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Non-destructive characterization of plasma-sprayed tungsten and braze-joints for fusion by ultra-sonic examination
Fusion Engineering and Design 42 (1998) 511 – 517
Non-destructive characterization of plasma-sprayed tungsten and braze-joints for fusion by ultra-sonic examination1 I. S& mid a,*, E. Kny a, M. Scheerer a, P.A. Hahn a, G. Korb a, J. Linke b, G. Vieider c a
Austrian Research Centre, A-2444 Seibersdorf, Austria Forschungszentrum Julich, D-52425 Julich, Germany c The NET Team, D-85748 Garching, Germany
b
Abstract High resolution non-destructive ultra-sonic testing has been applied for the characterization of joints and bulk materials. The measurements were carried out with an ultra-sonic inspection system of the type Panametrics MultiscanTM equipped with a four axis motorized manipulator for positioning the transducer (actuator/receiver unit). For scanning, the system is operated in pulse-echo mode, the desired penetration depth is adjusted by choosing the corresponding time window when recording the reflected signal. Transducer frequencies 5 – 100 MHz are available. A very fine resolution is achieved, with 100 MHz voids or pores in copper down to a size of 25 mm can be detected. Presently, objects can be scanned up to a dimension of 500 mm in steps ] 10 mm. Data acquisition and post processing is performed on a PC with a 1 GHz sonix digitizer card. By ultra-sonic inspection not only individual defects in fully dense materials, but also regions of higher or irregular porosity, such as in plasma sprayed coatings, can be detected. To eliminate interference with the residual porosity, the resolution needed for detection can be selected with the transducer frequency. By the use of a special transducer the Cu tube and braze interface in CFC armored divertor modules of monoblock geometry (‘tube-in-tile’) was examined successfully. For the first time it is possible to inspect in-vessel components for fusion non-destructively from the inside of the coolant channel — before high heat flux loading, reproducibly and reliably mapping all pores and imperfections down to 125 mm in size (when using 20 MHz working frequency). © 1998 Elsevier Science S.A. All rights reserved.
1. Introduction Depending on the geometry and location, under severe thermal loading voids and detachment in a braze joint can cause early failure. Prior to the final assembly of future fusion devices in-vessel
* Corresponding author. 1 Manuscript for: Fourth International Symposium on Fusion Nuclear Technology (ISFNT-4), Tokyo, 6– 11 April, 1997.
components are subjected to non-destructive inspection procedures. In the past, however, there was no established routine method which would guarantee with ease a highly reliable detection of small defects in the structural material and/or the joining interface. Simple heating/cooling with hot/cold water, electron- or ion-beam loading (combined with extended diagnostics, e.g. [1,2]), as well as X-ray radiography examination qualifies only for a coarse screening of multi-material components.
0920-3796/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PII S0920-3796(98)00327-5
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Other available methods are destructive, and as such can only be used for representative inspection of selected test pieces. Although the principles of ultra-sonic inspection were established decades ago, only recently transducers with working frequencies as high as 100 MHz were made commercially available. With this a resolution of 10 mm and better is possible. Naturally, the higher that frequency, the more powerful computer systems for data acquisition and processing are needed. Brazebonded carbon-armored copper tubes and tungsten-coated copper-base substrates, recently developed for the ITER-divertor, were non-destructively examined. With the applied system, using a 1 GHz processor card, scans of small/ medium size divertor mock-ups can be performed within 10 – 30 min. In plasma interactive components a reliable removal of heat across the joining interface even after long operation has to be ensured. During off-normal plasma operation (disruptions etc.) in a very short time big quantities of energy will be deposited locally onto very small areas. Not only the plasma facing surface of the armor will be damaged, also detachment from the heat sink is likely to occur, in particular if these off-normal heat loads take place repeatedly at the same location. To obtain a wider margin in passive safety (even after localized damage), three-dimensional CFC armor is used in the EU reference divertor design with monoblock geometry, because these armor tiles will stay in place even after detachment of the braze joint. The non-destructive examination of a divertor module with a flat interface between the armor and the heat-sink was reported recently [3].
1.1. Resolution
The resolution and the velocity of the ultrasonic wave are linearly correlated: the slower the ultra-sonic wave travels in a material, the finer the resolution. Also, higher transducer frequencies give better resolution: the resolution typically is not better than 1/2 of the wavelength (which is reciprocal to the frequency).
Further the intensity decreases exponentially with the penetration depth. Therefore the highest resolution is achieved in near-surface regions using high transducer frequencies, in materials with a low ultra-sonic velocity; vice-versa, a poorer resolution is achieved using low transducer frequencies, deep into e.g. beryllium.
2. Experimental In a homogeneous material, e.g. metal, only a negligible amount of the ultrasonic beam is reflected, permitting the beam to travel a long distance before having lost most of its intensity. At pores, voids, inhomogeneities, detachments, defects etc. the reflection becomes stronger, or even dominant. The areas in a test piece behind spots of stronger reflection are therefore not accessible. To generate a two-dimensional map of defects the intensity of reflection has to be plotted according to the coordinates of ultra-sonic scanning. Using multiple colors, in two pictures the relative intensity of a reflection, and the depth of its occurrence can be plotted. Ultra-sonic examination of weld, hip or braze joints between metallic substrates in plasma facing components was tried already some years ago [4,5]. The minimum size of detectable voids, however, was 1.5 mm. The system used for the present measurements was operated in pulse-echo mode. One transducer (actuator/receiver unit) is used to generate and receive the ultrasonic signal. The intensity of the reflected signal is recorded in ‘time-offlight’ measurements. Setting the time window allows to choose the location (i.e. depth) where the reflection took place. Such it is possible to examine the test piece from one side, recording in one step the relative intensity of the reflected signal, and the location of the defect/irregularity. The working frequency of one transducer is a consequence of its hardware and can not be varied. Of further importance is the focal length (depending on the working frequency, diameter, curvature etc.), which determines the typical
I. S& mid et al. / Fusion Engineering and Design 42 (1998) 511–517
working distance, consisting of the ultra-sonic wave’s path through water plus penetration depth into the test piece. In the present measurements the coupling media-to carry the ultra-sonic wave between the transducer and the test piece-was water. Since the measurements were performed fully under water, the size of the test pieces was limited by the dimensions of the available water tank, and the operation range of the four-dimensional manipulator.
2.1. Manufacture of monoblock di6ertor mock-ups (Fig. 1) First an opening, 14 mm in ¥, is drilled into the CFC tiles, and laser structuring [6] is applied to the surface prior to active metal casting (AMC), depositing 1 mm copper onto the activated CFC surface [7]. Subsequently a copper tube (Cu–Cr – Zr or DS-Cu) with 1 mm wall
513
thickness and 12 mm outer ¥, tightly fitting into the opening, is brazed with Ti. In the case of a Mo5Re coolant tube, after laser structuring the tube is placed into the opening; the joining is done in one step together with AMC, no further brazing is needed here. Such for all material combinations the maximum adhesion strength between the carbon armor and the coolant tube is achieved. The monoblock divertor modules were scanned from the inside of the coolant channel. For that the ultra-sonic beam started parallel to the axis of the coolant channel, to be reflected on a steel mirror inside the channel before and after hitting the test piece, which was rotated 360° (here the x coordinate) for every increment along the channel axis (the y coordinate). The scans of the vacuum plasma sprayed tungsten were recorded from a short distance, placing the transducer in front of the test piece.
3. Results and discussion The thermal conductivity, expansion and density of the examined three dimensional CFC armor, and of the heat sink candidate materials, is given in Table 1.
3.1. CFC armored monoblock di6ertor mock-ups
Fig. 1. Procedure for fabrication, ultra-sonic examination, and assembly or testing.
Due to the inhomogeneous micro-structure of carbon-fiber-reinforced carbon (CFC), it is not possible to perform high resolution ultra-sonic inspection through the carbon. Too much of the ultra-sonic wave’s intensity is lost due to multiple and diffuse reflection. In the case of divertor plates with a monoblock (‘tube-in-tile’) arrangement, the interface CFC/coolant-tube becomes easily accessible from the copper side, i.e. from the inside of the coolant channel. Due to the constant wall thickness of the tube a high accuracy in the detection of defects and inhomogeneities is achieved. Up to now typically one to two monoblock divertor mock-ups per material combination were scanned in the ultra-sonic device. The scat-
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Table 1 Properties of CFC armor and heat sink materials at room temperature
SEPCARB N112a SEPCARB N31Ca DUNLOP concept1a Mo5Re W (pure, dense) W-30%wt. Cu (50%vol each) Cu (pure) Cu – Cr – Zr (precipitation hardened) DS-Cu (dispersion hardened) CFR-Cu (carbon fiber reinforced) a
Thermal conductivity (W/mk) Expansion coefficient (10−6 K)
Density (g cm−3)
A&B: 280, C: 210 A: 260, B: 70, C:50 A: 340, B: 113, C:78 120 145 295 400 336 344 \200
1.98 1.86 1.92 10.7 19.3 14.0 8.9 8.9 8.9 5.6
A&B: 2.45, C: 3.78 A: 1.02, B: 3.06, C: 3.06 A: 0.5, B: 1.6, C: 3.8 5.2 4.2 11.5 16.6 16.5 16.8 4.5
Due to the anisotropy of these CFCs the thermal properties are different for each direction.
ter of results is too big for a clear distinction between the three armor materials considered.
3.2. Mo5Re heat sink The ultra-sonic scans of mock-ups with a Mo5Re heat sink show only a few spots in the interface with somewhat stronger reflection (i.e. inhomogeneities), see Table 2. The intensity of these—compared to the signal reflected at the outer surface of the tube (without CFC armor)—is rather low. Only a small impact, if any, on the thermal performance under high heat flux loading is to be expected. The likely reasons for the soundness of these joints are: (1) The thermal expansion of Mo5Re is similar to CFC, therefore during the important first cooldown after joining almost no tensile forces in the joining interface occur. (2) Due to the very high melting temperature of Mo5Re ( 2600°C), the Mo5Re tube is placed in the center of the hole in the CFC armor tiles, and joined in one step with AMC.
3.3. DS –Cu and Cu– Cr– Zr heat sink Important for a reliable heat removal under one-sided heating is a thermally sound joint between the armor tile and the coolant tube, in particular where intersecting the direct path of the heat, i.e. in a monoblock the ‘top segment’, see Table 2.
Of the examined mock-ups with a DS–Cu coolant tube, in the top segment only 1–5% of the joining area appears to be defective, see Table 2. In Fig. 2 such a scan for the material combination DS-Cu plus SEP-N112 is shown. All examined mock-ups with a Cu–Cr–Zr coolant tube show a higher amount of defects in the joining interface in the top segment, see Table 2. In addition the observed defects cover coherent areas; failure or detachment under severe thermal loading seems more probable.
3.4. Plasma sprayed tungsten The thickness of tungsten armor when facing the plasma near the strike-point should be 5–10 mm. In order to provide sufficient life time, a small variation in thickness is acceptable, but a low and constant porosity and a uniform microstructure have to be guaranteed. Due to the cooling requirements of divertor and first wall, a mismatch in thermal expansion between the tungsten layer and the copper heat-sink should be as small as possible. Therefore special copper based structural materials, such as carbon-fiberreinforced copper and W-Cu, and intermediate gradient layers are envisaged for the heat sink to achieve a better performance and higher lifetime. In the EU fusion program the vacuumplasma-spray (VPS) technique was chosen for
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Table 2 Amount of voids, porosity and/or defects in the joining interface of selected monoblock divertor mock-ups, as detected by ultra-sonic inspection; one mock-up tested for each material combination Average for 0–360° Mo5Re+Dunlop Mo5Re +SEP-N31C Mo5Re +SEP-N112 Cu – Cr – Zr+Dunlop Cu – Cr – Zr +SEP-N31C Cu – Cr – Zr +SEP-N 112 DS-Cu+Dunlop DS-Cu +SEP-N31C DS-Cu +SEP-N112
Top segment 135–225°
Right segment 225 – 315°
Bottom segment 315 – 45°
Left segment 45 – 135°
0.1 0.3
0.0 0.1
0.0 0.4
0.1 0.4
0.2 0.3
0.3
0.3
0.0
0.3
0.6
15.2
17.4
8.6
33.2
1.6
12.6
14.3
9.5
12.5
14.1
11.6
11.8
5.5
10.2
18.7
7.6 14.2
17 1.7
8.6 7.4
11.8 44.7
8.2 2.9
8.0
5.2
0.7
12.9
13.0
the deposition of W. Three substrate materials were considered in the present study: the pseudoalloy W-Cu, pure copper, and carbon-fiber-reinforced copper (CFR-Cu), see Table 1. W-30%wt. Cu is made by infiltration of porous tungsten with liquid copper. The resulting material is a composite, consisting of 50%vol W and 50%vol Cu. The size of individual tungsten and copper areas, with big differences in density and ultra-sonic velocity, appears to be in the order of the wavelength of the ultrasonic wave. Therefore an ultra-sonic beam generated with higher transducer frequencies can not be used for examination of interfaces from the W-Cu side; see also earlier in ‘3.1.CFC armored monoblock divertor mock-ups’. On the other hand the interface between VPSW coatings and pure copper can easily be accessed through the copper substrate. The best approach to examine the available VPS-W coatings proved to be using a sufficiently low ultrasonic frequency, such that individual pores are not resolved any more; when examining from the
side of the coating, regions of higher or irregular porosity can be detected. The VPS-W coatings on a pure copper substrate showed a somewhat higher porosity than on the W-Cu substrate. It is to be noted here, that for the material combination VPS-W/W-Cu there is a smaller mismatch in thermal expansion, and only here a graded layer W+ Cu, with a composition in between pure W and W-30%wt. Cu, was sprayed to the substrate prior to depositing the tungsten layer.
3.5. CFR-Cu Carbon-fiber-reinforced copper is presently being developed in Austrian Research Centre Seibersdorf for different areas of application, because it combines good thermal conductivity with higher strength as well as resistance to heat treatment, and above all the thermal expansion can be adjusted by setting the ratio Cu-content versus fiber-content. With the presently available prototype materials an impressive thermal conductivity, combined with a sufficiently low thermal expan-
I. S& mid et al. / Fusion Engineering and Design 42 (1998) 511–517
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Fig. 2. Mapping of the intensity of the ultra-sonic beam after reflection at inhomogeneities/defects in the braze joint of a CFC armored DS-Cu tube (left), and the location (i.e. the depth) where the reflection occurs.
sion (closely matching pure tungsten) was achieved, see Table 1. Therefore CFR-Cu is considered a potential candidate substrate for VPSW. As already mentioned for W-Cu and CFCs, by choosing the appropriate transducer frequency, the resolution of ultra-sonic examination can be set to selectively detect regions of higher or irregular porosity. Also with the ultra-sonic method areas of incomplete sintering (i.e. with incomplete bonding between the carbon fibers and the copper matrix) inside a substrate are easily detected. Contrary to W-Cu, for examining near surface regions in CFR-Cu the highest available transducer frequencies can be used, making visible the locations of the carbon fibers (typically 10 mm ¥, 100 mm long) in the copper matrix, at different depths into the material.
4. Conclusions
For the first time it is possible to inspect in-vessel components for fusion nondestructively from the inside of the coolant channel —before
high heat flux loading, reproducibly and reliably mapping all pores and imperfections down to 125 mm in copper (when using 20 MHz working frequency). Prior to the final assembly of future fusion devices in-vessel components will have to be subjected to non-destructive inspection. High resolution computerized ultra-sonic examination qualifies adequately for such inspections. Using multiple colors, in one step two pictures can be produced, mapping the relative intensity (corresponding to the degree of irregularity in a material) of the reflected ultra-sonic beam, and the location where the reflection took place. With the applied system, using a 1 GHz digitizer card, small/medium size divertor mockups can be examined within 10–30 min. Less than 0.5% of the interface in the examined CFC monoblock-armored Mo5Re coolant tubes shows weak irregularities in the ultra sonic scan. These joints are considered very sound. Up to 5% of the interface in the top segment of CFC/DS-Cu appears to be defect after joining, compared to 12–17% defects in CFC/Cu–
I. S& mid et al. / Fusion Engineering and Design 42 (1998) 511–517
Cr–Zr, where the observed defects typically cover coherent areas. The best approach to examine VPS-W coatings proved to be using a sufficiently low ultra-sonic frequency, such that individual pores are not resolved any more; when examining from the side of the coating, regions of higher or irregular porosity can be detected. For examining near surface regions of CFR-Cu the highest available transducer frequencies can be used, making visible the locations of the carbon fibers (typically 10 mm ¥, 100 mm long) in the copper matrix, at different depths into the material.
[3]
[4]
[5]
[6]
References [1] I. Smid, C.D. Croessmann, R.D. Watson, J. Linke, A. Cardella, H. Bolt, N. Reheis, E. Kny, Performance of brazed graphite, carbon-fiber composite and TZM materials for actively cooled structures: qualification tests, Fusion Technol. 19 (1991) 2035–2040. [2] I. Smid, A. Cardella, C.D. Croessmann, R.D. Watson, N.
.
[7]
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Reheis, G. Kneringer, Response to high heat fluxes and metallurgical examination of a brazed carbon-fiber-composite/refractory-metal divertor mock-up, Fusion Eng. Des. 18 (1991) 125 – 133. P.A. Hahn, M. Scheerer, I. S& mid, E. Kny, W. Hohenauer, J. Linke, N. Reheis, Non-destructive ultra-sonic examination of a CFC/TZM/Mo41Re divertor mock-up for next generation Tokamaks, 14th International Plansee Seminar, Reutte/Tirol, Austria, May 1997. G. Vieider, M. Harrison, F. Moons, NET plasma facing components, in: A.M. Van Ingen, A. Nijsen-Vis, H.T. Klippel (Eds.), Fusion Technology (1988), Elsevier, Amsterdam, 1989, pp. 125 – 137. B. Bielak, D. Besson, F. Carre, W. Daenner, M. Gouton, P. Hubert, J. Mercier, G. Stalport, P. Thomas, G. Vieider, Predesign and feasibility studies of the NET shielding blanket segments, in: B.E. Keen, M. Huguet, R. Hemsworth (Eds.), Fusion Technology (1990), Elsevier, Amsterdam, 1991, pp. 401 – 405. N. Reheis, M. Witwer, G. Kneringer, G. Vieider, A. Cardella, E. Di Pietro, Industrial aspects in the fabrication of prototypes for different divertor concepts, in: C. Ferro, M. Gasparotto, H. Knoepfel (Eds.), Fusion Technology (1992), Elsevier, Amsterdam, 1993, pp. 344 – 347. T. Huber, L. Plo¨chl, N. Reheis, J.P. Cocat, J. Schlosser, The manufacturing and testing of the toroidal pumped limiter prototype elements for Tore Supra, SOFE, Champaign/IL, USA, October 1995.
Non-destructive characterization of plasma-sprayed tungsten and braze-joints for fusion by ultra-sonic examination1 I. S& mid a,*, E. Kny a, M. Scheerer a, P.A. Hahn a, G. Korb a, J. Linke b, G. Vieider c a
Austrian Research Centre, A-2444 Seibersdorf, Austria Forschungszentrum Julich, D-52425 Julich, Germany c The NET Team, D-85748 Garching, Germany
b
Abstract High resolution non-destructive ultra-sonic testing has been applied for the characterization of joints and bulk materials. The measurements were carried out with an ultra-sonic inspection system of the type Panametrics MultiscanTM equipped with a four axis motorized manipulator for positioning the transducer (actuator/receiver unit). For scanning, the system is operated in pulse-echo mode, the desired penetration depth is adjusted by choosing the corresponding time window when recording the reflected signal. Transducer frequencies 5 – 100 MHz are available. A very fine resolution is achieved, with 100 MHz voids or pores in copper down to a size of 25 mm can be detected. Presently, objects can be scanned up to a dimension of 500 mm in steps ] 10 mm. Data acquisition and post processing is performed on a PC with a 1 GHz sonix digitizer card. By ultra-sonic inspection not only individual defects in fully dense materials, but also regions of higher or irregular porosity, such as in plasma sprayed coatings, can be detected. To eliminate interference with the residual porosity, the resolution needed for detection can be selected with the transducer frequency. By the use of a special transducer the Cu tube and braze interface in CFC armored divertor modules of monoblock geometry (‘tube-in-tile’) was examined successfully. For the first time it is possible to inspect in-vessel components for fusion non-destructively from the inside of the coolant channel — before high heat flux loading, reproducibly and reliably mapping all pores and imperfections down to 125 mm in size (when using 20 MHz working frequency). © 1998 Elsevier Science S.A. All rights reserved.
1. Introduction Depending on the geometry and location, under severe thermal loading voids and detachment in a braze joint can cause early failure. Prior to the final assembly of future fusion devices in-vessel
* Corresponding author. 1 Manuscript for: Fourth International Symposium on Fusion Nuclear Technology (ISFNT-4), Tokyo, 6– 11 April, 1997.
components are subjected to non-destructive inspection procedures. In the past, however, there was no established routine method which would guarantee with ease a highly reliable detection of small defects in the structural material and/or the joining interface. Simple heating/cooling with hot/cold water, electron- or ion-beam loading (combined with extended diagnostics, e.g. [1,2]), as well as X-ray radiography examination qualifies only for a coarse screening of multi-material components.
0920-3796/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PII S0920-3796(98)00327-5
I. S& mid et al. / Fusion Engineering and Design 42 (1998) 511–517
512
Other available methods are destructive, and as such can only be used for representative inspection of selected test pieces. Although the principles of ultra-sonic inspection were established decades ago, only recently transducers with working frequencies as high as 100 MHz were made commercially available. With this a resolution of 10 mm and better is possible. Naturally, the higher that frequency, the more powerful computer systems for data acquisition and processing are needed. Brazebonded carbon-armored copper tubes and tungsten-coated copper-base substrates, recently developed for the ITER-divertor, were non-destructively examined. With the applied system, using a 1 GHz processor card, scans of small/ medium size divertor mock-ups can be performed within 10 – 30 min. In plasma interactive components a reliable removal of heat across the joining interface even after long operation has to be ensured. During off-normal plasma operation (disruptions etc.) in a very short time big quantities of energy will be deposited locally onto very small areas. Not only the plasma facing surface of the armor will be damaged, also detachment from the heat sink is likely to occur, in particular if these off-normal heat loads take place repeatedly at the same location. To obtain a wider margin in passive safety (even after localized damage), three-dimensional CFC armor is used in the EU reference divertor design with monoblock geometry, because these armor tiles will stay in place even after detachment of the braze joint. The non-destructive examination of a divertor module with a flat interface between the armor and the heat-sink was reported recently [3].
1.1. Resolution
The resolution and the velocity of the ultrasonic wave are linearly correlated: the slower the ultra-sonic wave travels in a material, the finer the resolution. Also, higher transducer frequencies give better resolution: the resolution typically is not better than 1/2 of the wavelength (which is reciprocal to the frequency).
Further the intensity decreases exponentially with the penetration depth. Therefore the highest resolution is achieved in near-surface regions using high transducer frequencies, in materials with a low ultra-sonic velocity; vice-versa, a poorer resolution is achieved using low transducer frequencies, deep into e.g. beryllium.
2. Experimental In a homogeneous material, e.g. metal, only a negligible amount of the ultrasonic beam is reflected, permitting the beam to travel a long distance before having lost most of its intensity. At pores, voids, inhomogeneities, detachments, defects etc. the reflection becomes stronger, or even dominant. The areas in a test piece behind spots of stronger reflection are therefore not accessible. To generate a two-dimensional map of defects the intensity of reflection has to be plotted according to the coordinates of ultra-sonic scanning. Using multiple colors, in two pictures the relative intensity of a reflection, and the depth of its occurrence can be plotted. Ultra-sonic examination of weld, hip or braze joints between metallic substrates in plasma facing components was tried already some years ago [4,5]. The minimum size of detectable voids, however, was 1.5 mm. The system used for the present measurements was operated in pulse-echo mode. One transducer (actuator/receiver unit) is used to generate and receive the ultrasonic signal. The intensity of the reflected signal is recorded in ‘time-offlight’ measurements. Setting the time window allows to choose the location (i.e. depth) where the reflection took place. Such it is possible to examine the test piece from one side, recording in one step the relative intensity of the reflected signal, and the location of the defect/irregularity. The working frequency of one transducer is a consequence of its hardware and can not be varied. Of further importance is the focal length (depending on the working frequency, diameter, curvature etc.), which determines the typical
I. S& mid et al. / Fusion Engineering and Design 42 (1998) 511–517
working distance, consisting of the ultra-sonic wave’s path through water plus penetration depth into the test piece. In the present measurements the coupling media-to carry the ultra-sonic wave between the transducer and the test piece-was water. Since the measurements were performed fully under water, the size of the test pieces was limited by the dimensions of the available water tank, and the operation range of the four-dimensional manipulator.
2.1. Manufacture of monoblock di6ertor mock-ups (Fig. 1) First an opening, 14 mm in ¥, is drilled into the CFC tiles, and laser structuring [6] is applied to the surface prior to active metal casting (AMC), depositing 1 mm copper onto the activated CFC surface [7]. Subsequently a copper tube (Cu–Cr – Zr or DS-Cu) with 1 mm wall
513
thickness and 12 mm outer ¥, tightly fitting into the opening, is brazed with Ti. In the case of a Mo5Re coolant tube, after laser structuring the tube is placed into the opening; the joining is done in one step together with AMC, no further brazing is needed here. Such for all material combinations the maximum adhesion strength between the carbon armor and the coolant tube is achieved. The monoblock divertor modules were scanned from the inside of the coolant channel. For that the ultra-sonic beam started parallel to the axis of the coolant channel, to be reflected on a steel mirror inside the channel before and after hitting the test piece, which was rotated 360° (here the x coordinate) for every increment along the channel axis (the y coordinate). The scans of the vacuum plasma sprayed tungsten were recorded from a short distance, placing the transducer in front of the test piece.
3. Results and discussion The thermal conductivity, expansion and density of the examined three dimensional CFC armor, and of the heat sink candidate materials, is given in Table 1.
3.1. CFC armored monoblock di6ertor mock-ups
Fig. 1. Procedure for fabrication, ultra-sonic examination, and assembly or testing.
Due to the inhomogeneous micro-structure of carbon-fiber-reinforced carbon (CFC), it is not possible to perform high resolution ultra-sonic inspection through the carbon. Too much of the ultra-sonic wave’s intensity is lost due to multiple and diffuse reflection. In the case of divertor plates with a monoblock (‘tube-in-tile’) arrangement, the interface CFC/coolant-tube becomes easily accessible from the copper side, i.e. from the inside of the coolant channel. Due to the constant wall thickness of the tube a high accuracy in the detection of defects and inhomogeneities is achieved. Up to now typically one to two monoblock divertor mock-ups per material combination were scanned in the ultra-sonic device. The scat-
514
I. S& mid et al. / Fusion Engineering and Design 42 (1998) 511–517
Table 1 Properties of CFC armor and heat sink materials at room temperature
SEPCARB N112a SEPCARB N31Ca DUNLOP concept1a Mo5Re W (pure, dense) W-30%wt. Cu (50%vol each) Cu (pure) Cu – Cr – Zr (precipitation hardened) DS-Cu (dispersion hardened) CFR-Cu (carbon fiber reinforced) a
Thermal conductivity (W/mk) Expansion coefficient (10−6 K)
Density (g cm−3)
A&B: 280, C: 210 A: 260, B: 70, C:50 A: 340, B: 113, C:78 120 145 295 400 336 344 \200
1.98 1.86 1.92 10.7 19.3 14.0 8.9 8.9 8.9 5.6
A&B: 2.45, C: 3.78 A: 1.02, B: 3.06, C: 3.06 A: 0.5, B: 1.6, C: 3.8 5.2 4.2 11.5 16.6 16.5 16.8 4.5
Due to the anisotropy of these CFCs the thermal properties are different for each direction.
ter of results is too big for a clear distinction between the three armor materials considered.
3.2. Mo5Re heat sink The ultra-sonic scans of mock-ups with a Mo5Re heat sink show only a few spots in the interface with somewhat stronger reflection (i.e. inhomogeneities), see Table 2. The intensity of these—compared to the signal reflected at the outer surface of the tube (without CFC armor)—is rather low. Only a small impact, if any, on the thermal performance under high heat flux loading is to be expected. The likely reasons for the soundness of these joints are: (1) The thermal expansion of Mo5Re is similar to CFC, therefore during the important first cooldown after joining almost no tensile forces in the joining interface occur. (2) Due to the very high melting temperature of Mo5Re ( 2600°C), the Mo5Re tube is placed in the center of the hole in the CFC armor tiles, and joined in one step with AMC.
3.3. DS –Cu and Cu– Cr– Zr heat sink Important for a reliable heat removal under one-sided heating is a thermally sound joint between the armor tile and the coolant tube, in particular where intersecting the direct path of the heat, i.e. in a monoblock the ‘top segment’, see Table 2.
Of the examined mock-ups with a DS–Cu coolant tube, in the top segment only 1–5% of the joining area appears to be defective, see Table 2. In Fig. 2 such a scan for the material combination DS-Cu plus SEP-N112 is shown. All examined mock-ups with a Cu–Cr–Zr coolant tube show a higher amount of defects in the joining interface in the top segment, see Table 2. In addition the observed defects cover coherent areas; failure or detachment under severe thermal loading seems more probable.
3.4. Plasma sprayed tungsten The thickness of tungsten armor when facing the plasma near the strike-point should be 5–10 mm. In order to provide sufficient life time, a small variation in thickness is acceptable, but a low and constant porosity and a uniform microstructure have to be guaranteed. Due to the cooling requirements of divertor and first wall, a mismatch in thermal expansion between the tungsten layer and the copper heat-sink should be as small as possible. Therefore special copper based structural materials, such as carbon-fiberreinforced copper and W-Cu, and intermediate gradient layers are envisaged for the heat sink to achieve a better performance and higher lifetime. In the EU fusion program the vacuumplasma-spray (VPS) technique was chosen for
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Table 2 Amount of voids, porosity and/or defects in the joining interface of selected monoblock divertor mock-ups, as detected by ultra-sonic inspection; one mock-up tested for each material combination Average for 0–360° Mo5Re+Dunlop Mo5Re +SEP-N31C Mo5Re +SEP-N112 Cu – Cr – Zr+Dunlop Cu – Cr – Zr +SEP-N31C Cu – Cr – Zr +SEP-N 112 DS-Cu+Dunlop DS-Cu +SEP-N31C DS-Cu +SEP-N112
Top segment 135–225°
Right segment 225 – 315°
Bottom segment 315 – 45°
Left segment 45 – 135°
0.1 0.3
0.0 0.1
0.0 0.4
0.1 0.4
0.2 0.3
0.3
0.3
0.0
0.3
0.6
15.2
17.4
8.6
33.2
1.6
12.6
14.3
9.5
12.5
14.1
11.6
11.8
5.5
10.2
18.7
7.6 14.2
17 1.7
8.6 7.4
11.8 44.7
8.2 2.9
8.0
5.2
0.7
12.9
13.0
the deposition of W. Three substrate materials were considered in the present study: the pseudoalloy W-Cu, pure copper, and carbon-fiber-reinforced copper (CFR-Cu), see Table 1. W-30%wt. Cu is made by infiltration of porous tungsten with liquid copper. The resulting material is a composite, consisting of 50%vol W and 50%vol Cu. The size of individual tungsten and copper areas, with big differences in density and ultra-sonic velocity, appears to be in the order of the wavelength of the ultrasonic wave. Therefore an ultra-sonic beam generated with higher transducer frequencies can not be used for examination of interfaces from the W-Cu side; see also earlier in ‘3.1.CFC armored monoblock divertor mock-ups’. On the other hand the interface between VPSW coatings and pure copper can easily be accessed through the copper substrate. The best approach to examine the available VPS-W coatings proved to be using a sufficiently low ultrasonic frequency, such that individual pores are not resolved any more; when examining from the
side of the coating, regions of higher or irregular porosity can be detected. The VPS-W coatings on a pure copper substrate showed a somewhat higher porosity than on the W-Cu substrate. It is to be noted here, that for the material combination VPS-W/W-Cu there is a smaller mismatch in thermal expansion, and only here a graded layer W+ Cu, with a composition in between pure W and W-30%wt. Cu, was sprayed to the substrate prior to depositing the tungsten layer.
3.5. CFR-Cu Carbon-fiber-reinforced copper is presently being developed in Austrian Research Centre Seibersdorf for different areas of application, because it combines good thermal conductivity with higher strength as well as resistance to heat treatment, and above all the thermal expansion can be adjusted by setting the ratio Cu-content versus fiber-content. With the presently available prototype materials an impressive thermal conductivity, combined with a sufficiently low thermal expan-
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Fig. 2. Mapping of the intensity of the ultra-sonic beam after reflection at inhomogeneities/defects in the braze joint of a CFC armored DS-Cu tube (left), and the location (i.e. the depth) where the reflection occurs.
sion (closely matching pure tungsten) was achieved, see Table 1. Therefore CFR-Cu is considered a potential candidate substrate for VPSW. As already mentioned for W-Cu and CFCs, by choosing the appropriate transducer frequency, the resolution of ultra-sonic examination can be set to selectively detect regions of higher or irregular porosity. Also with the ultra-sonic method areas of incomplete sintering (i.e. with incomplete bonding between the carbon fibers and the copper matrix) inside a substrate are easily detected. Contrary to W-Cu, for examining near surface regions in CFR-Cu the highest available transducer frequencies can be used, making visible the locations of the carbon fibers (typically 10 mm ¥, 100 mm long) in the copper matrix, at different depths into the material.
4. Conclusions
For the first time it is possible to inspect in-vessel components for fusion nondestructively from the inside of the coolant channel —before
high heat flux loading, reproducibly and reliably mapping all pores and imperfections down to 125 mm in copper (when using 20 MHz working frequency). Prior to the final assembly of future fusion devices in-vessel components will have to be subjected to non-destructive inspection. High resolution computerized ultra-sonic examination qualifies adequately for such inspections. Using multiple colors, in one step two pictures can be produced, mapping the relative intensity (corresponding to the degree of irregularity in a material) of the reflected ultra-sonic beam, and the location where the reflection took place. With the applied system, using a 1 GHz digitizer card, small/medium size divertor mockups can be examined within 10–30 min. Less than 0.5% of the interface in the examined CFC monoblock-armored Mo5Re coolant tubes shows weak irregularities in the ultra sonic scan. These joints are considered very sound. Up to 5% of the interface in the top segment of CFC/DS-Cu appears to be defect after joining, compared to 12–17% defects in CFC/Cu–
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Cr–Zr, where the observed defects typically cover coherent areas. The best approach to examine VPS-W coatings proved to be using a sufficiently low ultra-sonic frequency, such that individual pores are not resolved any more; when examining from the side of the coating, regions of higher or irregular porosity can be detected. For examining near surface regions of CFR-Cu the highest available transducer frequencies can be used, making visible the locations of the carbon fibers (typically 10 mm ¥, 100 mm long) in the copper matrix, at different depths into the material.
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