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Temperature distribution in a cylindrical Al2O3-steel joint during the vacuum brazing cycle
Journal of
Materials Processing Technology
ELSEVIER
Journal of Materials Processing Technology 56 (1996) 945-954
TEMPERATURE DISTRIBUTION IN A CYLINDRICAL A1203-STEEL JOINT DURING THE VACUUM BRAZING CYCLE D. Golanski Welding Department of L T.B., Faculty of Production Engineering, Warsaw University of Technology, 85 Narbutta Str, 02-524 Warsaw, Poland
ABSTRACT This paper deals with the investigations of temperature distribution during vacuum brazing of cylindric AI203 and steel samples, using the AgCulnTi active filler metal. The thermovision system AGA 680S was used to register the temperature field during brazing cycle. Bonded joints were observed by the infrared scanner unit during heating, brazing and cooling.. Two types of joints with different shape of steel element were examined during temperature measurements. Additionally, a 2D finite element code "ADINA-T" was used to solve for the problem of nonlinear transient thermal analysis of temperature distribution in the axisymmetric models of A1203steel brazed joints. The experimental results revealed the presence of big transient axial temperature drops in the ceramic part. The change of metal shape resulted in a reduction of the transient temperature drops in the ceramic element. The finite element calculations corresponded well with the results of measurements and showed that the perfect thermal contact between adjacent surfaces plays the important role in heat transfer and can be a factor affecting temperature gradients in ceramic.
1.
INTRODUCTION
The process of bonding (brazing, diffusion bonding) ceramics to metals is an unsteady process in which the temperature as the main parameter is varying according to the specified cycle: heating-brazing,cooling High temperatures that are needed for bonding such materials can cause large temperature drops inside each material. This can refer especially to ceramics characterized by a low heat conductivity and poor diffusivity which make the barrier in the heat flow through the material. The result can be big transient temperature gradients in the ceramic part. The main drawback of these gradients is the increase of transient thermal stresses in ceramic leading to cracking of ceramic ref. [ 1]. This can be danger especially during cooling from brazing temperature to room temperature when the transient thermal stresses are summed up with the residual stresses in ceramic developed from the mismatch in coefficient of thermal expansions between ceramic and steel. The aim of this paper was to present the results of investigation of transient temperature distribution in ceramic to metal joints registered during vacuum brazing cycle of A1203-steel samples. The method used for evaluating the temperature changes in brazing cycle was the AGA thermovision system. To check for the possibility of calculating temperature distribution during brazing cycle a finite element code "ADINA-T" was used to solve for the problem of nonlinear transient thermal analysis of axisymmetric A1203-steel models. 0924-0136/96/$15.00 © 1996 Elsevier Science S.A. All rights reserved SSD10924-0136 ( 95 ) 0 1 9 0 6 - U
946
2.
D. Golanski / Journal of Materials Processing Technology 56 (1996) 945-954
EXPERIMENTAL TEMPERATURE MEASUREMENT
2.1. Specimens The ceramic to metal joints are configured of 98% alumina and construction steel (0.15%C, 0.35%Mn, 0.17%Si, Polish Standard Steel 15) samples. The brazing filler metal was Ag72.5Cu19.5In5Ti3 active filler metal (CB 1-Degussa) with thickness 0.1 mm. Two kinds of A1203-steel samples were used in the tests. They differ in the shape of steel surface to be bonded. The shape and dimensions of these samples are presented in Fig. 1. The ceramic and metal samples were placed in a vacuum brazing chamber and fixed between two graphite cylinders as shown in Fig. 2. The ends of graphite elements were placed inside the brazed tubes positioning them in vertical axis. The heat was generated in induction coils and transported to the brazed parts by graphite cylinders.
type-BZ
type-A Z
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Fig. 1. Shape and dimensions of vacuum brazed Al203-steel samples 2.2. Brazing parameters The brazing parameters: vacuum (1~2-10 -3 Pa), brazing time (26 minutes), brazing temperature (800°C) were similar in both types of joints except the heating and cooling rates. The average heating rate was 7.7°C/minute (330+800°C) for type-A joint and 25.6°C (370+800°C) for type-B joint, and the average cooling rate was 7.6 C/minute (800+300°C) for type-A joint and 8.8°C/minute (800+250°C) for type-B joint• The rate of temperature change was lowest at temperatures close to the brazing temperature. 2.3. Measuring equipment During brazing cycle the thermovision scanner unit AGA 680S (wave length 2+5 lam) was monitoring the temperature field on the surface of brazed materials through the quartz window installed in the wall of brazing chamber (Fig. 3).
Fig. 2. Settlement of A1203-steel samples before brazing
Fig. 3. The location of the infrared scanner unit against the quartz window installed in the vacuum brazing chamber
To equal the radiating conditions for materials with different emmisivity a part of circular surface of ceramic and steel samples were covered with a thin black layer withstanding high temperatures (soot layer with emmisivity above 0.97). Thus, the blackened circular surface was radiating like a flat one, because the emitting energy for the black body does not depend upon the angle of radiation. The quartz window allowed the infrared monitoring of the temperature changes but the quartz transmission characteristic cut out some signals from the wave range
D Golanski/Journal of Materials Processing Technology 56 (1996) 945-954
947
used by the thermovision detector. As a result, the highest signal was weakened, yet it was changing with temperature according to the Wien's rule. Therefore, in order to calibrate the results registered by thermovision equipment additional temperature measurement by a thermocouple attached to the steel surface was necessary. 2.4. Results of experiment The results of the test were temperature profiles along the surface of brazed elements registered in the whole brazing cycle. They were calculated from the registered signal profiles using calibration curves selected upon applied shutter and sensitivity level of infrared camera scanning. Because of the space limit only one typical profile for each part of brazing process (heating, brazing and cooling) was presented in this paper. They are shown in Fig. I (Appendix) for the joint type-A and type-B. Analysing these graphs it can be stated that during brazing cycle of alumina to steel elements big transient axial temperature drops occur in the ceramic part. They reach the highest magnitude in the highest temperatures of the process. Type-A joint reached 100°C axial temperature drop in ceramic while type-B joint about 70°C. In both types of joints the temperature drops in the steel element were 2 to 4 times lower than in the ceramic and didn't exceed 30°C. An important thing to notice was a sharp change of temperature profile across the ceramic-metal interface as was shown on the graphs for type-A joint (vertical line at z=0.0). This was also observed for the ceramicgraphite and steel-graphite faying surfaces, but not shown on the profiles. This was due to the non-perfect thermal contact between two adjacent materials (especially dissimilar materials). Only for a very intimate contact between two faying surfaces the temperatures on both of them can be the same. Here, it was reported mainly during heating when the braze metal was not bonded to the ceramic and metal. This seems to be an important factor in the heat transfer across the surface of separation between two materials ref. [2]. Interesting results gave the change of the transient axial temperature drop in ceramic during whole brazing process what was shown in Fig. 4a and 4b. It is clearly seen that the change in shape of steel element resulted in reducing the transient temperature drops in ceramic. Even the heating rate was 3 times higher for type-B joint the resulted transient temperature drop was nearly two times lower than in type-A joint. During cooling process the transient temperature drop in ceramic became to vanish beginning from -600°C in type-B joint and from -300°C in type-A joint. 120 "~ J
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Fig. 4 Transient axial surface temperature drop in ceramic occurred during heating (a) and cooling (b) stages of type-A and type-B vacuum brazed Al203-steel joints, registered by the infrared scanner unit. This can be very important when considering thermal and residual stresses in ceramic-metal joints developing during cooling process and causing cracking of ceramic. Here, after brazing cycle was completed the type-A joint revealed damage by circumferential crack in the ceramic while type-B joint was reported as free of cracks_ Of course, the reduced shape of steel part had a big influence on the magnitude of forces acting on brazed parts in the cooling stage. But the results of temperature measurements showed the importance of transient temperature drops in ceramic regarding brazing cycle of ceramic-to-metal joints.
948
3.
D. Golanski / Journal of Materials Processing Technology 56 (1996) 945-954
FINITE ELEMENT ANALYSIS OF TEMPERATURE FIELD
3.1. Assumptions for FEM calculations The analysis of finite element method was used for calculating temperature distribution in the ceramic to metal models. The scope of this analysis was to check whether this kind of evaluation could be implemented in investigated problem. The finite element code "ADINA-T" was used for computing temperature fields in the axisymmetric model of Al203-steel joints subjected to the thermal brazing cycle according to the experimental procedure. The governing nonlinear heat transfer equation together with initial and boundary conditions can be found in ref [2,3]. The most important assumptions to FEM calculations are summarized below: (a) The models used in calculations reflect the joints geometry used in the experiment and the graphite parts are leQ inside brazed elements (Fig. 5). (b) It was assumed that the heat sources were the uniform temperatures along the radius of the model acting on the external ends of ceramic and steel parts (temperature boundary condition). These temperatures followed the brazing cycles (heating, brazing and cooling) of Al203-steel joints used in the experiment. The other outer circular surfaces of ceramic, steel and filler metal were subjected to the radiative boundary condition only (the process was conducted in vacuum) and the rest surfaces do not participate in the heat transfer. (c) There is a perfect thermal contact between any laying surfaces of the model. (d) The heat and radiative material properties vary with temperature and were taken from the literature re£ [4]. The filler metal properties were found according to the percentage proportion of silver and copper elements in the braze metal. type-A z
~
type-B ~
graphite
filler metal
~z I//A~
Fig. 5. Models of Al203-steel joints used in finite element analysis. 3.2. Finite element model Because of symmetry only one-half of models shown in Fig. 5 was used for computation. It comprised of 8node axisymmetric conductive quadrilateral elements and 3-node axisymmetric radiative line elements creating the finite element mesh (1583 nodes in type-A model and 1820 nodes in type-B model) shown in Fig. II(a) and III(a) (Appendix) for type-A and type-B joint models respectively. The mesh was refined near the interface close to the outer surface of the model. Calculations were carried out with 60 seconds stepping using time-step implicit method (Euler-backward time integration method). 3.3. Results of finite element computations The result of finite element analysis was the distribution of transient temperature field in the analyzed models. Three selected temperature maps representing the temperature distribution during heating, brazing and cooling process were shown in Fig. II(b), II(c), II(d) and III(b), III(c), III(d) (Appendix). Also three temperature profiles along the surface of the model were presented in Fig. IV(a) and IV(b) (Appendix) to compare them with the experiment. The transient temperature fields showed the existence of both axial and radial temperature drops in ceramic. The radial drops were two times lower than axial and didn't exceed 30°C in both types of joints. The highest axial temperature drops in ceramic reached 82°C in type-A and 78°C in type-B model during brazing stage. Similar to measurements the temperature drops in steel part were very small which can be seen from profiles shown in Fig. IV (Appendix).
D. Golanski / Journal of Materials Processing Technology 56 (1996) 945-954
949
The assumption of the perfect thermal contact between all adjacent surfaces in analyzed models was the reason of smaller transient temperature drops in ceramic in comparison with the experiment. This was observed mainly during heating process, especially for type-A model joint as was shown in Fig. 6. 120
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Fig. 6 The comparison of transient axial surface temperature drop in ceramic occurred during heating stage of type-A (a) and type-B (b) vacuum brazed Al203-steel joints, measured by thermovision and calculated by FEM. In order to receive accurate results from the heating stage a simulation of non-perfect thermal contact between adjacent materials should be included. During cooling process (Fig. 7) when brazed parts were bonded, the convergence between calculated and measured axial temperature drops in ceramic was much better. It is seen especially for type-A joint. %20-
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Fig. 7 The comparison of transient axial surface temperature drop in ceramic occurred during cooling stage of type-A (a) and type-B (b) vacuum brazed A1203-steel joints, measured by thermovision and calculated by FEM
Here, the radiative boundary conditions used in FEM calculations were much simpler comparing to type-B model joint, because there were no any additional heat exchange at the interface. However, the assumption of no radiative heat exchange between the surfaces created by the changed shape of steel part in type-B joint resulted in certain deviation of temperature drop in ceramic from the results achieved by the measurement. Generally, the whole vacuum brazing process described in Fig. 8 by the nodal brazing thermal cycle (for the point r=12.5mm, z=0.0mm) showed good profile agreement for both types of analyzed A1203-steel joints.
D. Golanski /Journal of Materials Processing Technology 56 (1996) 945-954
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4
CONCLUSIONS
The results presenting the temperature distribution in vacuum brazed A1203-steel joints obtained from the experiment and computations gave some important remarks: (a) During vacuum brazing cycle of Al203-steel samples big transient temperature drops are formed in ceramic especially in high temperatures. This can affect the increase of the transient thermal stresses in ceramic that are responsible for cracking of this material. (b) A change in the shape of metal surface can advantage in lowering of transient temperature drops in ceramic part of Al203-steel brazed joints. (c) The elimination (reduction) of the thermal contact barrier existing between any faying surfaces participating in the heat transfer can be the method to reduce the transient temperature drops in the ceramic bonded to metal in the vacuum brazing cycle. (d) The finite element method used for calculation of temperature distribution in the vacuum brazed Al203-steei model joints gave acceptable results in comparison with the measurement. More precise results can be obtained by including in computations data representing the thermal contact barrier between any faying surfaces of a model.
ACKNOWLEDGEMENTS Author wishes to thank The Institute of Mechanics and Design of Warsaw University of Technology for helping to use the "ADINA-T" code, and PIRS Company for using AGA thermovision system.
REFERENCES [1] [2] [3] [4]
D.L. Hartsock and A.F. McLean, What the Designer with Ceramics Needs, Ceramic Bulletin, vol.63, No.2(1984), p.266 H.S. Carslaw and J.C. Jaeger, Conduction of Heat in Solids, Oxford Science Publications 2nd edition 1989, p.23 ADINA-T, Theory and modelling guide, ADINA corp. A. Goldsmith, TE. Waterman, H.J. Hirhorn, Handbook of Thermophysical Properties of Solid Materials, New York 1961
951
D Golanski/Journal of Materials Processing Technology 56 (1996) 945-954
APPENDIX 690
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Fig. I. Axial surface transient temperature profiles measured by thermovision scanner, selected from heating, brazing and cooling stages of vacuum brazing cycle oftype-A (a) and type-B (b) A1203-steel joints•
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D. Golanski / Journal of Materials Processing Technology 56 (1996) 945-954
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D. Golanski / Journal of Materials Processing Technology 56 (1996) 945-954
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Fig. IV. Axial surface transient temperature profiles calculated by finite element method, selected from heating brazing and cooling stages o f vacuum brazing cycle oftype-A (a) and type-B (b) Al203-steel joints.
Materials Processing Technology
ELSEVIER
Journal of Materials Processing Technology 56 (1996) 945-954
TEMPERATURE DISTRIBUTION IN A CYLINDRICAL A1203-STEEL JOINT DURING THE VACUUM BRAZING CYCLE D. Golanski Welding Department of L T.B., Faculty of Production Engineering, Warsaw University of Technology, 85 Narbutta Str, 02-524 Warsaw, Poland
ABSTRACT This paper deals with the investigations of temperature distribution during vacuum brazing of cylindric AI203 and steel samples, using the AgCulnTi active filler metal. The thermovision system AGA 680S was used to register the temperature field during brazing cycle. Bonded joints were observed by the infrared scanner unit during heating, brazing and cooling.. Two types of joints with different shape of steel element were examined during temperature measurements. Additionally, a 2D finite element code "ADINA-T" was used to solve for the problem of nonlinear transient thermal analysis of temperature distribution in the axisymmetric models of A1203steel brazed joints. The experimental results revealed the presence of big transient axial temperature drops in the ceramic part. The change of metal shape resulted in a reduction of the transient temperature drops in the ceramic element. The finite element calculations corresponded well with the results of measurements and showed that the perfect thermal contact between adjacent surfaces plays the important role in heat transfer and can be a factor affecting temperature gradients in ceramic.
1.
INTRODUCTION
The process of bonding (brazing, diffusion bonding) ceramics to metals is an unsteady process in which the temperature as the main parameter is varying according to the specified cycle: heating-brazing,cooling High temperatures that are needed for bonding such materials can cause large temperature drops inside each material. This can refer especially to ceramics characterized by a low heat conductivity and poor diffusivity which make the barrier in the heat flow through the material. The result can be big transient temperature gradients in the ceramic part. The main drawback of these gradients is the increase of transient thermal stresses in ceramic leading to cracking of ceramic ref. [ 1]. This can be danger especially during cooling from brazing temperature to room temperature when the transient thermal stresses are summed up with the residual stresses in ceramic developed from the mismatch in coefficient of thermal expansions between ceramic and steel. The aim of this paper was to present the results of investigation of transient temperature distribution in ceramic to metal joints registered during vacuum brazing cycle of A1203-steel samples. The method used for evaluating the temperature changes in brazing cycle was the AGA thermovision system. To check for the possibility of calculating temperature distribution during brazing cycle a finite element code "ADINA-T" was used to solve for the problem of nonlinear transient thermal analysis of axisymmetric A1203-steel models. 0924-0136/96/$15.00 © 1996 Elsevier Science S.A. All rights reserved SSD10924-0136 ( 95 ) 0 1 9 0 6 - U
946
2.
D. Golanski / Journal of Materials Processing Technology 56 (1996) 945-954
EXPERIMENTAL TEMPERATURE MEASUREMENT
2.1. Specimens The ceramic to metal joints are configured of 98% alumina and construction steel (0.15%C, 0.35%Mn, 0.17%Si, Polish Standard Steel 15) samples. The brazing filler metal was Ag72.5Cu19.5In5Ti3 active filler metal (CB 1-Degussa) with thickness 0.1 mm. Two kinds of A1203-steel samples were used in the tests. They differ in the shape of steel surface to be bonded. The shape and dimensions of these samples are presented in Fig. 1. The ceramic and metal samples were placed in a vacuum brazing chamber and fixed between two graphite cylinders as shown in Fig. 2. The ends of graphite elements were placed inside the brazed tubes positioning them in vertical axis. The heat was generated in induction coils and transported to the brazed parts by graphite cylinders.
type-BZ
type-A Z
stee] ~/
steel/~/
tt%
fillermetal
filler metal ...L_ r
g
0•65 x-~
x~
x.
r_~
xx x~
AI20)
xx
IAI203 ~
~x x x, ~,
_
~16.5 ¢25
i
~16.5 ~'25
" ,,...
Fig. 1. Shape and dimensions of vacuum brazed Al203-steel samples 2.2. Brazing parameters The brazing parameters: vacuum (1~2-10 -3 Pa), brazing time (26 minutes), brazing temperature (800°C) were similar in both types of joints except the heating and cooling rates. The average heating rate was 7.7°C/minute (330+800°C) for type-A joint and 25.6°C (370+800°C) for type-B joint, and the average cooling rate was 7.6 C/minute (800+300°C) for type-A joint and 8.8°C/minute (800+250°C) for type-B joint• The rate of temperature change was lowest at temperatures close to the brazing temperature. 2.3. Measuring equipment During brazing cycle the thermovision scanner unit AGA 680S (wave length 2+5 lam) was monitoring the temperature field on the surface of brazed materials through the quartz window installed in the wall of brazing chamber (Fig. 3).
Fig. 2. Settlement of A1203-steel samples before brazing
Fig. 3. The location of the infrared scanner unit against the quartz window installed in the vacuum brazing chamber
To equal the radiating conditions for materials with different emmisivity a part of circular surface of ceramic and steel samples were covered with a thin black layer withstanding high temperatures (soot layer with emmisivity above 0.97). Thus, the blackened circular surface was radiating like a flat one, because the emitting energy for the black body does not depend upon the angle of radiation. The quartz window allowed the infrared monitoring of the temperature changes but the quartz transmission characteristic cut out some signals from the wave range
D Golanski/Journal of Materials Processing Technology 56 (1996) 945-954
947
used by the thermovision detector. As a result, the highest signal was weakened, yet it was changing with temperature according to the Wien's rule. Therefore, in order to calibrate the results registered by thermovision equipment additional temperature measurement by a thermocouple attached to the steel surface was necessary. 2.4. Results of experiment The results of the test were temperature profiles along the surface of brazed elements registered in the whole brazing cycle. They were calculated from the registered signal profiles using calibration curves selected upon applied shutter and sensitivity level of infrared camera scanning. Because of the space limit only one typical profile for each part of brazing process (heating, brazing and cooling) was presented in this paper. They are shown in Fig. I (Appendix) for the joint type-A and type-B. Analysing these graphs it can be stated that during brazing cycle of alumina to steel elements big transient axial temperature drops occur in the ceramic part. They reach the highest magnitude in the highest temperatures of the process. Type-A joint reached 100°C axial temperature drop in ceramic while type-B joint about 70°C. In both types of joints the temperature drops in the steel element were 2 to 4 times lower than in the ceramic and didn't exceed 30°C. An important thing to notice was a sharp change of temperature profile across the ceramic-metal interface as was shown on the graphs for type-A joint (vertical line at z=0.0). This was also observed for the ceramicgraphite and steel-graphite faying surfaces, but not shown on the profiles. This was due to the non-perfect thermal contact between two adjacent materials (especially dissimilar materials). Only for a very intimate contact between two faying surfaces the temperatures on both of them can be the same. Here, it was reported mainly during heating when the braze metal was not bonded to the ceramic and metal. This seems to be an important factor in the heat transfer across the surface of separation between two materials ref. [2]. Interesting results gave the change of the transient axial temperature drop in ceramic during whole brazing process what was shown in Fig. 4a and 4b. It is clearly seen that the change in shape of steel element resulted in reducing the transient temperature drops in ceramic. Even the heating rate was 3 times higher for type-B joint the resulted transient temperature drop was nearly two times lower than in type-A joint. During cooling process the transient temperature drop in ceramic became to vanish beginning from -600°C in type-B joint and from -300°C in type-A joint. 120 "~ J
120 :j aAA~,A TYPE-A ooaao
HEATING
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,~ 20. / 300
400
500 600 700 800 Reference temperoture ot z----12.5mm (°C)
900
0 200
300
400 500 600 700 B00 Reference temperature at z = - 1 2 . 5 m m (°C)
9C}~3
Fig. 4 Transient axial surface temperature drop in ceramic occurred during heating (a) and cooling (b) stages of type-A and type-B vacuum brazed Al203-steel joints, registered by the infrared scanner unit. This can be very important when considering thermal and residual stresses in ceramic-metal joints developing during cooling process and causing cracking of ceramic. Here, after brazing cycle was completed the type-A joint revealed damage by circumferential crack in the ceramic while type-B joint was reported as free of cracks_ Of course, the reduced shape of steel part had a big influence on the magnitude of forces acting on brazed parts in the cooling stage. But the results of temperature measurements showed the importance of transient temperature drops in ceramic regarding brazing cycle of ceramic-to-metal joints.
948
3.
D. Golanski / Journal of Materials Processing Technology 56 (1996) 945-954
FINITE ELEMENT ANALYSIS OF TEMPERATURE FIELD
3.1. Assumptions for FEM calculations The analysis of finite element method was used for calculating temperature distribution in the ceramic to metal models. The scope of this analysis was to check whether this kind of evaluation could be implemented in investigated problem. The finite element code "ADINA-T" was used for computing temperature fields in the axisymmetric model of Al203-steel joints subjected to the thermal brazing cycle according to the experimental procedure. The governing nonlinear heat transfer equation together with initial and boundary conditions can be found in ref [2,3]. The most important assumptions to FEM calculations are summarized below: (a) The models used in calculations reflect the joints geometry used in the experiment and the graphite parts are leQ inside brazed elements (Fig. 5). (b) It was assumed that the heat sources were the uniform temperatures along the radius of the model acting on the external ends of ceramic and steel parts (temperature boundary condition). These temperatures followed the brazing cycles (heating, brazing and cooling) of Al203-steel joints used in the experiment. The other outer circular surfaces of ceramic, steel and filler metal were subjected to the radiative boundary condition only (the process was conducted in vacuum) and the rest surfaces do not participate in the heat transfer. (c) There is a perfect thermal contact between any laying surfaces of the model. (d) The heat and radiative material properties vary with temperature and were taken from the literature re£ [4]. The filler metal properties were found according to the percentage proportion of silver and copper elements in the braze metal. type-A z
~
type-B ~
graphite
filler metal
~z I//A~
Fig. 5. Models of Al203-steel joints used in finite element analysis. 3.2. Finite element model Because of symmetry only one-half of models shown in Fig. 5 was used for computation. It comprised of 8node axisymmetric conductive quadrilateral elements and 3-node axisymmetric radiative line elements creating the finite element mesh (1583 nodes in type-A model and 1820 nodes in type-B model) shown in Fig. II(a) and III(a) (Appendix) for type-A and type-B joint models respectively. The mesh was refined near the interface close to the outer surface of the model. Calculations were carried out with 60 seconds stepping using time-step implicit method (Euler-backward time integration method). 3.3. Results of finite element computations The result of finite element analysis was the distribution of transient temperature field in the analyzed models. Three selected temperature maps representing the temperature distribution during heating, brazing and cooling process were shown in Fig. II(b), II(c), II(d) and III(b), III(c), III(d) (Appendix). Also three temperature profiles along the surface of the model were presented in Fig. IV(a) and IV(b) (Appendix) to compare them with the experiment. The transient temperature fields showed the existence of both axial and radial temperature drops in ceramic. The radial drops were two times lower than axial and didn't exceed 30°C in both types of joints. The highest axial temperature drops in ceramic reached 82°C in type-A and 78°C in type-B model during brazing stage. Similar to measurements the temperature drops in steel part were very small which can be seen from profiles shown in Fig. IV (Appendix).
D. Golanski / Journal of Materials Processing Technology 56 (1996) 945-954
949
The assumption of the perfect thermal contact between all adjacent surfaces in analyzed models was the reason of smaller transient temperature drops in ceramic in comparison with the experiment. This was observed mainly during heating process, especially for type-A model joint as was shown in Fig. 6. 120
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Fig. 6 The comparison of transient axial surface temperature drop in ceramic occurred during heating stage of type-A (a) and type-B (b) vacuum brazed Al203-steel joints, measured by thermovision and calculated by FEM. In order to receive accurate results from the heating stage a simulation of non-perfect thermal contact between adjacent materials should be included. During cooling process (Fig. 7) when brazed parts were bonded, the convergence between calculated and measured axial temperature drops in ceramic was much better. It is seen especially for type-A joint. %20-
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Fig. 7 The comparison of transient axial surface temperature drop in ceramic occurred during cooling stage of type-A (a) and type-B (b) vacuum brazed A1203-steel joints, measured by thermovision and calculated by FEM
Here, the radiative boundary conditions used in FEM calculations were much simpler comparing to type-B model joint, because there were no any additional heat exchange at the interface. However, the assumption of no radiative heat exchange between the surfaces created by the changed shape of steel part in type-B joint resulted in certain deviation of temperature drop in ceramic from the results achieved by the measurement. Generally, the whole vacuum brazing process described in Fig. 8 by the nodal brazing thermal cycle (for the point r=12.5mm, z=0.0mm) showed good profile agreement for both types of analyzed A1203-steel joints.
D. Golanski /Journal of Materials Processing Technology 56 (1996) 945-954
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7000
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4
CONCLUSIONS
The results presenting the temperature distribution in vacuum brazed A1203-steel joints obtained from the experiment and computations gave some important remarks: (a) During vacuum brazing cycle of Al203-steel samples big transient temperature drops are formed in ceramic especially in high temperatures. This can affect the increase of the transient thermal stresses in ceramic that are responsible for cracking of this material. (b) A change in the shape of metal surface can advantage in lowering of transient temperature drops in ceramic part of Al203-steel brazed joints. (c) The elimination (reduction) of the thermal contact barrier existing between any faying surfaces participating in the heat transfer can be the method to reduce the transient temperature drops in the ceramic bonded to metal in the vacuum brazing cycle. (d) The finite element method used for calculation of temperature distribution in the vacuum brazed Al203-steei model joints gave acceptable results in comparison with the measurement. More precise results can be obtained by including in computations data representing the thermal contact barrier between any faying surfaces of a model.
ACKNOWLEDGEMENTS Author wishes to thank The Institute of Mechanics and Design of Warsaw University of Technology for helping to use the "ADINA-T" code, and PIRS Company for using AGA thermovision system.
REFERENCES [1] [2] [3] [4]
D.L. Hartsock and A.F. McLean, What the Designer with Ceramics Needs, Ceramic Bulletin, vol.63, No.2(1984), p.266 H.S. Carslaw and J.C. Jaeger, Conduction of Heat in Solids, Oxford Science Publications 2nd edition 1989, p.23 ADINA-T, Theory and modelling guide, ADINA corp. A. Goldsmith, TE. Waterman, H.J. Hirhorn, Handbook of Thermophysical Properties of Solid Materials, New York 1961
951
D Golanski/Journal of Materials Processing Technology 56 (1996) 945-954
APPENDIX 690
THEJIMOVISlON
i
87ol
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s
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Fig. I. Axial surface transient temperature profiles measured by thermovision scanner, selected from heating, brazing and cooling stages of vacuum brazing cycle oftype-A (a) and type-B (b) A1203-steel joints•
952
D. Golanski / Journal of Materials Processing Technology 56 (1996) 945-954
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Fig. III. The finite element mesh (a) and transient temperature distribution in type-B Al203-steel model joint selected from heating (b), brazing (c) and cooling (d) stages of vacuum brazing cycle.
D. Golanski / Journal of Materials Processing Technology 56 (1996) 945-954
954
690
a )
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670
-
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1
Fig. IV. Axial surface transient temperature profiles calculated by finite element method, selected from heating brazing and cooling stages o f vacuum brazing cycle oftype-A (a) and type-B (b) Al203-steel joints.