechanical properties and microstructure of 3D-printed high Co–Ni secondary hardening steel fabricated by laser melting deposition, Int. J. Miner. Metall. Mater. 24 (9) (2017) 1027. [6] C. Brunner-Schwer, T. Petrat, B. Graf, et al., High speed-plasma-laser-cladding of thin wear resistance coatings: a process approach as a hybrid metal depositiontechnology[J], Vacuum 166 (2019) 123–126. [7] M.M. Verdian, Fabrication of FeAl(Cu) intermetallic coatings by plasma spraying of vacuum annealed powders, Vacuum 132 (2016) 5–9. [8] Y. Chen, J. Zhang, B. Wang, et al., Comparative study of IN600 superalloy produced by two powder metallurgy technologies: argon Atomizing and Plasma Rotating Electrode Process, Vacuum 156 (2018) 302–309. [9] Y.M. Wang, J.J. Hao, Y.W. Sheng, Spheroidization of Nd-Fe-B powders by RF induction plasma processing, Rare Metal Mater. Eng. 42 (9) (2013) 1810.
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