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aluminium 5A06 dissimilar materials
Materials Letters 62 (2008) 4106–4108
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / m a t l e t
Microstructure and XRD analysis of FSW joints for copper T2/aluminium 5A06 dissimilar materials Peng Liu ⁎, Qingyu Shi, Wei Wang, Xin Wang, Zenglei Zhang Department of Mechanical Engineering, Tsinghua University, Beijing 100084, China, Key Laboratory for Advanced Materials Processing Technology Ministry of Education, China
A R T I C L E
I N F O
Article history: Received 10 January 2008 Accepted 3 June 2008 Available online 6 June 2008 Keywords: Cu/Al dissimilar materials Friction stir welding Microstructure Tensile strength X-ray technique
A B S T R A C T Copper (T2) and aluminium alloy (5A06) were welded by friction stir welding (FSW). The microstructure, mechanical properties and phase constituents of FSW joints were studied by metallography, tensile testing machine and X-ray diffraction. The results indicated that the high quality weld joint could be obtained when tool rotational speed is 950 rpm, and travel speed is 150 mm/min. The maximum value of tensile strength is about 296 MPa. The metal Cu and Al close to copper side in the weld nugget (WN) zone showed a lamellar alternating structure characteristic. However, a mixed structure characteristic of Cu and Al existed in the aluminium side of weld nugget (WN) zone. There were no new Cu–Al intermetallic compounds in the weld nugget zone. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Copper and aluminium are widely applied in engineering structure due to unique performances such as higher electric conductivity, heat conductivity, corrosion resistance and mechanical properties. However, the melting points of both materials have a significant difference (nearly 400 °C). This may lead to a large difference in microstructure and performance of Cu–Al joints if copper and aluminium would be joined. Moreover, the Al was easily oxidized at an elevated temperature, and some welding cracks existed easily in a joint of brazed or fusion welding Cu [1]. Therefore, a high quality weld joint of Cu/Al was difficult to obtain by means of conventional welding methods. During fusion welding or pressure welding (brazing, diffusion bonding, etc), the Cu-Al intermetallics, which resulted in decreased mechanical properties of joints, is very difficult to be avoided in Cu/Al dissimilar materials joint [2,3]. Friction stir welding (FSW) is a novel method of joining materials, patented by the welding institute (TWI) in 1991 [4]. During friction stir welding the weld metal experiences elevated temperature, large plastic deformation and stress-strain course. However, the weld metal does not melt, which helps to avoid the oxidization of weld metal, hot cracks and gas pores. Till now, joining of aluminium and aluminium alloys by FSW has been extensively studied [5,6]. However, a study on FSW Cu/Al dissimilar materials was reported less [7–9]. In this paper, copper T2 and aluminium 5A06 was welded successfully by friction stir welding (FSW). This study was focused on microstructure, phase constituents and mechanical
properties of FSW joint. The research findings would provide an important test basis for further studying of FSW Cu/Al dissimilar materials. 2. Experimental The test materials studied were copper (T2) and aluminium alloy (5A06). The dimensions of test materials were 150 mm× 60 mm× 3 mm. The chemical composition, thermo-physical and mechanical performances of test material were shown in Table 1. The oxidation film and oil soil on the surface of workpiece were removed before welding. Then, the two 3 mm thick sheets were butt welded. Experimental parameters were varied in the test in the following ranges: The tool rotational speed was 950–1180 rpm, and travel speed was 150–235 mm/min. The test results indicated that the Cu/Al joints with better formation of weld without obvious defects can be obtained when travel speed was 150 mm/min. In the following experiments, this joint was analyzed. The Cu/Al joints were cut into specimens, and then these specimens were made into metallographic samples and tensile samples. The Cu side of metallographic samples were etched using mixed solution FeCl2 (6 g)+ HCl (10 ml) + H2O (90 ml), and the Al side was etched using mixed solution HF (1 ml) + HCl (1.5 ml) + HNO3 (2.5 ml). The microstructure of Cu/Al joint was observed and analyzed by NEOPHOT32 metallographic microscope. The tensile strength of joints was measured using a CSS1100 tensile test machine. The phase constituents in stir zone of Cu/Al joint were analyzed by mean of X-ray diffractometer (XRD). 3. Results and analysis 3.1. Tensile strength of FSW joints
⁎ Corresponding author. Room 206, Welding building, Department of Mechanical Engineering, Tsinghua University, Beijing 100084, China. Tel.: +86 10 62784578; fax: +86 10 62773637. E-mail address: [email protected] (P. Liu). 0167-577X/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2008.06.004
The tensile strength of FSW joints for Cu/Al dissimilar materials was carried out. The test results were shown in Fig. 1. These joints were all fractured in the copper (T2) side of the weld nugget zone (WNZ). The test results were shown in Fig. 1b. The
P. Liu et al. / Materials Letters 62 (2008) 4106–4108
4107
Table 1 Chemical composition, thermophysical and mechanical performance of copper T2 and aluminium 5A06 Chemical composition (wt.%) Materials
Cu + Ag
Bi
Sb
As
Fe
Pb
S
other
T2 Materials 5A06
99.9 Al Other
0.001 Si 0.4
0.002 Fe 0.4
0.002 Cu 0.1
0.005 Mn 0.5–0.8
0.005 Mg 5.8–6.8
0.005 Zn 0.2
0.1 Ti 0.02–0.1
Be 0.001–0.005
Thermo-physical performance Materials
Tensile strength/MPa
Elongation/%
Heat conductivity/(W·m− 1·K− 1)
T2 5A06
295 315
3 16
352 (324 °C) 138 (400 °C)
maximum tensile strength was 296 MPa. It is about 100% of copper T2, and about 94% of 5A06 aluminium. 3.2. Microstructure in stir zone According to tensile test, the joints were fractured in Cu side of the weld. Therefore, in the weld of Cu side, especially in the weld nugget zone, the microstructure has an important effect on mechanical properties of Cu/Al joints. The microstructure of the stir zone in the weld for Cu/Al joints was shown in Fig. 2. The distribution between Cu and Al has an obvious boundary. And the metal in stir zone shows obvious plastic combination. An onion ring structure in stir zone can be observed clearly (see region C in Fig. 2a). Further observing the WNZ indicated that the metal Cu (gray) and Al (black) close to copper side in the WNZ showed a lamellar alternating structure characteristic (see Fig. 2b). However, in the aluminium side of WNZ the metal Cu (black) and Al (gray) showed a mixed structure characteristic (see Fig. 2c). Plastic stir action and friction heat of tool and flow of materials may induce the different of metal structure of both sides. In this experiment, the temperature of WNZ was obvious lower than the solidus temperature of either aluminium or Copper. At this time, the metal Al experienced a “continuous” dynamic recrystallization (CDRX) [10], and the grain in this region was refined obviously. However, the metal Cu in weld nugget zone also experienced plastic flow by stir action and friction heat, but the Cu would not experience the CDRX process since the temperature was lower than 500 °C (It was accepted that this temperature was the lower limit of recrystallization temperature for copper.) [11]. As a result, the weld nugget zone with lamellar alternating and mixed structure was formed by stir action and friction heat of tool. The different heat conductivity in both sides in WNZ may be also another major reason that induced different structure. Since the heat conductivity of copper T2 is 2.5 times of that of aluminium 5A06, more friction heat produced by stir action in Cu side lost into the nearby metal than in Al side. This brought to temperature reduction in this region and resulted in different structure of different sides of WNZ. As a result, no obvious metallurgical process between copper and aluminium would be produced during the
FSW. Therefore, the Cu and Al were closely combined in the Al side, and showed a mixed structure. However, the Cu and Al showed a lamellar alternating structure characteristic. And since the metal only experienced plastic deformation process in the friction stir weld, the fracture location of joints was in this region of Cu side. 3.3. XRD analysis in weld nugget zone During FSW, the plastic deformation and recrystallization of weld metal by stir action and friction heat was usually occurred. These may lead to a change of phase constituents in stir zone. Moreover, some intermetallics of Al–Cu system might be also formed in weld nugget zone. Those intermetallics can decrease the mechanical properties of FSW joints. Therefore, an XRD analysis of phase constituents in stir zone of the weld is quite important. In this experiment, the X-ray diffraction instrument of Rigaku-2500 type was used, and the weld nugget zone of FS weld and base metal was analyzed. The XRD analysis was carried out using Cu Kα target at a working voltage 40 kV and current 200 mA. The XRD results of weld nugget zone and base meal are shown in Fig. 3. According to Fig. 3, the X-ray diffraction results in WNZ exactly correspond to the addition of diffraction patterns of copper T2 and aluminium 5A06. This indicated that there are no new Cu–Al intermetallics formed in WNZ of FS weld for Cu/Al dissimilar materials. However, since the metal Cu and Al experienced a period of larger plastic deformation and stress-strain in WNZ, the lattice structure of Cu and Al was distorted. Therefore, a minor deviation of diffraction peak existed between the WNZ and base metals. The analysis results of phase constituents indicate that there was no new intermetallic compounds generated, which would influence the mechanical properties of FSW joint when joining Cu/Al dissimilar materials.
4. Conclusions The copper (T2) and aluminium alloy (5A06) were welded by friction stir welding technology. The test results indicated that the
Fig. 1. The schematic for tensile sample and test results of FSW joint for Cu/Al dissimilar materials. (a) The schematic of tensile sample (b) Tensile strength results.
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P. Liu et al. / Materials Letters 62 (2008) 4106–4108
Fig. 2. The microstructure in stir zone of FS weld for Cu/Al dissimilar materials. (a) Image of cross section of FS weld (b) Microstructure of Cu side in WNZ (c) Microstructure of Al side in WNZ.
high quality weld joint could be obtained when tool rotational speed is 950 rpm, and travel speed is 150 mm/min. The maximum value of tensile strength is about 296 MPa. It is about 100% of T2 copper, and about 94% of 5A06 aluminium. The Cu/Al FSW joint has a satisfactory higher service performance. The microstructure analysis indicated that the metal Cu and Al close to copper side in the weld nugget (WN) zone showed a lamellar alternating structure characteristic. However, a mixed structure characteristic of Cu and Al existed in the aluminium side of weld nugget (WN) zone. The stir action of tool, friction heat and heat conductivity of Cu and Al could induce the different structure of both sides in weld nugget zone. The XRD analysis indicated that there were no new Cu–Al intermetallics in the weld nugget zone. As a result, the structure of weld nugget zone was mainly plastic diffusion combination of Cu and Al. Acknowledgements This research was financially supported through the National High Technology Research and Development Program of China (863 Program, No. 2006AA04Z139). References [1] [2] [3] [4]
[5] [6] [7] [8] [9] [10] [11]
Fig. 3. The XRD diffraction patterns in WN zone of FS weld and base materials for Cu/Al dissimilar materials.
Metals Handbook. 9th ed. Metals Park, Ohio: ASM international; 1979. Li YJ, Wu HQ, Chen MA, Yang M, Feng T. Chin J Nonfer Met 2001;11(3):424–7. Chen XD, Song Q, Yu WY, Song CY. TransChina Weld Inst 2003;24(1):40–3. W.M.Thomas, E.D. Nicholas, J.C. Needham, M.G. Murch, P. Temple-Smith, C.J. Dawes, “Friction stir butt welding”: UK, International patent application no. PCTPGB92P02203 and GB patent application (no. 9125978. 8, 6 December 1991). Mahoney MW, Rhodes CG. Metall Mater Trans A 1998;28(7):1955–64. Sato YS, Kokawa H, Enomoto M. Metall Mater Trans A 1999;30(9):2429–37. Elrefaey A, Takahashi M, Ikeuchi K. Weld in the World 2005;49(3–4):93–101. Ke LM, Liu GP, Xing L, Xia C. Chin J Nonfer Met 2004;14(9):1534–8. Pietras A. Weld in the world 2005;49(9):122–33. Fratini L, Buffa G, Palmeri D, Hua J, Shivpuri R. J Eng MaterTechnol 2006;128 (3):428–35. Jeff D. Weld J 2006;85(3):42–4.
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / m a t l e t
Microstructure and XRD analysis of FSW joints for copper T2/aluminium 5A06 dissimilar materials Peng Liu ⁎, Qingyu Shi, Wei Wang, Xin Wang, Zenglei Zhang Department of Mechanical Engineering, Tsinghua University, Beijing 100084, China, Key Laboratory for Advanced Materials Processing Technology Ministry of Education, China
A R T I C L E
I N F O
Article history: Received 10 January 2008 Accepted 3 June 2008 Available online 6 June 2008 Keywords: Cu/Al dissimilar materials Friction stir welding Microstructure Tensile strength X-ray technique
A B S T R A C T Copper (T2) and aluminium alloy (5A06) were welded by friction stir welding (FSW). The microstructure, mechanical properties and phase constituents of FSW joints were studied by metallography, tensile testing machine and X-ray diffraction. The results indicated that the high quality weld joint could be obtained when tool rotational speed is 950 rpm, and travel speed is 150 mm/min. The maximum value of tensile strength is about 296 MPa. The metal Cu and Al close to copper side in the weld nugget (WN) zone showed a lamellar alternating structure characteristic. However, a mixed structure characteristic of Cu and Al existed in the aluminium side of weld nugget (WN) zone. There were no new Cu–Al intermetallic compounds in the weld nugget zone. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Copper and aluminium are widely applied in engineering structure due to unique performances such as higher electric conductivity, heat conductivity, corrosion resistance and mechanical properties. However, the melting points of both materials have a significant difference (nearly 400 °C). This may lead to a large difference in microstructure and performance of Cu–Al joints if copper and aluminium would be joined. Moreover, the Al was easily oxidized at an elevated temperature, and some welding cracks existed easily in a joint of brazed or fusion welding Cu [1]. Therefore, a high quality weld joint of Cu/Al was difficult to obtain by means of conventional welding methods. During fusion welding or pressure welding (brazing, diffusion bonding, etc), the Cu-Al intermetallics, which resulted in decreased mechanical properties of joints, is very difficult to be avoided in Cu/Al dissimilar materials joint [2,3]. Friction stir welding (FSW) is a novel method of joining materials, patented by the welding institute (TWI) in 1991 [4]. During friction stir welding the weld metal experiences elevated temperature, large plastic deformation and stress-strain course. However, the weld metal does not melt, which helps to avoid the oxidization of weld metal, hot cracks and gas pores. Till now, joining of aluminium and aluminium alloys by FSW has been extensively studied [5,6]. However, a study on FSW Cu/Al dissimilar materials was reported less [7–9]. In this paper, copper T2 and aluminium 5A06 was welded successfully by friction stir welding (FSW). This study was focused on microstructure, phase constituents and mechanical
properties of FSW joint. The research findings would provide an important test basis for further studying of FSW Cu/Al dissimilar materials. 2. Experimental The test materials studied were copper (T2) and aluminium alloy (5A06). The dimensions of test materials were 150 mm× 60 mm× 3 mm. The chemical composition, thermo-physical and mechanical performances of test material were shown in Table 1. The oxidation film and oil soil on the surface of workpiece were removed before welding. Then, the two 3 mm thick sheets were butt welded. Experimental parameters were varied in the test in the following ranges: The tool rotational speed was 950–1180 rpm, and travel speed was 150–235 mm/min. The test results indicated that the Cu/Al joints with better formation of weld without obvious defects can be obtained when travel speed was 150 mm/min. In the following experiments, this joint was analyzed. The Cu/Al joints were cut into specimens, and then these specimens were made into metallographic samples and tensile samples. The Cu side of metallographic samples were etched using mixed solution FeCl2 (6 g)+ HCl (10 ml) + H2O (90 ml), and the Al side was etched using mixed solution HF (1 ml) + HCl (1.5 ml) + HNO3 (2.5 ml). The microstructure of Cu/Al joint was observed and analyzed by NEOPHOT32 metallographic microscope. The tensile strength of joints was measured using a CSS1100 tensile test machine. The phase constituents in stir zone of Cu/Al joint were analyzed by mean of X-ray diffractometer (XRD). 3. Results and analysis 3.1. Tensile strength of FSW joints
⁎ Corresponding author. Room 206, Welding building, Department of Mechanical Engineering, Tsinghua University, Beijing 100084, China. Tel.: +86 10 62784578; fax: +86 10 62773637. E-mail address: [email protected] (P. Liu). 0167-577X/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2008.06.004
The tensile strength of FSW joints for Cu/Al dissimilar materials was carried out. The test results were shown in Fig. 1. These joints were all fractured in the copper (T2) side of the weld nugget zone (WNZ). The test results were shown in Fig. 1b. The
P. Liu et al. / Materials Letters 62 (2008) 4106–4108
4107
Table 1 Chemical composition, thermophysical and mechanical performance of copper T2 and aluminium 5A06 Chemical composition (wt.%) Materials
Cu + Ag
Bi
Sb
As
Fe
Pb
S
other
T2 Materials 5A06
99.9 Al Other
0.001 Si 0.4
0.002 Fe 0.4
0.002 Cu 0.1
0.005 Mn 0.5–0.8
0.005 Mg 5.8–6.8
0.005 Zn 0.2
0.1 Ti 0.02–0.1
Be 0.001–0.005
Thermo-physical performance Materials
Tensile strength/MPa
Elongation/%
Heat conductivity/(W·m− 1·K− 1)
T2 5A06
295 315
3 16
352 (324 °C) 138 (400 °C)
maximum tensile strength was 296 MPa. It is about 100% of copper T2, and about 94% of 5A06 aluminium. 3.2. Microstructure in stir zone According to tensile test, the joints were fractured in Cu side of the weld. Therefore, in the weld of Cu side, especially in the weld nugget zone, the microstructure has an important effect on mechanical properties of Cu/Al joints. The microstructure of the stir zone in the weld for Cu/Al joints was shown in Fig. 2. The distribution between Cu and Al has an obvious boundary. And the metal in stir zone shows obvious plastic combination. An onion ring structure in stir zone can be observed clearly (see region C in Fig. 2a). Further observing the WNZ indicated that the metal Cu (gray) and Al (black) close to copper side in the WNZ showed a lamellar alternating structure characteristic (see Fig. 2b). However, in the aluminium side of WNZ the metal Cu (black) and Al (gray) showed a mixed structure characteristic (see Fig. 2c). Plastic stir action and friction heat of tool and flow of materials may induce the different of metal structure of both sides. In this experiment, the temperature of WNZ was obvious lower than the solidus temperature of either aluminium or Copper. At this time, the metal Al experienced a “continuous” dynamic recrystallization (CDRX) [10], and the grain in this region was refined obviously. However, the metal Cu in weld nugget zone also experienced plastic flow by stir action and friction heat, but the Cu would not experience the CDRX process since the temperature was lower than 500 °C (It was accepted that this temperature was the lower limit of recrystallization temperature for copper.) [11]. As a result, the weld nugget zone with lamellar alternating and mixed structure was formed by stir action and friction heat of tool. The different heat conductivity in both sides in WNZ may be also another major reason that induced different structure. Since the heat conductivity of copper T2 is 2.5 times of that of aluminium 5A06, more friction heat produced by stir action in Cu side lost into the nearby metal than in Al side. This brought to temperature reduction in this region and resulted in different structure of different sides of WNZ. As a result, no obvious metallurgical process between copper and aluminium would be produced during the
FSW. Therefore, the Cu and Al were closely combined in the Al side, and showed a mixed structure. However, the Cu and Al showed a lamellar alternating structure characteristic. And since the metal only experienced plastic deformation process in the friction stir weld, the fracture location of joints was in this region of Cu side. 3.3. XRD analysis in weld nugget zone During FSW, the plastic deformation and recrystallization of weld metal by stir action and friction heat was usually occurred. These may lead to a change of phase constituents in stir zone. Moreover, some intermetallics of Al–Cu system might be also formed in weld nugget zone. Those intermetallics can decrease the mechanical properties of FSW joints. Therefore, an XRD analysis of phase constituents in stir zone of the weld is quite important. In this experiment, the X-ray diffraction instrument of Rigaku-2500 type was used, and the weld nugget zone of FS weld and base metal was analyzed. The XRD analysis was carried out using Cu Kα target at a working voltage 40 kV and current 200 mA. The XRD results of weld nugget zone and base meal are shown in Fig. 3. According to Fig. 3, the X-ray diffraction results in WNZ exactly correspond to the addition of diffraction patterns of copper T2 and aluminium 5A06. This indicated that there are no new Cu–Al intermetallics formed in WNZ of FS weld for Cu/Al dissimilar materials. However, since the metal Cu and Al experienced a period of larger plastic deformation and stress-strain in WNZ, the lattice structure of Cu and Al was distorted. Therefore, a minor deviation of diffraction peak existed between the WNZ and base metals. The analysis results of phase constituents indicate that there was no new intermetallic compounds generated, which would influence the mechanical properties of FSW joint when joining Cu/Al dissimilar materials.
4. Conclusions The copper (T2) and aluminium alloy (5A06) were welded by friction stir welding technology. The test results indicated that the
Fig. 1. The schematic for tensile sample and test results of FSW joint for Cu/Al dissimilar materials. (a) The schematic of tensile sample (b) Tensile strength results.
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P. Liu et al. / Materials Letters 62 (2008) 4106–4108
Fig. 2. The microstructure in stir zone of FS weld for Cu/Al dissimilar materials. (a) Image of cross section of FS weld (b) Microstructure of Cu side in WNZ (c) Microstructure of Al side in WNZ.
high quality weld joint could be obtained when tool rotational speed is 950 rpm, and travel speed is 150 mm/min. The maximum value of tensile strength is about 296 MPa. It is about 100% of T2 copper, and about 94% of 5A06 aluminium. The Cu/Al FSW joint has a satisfactory higher service performance. The microstructure analysis indicated that the metal Cu and Al close to copper side in the weld nugget (WN) zone showed a lamellar alternating structure characteristic. However, a mixed structure characteristic of Cu and Al existed in the aluminium side of weld nugget (WN) zone. The stir action of tool, friction heat and heat conductivity of Cu and Al could induce the different structure of both sides in weld nugget zone. The XRD analysis indicated that there were no new Cu–Al intermetallics in the weld nugget zone. As a result, the structure of weld nugget zone was mainly plastic diffusion combination of Cu and Al. Acknowledgements This research was financially supported through the National High Technology Research and Development Program of China (863 Program, No. 2006AA04Z139). References [1] [2] [3] [4]
[5] [6] [7] [8] [9] [10] [11]
Fig. 3. The XRD diffraction patterns in WN zone of FS weld and base materials for Cu/Al dissimilar materials.
Metals Handbook. 9th ed. Metals Park, Ohio: ASM international; 1979. Li YJ, Wu HQ, Chen MA, Yang M, Feng T. Chin J Nonfer Met 2001;11(3):424–7. Chen XD, Song Q, Yu WY, Song CY. TransChina Weld Inst 2003;24(1):40–3. W.M.Thomas, E.D. Nicholas, J.C. Needham, M.G. Murch, P. Temple-Smith, C.J. Dawes, “Friction stir butt welding”: UK, International patent application no. PCTPGB92P02203 and GB patent application (no. 9125978. 8, 6 December 1991). Mahoney MW, Rhodes CG. Metall Mater Trans A 1998;28(7):1955–64. Sato YS, Kokawa H, Enomoto M. Metall Mater Trans A 1999;30(9):2429–37. Elrefaey A, Takahashi M, Ikeuchi K. Weld in the World 2005;49(3–4):93–101. Ke LM, Liu GP, Xing L, Xia C. Chin J Nonfer Met 2004;14(9):1534–8. Pietras A. Weld in the world 2005;49(9):122–33. Fratini L, Buffa G, Palmeri D, Hua J, Shivpuri R. J Eng MaterTechnol 2006;128 (3):428–35. Jeff D. Weld J 2006;85(3):42–4.