Materials Chemistry and Physics

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1 Materials Chemistry and Physics 137 (2013) 681e688 Contents lists available at SciVerse ScienceDirect Materials Chemistry and Physics journal homepage: Review Evaluation of the splats properties and relation with droplets diameters in atomization process using a De Laval Nozzle Marie-Pierre Planche a, *, Othmane Khatim a, Lucas Dembinski a, Yannick Bailly b, Christian Coddet a a LERMPS-UTBM Site de Sévenans, Belfort, Cedex, France b Institute FEMTO-ST/ENISYS, University of Franche-Comté, UMR CNRS 6174, Belfort, France highlights graphical abstract < In this study, changes of splat characteristics were examined versus operating parameters. < Splat diameter decreases with the increase of atomizing pressure. < Splat diameter decreases with the decrease of the melt nozzle diameter. < Changes of droplet diameter are in good agreement with those of splat diameter. article info abstract Article history: Received 23 January 2012 Received in revised form 31 August 2012 Accepted 22 September 2012 Keywords: Metals Powder metallurgy Deformation Optical microscopy Properties of the powders developed during atomization process are essentially determined by process parameters. Experiments based on splat collection have been performed at different stages of the process evolution. Formation of splat formed as a result of impingement of the melted metallic droplets onto a substrate was studied. Splat characteristics have been determined from image analysis in function of process parameters and time progress. Droplet diameters were measured using laser Coulter analyzer. Direct relationships between splat diameter and droplet diameter were established. The atomization experiments point out the strong influence of two processing parameters (atomizing gas and melt nozzle diameter) on splat characteristics and particle size. It has been observed that increasing atomizing pressure leads to a decrease of mean splat diameter and width of the size distribution. Similar evolution was found for droplet diameter. Moreover, it is clearly shown that increasing melt nozzle diameter induces an increase of both splat diameters and droplet diameters. The results provide a better understanding of the influence of atomization parameters on the size distribution. Ó 2012 Elsevier B.V. All rights reserved. 1. Introduction Challenges in the growing metal powder market are to improve conventional atomization techniques and to develop new atomization processes with better productivity and economy. The * Corresponding author. Tel.: þ address: marie-pierre.planche@utbm.fr (M.-P. Planche). objectives are to reduce powder costs by controlling particle size and energy efficiency. For example, it is proven that a wide particle size distribution of the raw metal powder means a low productivity [1]. For now, water atomization is one of the economical processes but it is not adapt to produce clean and spherical powder [2]. Other techniques, such as close coupled atomization are currently used for the production of metallic powder but its gas consumption is high and the size distribution of the powder rather is wide leading to a low productivity [3,4]. Among the gas atomization processes, Nanoval system which uses a De Laval nozzle to atomize liquid /$ e see front matter Ó 2012 Elsevier B.V. All rights reserved.

2 682 M.-P. Planche et al. / Materials Chemistry and Physics 137 (2013) 681e688 metals is thought to be a more efficient technique than using the widespread close-coupled nozzles [5], because of the laminar flow in the vicinity of the atomizing area [6]. As a result, fine to medium sized metallic powder with ball-shaped particle size distributions are produced [7]. For this system, some numerical and experimental studies have been already performed to carry out the relation between the powder properties and the process parameters [8e10]. They generally focused on the main parameters influencing the final powders in terms of size distribution and morphology especially. Concerning the numerical aspect, a simply model was first experimented to predict the gas flow at the vicinity of the nozzle exit. It was proven that the flow changes from sonic to supersonic structure depending on the process parameters [11]. A more complex two phase model was developed to investigate the interaction between the liquid metal film and the gas flow in the same region. Again, process parameters influence strongly the liquid breakup and modify the properties of the droplets, consequently [12]. Concerning the experimental aspect, features of the droplet flow were visualized using the Particle Image Velocimetry technique. PIV measures were performed in the 5e15 cm area downstream the nozzle exit. Images of Al metal droplets were captured and the corresponding velocity vector fields were determined from images treatment. A direct relationship between process parameters and average velocities was demonstrated [13]. However and despite its industrial use, the relationships between processing parameters (nozzle design and diameter, atomizing pressures, etc.) and particle size remain rather bad defined. The aim of this study was to further the understanding of the atomization process by investigating the properties of the splats. This approach consists to collect splats onto glass or steel substrates. An experimental set-up was specifically designed to be integrated in the atomization chamber. Thus, the images of these splats were statistically treated and their properties were analyzed in terms of flattening and splashing factors. During this investigation, several operating parameters have been varied (nature of metal, melt nozzle diameter, atomizing pressures). The splat substrate interaction was studied leading to better understand the flattening process of droplets. Splat characteristics related to droplets morphology and size were finally considered. 2. Experimental 2.1. Principle of the Nanoval atomization technique The LERMPS laboratory has for some time a metallic atomization system developed by Nanoval society. It consists of using a de Laval nozzle to atomize liquid metals. By this technique, the metal is melted and overheated by inductive heating in a crucible installed in the pressurized autoclave. A schematic representation of this process is shown in Fig. 1. During the pressurization phase, the molten metal leaves the crucible via the melt nozzle leading to the creation of the metal film. It meets the argon gas flow at the exit of the melt nozzle and passes through the de Laval nozzle. Due to the interaction with the gas flow, the film thickness is continuously decreasing and accelerating in the flow direction and it starts breaking up and disintegrating into small droplets. That corresponds to the so called «first atomization». Farer from the exit of the de Laval Nozzle exit in the atomization chamber, a second disintegration of the droplets takes place reducing the size of the initially formed droplets. After their solidification during flight, these cold droplets are finally collected in a container at the bottom of the atomization. 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. The design of the atomizing system leads to the formation of a negative overpressure at the tip of the melt nozzle. This negative overpressure is due to a pressure difference between the top of the melt and the area located near the melt nozzle tip. The design of the atomization unit produces a negative overpressure just at the exit of the melt nozzle. The velocity of the metal film at this position can be accurately estimated by: v ¼ Fig. 1. Schematic representation of Nanoval atomizing process. sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2gh þ 2DP r g is the gravitational constant (9.81 m s 2 ), h is the melt height (m) e the melt height is evaluated at the melting temperature from the knowledge of the mass of the metallic bars put initially inside the crucible and from the diameter of the crucible, r is the metal density (kg m 3 ) and P is the negative overpressure at the melt tip. The converging diverging design of the de Laval nozzle enhances the interaction between molten metal and high velocity gas. The gas stream is kept laminar in the atomizing area reaching sonic or supersonic velocity depending on the relative pressure between the autoclave and the atomizing chamber. For example, the evolution of flow velocity is represented in Fig. 2 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. Consequently, powder particles have different morphology and diameter which correspond to those obtained under various process conditions [14]. A better understanding of such process is attempted in order to improve control of particle properties especially their size (1)

3 M.-P. Planche et al. / Materials Chemistry and Physics 137 (2013) 681e Fig. 2. Gas flow velocity for two atomization pressures. distribution. This way, in-situ characterization of the metal flow was retained to get online information. The solution was to collect splats (impacts of liquid droplets) on a substrate during the atomization spray, just before the solidification of droplets and the production of powder Collection of the splats The system used for collecting the splats is shown in Fig. 3. The system was composed of two mm 2 plates one supporting substrates and the other using as substrate protection against the continuous droplets flow. Some windows with the same standard dimension than that of the substrate (25 70 mm 2 )were drilled in this protective plate in such a way that its moving allows or not to cover the substrates during the atomizing process. A 500 mm distance was maintained from the atomizing point (from the de Laval nozzle exit) to this system. The substrates were 25 mm separated from one to another. The edge of the centered substrate was coinciding with the axis of the flow direction. This way, a radial collection of the splats could be obtained simultaneously. Different substrates were tested either polished (Ra < 0.5 mm) steel substrates or glass substrates. Fig. 3. Experimental set-up for collecting splats.

4 684 M.-P. Planche et al. / Materials Chemistry and Physics 137 (2013) 681e688 Fig. 4. a) Typical micrographics of Cu splat on polished steel stain substrate and b) on glass substrate (atomizing pressure of 1.0 MPa, diameter of melt nozzle of 1.5 mm). During the spreading, the protective plate was moving limiting the splats number on the substrate. A relatively short interaction time, approximately 3 s between the substrate and the droplet flow was enough long to obtain enough exploitable splats. It was considered that the substrate temperature was kept constant (i.e. 100 C measured by thermocouples). Whatever the operating conditions, it was also observed that only the centered substrate present impacts Analysis of the splat diameter The splat characteristics were measured using optical microscopy coupled with image analyzer [15]. The images were then analyzed using ImageJ analysis software leading to the calculation of their impact diameter, flattening and splashing degrees. More than 30 splats were randomly chosen for statistical analysis to ensure a good representation of these estimated parameters. The equivalent diameter (D eq ) corresponds to the diameter of the splat supposing the particle circular. This diameter can be calculated from the surface (S) and the perimeter (P) of the observed splat. It is defined from Eq. (1): rffiffiffiffiffi 4S D eq ¼ (2) P The scatter rate, which could be related to the splashing degree, is defined by the dimensionless parameter, Eq. (2): S d ¼ P2 4pS (3) For a disk-shaped splat, S d ¼ 1. When the splat is indented, S d increases. Whatever the substrate, it was quasi null as seen in Fig. 4. The statistical treatment of data provides information about the mean value and the standard deviation [15] Analysis of the droplet diameter At the end of the atomization process, size distributions were analyzed using MALVERN system (Mastersizer 2000 Analyzer Ò, Malvern Instruments Microscopy). For each run, the average value (d 50 ) and the standard deviation of droplet diameters can be deduced from the curve of the size distribution Operating parameters Two process parameters, melt nozzle diameter and autoclave pressure, were varied and their effect were studied on Copper splat characteristics. The values of these variable parameters were changed from 1.5 to 2 mm for the melt nozzle diameter and from 0.5 to 1.5 MPa for the atomization pressure. The diameter of the de Laval nozzle was 4 mm and the pressure in the atomizing chamber was the atmospheric pressure. To produce the copper droplet and powders, raw materials used is copper (99.90 atomic %). The melting was performed using the induction heating device under an argon atmosphere in the autoclave. Just before the atomization, the melting temperature was measured between 1300 and 1350 C. When collecting impacts, only a small quantity of Cu was melt (between 500 and 900 g) leading to a total 10e 20 s time for atomization. This way, the temperature inside the Fig. 5. Autoclave pressure at the start of the process. Fig. 6. Evolution of splat and droplet diameters versus time progress.

5 M.-P. Planche et al. / Materials Chemistry and Physics 137 (2013) 681e Table 1 Evolution of the splat diameter and of the corresponding powder diameter under different atomizing pressure of 0.5, 1.0 and 1.5 MPa e melt nozzle diameter of 2 mm. Atomizing pressures (MPa) Splat diameter D eq (mm) atomization chamber didn t significantly change during the process Evolution of the atomizing pressure versus time Powder diameter d 50 (mm) To avoid the solidification of the melt in the melt nozzle, the atomization process has to be started at an intermediate pressure. Otherwise, too much gas may lead to a partial freezing of the melt nozzle before the melt would have heated it up. During the pressurization time from the start of the melt flowing out of the crucible to the setting pressure, the particle size of the powder cannot be controlled. Furthermore, the static height of the melt decreases during the process, as the crucible is emptying. So, the process is never at steady state. The evolution of the atomizing pressure versus time is given in Fig. 5. To point out the effect of the atomizing pressure on the splat characteristics, three different instants were chosen to collect them: T 1 in the ramp of the pressure between the starting and the setting pressure, T 2 in the beginning of the setting pressure and T 3 in the well-established value of setting pressure. In correspondence, samples were taken from the complete powder container at the same stages of the process and the droplet diameters analyzed, respectively. 3 2,5 1,5 MPa 1,0 MPa 0,5 MPa Splashing degree 2 1,5 1 0, Splat diameter (µm) Fig. 7. Distribution of splat diameters sprayed on stainless steel substrates versus atomizing pressures. a D = 10,6 µm D = 62,4 µm D = 164,4 µm 0.5 MPa 1.0 MPa 1.5 MPa b Volumetric fraction (%) D = 8,5 µm D = 34 µm D = 118,6 µm D = 24,6 µm D = 98 µm D = 330,6 µm Droplet diameter (µm) Fig. 8. a) Distribution of droplet diameters versus atomizing pressures of 0.5, 1.0 and 1.5 MPa and b) corresponding SEM micrograph of powder atomized at 1 MPa e melt nozzle diameter of 2 mm.

6 686 M.-P. Planche et al. / Materials Chemistry and Physics 137 (2013) 681e688 Table 2 Evolution of the splat diameter and of the corresponding powder diameter obtained for different melt nozzle diameter of 1.5, 1.8 and 2 mm e atomizing pressure of 1.0 MPa. Melt nozzle diameter (mm) Splat diameter D eq (mm) Powder diameter d 50 (mm) Droplet velocity direction d 50 e D eq 3. Results and discussion 3.1. Evolution of splat/droplet diameter versus atomizing time The mean diameters of splats (D eq ) and droplets (d 50 ) are given in Fig. 6 for T 1, T 2 and T 3 stages. Whatever the stage for the splats collection (T 1, T 2 or T 3 ), splashing degree is quasi null. The mean diameter of splats is mm att 1, mm att 2 and mm att 3. These results were correlated to the powder analysis. Supposing that the powder present in the container is chronologically distributed (the bottom corresponding to T 1, the top to T 3 ), the powder was separated into three samples, each of them corresponding to the three given stages of the atomization process In Fig. 6, are also given the corresponding mean particle size diameters produced during different stages of the process. Correspondingly, the mean diameter of droplets is 98 mm att 1, 68.3 mm att 2 and 62.3 mm att 3. There is a decrease in the splat diameter versus time progress leading to a correlative decrease in the droplet diameter. The ratio between droplet diameter and splat diameter is quite constant about 0.35 for the three different instants. During the pressurization time (between 0 and T 2 including T 1 ), the pressure rise affects the gas velocity and leads to coarser powder because of a weaker friction force on the melt monofilament. At the setting pressure, the quantity of melt in the autoclave is progressively decreasing. Consequently, the Gas to Metal Ratio (GMR) is changing and the same gas pressure will lead to finer powder Evolution of splat/droplet diameter versus atomizing pressure Table 1 gives the splat diameters and the corresponding powder diameters obtained at different atomizing pressures 0.5, 1 and 1.5 MPa. The collection of the splats was performed during the stabilized period of the setting pressure, so between T 2 and T 3 instants. Substrate Fig. 10. Schematic of the idealized deformation of a molten droplet into a thin disc. The statistical analysis of splat and droplet (powder) diameters points out the correlation between them. It presents the constant decrease of both diameters with the atomizing pressure increase. The variation ranges from 257 to 94 for the splat diameter and from 98 to 34 mm for the droplet diameter. This increase in atomizing pressure results in increasing the gas velocity. As the gas velocity contributes to the forces exerted on the melt monofilament, it induces better atomization efficiency, therefore a reduction of mean diameter. Fig. 7 presents the distribution of the splashing degree versus corresponding mean splat diameter. The splashing degree still inferior to 2 which corresponds to relative low values whatever the pressures. On the other hand, the dispersion of splat diameter is influenced by the atomizing pressures because of its significant decrease with the pressure increase. The splat diameter distribution becomes quite large when the pressure is decreasing. The standard deviations of splat diameters change from 88 mm at 0.5 MPa to 17 mm at 1.5 MPa, respectively. That can be directly related to the process stabilization. Considering the ratio of upstream to downstream absolute pressures, that means that the PID (Proportional-Integral-Derivative) pressure regulation is more effective for a higher atomizing pressure. Fig. 8 shows the droplet size distribution at different atomizing pressures. The droplets are spherical as shown in the SEM micrograph for one of the raw powder atomizing at 1 MPa. Depending on the atomizing pressure, the corresponding distribution of droplet diameters presents the same evolution than that of the splat diameters. Even more important than the mean diameter is the standard deviation of the droplet size distribution. Fig. 9. a) Micrography and b) profile of Cu splat sprayed onto stainless steel substrate e atomizing pressure of 0.5 MPa, melt nozzle diameter of 2 mm.

7 M.-P. Planche et al. / Materials Chemistry and Physics 137 (2013) 681e Table 3 Comparison between powder diameters estimated from splat characteristics and powder diameters measured by laser coulter analyzer e melt nozzle diameter of 2 mm. Atomizing pressure (MPa) Measured values Estimated values from splat characteristics Splat thickness (mm) Splat diameter (mm) Powder diameter (mm) Powder diameter (mm) Evolution of splat/droplet diameter versus melt nozzle diameter Table 2 presents the evolution of splat diameter and of the corresponding powder diameter versus the melt nozzle diameter. For this study, the atomizing pressure was 1 MPa constant and 1.5, 1.8 and 2 mm melt nozzle diameters were tested. The splat collection was realized in the setting pressure period, between T 2 and T 3. According to results in Table 2, the mean splat diameter increases significantly from 110 mm to 205 mm when the melt nozzle diameter changes from 1.5 to 2 mm, respectively. Again, there is a close correlation between the splat diameters evolution and that of the droplet diameters versus the melt nozzle diameter. A change in the melt nozzle diameter induces a change of the initial diameter of the monofilament and if the amount of gas is kept constant, the particle size will change, consequently. The effect of sheared forces becomes more efficient and allows producing finer droplet Simulation of droplet diameter Estimation based on splat thickness The thickness of the splats obtained on steel substrates was measured using a 3D profilometry. Typical micrography and profile of Cu particle impacted onto polished stainless steel substrate are displayed in Fig. 9. The atomizing pressure was 0.5 MPa and the melt nozzle diameter was 2 mm. Thickness of this splat was then deduced. Using Eq. (4), it is possible to estimate a mean droplet diameter from the knowledge of the splat characteristics, i.e. equivalent diameter and thickness, Fig. 10. Furthermore, the powder produced using De Laval nozzle presents a spherical morphology with almost no splashing phenomena. The droplet diameter as a function of splat diameter (D eq ) and splat thickness (e) is calculated by: 3eDeq d 50 ¼ (4) It is then possible to estimate the droplet diameter from splat characteristics. Table 3 compares the experimental data with the calculated values for different atomizing pressures. The experimental and the calculated mean droplet diameters are in good agreement. A difference of 15% is observed between the two results. Splat thickness presents an evolution similar to those of splat and droplet diameter because of its decrease when atomizing pressure increases. Splat thickness is reduced from 14 mm to 4 mm when atomizing pressure varies from 0.5 to 1.5 MPa Estimation based on Reynolds number Deposition behavior of particle is evaluated by the flattening degree D eq /d 50, the ratio between the splat diameter (D eq ) and the powder diameter (d 50 ). It has been proved that the droplet flattening degree is influenced by droplet size, density, viscosity and impact velocity, which can be normalized as the influence of the Reynolds number on the flattening degree. Madejski [16], from which a relationship between splat diameter and Reynolds number was derived, has done the early theoretical model. Relationships are generally expressed in the case of liquid particles impacting and solidifying onto the substrate by the following equation: ðd eq =d 50 ¼ are b Þ. Particle Reynolds number is defined as Re ¼ðrnd 50 Þ=m where the determination of average Reynolds number is made by taking copper molten properties. r is the density (8960 kg m 3 ), v is the velocity at impact distance (m s 1 ) measured by PIV technique and m is the viscosity ( kg ms 1 ). Fig. 11 displays the evolution of the kinetic energy Ec ¼ð1=2Þmv 2 versus the atomizing pressure. Assuming the particles spherical, the droplet mass can be expressed by m ¼ rv with V ¼ðp=6Þd 3 50, the volume of the droplet. Table 4 summarizes the different values used for this calculation. Fig. 12 gives the logarithmic evolution of Reynolds number as a function of the logarithm of the ratio between the splat diameter and the droplet diameter. A significant decrease of droplet kinetic energy is observed versus the atomizing pressure. It points out the importance of the droplet diameter compared to its corresponding velocity. From the equation obtained by the linear regression, it comes D eq =d 50 ¼ 2:561Re 0:122. Then, the higher the Re numbers, the smaller the ratio D eq /d 50. And, Re numbers are the highest when the atomizing pressures are the lowest corresponding also to the biggest droplet diameters. It can be thought that faster thermal energy dissipation occurs when the thickness diminishes which is the case when the ratio D eq /d 50 increases for small droplets. Table 4 Mean velocity, mean droplet diameter and mean splat diameter versus atomizing pressure e melt nozzle diameter of 1.5 mm. Fig. 11. Evolution of the droplet kinetic energy versus the atomizing pressure at the impacted distance. Atomizing pressure (MPa) Droplet velocity at impacted distance (mm) Mean droplet diameter (mm) Re (10 3 ) Mean splat diameter (mm)

8 688 M.-P. Planche et al. / Materials Chemistry and Physics 137 (2013) 681e688 versus these same parameters. The distribution of corresponding standard deviations increasing for low atomizing pressure indicates that relatively a higher proportion of large particles are present in such powders. References Fig. 12. Logarithmic evolution of the ratio D eq /d 50 in function of the logarithm of Reynolds number. 4. Conclusion The work presented here is devoted to the influence of operating parameters (atomizing pressure and melt nozzle diameter) on the Nanoval atomization process. This process consists in atomizing liquid metal through a de Laval nozzle. The acceleration of Ar gas in contact with the liquid metal flow allows improving the properties of the generated atomized droplets. According to the results, powder size distribution seems to be controlled by the atomizing pressure and the melt nozzle diameter. The results obtained are consistent with splat characterization (equivalent diameter) and can be used for prediction purposes. The results demonstrate that the mean splat diameter decreases when increasing the atomizing pressure or decreasing the melt nozzle diameter. The mean droplet diameters show similar behavior [1] S. Lagutkin, L. Achelis, S. Sheikhaliev, V. Uhlenwinkel, V. Srivastava, J. Mater. Sci. Eng. A 383 (2004) 1e6. [2] L. Achelis, V. Uhlenwinkel, J. Mater. Sci. Eng. A 477 (2008) 15e20. [3] I.E. Anderson, R.L. Terpstra, S. Rau, Progress toward understanding of gas atomisation processing physics, in: K. Bauckhage, V. Uhlenwinkel (Eds.), Spray Forming, Kolloquium Band 5, Books on Demand GmbH, Norderstedt, 2001, ISBN , pp. 1e16. [4] A. Lawley, Atomization: The Production of Metal Powders, Metal Powder Industry, Princeton, [5] S. Mates, G. Settles, J. Atom. Sprays 15 (2005) 19e60. [6] M. Stobik, J. Kollokium, Sprühkompaktieren/Sprayforming 6 (2002) 65e80. [7] M. Stobik, Nanoval atomizing e superior flow design for finer powder, in: Proc. of the International Conference on Spray deposition and Melt Atomization (2000). [8] O. Khatim, M.P. Planche, L. Dembinski, Y. Bailly, C. Coddet, J. Surf. Coat. Tech. 205 (2010) 1171e1175. [9] A. Allimant, M.P. Planche, L. Dembinski, C. Coddet, J. Powder Technol. 190 (2009) 79e83. [10] I.E. Anderson, R.L. Terpstra, R.S. Figliola, J. Adv. Powder Metall. Part. Mater. 2 (2004) 26e36. [11] N. Zeoli, S. Gu, J. Comp. Mater. Sci. 43 (2008) 268e278. [12] H. Liu, Science and Engineering of Droplets Fundamentals and Applications, William Andrew Publishing/Notes, NewYork, [13] A.M. Mullis, I.N. McCarthy, R.F. Cochrane, J. Mater. Proc. Tech. 211 (2011) 1471e1477. [14] O. Khatim, M.P. Planche, L. Dembinski, C. Coddet, Characterization of atomized metallic powders using De Laval Nozzle, in: Proc. of Thermal Spray Conference (2009). [15] M. Fukumoto, I. Ohgitani, M. Shiba, T. Yasui, Effect of substrate surface change by heating in transition in flattening behavior of thermal sprayed droplets, in: Proceeding of International Thermal Spraying Conference, Osaka, Japan, [16] J. Madejski, J. Heat Mass Trans. 19 (1976) 1009e1013.