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Injection molding of tungsten powder treated by jet mill with high powder loading: A solution for fabrication of dense tungsten component at relative low temperature
Int. Journal of Refractory Metals and Hard Materials 62 (2017) 42–46
Contents lists available at ScienceDirect
Int. Journal of Refractory Metals and Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
Injection molding of tungsten powder treated by jet mill with high powder loading: A solution for fabrication of dense tungsten component at relative low temperature Rui Li, Mingli Qin ⁎, Chengcheng Liu, Hua Huang, Huifeng Lu, Pengqi Chen, Xuanhui Qu School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China
a r t i c l e
i n f o
Article history: Received 22 July 2016 Received in revised form 16 October 2016 Accepted 16 October 2016 Available online 17 October 2016 Keywords: Tungsten powder Jet mill Injection molding Powder loading
a b s t r a c t The greater demands on final product quality of powder metallurgy processing have led to an increased demand for metal powders with high quality. In this paper we report the successful preparation of tungsten powder with a near-spherical shape and surface morphology by jet milling process, with the disappearance of agglomeration and the improvement of dispersion, narrow particle size distribution of tungsten powder was achieved. The values of D10, D50 and D90 changed from 2.02 μm, 3.67 μm and 6.34 μm to the original tungsten powder to 1.36 μm, 2.13 μm and 3.19 μm for the treated tungsten powder, respectively. The process increases apparent density and tap density from 3.71 g/cm3, 5.54 g/cm3 to 5.71 g/cm3, 8.47 g/cm3. Due to the optimization of the powder characteristics, the powder loading of the feedstock was raised to 65 vol% for the treated powder. The sintered samples of 65 vol% powder loading is easy to get sinter densification with no cracks and has superior hardness and microstructure at a relative low temperature of 1900 °C. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction Tungsten plays an important role in many technology fields such as atomic energy, military, electro vacuum, crucible, heating element and other ultra-high temperature applications owing to its very high melting point (3390–3423 °C), the highest boiling point (5700 ± 200 °C), the lowest vapor pressure and low thermal expansivity. Due to the mechanical properties (hardness and strength) of tungsten, small dimensions of complicated shape tungsten parts cannot be processed by traditional metal working processes (e.g. milling or casting/pressing). Metal injection molding (MIM) has been established as a competitive manufacturing process for small components with high consistency and low cost over the past decades [1–3]. Thus, Metal injection molding (MIM) as a way for time and cost effective net shape fabrication has been developed for tungsten. Due to its high melting point, traditional sintering cycles for tungsten tend to rely on high temperatures and long time to induce densification, evaporate impurities, and reduce oxides. The rather high sintering temperature and long holding time means big grain size, poor mechanical properties and high energy consumption. Many efforts have been reported to improve the sinterability of tungsten for the purpose of decreasing the sintering temperature. Kiran employed high energy mechanical milling to produce fine tungsten powders and achieved 98% theoretical density after sintering at 1790 °C [4]. The mechanism of ⁎ Corresponding author. E-mail address: [email protected] (M. Qin).
http://dx.doi.org/10.1016/j.ijrmhm.2016.10.015 0263-4368/© 2016 Elsevier Ltd. All rights reserved.
MA was refinement of the powder to increase the sintering activity. The mechanical alloying method is apt to bring about impurities, and after MA the powder was sheared and compressed broken into pieces, which is not suitable for powder injection molding and cannot get high powder loading. Zhou used SHS assisted hot-shock consolidation to produce pure tungsten with the highest relative density of 96.7% and no cracks [5]. Wang used high-energy milling and pressed at rather high temperature to gain high green density which lead to densification at rather low temperature [6]. Superfine powder, advanced sintering method and high green density are the effective way to achieve low temperature densification. Nano powders with low packing density, prone to contamination and high price are not suitable for MIM nowadays. SPS, FAST and microwave sintering are not available for MIM industries for mass production particularly its complicate shape. Our previous work shows the pure tungsten compacts with complex shape were fabricated by metal injection molding (MIM), after sintering at 2300 °C for 180 min in pure hydrogen atmosphere, the relative density of pure tungsten part can reach 92.86%, the powder loading only can reach 54% for 3 μm tungsten powders [7]. Mamen used three different tungsten powders showed the final relative density of sintered W components produced by MIM depends mostly on the particle size of the used powders, the fine W powder component with 0.4 μm reached the highest sintered density (90– 94%) at a much reduced temperature of 1700 °C and a short holding time of 5 min, at the same time the W powder component with 3 μm and 7 μm reached 65% and 54% [8]. Antusch adopted 50% fine (0.7 μm) and 50% coarse powder (1.7 μm), and tungsten feedstock with 50 vol%
R. Li et al. / Int. Journal of Refractory Metals and Hard Materials 62 (2017) 42–46
solid loading was obtained. The successful finished samples showed a sintered density of approximately 99% and a hardness of 457HV0.1 [9]. Suri compared as-received and rod-milled tungsten powders mixed in a paraffin wax/polypropylene binder, respectively. The results show that the feedstock prepared with rod-milled tungsten powder possesses higher solids volume fraction, lower mixing torques, and improved homogeneity compared with as-received tungsten powder, emphasizing the importance of particle characteristics and mixing procedures in the PIM process [10]. Liu reported that a solid loading up to 64 vol% by a prior ball milling treatment of alumina powders with a small amount of SA before the traditional blending process, the agglomeration of the particles reduced by the surface modification of SA which effectively enhanced powder loading [11]. One of the key issues is to obtain a high solid loading in the feedstock system. Actually, there are three important issues related to the powder loading in metal injection molding process. The first is the excellent feedstock rheological properties for successful molding of intricate and delicate parts. The second is the small compact distortion and tight dimension tolerance control. The last is the good mechanical properties [12]. Traditional tungsten is difficult to achieve high loading capacity and difficult to sintering mainly due to the commercial tungsten powder is usually highly agglomerated. Agglomerates of tungsten, formed during the powder production stage, constitute particles sintered to form aggregates which remains in the powder-binder mixture, it can cause defects in the sintered parts and reduce the powder sinterability and mechanical properties of the final components. The pores among the aggregates are filled with binder which lead to low powder loading at mixing procedure and difficult to densification at sintering stage. Thus, a pretreatment of the appropriate of the applied tungsten powder is necessary. Jet milling is an innovative method developed in the last two decades that has many advantages such as the ability to produce microsized particles with a narrow particle size distribution, the absence of contamination, a low wear rate and noise level, and a small footprint [13–15]. The material being ground is itself the milling media which is very suitable for the processing of brittle powder, and minimal additional heat will be generated during the process by the gas. Jet milling process as a dry method, which is stable and suitable for mass production, has been taken for our experiments. Zeep reported that a tungsten feedstock with an optimised solid load of 55 vol.% lead to a sintered density of approximately 96% and a grain size of approximately 18um. Particularly the powders with a particle size b 1.9 μm FSSS are mainly build of sponge like agglomerates with a primary grain size of approximately 0.2 μm which is hard to dispersed [16]. So we choose the tungsten powder of 3 μm FSSS for the following experiments. Although jet milling has many advantages, the cohesive forces, especially the electrostatic attraction caused by friction of particles during jet milling process, may lead to the formation of aggregates that offset the
43
advantages of jet milling to a certain extent. To exploit its applications, it then becomes necessary to effectively reduce the aggregates and to enhance the dispersion of the powder after jet milling. Yin prepared micro-sized powder through the combination of jet milling and electrostatic dispersion, the method achieved the dual goal of producing fine powders and concurrently maintaining the dispersion of the powder product [17]. In the present work, tungsten powder with average size of 3 μm was used for our experiments, we found that the prior jet milling treatment resulted in deagglomeration of tungsten powder with near spherical morphology which led narrower particle size distribution, better flowability and higher packing and tap density. Powder and binder were mixed in a very short period of time and the feedstock with a higher solid loading up to 65% was obtained. The successful sintered samples showed a density of approximately 97.3%, a hardness of 496HV0.1 and a dense micro-structure with no cracks. 2. Experimental procedure 2.1. Powder Commercially available as-received tungsten powder with average size of 3μm, including the impurity elements of 0.02 wt.% C, 0.0005 wt.% Si, 0.0005 wt.% Al, 0.0005 wt.% Fe, 0.0005 wt.% V, was sourced from Xiamen Golden Egret Special Alloy Co. Ltd. 2.2. Experimental procedures Jet milling process was comminuted in high purity nitrogen by QLMR-150T fluidized bed jet mill equipped with a forced vortex classifier. The grinding process was determined by the operational parameters include the feed quantity, the grinding gas pressure, the rotating speed which decides the classifier frequency. In the process, the feed mass was kept constant at 5 kg, the grinding pressure of 0.70 Mpa, the rotating speed was set at 4200 rpm. Then tungsten powder was mixed in a SK-160 open mixer with a wax-based thermoplastic binder containing paraffin wax (PW), high density polyethylene (HDPE), polypropylene (PP) and stearic acid (SA) at the temperature of 165 °C for 120 min. After compounding, the feedstock was granulated in a PSJ32 granulator. Molding was performed with a CJ80-E injection molding machine. Debinding was taken in a two-step procedure starting with the solvent debinding in heptane and followed by the thermal debinding step. High-purity hydrogen atmosphere which play roles of reduction and purification was chosen as the sintering atmosphere for sintering, and samples were sintered in a tungsten coil furnace. 2.3. Experimental characterization The particle size distribution was measured by using a LMS-30 laser particle size analyzer. The packing density, tap density were tested by
Fig. 1. SEM photograph of surface morphology of tungsten powders (a) as-received and (b) treated.
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R. Li et al. / Int. Journal of Refractory Metals and Hard Materials 62 (2017) 42–46
Fig. 2. Particle size distributions of tungsten powders (a) as-received and (b) treated.
MT-1000 Multifunctional powder physical properties tester. The phase identifications of the tungsten powder were investigated by a Siemens D5000 X-ray diffraction meter. Microstructural observation was carried out on a LEO-1450 scanning electron microscope. Vickers hardness was tested on a M-400-HI hardness testing machine. The density of the sintered samples was calculated by Archimedes' principle.
3. Results and discussion 3.1. Particle surface morphology Fig. 1 shows scanning micrographs of surface morphology both asreceived (a) and the treated (b) tungsten powder. It can be seen from Fig. 1 that the agglomeration of the initial powder disappeared and irregular edges of the original tungsten particle became smooth after jet milling treatment, the impacting actions between the tungsten powder particles made the particle surface morphology tend to be nearly spherical. The dispersed tungsten powder with nearly spherical shape has good flowability, which is good for the uniformity of the mixing and injection molding process.
3.2. Particle size distribution and specific surface area The particle size distributions of as-received and the treated (after jet milled) tungsten powder are shown in Fig. 2. It is clear that the process of jet milling has a great impact on the particle size distribution. The size distribution (D10, D50, D90) and specific surface area before and after jet milling for tungsten powders are shown in Table 1. The particle size decreased and the size distribution was narrowed by the jet milling process. The raw particles are accelerated by means of a jet stream of high purity nitrogen, and mechanical impact collision drastically occurs among inter-particles from the same nozzle jet with different speed or inter-particles from different nozzle jets, resulting eventually in tungsten particles with a good dispersion and shape. The specific surface area automatically calculated by the particle size distributions have also increased noticeably under the jet milling treatment.
3.3. Phase constitution and impurity contents The phase constitution analysis results show that, before and after jet milling treatment, there is no distinct difference for the powder samples. The clear diffraction peaks suggest that the ground powders consist of pure W in Fig. 3. For the jet mill grinding, the particles are accelerated in nitrogen gas jet stream and reduced in size mainly during the inter particle collision. Large quantities of powder can be handled in very short time, and the oxygen content is about 0.033 wt.% for the treated powder with contrast to the initial content of 0.041 wt.%, indicating that the process is free of contamination.
3.4. Effect of jet milling of tungsten powder on solid loading of MIM Powder loading is one of the most critical factors which has important influence on metal injection molding process. In general, it is expected that MIM feedstock has high powder loading. Higher powder loading means small compact volume shrinkage and precisely dimension tolerance control, which is very important for the mass production of complex and delicate MIM parts. But too high powder loading is also unacceptable because it will lead to too high feedstock viscosity and result in the failure of injection molding. Metal powder characteristics, especially the average particle size and size distribution, the shape, and the surface morphology, all have a great influence on final products formed using the metal injection molding (MIM) process. The ideal
Table 1 Size distribution and specific surface area before and after jet mill. Powders
D10 (um)
D50 (um)
D90 (um)
Specific surface area (m2/g)
As-received Treated
2.02 1.79
4.43 3.02
9.95 5.54
1.754 2.569
Fig. 3. XRD pattern of tungsten powders (a) as-received and (b) treated.
R. Li et al. / Int. Journal of Refractory Metals and Hard Materials 62 (2017) 42–46 Table 2 Packing density, tap density and relative density of the sintered tungsten samples. Sintered samples
Packing density (g/cm3)
Tap density (g/cm3)
Relative density (%)
Made from the as-received powder Made from the treated powder
3.71 5.54
5.71 8.47
79.0 97.3
powder suitable for injection molding is average particle size below 20 μm, nearly spherical with a narrow particle distribution, no agglomeration and high packing density is also important for both reliable process control and high product performance. Table 2 shows the packing density, tap density and tightness of the sintered tungsten samples. The jet milling process increases packing density of tungsten powder from 3.71 g/cm3 to 5.54 g/cm3, the tap density from 5.71 g/cm3 to 8.47 g/cm3. Fig. 4 shows the relationship between feedstock density and powder solid loading of tungsten powder. Initially, the measured density follows the theoretical density line. Then at a certain point, there is a deviation of the measured density from the theoretical density line. This deviation point is then considered the critical solids loading [3]. The increase of critical powder loading from 54 vol% for as-received powder to 65 vol% of the treated powder. The effective treatment for the original powder is the key factor for achieving a suitable feedstock with high loading capacity. The prior jet milling treatment can result in dispersing of tungsten powders with near-spherical shape which lead to narrow particle size distribution, better flowability and high packing density, lead to a high powder loading.
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3.5. Effect of jet milling of tungsten powder on structure and hardness of sintered samples Sintering densification is sustained only as long as the pores remain coupled with the grain boundaries [18]. As the pores shrink their pinning and attachment diminish, leading to stranded pores and rapidly growing grain boundaries. Sintered tungsten from the treated powder shows higher sintered densification (97.3%), when compared to the sintered samples made of the as-received powder (79.0%). The enhanced sintering density during jet milling process can be explained as follows: (1) removing the agglomerates content could improve the properties of the feedstock, green bodies and sintered parts, a decrease in the amount of agglomerates increases the solid loadings and green density, which lead to densification at lower temperature with less grain growth; (2) removing the agglomerates content could eliminate the tungsten powder internal cavities and fractures, the existence of internal porosity of powder lead to the poor sinterability and need high temperature and long holding time; (3) jet milling process results in creation of new surfaces and thereby a higher surface energy of the powders, at the same time high-intensity energy delivered by the jet milling onto the particles also increases the activity of powder. Fig. 5 represents the comparison of microstructure after sintering, the sample made from the as-received tungsten powder has more irregular pores, micro-cracks and massive crystallites in Fig. 5(a), while the sintered sample made from the treated (after jet milled) tungsten powder has a homogeneous dense microstructure with no cracks. The tightness and the grain size of sintered product has direct influence on its mechanical performance. At the same time, Vickers hardness of 496HV0.1 was obtained in the treated samples. In contrast, the initial samples only reach Vickers hardness of 326HV0.1.
4. Conclusions
Fig. 4. Feedstock density versus powder loading of tungsten powders.
Tungsten particles with near spherical shape and surface morphology have been prepared by jet milling process. Jet milling process which is suitable for tungsten powder significantly changed the tungsten powder characteristics including the size distribution, the surface morphology, the specific surface area, the flowability, and the packing and tap density. The mixing of powder and binder in a very short period of time enhanced the dispersion, obtained the feedstock with a higher solid loading up to 65%, all leading to an improved sintered product quality. The energy delivered from jet milling process, creation of new surfaces and thereby a higher surface energy of the powders also promoted the sintering process. After sintered at a relative low temperature of 1900 °C for 120 min, the samples sintered from the treated (after jet milled) powder achieved the highest density of 97.3% and a Vickers hardness of 496HV0.1, which was much superior to those products made from as-received tungsten powder.
Fig. 5. Microstructures of W components sintered from (a) as-received and (b) treated powder.
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Acknowledgments This work is financially supported by the National Natural Science Foundation of China (51574031) and (51574029), the Natural Science Foundation of Beijing (2162027), the National 863 Program (2013AA031101), and the Fundamental Research Funds for the Central Universities (06109063). References [1] V. Piotter, B. Zeep, P. Norajitra, R. Ruprecht, A. von der Weth, J. Hausselt, Development of a powder metallurgy process for tungsten components, Fusion Eng. Des. 83 (2008) 1517–1520. [2] D.T. Blagoeva, J. Opschoor, J.G. van der Laan, C. Sârbu, G. Pintsuk, M. Jong, T. Bakker, P. Ten Pierick, H. Nolles, Development of tungsten and tungsten alloys for DEMO divertor applications via MIM technology, J. Nucl. Mater. 442 (2013) S198–S203. [3] R.M. German, A. Bose, Injection Molding of Metals and CeramicsMPIF 1997. [4] U. Ravi Kiran, M. Prem Kumar, M. Sankaranarayana, A.K. Singh, T.K. Nandy, High energy milling on tungsten powders, Int. J. Refract. Met. Hard Mater. 48 (2015) 74–81. [5] Q. Zhou, P. Chen, Fabrication and characterization of pure tungsten using the hotshock consolidation, Int. J. Refract. Met. Hard Mater. 42 (2014) 215–220. [6] X. Wang, Z. Zak Fang, M. Koopman, The relationship between the green density and as-sintered density of nano-tungsten compacts, Int. J. Refract. Met. Hard Mater. 53 (2015) 134–138. [7] T.G. Luo, X.H. Qu, M.L. Qin, M.L. Ouyang, Dimension precision of metal injection molded pure tungsten, Int. J. Refract. Met. Hard Mater. 27 (2009) 615–620. [8] B. Mamen, J. Song, T. Barriere, J. Gelin, Experimental and numerical analysis of the particle size effect on the densification behaviour of metal injection moulded tungsten parts during sintering, Powder Technol. 270 (2015) 230–243.
[9] S. Antusch, P. Norajitra, V. Piotter, H. Ritzhaupt-Kleissl, L. Spatafora, Powder injection molding – an innovative manufacturing method for He-cooled DEMO divertor components, Fusion Eng. Des. 86 (2011) 1575–1578. [10] P. Suri, S.V. Atre, R.M. German, J.P. de Souza, Effect of mixing on the rheology and particle characteristics of tungsten-based powder injection molding feedstock, Mater. Sci. Eng. A 356 (2003) 337–344. [11] W. Liu, Z. Xie, T. Bo, X. Yang, Injection molding of surface modified powders with high solid loadings: a case for fabrication of translucent alumina ceramics, J. Eur. Ceram. Soc. 31 (2011) 1611–1617. [12] Y. Li, L. Li, K.A. Khalil, Effect of powder loading on metal injection molding stainless steels, J. Mater. Process. Technol. 183 (2007) 432–439. [13] X. Lu, C. Liu, L. Zhu, X. Qu, Influence of process parameters on the characteristics of TiAl alloyed powders by fluidized bed jet milling, Powder Technol. 254 (2014) 235–240. [14] M. Djokić, K. Kachrimanis, L. Solomun, J. Djuriš, D. Vasiljević, S. Ibrić, A study of jetmilling and spray-drying process for the physicochemical and aerodynamic dispersion properties of amiloride HCl, Powder Technol. 262 (2014) 170–176. [15] N.V. Rama Rao, G.C. Hadjipanayis, Influence of jet milling process parameters on particle size, phase formation and magnetic properties of MnBi alloy, J. Alloys Compd. 629 (2015) 80–83. [16] B. Zeep, P. Norajitra, V. Piotter, J. Boehm, R. Ruprecht, J. Hausselt, Net shaping of tungsten components by micro powder injection moulding, Fusion Eng. Des. 82 (2007) 2660–2665. [17] P. Yin, R. Zhang, Y. Li, N. Li, J. Hu, The charging efficiency and flow dynamics of micropowder during jet milling/electrostatic dispersion, Powder Technol. 256 (2014) 450–461. [18] R.M. German, Sintering: From Empirical Observations to Scientific Principles, 2014.
Contents lists available at ScienceDirect
Int. Journal of Refractory Metals and Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
Injection molding of tungsten powder treated by jet mill with high powder loading: A solution for fabrication of dense tungsten component at relative low temperature Rui Li, Mingli Qin ⁎, Chengcheng Liu, Hua Huang, Huifeng Lu, Pengqi Chen, Xuanhui Qu School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China
a r t i c l e
i n f o
Article history: Received 22 July 2016 Received in revised form 16 October 2016 Accepted 16 October 2016 Available online 17 October 2016 Keywords: Tungsten powder Jet mill Injection molding Powder loading
a b s t r a c t The greater demands on final product quality of powder metallurgy processing have led to an increased demand for metal powders with high quality. In this paper we report the successful preparation of tungsten powder with a near-spherical shape and surface morphology by jet milling process, with the disappearance of agglomeration and the improvement of dispersion, narrow particle size distribution of tungsten powder was achieved. The values of D10, D50 and D90 changed from 2.02 μm, 3.67 μm and 6.34 μm to the original tungsten powder to 1.36 μm, 2.13 μm and 3.19 μm for the treated tungsten powder, respectively. The process increases apparent density and tap density from 3.71 g/cm3, 5.54 g/cm3 to 5.71 g/cm3, 8.47 g/cm3. Due to the optimization of the powder characteristics, the powder loading of the feedstock was raised to 65 vol% for the treated powder. The sintered samples of 65 vol% powder loading is easy to get sinter densification with no cracks and has superior hardness and microstructure at a relative low temperature of 1900 °C. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction Tungsten plays an important role in many technology fields such as atomic energy, military, electro vacuum, crucible, heating element and other ultra-high temperature applications owing to its very high melting point (3390–3423 °C), the highest boiling point (5700 ± 200 °C), the lowest vapor pressure and low thermal expansivity. Due to the mechanical properties (hardness and strength) of tungsten, small dimensions of complicated shape tungsten parts cannot be processed by traditional metal working processes (e.g. milling or casting/pressing). Metal injection molding (MIM) has been established as a competitive manufacturing process for small components with high consistency and low cost over the past decades [1–3]. Thus, Metal injection molding (MIM) as a way for time and cost effective net shape fabrication has been developed for tungsten. Due to its high melting point, traditional sintering cycles for tungsten tend to rely on high temperatures and long time to induce densification, evaporate impurities, and reduce oxides. The rather high sintering temperature and long holding time means big grain size, poor mechanical properties and high energy consumption. Many efforts have been reported to improve the sinterability of tungsten for the purpose of decreasing the sintering temperature. Kiran employed high energy mechanical milling to produce fine tungsten powders and achieved 98% theoretical density after sintering at 1790 °C [4]. The mechanism of ⁎ Corresponding author. E-mail address: [email protected] (M. Qin).
http://dx.doi.org/10.1016/j.ijrmhm.2016.10.015 0263-4368/© 2016 Elsevier Ltd. All rights reserved.
MA was refinement of the powder to increase the sintering activity. The mechanical alloying method is apt to bring about impurities, and after MA the powder was sheared and compressed broken into pieces, which is not suitable for powder injection molding and cannot get high powder loading. Zhou used SHS assisted hot-shock consolidation to produce pure tungsten with the highest relative density of 96.7% and no cracks [5]. Wang used high-energy milling and pressed at rather high temperature to gain high green density which lead to densification at rather low temperature [6]. Superfine powder, advanced sintering method and high green density are the effective way to achieve low temperature densification. Nano powders with low packing density, prone to contamination and high price are not suitable for MIM nowadays. SPS, FAST and microwave sintering are not available for MIM industries for mass production particularly its complicate shape. Our previous work shows the pure tungsten compacts with complex shape were fabricated by metal injection molding (MIM), after sintering at 2300 °C for 180 min in pure hydrogen atmosphere, the relative density of pure tungsten part can reach 92.86%, the powder loading only can reach 54% for 3 μm tungsten powders [7]. Mamen used three different tungsten powders showed the final relative density of sintered W components produced by MIM depends mostly on the particle size of the used powders, the fine W powder component with 0.4 μm reached the highest sintered density (90– 94%) at a much reduced temperature of 1700 °C and a short holding time of 5 min, at the same time the W powder component with 3 μm and 7 μm reached 65% and 54% [8]. Antusch adopted 50% fine (0.7 μm) and 50% coarse powder (1.7 μm), and tungsten feedstock with 50 vol%
R. Li et al. / Int. Journal of Refractory Metals and Hard Materials 62 (2017) 42–46
solid loading was obtained. The successful finished samples showed a sintered density of approximately 99% and a hardness of 457HV0.1 [9]. Suri compared as-received and rod-milled tungsten powders mixed in a paraffin wax/polypropylene binder, respectively. The results show that the feedstock prepared with rod-milled tungsten powder possesses higher solids volume fraction, lower mixing torques, and improved homogeneity compared with as-received tungsten powder, emphasizing the importance of particle characteristics and mixing procedures in the PIM process [10]. Liu reported that a solid loading up to 64 vol% by a prior ball milling treatment of alumina powders with a small amount of SA before the traditional blending process, the agglomeration of the particles reduced by the surface modification of SA which effectively enhanced powder loading [11]. One of the key issues is to obtain a high solid loading in the feedstock system. Actually, there are three important issues related to the powder loading in metal injection molding process. The first is the excellent feedstock rheological properties for successful molding of intricate and delicate parts. The second is the small compact distortion and tight dimension tolerance control. The last is the good mechanical properties [12]. Traditional tungsten is difficult to achieve high loading capacity and difficult to sintering mainly due to the commercial tungsten powder is usually highly agglomerated. Agglomerates of tungsten, formed during the powder production stage, constitute particles sintered to form aggregates which remains in the powder-binder mixture, it can cause defects in the sintered parts and reduce the powder sinterability and mechanical properties of the final components. The pores among the aggregates are filled with binder which lead to low powder loading at mixing procedure and difficult to densification at sintering stage. Thus, a pretreatment of the appropriate of the applied tungsten powder is necessary. Jet milling is an innovative method developed in the last two decades that has many advantages such as the ability to produce microsized particles with a narrow particle size distribution, the absence of contamination, a low wear rate and noise level, and a small footprint [13–15]. The material being ground is itself the milling media which is very suitable for the processing of brittle powder, and minimal additional heat will be generated during the process by the gas. Jet milling process as a dry method, which is stable and suitable for mass production, has been taken for our experiments. Zeep reported that a tungsten feedstock with an optimised solid load of 55 vol.% lead to a sintered density of approximately 96% and a grain size of approximately 18um. Particularly the powders with a particle size b 1.9 μm FSSS are mainly build of sponge like agglomerates with a primary grain size of approximately 0.2 μm which is hard to dispersed [16]. So we choose the tungsten powder of 3 μm FSSS for the following experiments. Although jet milling has many advantages, the cohesive forces, especially the electrostatic attraction caused by friction of particles during jet milling process, may lead to the formation of aggregates that offset the
43
advantages of jet milling to a certain extent. To exploit its applications, it then becomes necessary to effectively reduce the aggregates and to enhance the dispersion of the powder after jet milling. Yin prepared micro-sized powder through the combination of jet milling and electrostatic dispersion, the method achieved the dual goal of producing fine powders and concurrently maintaining the dispersion of the powder product [17]. In the present work, tungsten powder with average size of 3 μm was used for our experiments, we found that the prior jet milling treatment resulted in deagglomeration of tungsten powder with near spherical morphology which led narrower particle size distribution, better flowability and higher packing and tap density. Powder and binder were mixed in a very short period of time and the feedstock with a higher solid loading up to 65% was obtained. The successful sintered samples showed a density of approximately 97.3%, a hardness of 496HV0.1 and a dense micro-structure with no cracks. 2. Experimental procedure 2.1. Powder Commercially available as-received tungsten powder with average size of 3μm, including the impurity elements of 0.02 wt.% C, 0.0005 wt.% Si, 0.0005 wt.% Al, 0.0005 wt.% Fe, 0.0005 wt.% V, was sourced from Xiamen Golden Egret Special Alloy Co. Ltd. 2.2. Experimental procedures Jet milling process was comminuted in high purity nitrogen by QLMR-150T fluidized bed jet mill equipped with a forced vortex classifier. The grinding process was determined by the operational parameters include the feed quantity, the grinding gas pressure, the rotating speed which decides the classifier frequency. In the process, the feed mass was kept constant at 5 kg, the grinding pressure of 0.70 Mpa, the rotating speed was set at 4200 rpm. Then tungsten powder was mixed in a SK-160 open mixer with a wax-based thermoplastic binder containing paraffin wax (PW), high density polyethylene (HDPE), polypropylene (PP) and stearic acid (SA) at the temperature of 165 °C for 120 min. After compounding, the feedstock was granulated in a PSJ32 granulator. Molding was performed with a CJ80-E injection molding machine. Debinding was taken in a two-step procedure starting with the solvent debinding in heptane and followed by the thermal debinding step. High-purity hydrogen atmosphere which play roles of reduction and purification was chosen as the sintering atmosphere for sintering, and samples were sintered in a tungsten coil furnace. 2.3. Experimental characterization The particle size distribution was measured by using a LMS-30 laser particle size analyzer. The packing density, tap density were tested by
Fig. 1. SEM photograph of surface morphology of tungsten powders (a) as-received and (b) treated.
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R. Li et al. / Int. Journal of Refractory Metals and Hard Materials 62 (2017) 42–46
Fig. 2. Particle size distributions of tungsten powders (a) as-received and (b) treated.
MT-1000 Multifunctional powder physical properties tester. The phase identifications of the tungsten powder were investigated by a Siemens D5000 X-ray diffraction meter. Microstructural observation was carried out on a LEO-1450 scanning electron microscope. Vickers hardness was tested on a M-400-HI hardness testing machine. The density of the sintered samples was calculated by Archimedes' principle.
3. Results and discussion 3.1. Particle surface morphology Fig. 1 shows scanning micrographs of surface morphology both asreceived (a) and the treated (b) tungsten powder. It can be seen from Fig. 1 that the agglomeration of the initial powder disappeared and irregular edges of the original tungsten particle became smooth after jet milling treatment, the impacting actions between the tungsten powder particles made the particle surface morphology tend to be nearly spherical. The dispersed tungsten powder with nearly spherical shape has good flowability, which is good for the uniformity of the mixing and injection molding process.
3.2. Particle size distribution and specific surface area The particle size distributions of as-received and the treated (after jet milled) tungsten powder are shown in Fig. 2. It is clear that the process of jet milling has a great impact on the particle size distribution. The size distribution (D10, D50, D90) and specific surface area before and after jet milling for tungsten powders are shown in Table 1. The particle size decreased and the size distribution was narrowed by the jet milling process. The raw particles are accelerated by means of a jet stream of high purity nitrogen, and mechanical impact collision drastically occurs among inter-particles from the same nozzle jet with different speed or inter-particles from different nozzle jets, resulting eventually in tungsten particles with a good dispersion and shape. The specific surface area automatically calculated by the particle size distributions have also increased noticeably under the jet milling treatment.
3.3. Phase constitution and impurity contents The phase constitution analysis results show that, before and after jet milling treatment, there is no distinct difference for the powder samples. The clear diffraction peaks suggest that the ground powders consist of pure W in Fig. 3. For the jet mill grinding, the particles are accelerated in nitrogen gas jet stream and reduced in size mainly during the inter particle collision. Large quantities of powder can be handled in very short time, and the oxygen content is about 0.033 wt.% for the treated powder with contrast to the initial content of 0.041 wt.%, indicating that the process is free of contamination.
3.4. Effect of jet milling of tungsten powder on solid loading of MIM Powder loading is one of the most critical factors which has important influence on metal injection molding process. In general, it is expected that MIM feedstock has high powder loading. Higher powder loading means small compact volume shrinkage and precisely dimension tolerance control, which is very important for the mass production of complex and delicate MIM parts. But too high powder loading is also unacceptable because it will lead to too high feedstock viscosity and result in the failure of injection molding. Metal powder characteristics, especially the average particle size and size distribution, the shape, and the surface morphology, all have a great influence on final products formed using the metal injection molding (MIM) process. The ideal
Table 1 Size distribution and specific surface area before and after jet mill. Powders
D10 (um)
D50 (um)
D90 (um)
Specific surface area (m2/g)
As-received Treated
2.02 1.79
4.43 3.02
9.95 5.54
1.754 2.569
Fig. 3. XRD pattern of tungsten powders (a) as-received and (b) treated.
R. Li et al. / Int. Journal of Refractory Metals and Hard Materials 62 (2017) 42–46 Table 2 Packing density, tap density and relative density of the sintered tungsten samples. Sintered samples
Packing density (g/cm3)
Tap density (g/cm3)
Relative density (%)
Made from the as-received powder Made from the treated powder
3.71 5.54
5.71 8.47
79.0 97.3
powder suitable for injection molding is average particle size below 20 μm, nearly spherical with a narrow particle distribution, no agglomeration and high packing density is also important for both reliable process control and high product performance. Table 2 shows the packing density, tap density and tightness of the sintered tungsten samples. The jet milling process increases packing density of tungsten powder from 3.71 g/cm3 to 5.54 g/cm3, the tap density from 5.71 g/cm3 to 8.47 g/cm3. Fig. 4 shows the relationship between feedstock density and powder solid loading of tungsten powder. Initially, the measured density follows the theoretical density line. Then at a certain point, there is a deviation of the measured density from the theoretical density line. This deviation point is then considered the critical solids loading [3]. The increase of critical powder loading from 54 vol% for as-received powder to 65 vol% of the treated powder. The effective treatment for the original powder is the key factor for achieving a suitable feedstock with high loading capacity. The prior jet milling treatment can result in dispersing of tungsten powders with near-spherical shape which lead to narrow particle size distribution, better flowability and high packing density, lead to a high powder loading.
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3.5. Effect of jet milling of tungsten powder on structure and hardness of sintered samples Sintering densification is sustained only as long as the pores remain coupled with the grain boundaries [18]. As the pores shrink their pinning and attachment diminish, leading to stranded pores and rapidly growing grain boundaries. Sintered tungsten from the treated powder shows higher sintered densification (97.3%), when compared to the sintered samples made of the as-received powder (79.0%). The enhanced sintering density during jet milling process can be explained as follows: (1) removing the agglomerates content could improve the properties of the feedstock, green bodies and sintered parts, a decrease in the amount of agglomerates increases the solid loadings and green density, which lead to densification at lower temperature with less grain growth; (2) removing the agglomerates content could eliminate the tungsten powder internal cavities and fractures, the existence of internal porosity of powder lead to the poor sinterability and need high temperature and long holding time; (3) jet milling process results in creation of new surfaces and thereby a higher surface energy of the powders, at the same time high-intensity energy delivered by the jet milling onto the particles also increases the activity of powder. Fig. 5 represents the comparison of microstructure after sintering, the sample made from the as-received tungsten powder has more irregular pores, micro-cracks and massive crystallites in Fig. 5(a), while the sintered sample made from the treated (after jet milled) tungsten powder has a homogeneous dense microstructure with no cracks. The tightness and the grain size of sintered product has direct influence on its mechanical performance. At the same time, Vickers hardness of 496HV0.1 was obtained in the treated samples. In contrast, the initial samples only reach Vickers hardness of 326HV0.1.
4. Conclusions
Fig. 4. Feedstock density versus powder loading of tungsten powders.
Tungsten particles with near spherical shape and surface morphology have been prepared by jet milling process. Jet milling process which is suitable for tungsten powder significantly changed the tungsten powder characteristics including the size distribution, the surface morphology, the specific surface area, the flowability, and the packing and tap density. The mixing of powder and binder in a very short period of time enhanced the dispersion, obtained the feedstock with a higher solid loading up to 65%, all leading to an improved sintered product quality. The energy delivered from jet milling process, creation of new surfaces and thereby a higher surface energy of the powders also promoted the sintering process. After sintered at a relative low temperature of 1900 °C for 120 min, the samples sintered from the treated (after jet milled) powder achieved the highest density of 97.3% and a Vickers hardness of 496HV0.1, which was much superior to those products made from as-received tungsten powder.
Fig. 5. Microstructures of W components sintered from (a) as-received and (b) treated powder.
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Acknowledgments This work is financially supported by the National Natural Science Foundation of China (51574031) and (51574029), the Natural Science Foundation of Beijing (2162027), the National 863 Program (2013AA031101), and the Fundamental Research Funds for the Central Universities (06109063). References [1] V. Piotter, B. Zeep, P. Norajitra, R. Ruprecht, A. von der Weth, J. Hausselt, Development of a powder metallurgy process for tungsten components, Fusion Eng. Des. 83 (2008) 1517–1520. [2] D.T. Blagoeva, J. Opschoor, J.G. van der Laan, C. Sârbu, G. Pintsuk, M. Jong, T. Bakker, P. Ten Pierick, H. Nolles, Development of tungsten and tungsten alloys for DEMO divertor applications via MIM technology, J. Nucl. Mater. 442 (2013) S198–S203. [3] R.M. German, A. Bose, Injection Molding of Metals and CeramicsMPIF 1997. [4] U. Ravi Kiran, M. Prem Kumar, M. Sankaranarayana, A.K. Singh, T.K. Nandy, High energy milling on tungsten powders, Int. J. Refract. Met. Hard Mater. 48 (2015) 74–81. [5] Q. Zhou, P. Chen, Fabrication and characterization of pure tungsten using the hotshock consolidation, Int. J. Refract. Met. Hard Mater. 42 (2014) 215–220. [6] X. Wang, Z. Zak Fang, M. Koopman, The relationship between the green density and as-sintered density of nano-tungsten compacts, Int. J. Refract. Met. Hard Mater. 53 (2015) 134–138. [7] T.G. Luo, X.H. Qu, M.L. Qin, M.L. Ouyang, Dimension precision of metal injection molded pure tungsten, Int. J. Refract. Met. Hard Mater. 27 (2009) 615–620. [8] B. Mamen, J. Song, T. Barriere, J. Gelin, Experimental and numerical analysis of the particle size effect on the densification behaviour of metal injection moulded tungsten parts during sintering, Powder Technol. 270 (2015) 230–243.
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