- Email: [email protected]
Electron beam sintering of ceramics for additive manufacturing
Vacuum 169 (2019) 108933
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Electron beam sintering of ceramics for additive manufacturing a,∗
a
c
A.S. Klimov , I.Yu Bakeev , E.S. Dvilis , E.M. Oks a b c
a,b
, A.A. Zenin
T
a
Tomsk State University of Control Systems and Radioelectronics, Tomsk, Russia Institute of High Current Electronics, Russian Academy of Sciences, Tomsk, Russia Tomsk Polytechnic University, Tomsk, Russia
A B S T R A C T
We describe the results of electron-beam layer-by-layer sintering of a mixture of alumina powder and talc. An electron beam with high power density, generated by a forevacuum-pressure, plasma-cathode electron source, was used as heat source. A specific feature of the source is the option to generate an electron beam at pressure range 5–20 Pa. In this pressure range, a high density beam-plasma is formed in the electron beam transport region. Positive ions from this plasma neutralize the negative charge that the beam's electrons deposit to the surface of the target. Using an electron beam, the alumina/talc powder mixture was heated and sintering occurred at a temperature of 1350 °C. An additional layer of powder was subsequently poured onto the previous one and sintered in the same way. We formed an electron-beam-sintered 7-layer structure. We show that the porosity of the sintered layers depends on the layer number, with the least porous layers located in the upper part of the sample.
1. Introduction Additive technologies make possible complex-shaped 3D objects whose manufacture by conventional methods can be excessively material- and energy-consuming. Among such technologies are fused deposition modeling (FDM) [1], stereolithography (SLA or SLT) [2–4], laminated object manufacturing (LOM) [5], and selective laser sintering (SLS) [6–8] and melting (SLM) [9], all of which are successfully used in industry for manufacturing metal and metal alloy products. High-density ceramics are difficult to synthesize by additive methods because the material has a high melting point, low heat conductivity, and is brittle. Nevertheless some progress has been made in the field using SLS and SLM [10–12]. Note that in SLS and SLM, the efficiency of laser energy transfer strongly depends on the optical properties of the material, and for attaining the desired result with a ceramic, one should either increase the laser power or use special additives to enhance the material absorptivity [13]. Electron beams compare with laser beams in specific power, and the capability to independently control the electron beam current and energy makes electron beams attractive for welding [14], creating 3D objects from metals [15,16], alloys, and other current-conducting materials [17–20]. For efficient electron beam interaction with dielectric materials, and with ceramics in particular, one should provide efficient neutralization of the charge built up on their surface by accelerated electrons [21]. In forevacuum-pressure plasma-cathode electron beam sources [22], charge neutralization is attained due to the formation of a dense plasma in the region of electron beam transport [23]. The
∗
capabilities of such sources have been successfully demonstrated via the examples of electron beam welding, sintering, and surface modification of alumina and zirconium ceramics [24–26]. One possible approach to additive manufacturing of ceramic products can be layer-by-layer electron beam sintering of ceramic powders. Here we present research results which demonstrate the possibility of using electron beam sintering for additive manufacturing. 2. Experimental Fig. 1 shows a simplified schematic of the experimental arrangement for electron beam sintering of ceramic powder. Forevacuum plasma electron source 1 [27,28] produces a continuous electron beam of diameter 10 mm with current and electron energies in the range 2–100 mA and 1–20 keV, respectively at a pressure of 5–20 Pa. The test material is a layer of ceramic powder mixture 2 on pre-sintered substrate 3. The material is located in graphite crucible 5 and is sintered in the vacuum chamber at a residual pressure of 10 Pa. The temperature of the irradiated surface is measured with infrared pyrometer 6 (RAYTEK 1 MH) effective over the range 550–3000 °C. The electrode system of the electron source and its operational principles have been described in detail elsewhere [28]. For layer-by-layer sintering, we used Al2O3 powder, one of the most widespread ceramic materials in additive manufacturing. The powder was in two states differing in morphology: with fine particles of size 1–10 μm, and with particles of size up to 30 μm in dense granules sized to ≈100 μm (see Fig. 2). After sintering, we analyzed the morphology
Corresponding author. E-mail address: [email protected] (A.S. Klimov).
https://doi.org/10.1016/j.vacuum.2019.108933 Received 23 August 2019; Received in revised form 31 August 2019; Accepted 7 September 2019 Available online 08 September 2019 0042-207X/ © 2019 Published by Elsevier Ltd.
Vacuum 169 (2019) 108933
A.S. Klimov, et al.
Fig. 3. Electron beam power and surface temperature during sintering.
rate was 30 W/min, which allowed heat the powder to the required sintering temperature, region II in Fig. 3. According to pyrometer measurements, the sintering temperature was 1350 °C at a beam power of 700 W. The sintering temperature was set based on previously conducted experiments with alumina ceramics [30]. The sintering time at this temperature was 2 min, region III if Fig. 3. This exposure time was sufficient for sintering. Thereafter, the beam power was gradually decreased to zero at a rate of 45 W/min. The cooling rate at this level of power change was 55 deg/min, region IV in Fig. 3. At a higher cooling rate, the probability of deformation or cracking of sintered samples due to thermomechanical stresses increased. The next powder layer was applied after cooling of the previous layer and was sintered in the same way. Fig. 3 shows the electron beam power and surface temperature as a function of time.
Fig. 1. Schematic of the experimental arrangement: 1 – plasma electron source, 2 – powder mixture, 3 – pre-sintered substrate, 4 – vacuum chamber, 5 – graphite crucible, 6 – pyrometer, Ud – discharge power supply, Ua – accelerating voltage source.
of the specimens (upper surface layers, polished and cleaned lateral sections) on a JSM-7500FA scanning electron microscope (SEM), their elemental composition by energy dispersive X-ray spectroscopy (EDS) on a Hitachi S3400 N scanning electron microscope, and elastoplastic characteristics on a DUH-211S device. The optimum talc content in the ceramic was 30 wt%. This value was found experimentally. The powder on the substrate was applied with a special feeder and was leveled with a spatula without pressing. First, the powder layer was exposed to preliminary electron beam heating at a beam power of 250 W for 10 min (accelerating voltage of 5 kV, beam current of 50 mA, beam diameter in the region of exposure of 10 mm), область I на рис. 3. With such power of the electron beam and irradiation time, the powder was heated fairly uniformly. Electron beam exposure with a higher electron beam power led to the scattering of powder particles from the region of electron beam interaction due to the intensification of gas desorption from their surface [29]. Then the beam power was increased linearly to 400 W in 15 min by increasing the accelerating voltage. The rate of power increase in this case was 10 W/min and was selected experimentally. At such rate of increasing power, the volume of the powder warms up fairly evenly and there is no expansion of its particles since the adsorbed gas molecules manage to leave the surface of the powder particles located in the lower layers. When the powder heated to about 1000 °C the value of thermal radiation, determined by the Stefan-Boltzmann law and proportional to T4, from the heated surface of the powder became significant. This in turn requires a sharper increase in power. At this stage, the power increase
3. Results and discussion Fig. 4 shows the structure of the upper surface layer after electron beam sintering. As can be seen, the structure reveals melting and partial crystallization of the phases. On the surface one can distinguish alumina crystallites solidified in molten talc (melting temperature 1530 °C). The melting of one of the phases suggests that the temperature in the region of exposure is locally higher than the value measured by the pyrometer. The same was observed in silicon carbide specimens sintered by electron beam [31]. Obviously, more reliable methods compared to radiation pyrometers are needed to control the temperature of any material treated by electron beams. Our analysis of lateral sections shows that the specimen porosity depends not only on the initial powder morphology but also on the distance from the substrate. In all specimens, the first (lowest) layer shows high porosity (Fig. 5). For the specimen sintered from fine powder, the size of pores in this layer is larger (Fig. 5 a) and their number is smaller compared to the specimen sintered from granular
Fig. 2. SEM images of alumina particles for layer-by-layer electron beam sintering: a – fine powder, b – granular powder. The binder was low-melting-point talc (Mg3Si4O10(OH)2) with hexagonal or rhombic crystallites of different sizes no greater than 30 μm. 2
Vacuum 169 (2019) 108933
A.S. Klimov, et al.
Fig. 4. SEM image of surface structure in specimens sintered using (a) fine powder, and (b) granular Al2O3 powder.
Fig. 5. Porous structure of first (lowest) layer in specimens sintered (a) from fine, and (b) from granular Al2O3 powder.
Fig. 6. Upper (a, b) and middle part of lateral sections (c, d) in specimens sintered from fine (a, c) and granular Al2O3 powder (b, d).
sintering. For the fine powder, the bulk density, ceteris paribus, is lower than for the granular powder, and such differences in the density of the green body according to Ref. [32] lead to differences in porosity after sintering. In addition, in powders without additives used in the
powder. The average pore sizes in the samples sintered from a fine powder are in the range of 5–50 μm, while the pores in the samples sintered from a granular powder do not exceed 30 μm. Such difference in pore size may be due to the different porosity of these samples before
3
Vacuum 169 (2019) 108933
A.S. Klimov, et al.
Fig. 7. SEM image of specimen cross-section and EDS analysis data.
optimum temperature for heating the powder mixture is 1350 °C. Electron beam heating to this temperature leads to sintering of one layer of powder. The subsequent layers are formed by sintering in the same manner. The porosity of the sintered layers depends on the layer number. The least porous layers are formed in the upper part of the sintered sample. The dependence of the count of pores on the number of the sintered layer is explained by the low thermal conductivity of the material used for sintering. The microhardness of the sintered material, despite the high porosity of the initial layers, turns out to be comparable with the microhardness of alumina ceramics sintered by the traditional method. Thus our research demonstrates the possibility and prospects of using layer-by-layer electron beam sintering of alumina and other ceramic materials in additive technologies.
experiment, pore reduction does not occur quickly enough. As the number of layers is increased, the pores near the substrate become larger, while in the last (uppermost) layers (the sixth and seventh layers) the porosity is minimal (Fig. 6(a), (b)). The intermediate layers of the specimens sintered from fine powder reveal large pores in the peripheral and central regions (Fig. 6(c)). In the intermediate layers of the specimens sintered from granular powder, such pores are observed mostly at the periphery (Fig. 6(d)). This is likely because the packing of granular particles is closer than that of fine powder particles, and a better method of applying such particles is needed. The decrease in specimen porosity with increasing number of sintered layers could be due to different heating and cooling conditions during e-beam irradiation. The first layer was applied directly on the graphite crucible, and the beam energy was partially transferred to the holder via heat conduction. The energy loss of the electron beam on heating the crucible caused insufficient heating of the sintered material itself. Despite the fact that the surface temperature of the sintered layer was maintained at the same level from layer to layer, nevertheless, due to the low thermal conductivity of ceramics, a temperature gradient over its volume may exist. This led to differences in the heating of the surface of the first powder layer and the side of the layer to reverse irradiation. The second and subsequent layers were heated more efficiently due to the low conductivity of the previous layer, and the local heat release was sufficient for more complete sintering. In Fig. 6(b), an intergranular phase is detectable. In Fig. 7, showing a higher-contrast image of this region, one can see grain boundaries of the main phase which are filled with the other (molten) phase. The average microhardness of the main phase (grain center) is 1400 HV, and the microhardness of the intergranular phase is 700 HV. The microhardness of the main phase is comparable to its value 1600 HV in conventionally sintered alumina.
Acknowledgments This work was supported by the Ministry of Science and High Education, project No. 11.1550.2017/4.6. E.Oks is the executor of work on the topic "Organization of scientific research" No. 3.6441.2017/6.7. The scanning electron microscope was provided by Tomsk Polytechnic University. Special thanks to Dr. Ian Brown (Berkeley Lab) for English correction and helpful discussions. References [1] Harish Kumar Garg, Rupinder Singh, Development of new composite materials for rapid tooling using fused deposition modeling, Mater. Sci. Forum 808 (2015) 103–108. [2] E. Yasa, et al., The investigation of the influence of laser remelting on density, surface quality and microstructure of selective laser melting parts, Rapid Prototyp. J. 17 (5) (2011) 312–327. [3] Sabina L. Campanelli, et al., Capabilities and performances of the selective laser melting process, Book: New Trends in Technologies: Devices, Computer, Communication and Industrial Systems, 2010. [4] E. Louvis, et al., Selective laser melting of aluminum components, J. Mater. Process. Technol. 211 (2) (2011) 275–284. [5] Joon Park, Michael J. Tari, H. Thomas Hahn, Characterization of the laminated object manufacturing (LOM) process, Rapid Prototyp. J. 6 (1) (2000) 36–50. [6] Ph Bertrand, F. Bayle, C. Combe, P. Goeuriot, I. Smurov, Ceramic components manufacturing by selective laser sintering, Appl. Surf. Sci. 254 (2007) 989–992.
4. Conclusion Layer-by-layer electron-beam irradiation of a mixture of aluma ceramic and talc powders allows sintering multilayer samples. The 4
Vacuum 169 (2019) 108933
A.S. Klimov, et al.
Vacuum 167 (2019) 364–373. [21] É.I. Rau, E.N. Evstaf'eva, M.V. Andrianov, Mechanisms of charging of insulators under irradiation with medium-energy electron beams, Phys. Solid State 50 (4) (2008) 621–630. [22] V.A. Burdovitsin, A.S. Klimov, A.V. Medovnik, E.M. Oks, YuG. Yushkov, Forevacuum Plasma Electron Sources, Tomsk State Univ., 2014, p. 288. [23] V.A. Burdovitsin, A.S. Klimov, E. M Oks, On the possibility of electron-beam processing of dielectrics using a forevacuum plasma electron source, Tech. Phys. Lett. 35 (6) (2009) 511–513. [24] V.A. Burdovitsin, et al., Electron beam treatment of non-conducting materials by a forepump-pressure plasma-cathode electron beam source, Plasma Sources Sci. Technol. 19 (5) (2010) 055003. [25] A.K. Goreev, V.A. Burdovitsin, A.S. Klimov, E.M. Oks, Electron beam welding of ceramic to metal using fore-vacuum plasma electron source, Inorg. Mater. Appl. Res. 3 (5) (2012) 446–449. [26] A.S. Klimov, A.A. Zenin, E.M. Oks, M.V. Shandrikov, YuG. Yushkov, Electron beam evaporation of ceramics at forevacuum pressures, Prikl. Fiz. (3) (2016) 40–44. [27] A.A. Zenin, I.Yu Bakeev, YuA. Burachevsky, A.S. Klimov, E.M. Oks, Electron beam focusing features in a plasma electron source under forevacuum pressures, Phys. Tech. Lett. 42 (7) (2016) 712–714. [28] A.A. Zenin, A.S. Klimov, V.A. Burdovitsin, E.M. Oks, Generating stationary electron beams by a forevacuum plasma source at pressures up to 10 Pa, Tech. Phys. Lett. 39 (5) (2013) 454–456. [29] I.Yu Bakeev, A.A. Zenin, A.V. Tyun'kov, A.S. Klimov, Composition of the gas atmosphere during the electron beam interaction with the alumina powder in the forevacuum pressure range, Doklady Tomskogo gosudarstvennogo universiteta sistem upravleniya i radioelektroniki 19 (4) (2016) 13–16. [30] A.A. Zenin, A.S. Klimov, A.V. Kazakov, E.M. Oks, E.S. Dvilis, O.L. Khasanov, Sintering of alumina ceramics using plasma electron source, Izvestiya vysshih uchebnyh zavedenij, Fizika 55 (12–3) (2012) 216–219. [31] V. Burdovitsin, E. Dvilis, A. Zenin, A. Klimov, E. Oks, V. Sokolov, A. Kachaev, O. Khasanov, Electron beam sintering of zirconia ceramics, Adv. Mater. Res. 872 (2014) 150–156. [32] V.A. Ivensen, Densification of Metal Powders during Sintering, Consultants Bureau, New York, 1973https://doi.org/10.1007/978-1-4757-0106-7.
[7] B. Sustarsic, M. Godec, M. Jenko, T. Drglin, S. Dolinsek, Bulk and surface characterisation of metal powders for direct laser sintering, Vacuum 80 (1–3) (2005) 29–34. [8] A.N. Jinoop, C.P. Paul, S.K. Mishra, K.S. Bindra, Laser Additive Manufacturing using directed energy deposition of Inconel-718 wall structures with tailored characteristics, Vacuum 166 (2019) 270–278. [9] D.L. Bourell, H.L. Marcus, J.W. Barlow, J.J. Beaman, Laser sintering of metals and ceramics, Int. J. Powder Metall. 28 (4) (1992) 369–381. [10] R. Kniter, W. Bauer, Ceramic microfabrication by rapid prototyping process chains, Sadhana 28 (1–2) (2003) 307–318. [11] D.H. Matthiesen, W.T. Petuskey, Solidification microstructures of laser-melted alumina powder compacts, J. Am. Ceram. Soc. 68 (1985) 114–116. [12] Y. Wu, J. Du, K.-L. Choy, L.L. Hench, Laser densification of alumina powder beds generated using aerosol assisted spray deposition, J. Eur. Ceram. Soc. 27 (2007) 4727. [13] Y. Yang, X. Bai, Z. Xie, T. Kuang, Z. Liu, Y. Zhuang, B. Tong, Y. Liang, Influence of additive silica on the laser melting of the ceramic coatings, J. Mater. Sci. Technol. 19 (2003) 33–36. [14] M.S. Wȩglowski, S. Błacha, A. Phillips, Electron beam welding - techniques and trends – Review, Vacuum 130 (2016) 72–92. [15] S. Franchitti, R. Borrelli, C. Pirozzi, L. Carrino, W. Polini, L. Sorrentino, A. Gazzerro, Investigation on electron beam melting: dimensional accuracy and process repeatability, Vacuum 157 (2018) 340–348. [16] S. Valkov, R. Bezdushnyi, P. Petrov, Synthesis, structure and mechanical properties of Ti-Al-Nb coatings formed by electron beam additive technique, Vacuum 156 (2018) 140–145. [17] P.P. Edwards, A.A. O'Conner, M.M. Ramulu, Electron beam additive manufacturing of titanium components: properties and performance, J. Manuf. Sci. Eng. 135 (6) (2013) 061016-061016-7. [18] X. Gong, T. Anderson, K. Chou, Review on powder-based electron beam additive manufacturing technology, Manuf. Rev. 1 (2) (2014) 2–12, https://doi.org/10. 1051/mfreview/2014001. [19] P. Abraha, Y. Yoshikawa, Y. Katayama, Surface modification of steel surfaces by electron beam excited plasma processing, Vacuum 83 (3) (2008) 497–500. [20] J. Xu, J. Zhu, J. Fan, Q. Zhou, Y. Peng, S. Guo, Microstructure and mechanical properties of Ti–6Al–4V alloy fabricated using electron beam freeform fabrication,
5
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Electron beam sintering of ceramics for additive manufacturing a,∗
a
c
A.S. Klimov , I.Yu Bakeev , E.S. Dvilis , E.M. Oks a b c
a,b
, A.A. Zenin
T
a
Tomsk State University of Control Systems and Radioelectronics, Tomsk, Russia Institute of High Current Electronics, Russian Academy of Sciences, Tomsk, Russia Tomsk Polytechnic University, Tomsk, Russia
A B S T R A C T
We describe the results of electron-beam layer-by-layer sintering of a mixture of alumina powder and talc. An electron beam with high power density, generated by a forevacuum-pressure, plasma-cathode electron source, was used as heat source. A specific feature of the source is the option to generate an electron beam at pressure range 5–20 Pa. In this pressure range, a high density beam-plasma is formed in the electron beam transport region. Positive ions from this plasma neutralize the negative charge that the beam's electrons deposit to the surface of the target. Using an electron beam, the alumina/talc powder mixture was heated and sintering occurred at a temperature of 1350 °C. An additional layer of powder was subsequently poured onto the previous one and sintered in the same way. We formed an electron-beam-sintered 7-layer structure. We show that the porosity of the sintered layers depends on the layer number, with the least porous layers located in the upper part of the sample.
1. Introduction Additive technologies make possible complex-shaped 3D objects whose manufacture by conventional methods can be excessively material- and energy-consuming. Among such technologies are fused deposition modeling (FDM) [1], stereolithography (SLA or SLT) [2–4], laminated object manufacturing (LOM) [5], and selective laser sintering (SLS) [6–8] and melting (SLM) [9], all of which are successfully used in industry for manufacturing metal and metal alloy products. High-density ceramics are difficult to synthesize by additive methods because the material has a high melting point, low heat conductivity, and is brittle. Nevertheless some progress has been made in the field using SLS and SLM [10–12]. Note that in SLS and SLM, the efficiency of laser energy transfer strongly depends on the optical properties of the material, and for attaining the desired result with a ceramic, one should either increase the laser power or use special additives to enhance the material absorptivity [13]. Electron beams compare with laser beams in specific power, and the capability to independently control the electron beam current and energy makes electron beams attractive for welding [14], creating 3D objects from metals [15,16], alloys, and other current-conducting materials [17–20]. For efficient electron beam interaction with dielectric materials, and with ceramics in particular, one should provide efficient neutralization of the charge built up on their surface by accelerated electrons [21]. In forevacuum-pressure plasma-cathode electron beam sources [22], charge neutralization is attained due to the formation of a dense plasma in the region of electron beam transport [23]. The
∗
capabilities of such sources have been successfully demonstrated via the examples of electron beam welding, sintering, and surface modification of alumina and zirconium ceramics [24–26]. One possible approach to additive manufacturing of ceramic products can be layer-by-layer electron beam sintering of ceramic powders. Here we present research results which demonstrate the possibility of using electron beam sintering for additive manufacturing. 2. Experimental Fig. 1 shows a simplified schematic of the experimental arrangement for electron beam sintering of ceramic powder. Forevacuum plasma electron source 1 [27,28] produces a continuous electron beam of diameter 10 mm with current and electron energies in the range 2–100 mA and 1–20 keV, respectively at a pressure of 5–20 Pa. The test material is a layer of ceramic powder mixture 2 on pre-sintered substrate 3. The material is located in graphite crucible 5 and is sintered in the vacuum chamber at a residual pressure of 10 Pa. The temperature of the irradiated surface is measured with infrared pyrometer 6 (RAYTEK 1 MH) effective over the range 550–3000 °C. The electrode system of the electron source and its operational principles have been described in detail elsewhere [28]. For layer-by-layer sintering, we used Al2O3 powder, one of the most widespread ceramic materials in additive manufacturing. The powder was in two states differing in morphology: with fine particles of size 1–10 μm, and with particles of size up to 30 μm in dense granules sized to ≈100 μm (see Fig. 2). After sintering, we analyzed the morphology
Corresponding author. E-mail address: [email protected] (A.S. Klimov).
https://doi.org/10.1016/j.vacuum.2019.108933 Received 23 August 2019; Received in revised form 31 August 2019; Accepted 7 September 2019 Available online 08 September 2019 0042-207X/ © 2019 Published by Elsevier Ltd.
Vacuum 169 (2019) 108933
A.S. Klimov, et al.
Fig. 3. Electron beam power and surface temperature during sintering.
rate was 30 W/min, which allowed heat the powder to the required sintering temperature, region II in Fig. 3. According to pyrometer measurements, the sintering temperature was 1350 °C at a beam power of 700 W. The sintering temperature was set based on previously conducted experiments with alumina ceramics [30]. The sintering time at this temperature was 2 min, region III if Fig. 3. This exposure time was sufficient for sintering. Thereafter, the beam power was gradually decreased to zero at a rate of 45 W/min. The cooling rate at this level of power change was 55 deg/min, region IV in Fig. 3. At a higher cooling rate, the probability of deformation or cracking of sintered samples due to thermomechanical stresses increased. The next powder layer was applied after cooling of the previous layer and was sintered in the same way. Fig. 3 shows the electron beam power and surface temperature as a function of time.
Fig. 1. Schematic of the experimental arrangement: 1 – plasma electron source, 2 – powder mixture, 3 – pre-sintered substrate, 4 – vacuum chamber, 5 – graphite crucible, 6 – pyrometer, Ud – discharge power supply, Ua – accelerating voltage source.
of the specimens (upper surface layers, polished and cleaned lateral sections) on a JSM-7500FA scanning electron microscope (SEM), their elemental composition by energy dispersive X-ray spectroscopy (EDS) on a Hitachi S3400 N scanning electron microscope, and elastoplastic characteristics on a DUH-211S device. The optimum talc content in the ceramic was 30 wt%. This value was found experimentally. The powder on the substrate was applied with a special feeder and was leveled with a spatula without pressing. First, the powder layer was exposed to preliminary electron beam heating at a beam power of 250 W for 10 min (accelerating voltage of 5 kV, beam current of 50 mA, beam diameter in the region of exposure of 10 mm), область I на рис. 3. With such power of the electron beam and irradiation time, the powder was heated fairly uniformly. Electron beam exposure with a higher electron beam power led to the scattering of powder particles from the region of electron beam interaction due to the intensification of gas desorption from their surface [29]. Then the beam power was increased linearly to 400 W in 15 min by increasing the accelerating voltage. The rate of power increase in this case was 10 W/min and was selected experimentally. At such rate of increasing power, the volume of the powder warms up fairly evenly and there is no expansion of its particles since the adsorbed gas molecules manage to leave the surface of the powder particles located in the lower layers. When the powder heated to about 1000 °C the value of thermal radiation, determined by the Stefan-Boltzmann law and proportional to T4, from the heated surface of the powder became significant. This in turn requires a sharper increase in power. At this stage, the power increase
3. Results and discussion Fig. 4 shows the structure of the upper surface layer after electron beam sintering. As can be seen, the structure reveals melting and partial crystallization of the phases. On the surface one can distinguish alumina crystallites solidified in molten talc (melting temperature 1530 °C). The melting of one of the phases suggests that the temperature in the region of exposure is locally higher than the value measured by the pyrometer. The same was observed in silicon carbide specimens sintered by electron beam [31]. Obviously, more reliable methods compared to radiation pyrometers are needed to control the temperature of any material treated by electron beams. Our analysis of lateral sections shows that the specimen porosity depends not only on the initial powder morphology but also on the distance from the substrate. In all specimens, the first (lowest) layer shows high porosity (Fig. 5). For the specimen sintered from fine powder, the size of pores in this layer is larger (Fig. 5 a) and their number is smaller compared to the specimen sintered from granular
Fig. 2. SEM images of alumina particles for layer-by-layer electron beam sintering: a – fine powder, b – granular powder. The binder was low-melting-point talc (Mg3Si4O10(OH)2) with hexagonal or rhombic crystallites of different sizes no greater than 30 μm. 2
Vacuum 169 (2019) 108933
A.S. Klimov, et al.
Fig. 4. SEM image of surface structure in specimens sintered using (a) fine powder, and (b) granular Al2O3 powder.
Fig. 5. Porous structure of first (lowest) layer in specimens sintered (a) from fine, and (b) from granular Al2O3 powder.
Fig. 6. Upper (a, b) and middle part of lateral sections (c, d) in specimens sintered from fine (a, c) and granular Al2O3 powder (b, d).
sintering. For the fine powder, the bulk density, ceteris paribus, is lower than for the granular powder, and such differences in the density of the green body according to Ref. [32] lead to differences in porosity after sintering. In addition, in powders without additives used in the
powder. The average pore sizes in the samples sintered from a fine powder are in the range of 5–50 μm, while the pores in the samples sintered from a granular powder do not exceed 30 μm. Such difference in pore size may be due to the different porosity of these samples before
3
Vacuum 169 (2019) 108933
A.S. Klimov, et al.
Fig. 7. SEM image of specimen cross-section and EDS analysis data.
optimum temperature for heating the powder mixture is 1350 °C. Electron beam heating to this temperature leads to sintering of one layer of powder. The subsequent layers are formed by sintering in the same manner. The porosity of the sintered layers depends on the layer number. The least porous layers are formed in the upper part of the sintered sample. The dependence of the count of pores on the number of the sintered layer is explained by the low thermal conductivity of the material used for sintering. The microhardness of the sintered material, despite the high porosity of the initial layers, turns out to be comparable with the microhardness of alumina ceramics sintered by the traditional method. Thus our research demonstrates the possibility and prospects of using layer-by-layer electron beam sintering of alumina and other ceramic materials in additive technologies.
experiment, pore reduction does not occur quickly enough. As the number of layers is increased, the pores near the substrate become larger, while in the last (uppermost) layers (the sixth and seventh layers) the porosity is minimal (Fig. 6(a), (b)). The intermediate layers of the specimens sintered from fine powder reveal large pores in the peripheral and central regions (Fig. 6(c)). In the intermediate layers of the specimens sintered from granular powder, such pores are observed mostly at the periphery (Fig. 6(d)). This is likely because the packing of granular particles is closer than that of fine powder particles, and a better method of applying such particles is needed. The decrease in specimen porosity with increasing number of sintered layers could be due to different heating and cooling conditions during e-beam irradiation. The first layer was applied directly on the graphite crucible, and the beam energy was partially transferred to the holder via heat conduction. The energy loss of the electron beam on heating the crucible caused insufficient heating of the sintered material itself. Despite the fact that the surface temperature of the sintered layer was maintained at the same level from layer to layer, nevertheless, due to the low thermal conductivity of ceramics, a temperature gradient over its volume may exist. This led to differences in the heating of the surface of the first powder layer and the side of the layer to reverse irradiation. The second and subsequent layers were heated more efficiently due to the low conductivity of the previous layer, and the local heat release was sufficient for more complete sintering. In Fig. 6(b), an intergranular phase is detectable. In Fig. 7, showing a higher-contrast image of this region, one can see grain boundaries of the main phase which are filled with the other (molten) phase. The average microhardness of the main phase (grain center) is 1400 HV, and the microhardness of the intergranular phase is 700 HV. The microhardness of the main phase is comparable to its value 1600 HV in conventionally sintered alumina.
Acknowledgments This work was supported by the Ministry of Science and High Education, project No. 11.1550.2017/4.6. E.Oks is the executor of work on the topic "Organization of scientific research" No. 3.6441.2017/6.7. The scanning electron microscope was provided by Tomsk Polytechnic University. Special thanks to Dr. Ian Brown (Berkeley Lab) for English correction and helpful discussions. References [1] Harish Kumar Garg, Rupinder Singh, Development of new composite materials for rapid tooling using fused deposition modeling, Mater. Sci. Forum 808 (2015) 103–108. [2] E. Yasa, et al., The investigation of the influence of laser remelting on density, surface quality and microstructure of selective laser melting parts, Rapid Prototyp. J. 17 (5) (2011) 312–327. [3] Sabina L. Campanelli, et al., Capabilities and performances of the selective laser melting process, Book: New Trends in Technologies: Devices, Computer, Communication and Industrial Systems, 2010. [4] E. Louvis, et al., Selective laser melting of aluminum components, J. Mater. Process. Technol. 211 (2) (2011) 275–284. [5] Joon Park, Michael J. Tari, H. Thomas Hahn, Characterization of the laminated object manufacturing (LOM) process, Rapid Prototyp. J. 6 (1) (2000) 36–50. [6] Ph Bertrand, F. Bayle, C. Combe, P. Goeuriot, I. Smurov, Ceramic components manufacturing by selective laser sintering, Appl. Surf. Sci. 254 (2007) 989–992.
4. Conclusion Layer-by-layer electron-beam irradiation of a mixture of aluma ceramic and talc powders allows sintering multilayer samples. The 4
Vacuum 169 (2019) 108933
A.S. Klimov, et al.
Vacuum 167 (2019) 364–373. [21] É.I. Rau, E.N. Evstaf'eva, M.V. Andrianov, Mechanisms of charging of insulators under irradiation with medium-energy electron beams, Phys. Solid State 50 (4) (2008) 621–630. [22] V.A. Burdovitsin, A.S. Klimov, A.V. Medovnik, E.M. Oks, YuG. Yushkov, Forevacuum Plasma Electron Sources, Tomsk State Univ., 2014, p. 288. [23] V.A. Burdovitsin, A.S. Klimov, E. M Oks, On the possibility of electron-beam processing of dielectrics using a forevacuum plasma electron source, Tech. Phys. Lett. 35 (6) (2009) 511–513. [24] V.A. Burdovitsin, et al., Electron beam treatment of non-conducting materials by a forepump-pressure plasma-cathode electron beam source, Plasma Sources Sci. Technol. 19 (5) (2010) 055003. [25] A.K. Goreev, V.A. Burdovitsin, A.S. Klimov, E.M. Oks, Electron beam welding of ceramic to metal using fore-vacuum plasma electron source, Inorg. Mater. Appl. Res. 3 (5) (2012) 446–449. [26] A.S. Klimov, A.A. Zenin, E.M. Oks, M.V. Shandrikov, YuG. Yushkov, Electron beam evaporation of ceramics at forevacuum pressures, Prikl. Fiz. (3) (2016) 40–44. [27] A.A. Zenin, I.Yu Bakeev, YuA. Burachevsky, A.S. Klimov, E.M. Oks, Electron beam focusing features in a plasma electron source under forevacuum pressures, Phys. Tech. Lett. 42 (7) (2016) 712–714. [28] A.A. Zenin, A.S. Klimov, V.A. Burdovitsin, E.M. Oks, Generating stationary electron beams by a forevacuum plasma source at pressures up to 10 Pa, Tech. Phys. Lett. 39 (5) (2013) 454–456. [29] I.Yu Bakeev, A.A. Zenin, A.V. Tyun'kov, A.S. Klimov, Composition of the gas atmosphere during the electron beam interaction with the alumina powder in the forevacuum pressure range, Doklady Tomskogo gosudarstvennogo universiteta sistem upravleniya i radioelektroniki 19 (4) (2016) 13–16. [30] A.A. Zenin, A.S. Klimov, A.V. Kazakov, E.M. Oks, E.S. Dvilis, O.L. Khasanov, Sintering of alumina ceramics using plasma electron source, Izvestiya vysshih uchebnyh zavedenij, Fizika 55 (12–3) (2012) 216–219. [31] V. Burdovitsin, E. Dvilis, A. Zenin, A. Klimov, E. Oks, V. Sokolov, A. Kachaev, O. Khasanov, Electron beam sintering of zirconia ceramics, Adv. Mater. Res. 872 (2014) 150–156. [32] V.A. Ivensen, Densification of Metal Powders during Sintering, Consultants Bureau, New York, 1973https://doi.org/10.1007/978-1-4757-0106-7.
[7] B. Sustarsic, M. Godec, M. Jenko, T. Drglin, S. Dolinsek, Bulk and surface characterisation of metal powders for direct laser sintering, Vacuum 80 (1–3) (2005) 29–34. [8] A.N. Jinoop, C.P. Paul, S.K. Mishra, K.S. Bindra, Laser Additive Manufacturing using directed energy deposition of Inconel-718 wall structures with tailored characteristics, Vacuum 166 (2019) 270–278. [9] D.L. Bourell, H.L. Marcus, J.W. Barlow, J.J. Beaman, Laser sintering of metals and ceramics, Int. J. Powder Metall. 28 (4) (1992) 369–381. [10] R. Kniter, W. Bauer, Ceramic microfabrication by rapid prototyping process chains, Sadhana 28 (1–2) (2003) 307–318. [11] D.H. Matthiesen, W.T. Petuskey, Solidification microstructures of laser-melted alumina powder compacts, J. Am. Ceram. Soc. 68 (1985) 114–116. [12] Y. Wu, J. Du, K.-L. Choy, L.L. Hench, Laser densification of alumina powder beds generated using aerosol assisted spray deposition, J. Eur. Ceram. Soc. 27 (2007) 4727. [13] Y. Yang, X. Bai, Z. Xie, T. Kuang, Z. Liu, Y. Zhuang, B. Tong, Y. Liang, Influence of additive silica on the laser melting of the ceramic coatings, J. Mater. Sci. Technol. 19 (2003) 33–36. [14] M.S. Wȩglowski, S. Błacha, A. Phillips, Electron beam welding - techniques and trends – Review, Vacuum 130 (2016) 72–92. [15] S. Franchitti, R. Borrelli, C. Pirozzi, L. Carrino, W. Polini, L. Sorrentino, A. Gazzerro, Investigation on electron beam melting: dimensional accuracy and process repeatability, Vacuum 157 (2018) 340–348. [16] S. Valkov, R. Bezdushnyi, P. Petrov, Synthesis, structure and mechanical properties of Ti-Al-Nb coatings formed by electron beam additive technique, Vacuum 156 (2018) 140–145. [17] P.P. Edwards, A.A. O'Conner, M.M. Ramulu, Electron beam additive manufacturing of titanium components: properties and performance, J. Manuf. Sci. Eng. 135 (6) (2013) 061016-061016-7. [18] X. Gong, T. Anderson, K. Chou, Review on powder-based electron beam additive manufacturing technology, Manuf. Rev. 1 (2) (2014) 2–12, https://doi.org/10. 1051/mfreview/2014001. [19] P. Abraha, Y. Yoshikawa, Y. Katayama, Surface modification of steel surfaces by electron beam excited plasma processing, Vacuum 83 (3) (2008) 497–500. [20] J. Xu, J. Zhu, J. Fan, Q. Zhou, Y. Peng, S. Guo, Microstructure and mechanical properties of Ti–6Al–4V alloy fabricated using electron beam freeform fabrication,
5