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A thermal cycling test of tungsten copper bonds for divertor collector plates
Fusion Engineering and Design 9 (1989) 277-282 North-Holland, Amsterdam
277
A THERMAL CYCLING TEST OF TUNGSTEN COLLECTOR PLATES M. OGAWA ‘, M. SEKI ‘, K. FUKAYA’, ’ Japan Atomic Energy 2 JAERI, Naka Ibaraki ’ Toshiba Corporation,
COPPER BONDS FOR DIVERTOR
T. HORIE 2, and T. ARAKI
Research Institute (JAERJ), Tokai 31 I-02. Japan Yokohama Kanogawa 230, Japan
Ibaraki
319-11
3
Japan
A tungsten-copper (W-Cu) bond is a reference design concept for a divertor collector plate in the Fusion Experimental Reactor (FER). In the present study, durability of the W-Cu bond under thermal cycling was experimentally examined. A plasma spraying technique was applied to form a tungsten layer with a thickness of around 2 mm on a copper disk. This technique has advantages in that tungsten can be deposited on curved and complex surfaces of substrates and that the temperature of the substrate can be kept low during the deposition. Two kinds of copper materials, oxygen-free high conductivity copper (Cu/OFHC) and copper alloyed with beryllium (Cu/Be), were used. The tungsten surface of the W-Cu test piece was periodically irradiated with a high temperature argon plasma jet under the thermal stress condition corresponding to that expected at the normal operation of the FER. The thermal properties of the plasma sprayed tungsten were measured before the test, and the microstructure before and after the test was investigated with a scanning electron microscope. The sprayed tungsten layer was found stratified and had porosity of 2-3 I% before the test. The W-Cu/OFHC test piece survived for thermal cycle up to 6000, while the W-Cu/Be test piece broke between 4500 and 5000 cycles.
1. Introduction The Fusion Experimental Reactor (FER) [l] is now being designed at JAERJ as a next generation tokamak machine. The FER is equipped with a divertor as the impurity control system. Divertor collector plates are inherently exposed to high energy particles and high heat loads. A duplex structure, composed of a heat sink material and an armor material, has often been proposed for the divertor collector plates. The FER employs a copper heat sink with tungsten armor for the collector plate. A metallurgical bonding method is superior to a mechanical attaching method because of high thermal conductance at the interface, but it generates large thermal stresses due to the difference in the thermal expansion coefficients of tungsten and copper. To reduce the thermal stress, it is proposed to insert a buffer material with a thermal expansion coefficient intermediate between tungsten and copper. However, it is fist necessary to accumulate technical experience of bonding methods and to examine the durability of the W-Cu bond without the buffer layer, against thermal cycles. Since only limited data were available on the thermal fatigue of the W-Cu duplex structure [2,3], we have
started to carry out experiments on the durability of the W-Cu bond against thermal cycles. In the present paper, the experimental results of the thermal properties and the integrity of the plasma sprayed W-Cu bond against thermal cycles are reported.
2. Previous studies Promising methods to make the W-Cu duplex structures include brazing, direct casting, diffusion bonding, plasma spraying, and so on. Brossa et al. [3], having examined brazing, diffusion bonding, and direct casting methods, reported that the diffusion bonding method was not applicable to large scale structures. We have already carried out the experiments [2] using the test pieces made by brazing or casting methods as listed in table 1. Two kinds of tungsten materials were used for the brazed W-Cu test pieces. In the tungsten (W/Fe/K) which contains an extremely small quantity of impurities and was sintered at 3000 *C, microcracks were observed only in the brazing material after 3 700 thermal cycles as shown in fig. l(a). While in the tungsten (W/Ni-P/Fe/MO) which was sintered at 1100 o C, grain boundary microcracks were observed in the tungsten only after 200 thermal cycles as shown in
0920-3796/89/$03.50 0 Elsevier Science Publishers B.V.
218
M. Ogawa
e: al. / Thermal
cycling
lest of W-Cu
bona5
fig. l(b). A nickel brazing material indicated better wettability than silver, which was also reported by Brossa et al. [3]. They recommended the casting method because of its applicability to large scale structures, its economy,
W
Ni/Cr/P
Waterv Fig. 2. Experimental set-up.
w
Ag/Cu/Zn/Ni
Fig. 1. Micro structures of W-Cu test pieces brazed and direct casted after thermal cycles. (a) Brazed W-Cu test piece after 3700 cycles (the scale marker at the bottom right-hand side is 10 pm and the arrows indicate cracks); (b) Brazed W-Cu test piece after 200 cycles (the scale marker at the top left-hand side is 10 pm); (c) Direct casted w-01 test piece after 2200 cycles (the scale marker at the bottom right-hand side is 100 am).
and easiness. However, the bonding temperature for the casting method reached 1250 o C, close to the tungsten recrystallization temperature of 1300 o C. Therefore, they coated the tungsten with a thin nickel layer to keep the tungsten temperature below 1300 o C. We improved the casting method in which the bonding temperature does not exceed the recrystallization temperature without nickel coating [2]. However, after 2200 thermal cycles, visible cracks were found in the tungsten as shown in fig. l(c). Although it is generally said that the effective thickness of the buffer layer absorbing the thermal stress is about 0.5 to 1 mm, these results suggest that the layer of the brazing material with a thickness of about 50 pm plays an important role in mitigating the thermal stress. The annealing temperature for oxygen-free high-conductivity copper (Cu/OFHC) is about 200400 a C,
TC : lhrmamuple I mm1
Fig. 3. Test piece of W-Cu duplex structure (the tungsten thickness t is about 2nun).
M. Ogawa et al. / Thermal cycling test of W-Cu bonds Table 1 Features of W-0.1 Material
test pieces and results of durability
Copper Tungsten
219
tests
Cu/OFHC W/Ni-P/Fe/MO (sintered at llOO°C)
Cu/OFHC W/Fe/K (sintered at 3000 o C)
Cu/OFHC W/Fe/K (sintered at 3000°C)
Cu/OFHC W/Fe/K (powder)
Cu/Be W/Fe/K (powder)
Bonding method (intermediate layer)
brazing (Ag/Cr/Zn/Ni)
brazing (Ni/Cr/P)
casting
spraying (Ni coating)
wayin (Ni coating)
Thermal
200 and 1100
3700
2200
6000
5ooo
gram boundary microcrack tungsten
microcrack
visible crack tungsten
(stratifi cation) (tungsten)
rupture
PI
PI
PI
present study
cycles
Result
crack,
rapture
position
Ref.
brazing material
whereasin the brazing or direct casting process,the copper, beingheatedup to about 1000 oC, therefore, is annealed.Thus, its mechanicalstrength is largely reduced. To prevent the copper from annealing, the tungstenwas sprayedonto a copper disk at a temperature below 200 ‘C in the present study. The other advantageof the plasmaspraying method is its ability to deposit tungstenon curved and complex surfaces.In addition to Cu/OFHC, copper alloyed with beryllium (Cu/Be) is also used, which has superior mechanical strength to Cu/OFHC.
3.1. Experimental
set-up
The experimental set-up is shown in fig. 2. The tungstensurfacewasirradiated with a high temperature argon plasmajet and the copper wascooled by a water flow. The configurationsand dimensionsof the W-Cu test pieceare shownin fig. 3. The test pieceis a cylinder with a 50 mm outer diameter.The tungstenand copper thicknessesare 2 mm and 20 mm, respectively. The copper heat sink wasmachinedto accommodatea cooling and supportingstructure.Three thermocoupleswere mounted in the copper to measuretemperaturesin the vicinity of the interface (TC-3,5) and the cooledsurface (TC-4) asshownin fig. 3. The bonding condition is shown in table 2. The copper disk wascoated with a nickel thin layer to gain good wettability. The spraying processis describedin the following.
present study
(1) The copper surface is washed, blasted, and then coated with about a 100 pm thick nickel layer. (2) The powder tungsten with a particle size of about 30-75 pm is sprayed on the copper surfaceheld at about 200 o C. (3) The W-Cu bond is ground. Two test pieces,W-Cu/OFHC and W-Cu/Be, were used.Sincethe Young’s modulusof the Cu/Be is larger than that of the Cu/OFHC, a longerlife is expectedfor the W-Cu/Be bond than for the W-Cu/OFHC. 3.2. Experimental
3. Experiment
lower part test piece
condition
and procedure
The thermal propertiessuchasspecificheat, thermal conductivity, thermal diffusibility, and bulk density of the sprayed tungstenbefore the test were measuredby usinga thermalproperty measurement rig of a laser-flash type both in open air and in vacuum.
Table 2 Bonding condition Material
in plasma
spraying
powder particle
tungsten diameter
method sintered at 3000 o C 30-75 pm
Blasting
condition
pressure material
0.5 MPa sand/Al 2Os
Spraying
condition
arcing gas arcing current arcing voltage copper temperature tungsten thickness
argon and helium 8OOA 35 v about 200 o C about 2 mm
M. Ogawa et al. / Thermal cycling test of W-Cu bon&
280
-
ii
the radial distributions of the shear stress calculated by ADINA under the experimental condition. The maximum shear stress was about 100 MPa near the edge in tungsten. The heating and cooling periods were 20 s and 120s respectively as shown in fig. 5. W-Cu/OFHC and W-Cu/Be test pieces were explosed to 6000 and 5000 thermal cycles, respectively, as shown in table 1. Before and after the test, the test pieces were investigated with the aid of a scanning electron microscope (SEM).
-Tungsten
5, -100
I
I 0
I
I
10
center
I
I
20
r lmmt
edge
Fig. 4. Radial distributions of shear stress at the interface of tungsten and copper (calculated by ADINA). The experimental condition was determined to generate the thermal stress almost equivalent to that estimated in the FER design. The thermal stress of the test piece was predicted by using the elastic-plastic stress analysis code ADINA. In the calculation, the heat load from the plasma jet was given so that the calculated temperatures agreed with those measured. Fig. 4 shows 1”
150
-
-.-
“I””
I
’
TC-3 TC-I,
4. Results and discussion 4. I. Thermal properties Table 3 compares the thermal properties of the sprayed tungsten with those of the sintered tungsten used for the brazed W-Cu bond. The thermal conductivity of the sprayed tungsten is roughly one tenth of that of the sintered tungsten. The over-all coefficient of heat transmission of the sprayed test piece is half that of the brazed test piece. These poor thermal properties result from both the pore and stratified structure of the sprayed tungsten as shown in fig. 6(a) and (b). Thus, the bulk density of the sprayed tungsten was about four-fifths that of the sintered tungsten. 4.2. Thermal
Fig. 5. Temperature traces in the vicinity of the interface (TC-3,5, see fig. 2) and cooled surface (TC-4).
cycling test
The W-Cu/Be test piece broke at between 4 500 and 5 000 thermal cycles as shown in table 1. Fig. 7(a) shows the rupture part near the lower edge of the test piece. A thin tungsten layer remained on the copper surface. Fig. 7(b) shows this part observed by SEM. The W-Cu/OFHC test piece did not break for up to 6000 thermal cycles. The spraying process under atmospheric pressure makes the deposited tungsten layer stratified. Then, the
Fig, 6. Plasma sprayed W-Cu/OFHC test piece exposed to 6000 cycles: (a) Plasma sprayed test piece after 6000 cycles (the scale marker at the bottom right-hand side is 100 pm); (b) Expanded photograph of stratified tungsten layer (the scale marker at the bottom right-hand side is 10 pm).
kf. Ogawa
et al. / Thermal
cycling
test of W-Cu
281
bona5
Table 3 Thermal properties of sprayed tungsten Density (kg/m-‘) Specific heat (J/kg/K) Thermal conductivity (W/m/K) Thermal diffusibility (m’/s) Overall coefficient of heat transmission (W/m2/K)
Sprayed tungsten 15800 176 9.8 (in air) 8.7 (in vacuum) 3.5 x 10-6 (in air) 3.1 X 10e6 (in vacuum) 5.0x104
spraying processwastried under low pressure.But, the
thermal conductivity was hardly increased.If the hot isostatic pressing(HIP) processis applied after the spraying process,the tungsten layer is expected to become as denseas the sintered tungsten. However, the advantageof the spraying method is lost, becausethe copper is inevitably annealedin this process.It can be said that the plasmaspraying techniqueis not yet fully establishedto manufacturethe W-cu duplex structure at the presentstage. The effect of the beryllium was not definitely confirmed in the presenttest.
Brazed tungsten 19300 133 178 69.4x10-6 13.0-16.0 x lo4 (brazed W-Cu)
(6) not to worsen tungsten and/or copper materials, suchas to avoid stratified tungstenand not to form a brittle solid solution. The brazing and casting methods do not satisfy items (2) and (5), while the plasmaspraying method doesnot satisfy item (6). No bonding techniquesatisfies all items at the present stage. Consequently, future subjectsfor researchinclude the following: (1) to improve bonding techniquesand to carry out the durability test againstthermal cycles: (2) to establisha life-time prediction method for the duplex structure, that is, to clarify the main mechanism or parametersdominating the life.
4.3. Discussion
The bonding methodsof tungsten and copper are required: (1) to prevent the tungstenfrom recrystallization; (2) to keep the copper temperaturebelow 200 oC; (3) to be applicable to large scalestructure and complicated tungstenconfiguration; (4) to manufacturesimply, easily, and economically; (5) to generatesmallresidualthermal stress;and
5. Concluding
remarks
The durability test of the plasmasprayed W-Cu/ OFHC and W-Cu/Be test piecesexposedperiodically to an argon plasmajet was carried out for a thermal stress condition similar to that expected during the normal operation of the FER. The W-Cu/OFHC test piecedid not break for thermal cyclesup to 6000, while
Fig. 7. Plasma sprayed W&u/Be test piece exposed to 5000 cycles: (a) Plasma sprayed W-Cu/Be test pmce (outer diameter is 50mm); (b) Rupture part (the scale marker at the bottom right-hand side is 100 pm).
282
M. Ogowo
et al. / Thermoi ’ cycling
the W-&/Be test piece broke between 4 500 and 5 000 thermal cycles. It can be said that the plasma spraying technique is not yet fully established to manufacture the W-Cu duplex structure at the present stage. Future subjects include to improve the bonding technique and to carry out the durability test against thermal cycles, and to clarify the main mechanism or parameters dominating the life.
lest of W-Cu
bona5
References [l] T. Tone et al., Conceptual design of Fusion Experimental Reactor (FER), Nucl. Technol. Fusion 4(2) (1983) 573-578. [2] M. Seki, M. Ogawa, A. Minato, K. Fukaya, T.Tone, and N. Miki, A thermal cycling durability test of tungsten copper duplex structures for use as a divertor plate, Fusion Engrg. Des. 5(2) (1987) 205-213. [3] F. Brossa. P. Ghiselli, G. Tonunei, G. Piatti, and P. Schiller, Experimental tests concerning the use of the tungstencopper couple design concept on the clivertor system, Fusion Technol. 1 (1982) 491-496.
277
A THERMAL CYCLING TEST OF TUNGSTEN COLLECTOR PLATES M. OGAWA ‘, M. SEKI ‘, K. FUKAYA’, ’ Japan Atomic Energy 2 JAERI, Naka Ibaraki ’ Toshiba Corporation,
COPPER BONDS FOR DIVERTOR
T. HORIE 2, and T. ARAKI
Research Institute (JAERJ), Tokai 31 I-02. Japan Yokohama Kanogawa 230, Japan
Ibaraki
319-11
3
Japan
A tungsten-copper (W-Cu) bond is a reference design concept for a divertor collector plate in the Fusion Experimental Reactor (FER). In the present study, durability of the W-Cu bond under thermal cycling was experimentally examined. A plasma spraying technique was applied to form a tungsten layer with a thickness of around 2 mm on a copper disk. This technique has advantages in that tungsten can be deposited on curved and complex surfaces of substrates and that the temperature of the substrate can be kept low during the deposition. Two kinds of copper materials, oxygen-free high conductivity copper (Cu/OFHC) and copper alloyed with beryllium (Cu/Be), were used. The tungsten surface of the W-Cu test piece was periodically irradiated with a high temperature argon plasma jet under the thermal stress condition corresponding to that expected at the normal operation of the FER. The thermal properties of the plasma sprayed tungsten were measured before the test, and the microstructure before and after the test was investigated with a scanning electron microscope. The sprayed tungsten layer was found stratified and had porosity of 2-3 I% before the test. The W-Cu/OFHC test piece survived for thermal cycle up to 6000, while the W-Cu/Be test piece broke between 4500 and 5000 cycles.
1. Introduction The Fusion Experimental Reactor (FER) [l] is now being designed at JAERJ as a next generation tokamak machine. The FER is equipped with a divertor as the impurity control system. Divertor collector plates are inherently exposed to high energy particles and high heat loads. A duplex structure, composed of a heat sink material and an armor material, has often been proposed for the divertor collector plates. The FER employs a copper heat sink with tungsten armor for the collector plate. A metallurgical bonding method is superior to a mechanical attaching method because of high thermal conductance at the interface, but it generates large thermal stresses due to the difference in the thermal expansion coefficients of tungsten and copper. To reduce the thermal stress, it is proposed to insert a buffer material with a thermal expansion coefficient intermediate between tungsten and copper. However, it is fist necessary to accumulate technical experience of bonding methods and to examine the durability of the W-Cu bond without the buffer layer, against thermal cycles. Since only limited data were available on the thermal fatigue of the W-Cu duplex structure [2,3], we have
started to carry out experiments on the durability of the W-Cu bond against thermal cycles. In the present paper, the experimental results of the thermal properties and the integrity of the plasma sprayed W-Cu bond against thermal cycles are reported.
2. Previous studies Promising methods to make the W-Cu duplex structures include brazing, direct casting, diffusion bonding, plasma spraying, and so on. Brossa et al. [3], having examined brazing, diffusion bonding, and direct casting methods, reported that the diffusion bonding method was not applicable to large scale structures. We have already carried out the experiments [2] using the test pieces made by brazing or casting methods as listed in table 1. Two kinds of tungsten materials were used for the brazed W-Cu test pieces. In the tungsten (W/Fe/K) which contains an extremely small quantity of impurities and was sintered at 3000 *C, microcracks were observed only in the brazing material after 3 700 thermal cycles as shown in fig. l(a). While in the tungsten (W/Ni-P/Fe/MO) which was sintered at 1100 o C, grain boundary microcracks were observed in the tungsten only after 200 thermal cycles as shown in
0920-3796/89/$03.50 0 Elsevier Science Publishers B.V.
218
M. Ogawa
e: al. / Thermal
cycling
lest of W-Cu
bona5
fig. l(b). A nickel brazing material indicated better wettability than silver, which was also reported by Brossa et al. [3]. They recommended the casting method because of its applicability to large scale structures, its economy,
W
Ni/Cr/P
Waterv Fig. 2. Experimental set-up.
w
Ag/Cu/Zn/Ni
Fig. 1. Micro structures of W-Cu test pieces brazed and direct casted after thermal cycles. (a) Brazed W-Cu test piece after 3700 cycles (the scale marker at the bottom right-hand side is 10 pm and the arrows indicate cracks); (b) Brazed W-Cu test piece after 200 cycles (the scale marker at the top left-hand side is 10 pm); (c) Direct casted w-01 test piece after 2200 cycles (the scale marker at the bottom right-hand side is 100 am).
and easiness. However, the bonding temperature for the casting method reached 1250 o C, close to the tungsten recrystallization temperature of 1300 o C. Therefore, they coated the tungsten with a thin nickel layer to keep the tungsten temperature below 1300 o C. We improved the casting method in which the bonding temperature does not exceed the recrystallization temperature without nickel coating [2]. However, after 2200 thermal cycles, visible cracks were found in the tungsten as shown in fig. l(c). Although it is generally said that the effective thickness of the buffer layer absorbing the thermal stress is about 0.5 to 1 mm, these results suggest that the layer of the brazing material with a thickness of about 50 pm plays an important role in mitigating the thermal stress. The annealing temperature for oxygen-free high-conductivity copper (Cu/OFHC) is about 200400 a C,
TC : lhrmamuple I mm1
Fig. 3. Test piece of W-Cu duplex structure (the tungsten thickness t is about 2nun).
M. Ogawa et al. / Thermal cycling test of W-Cu bonds Table 1 Features of W-0.1 Material
test pieces and results of durability
Copper Tungsten
219
tests
Cu/OFHC W/Ni-P/Fe/MO (sintered at llOO°C)
Cu/OFHC W/Fe/K (sintered at 3000 o C)
Cu/OFHC W/Fe/K (sintered at 3000°C)
Cu/OFHC W/Fe/K (powder)
Cu/Be W/Fe/K (powder)
Bonding method (intermediate layer)
brazing (Ag/Cr/Zn/Ni)
brazing (Ni/Cr/P)
casting
spraying (Ni coating)
wayin (Ni coating)
Thermal
200 and 1100
3700
2200
6000
5ooo
gram boundary microcrack tungsten
microcrack
visible crack tungsten
(stratifi cation) (tungsten)
rupture
PI
PI
PI
present study
cycles
Result
crack,
rapture
position
Ref.
brazing material
whereasin the brazing or direct casting process,the copper, beingheatedup to about 1000 oC, therefore, is annealed.Thus, its mechanicalstrength is largely reduced. To prevent the copper from annealing, the tungstenwas sprayedonto a copper disk at a temperature below 200 ‘C in the present study. The other advantageof the plasmaspraying method is its ability to deposit tungstenon curved and complex surfaces.In addition to Cu/OFHC, copper alloyed with beryllium (Cu/Be) is also used, which has superior mechanical strength to Cu/OFHC.
3.1. Experimental
set-up
The experimental set-up is shown in fig. 2. The tungstensurfacewasirradiated with a high temperature argon plasmajet and the copper wascooled by a water flow. The configurationsand dimensionsof the W-Cu test pieceare shownin fig. 3. The test pieceis a cylinder with a 50 mm outer diameter.The tungstenand copper thicknessesare 2 mm and 20 mm, respectively. The copper heat sink wasmachinedto accommodatea cooling and supportingstructure.Three thermocoupleswere mounted in the copper to measuretemperaturesin the vicinity of the interface (TC-3,5) and the cooledsurface (TC-4) asshownin fig. 3. The bonding condition is shown in table 2. The copper disk wascoated with a nickel thin layer to gain good wettability. The spraying processis describedin the following.
present study
(1) The copper surface is washed, blasted, and then coated with about a 100 pm thick nickel layer. (2) The powder tungsten with a particle size of about 30-75 pm is sprayed on the copper surfaceheld at about 200 o C. (3) The W-Cu bond is ground. Two test pieces,W-Cu/OFHC and W-Cu/Be, were used.Sincethe Young’s modulusof the Cu/Be is larger than that of the Cu/OFHC, a longerlife is expectedfor the W-Cu/Be bond than for the W-Cu/OFHC. 3.2. Experimental
3. Experiment
lower part test piece
condition
and procedure
The thermal propertiessuchasspecificheat, thermal conductivity, thermal diffusibility, and bulk density of the sprayed tungstenbefore the test were measuredby usinga thermalproperty measurement rig of a laser-flash type both in open air and in vacuum.
Table 2 Bonding condition Material
in plasma
spraying
powder particle
tungsten diameter
method sintered at 3000 o C 30-75 pm
Blasting
condition
pressure material
0.5 MPa sand/Al 2Os
Spraying
condition
arcing gas arcing current arcing voltage copper temperature tungsten thickness
argon and helium 8OOA 35 v about 200 o C about 2 mm
M. Ogawa et al. / Thermal cycling test of W-Cu bon&
280
-
ii
the radial distributions of the shear stress calculated by ADINA under the experimental condition. The maximum shear stress was about 100 MPa near the edge in tungsten. The heating and cooling periods were 20 s and 120s respectively as shown in fig. 5. W-Cu/OFHC and W-Cu/Be test pieces were explosed to 6000 and 5000 thermal cycles, respectively, as shown in table 1. Before and after the test, the test pieces were investigated with the aid of a scanning electron microscope (SEM).
-Tungsten
5, -100
I
I 0
I
I
10
center
I
I
20
r lmmt
edge
Fig. 4. Radial distributions of shear stress at the interface of tungsten and copper (calculated by ADINA). The experimental condition was determined to generate the thermal stress almost equivalent to that estimated in the FER design. The thermal stress of the test piece was predicted by using the elastic-plastic stress analysis code ADINA. In the calculation, the heat load from the plasma jet was given so that the calculated temperatures agreed with those measured. Fig. 4 shows 1”
150
-
-.-
“I””
I
’
TC-3 TC-I,
4. Results and discussion 4. I. Thermal properties Table 3 compares the thermal properties of the sprayed tungsten with those of the sintered tungsten used for the brazed W-Cu bond. The thermal conductivity of the sprayed tungsten is roughly one tenth of that of the sintered tungsten. The over-all coefficient of heat transmission of the sprayed test piece is half that of the brazed test piece. These poor thermal properties result from both the pore and stratified structure of the sprayed tungsten as shown in fig. 6(a) and (b). Thus, the bulk density of the sprayed tungsten was about four-fifths that of the sintered tungsten. 4.2. Thermal
Fig. 5. Temperature traces in the vicinity of the interface (TC-3,5, see fig. 2) and cooled surface (TC-4).
cycling test
The W-Cu/Be test piece broke at between 4 500 and 5 000 thermal cycles as shown in table 1. Fig. 7(a) shows the rupture part near the lower edge of the test piece. A thin tungsten layer remained on the copper surface. Fig. 7(b) shows this part observed by SEM. The W-Cu/OFHC test piece did not break for up to 6000 thermal cycles. The spraying process under atmospheric pressure makes the deposited tungsten layer stratified. Then, the
Fig, 6. Plasma sprayed W-Cu/OFHC test piece exposed to 6000 cycles: (a) Plasma sprayed test piece after 6000 cycles (the scale marker at the bottom right-hand side is 100 pm); (b) Expanded photograph of stratified tungsten layer (the scale marker at the bottom right-hand side is 10 pm).
kf. Ogawa
et al. / Thermal
cycling
test of W-Cu
281
bona5
Table 3 Thermal properties of sprayed tungsten Density (kg/m-‘) Specific heat (J/kg/K) Thermal conductivity (W/m/K) Thermal diffusibility (m’/s) Overall coefficient of heat transmission (W/m2/K)
Sprayed tungsten 15800 176 9.8 (in air) 8.7 (in vacuum) 3.5 x 10-6 (in air) 3.1 X 10e6 (in vacuum) 5.0x104
spraying processwastried under low pressure.But, the
thermal conductivity was hardly increased.If the hot isostatic pressing(HIP) processis applied after the spraying process,the tungsten layer is expected to become as denseas the sintered tungsten. However, the advantageof the spraying method is lost, becausethe copper is inevitably annealedin this process.It can be said that the plasmaspraying techniqueis not yet fully establishedto manufacturethe W-cu duplex structure at the presentstage. The effect of the beryllium was not definitely confirmed in the presenttest.
Brazed tungsten 19300 133 178 69.4x10-6 13.0-16.0 x lo4 (brazed W-Cu)
(6) not to worsen tungsten and/or copper materials, suchas to avoid stratified tungstenand not to form a brittle solid solution. The brazing and casting methods do not satisfy items (2) and (5), while the plasmaspraying method doesnot satisfy item (6). No bonding techniquesatisfies all items at the present stage. Consequently, future subjectsfor researchinclude the following: (1) to improve bonding techniquesand to carry out the durability test againstthermal cycles: (2) to establisha life-time prediction method for the duplex structure, that is, to clarify the main mechanism or parametersdominating the life.
4.3. Discussion
The bonding methodsof tungsten and copper are required: (1) to prevent the tungstenfrom recrystallization; (2) to keep the copper temperaturebelow 200 oC; (3) to be applicable to large scalestructure and complicated tungstenconfiguration; (4) to manufacturesimply, easily, and economically; (5) to generatesmallresidualthermal stress;and
5. Concluding
remarks
The durability test of the plasmasprayed W-Cu/ OFHC and W-Cu/Be test piecesexposedperiodically to an argon plasmajet was carried out for a thermal stress condition similar to that expected during the normal operation of the FER. The W-Cu/OFHC test piecedid not break for thermal cyclesup to 6000, while
Fig. 7. Plasma sprayed W&u/Be test piece exposed to 5000 cycles: (a) Plasma sprayed W-Cu/Be test pmce (outer diameter is 50mm); (b) Rupture part (the scale marker at the bottom right-hand side is 100 pm).
282
M. Ogowo
et al. / Thermoi ’ cycling
the W-&/Be test piece broke between 4 500 and 5 000 thermal cycles. It can be said that the plasma spraying technique is not yet fully established to manufacture the W-Cu duplex structure at the present stage. Future subjects include to improve the bonding technique and to carry out the durability test against thermal cycles, and to clarify the main mechanism or parameters dominating the life.
lest of W-Cu
bona5
References [l] T. Tone et al., Conceptual design of Fusion Experimental Reactor (FER), Nucl. Technol. Fusion 4(2) (1983) 573-578. [2] M. Seki, M. Ogawa, A. Minato, K. Fukaya, T.Tone, and N. Miki, A thermal cycling durability test of tungsten copper duplex structures for use as a divertor plate, Fusion Engrg. Des. 5(2) (1987) 205-213. [3] F. Brossa. P. Ghiselli, G. Tonunei, G. Piatti, and P. Schiller, Experimental tests concerning the use of the tungstencopper couple design concept on the clivertor system, Fusion Technol. 1 (1982) 491-496.