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Surface & Coatings Technology 191 (2005) 375 – 383 www.elsevier.com/locate/surfcoat

Experimental determination of the relationship between flattening degree and Reynolds number for spray molten droplets Chang-Jiu Li a,*, Han-Lin Liao b, P. Gougeon b, G. Montavon b, C. Coddet b a

State Key Laboratory for Mechanical Behavior of Materials, School of Materials Science and Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, PR China b LERMPS, UTBM, 90010 Belfort Cedex, France Received 9 December 2003; accepted in revised form 9 April 2004 Available online 2 June 2004

Abstract Copper powders were selected to create splats of a regular disk shape by plasma spraying on a preheated flat substrate surface. Copper powders of a small size range with a regular spherical shape were used to ensure a valid mean particle size. The particle velocity and temperature were measured based on the thermal radiation from particle surface. The flattening degrees were estimated using the average diameter of splats and spray particles for different spray conditions. The relationship between the flattening degree (n) and the Reynolds number (Re) for the spray molten droplet was examined experimentally using exponential formulas with different power factors as reported in the literature. The results revealed that the equation n = 1.21Re0.125 determined by the experimental data rather than other equations with the power factors of 0.2 and 0.25 to Reynolds numbers is applicable to reasonably estimate the flattening degree of the spray molten droplet in a wide range of the Reynolds numbers. This range, from several hundreds to several ten thousands, covers typical molten metallic droplets and oxide ceramic droplets in thermal spraying. D 2004 Elsevier B.V. All rights reserved. Keywords: Plasma spraying; Molten droplet; Splat formation; Flattening degree; Reynolds numbers; Impact; Copper

1. Introduction The splat cooling is a fundamental process that concerns with materials processing processes involved in rapid cooling. These include thermal spray deposition, spray forming, melt spinning cooling, etc. During splat cooling, the cooling rate of a flattened molten droplet is inversely proportional to splat thickness by a power factor from 1 to 2 depending on the interfacial thermal contact conditions [1]. The rapid cooling feature determines the microstructure and property of subsequent splat. As a thermal spray coating is constituted of splats, the performance of the coating can be determined by the lamellar structure of the coating and the microstructure of individual splats [2]. Thus, splat formation is one of the most important fundamental topics involved in thermal spray technology.

* Corresponding author. Tel.: +86-29-82660970; fax: +86-2983237910. E-mail address: [email protected] (C.-J. Li). 0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2004.04.063

When a splat is deposited by a molten droplet on a flat substrate surface at an ambient atmosphere in thermal spraying, the splashing that occurs results in the formation of the splat in an irregular morphology [3,4]. The recent studies have revealed that the preheating of the substrate surface over 200 jC can lead to the formation of the disklike splat on a flat substrate surface [5,6], provided that the melting of the substrate surface resulting from impacting droplet does not occur [6,7]. As for the disk-like splat, the flattening degree, which is defined as the ratio of the diameter of the splat to the diameter of starting droplet, defines both the diameter and thickness of splat. As a result, the microstructure of the splat will be dominated by the flattening degree. Therefore, it is essentially important to understand how the droplet conditions, including velocity, temperature and size, influence the flattening degree of a spray molten droplet. The final size of a splat will be determined by three mechanisms: viscous and surface tension dissipations of the inertial energy, and the arrest of liquid flow by solidification [8,9]. The effect of droplet conditions on splat formation in terms of the flattening degree is

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usually normalized by the Reynolds number of spray droplet. Following the theoretical treatment by Jones [8], the following relation was obtained to estimate the flattening degree by our previous results [10]: n ¼ 1:06Re0:125

ð1Þ

where n is the flattening degree and Re is the Reynolds number. Re = qdv/g, q, d, v and g are the density, diameter, velocity and viscosity of the molten droplet, respectively. Theoretically, Madejski [9] derived a formula for the flattening degree as the function of the Reynolds numbers, Weber numbers and solidification effect. The following equation was obtained when the effects of both surface tension and solidification were neglected [9]: n ¼ 1:249Re0:2

ð2Þ

Furthermore, the following equation, which was obtained by following the approach proposed by Passandideh-Fard et al. [11], may also be used to estimate the flattening ratio in thermal spraying: n ¼ 0:5Re0:25

ð3Þ

The experimental correlation suggested that Eq. (1) gives a reasonable estimation of splat size according to our previous reports [10,12,13]. On the other hand, Vardelle et al. [14] reported the experimental data supporting Eq. (2). However, it has been widely argued that Eq. (2) overestimates the flattening degree [10,12,13,15,16]. Recent numerical simulation studies of droplet flattening process have shown that a coefficient close to about the unity was obtained instead of 1.249 in Eq. (2) when n was correlated using the formula Re0.2. Trapaga and Szekely [17] reported a coefficient of the unity and Liu et al. [18] gave a coefficient of 1.04. A coefficient of 0.925 was proposed by Bertagnolli et al. [16]. Yoshida et al. [19] reported a coefficient of 0.83. Our recent simulation also yielded a coefficient of 1.025 [20]. Therefore, the discrepancy among researchers is obvious with regard to the relationship between the Reynolds numbers and the flattening degree of spray droplet. Table 1 summarizes the coefficients reported Table 1 Coefficients reported for relationship n = aRe0.2 by different investigators Coefficient (a)

Investigators

Remarks

1.249 1

Theoretical model Simulated at isothermal conditions

0.83 1.04

Madejski [9] Trapaga and Szekely [17] Yoshida et al. [19] Liu et al. [18]

0.925

Bertagnolli et al. [16]

1.025

Li et al. [23]

Simulated at isothermal conditions Simulated by taking account of solidification effect Simulated at isothermal conditions

by different researchers when the flattening degree is correlated using the exponential function of Re with a power of 0.2. Due to the occurrence of splashing during splat formation, the experimental correlation results in inaccuracy. Only the regular splats without any significant splashing present their exact dimensions. It was widely confirmed only recently that a regular disclike splat could be formed on a flat surface only if the surface is preheated to over about 200 jC. Another reason is due to the difficulty to control the particle size of spray droplets. This is because that the diameter of spray powders usually presents a certain distribution. But the average particle diameter is generally used to calculate the flattening degree. When spray particles of small diameter are used in the case of oxide ceramic powders, the control of particle diameter becomes difficult and the relative error resulting from the inaccuracy of particle size would be significant. Therefore, to achieve a reliable correlation, it is necessary to control particle diameter and measure the velocity and temperature of spray particles in a sufficient accuracy. In the present study, the powder with spherical shape was carefully filtered into two small size ranges. The copper powder was selected to limit the difference of the measured surface temperature from the mean temperature of the droplet in a completely molten state. Through the preheating of the substrate surface and deposition conditions, the disktype splats were created. Thereafter, the relationship between the Reynolds numbers and flattening degree for thermal spray molten droplets was examined.

2. Materials and experimental procedures 2.1. Materials Copper powders were selected as the spray material because of their excellent thermal conductivity. The high thermal conductivity will reduce the temperature gradient within the molten droplet. Accordingly, the measured surface temperature can approximately represent the mean temperature of the molten droplet. Another advantage of using copper powder is due to the regular morphology of powder manufactured by the gas atomization process. Such morphology can effectively limit the particle diameter range. The powder was sieved into two different particle diameter ranges: 45 –50 and 80– 90 Am from a commercial Cu powder (Amdry 3260). The original powder had a nominal particle diameter range from 45 to 90 Am. Most powder particles had a spherical morphology that was confirmed by scanning electron microscopy (SEM) as shown in Fig. 1. The particle diameter was statistically estimated using SEM images of particles by image analysis technique. The measurement yielded the mean particle diameters of 48.4 F 6.8 and 89.2 F 8.2 Am for the two powders.


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2.2. Substrate conditions for splat deposition

Table 2 Operating conditions for Ar – H2 plasma

Stainless steel plate of 3 mm in thickness was used as a substrate. The surface of the substrate was polished and then was preheated using a flame torch to 350 jC prior to splat deposition. The preheating temperature was monitored by the thermocouples attached to the surface of the substrate for the deposition of splats. After the preheating temperature reached to 350 jC, the substrate was carried by a robot and set to move across spray particle stream vertically for the deposition of splats.

Plasma torch

Plasma-Tech F4 model

Primary plasma gas (Ar)

Pressure Flow Pressure Flow

2.3. Plasma spray conditions for the generation of molten droplets Commercial plasma spray torch (Plasma-Tech F4 model) was used for the generation of molten spray droplets. The torch was operated using Ar as a primary plasma gas and H2 as an auxiliary gas at an arc power of 32.4 kW. Table 2 shows the primary Ar – H2 plasma generating conditions. The particle velocity was adjusted through the change of spray distance. For small powders, the splats were deposited at three spray distances of 140, 200 and 250 mm. For large powders, the splats were deposited at the distances of 140, 220 and 300 mm. 2.4. Deposition of splats To deposit the isolated splats, a molybdenum shielding plate with a hole of 5 mm in diameter was placed perpendicularly to the plasma jet at a distance of 130 mm from the plasma torch exit to permit the limited particles to reach to the substrate. The center axis in the hole of the shielding plate was adjusted to align with the center of particle stream, which was about 6 mm lower with respect to the plasma jet axis when the powders were fed into the plasma jet

Auxiliary plasma gas (H2) Arc current Arc voltage Powder carrier gas (Ar) Gun traverse speed

0.3 MPa 45.5 l/min 0.3 MPa 5 l/min 540 A 60 V 1.8 l/min 500 mm/s

downward vertically. During plasma generation, the plasma torch and shielding plate were fixed stationary. The preheating of substrate surface was carried out by heating the back of the substrate with an oxy-acetylene flame held about 300 mm away from the substrate. The thermocouples were attached to the surface of the substrate to monitor the substrate temperature. When the temperature of the substrate reached to 350 jC, the substrate was moved vertically crossing particle stream at a speed of 500 mm/min by the robot. 2.5. The measurement of the particle velocity and temperature and splat size The simultaneous measurements of the velocity and temperature of particles passing through the shielding plate were carried out through employing a commercial system DPV-2000 (Technar, Canada). With this system, the thermal radiation of a molten spray particle is modulated by double slits to a double-pulse signal. The particle velocity is obtained by detecting the time interval between two pulses. The temperature is measured by the two-color pyrometer. The calibration was made for the temperature measurement to ensure the validity of the data. The morphology of splats was examined using optical microscope (EPIPHOT, Nikon) and scanning electron microscope (SEM). Using a digital camera (Cool-Pix 995, Nikon) attached to the optical microscope, the images of splats were taken for the quantitative estimation of the diameter of individual splats. The arithmetical mean splat diameter was calculated from all splats collected for each condition.

3. Experimental results 3.1. Particle velocity and temperature

Fig. 1. Morphology of copper powders of nominal diameter ranges from 80 to 90 Am.

Table 3 shows the measurement results of the particle velocity and temperature. Using Ar –H2 plasma, the average velocity of copper droplets of an average diameter 89.2 Am changed from 88 to 112 m/s with spray distance. The particle velocity changed from 108 to 152 m/s for smaller particles of an average diameter of 48.4 Am. It was found

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Table 3 Particle size, velocity and temperature measured under different spray conditions and corresponding Reynolds numbers Test runs

Mean particle size (Am)

Spray distance (mm)

Particle velocity (m/s)

Mean temperature (jC)

Reynolds numbers

1 2 3 4 5 6

89.2 89.2 89.2 48.4 48.4 48.4

140 220 300 140 200 250

112 F 16 95 F 9.1 88 F 5 152 F 21 128 F 15 108 F 10

1996 1950 1720 2174 1964 1791

50,404 40,391 31,879 39,873 29,887 22,491

that the variation of the particle velocity was less than 15% for each case. It was typically about 10% as a result of the limited particle diameter distribution and relative concen-

tration of particle trajectories, which was achieved using the shielding plate. On the contrary, no significant change was observed for the surface temperature of copper droplets. Most droplets reached an average temperature within a range of 1720 to about 2000 jC. 3.2. Reynolds number of the copper droplets produced by plasma spraying With the high thermal conductivity of copper and the relative long flight distance of particles, it can be considered that the surface temperature of the droplet observed at the present study represented the through-particle temperature. It was also observed that the difference of the mean

Fig. 2. Typical morphologies of Cu splats collected at different test runs: (a) run 6; (b) run 5; (c) run 4; (d) run 3; (e) run 2.


temperature range for the copper droplets under all conditions in the present study was limited to about 300 jC as shown in Table 3, except that of the small particles that traveled a short spray distance. The viscosity values of 4.04 10 3 and 1.95 10 3 Pa s were reported for molten copper at temperatures of 1083 and 1725 jC, respectively [21]. Following the exponential law of viscosity of liquid metal with the change of temperature [22], the viscosity of molten copper droplet at different temperatures was determined. Accordingly, the Reynolds numbers of spray molten copper particles at different conditions are shown in Table 3. It can be clearly seen that the Reynolds numbers of the copper particles used in the present study varied from 22,491 to 50,404.

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Fig. 3. Comparison of the relationship between the Reynolds numbers and flattening degree obtained by the correlated relationship with the observed ones.

3.3. Average diameter of splats Most experimental observations in the past reported that the splashing during splatting is suppressed and, consequently, a regular disclike splat is formed when a splat is formed on a flat surface preheated over 200 jC. Fig. 2 shows typical Cu splats deposited by the droplets under different conditions. It was found that the splats formed under the conditions at which the Reynolds numbers were lower than 50,404 presented regular disk morphology. The average splat diameters and splat numbers collected at different conditions are shown in Table 4. It is evident that the mean splat diameter depended mainly on the diameter of the droplet. 3.4. Relationship between the flattening degree and Reynolds number The average flattening degree for splats deposited under each spray condition was obtained through dividing the average diameter of splats by the average powder particle diameter. The results are shown in Table 4. It was found that the flattening degree was increased with the increase of the Reynolds numbers of molten spray particles. As the Reynolds numbers of the droplet increased to over 50,000, it was found that the splashing occurred at the periphery of the splat regardless of preheating of the substrate surface [23]. Such splashing may lead to the decrease of the splat diameter. To ensure the statistical reliability, only the data

Table 4 Mean diameter of splats and flattening degree observed at different droplet conditions Test runs

Mean splat diameter (Am)

Number of splats

Flattening degree

1 2 3 4 5 6

400 F 30 405 F 31 393 F 32 222 F 28 213 F 25 208 F 29

195 97 40 162 214 180

4.48 4.54 4.41 4.59 4.40 4.30

of the test runs 2 –6 were used to fit the experimental data for the relationship between the flattening degree and Reynolds numbers. By using the exponential coefficients in Madejski’s model and Pasandideh-Fard’s model, the following correlations were obtained for the flattening degree against the Reynolds numbers: n ¼ 0:55Re0:2 n¼

1 0:25 Re 3

ð4Þ ð5Þ

Compared with the coefficients of 1.249 and 0.5 in the original models, the present results led to much smaller ones. However, through using exponential equation derived following the model suggested by Jones [8] with a power coefficient of 0.125, the following correlation equation was obtained: n ¼ 1:21Re0:125

ð6Þ

In comparison of the coefficient of 1.06 in the theoretical model with the coefficient in Eq. (6) obtained by the experimental data, it is clear that the Jones’ model gave an approximately better description of the flattening degree of spray particle. Fig. 3 shows the comparison of the experimental data with those calculated by Eq. (6). For comparison, the experimental data of the test run 1 were also plotted in the graph. It is obvious that Eq. (6) describes the relationship between the flattening degree and Reynolds numbers for the molten droplet splatting well.

4. Discussion The experimental determination of the relationship between the flattening degree and Reynolds numbers for impacting molten droplets is important to predict the microstructure of splat. With the increase of the concerns to

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Fig. 4. Particle diameter distributions of two powders: (a) fine particles; (b) coarse particles.

model and simulate the formation of thermal spray coatings [24 –29], the relationships among the flattening degree and spray parameters and droplet conditions became more and more necessary. This is because the splat size and its microstructure depend on the flattening degree. The experimental determination of the accurate relationship first requires the control of spray particle diameter and the formation of regular disk-type splats, which permits the acquisition of the reliable splat diameter. Accordingly, the accurate flattening degree can be obtained experimentally. On the other hand, the measurement of particle velocity and temperature, and the control of particle diameter distribution as well, are required for an accurate estimation of the Reynolds numbers of molten droplet. Because the temperature of molten spray droplet is used to estimate the viscosity of liquid droplet, the surface temperature observed experimentally should reasonably represent the mean temperature of whole molten droplet. Two sets of powders of the diameter ranges from 45 to 50 Am and from 80 to 90 Am were sieved out from a commercial Cu powder of nominal diameter range from 45 to 90 Am to control powder particle diameter distribution in the present study. To ensure the cut-off diameters of the powders, two sieves of opening sizes of 44 and 90 Am were used to screen out the particles either smaller than 44 Am or larger than 90 Am to ensure the cut-off diameters of the powders. Fig. 4(a) and (b) shows the size distributions obtained by measuring particle diameters using image-

analyzing soft. Despite a careful screening, it was found that the diameter distributions were larger than those expected from the opening sizes of sieves for both powders. Fig. 5 shows the relative particle diameter distribution for the two types of powders, in which the particle diameter was normalized by the mean particle diameter. It can be found that the distribution of the particle diameter for the small powder particles was wider than that for the large powder particles. The fractions of relative particle diameter in the range from 0.9 to 1.1, i.e., the range of mean F 10%, were 0.65 and 0.8 for the small powders and the large powders, respectively. This means that the small powders lead likely to a larger deviation from the mean particle size than large powders. On the other hand, in the present study, the preheating of the flat substrate surface ensured the formation of regular disk type of splats as illustrated in Fig. 2. The measurement of splat diameters yielded the relative splat diameter distributions as shown in Fig. 6. If the results of powder particle diameter distributions (Fig. 5) are compared with those of splat diameters, it can be recognized that the relative splat diameter range is comparable to that of powder particles. Those results suggest that the splat of a relative diameter was formed likely by a spray molten particle of the corresponding relative diameter. Therefore, it is reasonable to estimate statistically the flattening ratio by using the mean splat diameter and particle diameter for each powder.

Fig. 5. Normalized particle diameter distribution for two particles: (a) fine particles; (b) coarse particles.


Fig. 6. Normalized relative splat diameter distribution obtained for (a) fine particles and (b) coarse particles.

The viscosity of a molten droplet is determined by droplet temperature. The measurement based on the particle surface thermal radiation using the tester DPV-2000 gave the surface temperature of the molten droplet. Due to high thermal conductivity of copper and relatively long flight distance of particles, the surface temperature measured was reasonably considered the mean temperature of the molten droplet. The measurement of the particle velocity showed that the mean particle velocity presented a standard deviation of about 10% for most conditions. Taking into account the fact that a small particle accelerates to a higher velocity during in-flight and the deviations in both particle diameter and particle velocity, the Reynolds numbers of the spray molten droplet under a certain spray condition could be reasonably estimated by using the mean velocity and diameter of spray particles. When the different models were fitted with experimental data observed in the present study, the relationships between the flattening degree and Reynolds number were obtained for different models as given by the Eqs. (4) –(6). Through the modification of the coefficients in the different equations, the flattening degree could be reasonably estimated for the copper droplets created by plasma spraying. Fig. 7 shows the change of the flattening degree verses the Reynolds numbers calculated from Eqs. (4) – (6) for a relatively large Reynolds number range. The experimental data were also shown for comparison. It can be seen that

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Eqs. (4) – (6) are all valid for the molten copper droplets in the relatively large Reynolds numbers range to certain accuracy because those formulas were all obtained by correlating the experimental data. However, it can be clearly found that the difference among the flattening degrees calculated using those equations becomes significantly large with the decrease of the Reynolds numbers towards the values of molten oxide ceramics droplets. Therefore, the question is whether those modified the formulas and which one could be applied to spray droplets in such large extent of the Reynolds numbers as given in Fig. 7. Regarding commonly used thermal spray materials, the Reynolds numbers of the molten metallic droplets may be higher by a factor of 2 orders than those of molten oxide ceramic droplets. This difference results mainly from the large difference in the viscosity of two types of materials in a molten state. As a result, the Reynolds numbers may vary from several hundreds for molten oxide spray droplets to several ten thousands for molten metallic droplets observed in this study. It should be pointed out that the empirical equations (Eqs. (4) –(6)) are valid for the molten metallic equations. However, it is necessary to examine whether one of those formulas is applicable to oxide ceramic droplet of low Reynolds numbers. Vardelle et al. [30] examined the flattening behavior of Y2O3 stabilized ZrO2 powders (YSZ) of the diameter from 22 to 45 Am with a mean diameter of 42 Am under three different mean particle velocities of 105, 190 and 230 m/s. For the plasma conditions used in the case of the substrate preheating, the mean velocity and temperature of the particle were 230 m/s and 2727 jC. By using the viscosity data of 0.0386 Pa s, which can be calculated by the data reported elsewhere by the same authors [14], the Reynolds numbers of 1426 can be estimated for YSZ particles of the mean size under this condition. However, it can be found from the literature [30] that the average flattening degree was approximately 3.2 when its mean velocity was 230 m/s.

Fig. 7. Comparison of the observed experimental data with those estimated by using the correlations obtained by the different models: the results are those obtained by this study for copper (o), our previous study for alumina [12] (E) and Vardelle et al. [30] ( ).

.

382


Although a high flattening degree for YSZ particles was reported by Vardelle et al. [14], this is reasonably consistent with the 2.96 estimated from Eq. (6). As for plasma-sprayed alumina coatings deposited by the powder with a mean particle diameter of 20 Am, our previous study showed that the mean lamellar thickness under a typical deposition condition was 1.75 Am [12]. This result corresponds to a flattening degree of 2.76. When using molten alumina with a viscosity of 0.05 Pa s at the melting point of 2050 jC and velocity of 300 m/s, Reynolds numbers of 477 can be estimated. Both this result and that for YSZ particles from the literature [30] were plotted in Fig. 7 for comparison. From the results shown in Fig. 7, it can be clearly recognized that only the relation that is related to the Reynolds numbers by a power factor of 0.125 can be reasonably applied to estimate the flattening degree. The limited difference between the observed data and those estimated may be possibly due to the underestimation of the Reynolds numbers by using the viscosity of the molten droplet estimated from the temperature near/at their melting points. In fact, a slightly higher temperature than the melting points was achieved by the particles. However, it can found that when the power factors of 0.2 and 0.25 to the Reynolds numbers are used corresponding to Madejski’s model and Pasandideh-Fard’s model, the flattening degree of oxide ceramic spray particles will be remarkably underestimated. The present results evidently revealed that only the relation with the Reynolds numbers by a power factor close to 0.1 can be practically applied to the flattening of the molten spray particles from oxide ceramic particles to metallic particles, which covers a relatively large Reynolds numbers range span of nearly 3 orders in magnitude, although the modeling study of dynamics of the flattening process is required to explain theoretically the power factor for the Reynolds numbers.

5. Conclusions Disk-type copper splats were created by plasma spraying under different particle conditions on a flat substrate surface through the preheating of the substrate. The diameters of individual splats were measured. The particle velocity and temperature were measured under different spray conditions. The Reynolds numbers of spray particles were calculated using droplet parameters including velocity and temperature of in-flight particles. The correlation of the exponential equation with the Reynolds numbers by a power of 0.125 using the observed flattening degree data for copper droplets yielded the following equation: n ¼ 1:21Re0:125 By applying this equation to oxide ceramic droplets of lower Reynolds numbers by using reported data, it was revealed that the flattening degree can be reasonably predicted for

molten spray droplets in a wide range of the Reynolds numbers from several hundreds to several ten thousands, which covers Reynolds numbers range of general thermal spray droplets. On the other hand, it was found that the correlations obtained using metallic droplets in the present study following Madejski’s model and Pasandideh-Fard’s model were not applicable to ceramic droplet. Therefore, as a general formula to predict the flattening degree of spray molten droplet by the Reynolds numbers, the equation n = 1.21Re0.125 is found to be more suitable.

Acknowledgements The present work was partially supported by the Ministry of Education of France.

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