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Investigation of the impact of fabrication methods on the microstructure features of W-components of a He-cooled divertor
Fusion Engineering and Design 81 (2006) 259–264
Investigation of the impact of fabrication methods on the microstructure features of W-components of a He-cooled divertor W. Krauss a,∗ , N. Holstein a , J. Konys a , I. Mazul b a
Forschungszentrum Karlsruhe, Institut f¨ur Materialforschung III, P.O. Box 3640, D-76021 Karlsruhe, Germany b D.V. Efremov Institute, Scientific Technical Centre “Sintez”, 196641 St. Petersburg, Russia Received 1 February 2005; received in revised form 9 September 2005; accepted 9 September 2005 Available online 20 December 2005
Abstract Within the EU framework of the power plant conceptual study (PPCS), a He-cooled modular divertor concept to remove the expected heat loads of up to 15 MW/m2 is investigated at Forschungszentrum Karlsruhe. These high loads require sufficient cooling of the divertor components, which can only be obtained by an adapted design together with a close interaction with materials issues and development of manufacturing processes. Physical aspects favor tungsten as a functional and structural material. The design work performed indicates that sufficient heat removal by He requires microstructured W-surfaces in the shape of pin or slot arrays, or else a multi-jet cooling technology. In this work, manufacturing processes (e.g. EDM, laser etching, PIM, ECM) were analyzed for their applicability and cost effectiveness for shaping of microstructured W-arrays. In a second step, their impact on the microstructure and, thus, on stability and function of the parts were investigated. First test arrays were fabricated by EDM and brazed into the designed finger-like cooling structures. However, testing showed clearly that further development of the structuring processes (e.g. PIM, ECM) for W-components and of improved W-alloys are necessary. © 2005 Elsevier B.V. All rights reserved. Keywords: Tungsten alloys; Microstructuring; Re-crystallization; Crack formation; Manufacturing technologies; He-cooled divertor
1. Introduction The He-cooled divertor concept proposed, here, is based on a modular arrangement of small cooling fingers with integrated heat promoters for enhanced heat transfer. The modular structured surface in the form of ∗ Corresponding author. Tel.: +49 7247 82 3721; fax: +49 7247 82 3956. E-mail address: [email protected] (W. Krauss).
0920-3796/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2005.09.016
cooling fingers required to keep thermally introduced stresses within acceptable limits. The top parts of the cooling fingers have to be made from W or W-alloys to withstand the high heat loading and to control sputtering yields. These components will be mounted onto a manifold fabricated from a low activation steel (EUROFER). A more detailed description of the concept and its layout is given in Ref. [1]. It should be noted that the actual design is based on state-of-the-art parameters for standard, commercially available unirradiated
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Fig. 1. Principle divertor layout (left) with different types of array structures (right).
materials [2] and it is clear that material improvements (e.g. re-crystallization behavior, DBTT, strength or ductility) is absolutely required so that operation under expected loads including fast neutron irradiation is possible. The excellent heat transfer required of the cooling gas is realized in the proposed HEMP or HEMS (He-cooled modular pin or slot array) concept by the installation of heat transfer promoting features in the form of pin or slot arrays into the cooling fingers. For both concepts, microstructural shaping of tungsten surfaces is necessary. Due to the mechanical properties (hardness and strength) of W and the small dimensions of the microfeature arrays, ordinary metal working processes (e.g. milling or forging/pressing) cannot be applied. Thus, more advanced processes have to be tested and developed for fabrication of such components, which are essential for successful implementation of the designed cooling concept.
2. Design requirements on fabrication and materials The design of the He-cooled divertor concept is based on a modular arrangement of cooling fingers consisting of several parts: a tile acting as sacrificial layer, a thimble cooled by flowing high pressurized He (10 MPa), and a specially-inserted microstructured heat transfer promoting features (pin or slot arrays) for enhanced heat transfer. A sketch drawing of this arrangement and different types of arrays is given in Fig. 1. The diameter of the microstructured W-discs
in the shape of pin or slot arrays is about 12 mm. Gap widths are near 0.2 mm and aspect ratios (gap width to pin height) are of the order 1–10. The arrays have to exhibit high density and strength to guarantee high heat conductivity (λ > 100 W/m K) and to withstand the thermally introduced stresses. These restrictions lead to selection of pure W or W–1% La2 O3 material for fabrication of the arrays. The arrays will be joined by brazing into the cooling fingers (thimbles) with an outer diameter of 15 mm. The surface temperature at the top of the thimbles is expected to be near 1250 ◦ C. Tiles will be fabricated from pure W-discs cut from rods to reduce lamination effects, whereas thimbles should be manufactured from cross rolled W–1% La2 O3 sheets to maximize their strength.
3. Microstructuring processes for pin and slot arrays The required high heat transfer rates to the He-gas can only be achieved by applying microstructured Wsurfaces. However, no standard fabrication process is available to manufacture the designed heat transfer promoting features due to the high hardness and strength of W, the small gap distances and the unique metallurgical and material properties of W. An assessment of alternative manufacturing methods was performed and the most promising processes (Table 1) were selected for testing in shaping W-arrays. The column ‘problem’ indicates in short form the most critical effect occurring during testing or the limitation of the process for W shaping at the initiation of this work. The column
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Table 1 Selected processes for trial fabrication of W-arrays Process
Problem
Valuation
Sinking EDM Wire EDM PIM Laser etching ECM
Microcracks, tool erosion Limited to straight slots Density, grain growth, feedstock Crack formation, grain growth Chemical resistance of W
Too expensive, physical tests only Too expensive, physical tests only First results, development required First results, development required First results, development required
‘valuation’ reflects the actual status of these processes for mass production shaping of W-arrays under consideration. In the following paragraphs, the results and limits of the tested processes will be explained in detail. 3.1. Sinking EDM Sinking EDM (electro-discharge-machining) is a process well known from steel tool shaping technology and was used for the first manufacturing of W-arrays. Fig. 2 depicts the first tests of a W-pin-array. Pins with the smallest diameter (∅ = 0.6 mm) show strong defects, however, this limitation can be at least partly overcome by adjusting parameters (current, pulse length, voltage, electrode material, etc.) and the use of betteroptimized starting material. The center image in Fig. 2 illustrates the quality improvement achieved by utilizing reduced (half) current loads, and raw material in the form of rod sections, which is worked in the direction of pin orientation, instead of material cut from plates. Despite these improvements, surface roughness is still high (about ± 15 m, Fig. 2 right image) and the product contains microcracks, strong erosion defects
(50–100 m in depth), and corrosion layers. The evaluation of the sinking EDM process for pin array manufacturing is as follows: fabrication of test parts is possible, but due to the long processing time of about 1 day per 1 mm depth it is too expensive and unsuitable for mass production. Further quality improvement requires finer grained W-raw materials and further reduction in current loads, so that sparks will not generate microcracks. The application of W–1% La2 O3 with higher crack resistance might, also, offer improvement. 3.2. Wire EDM EDM wire cutting is more efficient compared to sinking erosion by a factor of ten in machining speed, however, this tool can be used only for fabrication of straight slot arrays. First slot arrays could be fabricated within most of the tolerance limits given by design (Fig. 3, left side). However, surface roughness (Rmax ) ranges up to 10 m and will be a field for improvement. Also, the edges of the slots must be improved to reduce pressure losses. Fig. 3, right side, depicts optimized slot arrays with small outlet cones at the
Fig. 2. Microstructured W-arrays produced by sinking EDM. (Left) Structure eroded into W-plate. Pin diameter 1.0, 0.8 and 0.6 mm, respectively. (Centre) Adapted EDM parameters and W-qualities. Defect of pin with diameter 0.6 mm. (Right) Roughness of EDM eroded surface. Orientation 0◦ , 45◦ and 90◦ of scan, respectively.
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Fig. 3. Flat straight slot array with a gap width of 0.3 mm. Right side shows parts optimized for pressure loss.
outer edges to reduce pressure loss as predicted by thermo-hydraulic calculations. The arrays with 24 slots are ready for brazing into the thimbles. The goal is to reach by joining with suitable filler metals a decoupling of thermal stresses between the array and the pressure loaded thimbles, which are fabricated from W–La2 O3 (WL10 grade) supplied by Plansee. First pressure loss results from pretests at low temperature will be reported in Ref. [3]. In general, wire EDM is evaluated as a good tool to fabricate first real W-based mock-ups for physical testing and validation of heat transfer calculations. However, wire EDM will never be a mass production process. Additionally, more complex designs including curved or domed slots cannot be fabricated and require innovative, improved technologies like powder injection molding (PIM), electro-chemical machining (ECM) or laser etching (LE).
commercially available W-powders showed that rheological behavior of W-feedstocks has to be investigated and improved first before entering into the second product quality-determining step of sintering. In the meantime, inject-able feedstocks were developed and first test disks were sintered [6,7]. Rather high densities (about 90% TD) could be achieved at low sintering temperatures (about 1600 ◦ C) using fine-grained Wpowders (1 m range). These tests can be evaluated as optimistic with respect to feedstock development and molding. However, alloy development and optimization of sintering parameters will be necessary to suppress the observed strong grain growth.
3.3. Powder injection molding (PIM) All the innovative technologies mentioned have the same problem, they are not used in W processing and, thus, first testing, qualification and development of these tools for microstructuring of W has to start at a rather low level of technological knowledge. Interactions with, and dependence on, chemical, physical and metallurgical behavior of W have to be investigated to enable these processes for W shaping. PIM is a mass production technology in fabrication of microstructured parts [4,5]. However, tests with
Fig. 4. ECM processed W-surface. Pin diameters 1.0, 0.8 and 0.6 mm, respectively.
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Fig. 5. Laser etching of grooves in pure W (left) and strong heating leads to re-crystallization and formation of microcracks (right).
3.4. Electro-chemical machining (ECM)
3.5. Laser etching (LE)
During recent years the technology of ECM and its associated equipment were developed for use in shaping processes mainly applied e.g. in steel die fabrication [8]. In contrast to EDM the cutting speeds for ECM are faster by a factor of up to 100 and fewer defects are produced. However, the first scoping tests in this program failed these trials, which were performed at production facilities with solutions commonly used in steel working as e.g. a 10 wt.% NaNO3 electrolyte. By performed electro-chemical and galvanostatic analyses the failure could be correlated to formation of short circuits caused by insufficient solubility of W or to precipitation of W in the working gap. At low voltages, a current supported material takeoff sufficient for microstructuring of W could be realized (Fig. 4). These positive results indicate that ECM is a promising future candidate for shaping W-surfaces. No microcracks could be detected at the processed surfaces, however, process and equipment development is still necessary.
The method of laser etching (LE) was included in the list of favored processes for array manufacturing due to its flexibility in shaping complex structures. The first LE tests were performed with two main goals. The first was to demonstrate that W could be etched by laser despite its high melting point. The second interest was to study the impact of local heating on the microstructure. Most of the tests were performed using Nd-lasers (λ = 1064 nm) with an average power of about 100 W, but with different pulse durations (10 and 0.1 s, respectively). All tests were performed in air without enhanced cooling of the work piece. The general macroscopic result is that grooves (slots) can be etched into Wdiscs (Fig. 5). Microstructural analyses deliver different and more detailed results with strong impact on processing parameters. The tests with long pulse duration exhibit dramatic grain growth (re-crystallization) and microcrack formation in the etched area. Shorter
Fig. 6. Laser etching of a ring slot (∅ = 4 mm) in rotating pure W-disc. Grooves of 1 mm depth were realized, however, re-crystallization and crack formation is still present.
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locally acting laser pulses (e.g. realized by, also, rotating the disc, additionally) generate smaller microstructural changes in pure W (Fig. 6). The microstructural analyses clearly indicate that for further LE tests on W–1% La2 O3 are necessary due to its higher recrystallization temperature and improved mechanical properties. The microstructural analyses show a redeposition at the groove corners at too high laser power, which is correlated with insufficient formation of volatile oxides. Special measures (e.g. array cooling, reactive etching) to reduce these effects are under development.
4. Discussion and conclusion Work on different microstructural shaping technologies has been started. First positive results for fabrication of demonstration parts for scientific measurements were achieved and first physical tests are underway. The analyses performed on microstructuring have shown that for efficient structuring of W-alloys process development is still necessary. At the moment, none of the innovative methods tested (PIM, ECM, LE) has clear drawback criteria. However, ECM will have the smallest impact on the base material microstructure due to the smallest energy input during processing. The metallurgical investigations performed on commercially available W-products show clearly that alloy improvements are, also, urgently needed and have to be undertaken for the successful development of a He-cooled divertor.
Acknowledgements This work has been performed in the framework of the Nuclear Fusion Program of the Forschungszentrum Karlsruhe and was supported by the European Communities within the European Fusion Technology Program. References [1] P. Norajitra, L.V. Boccaccini, E. Diegele, V. Filatov, A. Gervash, R. Giniyatulin, et al., Development of a helium-cooled divertor concept: design-related requirements on materials and fabrication technology, J. Nucl. Mater. 329–333 (2004) 1594–1598. [2] Metallwerk Plansee, Product information, Austria, 2002. http://www.plansee.com. [3] R. Giniyatulin, T. Ihli, G. Janeschitz, A. Komarov, R. Kruessmann, V. Kuznetsov, et al., Experimental study of DEMO divertor target mock-ups to estimate their thermal and pumping efficiencies, This Conference, Paper ID: 01–382, 2005. [4] L. Merz, S. Rath, V. Piotter, R. Ruprecht, J. Hausselt, Powder injection molding of metallic and ceramic microparts, J. Microsyst. Techn. 10 (2004) 202–204. [5] V. Piotter, T. Gietzelt, L. Merz, R. Ruprecht, J. Hausselt, PIM enters microsystem technology, P/M Sci. Techn. Briefs 4 (1) (2002). [6] B. Zeep, V. Piotter, R. Ruprecht, J. Hausselt, Metal injection moulding of microstructured tungsten components for heat transfer promoters in a helium-cooled divertor, in: Proceedings of the Junior Euromat, Lausanne, Swiss, September 6–9, 2004. [7] B. Zeep, S. Rath, T. Ihli, V. Piotter, R. Ruprecht, J. Hausselt, Powder injection moulding of tungsten components for a Hecooled divertor, in: Proceedings of the 16th Int. Plansee Seminar, Reutte, Austria, May 30–June 3, 2005. [8] W. K¨onig, F. Klocke, Elektrochemisches Abtragen (ECM), Fertigungsverfahren, vol. 3, Springer, 1997, pp. 91–121.
Investigation of the impact of fabrication methods on the microstructure features of W-components of a He-cooled divertor W. Krauss a,∗ , N. Holstein a , J. Konys a , I. Mazul b a
Forschungszentrum Karlsruhe, Institut f¨ur Materialforschung III, P.O. Box 3640, D-76021 Karlsruhe, Germany b D.V. Efremov Institute, Scientific Technical Centre “Sintez”, 196641 St. Petersburg, Russia Received 1 February 2005; received in revised form 9 September 2005; accepted 9 September 2005 Available online 20 December 2005
Abstract Within the EU framework of the power plant conceptual study (PPCS), a He-cooled modular divertor concept to remove the expected heat loads of up to 15 MW/m2 is investigated at Forschungszentrum Karlsruhe. These high loads require sufficient cooling of the divertor components, which can only be obtained by an adapted design together with a close interaction with materials issues and development of manufacturing processes. Physical aspects favor tungsten as a functional and structural material. The design work performed indicates that sufficient heat removal by He requires microstructured W-surfaces in the shape of pin or slot arrays, or else a multi-jet cooling technology. In this work, manufacturing processes (e.g. EDM, laser etching, PIM, ECM) were analyzed for their applicability and cost effectiveness for shaping of microstructured W-arrays. In a second step, their impact on the microstructure and, thus, on stability and function of the parts were investigated. First test arrays were fabricated by EDM and brazed into the designed finger-like cooling structures. However, testing showed clearly that further development of the structuring processes (e.g. PIM, ECM) for W-components and of improved W-alloys are necessary. © 2005 Elsevier B.V. All rights reserved. Keywords: Tungsten alloys; Microstructuring; Re-crystallization; Crack formation; Manufacturing technologies; He-cooled divertor
1. Introduction The He-cooled divertor concept proposed, here, is based on a modular arrangement of small cooling fingers with integrated heat promoters for enhanced heat transfer. The modular structured surface in the form of ∗ Corresponding author. Tel.: +49 7247 82 3721; fax: +49 7247 82 3956. E-mail address: [email protected] (W. Krauss).
0920-3796/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2005.09.016
cooling fingers required to keep thermally introduced stresses within acceptable limits. The top parts of the cooling fingers have to be made from W or W-alloys to withstand the high heat loading and to control sputtering yields. These components will be mounted onto a manifold fabricated from a low activation steel (EUROFER). A more detailed description of the concept and its layout is given in Ref. [1]. It should be noted that the actual design is based on state-of-the-art parameters for standard, commercially available unirradiated
260
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Fig. 1. Principle divertor layout (left) with different types of array structures (right).
materials [2] and it is clear that material improvements (e.g. re-crystallization behavior, DBTT, strength or ductility) is absolutely required so that operation under expected loads including fast neutron irradiation is possible. The excellent heat transfer required of the cooling gas is realized in the proposed HEMP or HEMS (He-cooled modular pin or slot array) concept by the installation of heat transfer promoting features in the form of pin or slot arrays into the cooling fingers. For both concepts, microstructural shaping of tungsten surfaces is necessary. Due to the mechanical properties (hardness and strength) of W and the small dimensions of the microfeature arrays, ordinary metal working processes (e.g. milling or forging/pressing) cannot be applied. Thus, more advanced processes have to be tested and developed for fabrication of such components, which are essential for successful implementation of the designed cooling concept.
2. Design requirements on fabrication and materials The design of the He-cooled divertor concept is based on a modular arrangement of cooling fingers consisting of several parts: a tile acting as sacrificial layer, a thimble cooled by flowing high pressurized He (10 MPa), and a specially-inserted microstructured heat transfer promoting features (pin or slot arrays) for enhanced heat transfer. A sketch drawing of this arrangement and different types of arrays is given in Fig. 1. The diameter of the microstructured W-discs
in the shape of pin or slot arrays is about 12 mm. Gap widths are near 0.2 mm and aspect ratios (gap width to pin height) are of the order 1–10. The arrays have to exhibit high density and strength to guarantee high heat conductivity (λ > 100 W/m K) and to withstand the thermally introduced stresses. These restrictions lead to selection of pure W or W–1% La2 O3 material for fabrication of the arrays. The arrays will be joined by brazing into the cooling fingers (thimbles) with an outer diameter of 15 mm. The surface temperature at the top of the thimbles is expected to be near 1250 ◦ C. Tiles will be fabricated from pure W-discs cut from rods to reduce lamination effects, whereas thimbles should be manufactured from cross rolled W–1% La2 O3 sheets to maximize their strength.
3. Microstructuring processes for pin and slot arrays The required high heat transfer rates to the He-gas can only be achieved by applying microstructured Wsurfaces. However, no standard fabrication process is available to manufacture the designed heat transfer promoting features due to the high hardness and strength of W, the small gap distances and the unique metallurgical and material properties of W. An assessment of alternative manufacturing methods was performed and the most promising processes (Table 1) were selected for testing in shaping W-arrays. The column ‘problem’ indicates in short form the most critical effect occurring during testing or the limitation of the process for W shaping at the initiation of this work. The column
W. Krauss et al. / Fusion Engineering and Design 81 (2006) 259–264
261
Table 1 Selected processes for trial fabrication of W-arrays Process
Problem
Valuation
Sinking EDM Wire EDM PIM Laser etching ECM
Microcracks, tool erosion Limited to straight slots Density, grain growth, feedstock Crack formation, grain growth Chemical resistance of W
Too expensive, physical tests only Too expensive, physical tests only First results, development required First results, development required First results, development required
‘valuation’ reflects the actual status of these processes for mass production shaping of W-arrays under consideration. In the following paragraphs, the results and limits of the tested processes will be explained in detail. 3.1. Sinking EDM Sinking EDM (electro-discharge-machining) is a process well known from steel tool shaping technology and was used for the first manufacturing of W-arrays. Fig. 2 depicts the first tests of a W-pin-array. Pins with the smallest diameter (∅ = 0.6 mm) show strong defects, however, this limitation can be at least partly overcome by adjusting parameters (current, pulse length, voltage, electrode material, etc.) and the use of betteroptimized starting material. The center image in Fig. 2 illustrates the quality improvement achieved by utilizing reduced (half) current loads, and raw material in the form of rod sections, which is worked in the direction of pin orientation, instead of material cut from plates. Despite these improvements, surface roughness is still high (about ± 15 m, Fig. 2 right image) and the product contains microcracks, strong erosion defects
(50–100 m in depth), and corrosion layers. The evaluation of the sinking EDM process for pin array manufacturing is as follows: fabrication of test parts is possible, but due to the long processing time of about 1 day per 1 mm depth it is too expensive and unsuitable for mass production. Further quality improvement requires finer grained W-raw materials and further reduction in current loads, so that sparks will not generate microcracks. The application of W–1% La2 O3 with higher crack resistance might, also, offer improvement. 3.2. Wire EDM EDM wire cutting is more efficient compared to sinking erosion by a factor of ten in machining speed, however, this tool can be used only for fabrication of straight slot arrays. First slot arrays could be fabricated within most of the tolerance limits given by design (Fig. 3, left side). However, surface roughness (Rmax ) ranges up to 10 m and will be a field for improvement. Also, the edges of the slots must be improved to reduce pressure losses. Fig. 3, right side, depicts optimized slot arrays with small outlet cones at the
Fig. 2. Microstructured W-arrays produced by sinking EDM. (Left) Structure eroded into W-plate. Pin diameter 1.0, 0.8 and 0.6 mm, respectively. (Centre) Adapted EDM parameters and W-qualities. Defect of pin with diameter 0.6 mm. (Right) Roughness of EDM eroded surface. Orientation 0◦ , 45◦ and 90◦ of scan, respectively.
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Fig. 3. Flat straight slot array with a gap width of 0.3 mm. Right side shows parts optimized for pressure loss.
outer edges to reduce pressure loss as predicted by thermo-hydraulic calculations. The arrays with 24 slots are ready for brazing into the thimbles. The goal is to reach by joining with suitable filler metals a decoupling of thermal stresses between the array and the pressure loaded thimbles, which are fabricated from W–La2 O3 (WL10 grade) supplied by Plansee. First pressure loss results from pretests at low temperature will be reported in Ref. [3]. In general, wire EDM is evaluated as a good tool to fabricate first real W-based mock-ups for physical testing and validation of heat transfer calculations. However, wire EDM will never be a mass production process. Additionally, more complex designs including curved or domed slots cannot be fabricated and require innovative, improved technologies like powder injection molding (PIM), electro-chemical machining (ECM) or laser etching (LE).
commercially available W-powders showed that rheological behavior of W-feedstocks has to be investigated and improved first before entering into the second product quality-determining step of sintering. In the meantime, inject-able feedstocks were developed and first test disks were sintered [6,7]. Rather high densities (about 90% TD) could be achieved at low sintering temperatures (about 1600 ◦ C) using fine-grained Wpowders (1 m range). These tests can be evaluated as optimistic with respect to feedstock development and molding. However, alloy development and optimization of sintering parameters will be necessary to suppress the observed strong grain growth.
3.3. Powder injection molding (PIM) All the innovative technologies mentioned have the same problem, they are not used in W processing and, thus, first testing, qualification and development of these tools for microstructuring of W has to start at a rather low level of technological knowledge. Interactions with, and dependence on, chemical, physical and metallurgical behavior of W have to be investigated to enable these processes for W shaping. PIM is a mass production technology in fabrication of microstructured parts [4,5]. However, tests with
Fig. 4. ECM processed W-surface. Pin diameters 1.0, 0.8 and 0.6 mm, respectively.
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Fig. 5. Laser etching of grooves in pure W (left) and strong heating leads to re-crystallization and formation of microcracks (right).
3.4. Electro-chemical machining (ECM)
3.5. Laser etching (LE)
During recent years the technology of ECM and its associated equipment were developed for use in shaping processes mainly applied e.g. in steel die fabrication [8]. In contrast to EDM the cutting speeds for ECM are faster by a factor of up to 100 and fewer defects are produced. However, the first scoping tests in this program failed these trials, which were performed at production facilities with solutions commonly used in steel working as e.g. a 10 wt.% NaNO3 electrolyte. By performed electro-chemical and galvanostatic analyses the failure could be correlated to formation of short circuits caused by insufficient solubility of W or to precipitation of W in the working gap. At low voltages, a current supported material takeoff sufficient for microstructuring of W could be realized (Fig. 4). These positive results indicate that ECM is a promising future candidate for shaping W-surfaces. No microcracks could be detected at the processed surfaces, however, process and equipment development is still necessary.
The method of laser etching (LE) was included in the list of favored processes for array manufacturing due to its flexibility in shaping complex structures. The first LE tests were performed with two main goals. The first was to demonstrate that W could be etched by laser despite its high melting point. The second interest was to study the impact of local heating on the microstructure. Most of the tests were performed using Nd-lasers (λ = 1064 nm) with an average power of about 100 W, but with different pulse durations (10 and 0.1 s, respectively). All tests were performed in air without enhanced cooling of the work piece. The general macroscopic result is that grooves (slots) can be etched into Wdiscs (Fig. 5). Microstructural analyses deliver different and more detailed results with strong impact on processing parameters. The tests with long pulse duration exhibit dramatic grain growth (re-crystallization) and microcrack formation in the etched area. Shorter
Fig. 6. Laser etching of a ring slot (∅ = 4 mm) in rotating pure W-disc. Grooves of 1 mm depth were realized, however, re-crystallization and crack formation is still present.
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locally acting laser pulses (e.g. realized by, also, rotating the disc, additionally) generate smaller microstructural changes in pure W (Fig. 6). The microstructural analyses clearly indicate that for further LE tests on W–1% La2 O3 are necessary due to its higher recrystallization temperature and improved mechanical properties. The microstructural analyses show a redeposition at the groove corners at too high laser power, which is correlated with insufficient formation of volatile oxides. Special measures (e.g. array cooling, reactive etching) to reduce these effects are under development.
4. Discussion and conclusion Work on different microstructural shaping technologies has been started. First positive results for fabrication of demonstration parts for scientific measurements were achieved and first physical tests are underway. The analyses performed on microstructuring have shown that for efficient structuring of W-alloys process development is still necessary. At the moment, none of the innovative methods tested (PIM, ECM, LE) has clear drawback criteria. However, ECM will have the smallest impact on the base material microstructure due to the smallest energy input during processing. The metallurgical investigations performed on commercially available W-products show clearly that alloy improvements are, also, urgently needed and have to be undertaken for the successful development of a He-cooled divertor.
Acknowledgements This work has been performed in the framework of the Nuclear Fusion Program of the Forschungszentrum Karlsruhe and was supported by the European Communities within the European Fusion Technology Program. References [1] P. Norajitra, L.V. Boccaccini, E. Diegele, V. Filatov, A. Gervash, R. Giniyatulin, et al., Development of a helium-cooled divertor concept: design-related requirements on materials and fabrication technology, J. Nucl. Mater. 329–333 (2004) 1594–1598. [2] Metallwerk Plansee, Product information, Austria, 2002. http://www.plansee.com. [3] R. Giniyatulin, T. Ihli, G. Janeschitz, A. Komarov, R. Kruessmann, V. Kuznetsov, et al., Experimental study of DEMO divertor target mock-ups to estimate their thermal and pumping efficiencies, This Conference, Paper ID: 01–382, 2005. [4] L. Merz, S. Rath, V. Piotter, R. Ruprecht, J. Hausselt, Powder injection molding of metallic and ceramic microparts, J. Microsyst. Techn. 10 (2004) 202–204. [5] V. Piotter, T. Gietzelt, L. Merz, R. Ruprecht, J. Hausselt, PIM enters microsystem technology, P/M Sci. Techn. Briefs 4 (1) (2002). [6] B. Zeep, V. Piotter, R. Ruprecht, J. Hausselt, Metal injection moulding of microstructured tungsten components for heat transfer promoters in a helium-cooled divertor, in: Proceedings of the Junior Euromat, Lausanne, Swiss, September 6–9, 2004. [7] B. Zeep, S. Rath, T. Ihli, V. Piotter, R. Ruprecht, J. Hausselt, Powder injection moulding of tungsten components for a Hecooled divertor, in: Proceedings of the 16th Int. Plansee Seminar, Reutte, Austria, May 30–June 3, 2005. [8] W. K¨onig, F. Klocke, Elektrochemisches Abtragen (ECM), Fertigungsverfahren, vol. 3, Springer, 1997, pp. 91–121.