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Improvements in W-Eurofer first wall brazed joint using alloyed powders fillers
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ARTICLE IN PRESS
FUSION-9307; No. of Pages 4
Fusion Engineering and Design xxx (2017) xxx–xxx
Contents lists available at ScienceDirect
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Improvements in W-Eurofer first wall brazed joint using alloyed powders fillers ˜ J. de Prado ∗ , M. Sánchez, A. Urena Materials Science and Engineering Area, ESCET, Rey Juan Carlos University, C/Tulipán s/n, 28933 Móstoles, Madrid, Spain
h i g h l i g h t s • The use of alloyed powders as filler material could improve the productivity of the brazing procedure. • The advantages allow shortening the heating stage up to 3 times than if pure powders were used. • The lower melting range makes possible to work with lower brazing temperature.
a r t i c l e
a b s t r a c t
i n f o
Article history: Received 3 October 2016 Received in revised form 23 January 2017 Accepted 22 March 2017 Available online xxx Keywords: Plasma facing component Tungsten Eurofer Brazing Joining
This paper studies the improvements of 80Cu-20Ti alloyed powders as filler material for W-Eurofer joints of the next generation of fusion power plants. The advantages derived from their alloyed composition allow shortening the heating stage up to 3 times with respect to pure powders. Besides, the better flowing capabilities and lower melting range make possible to work with lower brazing temperature. Operational brazeability of W-Eurofer joints using a mixture of 80Cu-20Ti alloyed powders has been achieved for brazing temperatures of 960 ◦ C and 940 ◦ C, holding the temperature during 1–5 and 10 min, respectively. Hardening and softening processes of the Eurofer occurs during the brazing cycle, but the as received hardness conditions could be recovered by the application of a post-weld annealing treatment. The highest shear strength was obtained for the sample fabricated at 960 ◦ C for 5 min. © 2017 Elsevier B.V. All rights reserved.
1. Introduction The development of the next generation of fusion power plants involves the selection and development of new materials capable to withstand the severe conditions inside the reactor such as high thermal loads, high neutron fluxes and sputtering. New processing technologies have to be implemented in order to meet the requirements of the reactor environment. Joining technologies have become an important challenge for the scientists due to the lack of knowledge in joining tungsten with other materials, such as low activation ferritic-martensitic steels (i.e. Eurofer). At the first wall (FW) of the DEMO reactor, the plasma facing components consist of a sacrificial layer made of tungsten, which is also supported by a Eurofer structure [1]. The existing literature about W-Eurofer joints has proved that soldering promotes grain coarsening and recrystallization in base materials and, therefore, the efforts were focused on brazing and diffusion bonding technologies.
∗ Corresponding author. E-mail address: [email protected] (J. de Prado).
The present work proposes an alternative procedure using 80Cu-20Ti powders as metal filler, fabricated by means of mechanical alloying, with advanced thermal properties. Previous studies using this filler composition, but using a mixture of pure powders instead of alloyed one, showed that successful joints were achieved by brazing at 960 ◦ C for 10 min [2]. Some of the benefits of using pre-alloyed powders are narrower melting ranges and enhanced flowing and spreading capabilities that open the possibilities to work with lower temperatures, shorter dwell times or higher heating rates. A study conducted by Ivanov et al. [3] showed that the use of mechanically alloyed powders could cut down the cost of the brazing procedure by 40–50%. Although it is important to take into account the increase of the cost associated to the manufacturing of alloyed powders. In order to evaluate and characterize the named benefits, this work focusses on possible correlations of the microstructure with the mechanical properties using different brazing conditions. The objective is to optimize the brazing procedure, which could lead to less base material modification, and to improve the productivity, making it more attractive to the brazing industry.
http://dx.doi.org/10.1016/j.fusengdes.2017.03.126 0920-3796/© 2017 Elsevier B.V. All rights reserved.
Please cite this article in press as: J. de Prado, et al., Improvements in W-Eurofer first wall brazed joint using alloyed powders fillers, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.03.126
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2. Experimental procedure 2.1. Materials Tungsten and Eurofer were the base materials used in brazing tests. W was supplied by Plansee (>99.97 %) in a 12.7 mm diameter rod. Eurofer was supplied by Karlsruhe Institute of Technology (KIT) and had the standard composition and microstructure [4]. 80Cu-20Ti fillers were fabricated using alloyed and pure powders. Alloyed powders were supplied by Goodfellow and manufactured by means of mechanical alloying, and the particles were screened using a 200 m sieve. Ti powders (99.95% purity, −200 mesh) were supplied by Alfa Aesar and Cu ones (99.9% purity, 100 mesh) by Stream Chemical. Mixtures of both pure powders in the studied proportion were mixed by means of a rotatory milling process. To manufacture the fillers, the powders were mixed with an organic binder (powder/binder weight ratio: 95/5) and laminated to obtain flexible tapes of 250 m thickness. The binder used was polypropylene carbonate (PPC, QPAC 40) supplied by Empower Materials in pellets form.
Fig. 1. DTA curves in the melting range of laminated alloyed powders, laminated pure powders and pressed pure powders subjected to different heating ramps.
2.2. Brazing tests W substrates were cut from a 12.7 mm diameter bar in slices of about 1.5 mm thickness. Eurofer pieces were cut from a plate with an approximately exposed area of 7.5 × 12 mm2 and a thickness of 1.5 mm. Base material surfaces were ground to grit size P4000 with a silicon carbide paper. The laminated filler made of 80Cu-20Ti alloyed powders was placed between the two parent substrates. Brazing tests were carried out in a high vacuum furnace at the residual pressure of 10−6 mbar. Brazing temperature and dwell times were optimized taking into account previous studies in which successful joints were obtained using brazing temperatures of ∼50 ◦ C over the experimental liquidus temperature of the filler (960 ◦ C) holding the temperature for 10 min [2]. Thus, two different strategies were followed to optimize the brazing cycle: 1) reduce the dwell time to 1 and 5 min maintaining the brazing temperature (960 ◦ C); and 2) reduce the brazing temperature to 940 ◦ C maintaining the dwell time during 10 min. In both cases, the heating and cooling rates were 5 ◦ C/min.
Table 1 Solidus and liquidus temperatures. Filler
Heating rate (◦ C/min)
Tsol (◦ C)
Tliq (◦ C)
Alloyed laminated
20 30 20 20 30
884.3 881.8 NM 894.1 *NM
912.1 911.1 NM 907.3 *NM
Pure laminated Pressed Pure *NM: Not melted.
imprint sizes. Shear strength values were obtained using a shear fixture that was placed between compress platens in a Universal Testing Machine (Zwick Z100) at a speed of 1 mm/min. Three samples were measured in order to ensure accuracy. 3. Results
2.3. Characterization techniques
3.1. Thermal characterization of the filler
Solidus and liquidus temperatures of the fillers made of alloyed and a mixture of pure powders with 80Cu-20Ti composition were obtained by differential thermal analysis (DTA) measurements done with a Setaram Thermic Analyser Setsys 16/18 in argon atmosphere. The thermal characterization of a mixture of pure powders without binder was also made for comparative purposes and this sample was cold-pressed compacted under 232 MPa before the test. 20 mg of each material were subjected to a different heating rates (20, 30 ◦ C/min) up to 100 ◦ C over the theoretical liquidus temperature [5]. The heat exchanged throughout the process was recorded. The microstructural analysis of cross sections of the brazed samples was performed using Scanning Electron Microscopy (SEM, S3400 Hitachi) equipped with Energy Dispersive X-ray analysis (EDX). The samples were metallographically prepared following the standard polishing technique. The mechanical properties of the joints were evaluated by means of microhardness and shear tests. The hardness study gives information about the effect of the brazing process in the mechanical properties of the base materials. Thus, microhardness profile was traced across tungsten-Eurofer joint with a MHV-2SHIMADZU equipment. A 100 g load was applied during 15 s and three measurements were obtained for each position. Distances between neighbour indentations were longer than three times the residual
Fig. 1 shows the DTA results in the melting range of i) laminated mechanically alloyed and mixture of pure powders, and ii) pressed mixture of pure powders, under heating rates of 20 and 30 ◦ C/min. The results showed a significant difference between the two powders prepared by lamination route. Alloyed powders melted for both tested heating rates, at the temperatures reported in the bibliography [5], while the mixture of the pure powders did not. The alloyed powders had the 80Cu-20Ti composition and melted during heating. However, the mixture of pure powders was constituted at the beginning by Ti and Cu pure powders, and the heating rates used for the experiment did not allow reaching the desired composition by means of diffusion phenomena. On the other hand, if the pure powders were previously cold compacted, the particles were then in a more intimate contact, which favoured the diffusion process. In this case, the melting occurred at heating rates of 20 ◦ C/min but not for 30 ◦ C/min, as it happened for the alloyed powders (Table 1). This study gives an important information to bear in mind since the use of higher heating rates could imply an improvement in the productivity of the brazing procedure. The heating stage could be reduced by a factor of three compared to a brazing process developed for a mixture of pure powders that used a heating rate of 10 ◦ C/min [2].
Please cite this article in press as: J. de Prado, et al., Improvements in W-Eurofer first wall brazed joint using alloyed powders fillers, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.03.126
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Fig. 2. Microstructure of the W–Eurofer joints using 80Cu-20Ti 95/5 PPC fillers at the following brazing conditions: (a) 960 ◦ C-5 min, (b) 960 ◦ C-1 min and (c) 940 ◦ C-10 min.
3.2. Microstructural characterization of brazed joints The joints were continuous with an average thickness of 75 m (Fig. 2). The fillers melted and spread filling the joint clearance and reaching 100% of metallic contact at both interfaces. “The microstructure of the braze of the joints was formed by the same phases”. The microstructure of the Eurofer–braze interface was the result of the reaction between Eurofer and the titanium of the filler due to the active character of this element. Several phases composed of Cu-Ti-Fe (EDX microanalysis) were formed (phases 1 in Fig. 2(a)). The temperatures reached during brazing cycle were high enough to activate diffusion phenomena of some alloying elements of the Eurofer, which diffused from the Eurofer to the interface forming a continuous layer (at.% 65.9Fe-12.8Cr-11.1Ti-8.1W-2.1C) (light gray layer arrowed as 2 in Fig. 2(a)). The microstructure at the center of the braze was constituted by several Cu-Ti-Fe phases and acicular structures identified as Cu4 Ti (EDX analysis) in a Cu matrix. Finally, a continuous layer also composed of Cu4 Ti is observed at the W-braze interface (phase 3 in Fig. 2). Some cracks were identified in the sample fabricated at 960 ◦ C holding the brazing temperature for 1 min (arrowed in Fig. 2(b)). The brazing time has not been long enough to produce a chemical reaction between W and the molten filler. Furthermore, the shorter thermal cycle applied produced the formation of a larger Cu4 Ti intermetallic layer at the W-braze interface. The brittle character of the intermetallic favoured the propagation of cracks, which occasionally penetrated into the solidified filler due to the weak interaction between filler and W in this case. Longer times or higher temperatures favour the penetration of Fe into the braze alloy, by means of diffusion phenomena, which causes the modification of the equilibrium composition giving rise to a thinner Cu4 Ti layer as happened in the 960 ◦ C-5 min sample. 3.3. Mechanical characterization Fig. 3(a) shows the microhardness profile obtained from Eurofer base material to tungsten one across the joint. A softening process in Eurofer at distances lower than 400 m from the interface was observed. It may be due to the diffusion of alloying elements of the Eurofer towards the braze, especially carbon, during the brazing cycle causing a softening zone in the steel at the proximity of the interface [2]. The hardness of Eurofer at distances >400 m indicates that all the conditions gave rise to values higher than as received Eurofer (210 HV [4]). The 960 C-5 min brazing condition even gave rise to the typical hardness of Eurofer after the latter being subjected to an austenization treatment (∼390 HV0.1 [6]). However, the other two conditions produced lower hardness values (∼300 HV0.1 ). It is due to, although the steel is in the austenitization field (A1 = 895 ◦ C [6]) during brazing cycle, the time or temperatures chosen were not long or high enough to produce homogenization of the steel.
Fig. 3. (a) Microhardness profiles across the joints. (b) Shear strength results obtained from the joints.
The manufacturing of Eurofer consists of two steps: austenization and annealing treatment. Therefore, if the as-received hardness of Eurofer has to be recovered after the joining process, an annealing treatment has to be applied [6]. Thus, the sample joined at 960 ◦ C5 min was subjected to an annealing treatment similar to the to the Eurofer fabrication process (780 ◦ C/90 min). The result (purple line in Fig. 3(a)) showed the recovery of the as-received hardness at distances higher than 400 m from the joint.
Please cite this article in press as: J. de Prado, et al., Improvements in W-Eurofer first wall brazed joint using alloyed powders fillers, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.03.126
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On the other hand, the average microhardness values of tungsten (450 HV) are similar to those found in bibliography for polycrystalline tungsten [7], which demonstrated that brazing cycle does not affect tungsten; neither softening process nor recrystallization phenomenon have been detected. The hardness associated to recrystallized tungsten material is 300 HV [7]. The shear strength values measured for all tested conditions are showed in Fig. 3(b). The highest strength values were obtained for the sample fabricated at 960 ◦ C for 5 min, whereas the other conditions (960 ◦ C-1 min and 940 ◦ C-10 min) reported lower values due to the low adhesion between W and the braze as it was showed in the microstructure study. The obtained strength for the sample subjected to an annealing treatment reported also lower value, which could be associated to the continuous Cu4 Ti layer formed at the W-braze interface, which could not absorb the residual stresses generated during the cooling stage of both cycles (brazing and annealing) and caused a detriment in the strength of the joint. 4. Conclusions The use of alloyed powders instead of a mixture of pure powders with the same Cu/Ti ratio as filler material implied an improvement in the productivity of the brazing procedure. The advantages derived from its alloyed composition allowed shortening the heating stage up to 3 times. Operational brazeability of W-Eurofer joints using 80Cu-20Ti 95/5 PPC filler was achieved for 960 ◦ C – 5 min and 940 ◦ C -10 min conditions. The microstructure was constituted by ternary Cu-TiFe and Cu4 Ti phases in a Cu matrix. Additionally, several reaction layers were formed at the Eurofer-braze interface as a result of the active character of Ti. The mechanical study of the joints showed a hardening process of the Eurofer, in the 960 ◦ C – 5 min sample, typical of this steel after an austenitization treatment; however, the as received hardness could be recovered by the application of an annealing treatment in this case. It was also observed a softening process in Eurofer at the proximity of the interface (<400 m). It is caused by the loss of the alloying elements of the steel during the brazing cycle caused by diffusion phenomena.
The highest shear strength was obtained for the sample fabricated at 960 ◦ C for 5 min. However, the application of the annealing treatment seemed to cause a detriment in the strength. The other conditions reported lower strength values due to a lower adhesion achieved between filler and base materials. Acknowledgments The authors would like to acknowledge the financial support from the European Union’s Horizon 2020 research and innovation program under grant agreement number 633053 and the MULTIMAT-CHALLENGE program (S2013/MIT-2862). References [1] T.R. Barrett, G. Ellwood, G. Pérez, M. Kovari, M. Fursdon, F. Domptail, S. Kirk, S.C. McIntosh, S. Roberts, S. Zheng, L.V. Boccaccini, J.H. You, C. Bachmann, J. Reiser, M. Rieth, E. Visca, G. Mazzone, F. Arbeiter, P.K. Domalapally, Progress in the engineering design and assessment of the European DEMO first wall and divertor plasma facing components, Fusion Eng. Des. 109–111 (2016) 917–924, Part A November 1. ˜ Development of brazing process for [2] J.D. Prado, M. Sánchez, A. Urena, W–EUROFER joints using Cu-based fillers, Phys. Scr. 2016 (2016) 014022. [3] E.Y. Ivanov, A.E. Shapiro, M.G. Horne, Exploring solid-state synthesis of powder filler metals for vacuum brazing of titanium alloys, Weld. J. 85 (2006), 196S-199S. ˜ M. Hernández-Mayoral, Metallurgical [4] P. Fernández, A.M. Lancha, J. Lapena, characterization of the reduced activation ferritic/martensitic steel Eurofer’97 on as-received condition, Fusion Eng. Des. 58–59 (2001) 787–792. [5] J.L. Murray, The Cu-Ti (Copper-Titanium) system, Bull. Alloy Phase Diagr. 4 (1983) 81–95. [6] M. Rieth, M. Schirra, A. Falkenstein, P. Graf, S. Heger, H. Kempe, R. Lindau, H. Zimmermann, EUROFER 97 Tensile, Charpy, Creep and Structural Tests, 2003, M1 – FZKA–6911. [7] E. Lassner, W.-D. Schubert, Tungsten Properties, Chemistry, Technology of the Element, Alloys, and Chemical Compounds, Springer, US, Boston, 1999.
Please cite this article in press as: J. de Prado, et al., Improvements in W-Eurofer first wall brazed joint using alloyed powders fillers, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.03.126
ARTICLE IN PRESS
FUSION-9307; No. of Pages 4
Fusion Engineering and Design xxx (2017) xxx–xxx
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Improvements in W-Eurofer first wall brazed joint using alloyed powders fillers ˜ J. de Prado ∗ , M. Sánchez, A. Urena Materials Science and Engineering Area, ESCET, Rey Juan Carlos University, C/Tulipán s/n, 28933 Móstoles, Madrid, Spain
h i g h l i g h t s • The use of alloyed powders as filler material could improve the productivity of the brazing procedure. • The advantages allow shortening the heating stage up to 3 times than if pure powders were used. • The lower melting range makes possible to work with lower brazing temperature.
a r t i c l e
a b s t r a c t
i n f o
Article history: Received 3 October 2016 Received in revised form 23 January 2017 Accepted 22 March 2017 Available online xxx Keywords: Plasma facing component Tungsten Eurofer Brazing Joining
This paper studies the improvements of 80Cu-20Ti alloyed powders as filler material for W-Eurofer joints of the next generation of fusion power plants. The advantages derived from their alloyed composition allow shortening the heating stage up to 3 times with respect to pure powders. Besides, the better flowing capabilities and lower melting range make possible to work with lower brazing temperature. Operational brazeability of W-Eurofer joints using a mixture of 80Cu-20Ti alloyed powders has been achieved for brazing temperatures of 960 ◦ C and 940 ◦ C, holding the temperature during 1–5 and 10 min, respectively. Hardening and softening processes of the Eurofer occurs during the brazing cycle, but the as received hardness conditions could be recovered by the application of a post-weld annealing treatment. The highest shear strength was obtained for the sample fabricated at 960 ◦ C for 5 min. © 2017 Elsevier B.V. All rights reserved.
1. Introduction The development of the next generation of fusion power plants involves the selection and development of new materials capable to withstand the severe conditions inside the reactor such as high thermal loads, high neutron fluxes and sputtering. New processing technologies have to be implemented in order to meet the requirements of the reactor environment. Joining technologies have become an important challenge for the scientists due to the lack of knowledge in joining tungsten with other materials, such as low activation ferritic-martensitic steels (i.e. Eurofer). At the first wall (FW) of the DEMO reactor, the plasma facing components consist of a sacrificial layer made of tungsten, which is also supported by a Eurofer structure [1]. The existing literature about W-Eurofer joints has proved that soldering promotes grain coarsening and recrystallization in base materials and, therefore, the efforts were focused on brazing and diffusion bonding technologies.
∗ Corresponding author. E-mail address: [email protected] (J. de Prado).
The present work proposes an alternative procedure using 80Cu-20Ti powders as metal filler, fabricated by means of mechanical alloying, with advanced thermal properties. Previous studies using this filler composition, but using a mixture of pure powders instead of alloyed one, showed that successful joints were achieved by brazing at 960 ◦ C for 10 min [2]. Some of the benefits of using pre-alloyed powders are narrower melting ranges and enhanced flowing and spreading capabilities that open the possibilities to work with lower temperatures, shorter dwell times or higher heating rates. A study conducted by Ivanov et al. [3] showed that the use of mechanically alloyed powders could cut down the cost of the brazing procedure by 40–50%. Although it is important to take into account the increase of the cost associated to the manufacturing of alloyed powders. In order to evaluate and characterize the named benefits, this work focusses on possible correlations of the microstructure with the mechanical properties using different brazing conditions. The objective is to optimize the brazing procedure, which could lead to less base material modification, and to improve the productivity, making it more attractive to the brazing industry.
http://dx.doi.org/10.1016/j.fusengdes.2017.03.126 0920-3796/© 2017 Elsevier B.V. All rights reserved.
Please cite this article in press as: J. de Prado, et al., Improvements in W-Eurofer first wall brazed joint using alloyed powders fillers, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.03.126
G Model FUSION-9307; No. of Pages 4
ARTICLE IN PRESS J. de Prado et al. / Fusion Engineering and Design xxx (2017) xxx–xxx
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2. Experimental procedure 2.1. Materials Tungsten and Eurofer were the base materials used in brazing tests. W was supplied by Plansee (>99.97 %) in a 12.7 mm diameter rod. Eurofer was supplied by Karlsruhe Institute of Technology (KIT) and had the standard composition and microstructure [4]. 80Cu-20Ti fillers were fabricated using alloyed and pure powders. Alloyed powders were supplied by Goodfellow and manufactured by means of mechanical alloying, and the particles were screened using a 200 m sieve. Ti powders (99.95% purity, −200 mesh) were supplied by Alfa Aesar and Cu ones (99.9% purity, 100 mesh) by Stream Chemical. Mixtures of both pure powders in the studied proportion were mixed by means of a rotatory milling process. To manufacture the fillers, the powders were mixed with an organic binder (powder/binder weight ratio: 95/5) and laminated to obtain flexible tapes of 250 m thickness. The binder used was polypropylene carbonate (PPC, QPAC 40) supplied by Empower Materials in pellets form.
Fig. 1. DTA curves in the melting range of laminated alloyed powders, laminated pure powders and pressed pure powders subjected to different heating ramps.
2.2. Brazing tests W substrates were cut from a 12.7 mm diameter bar in slices of about 1.5 mm thickness. Eurofer pieces were cut from a plate with an approximately exposed area of 7.5 × 12 mm2 and a thickness of 1.5 mm. Base material surfaces were ground to grit size P4000 with a silicon carbide paper. The laminated filler made of 80Cu-20Ti alloyed powders was placed between the two parent substrates. Brazing tests were carried out in a high vacuum furnace at the residual pressure of 10−6 mbar. Brazing temperature and dwell times were optimized taking into account previous studies in which successful joints were obtained using brazing temperatures of ∼50 ◦ C over the experimental liquidus temperature of the filler (960 ◦ C) holding the temperature for 10 min [2]. Thus, two different strategies were followed to optimize the brazing cycle: 1) reduce the dwell time to 1 and 5 min maintaining the brazing temperature (960 ◦ C); and 2) reduce the brazing temperature to 940 ◦ C maintaining the dwell time during 10 min. In both cases, the heating and cooling rates were 5 ◦ C/min.
Table 1 Solidus and liquidus temperatures. Filler
Heating rate (◦ C/min)
Tsol (◦ C)
Tliq (◦ C)
Alloyed laminated
20 30 20 20 30
884.3 881.8 NM 894.1 *NM
912.1 911.1 NM 907.3 *NM
Pure laminated Pressed Pure *NM: Not melted.
imprint sizes. Shear strength values were obtained using a shear fixture that was placed between compress platens in a Universal Testing Machine (Zwick Z100) at a speed of 1 mm/min. Three samples were measured in order to ensure accuracy. 3. Results
2.3. Characterization techniques
3.1. Thermal characterization of the filler
Solidus and liquidus temperatures of the fillers made of alloyed and a mixture of pure powders with 80Cu-20Ti composition were obtained by differential thermal analysis (DTA) measurements done with a Setaram Thermic Analyser Setsys 16/18 in argon atmosphere. The thermal characterization of a mixture of pure powders without binder was also made for comparative purposes and this sample was cold-pressed compacted under 232 MPa before the test. 20 mg of each material were subjected to a different heating rates (20, 30 ◦ C/min) up to 100 ◦ C over the theoretical liquidus temperature [5]. The heat exchanged throughout the process was recorded. The microstructural analysis of cross sections of the brazed samples was performed using Scanning Electron Microscopy (SEM, S3400 Hitachi) equipped with Energy Dispersive X-ray analysis (EDX). The samples were metallographically prepared following the standard polishing technique. The mechanical properties of the joints were evaluated by means of microhardness and shear tests. The hardness study gives information about the effect of the brazing process in the mechanical properties of the base materials. Thus, microhardness profile was traced across tungsten-Eurofer joint with a MHV-2SHIMADZU equipment. A 100 g load was applied during 15 s and three measurements were obtained for each position. Distances between neighbour indentations were longer than three times the residual
Fig. 1 shows the DTA results in the melting range of i) laminated mechanically alloyed and mixture of pure powders, and ii) pressed mixture of pure powders, under heating rates of 20 and 30 ◦ C/min. The results showed a significant difference between the two powders prepared by lamination route. Alloyed powders melted for both tested heating rates, at the temperatures reported in the bibliography [5], while the mixture of the pure powders did not. The alloyed powders had the 80Cu-20Ti composition and melted during heating. However, the mixture of pure powders was constituted at the beginning by Ti and Cu pure powders, and the heating rates used for the experiment did not allow reaching the desired composition by means of diffusion phenomena. On the other hand, if the pure powders were previously cold compacted, the particles were then in a more intimate contact, which favoured the diffusion process. In this case, the melting occurred at heating rates of 20 ◦ C/min but not for 30 ◦ C/min, as it happened for the alloyed powders (Table 1). This study gives an important information to bear in mind since the use of higher heating rates could imply an improvement in the productivity of the brazing procedure. The heating stage could be reduced by a factor of three compared to a brazing process developed for a mixture of pure powders that used a heating rate of 10 ◦ C/min [2].
Please cite this article in press as: J. de Prado, et al., Improvements in W-Eurofer first wall brazed joint using alloyed powders fillers, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.03.126
G Model FUSION-9307; No. of Pages 4
ARTICLE IN PRESS J. de Prado et al. / Fusion Engineering and Design xxx (2017) xxx–xxx
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Fig. 2. Microstructure of the W–Eurofer joints using 80Cu-20Ti 95/5 PPC fillers at the following brazing conditions: (a) 960 ◦ C-5 min, (b) 960 ◦ C-1 min and (c) 940 ◦ C-10 min.
3.2. Microstructural characterization of brazed joints The joints were continuous with an average thickness of 75 m (Fig. 2). The fillers melted and spread filling the joint clearance and reaching 100% of metallic contact at both interfaces. “The microstructure of the braze of the joints was formed by the same phases”. The microstructure of the Eurofer–braze interface was the result of the reaction between Eurofer and the titanium of the filler due to the active character of this element. Several phases composed of Cu-Ti-Fe (EDX microanalysis) were formed (phases 1 in Fig. 2(a)). The temperatures reached during brazing cycle were high enough to activate diffusion phenomena of some alloying elements of the Eurofer, which diffused from the Eurofer to the interface forming a continuous layer (at.% 65.9Fe-12.8Cr-11.1Ti-8.1W-2.1C) (light gray layer arrowed as 2 in Fig. 2(a)). The microstructure at the center of the braze was constituted by several Cu-Ti-Fe phases and acicular structures identified as Cu4 Ti (EDX analysis) in a Cu matrix. Finally, a continuous layer also composed of Cu4 Ti is observed at the W-braze interface (phase 3 in Fig. 2). Some cracks were identified in the sample fabricated at 960 ◦ C holding the brazing temperature for 1 min (arrowed in Fig. 2(b)). The brazing time has not been long enough to produce a chemical reaction between W and the molten filler. Furthermore, the shorter thermal cycle applied produced the formation of a larger Cu4 Ti intermetallic layer at the W-braze interface. The brittle character of the intermetallic favoured the propagation of cracks, which occasionally penetrated into the solidified filler due to the weak interaction between filler and W in this case. Longer times or higher temperatures favour the penetration of Fe into the braze alloy, by means of diffusion phenomena, which causes the modification of the equilibrium composition giving rise to a thinner Cu4 Ti layer as happened in the 960 ◦ C-5 min sample. 3.3. Mechanical characterization Fig. 3(a) shows the microhardness profile obtained from Eurofer base material to tungsten one across the joint. A softening process in Eurofer at distances lower than 400 m from the interface was observed. It may be due to the diffusion of alloying elements of the Eurofer towards the braze, especially carbon, during the brazing cycle causing a softening zone in the steel at the proximity of the interface [2]. The hardness of Eurofer at distances >400 m indicates that all the conditions gave rise to values higher than as received Eurofer (210 HV [4]). The 960 C-5 min brazing condition even gave rise to the typical hardness of Eurofer after the latter being subjected to an austenization treatment (∼390 HV0.1 [6]). However, the other two conditions produced lower hardness values (∼300 HV0.1 ). It is due to, although the steel is in the austenitization field (A1 = 895 ◦ C [6]) during brazing cycle, the time or temperatures chosen were not long or high enough to produce homogenization of the steel.
Fig. 3. (a) Microhardness profiles across the joints. (b) Shear strength results obtained from the joints.
The manufacturing of Eurofer consists of two steps: austenization and annealing treatment. Therefore, if the as-received hardness of Eurofer has to be recovered after the joining process, an annealing treatment has to be applied [6]. Thus, the sample joined at 960 ◦ C5 min was subjected to an annealing treatment similar to the to the Eurofer fabrication process (780 ◦ C/90 min). The result (purple line in Fig. 3(a)) showed the recovery of the as-received hardness at distances higher than 400 m from the joint.
Please cite this article in press as: J. de Prado, et al., Improvements in W-Eurofer first wall brazed joint using alloyed powders fillers, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.03.126
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On the other hand, the average microhardness values of tungsten (450 HV) are similar to those found in bibliography for polycrystalline tungsten [7], which demonstrated that brazing cycle does not affect tungsten; neither softening process nor recrystallization phenomenon have been detected. The hardness associated to recrystallized tungsten material is 300 HV [7]. The shear strength values measured for all tested conditions are showed in Fig. 3(b). The highest strength values were obtained for the sample fabricated at 960 ◦ C for 5 min, whereas the other conditions (960 ◦ C-1 min and 940 ◦ C-10 min) reported lower values due to the low adhesion between W and the braze as it was showed in the microstructure study. The obtained strength for the sample subjected to an annealing treatment reported also lower value, which could be associated to the continuous Cu4 Ti layer formed at the W-braze interface, which could not absorb the residual stresses generated during the cooling stage of both cycles (brazing and annealing) and caused a detriment in the strength of the joint. 4. Conclusions The use of alloyed powders instead of a mixture of pure powders with the same Cu/Ti ratio as filler material implied an improvement in the productivity of the brazing procedure. The advantages derived from its alloyed composition allowed shortening the heating stage up to 3 times. Operational brazeability of W-Eurofer joints using 80Cu-20Ti 95/5 PPC filler was achieved for 960 ◦ C – 5 min and 940 ◦ C -10 min conditions. The microstructure was constituted by ternary Cu-TiFe and Cu4 Ti phases in a Cu matrix. Additionally, several reaction layers were formed at the Eurofer-braze interface as a result of the active character of Ti. The mechanical study of the joints showed a hardening process of the Eurofer, in the 960 ◦ C – 5 min sample, typical of this steel after an austenitization treatment; however, the as received hardness could be recovered by the application of an annealing treatment in this case. It was also observed a softening process in Eurofer at the proximity of the interface (<400 m). It is caused by the loss of the alloying elements of the steel during the brazing cycle caused by diffusion phenomena.
The highest shear strength was obtained for the sample fabricated at 960 ◦ C for 5 min. However, the application of the annealing treatment seemed to cause a detriment in the strength. The other conditions reported lower strength values due to a lower adhesion achieved between filler and base materials. Acknowledgments The authors would like to acknowledge the financial support from the European Union’s Horizon 2020 research and innovation program under grant agreement number 633053 and the MULTIMAT-CHALLENGE program (S2013/MIT-2862). References [1] T.R. Barrett, G. Ellwood, G. Pérez, M. Kovari, M. Fursdon, F. Domptail, S. Kirk, S.C. McIntosh, S. Roberts, S. Zheng, L.V. Boccaccini, J.H. You, C. Bachmann, J. Reiser, M. Rieth, E. Visca, G. Mazzone, F. Arbeiter, P.K. Domalapally, Progress in the engineering design and assessment of the European DEMO first wall and divertor plasma facing components, Fusion Eng. Des. 109–111 (2016) 917–924, Part A November 1. ˜ Development of brazing process for [2] J.D. Prado, M. Sánchez, A. Urena, W–EUROFER joints using Cu-based fillers, Phys. Scr. 2016 (2016) 014022. [3] E.Y. Ivanov, A.E. Shapiro, M.G. Horne, Exploring solid-state synthesis of powder filler metals for vacuum brazing of titanium alloys, Weld. J. 85 (2006), 196S-199S. ˜ M. Hernández-Mayoral, Metallurgical [4] P. Fernández, A.M. Lancha, J. Lapena, characterization of the reduced activation ferritic/martensitic steel Eurofer’97 on as-received condition, Fusion Eng. Des. 58–59 (2001) 787–792. [5] J.L. Murray, The Cu-Ti (Copper-Titanium) system, Bull. Alloy Phase Diagr. 4 (1983) 81–95. [6] M. Rieth, M. Schirra, A. Falkenstein, P. Graf, S. Heger, H. Kempe, R. Lindau, H. Zimmermann, EUROFER 97 Tensile, Charpy, Creep and Structural Tests, 2003, M1 – FZKA–6911. [7] E. Lassner, W.-D. Schubert, Tungsten Properties, Chemistry, Technology of the Element, Alloys, and Chemical Compounds, Springer, US, Boston, 1999.
Please cite this article in press as: J. de Prado, et al., Improvements in W-Eurofer first wall brazed joint using alloyed powders fillers, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.03.126