Influence of Oxide Film on the Bondability of Aluminum in

[Quarterly Journal of Japan Welding Society, Vol. 22, No. 3, pp. 355-363 (2004)]

Influence of Oxide Film on the Bondability of Aluminum in Ultrasonic Bonding and Materials Evaluation of Bonds*

by Hamed ABDEL-ALEEM**, KATOH Mitsuaki***, NISHIO Kazumasa***, YAMAGUCHI Tomiko***, and YAMAZAKI Yoshihiko***

Abstract

We performed ultrasonic bonding on sheets of 1050 and 5052 with different oxide thickness of 4, 15 and 30 µm. The oxide films macroscopically bent and distributed like whirl-shaped near bond interfaces. Microscopically, we observed two kinds of features of the oxide films, dot-shaped and line-shaped. We evaluated the bonding situation by an immersion method of ultrasonic testing (C-scope mode). The ratio of good bond area was ultrasonically obtained and depended on the thickness of the oxide films and input energy in bonding. The maximum tensile shear load decreased with the increase in the thickness of the oxide films. By using the results of both a tensile shear test and the good bond area obtained ultrasonically we could estimate the tensile shear strength of bonds.

Key Words: ultrasonic bonding, aluminum, oxide film, ultrasonic testing, C-scope mode, shear test.

1. Introduction

When we want to perform welding on aluminum and aluminum alloys, it is first of necessary to remove oxide films on their surfaces by using appropriate methods. In conventional fusion welding, for example GTA and GMA welding, cleaning action by inert gas is very useful for removing them. In vacuum roll bonding technique we can obtain good bonding between aluminum and other dissimilar materials because they can be destroyed by rolling during bonding. In ultrasonic bonding process, they say that this problem is also cleared through ultrasonic vibration1－7).

In ultrasonic bonding of metals, the process for bonding is as follows : First two separate metal sheets for bonding are rubbed at the interface, adsorbate and oxide films are destroyed, and then the bonding starts. It is considered that the bondability depends on the thickness of the oxide films. Though Watanabe et al.8) reported that aluminum oxide films whose thickness was about 1 µm did not affect bonding strength of Al/Cu and Al/SUS304 and their behavior when their thickness is larger is not clear.

In this study we performed the ultrasonic bonding on aluminum sheets with different oxide film thickness of 4, 15 and 30 µm in order to make clear the influence of the oxide films on the bondability of bonds. We observed the oxide film distribution macroscopically and microscopically when we performed bonding by different bonding conditions. We tried to evaluate the bonding situation by an ultrasonic immersion method (C-scope mode)9－11). It was possible to determine quantitatively the ratio of good bond area in the bonded area by selecting the appropriate threshold level. After that we performed a tensile

shear test to study the influence of the oxide films on the strength of the bonds and tried to obtain the correlation between the results of ultrasonic testing and tensile shear tests.

2. Material Used and Experimental Procedures

It is natural that aluminum surface is covered with thin oxide film. In this paper we used a symbol “N” to represent the specimen without anodically-oxidizing film while, we used a symbol “W” to represent the specimen with anodically-oxidizing film. Materials used in this study are commercially pure aluminum 1050N, 1050W, aluminum alloy 5052N and 5052W. The thickness of the oxide films was changed to t0＝4, 15 and 30 µm by an anodizing method (the original dimensions of the specimen are as follows : t1.5×w100×L250 mm). Two combinations of metals were bonded together, 1050W to 1050N and 5052W to 5052N (dimensions of the specimen for the bonding are as follows : t1.5×w20×L100 mm). Before the bonding, the specimens were cleaned by ultrasonic cleaning in acetone. After that the bonding was performed using an ultrasonic spot welding machine and lap joints (over lapped length : 20 mm) were obtained near the center of the lapped area. Aluminum sheets with anodically-oxidizing film were set in the upper side, which contacted with a horn of the machine, while aluminum sheets without anodically-oxidizing film were set in the lower side, which contacted with an anvil.

Table 1 shows conditions of the ultrasonic bonding. For each combination, bonding pressure (P＝ 0.31 and 0.28 MPa for 1050W/1050N and 5052W/5052N, respectively, based on the preliminary test) and amplitude (A＝68 µm) were kept constant. The value of the amplitude shown in this paper is the one of free

*Received：2003. 11. 21 **Student Member, Kyushu Institute of Technology ***Member, Kyushu Institute of Technology

356 研究論文 Hamed ABDEL-ALEEM et al. : Influence of Oxide Film on the Bondability of Aluminum in Ultrasonic Bonding and Materials Evaluation of Bonds

horn tip vibration without load. The vibration direction was parallel to the specimen surface. Input energy, Ei , for bonding was changed by 3 and 4 steps for the 1050W/1050N and 5052W/5052N combinations, respectively. When we input the value of input energy into the machine, it is automatically controlled by the area surrounded by the power and the bonding time curve during the bonding.

First we observed macrostructures of the specimens as they were. After that we observed bond cross-sections macroscopically, and microscopically using an optical microscope. We also analyzed distributions of the oxide films near the bond interfaces by EPMA. Moreover, we evaluated the bonding

situation by ultrasonic testing (immersion method, C-scope mode) using ultrasonic imaging equipment (AT7000 fabricated by Hitachi Construction Machine Co., Ltd). Table 2 shows the specifications of the probe used. Mechanical properties of the bonds were evaluated by performing tensile shear tests.

3. Experimental Results and Discussion

3.1 Features of bonds and their cross sections

Figure 1 shows an example of macroscopic features of the bonded specimens of 1050W/1050N. Knurling marks observed on the top of the bonded specimens were due to the horn of the ultrasonic welding machine. It is necessary that the tip of the horn should be knurled to transmit efficiently ultrasonic vibrations to specimens. It was also noticed that in all bonding conditions

Table 1 Conditions of ultrasonic bonding.

Fig. 1 An example of macroscopic features of specimens bonded (1050W/1050N, t0＝15 µm, Ei＝500 J).

Table 2 Specifications of the probe used for ultrasonic testing.

Fig. 2 Features of cross sections near the bond interfaces of 1050W/1050N when t0＝4 µm.

溶接学会論文集第 22 巻（2004）第３号 357

there were flashes around the circular bond area. The flashes were formed as the result of ultrasonic vibrations during the bonding process and increased with the increase in bonding input energy. Since the knurling marks and the flash around the bonding area make the specimen surface rough, it is difficult to transmit ultrasonic waves through this side of the specimen in ultrasonic testing, as will be mentioned in 3.3. Good bonding was obtained by using the appropriate range of input energies. When the input energy exceeded the range, a hole was developed in the bond area. We could not also use the input energy less than the range because it was not enough to bond the materials.

We observed cross sections of the bonds of 1050W/1050N and 5052W/5052N for different thickness of the oxide films by changing the input energies. Figures 2, 3, 4 and 5 show examples of the cross sections near the bond interfaces of

1050W/1050N and 5052W/5052N when t0 are 4 and 30 µm for several input energies, respectively. In Fig. 2 when t0＝4 µm and Ei＝350 J in 1050W/1050N the oxide film observed as a black line was almost straight, but this became wavy with the increase in the input energy. Though the oxide films looked like macroscopically successive those distributed microscopically discretely as will be shown in Fig. 8 ⒜ and ⒝. In Fig. 3 when t0＝30 µm the oxide films partly cohered and wound, that is, distributed complicatedly. This shows that two interfaces to be bonded moved to several directions including the parallel direction to the specimen surfaces during the bonding.

In Fig. 4 when t0＝4 µm in 5052W/5052N it was necessary to increase the input energy compared with the case in 1050W/1050N to obtain the good bonding and this lead to the more complex distribution of the oxide films. Since we could




observe the oxide films even in the 5052N side this means that the specimens in both sides vibrated strongly during the bonding. Moreover, in Fig.5 when t0＝30 µm in 5052W/5052N we needed the larger input energies than in the cases shown in Fig. 4 and we could observe the oxide films cut into the 5052N side about 1 mm at the maximum. The surface of the 5052W side became depressed and the fracture occurred along the oxide films after tensile shear tests.

Figure 6 shows a macroscopic feature of 1050/1050 (Ei＝450 J), which was not anodized, near the bond interface and we could observe almost the straight interface. Hence it is considered that the oxide films, which stuck firmly on the specimen surface, resisted the ultrasonic vibrations and those distributed complicatedly near the bond interface when we applied the sufficient input energy to overcome the resistance.

3.2 Results by Electronic Probe Micro Analyzer (EPMA)

Figure 7 ⒜ and ⒝ show examples of microstructures near the bond interfaces of 1050W/1050N (t0＝4 µm, Ei＝400 J). There were two kinds of features of structures observed black, that is, we observed the one dot-shaped and line-shaped. This phenomenon was also the case in 5052W/5052N.

Figure 8 ⒜ and ⒝ show the results of area analysis of Al and O by EPMA near the bond interfaces of 1050W/1050N (t0＝4 µm, Ei＝500 J) of the structures dot-shaped and line-shaped shown in Fig. 7. We could observe many small dots in Fig. 8 ⒜ and small discrete bar-shaped structures in Fig. 8 ⒝. Moreover, we could confirm that the structures observed black in Fig. 7 were Al oxide since we could observe high intensity of O and low intensity of Al at the same position. Regions where the oxide film is not observed in Fig. 8 are the one where we could obtain the good bonding.

3.3 Ultrasonic testing

We tried to evaluate the bonding situation by ultrasonic testing. Working sensitivity in ultrasonic testing is affected by the combination of ultrasonic test equipment and probe frequency. In order to perform the examination under the same condition it is necessary to define the standard echo height using the standard test block and adjust the working sensitivity using the echo height. The working sensitivity was set to 100% by using a bottom echo. In the case of the good bonding, ultrasonic waves will be transmitted through the bond interface without reflection and reflect from the bottom surface of a lower sheet. That is, we can


Fig. 6 Macroscopic feature of 1050/1050 near the bond interface which was not anodized.


obtain the back wall echo whose height is 100% on CRT of the equipment but no interface echo. In the case of bad bonding, however, the ultrasonic waves will be reflected from the bonding interface and we can obtain an interface echo whose height depends on the situation of the interface. Since we could not obtain the sufficient echo height due to the attenuation of the ultrasonic waves when we used probe frequency larger than 20 MHz we used a 10 MHz probe.

In the ultrasonic testing equipment the difference between flaw echo heights is represented by the difference in color tones of 16 levels. Good and bad bond areas are represented by blue and red, respectively. Depending on the bond quality the color of the bond area will be in the range between blue and red. The region of the base metal was represented by red because the transmitted waves were reflected from the base metal back surface and the bottom echo was high. The region of water (immersion coupling) was represented by blue because there was no reflection from it.

Figure 9 ⒜ and ⒝ show comparison of C-scope images as

bonded with that after grinding the surface. First we tried to transmit the ultrasonic waves from the specimen surface which contacted the anvil but we could not get a good result because the specimen surface is not smooth enough as shown in the upper figure of Fig. 9 ⒜ and most of the ultrasonic waves were scattered from the surface. When we smoothed the surface by grinding we could obtain good results and we could detect the existence of flaws due to the oxide films in the bond area as shown in Fig. 9 ⒝. Hence it was necessary to remove the influence of the specimen surface when we wanted to apply ultrasonic testing for evaluating the bond quality of ultrasonic bonds.

Figure 10 ⒜ , ⒝ , ⒞ and ⒟ show examples of C-scope

Fig. 8 Results of area analysis of Al and O near the bond interfaces of 1050W/1050N combination (t0＝4 µm, Ei

＝500J).

Fig. 9 Comparison of C-scope images as bonded with that after grinding the surface.

Fig. 7 Microstructures near the bond interface of 1050W/1050N (t0＝4 µm, Ei＝500J).


images from the bond interfaces for the anodically-oxidizing film thickness of 0, 4, 15 and 30 µm, respectively, in 1050W/1050N (Ei＝400 J). The area of dark blue region corresponding to the good bonding decreased with the increase in the oxide film thickness for the same input energy. Then we tried to evaluate quantitatively the ratio of good bond area. For this it is convenient to binarize the C-scope images by using the appropriate threshold echo height level. Figure 11 ⒜ and ⒝ show examples of the images before and after the binarization, respectively. In Fig. 11 ⒝ the threshold echo height level was set to 80 (in this equipment we could obtain the 256 levels from 0 (blue) to 255 (red)). The region less than the echo height level of 80 represented by gray in a circle (the circle shows the region of the horn) corresponds to the good bond area and the region larger than 80 represented by red corresponds to the bad bond area. It is possible to obtain the ratio of good bond area by obtaining the area represented by gray. As shown in Fig. 11, the input energy was quite high (Ei＝1400 J) and there is a big deformation of Al side which contacted with the horn (the thickness was decreased). Therefore, the bottom echo also would be decreased and this is why we obtained image outside of the horn region. Moreover, by observing the cross section of bond area at high magnification we obtained the diameter of bond area is nearly almost the same as the horn diameter.

Since the ratio of good bond area depends on the threshold level in the binarization we obtained the ratio of good bond area by changing the threshold levels from the level 20 to 220 every 20

levels. Figure 12 shows examples of C-scope images of 1050W/1050N after the binarization at different threshold levels (t0＝4 µm, Ei＝350J). We could observe that the gray region corresponding to the good bond area increased with the increase in the threshold level. The relations between the ratio of good bond area and the threshold level are shown in Fig. 13 ⒜ and ⒝ for 1050W/1050N (t0＝15 µm) and 5052W/5052N (t0＝15 µm), respectively. In general the ratio of good bond area increases with the increase in the input energy at the same threshold level because in the high input energy it was easy to break the oxide films and get better bonding than in the low input energy. The ratio of good bond area increased with the increase in the threshold level. Hence it is necessary to determine the appropriate threshold level when we want to evaluate the bond quality by ultrasonic testing. For this we obtained the strength of

Fig. 10 Examples of C-scope images from the bond interfaces for different oxide film thickness in 1050W/1050N (Ei＝400 J).

Fig. 11 Comparison of C-scope images before the binarization with that after the Binarization of 5052W/5052N (t0＝30 µm, Ei＝1400 J).


bonds of t0＝0 µm of both 1050 and 5052 materials when the good bond area had the maximum by dividing the maximum load in tensile shear tests by the good bond area and determined the threshold level so that the strength of the bonds was about 80% of the base metals. The threshold levels thus obtained were 200 for both 1050 and 5052, and we used these values for further study.

Figure 14 ⒜ and ⒝ show the relation between the ratio of good bond area and input energy of 1050W/1050N and 5052W/5052N, respectively. We used 3 specimens for each bonding condition for ultrasonic testing. Though the scattering of the data was fairly large the ratio of good bond area was more than 85% in average except for the case of t0＝30 µm in 1050W/1050N. In the case of t0＝30 µm, however, the ratio tended to increase with the increase in the input energy. In the case of 5052W/5052N the ratio of good bond area was more than 90% in average except for the case of t0＝0 µm in Ei＝1200 J.

3.4 Shear test

We performed tensile shear tests to study the influence of the oxide films on mechanical properties of the bonds. Figure 15 ⒜

Fig. 12 Examples of C-scope images of 1050W/1050N after the binarization at different threshold levels (t0＝4 µm, Ei＝350 J).

Fig. 13 Relations between the ratio of good bond area and the threshold level.

Fig. 14 Relations between the ratio of good bond area and input energy.


and ⒝ show the relations between maximum tensile shear load and input energy of 1050W/1050N and 5052W/5052N, respectively. The maximum tensile shear load tended to increase with the increase in the input energy. The maximum tensile shear load in the case of t0＝0 µm in Ei＝1200 J for 5052 was also low with large scatter.

We tried to estimate the tensile shear strength based on the results of both ultrasonic testing and tensile shear load. Figure 16 ⒜ and ⒝ show the relations between maximum tensile shear strength and input energy of 1050W/1050N and 5052W/5052N, respectively. The tensile shear strength was obtained by dividing the maximum tensile shear load by the good bond area obtained ultrasonically. In 1050W/1050N the tensile shear strength was independent of the input energy and this means there was good correlation between the results of the good bond area and the maximum tensile shear load. The strength in t0＝4 µm was a little larger than that when t0＝0 µm and this shows that the oxide film of about 4 µm did not influence the bond quality. When t0 increased to 15 and 30 µm the strength decreased. The strength in t0＝30 µm was about the half of the base metal. The tendency in 5052W/5052N was somewhat complex and the

scatter of the data was larger in this case due to the complex distribution of the oxide films. In every case the fracture was observed along the oxide films and we could observe the oxide on the fracture surfaces. Moreover, we could observe dimple patterns in the regions where the good bonding was obtained.

4. Conclusions

⑴ Even in the same oxide film thickness larger input energy was necessary in 5052W/5052N than in 1050W/1050N to obtain the good bonding in ultrasonic bonding. ⑵ The oxide film distributed complicatedly near the bond interface, that is, wavy, partly cohered and wound. The bond interface deformed due to the existence of the oxide films and the deformation increased with the increase in the input energy, especially in 5052W/5052N. ⑶ After bonding, the oxide films distributed microscopically dot-shaped and line-shaped, and we could obtain good bonding at the region where the oxide films were not observed. ⑷ It is necessary to select the appropriate threshold level of

Fig. 15 Relations between maximum tensile shear load and input energy.

Fig. 16 Relations between maximum tensile shear strength and input energy.

溶接学会論文集第 22 巻（2004）第３号 363 echo height when we evaluate the bond quality by an ultrasonic C-scope method. Otherwise it is easy to obtain meaningless results. ⑸ When we performed tensile shear tests on bonds the fracture occurred along the oxide film independently of the oxide film thickness. There was good correlation between the good bond area estimated ultrasonically and the maximum tensile shear load. ⑹ The tensile shear strength when t0＝4 µm was a little larger than that when t0＝0 µm and tended to decrease when t0 increased to 15 and 30 µm in 1050W/1050N.

Acknowledgement

We would like to express our sincere gratitude to Dr. T. Aiura in Kobe Steel, Ltd. for anodizing the specimens and Mr. M. Sudo in Branson Ultrasonics Division of Emerson Japan Corporation for performing ultrasonic bonding on the specimens.

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Influence of Oxide Film on the Bondability of Aluminum in