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Joining of stainless steel (SS304) and OFE copper by vacuum brazing
Materials Today: Proceedings xxx (xxxx) xxx
Contents lists available at ScienceDirect
Materials Today: Proceedings journal homepage: www.elsevier.com/locate/matpr
Joining of stainless steel (SS304) and OFE copper by vacuum brazing Vikas Mangla a,b,⇑, J.D. Sharma b, Sandeep Kumar a, Pal Dinesh Kumar a, Arun Agarwal a a b
Terminal Ballistics Research Laboratory, Sec-30, Chandigarh 160030, India Punjab Engineering College, Sec-12, Chandigarh 160012, India
a r t i c l e
i n f o
Article history: Received 11 November 2019 Accepted 3 January 2020 Available online xxxx Keywords: SS304 OFE copper Vacuum brazing Microstructure Hermeticity Micro hardness
a b s t r a c t This paper describes the vacuum brazing process for joining OFE copper with stainless steel (SS 304) using silver based filler material (68Ag27Cu5Pd). Vacuum brazing of SS304 and OFE copper was carried out in a vacuum furnace at a vacuum level of 10 6 mbar. The peak brazing temperature was kept as 920 °C with a soaking time of 10 min. This combination is often used in bi-metallic joints for high vacuum applications. Optimization of brazing parameters was done to produce defect free joints. Micro structural analysis of the brazed joint was carried out using optical and scanning electron microscope. The flow of filler material and diffusion of its elements into substrates was observed. The brazed joint was found to be of good quality and free from any major cracks. The micro hardness was evaluated across the interface and it was observed that hardness decreases towards the interface. The hermeticity of the joint was investigated using helium spray method. Ó 2020 Elsevier Ltd. All rights reserved. Selection and of the scientific committee of the 10th International Conference of Materials Processing and Characterization.
1. Introduction Welding, brazing, soldering, riveting, adhesive joining etc. are some of the widely used metal joining processes. The selection of material joining technique depends on the application of the product produced. Welding is basically used to join metals which have close melting points. In case of joining the metals having large difference in their physical and mechanical properties, brazing is generally used. Brazing involves the joining of two materials with the help of third material called as filler material. In brazing the joint is heated to a required temperature in the presence of filler material having liquidus temperature above 450 °C (840 °F) but below the solidus temperature of substrates. The filler can be used in various forms like powder, thin sheet etc. depending on the type of joint to be brazed. In the entire process only filler is melted and substrates remain in the solid state. The brazing filler metal is distributed between the closely fitted faying surfaces of the joint by capillary action. Joining takes place by formation of interatomic bonds like metallic in metals, ionic or covalent or mixed in ceramics [1,2]. ⇑ Corresponding author at: Terminal Ballistics Research Laboratory, Sec-30, Chandigarh 160030, India. E-mail address: [email protected] (V. Mangla).
The various elements like filler metal flow, base metal characteristics, surface preparation, joint design and clearance, brazing temperature and time, rate and source of heating etc. are to be clearly understood to produce good quality brazed joints [3]. The formation of interatomic bonds in brazing takes place by inter diffusion of atoms between filler and substrates. The brazing of substrates when carried out under vacuum is called vacuum brazing. This type of brazing is carried out in a vacuum furnace. There are several advantages of vacuum brazing over conventional brazing which are being listed below: Purity of atmosphere (i.e. vacuum) ensures less availability of oxygen to contaminate the work piece. The flow of filler material is uniform which results in better joint properties. Flux-free brazed joints are formed with superior joint strength. Minimum distortion of substrates due to controlled heating and cooling rates [4]. In the present work, the joining of SS304 and OFE copper has been carried out using vacuum brazing process. The characterization of the brazed joint has been done to evaluate metallurgical, mechanical properties and hermeticity of the joint.
https://doi.org/10.1016/j.matpr.2020.01.016 2214-7853/Ó 2020 Elsevier Ltd. All rights reserved. Selection and of the scientific committee of the 10th International Conference of Materials Processing and Characterization.
Please cite this article as: V. Mangla, J. D. Sharma, S. Kumar et al., Joining of stainless steel (SS304) and OFE copper by vacuum brazing, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2020.01.016
2
V. Mangla et al. / Materials Today: Proceedings xxx (xxxx) xxx
2. Experimental work The metals under investigation were SS304 and OFE copper solid cylinders both having same dimensions of / 10 and height 10 mm. The filler material used was in the form of disc of foil of dimensions / 10 and thickness 100 mm. Clean and oxide free surfaces of the substrates and filler are necessary to achieve good quality brazed joints. Uniform capillary flow between the substrates can be obtained only when all grease, oil, dirt and oxides have been removed from the filler material and substrates before brazing. The samples were thus ultrasonically cleaned with the suitable solvent. The degassing of the substrates and filler material was done at 600 °C to remove the entrapped gases which can otherwise can lead to defects like porosity in the final brazed product. The substrates and filler has been shown in Fig. 1a. The filler material was sandwiched between the substrates and kept in the hot zone of the vacuum furnace for brazing as shown in Fig. 1b. Vacuum brazing involves heating the component in a controlled vacuum environment. Brazing cycles plays a very crucial role in the vacuum brazing process for formation of strong joints. Heating rate was varied from 2 to 10 °C in the brazing cycle. In general, soaking time at brazing peak temperature should be long enough to ensure all sections of the work piece and all parts within the load reach the desired brazing temperature. The brazed sample was cut from the centre in the direction perpendicular to the longitudinal axis of the brazed joint for metallographic examination. The optical microscope (Leica, Germany, DM 2700P) and scanning electron microscope (Carl Zeiss, USA, EVO15) were used for the analysis. Energy-dispersive X-ray dot mapping analysis (EDX) was also carried out. The hardness evaluation across the interface was carried out using Vicker’s micro hardness tester (Radical, India, RMHT201). The hermeticity of the joint was checked using Helium leak detector.
Fig. 2. Samples after vacuum brazing.
Fig. 3. Optical micrograph at 100.
3. Results and discussions The experimental results obtained shows successful brazing of SS304 and OFE copper was achieved by vacuum brazing process (Fig. 2). Initially the sample was observed under optical microscope. The optical micrographs at 100X are being shown in Fig. 3. The interface was found to be continuous throughout the brazed joint with the absence of any major cracks or voids. The white phase was observed near to the interface on the copper side. To analyze the brazed zone further, the sample was investigated using scanning electron microscope (SEM) in backscattered mode at 1500 as shown in Fig. 4. The diffusion layer of different thickness between the filler and substrates was observed. Wider diffusion layers observed in the OFE copper side as compared to stainless steel (SS304) side. The thickness of brazing layer of
Fig. 4. SEM image in backscattered mode at 1500.
Fig. 1. (a) Substrates and filler; (b) Samples in hot zone of furnace.
Please cite this article as: V. Mangla, J. D. Sharma, S. Kumar et al., Joining of stainless steel (SS304) and OFE copper by vacuum brazing, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2020.01.016
V. Mangla et al. / Materials Today: Proceedings xxx (xxxx) xxx
3
Fig. 5. SEM image and EDX analysis of brazed joint (a) Brazed interface at 1500; (b) Dot mapping showing distribution of Ag; (c) Dot mapping showing distribution of Cu; (d) Dot mapping showing distribution of Cr; (e) Dot mapping showing distribution of Fe.
Fig. 6. Micro hardness profile.
copper and filler was around 40–50 mm and that of stainless steel and filler was around 4–5 mm. The region near to the interface found to be consisting of white and grey phases. The results obtained are similar to the results obtained by brazing copper and stainless steel with B-Ag72Cu filler [5]. The vacuum brazing is a diffusion process. To study the phases formed and diffusion of elements of filler and substrates into each other Energy Dispersive X ray analysis (EDX) was carried out. Fig. 5a shows the image of the sample taken for dot mapping analysis. The dot mapping of the elements i.e. silver (Ag), copper (Cu), chromium (Cr) and iron (Fe) to study the diffusion of elements from the filler to substrates is shown in (Fig. 5b, 5c, 5d, 5e). It can be seen from the Fig. 5b that the white phase is a silver rich region. The white region is predominant in the copper side while it is negligible on the stainless steel side. However, the grey region near to the brazed zone is a copper rich region. This implies there is a solid solution formation in the brazed zone. The white region is a solid solution of copper (solute) in silver (solvent) while grey
Fig. 7. (a) Schematic of sample; (b) Sectional view of sample (all dimensions in mm).
Please cite this article as: V. Mangla, J. D. Sharma, S. Kumar et al., Joining of stainless steel (SS304) and OFE copper by vacuum brazing, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2020.01.016
4
V. Mangla et al. / Materials Today: Proceedings xxx (xxxx) xxx
Fig. 8. (a) Sample for leak testing; (b) Helium leak detector.
region is a solid solution of silver (solute) in copper (solvent). The observations can be correlated to the results obtained by Anibal Guedes and Ana Maria Pires Pinto [6]. The hardness across the interface was evaluated using a micro hardness tester. The brazing zone was around 40–50 mm in the copper side. The hardness in this zone was found to be comparatively lower as compared to region beyond 40–50 mm. The hardness becomes nearly constant on moving away from brazed zone. As there is very less diffusion of the filler in SS304 side, there is no appreciable difference in the hardness near to the brazed zone as well in bulk of the material in Fig. 6. Hardness of both materials SS 304 and OFE copper decreased after brazing. The average hardness of original materials, i.e. SS304 and OFE copper were 330 HV and 110 HV respectively. After brazing beyond the brazing zone average hardness reduces to 190 HV and 85 HV of SS304 and copper respectively. This decrease in hardness can be attributed to annealing of the material during brazing. Helium leak detector was used to check the hermeticity of the brazed samples. The sample was fabricated and fitted in the machine to check the hermeticity of the joint. The schematic and sectional view of the sample is shown in Fig. 7(a-b). The fabricated sample and helium leak detector is shown in Fig. 8(a-b). The sample was evacuated to check the leak rate. Then, the helium gas was sprayed on the sample to check the hermeticity. The leak rate was found to be 3.5 10 10 mbar litres/second after spraying of helium gas. This test confirms the hermeticity of the brazed sample. 4. Conclusion SS304 and OFE copper was successfully joined by vacuum brazing process. The brazed interface was free from any major cracks as well as voids. The silver rich white phase and copper rich grey was formed in the brazing zone. The thickness of brazing layer of copper and filler was around 40–50 mm and that of stainless steel and filler was around 4–5 mm. The hardness in the brazed zone was less
in comparison to the region beyond brazing zone. However, the hardness of the substrates i.e. SS304 and OFE copper decreases by 42%and 23% of the original hardness respectively. The hermeticity of the sample was of the order of 3.5 10 10 mbar.litres/second. These hermetically brazed joints find critical applications in different technologies. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The financial support of Defence Research & Development Organization (DRDO), Ministry of Defence, Government of India is gratefully acknowledged. The authors would also like to express sincere thanks to Dr. Manjit Singh, Director, TBRL, Chandigarh for his continuous support and guidance during the study and experimental work carried out at TBRL. We are also grateful to Main Workshop for fabrication of samples, EED division for optical microscope analysis and QAC division of TBRL for carrying out SEM analysis. References [1] David M. Jacobson, Giles Humpston, Principles of Brazing, ASM International, USA, 2005. [2] Robert W. Messier, Joining of Advance Materials, Reed Publishing, USA, 1993. [3] Welding Brazing and Soldering, Vol. 6, ASM International, USA, 1993. [4] Janusz Kowalewski, Janusz Szczurek, Issues in Vacuum Brazing, in: Proc. International Brazing and Soldering Conference, 2006, pp. 326–333. [5] Changquing Ye, Experimental Investigation of Copper/Stainless Steel joints formed by Vacuum Brazing, in: Proc. ICONE18, China, 2010, pp. 47–55. [6] Anibal Guedes, Ana Maria Pires Pinto, Active Metal Brazing of Machinable Aluminium-Based Ceramic to Stainless Steel, J. Mater. Engg. Perfor. 21 (5) (2012) 671–677.
Please cite this article as: V. Mangla, J. D. Sharma, S. Kumar et al., Joining of stainless steel (SS304) and OFE copper by vacuum brazing, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2020.01.016
Contents lists available at ScienceDirect
Materials Today: Proceedings journal homepage: www.elsevier.com/locate/matpr
Joining of stainless steel (SS304) and OFE copper by vacuum brazing Vikas Mangla a,b,⇑, J.D. Sharma b, Sandeep Kumar a, Pal Dinesh Kumar a, Arun Agarwal a a b
Terminal Ballistics Research Laboratory, Sec-30, Chandigarh 160030, India Punjab Engineering College, Sec-12, Chandigarh 160012, India
a r t i c l e
i n f o
Article history: Received 11 November 2019 Accepted 3 January 2020 Available online xxxx Keywords: SS304 OFE copper Vacuum brazing Microstructure Hermeticity Micro hardness
a b s t r a c t This paper describes the vacuum brazing process for joining OFE copper with stainless steel (SS 304) using silver based filler material (68Ag27Cu5Pd). Vacuum brazing of SS304 and OFE copper was carried out in a vacuum furnace at a vacuum level of 10 6 mbar. The peak brazing temperature was kept as 920 °C with a soaking time of 10 min. This combination is often used in bi-metallic joints for high vacuum applications. Optimization of brazing parameters was done to produce defect free joints. Micro structural analysis of the brazed joint was carried out using optical and scanning electron microscope. The flow of filler material and diffusion of its elements into substrates was observed. The brazed joint was found to be of good quality and free from any major cracks. The micro hardness was evaluated across the interface and it was observed that hardness decreases towards the interface. The hermeticity of the joint was investigated using helium spray method. Ó 2020 Elsevier Ltd. All rights reserved. Selection and of the scientific committee of the 10th International Conference of Materials Processing and Characterization.
1. Introduction Welding, brazing, soldering, riveting, adhesive joining etc. are some of the widely used metal joining processes. The selection of material joining technique depends on the application of the product produced. Welding is basically used to join metals which have close melting points. In case of joining the metals having large difference in their physical and mechanical properties, brazing is generally used. Brazing involves the joining of two materials with the help of third material called as filler material. In brazing the joint is heated to a required temperature in the presence of filler material having liquidus temperature above 450 °C (840 °F) but below the solidus temperature of substrates. The filler can be used in various forms like powder, thin sheet etc. depending on the type of joint to be brazed. In the entire process only filler is melted and substrates remain in the solid state. The brazing filler metal is distributed between the closely fitted faying surfaces of the joint by capillary action. Joining takes place by formation of interatomic bonds like metallic in metals, ionic or covalent or mixed in ceramics [1,2]. ⇑ Corresponding author at: Terminal Ballistics Research Laboratory, Sec-30, Chandigarh 160030, India. E-mail address: [email protected] (V. Mangla).
The various elements like filler metal flow, base metal characteristics, surface preparation, joint design and clearance, brazing temperature and time, rate and source of heating etc. are to be clearly understood to produce good quality brazed joints [3]. The formation of interatomic bonds in brazing takes place by inter diffusion of atoms between filler and substrates. The brazing of substrates when carried out under vacuum is called vacuum brazing. This type of brazing is carried out in a vacuum furnace. There are several advantages of vacuum brazing over conventional brazing which are being listed below: Purity of atmosphere (i.e. vacuum) ensures less availability of oxygen to contaminate the work piece. The flow of filler material is uniform which results in better joint properties. Flux-free brazed joints are formed with superior joint strength. Minimum distortion of substrates due to controlled heating and cooling rates [4]. In the present work, the joining of SS304 and OFE copper has been carried out using vacuum brazing process. The characterization of the brazed joint has been done to evaluate metallurgical, mechanical properties and hermeticity of the joint.
https://doi.org/10.1016/j.matpr.2020.01.016 2214-7853/Ó 2020 Elsevier Ltd. All rights reserved. Selection and of the scientific committee of the 10th International Conference of Materials Processing and Characterization.
Please cite this article as: V. Mangla, J. D. Sharma, S. Kumar et al., Joining of stainless steel (SS304) and OFE copper by vacuum brazing, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2020.01.016
2
V. Mangla et al. / Materials Today: Proceedings xxx (xxxx) xxx
2. Experimental work The metals under investigation were SS304 and OFE copper solid cylinders both having same dimensions of / 10 and height 10 mm. The filler material used was in the form of disc of foil of dimensions / 10 and thickness 100 mm. Clean and oxide free surfaces of the substrates and filler are necessary to achieve good quality brazed joints. Uniform capillary flow between the substrates can be obtained only when all grease, oil, dirt and oxides have been removed from the filler material and substrates before brazing. The samples were thus ultrasonically cleaned with the suitable solvent. The degassing of the substrates and filler material was done at 600 °C to remove the entrapped gases which can otherwise can lead to defects like porosity in the final brazed product. The substrates and filler has been shown in Fig. 1a. The filler material was sandwiched between the substrates and kept in the hot zone of the vacuum furnace for brazing as shown in Fig. 1b. Vacuum brazing involves heating the component in a controlled vacuum environment. Brazing cycles plays a very crucial role in the vacuum brazing process for formation of strong joints. Heating rate was varied from 2 to 10 °C in the brazing cycle. In general, soaking time at brazing peak temperature should be long enough to ensure all sections of the work piece and all parts within the load reach the desired brazing temperature. The brazed sample was cut from the centre in the direction perpendicular to the longitudinal axis of the brazed joint for metallographic examination. The optical microscope (Leica, Germany, DM 2700P) and scanning electron microscope (Carl Zeiss, USA, EVO15) were used for the analysis. Energy-dispersive X-ray dot mapping analysis (EDX) was also carried out. The hardness evaluation across the interface was carried out using Vicker’s micro hardness tester (Radical, India, RMHT201). The hermeticity of the joint was checked using Helium leak detector.
Fig. 2. Samples after vacuum brazing.
Fig. 3. Optical micrograph at 100.
3. Results and discussions The experimental results obtained shows successful brazing of SS304 and OFE copper was achieved by vacuum brazing process (Fig. 2). Initially the sample was observed under optical microscope. The optical micrographs at 100X are being shown in Fig. 3. The interface was found to be continuous throughout the brazed joint with the absence of any major cracks or voids. The white phase was observed near to the interface on the copper side. To analyze the brazed zone further, the sample was investigated using scanning electron microscope (SEM) in backscattered mode at 1500 as shown in Fig. 4. The diffusion layer of different thickness between the filler and substrates was observed. Wider diffusion layers observed in the OFE copper side as compared to stainless steel (SS304) side. The thickness of brazing layer of
Fig. 4. SEM image in backscattered mode at 1500.
Fig. 1. (a) Substrates and filler; (b) Samples in hot zone of furnace.
Please cite this article as: V. Mangla, J. D. Sharma, S. Kumar et al., Joining of stainless steel (SS304) and OFE copper by vacuum brazing, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2020.01.016
V. Mangla et al. / Materials Today: Proceedings xxx (xxxx) xxx
3
Fig. 5. SEM image and EDX analysis of brazed joint (a) Brazed interface at 1500; (b) Dot mapping showing distribution of Ag; (c) Dot mapping showing distribution of Cu; (d) Dot mapping showing distribution of Cr; (e) Dot mapping showing distribution of Fe.
Fig. 6. Micro hardness profile.
copper and filler was around 40–50 mm and that of stainless steel and filler was around 4–5 mm. The region near to the interface found to be consisting of white and grey phases. The results obtained are similar to the results obtained by brazing copper and stainless steel with B-Ag72Cu filler [5]. The vacuum brazing is a diffusion process. To study the phases formed and diffusion of elements of filler and substrates into each other Energy Dispersive X ray analysis (EDX) was carried out. Fig. 5a shows the image of the sample taken for dot mapping analysis. The dot mapping of the elements i.e. silver (Ag), copper (Cu), chromium (Cr) and iron (Fe) to study the diffusion of elements from the filler to substrates is shown in (Fig. 5b, 5c, 5d, 5e). It can be seen from the Fig. 5b that the white phase is a silver rich region. The white region is predominant in the copper side while it is negligible on the stainless steel side. However, the grey region near to the brazed zone is a copper rich region. This implies there is a solid solution formation in the brazed zone. The white region is a solid solution of copper (solute) in silver (solvent) while grey
Fig. 7. (a) Schematic of sample; (b) Sectional view of sample (all dimensions in mm).
Please cite this article as: V. Mangla, J. D. Sharma, S. Kumar et al., Joining of stainless steel (SS304) and OFE copper by vacuum brazing, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2020.01.016
4
V. Mangla et al. / Materials Today: Proceedings xxx (xxxx) xxx
Fig. 8. (a) Sample for leak testing; (b) Helium leak detector.
region is a solid solution of silver (solute) in copper (solvent). The observations can be correlated to the results obtained by Anibal Guedes and Ana Maria Pires Pinto [6]. The hardness across the interface was evaluated using a micro hardness tester. The brazing zone was around 40–50 mm in the copper side. The hardness in this zone was found to be comparatively lower as compared to region beyond 40–50 mm. The hardness becomes nearly constant on moving away from brazed zone. As there is very less diffusion of the filler in SS304 side, there is no appreciable difference in the hardness near to the brazed zone as well in bulk of the material in Fig. 6. Hardness of both materials SS 304 and OFE copper decreased after brazing. The average hardness of original materials, i.e. SS304 and OFE copper were 330 HV and 110 HV respectively. After brazing beyond the brazing zone average hardness reduces to 190 HV and 85 HV of SS304 and copper respectively. This decrease in hardness can be attributed to annealing of the material during brazing. Helium leak detector was used to check the hermeticity of the brazed samples. The sample was fabricated and fitted in the machine to check the hermeticity of the joint. The schematic and sectional view of the sample is shown in Fig. 7(a-b). The fabricated sample and helium leak detector is shown in Fig. 8(a-b). The sample was evacuated to check the leak rate. Then, the helium gas was sprayed on the sample to check the hermeticity. The leak rate was found to be 3.5 10 10 mbar litres/second after spraying of helium gas. This test confirms the hermeticity of the brazed sample. 4. Conclusion SS304 and OFE copper was successfully joined by vacuum brazing process. The brazed interface was free from any major cracks as well as voids. The silver rich white phase and copper rich grey was formed in the brazing zone. The thickness of brazing layer of copper and filler was around 40–50 mm and that of stainless steel and filler was around 4–5 mm. The hardness in the brazed zone was less
in comparison to the region beyond brazing zone. However, the hardness of the substrates i.e. SS304 and OFE copper decreases by 42%and 23% of the original hardness respectively. The hermeticity of the sample was of the order of 3.5 10 10 mbar.litres/second. These hermetically brazed joints find critical applications in different technologies. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The financial support of Defence Research & Development Organization (DRDO), Ministry of Defence, Government of India is gratefully acknowledged. The authors would also like to express sincere thanks to Dr. Manjit Singh, Director, TBRL, Chandigarh for his continuous support and guidance during the study and experimental work carried out at TBRL. We are also grateful to Main Workshop for fabrication of samples, EED division for optical microscope analysis and QAC division of TBRL for carrying out SEM analysis. References [1] David M. Jacobson, Giles Humpston, Principles of Brazing, ASM International, USA, 2005. [2] Robert W. Messier, Joining of Advance Materials, Reed Publishing, USA, 1993. [3] Welding Brazing and Soldering, Vol. 6, ASM International, USA, 1993. [4] Janusz Kowalewski, Janusz Szczurek, Issues in Vacuum Brazing, in: Proc. International Brazing and Soldering Conference, 2006, pp. 326–333. [5] Changquing Ye, Experimental Investigation of Copper/Stainless Steel joints formed by Vacuum Brazing, in: Proc. ICONE18, China, 2010, pp. 47–55. [6] Anibal Guedes, Ana Maria Pires Pinto, Active Metal Brazing of Machinable Aluminium-Based Ceramic to Stainless Steel, J. Mater. Engg. Perfor. 21 (5) (2012) 671–677.
Please cite this article as: V. Mangla, J. D. Sharma, S. Kumar et al., Joining of stainless steel (SS304) and OFE copper by vacuum brazing, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2020.01.016