On the improvement of the manufacturing process of tungsten carbide

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On the improvement of the manufacturing process of tungsten carbide–cobalt hard metals by the application of sound assisted fluidization for the mixing of the powders Federica Raganati, Fabio Scherillo, Antonino Squillace, Riccardo Chirone, and Paola Ammendola Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b04627 • Publication Date (Web): 20 Dec 2017 Downloaded from http://pubs.acs.org on December 22, 2017

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Industrial & Engineering Chemistry Research

On the improvement of the manufacturing process of tungsten carbide–cobalt hard metals by the application of sound assisted fluidization for the mixing of the powders Federica Raganatia, Fabio Scherillob, Antonino Squillaceb, Riccardo Chironea, Paola Ammendolaa* a

b

Istituto di Ricerche sulla Combustione (IRC)-CNR, Piazzale Tecchio 80, 80125 Naples, Italy

Dipartimento di Ingegneria Chimica, dei Materiali e della Produzione Industriale - Università degli Studi di Napoli Federico II, Piazzale Tecchio 80, 80125 Naples, Italy

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT

In this work, the technology of sound assisted fluidization has been used as an alternative mixing methods to obtain homogeneous powder mixtures (WC, Co and polyethylene glycol - PEG) to be used in the standard steps of hard metals production. A preliminary experimental campaign was carried out to optimize the process parameters (sound intensity/frequency and fluidization velocity) to perform the mixing process. After that, the obtained powder mixture was pressed and subjected to the sintering process. The quality of the final sintered products was assessed by using different techniques (SEM analysis, magnetic properties analysis, relative density and Vickers hardness). The effect of two operating variable, mixing temperature and shaping pressure, was also studied aiming at improving the quality of the final products in terms of reduced porosity and enhanced density and hardness. Finally, also the effect of the addition of an inhibitor (Cr3C2) to the mixture was assessed.

Keywords: Sound assisted fluidization; Hard metals; Fine powders; Mixing; Tungsten Carbide/Cobalt.

*

Corresponding author

Tel.:+39 0817682237; fax:+39 0815936936. E-mail address: [email protected] ACS Paragon Plus Environment

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1.

Introduction Tungsten carbide-cobalt (CW-Co) or cemented carbide, also known as hard metal, is a material

that has been used in the manufacture of cutting tools since the 1920’s. It is still today a very important material in modern technology. Indeed, it finds application in such industries as metalworking, oil and gas industry, mining, engineering, cutting tool industry, production of structural parts and special purpose products, and other fields requiring high-performance materials with high wear resistance (e.g. sand blast/spray nozzles, seals in slurry pumps, cutting inserts that have to endure high mechanical and thermal loads during application).1–4 Hard metal alloys most simple constitution is a hard phase, tungsten carbide (WC), together with a metal which plays the role of binder.5 Morphologically, they consist of a high volume fraction of the hard hexagonal WC phase embedded within the soft and tough binder phase. Cobalt is the most frequently used metal, but nickel can also be used. The tribological, mechanical (hardness and strength) and thermal properties of WC cemented carbide substantially depend on its composition and WC particle size.6–8 In cemented carbides usually used the WC grain size ordinarily ranges from 0.1µm to 10µm, and Co powder is contained about 4–30%wt. Decrease of WC grain size increases hardness.9 Recently, ultra-fine particles of WC less than 0.1µm grain size have been developed.10 Although the wear property of cemented carbide is improved with smaller grain size and resulting higher hardness6, it is also reported that that for a specific hardness, the coarser grades showed superior wear resistance, but only over the hardness range 1000–1600HV.9 Therefore, cemented carbide does not always show better good wear resistance when the WC grain size becomes too small. Regarding the composition, increasing the volume fraction of Co generally increases the fracture toughness at the expense of hardness and wear resistance.1

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Considering the entire manufacturing process (production of the powders, mixing, pressing, presintering and sintering), several critical points can be found that end up in reducing the efficiency of the process and/or the quality of the final product. With reference to the pressing, a lubricant (2-3%wt), typically wax, is generally added to the powders to reduce the friction between the powder mixture and the surfaces of the shapes and also to minimize the tendency to form cracks.1 In recent times, the wax in the powder has been replaced by polyethylene glycol (PEG) by most manufacturers as this increases the quality of the final product and is easier to remove in the furnaces.11 Regarding the sintering step, abnormal grain growth, that is the exaggerated growth of a few grains at the expense of the main matrix grains, may occur, thus, negatively affecting the mechanical properties of the sintered product. Therefore, the control of grain growth of the carbide phase during the sintering is an important objective. This is primarily accomplished by the addition of grain-growth inhibitors (GGIs), such as V, Cr or Ta.3 Besides the above-mentioned critical points, one of the main issue concerns the mixing of the powder. This is generally achieved by wet milling in ball mills. This step has a duration of 10 – 60 h, thus strongly slowing down the entire production process. Moreover, the final result of the milling process is a homogenous slurry that must be dryed, thus further increasing the duration of the process of 10 – 20 h. Therefore, finding an alternative and more efficient mixing technique would improve the efficiency of the entire production process. Classical mixing methods, such as tumbling mixers, convective mixers, high-shear mixers, including media mills and hammer mills, are generally suitable for large, non-cohesive particles, i.e. mean particle sizes greater than 30µm, but not for particles smaller than 10µm in size (as in the case of WC and Co powders), always agglomerated due to strong interparticle forces.12–15 Alternatively, Wei et al.16 proposed mixing techniques for fine particles classified in wet (consisting in simply suspending, with agitation, the different powders in a liquid solvent with other additives, i.e. salts 4 ACS Paragon Plus Environment

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and surfactants) and dry (magnetically assisted impact mixing - MAIM, hybridization system HYB, mechanofusion - MF, rapid expansion of high pressure and supercritical suspensions REHPS) mixing. However, all these technological alternatives suffer from different disadvantages: additional steps of filtration and drying are needed in the case of wet methods and dry mixing methods generally involves the damaging/contamination and/or the alteration of the granulometry of the original powders. In this respect, the use of fluidization makes it possible to overcome these problems, since it is able to handle and process large quantities of powders. However, since these fine powders fall under the Geldart’s group C ( 0.8). However, frequencies in the range 40 – 80 Hz were able to maximize the fluidization quality, i.e. ∆P/∆P0 = 1 and suppression of the instabilities at low superficial gas velocities. The above-discussed effects of SPL and frequency on the fluidization quality were more readily expressed in terms of umf and ∆Pmax/∆P0, i.e. the asymptotic value of the pressure drops, reported in Fig 4A and B highlighting the effect of SPL and f, respectively. In particular, increasing SPLs led to better fluidization quality as confirmed by increasing ∆Pmax/∆P0 (=1 for SPL ≥ 145dB) and decreasing umf values (Figure 5A). These experimental evidences are in agreement with several works available in literature on the fluidization of fine cohesive powders 12– 15

, reporting that increasing the sound intensity means to increase the external energy introduced

inside the bed and, therefore, the efficiency of the break-up mechanism. With reference to sound frequency, its effect was not monotone (Figure 5B); it was possible to find an optimum range (40 – 80 Hz) giving the best fluidization quality in terms of highest values of ∆Pmax/∆P0 and lowest values of umf. This result was due to either the inability of the sound in properly penetrating inside the bed for too high frequencies (> 80 Hz) or the absence of relative motion between aggregates of different size, giving rise to the break-up mechanism, for too low frequencies (< 40 Hz).12–15 3.3

Mixing tests

Figure 6 reports the time evolution of the Co mean concentration in the mixture for A, B and C mixing tests. Regardless of the acoustic parameters, the Co weight percentage reached the 13 ACS Paragon Plus Environment


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theoretical value of 10.6%wt on a C-free base in about 7 min. This means that the mixing on a macroscopic scale actually occurred in very short times even using non-optimized mixing operating conditions in terms of acoustic parameters. With reference to the mixing at the microscopic scale, Figure 7 reports, for instance, a SEM image, with highlighted the aggregates composition, obtained for A test after 2 min of mixing. All aggregates were formed by both WC and Co particles, thus confirming that the mixing process actually occurred also on a microscopic level: i.e. not only between aggregates made of only one powder, but also inside aggregates, thus leading to the formation of hybrid aggregates made of both the powders. Figure 8 reports the time evolution of the coefficient of variation σ* obtained for A, B and C mixing tests. With increasing time σ* decreased, thus meaning that the mixture became more homogeneous. Moreover, the characteristic time of the mixing at the microscopic scale was strongly dependent on the acoustic parameters. In particular, with optimal values of SPL and frequency (A mixing test) the time needed to achieve a complete mixing also at the microscopic level is remarkably lower (about 60 min) than that necessary when using non-optimal acoustic parameters (> 180 min). This result is due to the fact that the achievement of an intimate mixing is only possible with a very efficient break-up and re-aggregation mechanism that is promoted by the application of acoustic fields. In this framework, the application of acoustic fields with optimal characteristics (SPL and f) could greatly enhance this mechanism, thus reducing the time needed to complete the mixing process. It is also clear from the results obtained that the mixing process was characterized by dynamics evolving in different times depending on whether it was observed from a macroscopic or microscopic point of view. In the former case, just few minutes were needed to reach the theoretical mean composition, whereas, in the latter case longer times were necessary (tens of minutes). Indeed, even though the macroscopic mixing was complete after just few minutes (i.e. the Co mean composition reached the theoretical value), the analyzed aggregates still presented


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a certain inhomogeneity (as confirmed by the high values of σ*), meaning that the microscopic mixing was not complete yet. The ICP-MS analyses on samples taken from the bed at the end of each mixing tests confirmed the efficiency of the mixing on a macroscopic scale in terms of mean concentration of the bed (%wt Co = 10.7 - 11%wt). 3.4

Characterization of the sintered specimens

Once identified the optimal operating conditions (150 dB - 80 Hz) several mixtures were prepared to study the effect of mixing temperature and the shaping pressure on the quality of the final sintered specimens. A typical specimen is shown in the photograph reported in Figure 9. All the sintered specimens were compared to that manufactured according to a standard cycle of Nashira Hard Metals, whose metallographic/microstructural characterization and properties are reported in Figure 10 and Table 1, respectively. In particular, a clear compactness and absence of porosity can be observed in Figure 10A, whereas, Figure 10B shows the homogeneous distribution of WC grains (white) inside the Co matrix (black). The relative density and the Vickers hardness of WC-Co sintered products are typically 14.5 kg/m3 and 1450 HV, respectively, as confirmed by the values obtained for the reference specimen (Table 1). 3.4.1 Effect of the mixing temperature Figure 11 reports the 50x magnification images of the four specimens crafted with PEG and using four different mixing temperatures. The analysis of these images reveals for each specimen the presence of a clear macroporosity that is absent in the reference specimen (Figure 10). However, it is also evident a reduction of the size of this porosity with increasing mixing temperatures. Most likely, during the mixing process PEG particles, much less dense than WC and Co ones, tended to stay in the form of aggregates, thus being unable to perform their lubricant role (i.e. coating of the metal particles) necessary in the following steps of the process and leading to undesired porosity in 15 ACS Paragon Plus Environment


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the final product. Indeed, in the following pre-sintering step PEG particles evaporated, thus giving rise to these cavities that remain in the final specimen in the form of the macroporosities shown in Figure 11. The lubricant role of PEG was particularly difficult also due to the morphology of WC particles: their pronounced roundness led to the formation of cavities much more easily than in the case of irregularly shaped particles (Figure 3A). Therefore, the positive effect played by the mixing temperature on the quality of the final sintered specimen (in terms of reduced porosity) was basically due to the progressive softening of PEG particles up to the melting point (68 °C) achieved in the test performed at 70 °C. In particular, the melting process made it possible to more homogeneously disperse and mix PEG particles, thus avoiding the formation of macro-aggregates. As matter of fact, a mixing temperature of 50 °C was not high enough to remarkably modify the lubricant state of aggregation. Therefore, PEG particles stayed in the solid state as in the test performed at ambient temperature. On the contrary, at 60 °C, even though they did not melt, PEG particles started to soften that led to a remarkable reduction of aggregates size and, consequently, of porosity. At 70 °C the melting occurred and the formation of aggregates was entirely avoided, thus letting PEG to perform its desired lubricant function through the coating of metal particles. Figure 12 reports 1500x magnification images of the crafted specimens. From the analysis of these images it is clear that the WC grains were uniformly distributed inside the Co matrix, thus confirming the efficiency of the mixing process performed under sound assisted fluidization conditions. However, it is also evident that WC grains were characterized by a less uniform size distribution than that of the reference specimen (Figure 10B). This inconvenience was probably due to the fact that mixing in sound assisted fluidized beds did not involve any kind of grinding process of the original particles, in contrast to the standard mixing process carried out in ball mills. Therefore, without this prior grains homogenization, the presence of differently sized grains (according to the granulometric distribution reported in Figure 2) in the final sintered specimen was unavoidable. In particular, the largest grains found more difficult to fill the voids during the shaping 16 ACS Paragon Plus Environment

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process, thus determining the formation of WC-free zones in which only Co is present. From the analysis of Figure 12 it is also clear the presence of regions made by the fusion of several WC grains. This structural flaw was due to the phenomenon of abnormal grain growth of the carbide phase during the sintering step that is typically controlled by the addition grain-growth inhibitors, as also done for the manufacture of the reference specimen. This phenomenon can also explain a part of the porosities observed in Figure 11, more specifically the smallest ones, whereas, the largest were more likely due to PEG aggregates. The properties of all the specimen are reported in Table 2. From the analysis of Table 2 it is clear that the mixing temperature had a positive effect on both the relative density and Vickers hardness, as confirmed by the increased trends obtained with increasing temperatures. In particular, the specimens manufactured at mixing temperatures lower than 60 °C were characterized by values remarkably lower than those of the reference specimen. This result is due to the strong spatial inhomogeneity and porosity, as shown by the metallographic analysis. The high standard deviations values confirmed the irregular and uneven nature of these specimens, due to the non-optimal lubricant function performed by PEG at these low mixing temperatures. On the contrary, using a mixing temperature of 70 °C it was possible to obtain a specimen whose characteristics were close to the desired values, coherently with the observed structural improvement. In particular, the still slightly lower values of the Vickers hardness with respect to the reference value is most likely due to the above-mentioned presence of regions with larger WC grains. This is due to both the heterogeneous granulometric distribution of WC powder (Figure 2), as a consequence of the lack of grinding process, and the grain growth phenomenon, that was not properly hindered by the addition of inhibitors. With reference to the magnetic properties, for all the specimens good values of percentage of magnetic Co, i.e. the amount of Co that actually made up the binder matrix, were obtained.



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Besides, using the reported characterization techniques (metallographic and microstructural examination) we did not observe any uneven shrinkage phenomena and/or cracks in the samples both with and without the use of PEG. The lack of both phenomena in this study may be due to the geometry of the samples. 3.4.2 Effect of the shaping pressure Figure 13 reports the 50x magnification images of the three PEG-free specimens crafted using three different shaping pressures and an ambient mixing temperature. From the analysis of these images it can be observed the total absence of the macroporosities observed for specimen obtained using the lubricant (Figure 11), and the presence of a reduced number of small cavities. As a matter of fact, the problem of the aggregation of PEG particles (i.e. the formation of macroporosities) was entirely avoided using only WC and Co powders, thus overcoming also the formation of the porosities in the final products. In particular, the PEG-free specimens were qualitatively very similar to that crafted using the lubricant and a mixing temperature of 70 °C. Moreover, even increasing the shaping pressure from 400 to 480 bar, it was not possible to obtain a specimen without porosity. With reference to this point, it must be considered that, even though the PEG itself activates the formation of the macroporosity, the absence of lubricant during the shaping step made more difficult the relative motion between the particles, thus involving the formation of little voids in the structure of the final product. From the analysis of the 1500x magnification images (Figure 14), as observed for the specimens crafted with PEG, it is clear the presence of little structural flaws caused by the granulometric inhomogeneity of WC powder and by the abnormal grain growth. This grain growth phenomenon can also be responsible of the little porosities observed in Figure 13. Likewise, from the analysis of the data reported in Table 3, it can be observed that, regardless of the shaping pressure, the relative density of all the specimens is very close to the reference value (Table 1), coherently with the good quality of the specimens in terms of porosity. On the contrary, it 18 ACS Paragon Plus Environment

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can be observed a positive effect of increasing shaping pressures on the Vickers hardness, even though also at 480 bar the measured value was lower than the reference one, mainly due to the inhomogeneity of WC grain size distribution. Also in this case, as for the specimen realized at different mixing temperatures, good values of percentage of magnetic Co were obtained, regardless of the shaping pressure. 3.4.3 Effect of Cr3C2 addition to the mixture Figure 15A reports the 50x magnification image of the PEG-free specimen crafted adding to the mixture the inhibitor (Cr3C2) and using an ambient mixing temperature. In line with the results obtained for the other PEG-free specimens, from the analysis of these images it can be observed the total absence of the macroporosities observed using the lubricant (Figure 11). However, the presence of a small number of little cavities can still be observed, which is due to the fact that the absence of lubricant during the shaping step made more difficult the relative motion between the particles, thus involving the formation of little voids in the structure of the final product. From the analysis of the 1500x magnification images (Figure 15B), it is clear that the addition of the inhibitor made it possible to completely avoid the phenomenon of the abnormal grain growth (i.e. absence of regions made by the fusion of several WC grains). In particular, the obtained specimen was very similar to the reference one, except for the heterogeneity of WC grains, which is obviously due to the granulometric inhomogeneity of WC powder. Likewise, from the analysis of the data reported in Table 4, it can be observed that, the addition of Cr3C2 made it possible to increase the value of Vickers hardness, achieving the highest values among all the manufactured specimens, coherently with the good quality of the specimens in terms of porosity and structure. The slight difference between the measured and the reference values was still mainly due to the inhomogeneity of WC grain size distribution. With reference to the relative density and percentage of magnetic Co, the measured values confirmed the good quality of the specimen. 19 ACS Paragon Plus Environment


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Conclusions The results obtained showed that sound assisted fluidization is a very promising technique to be

used for the mixing phase in the production process of sintered hard metal pieces. Indeed, it was possible to obtain sintered specimens whose structural characteristics are very similar to the reference ones. This mixing method has the major advantage of remarkably increasing the process efficiency in terms of a strong reduction of duration: conventional mixing techniques typically require 10 – 60 h, whereas the proposed technology is capable of efficiently mixing the powders in about 1 – 2h. The effect of two operating variable, mixing temperature and shaping pressure, was studied aiming at improving the quality of the final products in terms of reduced porosity and enhanced density and hardness. In particular, the specimens crafted using either a mixing temperature of 70 °C or a shaping pressure of 480 bar (without PEG) are very similar from a structural point of view and also in terms of relative density and Vickers hardness. Finally, another PEG-free specimen was crafted adding an inhibitor (Cr3C2) the mixture. The results obtained confirmed the absence of abnormal grain growth phenomena. In conclusion, acting on either the mixing temperature or shaping pressure and adding an inhibitor to the mixture, it was possible to remarkably enhance the quality of the final products, obtaining sintered specimens very similar to the reference one. The further improvement of the proposed production process could involve the reduction of the inherent inhomogeneity of WC grain size distribution, responsible of the residual micro-porosity. 5.

Acknowledgements The authors acknowledge Mr. Luciano Cortese for SEM/EDS analysis and Mr. Dino Stanzione

and Dr. Maria Rosaria Scotto di Vettimo for ICP/MS analysis. The Authors acknowledge Mr. Ranesh Thanikasalam for his support in the experimentation. The Authors also acknowledge Mr. Vito Campagnuolo and Nashira Hard Metals s.r.l. for providing the materials and equipment for the specimen characterization. 20 ACS Paragon Plus Environment

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Figure caption Figure 1. Cumulative size distribution of WC, Co, PEG and Cr3C2 powders with and without ultrasound application. Figure 2. Experimental apparatus: (1) N2 cylinder; (2) flow meter; (3) mass flow controller; (4) 40mm ID fluidization column; (5) microphone; (6) sound guide; (7) wind-box; (8) pressure transducer; (9) loudspeaker; (10) stack; (11) thermocouple; (12) temperature controller; (13) heating jacket. Figure 3. SEM images of the powders: A) WC; B) Co; C) PEG; D) Cr3C2. Figure 4. Fluidization curves of WC powder. a) ordinary conditions; b) effect of SPL at fixed frequency (80 Hz); c) effect of frequency at fixed SPL (145 dB). Figure 5. Trends of ∆Pmax/∆P0 and umf as functions of: A) SPL at fixed frequency (80 Hz); B) frequency at fixed SPL (145 dB). Figure 6. Time evolution of the Co mean concentration in the mixture for A, B and C mixing tests. Figure 7. A mixing test. SEM images of WC and Co aggregates and their composition (sample taken after 2 minutes of mixing). Figure 8. Time evolution of the coefficient of variation (σ*) for A, B and C mixing tests. Figure 9. Photograph of a sintered specimen. Figure 10. 50x (A) and 1500x (B) magnification images of the reference specimen manufactured according to a standard cycle of Nashira Hard Metals. Figure 11. 50x magnification images of the sintered specimens manufactured using different mixing temperatures. 24 ACS Paragon Plus Environment

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Figure 12. 1500x magnification images of the sintered specimens manufactured with PEG and using different mixing temperatures. Figure 13. 50x magnification images of the sintered specimens manufactured without PEG and using different shaping pressures. Figure 14. 1500x magnification images of the sintered specimens manufactured without PEG and using different shaping pressures. Figure 15. 50x (A) and 1500x (B) magnification images of the specimen manufactured without PEG and adding the inhibitor (Cr3C2) to the mixture.



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Figures

Figure 1. Experimental apparatus: (1) N2 cylinder; (2) flow meter; (3) mass flow controller; (4) 40mm ID fluidization column; (5) microphone; (6) sound guide; (7) wind-box; (8) pressure transducer; (9) loudspeaker; (10) stack; (11) thermocouple; (12) temperature controller; (13) heating jacket.


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


cumulative size distribution, %

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100

80 WC noUS WC US Co noUS Co US PEG noUS PEG US Cr3C2 noUS

60

40

Cr3C2 US

20

0 0.01

0.1

1

10

100

1000

10000

d, µm Figure 2. Cumulative size distribution of WC, Co, PEG and Cr3C2 powders with and without ultrasound application.



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Figure 3. SEM images of the powders: A) WC; B) Co; C) PEG; D) Cr3C2.


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1.0

∆P/∆ ∆P0, -

0.8

A

0.6

0.4

0.2

0.0 0

2

4

6

8

10

u, cm/s 1.0

∆P/∆ ∆P0, -

0.8

B 130dB 135dB 140dB 145dB 150dB

0.6

0.4

0.2

0.0 0

2

4

6

8

u, cm/s 1.0

0.8

∆P/∆ ∆P0, -

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


C 20Hz 40Hz 80Hz 100Hz 120Hz 150HZ

0.6

0.4

0.2

0.0 0

2

4

6

8

u, cm/s Figure 4. Fluidization curves of WC powder. a) ordinary conditions; b) effect of SPL at fixed frequency (80 Hz); c) effect of frequency at fixed SPL (145 dB). 29 ACS Paragon Plus Environment


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Figure 5. Trends of ∆Pmax/∆P0 and umf as functions of: A) SPL at fixed frequency (80 Hz); B) frequency at fixed SPL (145 dB).


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30

Co concentration, %wt

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


A mixing test B mixing test C mixing test Theoretical Co concentration

25

20

15

10

5

0 0

50

100

150

200

t, min Figure 6. Time evolution of the Co mean concentration in the mixture for A, B and C mixing tests.



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Figure 7. A mixing test. SEM images of WC and Co aggregates and their composition (sample taken after 2 minutes of mixing).


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1.8 A mixing test B mixing test C mixing test

1.6

*

Coefficient of variation (σ σ ), -

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 0

50

100

150

200

t, min Figure 8. Time evolution of the coefficient of variation (σ*) for A, B and C mixing tests.



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Figure 9. Photograph of a sintered specimen.


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Figure 10. 50x (A) and 1500x (B) magnification images of the reference specimen manufactured according to a standard cycle of Nashira Hard Metals.



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Figure 11. 50x magnification images of the sintered specimens manufactured using different mixing temperatures.


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Figure 12. 1500x magnification images of the sintered specimens manufactured with PEG and using different mixing temperatures.



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Figure 13. 50x magnification images of the sintered specimens manufactured without PEG and using different shaping pressures.


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Figure 14. 1500x magnification images of the sintered specimens manufactured without PEG and using different shaping pressures.



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Figure 15. 50x (A) and 1500x (B) magnification images of the specimen manufactured without PEG and adding the inhibitor (Cr3C2) to the mixture.


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Table caption

Table 1. Properties of the reference specimen manufactured with the addition of Cr3C2 and using the standard cycle of Nashira Hard Metals s.r.l. Table 2. Properties of the specimens manufactured with PEG and using different mixing temperatures. Table 3. Properties of the specimens manufactured without PEG and using different shaping pressures. Table 4. Properties of the specimen manufactured without PEG and adding the inhibitor (Cr3C2) to the mixture.



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Tables Table 1. Properties of the reference specimen manufactured with the addition of Cr3C2 and using the standard cycle of Nashira Hard Metals s.r.l. Relative density

Vickers hardness (HV)

Magnetic Co

(g/cm3)

Mean

Standard Dev.

(%wt)

14.51

1455

±36

9.7


Page 43 of 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 2. Properties of the specimens manufactured with PEG and using different mixing temperatures. Vichers hardness (HV)

Mixing temperature

Relative density

(°C)

(g/cm3)

Mean

Standard Dev.

(%wt)

20

13.55

1172

±245

8.9

50

13.59

1194

±203

9.7

60

13.95

1252

±201

9.1

70

14.31

1337

±25

9.7

Magnetic Co



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Table 3. Properties of the specimens manufactured without PEG and using different shaping pressures. Vichers hardness (HV)

Shaping pressures

Relative density

(bar)

(g/cm3)

Mean

Standard Dev.

(%wt)

400

14.56

1302

±33

9.3

440

14.48

1313

±13

9.3

480

14.51

1335

±14

9.3

Magnetic Co


Page 45 of 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 4. Properties of the specimens manufactured without PEG and adding the inhibitor (Cr3C2) to the mixture. Relative density

Vichers hardness (HV)

Magnetic Co

(g/cm3)

Mean

Standard Dev.

(%wt)

14.41

1372

±26

9.1



Page 46 of 46

For Table of Contents Only


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