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Velocities of copper droplets in the De Laval atomization process
Powder Technology 229 (2012) 191–198
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Powder Technology journal homepage: www.elsevier.com/locate/powtec
Velocities of copper droplets in the De Laval atomization process M.P. Planche a,⁎, O. Khatim a, L. Dembinski a, C. Coddet a, L. Girardot b, Y. Bailly b a b
LERMPS- UTBM, 90010 Belfort Cedex, France Institute FEMTO-ST/ENISYS, University of Franche-Comté, Belfort, France
a r t i c l e
i n f o
Article history: Received 13 February 2012 Received in revised form 16 May 2012 Accepted 18 June 2012 Available online 2 July 2012 Keyword: Close coupled process Copper atomization PIV measures
a b s t r a c t Atomization experiments were carried out at the Nanoval atomization unit. Particle image velocimetry (PIV) was used to determine the velocity vector fields of copper powder production via De Laval gas atomization. The experimental set-up designed to access the area closest to the atomization point was described in detail. Velocity vector fields were constructed by taking into account the distance from the nozzle exit. Instantaneous velocities on the atomization axis were calculated as a function of the atomization duration. Results from this study indicate a strong influence of the atomization pressures and the melt nozzle diameters on velocity characteristics. After processing, the droplet sizes were analyzed in relation to the working conditions using a Spraytec analyzer. A close relationship between velocity values and droplet sizes, depending on the operating parameters, was demonstrated. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Metal materials in the form of powders are the basis of powder metallurgy and have a large variety of applications, including sintering and thermal spraying [1]. The gas atomization process remains a good choice among the different methods of metal powder production. There are generally two types of gas atomization configurations: free-fall configuration and close-coupled configuration [2]. A closecoupled atomizer based on the use of a De Laval nozzle was considered in the present paper [3]. The principle of this high-pressure gas atomization technique is to transfer kinetic energy from a high-velocity gas flow expanded through a convergent–divergent nozzle to the liquid metal stream in order to constrict the liquid monofilament and then break it up into spheroidized droplets. The mean size and distribution of the resulting particles may vary, depending on the operating parameters [4,5]. In particular, changes in the gas atomization nature, atomization pressure, and melt flow were studied to observe the final modifications of powder characteristics [6–8]. Studies of gas atomization have focused on correlations between powder properties and process parameters. However, prediction of the droplet characteristics in terms of mean size and distribution during atomization requires an understanding of the physical phenomena involved in the process. On-line observation of the breakup of the molten metal stream appears to be a good way to better understand this atomization process. Further, it could help to establish direct relationships between droplet formation and operating parameters, to examine powder properties dependent on process parameters, and to enhance powder production capability. ⁎ Corresponding author. E-mail address: [email protected] (M.P. Planche). 0032-5910/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2012.06.031
Until now, only a few optical methods were developed and applied to the atomization process in order to observe droplets [9,10]. Indeed, the visualization of liquid metal atomization is impeded by limited access to nozzles due to the strongly confined environment and by the presence of dust in the atomization chamber (on optical windows) preventing online observation. In the present study, copper atomization was considered. The particle image velocimetry (PIV) technique was successfully implemented in the atomization chamber. On-line droplet velocities were measured at different axial distances from the De Laval nozzle exit. The influence of the atomizing gas pressure on particle velocity and size was studied, since the pressure of the atomizing gas is one of the important processing parameters of atomization. The second operating parameter investigated was the melt nozzle diameter. 1.1. Atomizer In the Nanoval process, the use of a De Laval nozzle to atomize liquid metals is thought to be an efficient technique because the gas stream is kept laminar in the atomizing area, reaching sonic or supersonic speed [11]. The atomizing system is composed of a melt nozzle and a De Laval nozzle, and its design leads to the formation of a negative overpressure at the tip of the melt nozzle. The metal is melted in an induction-heated crucible installed in an autoclave. The autoclave is separated from the atomization chamber by the De Laval nozzle so that the whole autoclave is pressurized during the atomizing process and also acts as a buffer tank. Once the metal is melted, the liquid stream flows out of the bottom of the crucible, influenced by both the static height of the melt in the crucible and the negative overpressure at the melt nozzle tip. At the exit of the melt nozzle, the liquid metal stream meets the cold and pressurized gas
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Atomizing gas entry
Autoclave
Water cooled wall
(600 mm in diameter)
Crucible
Autoclave Gas atomization access
Atomizing chamber (1000 mm in diameter)
Autoclave controls :
Induction heating
• Melting temperature
Atomizing gas exit
• Pressure
Melt nozzle De Laval nozzle Atomization chamber
Powder container
Fig. 1. Nanoval atomization system.
flow. The melt monofilament gets thinner due to shear forces exerted by the environmental gas flow, and the atomization starts, i.e., the monofilament explodes into many monofilaments or droplets. Finally, the droplets solidify during their flight downstream in the spray chamber. The powders produced are continuously collected in containers. The Nanoval atomization system is illustrated in Fig. 1, with a close-up view of the autoclave. Avoiding melt freezing in the duct is of critical importance to enable melt flow to exit the duct and consequently to atomize [12]. Therefore, gas pressure in the autoclave cannot be held constant for the entire process but must be increased from the pressure at the start of melt flow to the desired atomization pressure, as shown in Fig. 2. Thus, the melt flow is initiated at a pressure below the setting pressure in order to avoid solidification of the melt in the melt nozzle. Once the setting pressure was reached, a good stability of this pressure is obtained with less than 3% fluctuations.
1.2. Operating parameters
Laval nozzle diameter was 4 mm. In the different atomizations, about 2 kg of copper was melted and overheated at 1300 °C. The atomizing pressure was set using a gas pressure regulator. After processing, the particle sizes were analyzed using a Malvern Mastersizer 2000 (Malvern, UK) particle sizer. This is a light scattering instrument, which operates on the principle of the Fraunhofer diffraction theory. The size distribution is based on volume and the average size is quoted as the median based on volume equivalent diameter. Once the atomization has finished, a sample is taking from the whole quantity of produced powder. Average median diameters and standard deviations were then determined.
1.3. PIV 1.3.1. Principle of PIV PIV is based on the illumination of flying particles by two laser flashes in quick succession and the detection of backscattered light by a camera [13]. Hence, two images are taken for each laser flash. The synchronizer is a transistor–transistor logic (TTL) signal generator
Atomization pressures ranged from 0.5 to 1.5 MPa. Argon was used as the unique atomization gas. Melting took place in a ceramic crucible with a capacity of 5 kg of metal. The melting temperature was controlled by a K-thermocouple immersed in the liquid metal. The melt nozzle was 1 mm, 1.5 mm, or 2 mm in diameter. The De
Fig. 2. Scheme of pressure adjustment.
Fig. 3. Photograph of the spray jet.
M.P. Planche et al. / Powder Technology 229 (2012) 191–198
193
55 mm
atomization point observation field 55 mm 110 mm
protective envelop
atomization axis Axe d’atomisation
Image
caloric window Fenêtrecalorique
Diaphragme
scale
mirors Miroirs = 100 mm
Fig. 4. Optical trajectory of the observation fields and corresponding design of the system.
that allows the laser and the camera to be controlled according to a schedule that is set by the software. Finally, the registration system stores two successive images separately and rapidly. In PIV images, a particle appears as a particle image pair. As the spatial distance between the partners in the pair, the time delay between the laser pulses, and the flight direction of the particle are known, particle image pairs can be used to calculate particle velocities. Finally, the direction and the magnitude of a velocity vector can be calculated. 1.3.2. Installation of the PIV system in the atomizing chamber The area closest to the nozzle exit is of great interest in gaining a better understanding of the atomization phenomenon. However, as shown in Fig. 3, the PIV system cannot be installed in the vicinity of the atomization area because of the space constraints, temperature, and divergence of the spray. This is why an additional optical system was designed and installed inside the spray chamber to access to this specific area. Its function was to deflect the optical field towards the desired observation area, as illustrated in Fig. 4. It is composed of
two mirrors and a caloric window, which are placed inside a watertight stainless steel envelop to protect them from dust.
1.3.3. Experimental set-up of PIV system Droplets coming from the breakup of the melt stream during atomization were used as tracer particles. Individual droplets were detected and were associated with a corresponding velocity vector. An Nd-YAG double laser cavity— Twins Big Sky Laser 200 from Quantel—which releases a very short range of pulses was used. The energy in each pulse was set to 200 mJ, and the time between two pulses was 8 ns at a frequency of 15 Hz. The PCO Sensicam camera is a charge-coupled device (CCD) matrix of 1280 × 1024 pixels and 6.7 μm aside. The time between the end of the first frame and the beginning of the second frame is 500 ns. The frequency acquisition of pairs of images is 8 Hz. Moreover, undesired illumination coming from the stream is almost filtered out by using a special filter on the camera. The schematic set-up is shown in Fig. 5.
Autoclave
Z X
Atomization point Copper particles
Cylindrical Lens
Beam 2
Beam 1
Central unit
Synchroniser
Cavity Cavity alimentation 1 Alimentation 2
Fig. 5. PIV measurement device with the atomization system.
Image processing
Y
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Atomization point
PIV measure area 50 mm
V (m/s) 210 165 126
Atomization axis
Atomization area
84 42 0
110 mm
PIV measure area
Raw image
Velocity vector field
Fig. 6. PIV capture and associated velocity vector field.
2. Experimental results 2.1. PIV results and exploitation For the area analyzed by the PIV computer software, the instantaneous velocity vector map can be plotted. As shown in Fig. 6, the observed area was included in the 1024 × 1280 pixel image, and the analysis was delimited in the associated 864 × 1056 pixel area (red dotted line). Then, the area analyzed by the PIV computer software was deconstructed into 28 horizontal lines and 34 vertical lines. At each intersection of vertical and horizontal lines, the axial and radial components of the velocity vector were calculated such that a velocity vector field was obtained. As presented in the above figure, Fig. 7, the instantaneous velocity vector map can be plotted from each image. Then, this velocity vector field can be decomposed in order to determine two characteristics: the maximum velocity for each horizontal line and the maximum velocity value deduced from the complete field. That way, the evolution of both the maximum velocity for each line and the global maximum velocity can be obtained as a function of the atomization time.
2.2. Particle velocity A complete mapping of vectors was obtained between 50 and 110 mm and between 470 and 530 mm from the De Laval nozzle exit. Three types of values were defined, depending on the desired characterizations:
Fig. 7. Analysis of the field of the velocity vectors for each line and for the whole image.
– The velocities measured at 500 mm from the De Laval nozzle exit were denoted by V500. The PIV measures were obtained in the range 470 b x b 530 mm and − 30 b y b 30 mm. From these instantaneous values, an average spatial value, which also corresponded to average intensity, was deduced. – The velocities measured at 80 mm from the De Laval nozzle exit were denoted by V80. The PIV measures were obtained in the range 50 b x b 110 mm and − 30 b y b 30 mm. From these instantaneous values, an average spatial value, which also corresponded to average intensity, was deduced. – The velocity amplitude along the Ox axis was denoted by Vcenterline. Taking into account the axisymmetry of the nozzle, the profile of the velocity amplitude corresponds to the flow axis and can be defined as its vertical component. Vcenterline is evaluated in the 50 to 110 mm interval from the nozzle exit.
M.P. Planche et al. / Powder Technology 229 (2012) 191–198
195
2 Melt nozzle tip Mass flow inlet
1
Converging part of De Laval nozzle Finer zone grid
2
Throat of De Laval nozzle Diverging part of De Laval nozzle
Atomization and symmetry axis
1
Pressure outlet
Fig. 8. Domain defined for the gas flow modeling.
3. Results and discussion 3.1. Estimation of the regime and properties of the gas flow in the region of the De Laval nozzle exit Assuming the flow isentropic, the flow regime can be estimated firstly depending on the process parameters by calculating Reynolds number from the equation Re ¼ ρ vμd. The temperature at the nozzle section (d = 4 mm) is 216 K, the dynamic viscosity 1727.10 −5 Pa.s and the velocity 271.3 m/s. In the range of the atomization pressure from 0.5 MPa (giving a pressure of 0.24 MPa and a density of 5.35 kg/m 3 at the nozzle section) to 1.5 MPa leading to a pressure of 0.73 MPa and a density of 16.27 kg/m 3 accordingly, Reynolds numbers were estimated superior to 336100 corresponding to a turbulent
Mass flowrate= 90 g/s P = 2.4 MPa
flow. In addition, the profile of the gas flow in the region of the De Laval nozzle exit was modeled using Fluent code and the domain is represented in Fig. 8. The evolutions of flow velocity are represented in Fig. 9 for two different atomization pressures. Whatever the atomization pressure, the axial velocity of the gas flow displays a clear transition zone between a subsonic flow upstream the De Laval nozzle section and a supersonic flow downstream this section. The supersonic area is larger and the maximum velocity in this region is higher when the atomization pressure is more important. This supersonic area, longer and larger, is also moving downstream to the nozzle exit with the increase of the atomization pressure. The length of the supersonic area is then clearly dependent of the atomization pressure. Indeed, the gas stream is considered as a perfect gas meaning that the gas density is proportional to the
Mass flowrate = 10 g/s P = 0.25 MPa Mach < 1
Vmax = 513 m/s
Length of the supersonic area
Length of the supersonic area
Vmax = 335 m/s
Mach > 1
Fig. 9. Gas flow velocity for two atomization pressures.
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Transient phase
Stabilized phase
Stabilized phase
Transient phase
120
Maximum velocity: 60 m/s
Maximum velocity: 120 m/s
60 50
80
V500 [m/s]
V80 [m/s]
100
Minimum velocity: 80 m/s
60 40 20
40 Minimum velocity: 42 m/s
30 20 10
30
20
40
60
50
70
80
90
100
0
10
Atomization duration (S)
20
30
40
50
60
Atomization duration (S)
Fig. 10. Instantaneous droplet velocity versus the atomization duration — melt nozzle diameter of 1 mm.
atomization pressure. This way, a low gas density corresponds also to increase the flow velocity. In correspondence to these velocity evolutions, it was also observed a great dependence between a more important atomization pressure and a more extended area at low pressure and supersonic velocity. Finally, the presence of the chock area could be also related to the decrease of Mach number and flow velocity. According to Fristching et al. [14], the chock area can be viewed as a Mach disk whose thickness depends on the atomization pressure. 3.2. Evolution of the droplet velocity during the transient phase of the atomization pressure The instantaneous droplet velocity was studied as a function of the atomization duration for copper atomized with 0.5 MPa autoclave pressure and 1 mm melt nozzle diameter. Fig. 10 displays the evolutions of droplet velocity measured at 80 mm (V80) and 500 mm (V500). For these atomizations whose time duration was finally relatively short (less than 2 min), it could be approximated that the transient time represents no more than 10% of the time duration of the whole atomization. During this phase, the increase in atomization pressure induces an increase of the shear forces applied on the melt monofilament. Finally, the velocity of the droplets increases simultaneously with a decrease of the droplet diameter. It is remarkable that the droplet velocity follows the evolution of the atomization pressure. An increase in the maximum velocity is directly related to the increase of the atomizing pressure during the
transient phase (about 10 s for the two experiments), with a more homogeneous trend for measures further from the nozzle exit certainly due to a better pressure definition at this point. During the stabilized phase, most of the velocities are between the minimum and maximum values, i.e., between 80 m/s and 120 m/s for measures at 80 mm and between 42 m/s and 60 m/s for measures at 500 mm. The velocities are continuously decreasing versus time and distance from the nozzle exit. This could be explained by the decrease of the gas velocity with increasing distance from the nozzle exit due to viscous dissipation. This process cannot be definitively considered as a steady process since the crucible is regularly emptying, thus affecting the formation of the droplets. Fig. 9 shows the evolution of the mean diameter during atomization duration (Fig. 11). The results demonstrate that the evolution of the velocity can be inversely associated with that of the diameter. During the pressurization time (transient phase), the pressure rise affects the gas velocity and leads to coarser powder due to a weaker friction force on the melt monofilament. At the setting pressure (stabilized phase), the quantity of melt in the autoclave progressively decreases. Consequently, the gas-to-metal ratio (GMR) changes, so the same gas pressure will lead to finer powder.
3.3. Influence of the atomizing gas pressure on droplet velocity The influence of atomizing gas pressure on particle velocity was studied at 0.5 and 1 MPa in the stabilized phase, for a melt nozzle 0.5 MPa 1.0 MPa
250
VP = -0.24 t + 187
d50 = 68,3 µm
d50 = 62,3µm
1,0 d50 = 98 µm
Vcenterline [m/s]
Autoclave pressure (MPa)
200
150
100
50
2s
5s
11s
Atomizationduration (s) Fig. 11. Mean droplet diameter versus the atomization duration — atomization pressure of 1 MPa , melt nozzle diameter of 2 mm.
0 10
VP = -0.25 t + 98
15
20
25
30
35
40
45
50
Atomization duration [s] Fig. 12. Evolution of the droplet velocity along the atomization axis for 0.5 and 1 MPa atomization pressures.
M.P. Planche et al. / Powder Technology 229 (2012) 191–198
0.5 MPa pressure
197
1 MPa pressure
1.5 MPa pressure
Fig. 13. Droplet velocity profiles at 60, 100 and 120 mm from the nozzle exit for 0.5, 1 and 1.5 MPa atomization pressures.
diameter of 1 mm. Fig. 12 presents the evolution of the droplet velocity along the atomization axis. Whatever the setting pressure, a slight decrease of the droplet velocity measured along the atomization was noted and could be analyzed in relation to the decrease of the metal height versus time. The velocity increased with increasing setting pressure, resulting in an increased GMR. The increase in atomization pressure from 0.5 to 1 MPa induced an increase in the maximum velocity from almost 110 m/s to almost 200 m/s. This could be due to the gas velocity effect, which is dependent on the atomization pressure: the higher the atomization pressure, the higher the gas velocity. As the gas velocity contributes to enhance the forces exerted on the melt monofilament, it induces better atomization efficiency and therefore a reduction of mean diameter. Thus, the melt flow rate is reduced by increasing the atomization gas pressure. It can be concluded that higher gas velocities promote the acceleration of atomized droplets and consequently the formation of smaller droplet sizes. In addition to these evolutions, Fig. 13 shows the profiles of the velocity vectors in a cross section of the jet at different distances from the nozzle exit (60, 100, and 120 mm) and different atomization pressures (0.5, 1, and 1.5 MPa). The melt nozzle diameter was kept constant at 1.5 mm to provide comparable results. In the first analysis of the velocity profile, homogeneous and symmetrical profiles were observed around the flow axis supported by colinear vectors. Even if the flow is turbulent, the flat velocity profiles attest that there was then a jet down-flow with no evidence of
recirculation. Velocities were centered on the atomization axis and their intensities decreased continuously from the axis to the edge of the flow. Both the distance from the nozzle exit and the atomization pressures induced effects on the maximum velocity values. The shorter the distance and the higher the pressure, the higher the maximum value. Fig. 14 shows the effect of atomization pressures on the average velocity and powder size. The results are for copper atomizations performed with 1.5 mm melt nozzle diameter. It is clearly shown that higher atomization pressures result in higher velocities and finer powder sizes. The minimum velocity of 88 m/s was measured at 0.5 MPa pressure, and the maximum velocity was obtained at 1.5 MPa pressure. Correlatively, the minimum diameter of 23 μm corresponds to 1.5 MPa and the maximum diameter of 52 μm corresponds to 0.5 MPa. There exists a close relationship between the velocity and powder size. When the pressure of the atomization gas is higher, it increases the gas velocity, which in turn increases the relative velocity between the atomizing gas and the moving liquid droplets. Consequently, it contributes to the generation of finer powder through a better shear disintegration of the melt.
3.4. Effect of both atomization pressure and melt nozzle diameter on droplet properties
50 Particle velocity (m/s)
40
150
35 30
100
25 20
50
15 10 0.5
1
1,5
0
Atomization pressure (MPa) Fig. 14. Average velocity and powder size versus atomization pressures. Melt nozzle diameter of 1.5 mm.
Average velocity Vcenterline [m/s]
200
Mean median diameter (µm)
45
Particle Velocity (m/s)
Mean median diameter d 50 (µm)
Fig. 15 shows the average velocity on the atomization axis versus the average droplet diameter.
1.5 MPa
350 300
1.0 MPa
250 200
0.5 MPa
150
1.0 mm 1.5 mm
100 50 0 0
10
20
30
40
50
Average size d50 [m/s] Fig. 15. Average velocity versus powder size for different atomization pressures (0.5, 1, 1.5 MPa) and different melt nozzle diameters (1 and 1.5 mm).
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Table 1 Comparison of copper and aluminium velocities for different atomization pressures and melt nozzle diameters. Copper
Aluminium
Density: 8.96 g/cm3
Density: 2.7 g/cm3
Melting temperature (θf): 1084 °C
Melting temperature (θf): 660 °C
Viscosity at (θf): 3.05 Pa.s
Viscosity at (θf): 1.23 Pa.s
Atomization pressure
Melt nozzle
Melt nozzle
Melt nozzle
Melt nozzle
(MPa)
Diameter: 1 mm
Diameter: 1.5 mm
Diameter: 1 mm
Diameter: 1.5 mm
0.5
90 +/− 10 m/s 38 +/− 5 μm 200 +/− 22 m/s 23 +/− 4 μm 260 +/− 32 m/s 17 +/− 3 μm
88 +/− 10 m/s 46 +/− 7 μm 119 +/− 10 m/s 29 +/− 6 μm 155 +/− 25 m/s 19 +/− 2 μm
180 +/− 11 m/s
169 +/− 12 m/s
–
206 +/− 15 m/s
1 1.5
For a constant melt nozzle diameter, the increase in the atomization pressure is correlated to an increase in the gas velocity. For example, in the case of the 1 mm melt nozzle diameter, the average velocity ranged from 260 m/s for 1.5 MPa pressure to almost 90 m/s for 0.5 MPa pressure, and the droplet size varied from 17 μm to 38 μm. This leads to an increase in GMR and to the production of finer powder droplets. For a constant atomization pressure, the decrease in the melt nozzle diameter is expressed by an increase in the GMR, inducing an increase in the shear forces applied on the metal liquid stream. Consequently, the velocity increases and the droplet size decreases. Finally, the melt nozzle diameter appears to be directly linked to the dispersion in the velocity distribution. A narrower velocity distribution was observed when atomizing using the bigger melt nozzle diameter, which suggests that the bigger melt nozzle diameter allowed the production of a more regular stream with more regular droplet diameter and broader melt filament. Consequently, the range of velocities was narrower. Some additional experiments were realized for aluminium atomizations. The results obtained in velocity and mean diameter are also dependent of the process parameters and present the same evolutions compared to that obtained for copper atomizations. Table 1 give a summary of these results as well as some properties of both materials. It seems that the density of the atomized material as its viscosity may influence the velocity of the droplets leading to clearly increase (almost twice) this value. 4. Conclusion On-line control of droplets during atomization is a good way to examine the physics of metal production. Measurements were successfully achieved by the PIV technique. The conclusions of this study are summarized as follows: – The symmetry in the flow pattern on the atomization axis was noted. Parallel velocity vectors were found in the measured field, indicating that no recirculation was created in the flow. – A great correlation was observed between the increase in the atomization pressure up to the setting value and the instantaneous velocity. During the first transient, velocity intensities followed the rise of the atomization pressure. During the second stabilized phase, a slight decrease in the velocity was measured due to the progressive change of the gas-to-liquid ratio in time.
– The atomization pressure had a strong influence on droplet velocity and size. Droplet velocity increased with increasing atomizing pressure. Accordingly, the droplet size decreased. This is explained by the reduction of the melt flow rate in this case. – Significant effects on the velocity and droplet size were also noted by changing the melt nozzle diameter. A decrease in the melt nozzle diameter was associated with an increase in average velocity and a corresponding decrease in average droplet diameter. This is attributed to the increase of the shear forces on the liquid filament, leading to increased gas velocity and therefore droplet velocity. References [1] S. Lagutkin, L. Achelis, S. Scheikhaliev, V. Uhlenwinkel, V. Srivastava, Atomization process for metal powder, Materials Science and Engineering A 383 (1) (2004) 1–6. [2] L. Achelis, V. Uhlenwinkel, Characterization of metal powders generated by a pressure gas atomizer, Materials Science and Engineering A 477 (1–2) (2008) 15–20. [3] M. Stobik, Nanoval atomizing – superior flow design for finer powder, In: Proceedings of the International Conference on Spray Deposition and Melt Atomization, Bremen, Germany, 2000, pp. 511–520. [4] I. Anderson, R. Terpstra, Progress toward gas atomization processing with increased uniformity and control, Materials Science and Engineering A 326 (1) (2002) 101–109. [5] A. Unal, Effect of processing variables on particle size in gas atomization of rapidly solidified aluminum powders, Materials Science and Technology 3 (1987) 49–55. [6] R. Metz, C. Machado, M. Houabes, J. Pansiot, M. Elkhatib, R. Puyane, M. Hassanzadeh, Nitrogen spray atomization of molten tin metal: powder morphology characteristics, Materials Processing Technology 189 (1–3) (2007) 132–137. [7] A. Allimant, M.P. Planche, L. Dembinski, C. Coddet, Progress of the gas atomization of liquid metals by means of a De Laval nozzle, Journal of Powder Technology 190 (2009) 79–83. [8] K. Rao, S. Mehrotra, Effect of process variables on atomization of metals and alloys, Modern Developments in Powder Metallurgy 12 (1981) 113–130. [9] J. Ting, J. Connor, S. Ridder, High speed cinematography of gas metal atomization, Materials Science and Engineering A 390 (1–2) (2005) 452–460. [10] A. Mullis, I. McCarthy, R. Cochrane, High speed imaging of the flow during close coupled atomization: effect of melt delivery nozzle geometry, Materials Processing Technology 211 (2011) 1471–1477. [11] M. Stobik, Nanoval atomizing: capabilities, applications and related processes, In: Kollokium Sprühkompaktieren / Sprayforming, Bauckhage and Uhlenwinkel, Bremen, Germany, 6, 2002, pp. 65–80. [12] S.P. Mates, G.S. Settles, A study of liquid atomization using close-coupled nozzles, Part 1: Gas dynamic behavior, Atomization and Sprays 15 (1) (2005) 19–40. [13] A. Mullis, N. Adkins, Z. Aslam, I. McCarthy, R. Cochrane, High frame rate analysis of the spray cone geometry during close-coupled gas atomization, Powder Metallurgy 44 (2008) 55–64. [14] U. Fritsching, Droplets and particles in sprays : tailoring particle properties within spray processes, China Particuology 3 (2005) 125–133.
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Powder Technology journal homepage: www.elsevier.com/locate/powtec
Velocities of copper droplets in the De Laval atomization process M.P. Planche a,⁎, O. Khatim a, L. Dembinski a, C. Coddet a, L. Girardot b, Y. Bailly b a b
LERMPS- UTBM, 90010 Belfort Cedex, France Institute FEMTO-ST/ENISYS, University of Franche-Comté, Belfort, France
a r t i c l e
i n f o
Article history: Received 13 February 2012 Received in revised form 16 May 2012 Accepted 18 June 2012 Available online 2 July 2012 Keyword: Close coupled process Copper atomization PIV measures
a b s t r a c t Atomization experiments were carried out at the Nanoval atomization unit. Particle image velocimetry (PIV) was used to determine the velocity vector fields of copper powder production via De Laval gas atomization. The experimental set-up designed to access the area closest to the atomization point was described in detail. Velocity vector fields were constructed by taking into account the distance from the nozzle exit. Instantaneous velocities on the atomization axis were calculated as a function of the atomization duration. Results from this study indicate a strong influence of the atomization pressures and the melt nozzle diameters on velocity characteristics. After processing, the droplet sizes were analyzed in relation to the working conditions using a Spraytec analyzer. A close relationship between velocity values and droplet sizes, depending on the operating parameters, was demonstrated. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Metal materials in the form of powders are the basis of powder metallurgy and have a large variety of applications, including sintering and thermal spraying [1]. The gas atomization process remains a good choice among the different methods of metal powder production. There are generally two types of gas atomization configurations: free-fall configuration and close-coupled configuration [2]. A closecoupled atomizer based on the use of a De Laval nozzle was considered in the present paper [3]. The principle of this high-pressure gas atomization technique is to transfer kinetic energy from a high-velocity gas flow expanded through a convergent–divergent nozzle to the liquid metal stream in order to constrict the liquid monofilament and then break it up into spheroidized droplets. The mean size and distribution of the resulting particles may vary, depending on the operating parameters [4,5]. In particular, changes in the gas atomization nature, atomization pressure, and melt flow were studied to observe the final modifications of powder characteristics [6–8]. Studies of gas atomization have focused on correlations between powder properties and process parameters. However, prediction of the droplet characteristics in terms of mean size and distribution during atomization requires an understanding of the physical phenomena involved in the process. On-line observation of the breakup of the molten metal stream appears to be a good way to better understand this atomization process. Further, it could help to establish direct relationships between droplet formation and operating parameters, to examine powder properties dependent on process parameters, and to enhance powder production capability. ⁎ Corresponding author. E-mail address: [email protected] (M.P. Planche). 0032-5910/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2012.06.031
Until now, only a few optical methods were developed and applied to the atomization process in order to observe droplets [9,10]. Indeed, the visualization of liquid metal atomization is impeded by limited access to nozzles due to the strongly confined environment and by the presence of dust in the atomization chamber (on optical windows) preventing online observation. In the present study, copper atomization was considered. The particle image velocimetry (PIV) technique was successfully implemented in the atomization chamber. On-line droplet velocities were measured at different axial distances from the De Laval nozzle exit. The influence of the atomizing gas pressure on particle velocity and size was studied, since the pressure of the atomizing gas is one of the important processing parameters of atomization. The second operating parameter investigated was the melt nozzle diameter. 1.1. Atomizer In the Nanoval process, the use of a De Laval nozzle to atomize liquid metals is thought to be an efficient technique because the gas stream is kept laminar in the atomizing area, reaching sonic or supersonic speed [11]. The atomizing system is composed of a melt nozzle and a De Laval nozzle, and its design leads to the formation of a negative overpressure at the tip of the melt nozzle. The metal is melted in an induction-heated crucible installed in an autoclave. The autoclave is separated from the atomization chamber by the De Laval nozzle so that the whole autoclave is pressurized during the atomizing process and also acts as a buffer tank. Once the metal is melted, the liquid stream flows out of the bottom of the crucible, influenced by both the static height of the melt in the crucible and the negative overpressure at the melt nozzle tip. At the exit of the melt nozzle, the liquid metal stream meets the cold and pressurized gas
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Atomizing gas entry
Autoclave
Water cooled wall
(600 mm in diameter)
Crucible
Autoclave Gas atomization access
Atomizing chamber (1000 mm in diameter)
Autoclave controls :
Induction heating
• Melting temperature
Atomizing gas exit
• Pressure
Melt nozzle De Laval nozzle Atomization chamber
Powder container
Fig. 1. Nanoval atomization system.
flow. The melt monofilament gets thinner due to shear forces exerted by the environmental gas flow, and the atomization starts, i.e., the monofilament explodes into many monofilaments or droplets. Finally, the droplets solidify during their flight downstream in the spray chamber. The powders produced are continuously collected in containers. The Nanoval atomization system is illustrated in Fig. 1, with a close-up view of the autoclave. Avoiding melt freezing in the duct is of critical importance to enable melt flow to exit the duct and consequently to atomize [12]. Therefore, gas pressure in the autoclave cannot be held constant for the entire process but must be increased from the pressure at the start of melt flow to the desired atomization pressure, as shown in Fig. 2. Thus, the melt flow is initiated at a pressure below the setting pressure in order to avoid solidification of the melt in the melt nozzle. Once the setting pressure was reached, a good stability of this pressure is obtained with less than 3% fluctuations.
1.2. Operating parameters
Laval nozzle diameter was 4 mm. In the different atomizations, about 2 kg of copper was melted and overheated at 1300 °C. The atomizing pressure was set using a gas pressure regulator. After processing, the particle sizes were analyzed using a Malvern Mastersizer 2000 (Malvern, UK) particle sizer. This is a light scattering instrument, which operates on the principle of the Fraunhofer diffraction theory. The size distribution is based on volume and the average size is quoted as the median based on volume equivalent diameter. Once the atomization has finished, a sample is taking from the whole quantity of produced powder. Average median diameters and standard deviations were then determined.
1.3. PIV 1.3.1. Principle of PIV PIV is based on the illumination of flying particles by two laser flashes in quick succession and the detection of backscattered light by a camera [13]. Hence, two images are taken for each laser flash. The synchronizer is a transistor–transistor logic (TTL) signal generator
Atomization pressures ranged from 0.5 to 1.5 MPa. Argon was used as the unique atomization gas. Melting took place in a ceramic crucible with a capacity of 5 kg of metal. The melting temperature was controlled by a K-thermocouple immersed in the liquid metal. The melt nozzle was 1 mm, 1.5 mm, or 2 mm in diameter. The De
Fig. 2. Scheme of pressure adjustment.
Fig. 3. Photograph of the spray jet.
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193
55 mm
atomization point observation field 55 mm 110 mm
protective envelop
atomization axis Axe d’atomisation
Image
caloric window Fenêtrecalorique
Diaphragme
scale
mirors Miroirs = 100 mm
Fig. 4. Optical trajectory of the observation fields and corresponding design of the system.
that allows the laser and the camera to be controlled according to a schedule that is set by the software. Finally, the registration system stores two successive images separately and rapidly. In PIV images, a particle appears as a particle image pair. As the spatial distance between the partners in the pair, the time delay between the laser pulses, and the flight direction of the particle are known, particle image pairs can be used to calculate particle velocities. Finally, the direction and the magnitude of a velocity vector can be calculated. 1.3.2. Installation of the PIV system in the atomizing chamber The area closest to the nozzle exit is of great interest in gaining a better understanding of the atomization phenomenon. However, as shown in Fig. 3, the PIV system cannot be installed in the vicinity of the atomization area because of the space constraints, temperature, and divergence of the spray. This is why an additional optical system was designed and installed inside the spray chamber to access to this specific area. Its function was to deflect the optical field towards the desired observation area, as illustrated in Fig. 4. It is composed of
two mirrors and a caloric window, which are placed inside a watertight stainless steel envelop to protect them from dust.
1.3.3. Experimental set-up of PIV system Droplets coming from the breakup of the melt stream during atomization were used as tracer particles. Individual droplets were detected and were associated with a corresponding velocity vector. An Nd-YAG double laser cavity— Twins Big Sky Laser 200 from Quantel—which releases a very short range of pulses was used. The energy in each pulse was set to 200 mJ, and the time between two pulses was 8 ns at a frequency of 15 Hz. The PCO Sensicam camera is a charge-coupled device (CCD) matrix of 1280 × 1024 pixels and 6.7 μm aside. The time between the end of the first frame and the beginning of the second frame is 500 ns. The frequency acquisition of pairs of images is 8 Hz. Moreover, undesired illumination coming from the stream is almost filtered out by using a special filter on the camera. The schematic set-up is shown in Fig. 5.
Autoclave
Z X
Atomization point Copper particles
Cylindrical Lens
Beam 2
Beam 1
Central unit
Synchroniser
Cavity Cavity alimentation 1 Alimentation 2
Fig. 5. PIV measurement device with the atomization system.
Image processing
Y
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Atomization point
PIV measure area 50 mm
V (m/s) 210 165 126
Atomization axis
Atomization area
84 42 0
110 mm
PIV measure area
Raw image
Velocity vector field
Fig. 6. PIV capture and associated velocity vector field.
2. Experimental results 2.1. PIV results and exploitation For the area analyzed by the PIV computer software, the instantaneous velocity vector map can be plotted. As shown in Fig. 6, the observed area was included in the 1024 × 1280 pixel image, and the analysis was delimited in the associated 864 × 1056 pixel area (red dotted line). Then, the area analyzed by the PIV computer software was deconstructed into 28 horizontal lines and 34 vertical lines. At each intersection of vertical and horizontal lines, the axial and radial components of the velocity vector were calculated such that a velocity vector field was obtained. As presented in the above figure, Fig. 7, the instantaneous velocity vector map can be plotted from each image. Then, this velocity vector field can be decomposed in order to determine two characteristics: the maximum velocity for each horizontal line and the maximum velocity value deduced from the complete field. That way, the evolution of both the maximum velocity for each line and the global maximum velocity can be obtained as a function of the atomization time.
2.2. Particle velocity A complete mapping of vectors was obtained between 50 and 110 mm and between 470 and 530 mm from the De Laval nozzle exit. Three types of values were defined, depending on the desired characterizations:
Fig. 7. Analysis of the field of the velocity vectors for each line and for the whole image.
– The velocities measured at 500 mm from the De Laval nozzle exit were denoted by V500. The PIV measures were obtained in the range 470 b x b 530 mm and − 30 b y b 30 mm. From these instantaneous values, an average spatial value, which also corresponded to average intensity, was deduced. – The velocities measured at 80 mm from the De Laval nozzle exit were denoted by V80. The PIV measures were obtained in the range 50 b x b 110 mm and − 30 b y b 30 mm. From these instantaneous values, an average spatial value, which also corresponded to average intensity, was deduced. – The velocity amplitude along the Ox axis was denoted by Vcenterline. Taking into account the axisymmetry of the nozzle, the profile of the velocity amplitude corresponds to the flow axis and can be defined as its vertical component. Vcenterline is evaluated in the 50 to 110 mm interval from the nozzle exit.
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195
2 Melt nozzle tip Mass flow inlet
1
Converging part of De Laval nozzle Finer zone grid
2
Throat of De Laval nozzle Diverging part of De Laval nozzle
Atomization and symmetry axis
1
Pressure outlet
Fig. 8. Domain defined for the gas flow modeling.
3. Results and discussion 3.1. Estimation of the regime and properties of the gas flow in the region of the De Laval nozzle exit Assuming the flow isentropic, the flow regime can be estimated firstly depending on the process parameters by calculating Reynolds number from the equation Re ¼ ρ vμd. The temperature at the nozzle section (d = 4 mm) is 216 K, the dynamic viscosity 1727.10 −5 Pa.s and the velocity 271.3 m/s. In the range of the atomization pressure from 0.5 MPa (giving a pressure of 0.24 MPa and a density of 5.35 kg/m 3 at the nozzle section) to 1.5 MPa leading to a pressure of 0.73 MPa and a density of 16.27 kg/m 3 accordingly, Reynolds numbers were estimated superior to 336100 corresponding to a turbulent
Mass flowrate= 90 g/s P = 2.4 MPa
flow. In addition, the profile of the gas flow in the region of the De Laval nozzle exit was modeled using Fluent code and the domain is represented in Fig. 8. The evolutions of flow velocity are represented in Fig. 9 for two different atomization pressures. Whatever the atomization pressure, the axial velocity of the gas flow displays a clear transition zone between a subsonic flow upstream the De Laval nozzle section and a supersonic flow downstream this section. The supersonic area is larger and the maximum velocity in this region is higher when the atomization pressure is more important. This supersonic area, longer and larger, is also moving downstream to the nozzle exit with the increase of the atomization pressure. The length of the supersonic area is then clearly dependent of the atomization pressure. Indeed, the gas stream is considered as a perfect gas meaning that the gas density is proportional to the
Mass flowrate = 10 g/s P = 0.25 MPa Mach < 1
Vmax = 513 m/s
Length of the supersonic area
Length of the supersonic area
Vmax = 335 m/s
Mach > 1
Fig. 9. Gas flow velocity for two atomization pressures.
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Transient phase
Stabilized phase
Stabilized phase
Transient phase
120
Maximum velocity: 60 m/s
Maximum velocity: 120 m/s
60 50
80
V500 [m/s]
V80 [m/s]
100
Minimum velocity: 80 m/s
60 40 20
40 Minimum velocity: 42 m/s
30 20 10
30
20
40
60
50
70
80
90
100
0
10
Atomization duration (S)
20
30
40
50
60
Atomization duration (S)
Fig. 10. Instantaneous droplet velocity versus the atomization duration — melt nozzle diameter of 1 mm.
atomization pressure. This way, a low gas density corresponds also to increase the flow velocity. In correspondence to these velocity evolutions, it was also observed a great dependence between a more important atomization pressure and a more extended area at low pressure and supersonic velocity. Finally, the presence of the chock area could be also related to the decrease of Mach number and flow velocity. According to Fristching et al. [14], the chock area can be viewed as a Mach disk whose thickness depends on the atomization pressure. 3.2. Evolution of the droplet velocity during the transient phase of the atomization pressure The instantaneous droplet velocity was studied as a function of the atomization duration for copper atomized with 0.5 MPa autoclave pressure and 1 mm melt nozzle diameter. Fig. 10 displays the evolutions of droplet velocity measured at 80 mm (V80) and 500 mm (V500). For these atomizations whose time duration was finally relatively short (less than 2 min), it could be approximated that the transient time represents no more than 10% of the time duration of the whole atomization. During this phase, the increase in atomization pressure induces an increase of the shear forces applied on the melt monofilament. Finally, the velocity of the droplets increases simultaneously with a decrease of the droplet diameter. It is remarkable that the droplet velocity follows the evolution of the atomization pressure. An increase in the maximum velocity is directly related to the increase of the atomizing pressure during the
transient phase (about 10 s for the two experiments), with a more homogeneous trend for measures further from the nozzle exit certainly due to a better pressure definition at this point. During the stabilized phase, most of the velocities are between the minimum and maximum values, i.e., between 80 m/s and 120 m/s for measures at 80 mm and between 42 m/s and 60 m/s for measures at 500 mm. The velocities are continuously decreasing versus time and distance from the nozzle exit. This could be explained by the decrease of the gas velocity with increasing distance from the nozzle exit due to viscous dissipation. This process cannot be definitively considered as a steady process since the crucible is regularly emptying, thus affecting the formation of the droplets. Fig. 9 shows the evolution of the mean diameter during atomization duration (Fig. 11). The results demonstrate that the evolution of the velocity can be inversely associated with that of the diameter. During the pressurization time (transient phase), the pressure rise affects the gas velocity and leads to coarser powder due to a weaker friction force on the melt monofilament. At the setting pressure (stabilized phase), the quantity of melt in the autoclave progressively decreases. Consequently, the gas-to-metal ratio (GMR) changes, so the same gas pressure will lead to finer powder.
3.3. Influence of the atomizing gas pressure on droplet velocity The influence of atomizing gas pressure on particle velocity was studied at 0.5 and 1 MPa in the stabilized phase, for a melt nozzle 0.5 MPa 1.0 MPa
250
VP = -0.24 t + 187
d50 = 68,3 µm
d50 = 62,3µm
1,0 d50 = 98 µm
Vcenterline [m/s]
Autoclave pressure (MPa)
200
150
100
50
2s
5s
11s
Atomizationduration (s) Fig. 11. Mean droplet diameter versus the atomization duration — atomization pressure of 1 MPa , melt nozzle diameter of 2 mm.
0 10
VP = -0.25 t + 98
15
20
25
30
35
40
45
50
Atomization duration [s] Fig. 12. Evolution of the droplet velocity along the atomization axis for 0.5 and 1 MPa atomization pressures.
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0.5 MPa pressure
197
1 MPa pressure
1.5 MPa pressure
Fig. 13. Droplet velocity profiles at 60, 100 and 120 mm from the nozzle exit for 0.5, 1 and 1.5 MPa atomization pressures.
diameter of 1 mm. Fig. 12 presents the evolution of the droplet velocity along the atomization axis. Whatever the setting pressure, a slight decrease of the droplet velocity measured along the atomization was noted and could be analyzed in relation to the decrease of the metal height versus time. The velocity increased with increasing setting pressure, resulting in an increased GMR. The increase in atomization pressure from 0.5 to 1 MPa induced an increase in the maximum velocity from almost 110 m/s to almost 200 m/s. This could be due to the gas velocity effect, which is dependent on the atomization pressure: the higher the atomization pressure, the higher the gas velocity. As the gas velocity contributes to enhance the forces exerted on the melt monofilament, it induces better atomization efficiency and therefore a reduction of mean diameter. Thus, the melt flow rate is reduced by increasing the atomization gas pressure. It can be concluded that higher gas velocities promote the acceleration of atomized droplets and consequently the formation of smaller droplet sizes. In addition to these evolutions, Fig. 13 shows the profiles of the velocity vectors in a cross section of the jet at different distances from the nozzle exit (60, 100, and 120 mm) and different atomization pressures (0.5, 1, and 1.5 MPa). The melt nozzle diameter was kept constant at 1.5 mm to provide comparable results. In the first analysis of the velocity profile, homogeneous and symmetrical profiles were observed around the flow axis supported by colinear vectors. Even if the flow is turbulent, the flat velocity profiles attest that there was then a jet down-flow with no evidence of
recirculation. Velocities were centered on the atomization axis and their intensities decreased continuously from the axis to the edge of the flow. Both the distance from the nozzle exit and the atomization pressures induced effects on the maximum velocity values. The shorter the distance and the higher the pressure, the higher the maximum value. Fig. 14 shows the effect of atomization pressures on the average velocity and powder size. The results are for copper atomizations performed with 1.5 mm melt nozzle diameter. It is clearly shown that higher atomization pressures result in higher velocities and finer powder sizes. The minimum velocity of 88 m/s was measured at 0.5 MPa pressure, and the maximum velocity was obtained at 1.5 MPa pressure. Correlatively, the minimum diameter of 23 μm corresponds to 1.5 MPa and the maximum diameter of 52 μm corresponds to 0.5 MPa. There exists a close relationship between the velocity and powder size. When the pressure of the atomization gas is higher, it increases the gas velocity, which in turn increases the relative velocity between the atomizing gas and the moving liquid droplets. Consequently, it contributes to the generation of finer powder through a better shear disintegration of the melt.
3.4. Effect of both atomization pressure and melt nozzle diameter on droplet properties
50 Particle velocity (m/s)
40
150
35 30
100
25 20
50
15 10 0.5
1
1,5
0
Atomization pressure (MPa) Fig. 14. Average velocity and powder size versus atomization pressures. Melt nozzle diameter of 1.5 mm.
Average velocity Vcenterline [m/s]
200
Mean median diameter (µm)
45
Particle Velocity (m/s)
Mean median diameter d 50 (µm)
Fig. 15 shows the average velocity on the atomization axis versus the average droplet diameter.
1.5 MPa
350 300
1.0 MPa
250 200
0.5 MPa
150
1.0 mm 1.5 mm
100 50 0 0
10
20
30
40
50
Average size d50 [m/s] Fig. 15. Average velocity versus powder size for different atomization pressures (0.5, 1, 1.5 MPa) and different melt nozzle diameters (1 and 1.5 mm).
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Table 1 Comparison of copper and aluminium velocities for different atomization pressures and melt nozzle diameters. Copper
Aluminium
Density: 8.96 g/cm3
Density: 2.7 g/cm3
Melting temperature (θf): 1084 °C
Melting temperature (θf): 660 °C
Viscosity at (θf): 3.05 Pa.s
Viscosity at (θf): 1.23 Pa.s
Atomization pressure
Melt nozzle
Melt nozzle
Melt nozzle
Melt nozzle
(MPa)
Diameter: 1 mm
Diameter: 1.5 mm
Diameter: 1 mm
Diameter: 1.5 mm
0.5
90 +/− 10 m/s 38 +/− 5 μm 200 +/− 22 m/s 23 +/− 4 μm 260 +/− 32 m/s 17 +/− 3 μm
88 +/− 10 m/s 46 +/− 7 μm 119 +/− 10 m/s 29 +/− 6 μm 155 +/− 25 m/s 19 +/− 2 μm
180 +/− 11 m/s
169 +/− 12 m/s
–
206 +/− 15 m/s
1 1.5
For a constant melt nozzle diameter, the increase in the atomization pressure is correlated to an increase in the gas velocity. For example, in the case of the 1 mm melt nozzle diameter, the average velocity ranged from 260 m/s for 1.5 MPa pressure to almost 90 m/s for 0.5 MPa pressure, and the droplet size varied from 17 μm to 38 μm. This leads to an increase in GMR and to the production of finer powder droplets. For a constant atomization pressure, the decrease in the melt nozzle diameter is expressed by an increase in the GMR, inducing an increase in the shear forces applied on the metal liquid stream. Consequently, the velocity increases and the droplet size decreases. Finally, the melt nozzle diameter appears to be directly linked to the dispersion in the velocity distribution. A narrower velocity distribution was observed when atomizing using the bigger melt nozzle diameter, which suggests that the bigger melt nozzle diameter allowed the production of a more regular stream with more regular droplet diameter and broader melt filament. Consequently, the range of velocities was narrower. Some additional experiments were realized for aluminium atomizations. The results obtained in velocity and mean diameter are also dependent of the process parameters and present the same evolutions compared to that obtained for copper atomizations. Table 1 give a summary of these results as well as some properties of both materials. It seems that the density of the atomized material as its viscosity may influence the velocity of the droplets leading to clearly increase (almost twice) this value. 4. Conclusion On-line control of droplets during atomization is a good way to examine the physics of metal production. Measurements were successfully achieved by the PIV technique. The conclusions of this study are summarized as follows: – The symmetry in the flow pattern on the atomization axis was noted. Parallel velocity vectors were found in the measured field, indicating that no recirculation was created in the flow. – A great correlation was observed between the increase in the atomization pressure up to the setting value and the instantaneous velocity. During the first transient, velocity intensities followed the rise of the atomization pressure. During the second stabilized phase, a slight decrease in the velocity was measured due to the progressive change of the gas-to-liquid ratio in time.
– The atomization pressure had a strong influence on droplet velocity and size. Droplet velocity increased with increasing atomizing pressure. Accordingly, the droplet size decreased. This is explained by the reduction of the melt flow rate in this case. – Significant effects on the velocity and droplet size were also noted by changing the melt nozzle diameter. A decrease in the melt nozzle diameter was associated with an increase in average velocity and a corresponding decrease in average droplet diameter. This is attributed to the increase of the shear forces on the liquid filament, leading to increased gas velocity and therefore droplet velocity. References [1] S. Lagutkin, L. Achelis, S. Scheikhaliev, V. Uhlenwinkel, V. Srivastava, Atomization process for metal powder, Materials Science and Engineering A 383 (1) (2004) 1–6. [2] L. Achelis, V. Uhlenwinkel, Characterization of metal powders generated by a pressure gas atomizer, Materials Science and Engineering A 477 (1–2) (2008) 15–20. [3] M. Stobik, Nanoval atomizing – superior flow design for finer powder, In: Proceedings of the International Conference on Spray Deposition and Melt Atomization, Bremen, Germany, 2000, pp. 511–520. [4] I. Anderson, R. Terpstra, Progress toward gas atomization processing with increased uniformity and control, Materials Science and Engineering A 326 (1) (2002) 101–109. [5] A. Unal, Effect of processing variables on particle size in gas atomization of rapidly solidified aluminum powders, Materials Science and Technology 3 (1987) 49–55. [6] R. Metz, C. Machado, M. Houabes, J. Pansiot, M. Elkhatib, R. Puyane, M. Hassanzadeh, Nitrogen spray atomization of molten tin metal: powder morphology characteristics, Materials Processing Technology 189 (1–3) (2007) 132–137. [7] A. Allimant, M.P. Planche, L. Dembinski, C. Coddet, Progress of the gas atomization of liquid metals by means of a De Laval nozzle, Journal of Powder Technology 190 (2009) 79–83. [8] K. Rao, S. Mehrotra, Effect of process variables on atomization of metals and alloys, Modern Developments in Powder Metallurgy 12 (1981) 113–130. [9] J. Ting, J. Connor, S. Ridder, High speed cinematography of gas metal atomization, Materials Science and Engineering A 390 (1–2) (2005) 452–460. [10] A. Mullis, I. McCarthy, R. Cochrane, High speed imaging of the flow during close coupled atomization: effect of melt delivery nozzle geometry, Materials Processing Technology 211 (2011) 1471–1477. [11] M. Stobik, Nanoval atomizing: capabilities, applications and related processes, In: Kollokium Sprühkompaktieren / Sprayforming, Bauckhage and Uhlenwinkel, Bremen, Germany, 6, 2002, pp. 65–80. [12] S.P. Mates, G.S. Settles, A study of liquid atomization using close-coupled nozzles, Part 1: Gas dynamic behavior, Atomization and Sprays 15 (1) (2005) 19–40. [13] A. Mullis, N. Adkins, Z. Aslam, I. McCarthy, R. Cochrane, High frame rate analysis of the spray cone geometry during close-coupled gas atomization, Powder Metallurgy 44 (2008) 55–64. [14] U. Fritsching, Droplets and particles in sprays : tailoring particle properties within spray processes, China Particuology 3 (2005) 125–133.