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Joint strength of aluminum ultrasonic soldered under liquidus temperature of Sn–Zn hypereutectic solder
Journal of Materials Processing Technology 209 (2009) 5054–5059
Contents lists available at ScienceDirect
Journal of Materials Processing Technology journal homepage: www.elsevier.com/locate/jmatprotec
Joint strength of aluminum ultrasonic soldered under liquidus temperature of Sn–Zn hypereutectic solder Toru Nagaoka a,∗ , Yoshiaki Morisada a,1 , Masao Fukusumi a,1 , Tadashi Takemoto b,2 a b
Osaka Municipal Technical Research Institute, 1-6-50 Morinomiya, Joto-ku, Osaka, 536-8553, Japan Joining and Welding Research Institute, Osaka University, 11-1 Mihogaoka, Ibaraki-shi, Osaka, 567-0047, Japan
a r t i c l e
i n f o
Article history: Received 20 November 2008 Received in revised form 28 January 2009 Accepted 8 February 2009 Keywords: Ultrasonic soldering 1070 aluminum Sn–Zn solder Joint strength Tensile strength
a b s t r a c t In order to obtain high-strength aluminum joints, ultrasonic soldering of 1070 aluminum was conducted under liquidus temperature of Sn–Zn hypereutectic solder. A device for ultrasonic soldering was assembled, which propagated ultrasonic vibrations in a direction perpendicular to joining surfaces. This device joined 1070-Al using quasi-melting Sn–Zn hypereutectic solder without using any artificial spacers. The strength of the solder joints was evaluated by tensile tests. The optimum joining conditions were determined, and the effects of solder compositions and soldering temperature on the joint strength and the solder layer thickness were examined. In this ultrasonic soldering process, the highest tensile strength was obtained for the solder joints fabricated at 220 ◦ C for the Sn–23Zn and Sn–40Zn solder compositions. The joint strength was equivalent to that of 1070-Al heat treated at 220 ◦ C. The sound joints were obtained at 300 ◦ C using Sn–82Zn solder, the liquid phase volume fraction of which was theoretically only 0.24. The present work also revealed that the thickness of retained solder layer in the joint after ultrasonic soldering could be estimated. Accordingly, ultrasonic soldering under the liquidus temperature of Sn–Zn hypereutectic solder could be a spacer-free soldering method to obtain high-strength aluminum joints. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Severe plastic deformation techniques have been investigated in the last decade as strengthening methods for non-heat-treatable aluminum alloys. Utsunomiya et al. (2004) showed that ultrafine grained 1100 aluminum is obtained by equal channel angular pressing (ECAP). Saito et al. (1998) revealed that ultrafine grained 1100 aluminum produced by accumulative roll-bonding (ARB) process shows the strength higher than 300 MPa. The non-heat-treatable aluminum alloys which are severely plastic deformed will be commercially used in various fields in the immediate future. However, Zhao et al. (2004) indicated that the hardness of ultrafine grained aluminum alloy decreases after annealing because of significant decrease of dislocation density. This result implies that it is difficult to obtain a high-strength joint of severely cold-worked aluminum alloy. In order to solve this problem, a low temperature joining process should be developed.
∗ Corresponding author. Tel.: +81 6 6963 8157; fax: +81 6 6963 8145. E-mail addresses: [email protected] (T. Nagaoka), [email protected] (Y. Morisada), [email protected] (M. Fukusumi), [email protected] (T. Takemoto). 1 Tel.: +81 6 6963 8157; fax: +81 6 6963 8145. 2 Tel.: +81 6 6879 8685; fax: +81 6 6879 8689 (Admin. Office). 0924-0136/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2009.02.003
Ultrasonic soldering, which has been well known as a fluxless soldering method, is very attractive as a low temperature joining process. In an ordinary ultrasonic soldering method, substrates are immersed in a solder bath and joined by ultrasonic vibrations propagated through melted solder. Hunicke (1976) showed that aluminum alloys are readily wetted in a solder bath activated by ultrasonic energy. However, Hosking (1992) indicated that ultrasonic soldering is materials-dependent method. Additionally, solder must contact the faying surfaces in advance to assist the wetting between solder and substrates (Vianco et al., 1996). Recently, new ultrasonic soldering methods have been developed. This process propagates ultrasonic vibrations through substrates directly without using a solder bath (Faridi et al., 2000). Kago et al. (2004) conducted this type of ultrasonic soldering for joining Cu substrates. Naka and Hafez (2003) reported that joints of Al2 O3 to Cu using Zn–Al filler alloy were successfully obtained. Xu et al. (2005a,b) investigated ultrasonic aided interactions between Zn–Al alloy and Al2 O3p /6061 Al composites. The method without using a solder bath could be a short time process and useful for preventing softening of aluminum base metal. These ultrasonic soldering processes have never produced highstrength joints of pure commercial aluminum. Since ultrasonic soldering using Zn filler metal was conducted at high temperature of about 400 ◦ C, softening of cold-worked aluminum occurred during the joining process (Watanabe et al., 1992a,b). Ultrasonic
T. Nagaoka et al. / Journal of Materials Processing Technology 209 (2009) 5054–5059
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Table 1 Chemical compositions and mechanical properties of 1070 aluminum. Material
1070-Al
Compositions (mass%) Si
Fe
Al
0.04
0.13
Bal.
Tensile strength (MPa)
Elongation (%)
168.9
20.0
Table 2 Chemical compositions and solidus (TS ) and liquidus temperatures (TL ) of Sn–Zn solders compositions (mass%). Solder
Sn
Zn
O
Others
TS (◦ C)
TL (◦ C)
Sn–9Zn Sn–23Zn Sn–40Zn Sn–82Zn
91.2 77.0 60.5 18.3
8.70 22.9 39.4 81.6
0.0029 0.0024 0.0021 0.0020
Bal. Bal. Bal. Bal.
199 199 199 199
199 292 340 384
soldering at 200 ◦ C using Sn–30Pb solder also produced low strength joints (Faridi et al., 2000). It is seemed that the filler alloy should have low strength itself. These results imply that high-strength filler alloy should be used at low temperature in order to obtain high-strength aluminum joints. In this study, ultrasonic soldering under liquidus temperature of Sn–Zn hypereutectic solder alloy was conducted without using a solder bath. Since Sn–Zn hypereutectic solder alloys with high Zn content had higher strength than Sn–9Zn eutectic solder, highstrength aluminum joints could be obtained by using these alloys. In order to prevent softening of cold-worked aluminum during the soldering process, these solder alloys were used under the liquidus temperature, respectively. A device for this ultrasonic soldering method was assembled, which propagated ultrasonic vibrations in a direction perpendicular to joining surfaces. The strength of the resultant joints was evaluated by tensile tests. The optimum joining conditions were determined, and the effects of the solder compositions and soldering temperature on the joint strength were discussed. In addition, the effect of the volume fraction of unmelted phase during ultrasonic soldering on the solder layer thickness was also studied. 2. Experimental Highly drawn 1070 aluminum bars with 5 mm in diameter were used for ultrasonic soldering tests. Table 1 shows chemical compositions and mechanical properties of as-received 1070-Al. The mechanical properties were measured by tensile tests at room temperature. Sn–xZn (x = 23, 40, and 82 mass%) hypereutectic solders and Sn–9Zn eutectic solder were prepared by melting pure tin (99.9 %) and pure zinc (99.99 %) in a graphite crucible at 600 ◦ C in air. They were cast into bar-shaped copper molds, and then, each ingot bar was cold rolled into 100-m thick foil. Table 2 shows chemical compositions and solidus and liquidus temperatures of the prepared Sn–Zn solders. Solidus and liquidus temperatures were measured by DTA with the heating rate of 0.17 ◦ C/s. The ultrasonic soldering process conducted in this study is schematically shown in Fig. 1. A horn was installed in a horizontal
Fig. 2. Shape and size of specimens for tensile tests: (a) and (b) for solder joint, and (c) for aluminum rod.
direction for fixing substrates easily, and thereby it is unnecessary to use any clamping jigs such as a screw bolt or a knurled surface. The dimensions of aluminum test pieces were 5 mm in diameter and 35 mm in length. The faying surface finish was obtained by buffing. The thickness of Sn–xZn (x = 9, 23, 40, and 82 mass%) foils were 100 m and the foils were cut into disks with 5 mm in diameter. A couple of aluminum test pieces were mounted into the holder and the tip of the horn. The Sn–Zn solder disk was inserted between the faying surfaces. A K-type thermocouple was installed 1 mm away from the joining interfaces. The induction coil heated test pieces in air to the soldering temperature ranging from 180 to 300 ◦ C. The average heating rate was 9.2 ◦ C/s. Ultrasonic vibrations of 30–360 W at the frequency of 19 kHz were applied at each soldering temperature for the predetermined time. They were propagated in a direction perpendicular to faying surfaces through a 1070-Al substrate. Constant pressure was also applied during the ultrasonic soldering. After the ultrasonic soldering process, the test pieces were air cooled to room temperature. The optimum condition of ultrasonic soldering was determined on the results of tensile tests using the specimen shown in Fig. 2(a). The specimen shown in Fig. 2(b) was used for solder joints to be compared with aluminum rods. The specimen shown in Fig. 2(c) was used for evaluating the strength of 1070-Al which employed the same heat treatment as ultrasonic soldering processes. The tensile test specimens in Fig. 2 are in accordance with ISO 6892. The tensile strength was measured at the crosshead speed of 1.0 mm/min at 23 ◦ C. Microstructural observations and elemental analyses of the cross-section of the soldered interfaces were carried out with an SEM–EDX (JSM-6460LA). Fracture surfaces of the joints after tensile tests were also observed with the same equipment. Thickness fraction of retained solder layer (fT ) after ultrasonic soldering was calculated using the following equation: fT =
h h0
(1)
where h0 is the thickness of the solder disk before ultrasonic soldering and h is the thickness of the solder layer after ultrasonic soldering. 3. Results and discussion 3.1. Optimization of ultrasonic soldering condition
Fig. 1. Schematic illustration of ultrasonic soldering.
Prior to determine optimum condition for ultrasonic soldering, the vibration amplitude of the faying surface of a 1070-Al specimen mounted on the horn was measured with a strain gage. Fig. 3 shows
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Fig. 3. Relationship between ultrasonic power and vibration amplitude of faying surface of 1070-Al mounted on the horn.
Fig. 4. Relationship between ultrasonic power and tensile strength of joints ultrasonically soldered at 220 ◦ C with Sn–23Zn solder.
Fig. 5. Relationship between applied pressure and tensile strength of joints ultrasonically soldered at 220 ◦ C with Sn–23Zn solder.
Fig. 6. Relationship between ultrasonic vibration time and tensile strength of joints ultrasonically soldered at 220 ◦ C with Sn–23Zn solder.
Fig. 7. Fracture surfaces of joints ultrasonically soldered at 220 ◦ C with Sn–23Zn solder for various vibration time: (a) 1 s, (b) 2 s, (c) 4 s, and (d) 6 s.
T. Nagaoka et al. / Journal of Materials Processing Technology 209 (2009) 5054–5059
the relationship between ultrasonic power and vibration amplitude. Under the pressure of 0 Pa, the vibration amplitude increased in proportion to ultrasonic power. The vibration amplitude during ultrasonic soldering under some pressure should be less than these data even at the same power. In order to determine the optimum condition for ultrasonic soldering under the liquidus temperatures of Sn–Zn hypereutectic solders, effects of ultrasonic power, applied pressure and vibration time on joint strength were examined by tensile tests. Ultrasonic soldering was conducted at 220 ◦ C using Sn–23Zn solder (TS = 199 ◦ C, TL = 292 ◦ C). The specimen shown in Fig. 2(a) was used for tensile tests. Fig. 4 shows the relationship between ultrasonic power and the tensile strength of the joints. The vibration time and the applied pressure were set at 4 s and 0.5 MPa, respectively. The maximum joint strength was obtained under the ultrasonic power of 120 W. Joining could not be performed under the ultrasonic power less than 60 W. It seems to be insufficient to make sound joints. The joint strength decreased with increase of the ultrasonic power more than 240 W. The reason of the decrease of joint strength can be attributed to the solder pressed out from the joint gap during the joining process owing to the intense vibration. Fig. 5 shows the relationship between applied pressure and the tensile strength of joints. The ultrasonic power and the vibration time were set at 120 W and 4 s, respectively. The almost constant tensile strength of about 130 MPa was obtained irrespective of the applied pressure. The joint strength seems to be determined by the balance of the vibration amplitude and the applied pressure. Increasing of the applied pressure enhances the contact of faying surfaces but it simultaneously decreases vibration amplitude. Fig. 6 shows the effect of vibration time on the tensile strength of joints. The ultrasonic power and the applied pressure were set at 120 W and 0.5 MPa, respectively. The joint strength increased with increase of the vibration time and reached 130 MPa at the vibration time of 4 s or more. The joints soldered at the vibration time under 2 s showed insufficient strength, because the strength of 1070-Al base metal was 168.9 MPa as shown in Table 1. Unbonded areas indicated by arrows in Fig. 7 were observed around the edge of the fracture surface in the case of the vibration time under 2 s. This result indicates that the soldering process starts at the center of joining surfaces and proceeds to the edges.
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Fig. 8. Relationship between soldering temperature and tensile strength of joints.
These results provided the optimum condition for the ultrasonic soldering under the liquidus temperature of Sn–23Zn solder. In subsequent experiments, accordingly, ultrasonic soldering was carried out under the condition of ultrasonic power of 120 W, vibration time of 4 s, and applied pressure of 0.5 MPa. 3.2. Effect of soldering temperature and solder composition Fig. 8 shows the relationship between soldering temperature and the tensile strength of joints ultrasonically soldered with each solder. To evaluate the strength of joints including the edge of the faying surfaces, the specimen as shown in Fig. 2(a) was used. No joint was obtained at soldering temperature of 180 ◦ C. It was necessary to increase the soldering temperature higher than 220 ◦ C to obtain high-strength aluminum joints. The strength of joints soldered at temperatures higher than 220 ◦ C with Sn–23Zn (TL = 292 ◦ C) or Sn–40Zn solder (TL = 340 ◦ C) was higher than that of the joints soldered with Sn–9Zn eutectic solder. The joint soldered at 220 or 260 ◦ C with Sn–82Zn solder (TL = 384 ◦ C) showed insufficient strength. However, the strength of the joint soldered at 300 ◦ C was equivalent to those of the joints soldered at 300 ◦ C with other Sn–Zn system solders. SEM–EDX line analyses of the soldered interface on the joint soldered with
Fig. 9. SEM–EDX analysis of the soldered interface of a joint ultrasonically soldered with Sn–82Zn solder: (a) soldered at 220 ◦ C and (b) soldered at 300 ◦ C.
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Sn–82Zn solder are shown in Fig. 9. The volume fraction of liquid phase is 0.20 at 220 ◦ C and 0.24 at 300 ◦ C, according to the calculation based on Sn–Zn phase diagram (Baker, 1992b). The density data at 220–300 ◦ C of Sn–9Zn solder (Wang and Ai-Ping, 2005) was used as the density of the liquid phase of each Sn–Zn system solder. Numerous ␣-Zn phases which should be solid solution during ultrasonic soldering were observed in the solder layer. A gap on the interface between 1070-Al and Sn–82Zn solder was observed in the joint soldered at 220 ◦ C. The line analysis detected oxygen at interface. The presence of a gap at 220 ◦ C is the result of the low wettability caused by the low soldering temperature. Furthermore at 220 ◦ C there is partial melting of the solder that causes insufficient liquid to wet the 1070-Al substrate. No gap was observed in the joint soldered at 300 ◦ C but Sn was detected at interface. It seems that the wettability should be improved due to larger amount of liquid phase at this temperature. Some Al-rich particles including Zn were also observed in the solder layer. These particles correspond to the Al-rich phase in the Al–Zn phase diagram (Baker, 1992a). This result implies that the liquid phase during ultrasonic soldering should contribute to join 1070-Al and the solder, and Al diffuses into the solder metal at 300 ◦ C. These Al-rich particles were not observed in the joint soldered at 220 ◦ C due to the lower processing temperature and to the fact that a gap existed between the Al substrate and the solder. Therefore, the ultrasonic soldering process using Sn–82Zn solder was insufficiently accomplished at 220 ◦ C. Fig. 10 shows the tensile strength of the joints and 1070-Al rods using the specimen shown in Fig. 2(b). Prior to measuring tensile strength, some of 1070-Al rods were heat treated at 220–300 ◦ C for 4 s by induction heating. All of the solder joints fabricated at 220–300 ◦ C fractured in each solder layer. Their strength gradually decreased with increase of heat treatment temperature. The strength of joints using Sn–23Zn or Sn–40Zn solder was about 150 MPa, which were equivalent to heat-treated 1070-Al rods. This result indicates that the strength of the joints soldered with Sn–23Zn or Sn–40Zn solder depends on the strength of heat-affected 1070-Al. Accordingly, ultrasonic soldering at lower temperature is preferable to obtain high-strength aluminum joints.
Fig. 10. Tensile strength of joints and heat-treated 1070-Al rods.
3.3. Solder layer thickness after ultrasonic soldering under liquidus temperature Fig. 11 shows the cross-section of the joints ultrasonically soldered at 300 ◦ C. The thickness of the solder layer in the joint with Sn–9Zn eutectic solder was much thinner than that of the solder disk before ultrasonic soldering. Most of the solder was squeezed out from the joint gap by the applied pressure because the eutectic solder completely became a liquid phase at 300 ◦ C. On the contrary, the solder layer thickness in the joint with Sn–Zn system hypereutectic solder increased with increase of Zn content. It is considered that ␣-Zn solid solution played a role as a spacer and prevented the solder from being squeezed out from the joint gap. Fig. 12 shows the plots between the calculated volume fraction of ␣-Zn (fV ) at soldering temperature and the measured thickness fraction of retained solder layer (fT ). The value of fV was calculated on the basis of the Sn–Zn binary phase diagram (Baker, 1992b). The measured thickness fraction (fT ) increased with increase of fV . In addition, the deviation of Sn–23Zn and Sn–40Zn solders was larger than that of Sn–82Zn solder. This is because some of the liquid phase
Fig. 11. Micrographs across the soldered interfaces of joints ultrasonically soldered at 300 ◦ C: (a) Sn–9Zn, (b) Sn–23Zn, (c) Sn–40Zn, and (d) Sn–82Zn.
T. Nagaoka et al. / Journal of Materials Processing Technology 209 (2009) 5054–5059
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(2) The joints ultrasonically soldered at 220 ◦ C with Sn–23Zn or Sn–40Zn solder showed the highest strength of about 150 MPa. The strength was equivalent to that of 1070-Al which employed the heat treatment at 220 ◦ C for 4 s. (3) The strength of joints ultrasonically soldered at 300 ◦ C with Sn–82Zn solder, the liquid phase volume fraction of which was theoretically only 0.24, was equivalent to those of the joints soldered at 300 ◦ C with other Sn–Zn system solders. (4) The solder layer thickness increased with increase of the volume fraction of proeutectic phase at soldering temperature. Ultrasonic soldering under the liquidus temperature of Sn–Zn hypereutectic solder could be a spacer-free soldering method to obtain high-strength aluminum joints. References Fig. 12. Plots between calculated volume fraction of ␣-Zn (fV ) at soldering temperature and measured thickness fraction of retained solder layer (fT ).
was squeezed out from the joint gap by the applied pressure. The variation of fT of Sn–23Zn and Sn–40Zn solders should depend on the shape and the distribution of ␣-Zn solid solution during ultrasonic soldering; accordingly it should cause some deviation of fT of Sn–23Zn and Sn–40Zn solders. On the contrary, fT of Sn–82Zn solder was almost 1.0. This result indicates that ␣-Zn solid solutions were originally arranged as a close-packed structure owing to the high volume fraction. The data suggests that ultrasonic soldering under the liquidus temperature of Sn–Zn hypereutectic solder could be a spacer-free soldering method to obtain high-strength aluminum joints. 4. Conclusion Ultrasonic soldering of 1070 aluminum using Sn–xZn (x = 23, 40, and 82 mass%) solders were carried out under the liquidus temperatures of the solders. The process conditions to obtain high-strength aluminum joints were optimized and the effects of the solder compositions and soldering temperature on the tensile strength and the solder layer thickness of joints were examined in detail. The obtained results can be summarized as follows: (1) The optimum conditions of ultrasonic soldering at 220 ◦ C using Sn–23Zn solder were the ultrasonic power of 120 W and the ultrasonic vibration time of 4 s.
Baker, H., 1992a. ASM Handbook, Binary Alloy Phase Diagrams, vol. 3. ASM International, Materials Park, OH, p. 56. Baker, H., 1992b. ASM Handbook, Binary Alloy Phase Diagrams, vol. 3. ASM International, Materials Park, OH, p. 372. Faridi, H.R., Devletian, J.H., Le, H.P., 2000. A new look at flux-free ultrasonic soldering. Weld. J. 79, 41–45. Hosking, F.M., 1992. Fluxless soldering as an environmentally compatible manufacturing alternative. Int. J. Environ. Consc. Manufact. 1, 53–64. Hunicke, R.L., 1976. Ultrasonic soldering pots for fluxless production soldering. Weld. J. 55, 191–194. Kago, K., Suetsugu, K., Hibino, S., Ikari, T., Frusawa, A., Takano, H., Horiuchi, T., Ishida, K., Sakaguchi, T., Kikuchi, S., Matsushige, K., 2004. Novel ultrasonic soldering technique for lead-free solders. Mater. Trans. 45, 703–709. Naka, M., Hafez, K.M., 2003. Applying of ultrasonic waves on brazing of alumina to copper using Zn–Al filler alloy. J. Mater. Sci. 38, 3491–3494. Saito, Y., Tsuji, N., Utsunomiya, H., Sakai, T., Hong, R.G., 1998. Ultra-fine grained bulk aluminum produced by accumulative roll-bonding (ARB) process. Scripta Mater. 39, 1221–1227. Utsunomiya, H., Hatsuda, K., Sakai, T., Saito, Y., 2004. Continuous grain refinement of aluminum strip by conshearing. Mater. Sci. Eng. A 372, 199–206. Vianco, P.T., Hosking, F.M., Rejent, J.A., 1996. Ultrasonic soldering for structural and electronic applications. Weld. J. 75, 343–355. Wang, L., Ai-Ping, X., 2005. Density measurement of Sn–40Pb, Sn–57Bi, and Sn–9Zn by indirect archimedean method. J. Electron. Mater. 34, 1414–1419. Watanabe, T., Tanaka, Y., Yanagisawa, A., Onuma, S., Sato, S., 1992a. Q. J. Jpn. Weld. Soc. 10, 470–476. Watanabe, T., Tanaka, Y., Yanagisawa, A., Onuma, S., Sato, S., 1992b. Q. J. Jpn. Weld. Soc. 10, 476–483. Xu, Z.W., Yan, J., Wu, G., Kong, X., Yang, S., 2005a. Interface structure and strength of ultrasonic vibration liquid phase bonded joints of Al2 O3 /6061 Al composites. Scripta Mater. 53, 835–839. Xu, Z.W., Yan, J., Wu, G., Kong, X., Yang, S., 2005b. Interface structure of ultrasonic vibration aided interaction between Zn–Al alloy and Al2 O3 /6061 Al composite. Compos. Sci. Technol. 65, 1959–1963. Zhao, Y.H., Liao, X.Z., Jin, Z.Z., Valiev, R.Z., Zhu, Y.T., 2004. Microstructures and mechanical properties of ultrafine grained 7075 Al alloy processed by ECAP and their evolutions during annealing. Acta Mater. 52, 4589–4599.
Contents lists available at ScienceDirect
Journal of Materials Processing Technology journal homepage: www.elsevier.com/locate/jmatprotec
Joint strength of aluminum ultrasonic soldered under liquidus temperature of Sn–Zn hypereutectic solder Toru Nagaoka a,∗ , Yoshiaki Morisada a,1 , Masao Fukusumi a,1 , Tadashi Takemoto b,2 a b
Osaka Municipal Technical Research Institute, 1-6-50 Morinomiya, Joto-ku, Osaka, 536-8553, Japan Joining and Welding Research Institute, Osaka University, 11-1 Mihogaoka, Ibaraki-shi, Osaka, 567-0047, Japan
a r t i c l e
i n f o
Article history: Received 20 November 2008 Received in revised form 28 January 2009 Accepted 8 February 2009 Keywords: Ultrasonic soldering 1070 aluminum Sn–Zn solder Joint strength Tensile strength
a b s t r a c t In order to obtain high-strength aluminum joints, ultrasonic soldering of 1070 aluminum was conducted under liquidus temperature of Sn–Zn hypereutectic solder. A device for ultrasonic soldering was assembled, which propagated ultrasonic vibrations in a direction perpendicular to joining surfaces. This device joined 1070-Al using quasi-melting Sn–Zn hypereutectic solder without using any artificial spacers. The strength of the solder joints was evaluated by tensile tests. The optimum joining conditions were determined, and the effects of solder compositions and soldering temperature on the joint strength and the solder layer thickness were examined. In this ultrasonic soldering process, the highest tensile strength was obtained for the solder joints fabricated at 220 ◦ C for the Sn–23Zn and Sn–40Zn solder compositions. The joint strength was equivalent to that of 1070-Al heat treated at 220 ◦ C. The sound joints were obtained at 300 ◦ C using Sn–82Zn solder, the liquid phase volume fraction of which was theoretically only 0.24. The present work also revealed that the thickness of retained solder layer in the joint after ultrasonic soldering could be estimated. Accordingly, ultrasonic soldering under the liquidus temperature of Sn–Zn hypereutectic solder could be a spacer-free soldering method to obtain high-strength aluminum joints. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Severe plastic deformation techniques have been investigated in the last decade as strengthening methods for non-heat-treatable aluminum alloys. Utsunomiya et al. (2004) showed that ultrafine grained 1100 aluminum is obtained by equal channel angular pressing (ECAP). Saito et al. (1998) revealed that ultrafine grained 1100 aluminum produced by accumulative roll-bonding (ARB) process shows the strength higher than 300 MPa. The non-heat-treatable aluminum alloys which are severely plastic deformed will be commercially used in various fields in the immediate future. However, Zhao et al. (2004) indicated that the hardness of ultrafine grained aluminum alloy decreases after annealing because of significant decrease of dislocation density. This result implies that it is difficult to obtain a high-strength joint of severely cold-worked aluminum alloy. In order to solve this problem, a low temperature joining process should be developed.
∗ Corresponding author. Tel.: +81 6 6963 8157; fax: +81 6 6963 8145. E-mail addresses: [email protected] (T. Nagaoka), [email protected] (Y. Morisada), [email protected] (M. Fukusumi), [email protected] (T. Takemoto). 1 Tel.: +81 6 6963 8157; fax: +81 6 6963 8145. 2 Tel.: +81 6 6879 8685; fax: +81 6 6879 8689 (Admin. Office). 0924-0136/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2009.02.003
Ultrasonic soldering, which has been well known as a fluxless soldering method, is very attractive as a low temperature joining process. In an ordinary ultrasonic soldering method, substrates are immersed in a solder bath and joined by ultrasonic vibrations propagated through melted solder. Hunicke (1976) showed that aluminum alloys are readily wetted in a solder bath activated by ultrasonic energy. However, Hosking (1992) indicated that ultrasonic soldering is materials-dependent method. Additionally, solder must contact the faying surfaces in advance to assist the wetting between solder and substrates (Vianco et al., 1996). Recently, new ultrasonic soldering methods have been developed. This process propagates ultrasonic vibrations through substrates directly without using a solder bath (Faridi et al., 2000). Kago et al. (2004) conducted this type of ultrasonic soldering for joining Cu substrates. Naka and Hafez (2003) reported that joints of Al2 O3 to Cu using Zn–Al filler alloy were successfully obtained. Xu et al. (2005a,b) investigated ultrasonic aided interactions between Zn–Al alloy and Al2 O3p /6061 Al composites. The method without using a solder bath could be a short time process and useful for preventing softening of aluminum base metal. These ultrasonic soldering processes have never produced highstrength joints of pure commercial aluminum. Since ultrasonic soldering using Zn filler metal was conducted at high temperature of about 400 ◦ C, softening of cold-worked aluminum occurred during the joining process (Watanabe et al., 1992a,b). Ultrasonic
T. Nagaoka et al. / Journal of Materials Processing Technology 209 (2009) 5054–5059
5055
Table 1 Chemical compositions and mechanical properties of 1070 aluminum. Material
1070-Al
Compositions (mass%) Si
Fe
Al
0.04
0.13
Bal.
Tensile strength (MPa)
Elongation (%)
168.9
20.0
Table 2 Chemical compositions and solidus (TS ) and liquidus temperatures (TL ) of Sn–Zn solders compositions (mass%). Solder
Sn
Zn
O
Others
TS (◦ C)
TL (◦ C)
Sn–9Zn Sn–23Zn Sn–40Zn Sn–82Zn
91.2 77.0 60.5 18.3
8.70 22.9 39.4 81.6
0.0029 0.0024 0.0021 0.0020
Bal. Bal. Bal. Bal.
199 199 199 199
199 292 340 384
soldering at 200 ◦ C using Sn–30Pb solder also produced low strength joints (Faridi et al., 2000). It is seemed that the filler alloy should have low strength itself. These results imply that high-strength filler alloy should be used at low temperature in order to obtain high-strength aluminum joints. In this study, ultrasonic soldering under liquidus temperature of Sn–Zn hypereutectic solder alloy was conducted without using a solder bath. Since Sn–Zn hypereutectic solder alloys with high Zn content had higher strength than Sn–9Zn eutectic solder, highstrength aluminum joints could be obtained by using these alloys. In order to prevent softening of cold-worked aluminum during the soldering process, these solder alloys were used under the liquidus temperature, respectively. A device for this ultrasonic soldering method was assembled, which propagated ultrasonic vibrations in a direction perpendicular to joining surfaces. The strength of the resultant joints was evaluated by tensile tests. The optimum joining conditions were determined, and the effects of the solder compositions and soldering temperature on the joint strength were discussed. In addition, the effect of the volume fraction of unmelted phase during ultrasonic soldering on the solder layer thickness was also studied. 2. Experimental Highly drawn 1070 aluminum bars with 5 mm in diameter were used for ultrasonic soldering tests. Table 1 shows chemical compositions and mechanical properties of as-received 1070-Al. The mechanical properties were measured by tensile tests at room temperature. Sn–xZn (x = 23, 40, and 82 mass%) hypereutectic solders and Sn–9Zn eutectic solder were prepared by melting pure tin (99.9 %) and pure zinc (99.99 %) in a graphite crucible at 600 ◦ C in air. They were cast into bar-shaped copper molds, and then, each ingot bar was cold rolled into 100-m thick foil. Table 2 shows chemical compositions and solidus and liquidus temperatures of the prepared Sn–Zn solders. Solidus and liquidus temperatures were measured by DTA with the heating rate of 0.17 ◦ C/s. The ultrasonic soldering process conducted in this study is schematically shown in Fig. 1. A horn was installed in a horizontal
Fig. 2. Shape and size of specimens for tensile tests: (a) and (b) for solder joint, and (c) for aluminum rod.
direction for fixing substrates easily, and thereby it is unnecessary to use any clamping jigs such as a screw bolt or a knurled surface. The dimensions of aluminum test pieces were 5 mm in diameter and 35 mm in length. The faying surface finish was obtained by buffing. The thickness of Sn–xZn (x = 9, 23, 40, and 82 mass%) foils were 100 m and the foils were cut into disks with 5 mm in diameter. A couple of aluminum test pieces were mounted into the holder and the tip of the horn. The Sn–Zn solder disk was inserted between the faying surfaces. A K-type thermocouple was installed 1 mm away from the joining interfaces. The induction coil heated test pieces in air to the soldering temperature ranging from 180 to 300 ◦ C. The average heating rate was 9.2 ◦ C/s. Ultrasonic vibrations of 30–360 W at the frequency of 19 kHz were applied at each soldering temperature for the predetermined time. They were propagated in a direction perpendicular to faying surfaces through a 1070-Al substrate. Constant pressure was also applied during the ultrasonic soldering. After the ultrasonic soldering process, the test pieces were air cooled to room temperature. The optimum condition of ultrasonic soldering was determined on the results of tensile tests using the specimen shown in Fig. 2(a). The specimen shown in Fig. 2(b) was used for solder joints to be compared with aluminum rods. The specimen shown in Fig. 2(c) was used for evaluating the strength of 1070-Al which employed the same heat treatment as ultrasonic soldering processes. The tensile test specimens in Fig. 2 are in accordance with ISO 6892. The tensile strength was measured at the crosshead speed of 1.0 mm/min at 23 ◦ C. Microstructural observations and elemental analyses of the cross-section of the soldered interfaces were carried out with an SEM–EDX (JSM-6460LA). Fracture surfaces of the joints after tensile tests were also observed with the same equipment. Thickness fraction of retained solder layer (fT ) after ultrasonic soldering was calculated using the following equation: fT =
h h0
(1)
where h0 is the thickness of the solder disk before ultrasonic soldering and h is the thickness of the solder layer after ultrasonic soldering. 3. Results and discussion 3.1. Optimization of ultrasonic soldering condition
Fig. 1. Schematic illustration of ultrasonic soldering.
Prior to determine optimum condition for ultrasonic soldering, the vibration amplitude of the faying surface of a 1070-Al specimen mounted on the horn was measured with a strain gage. Fig. 3 shows
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Fig. 3. Relationship between ultrasonic power and vibration amplitude of faying surface of 1070-Al mounted on the horn.
Fig. 4. Relationship between ultrasonic power and tensile strength of joints ultrasonically soldered at 220 ◦ C with Sn–23Zn solder.
Fig. 5. Relationship between applied pressure and tensile strength of joints ultrasonically soldered at 220 ◦ C with Sn–23Zn solder.
Fig. 6. Relationship between ultrasonic vibration time and tensile strength of joints ultrasonically soldered at 220 ◦ C with Sn–23Zn solder.
Fig. 7. Fracture surfaces of joints ultrasonically soldered at 220 ◦ C with Sn–23Zn solder for various vibration time: (a) 1 s, (b) 2 s, (c) 4 s, and (d) 6 s.
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the relationship between ultrasonic power and vibration amplitude. Under the pressure of 0 Pa, the vibration amplitude increased in proportion to ultrasonic power. The vibration amplitude during ultrasonic soldering under some pressure should be less than these data even at the same power. In order to determine the optimum condition for ultrasonic soldering under the liquidus temperatures of Sn–Zn hypereutectic solders, effects of ultrasonic power, applied pressure and vibration time on joint strength were examined by tensile tests. Ultrasonic soldering was conducted at 220 ◦ C using Sn–23Zn solder (TS = 199 ◦ C, TL = 292 ◦ C). The specimen shown in Fig. 2(a) was used for tensile tests. Fig. 4 shows the relationship between ultrasonic power and the tensile strength of the joints. The vibration time and the applied pressure were set at 4 s and 0.5 MPa, respectively. The maximum joint strength was obtained under the ultrasonic power of 120 W. Joining could not be performed under the ultrasonic power less than 60 W. It seems to be insufficient to make sound joints. The joint strength decreased with increase of the ultrasonic power more than 240 W. The reason of the decrease of joint strength can be attributed to the solder pressed out from the joint gap during the joining process owing to the intense vibration. Fig. 5 shows the relationship between applied pressure and the tensile strength of joints. The ultrasonic power and the vibration time were set at 120 W and 4 s, respectively. The almost constant tensile strength of about 130 MPa was obtained irrespective of the applied pressure. The joint strength seems to be determined by the balance of the vibration amplitude and the applied pressure. Increasing of the applied pressure enhances the contact of faying surfaces but it simultaneously decreases vibration amplitude. Fig. 6 shows the effect of vibration time on the tensile strength of joints. The ultrasonic power and the applied pressure were set at 120 W and 0.5 MPa, respectively. The joint strength increased with increase of the vibration time and reached 130 MPa at the vibration time of 4 s or more. The joints soldered at the vibration time under 2 s showed insufficient strength, because the strength of 1070-Al base metal was 168.9 MPa as shown in Table 1. Unbonded areas indicated by arrows in Fig. 7 were observed around the edge of the fracture surface in the case of the vibration time under 2 s. This result indicates that the soldering process starts at the center of joining surfaces and proceeds to the edges.
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Fig. 8. Relationship between soldering temperature and tensile strength of joints.
These results provided the optimum condition for the ultrasonic soldering under the liquidus temperature of Sn–23Zn solder. In subsequent experiments, accordingly, ultrasonic soldering was carried out under the condition of ultrasonic power of 120 W, vibration time of 4 s, and applied pressure of 0.5 MPa. 3.2. Effect of soldering temperature and solder composition Fig. 8 shows the relationship between soldering temperature and the tensile strength of joints ultrasonically soldered with each solder. To evaluate the strength of joints including the edge of the faying surfaces, the specimen as shown in Fig. 2(a) was used. No joint was obtained at soldering temperature of 180 ◦ C. It was necessary to increase the soldering temperature higher than 220 ◦ C to obtain high-strength aluminum joints. The strength of joints soldered at temperatures higher than 220 ◦ C with Sn–23Zn (TL = 292 ◦ C) or Sn–40Zn solder (TL = 340 ◦ C) was higher than that of the joints soldered with Sn–9Zn eutectic solder. The joint soldered at 220 or 260 ◦ C with Sn–82Zn solder (TL = 384 ◦ C) showed insufficient strength. However, the strength of the joint soldered at 300 ◦ C was equivalent to those of the joints soldered at 300 ◦ C with other Sn–Zn system solders. SEM–EDX line analyses of the soldered interface on the joint soldered with
Fig. 9. SEM–EDX analysis of the soldered interface of a joint ultrasonically soldered with Sn–82Zn solder: (a) soldered at 220 ◦ C and (b) soldered at 300 ◦ C.
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Sn–82Zn solder are shown in Fig. 9. The volume fraction of liquid phase is 0.20 at 220 ◦ C and 0.24 at 300 ◦ C, according to the calculation based on Sn–Zn phase diagram (Baker, 1992b). The density data at 220–300 ◦ C of Sn–9Zn solder (Wang and Ai-Ping, 2005) was used as the density of the liquid phase of each Sn–Zn system solder. Numerous ␣-Zn phases which should be solid solution during ultrasonic soldering were observed in the solder layer. A gap on the interface between 1070-Al and Sn–82Zn solder was observed in the joint soldered at 220 ◦ C. The line analysis detected oxygen at interface. The presence of a gap at 220 ◦ C is the result of the low wettability caused by the low soldering temperature. Furthermore at 220 ◦ C there is partial melting of the solder that causes insufficient liquid to wet the 1070-Al substrate. No gap was observed in the joint soldered at 300 ◦ C but Sn was detected at interface. It seems that the wettability should be improved due to larger amount of liquid phase at this temperature. Some Al-rich particles including Zn were also observed in the solder layer. These particles correspond to the Al-rich phase in the Al–Zn phase diagram (Baker, 1992a). This result implies that the liquid phase during ultrasonic soldering should contribute to join 1070-Al and the solder, and Al diffuses into the solder metal at 300 ◦ C. These Al-rich particles were not observed in the joint soldered at 220 ◦ C due to the lower processing temperature and to the fact that a gap existed between the Al substrate and the solder. Therefore, the ultrasonic soldering process using Sn–82Zn solder was insufficiently accomplished at 220 ◦ C. Fig. 10 shows the tensile strength of the joints and 1070-Al rods using the specimen shown in Fig. 2(b). Prior to measuring tensile strength, some of 1070-Al rods were heat treated at 220–300 ◦ C for 4 s by induction heating. All of the solder joints fabricated at 220–300 ◦ C fractured in each solder layer. Their strength gradually decreased with increase of heat treatment temperature. The strength of joints using Sn–23Zn or Sn–40Zn solder was about 150 MPa, which were equivalent to heat-treated 1070-Al rods. This result indicates that the strength of the joints soldered with Sn–23Zn or Sn–40Zn solder depends on the strength of heat-affected 1070-Al. Accordingly, ultrasonic soldering at lower temperature is preferable to obtain high-strength aluminum joints.
Fig. 10. Tensile strength of joints and heat-treated 1070-Al rods.
3.3. Solder layer thickness after ultrasonic soldering under liquidus temperature Fig. 11 shows the cross-section of the joints ultrasonically soldered at 300 ◦ C. The thickness of the solder layer in the joint with Sn–9Zn eutectic solder was much thinner than that of the solder disk before ultrasonic soldering. Most of the solder was squeezed out from the joint gap by the applied pressure because the eutectic solder completely became a liquid phase at 300 ◦ C. On the contrary, the solder layer thickness in the joint with Sn–Zn system hypereutectic solder increased with increase of Zn content. It is considered that ␣-Zn solid solution played a role as a spacer and prevented the solder from being squeezed out from the joint gap. Fig. 12 shows the plots between the calculated volume fraction of ␣-Zn (fV ) at soldering temperature and the measured thickness fraction of retained solder layer (fT ). The value of fV was calculated on the basis of the Sn–Zn binary phase diagram (Baker, 1992b). The measured thickness fraction (fT ) increased with increase of fV . In addition, the deviation of Sn–23Zn and Sn–40Zn solders was larger than that of Sn–82Zn solder. This is because some of the liquid phase
Fig. 11. Micrographs across the soldered interfaces of joints ultrasonically soldered at 300 ◦ C: (a) Sn–9Zn, (b) Sn–23Zn, (c) Sn–40Zn, and (d) Sn–82Zn.
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(2) The joints ultrasonically soldered at 220 ◦ C with Sn–23Zn or Sn–40Zn solder showed the highest strength of about 150 MPa. The strength was equivalent to that of 1070-Al which employed the heat treatment at 220 ◦ C for 4 s. (3) The strength of joints ultrasonically soldered at 300 ◦ C with Sn–82Zn solder, the liquid phase volume fraction of which was theoretically only 0.24, was equivalent to those of the joints soldered at 300 ◦ C with other Sn–Zn system solders. (4) The solder layer thickness increased with increase of the volume fraction of proeutectic phase at soldering temperature. Ultrasonic soldering under the liquidus temperature of Sn–Zn hypereutectic solder could be a spacer-free soldering method to obtain high-strength aluminum joints. References Fig. 12. Plots between calculated volume fraction of ␣-Zn (fV ) at soldering temperature and measured thickness fraction of retained solder layer (fT ).
was squeezed out from the joint gap by the applied pressure. The variation of fT of Sn–23Zn and Sn–40Zn solders should depend on the shape and the distribution of ␣-Zn solid solution during ultrasonic soldering; accordingly it should cause some deviation of fT of Sn–23Zn and Sn–40Zn solders. On the contrary, fT of Sn–82Zn solder was almost 1.0. This result indicates that ␣-Zn solid solutions were originally arranged as a close-packed structure owing to the high volume fraction. The data suggests that ultrasonic soldering under the liquidus temperature of Sn–Zn hypereutectic solder could be a spacer-free soldering method to obtain high-strength aluminum joints. 4. Conclusion Ultrasonic soldering of 1070 aluminum using Sn–xZn (x = 23, 40, and 82 mass%) solders were carried out under the liquidus temperatures of the solders. The process conditions to obtain high-strength aluminum joints were optimized and the effects of the solder compositions and soldering temperature on the tensile strength and the solder layer thickness of joints were examined in detail. The obtained results can be summarized as follows: (1) The optimum conditions of ultrasonic soldering at 220 ◦ C using Sn–23Zn solder were the ultrasonic power of 120 W and the ultrasonic vibration time of 4 s.
Baker, H., 1992a. ASM Handbook, Binary Alloy Phase Diagrams, vol. 3. ASM International, Materials Park, OH, p. 56. Baker, H., 1992b. ASM Handbook, Binary Alloy Phase Diagrams, vol. 3. ASM International, Materials Park, OH, p. 372. Faridi, H.R., Devletian, J.H., Le, H.P., 2000. A new look at flux-free ultrasonic soldering. Weld. J. 79, 41–45. Hosking, F.M., 1992. Fluxless soldering as an environmentally compatible manufacturing alternative. Int. J. Environ. Consc. Manufact. 1, 53–64. Hunicke, R.L., 1976. Ultrasonic soldering pots for fluxless production soldering. Weld. J. 55, 191–194. Kago, K., Suetsugu, K., Hibino, S., Ikari, T., Frusawa, A., Takano, H., Horiuchi, T., Ishida, K., Sakaguchi, T., Kikuchi, S., Matsushige, K., 2004. Novel ultrasonic soldering technique for lead-free solders. Mater. Trans. 45, 703–709. Naka, M., Hafez, K.M., 2003. Applying of ultrasonic waves on brazing of alumina to copper using Zn–Al filler alloy. J. Mater. Sci. 38, 3491–3494. Saito, Y., Tsuji, N., Utsunomiya, H., Sakai, T., Hong, R.G., 1998. Ultra-fine grained bulk aluminum produced by accumulative roll-bonding (ARB) process. Scripta Mater. 39, 1221–1227. Utsunomiya, H., Hatsuda, K., Sakai, T., Saito, Y., 2004. Continuous grain refinement of aluminum strip by conshearing. Mater. Sci. Eng. A 372, 199–206. Vianco, P.T., Hosking, F.M., Rejent, J.A., 1996. Ultrasonic soldering for structural and electronic applications. Weld. J. 75, 343–355. Wang, L., Ai-Ping, X., 2005. Density measurement of Sn–40Pb, Sn–57Bi, and Sn–9Zn by indirect archimedean method. J. Electron. Mater. 34, 1414–1419. Watanabe, T., Tanaka, Y., Yanagisawa, A., Onuma, S., Sato, S., 1992a. Q. J. Jpn. Weld. Soc. 10, 470–476. Watanabe, T., Tanaka, Y., Yanagisawa, A., Onuma, S., Sato, S., 1992b. Q. J. Jpn. Weld. Soc. 10, 476–483. Xu, Z.W., Yan, J., Wu, G., Kong, X., Yang, S., 2005a. Interface structure and strength of ultrasonic vibration liquid phase bonded joints of Al2 O3 /6061 Al composites. Scripta Mater. 53, 835–839. Xu, Z.W., Yan, J., Wu, G., Kong, X., Yang, S., 2005b. Interface structure of ultrasonic vibration aided interaction between Zn–Al alloy and Al2 O3 /6061 Al composite. Compos. Sci. Technol. 65, 1959–1963. Zhao, Y.H., Liao, X.Z., Jin, Z.Z., Valiev, R.Z., Zhu, Y.T., 2004. Microstructures and mechanical properties of ultrafine grained 7075 Al alloy processed by ECAP and their evolutions during annealing. Acta Mater. 52, 4589–4599.