Interfacial Morphology of Magnetic Pulse Welded Aluminum/Aluminum and Copper/Copper Lap Joints

Transcription

1 Materials Transactions, Vol. 50, No. 2 (2009) pp. 286 to 292 #2009 The Japan Institute of Light Metals Interfacial Morphology of Magnetic Pulse Welded Aluminum/Aluminum and Copper/Copper Lap Joints Mitsuhiro Watanabe* and Shinji Kumai Department of Materials Science and Engineering, Tokyo Institute of Technology, Yokohama , Japan In order to investigate interfacial morphology and their welding condition dependency, Al/Al and Cu/Cu lap joints were fabricated by magnetic pulse welding under various discharge energies. A part of flyer plate along the longitudinal direction of the coil bulged toward a parent plate and hit the parent plate. Two parallel seam-welded areas were formed along the side edges of coil, but the area between them was left unwelded. The welding interface exhibited characteristic wavy morphology, which was similar to that of explosive welding. Wavelength and amplitude of the interfacial wave were not uniform, but gradually changed through the interface. In addition, the maximum wavelength and amplitude increased with increasing discharge energy. Both macro- and microscopic features of interfacial morphology are considered to be due to the oblique collision behavior between the plates, in which traveling velocity, collision and collision pressure of the plates gradually change during the welding for a few microseconds. [doi: /matertrans.l-mra ] (Received July 11, 2008; Accepted October 18, 2008; Published December 25, 2008) Keywords: magnetic pulse welding, wavy interface, amplitude, wavelength, collision pressure 1. Introduction Magnetic pulse welding is one of the solid-state welding processes, in which high-speed collision between two or multiple metal plates is utilized for lap joining. This welding process can be applied for materials widely differing in physical and mechanical properties such as melting point, heat conductivity and hardness. Sound metallurgical bonding can be obtained between a wide variety of similar and dissimilar metals and alloys without any external application of heat or the use of any intermediate metals. The principle of the magnetic pulse welding is briefly summarized as follows. A discharge circuit including a flat one-turn coil is used for the welding. A flyer plate is set over the coil. A parent plate is set over the flyer plate with a small gap. Electromagnetic force is generated by the interaction among discharge pulse, induced magnetic flux and eddy current produced in the plate. Since the discharge pulse has high-frequency, the eddy current is mainly generated at the plate surface due to the so-called surface effect. The generated electromagnetic force drives the flyer plate to the parent plate at a high velocity and the collision takes place between two plates. The welding is normally achieved within 10 ms with a negligible temperature increase. Metal plates with high electrical conductivity such as Al and Cu are suitable for the flyer plate because they can generate large electromagnetic force. Strong lap joints of Al/Al, Al/Mg, Al/Cu, Al/Ni and Al/ steel have been fabricated using the magnetic pulse welding. They seldom failed at the joint interface in tensile-shear tests. 1 8) The welding interface exhibits characteristic wavy morphology. 4,6 8) Such an interfacial morphology is similar to those of explosive welding, 9 12) water jet spot welding 13,14) and gas-gun impact welding. 15,16) For the explosive welding, a number of studies were performed in order to examine the mutual relationship among interfacial morphology, collision *Present address: Materials and Structures Laboratory, Tokyo Institute of Technology, Yokohama , Japan velocity of the flyer plate and collision of the plate surfaces. 17,18) In the magnetic pulse welding, however, a limited study has been done concerning the interfacial morphology and their welding condition dependency. In the present study, Al/Al and Cu/Cu similar metal joints were fabricated by the magnetic pulse welding. Both macroscopic and microscopic morphologies of the joint interface were examined in detail. behavior between two plates was investigated and the relationship among collision pressure, traveling velocity of the flyer plate and interfacial wavy morphology was discussed. 2. Experimental Procedures 2.1 Materials A pure aluminum (1050, hereafter Al) plate (100 mm 100 mm 1:0 mm) and a pure copper (oxygen free copper, hereafter Cu) plate (100 mm 100 mm 1:2 mm) were used in the present study. Their chemical composition is shown in Table 1. Before welding, the Al plates were annealed at 813 K for 2 h and the Cu plates of in size were also annealed at 873 K for 2 h to obtain a coarsegrained structure. Figure 1 shows optical micrographs of the grain structure. The average grain size is 50 mm in diameter for Al and 120 mm for Cu. The annealed plates were mechanically ground to remove the oxide scale with a waterproof abrasive paper, finally rinsed in acetone with an ultrasonic cleaner. Table 1 Chemical composition of the present materials. mass% Mn Si Cu Mg Fe Al Al (1050) Bal. ppm mass% O S Ag Fe Bi Sb Zn P Cu Cu (OFC) Bal.

2 Interfacial Morphology of Magnetic Pulse Welded Aluminum/Aluminum and Copper/Copper Lap Joints 287 Fixture Insulator G C Current E-shaped coil Electro magnetic force Eddy current Discharge pulse Magnetic flux Fig. 2 Schematic illustrations of welding process of magnetic pulse welding. Set-up of the lapped plates over the E-shaped one-turn coil. Principle of the magnetic pulse welding. Fig. 1 Optical micrographs of grain structure of annealed Al plate and Cu plate. 2.2 Magnetic pulse welding Figure 2 shows a schematic diagram of a discharge circuit. The circuit for the present method consists of a capacitor for a supply of electrical energy, a discharge gap switch and an E-shaped one-turn flat coil. The two plates are placed above the coil, with a little space between them. The plate nearest the coil is termed the flyer plate and the plate above it, which is fixed firmly in place, is referred to as the parent plate. When the capacitor is charged and the discharge gap switch is closed, a discharge pulse is released to the coil. The electrical energy stored in the capacitor is called discharge energy, the welding conditions are mainly controlled by this parameter. Figure 2 shows a close-up around middle section of the coil. When a discharge pulse passes through the coil from the capacitor, it induces a high-density magnetic flux around the coil. The generated magnetic flux lines intersect with the flyer plate, and in accordance with Lentz s law, eddy currents are excited in the surface of the flyer plate adjacent to the coil. In accordance to Fleming s left-hand rule, the eddy current and the magnetic flux generated around the coil induce an electromagnetic force upward. The electromagnetic force drives the flyer plate toward the parent plate at a high velocity and the flyer plate is welded to the parent plate. A pair of plates was set as shown in Fig. 2 to produce Al/ Al and Cu/Cu lap joints. The preset space between them was Insulator Fig. 3 G C Fixture Rogowski coil Oscilloscope E-shaped coil Set-up of an additional circuit including a Rogowski coil. fixed to be 0.8 mm. The discharge energy was controlled in the range from 1.5 to 4.0 kj. 2.3 Estimation of the traveling velocity of the flyer plate In order to estimate a traveling velocity of the flyer plate, the time required for the flyer plate to collide with the parent plate after the release of current into the coil was measured. For this purpose, an additional circuit including a Rogowski coil was constructed, as shown in Fig. 3. This circuit is open at the beginning of the welding process since there is a gap between the plates. When a discharge pulse passes through the coil, eddy currents are induced in the flyer plate, but no currents flow in the parent plate. When the flyer plate with the eddy currents hits the parent plate, the circuit is closed, and currents are induced due to the original difference of electrical potential between the plates. This gives a characteristic signal in a current-time curve recorded by an oscilloscope and from which, we can know the exact time

3 288 M. Watanabe and S. Kumai 200 ka Current, I / ka Time, t / µs 5 µs Bulged region Onset of discharge Fig. 4 Waveforms of discharge current (upper) and collision time (lower). when the flyer plate hits the parent plate after the discharge pulse starts to flow. Figure 4 shows a representative recorded signal. This is for Al/Al joining at the discharge energy of 2.5 kj. The upper curve indicates relationship between the current in the coil and time (hereafter current curve). The lower curve indicates relationship between the current in the circuit including the plates and time (hereafter collision signal curve). This result indicates that the time required for colliding of the plates was about 6 ms from the release of current into the coil. Assuming that the flyer plate travels and hits the parent plate at a constant velocity, we can estimate mean traveling velocity of the flyer plate. In case of Al/Al joining at the discharge energy of 2.5 kj as shown in Fig. 4, the traveling velocity of the flyer plate is estimated to about 130 m/s. 2.4 Metallography The obtained lap joint was cut perpendicular to the seam direction and the cross section was polished for optical microscopy. In order to reveal morphological change in the lap welding interface and the adjacent grain structure, the polished cross section was chemically etched in solutions. The solutions used are HF : H 2 O ¼ 1:50in volume for Al/ Al joints and H 2 O 2 : NH 3 : H 2 O ¼ 1 : 50 : 50 for Cu/Cu joints. 3. Results 3.1 Macroscopic appearance of the lap joint plates and their welding interface Figure 5 shows an external appearance of the magnetic pulse welded Al/Al lap joint. A part of the flyer plate along the longitudinal direction of the coil bulged toward the parent plate. The welded areas were a part of the bulged region. Figure 6 shows an optical micrograph of the cross section of the bulged region in the joint. The upper side is the parent plate and the lower side is the flyer plate. The bulged region of the flyer plate was about 5 mm wide, which corresponded to width of the coil. Figures 6, (c) and (d) are optical micrographs of the interfacial morphology obtained at the Fig. 5 Macroscopic appearance and illustration of magnetic pulse welded Al/Al lap joint. The welded areas were a part of the bulged region. position of B, C and D in Fig. 6, respectively. It was found that welding was not attained completely throughout the bulged region. The middle section of the bulged region was not welded as shown in Fig. 6. On the other hand, two areas corresponding to side edges of the coil were seamwelded as shown in Figs. 6(c) and (d). The interface of these welded areas exhibited characteristic wavy morphology, which was similar to that of the explosive welding. Morphology of the grains located at the surface of un-welded area remained unchanged comparing to that of the base metal. In contrast, grains in the vicinity of the welding interface were heavily deformed and elongated along the wavy pattern of the interface. The wavy patterns formed at each area showed mirror symmetry each other across the vertical centerline of the bulged region. These findings were common for s. 3.2 Morphological change of waveform through the joint interface Figures 7 and show optical micrographs of the welding interface throughout one of the welded area (righthand-side in this case) in the Al/Al and s. The left side of the picture corresponds to the central position of the bulged region (i.e., the un-welded area). Both wavelength and amplitude of the interfacial wave varied gradually through the welding interface. Wavelength increased with increasing distance from the vertical centerline of the bulged region, i.e., toward outside of the bulged region as shown in Fig. 8. Amplitude once increased toward outside, but it showed the maximum value at a certain position and then

4 Interfacial Morphology of Magnetic Pulse Welded Aluminum/Aluminum and Copper/Copper Lap Joints 289 C B D (c) (d) Fig. 6 Optical micrographs of the cross sectional view of the Al/Al lap joint. Bulged region. Central position of the bulged region. (c) Welding interface (right-hand-side). (d) Welding interface (left-hand-side). Un-welded area Fig. 7 Optical micrographs of the welding interface (right-hand-side)... decreased, as shown in Fig. 8. In Al/Al and s, both magnitude of the wavelength and the amplitude was similar. 3.3 Effects of discharge energy on wavelength and amplitude of the interfacial wave As shown in Fig. 8, both amplitude and wavelength of the interfacial wave gradually changed throughout the welding interface. This is a common feature for the all lap joints produced at various discharge energies. In the present study, the maximum wavelength and the maximum amplitude values were quantified in order to characterize the wave form. Figure 9 shows the relationship among the maximum values of wavelength and amplitude and the discharge energy. They increased with increasing discharge energy for both the Al/ Al and s. 3.4 Effects of discharge energy on traveling velocity of the flyer plate Table 2 shows the relationship among discharge energy, collision time, estimated traveling velocity of the flyer plate and collision pressure generated between the plate surfaces. The collision time was decreased with increasing discharge energy. The traveling velocity of the flyer plate increased

5 290 M. Watanabe and S. Kumai Wavelength, λ / µm Maximum wavelength, λ max / µm Amplitude, A/ µm with an increase in discharge energy. In the explosive welding, the collision pressure generated at the interface is directly proportional to the collision velocity of the flyer plate. 19) pressure is expressed by the following equation: 19) P ¼ðV p V s Þ=2 ð1þ where P is collision pressure, is density of a flyer plate metal, V p is collision velocity of a flyer plate, and V s is sound velocity in a flyer plate metal. According to this equation, the collision pressure increased with increasing discharge energy. 4. Discussion Distance from vertical centerline of bulged region, x / mm Distance from vertical centerline of bulged region, x / mm 4.1 Characteristic collision behavior between two plates in the magnetic pulse welding In the magnetic pulse welded joint, two parallel seam welded areas were formed. They were formed along both side edges of the coil and no welding took place in the area between them. In the both welded areas, characteristic wave form was observed. Wavelength and amplitude of the interfacial wave were not uniform, but gradually changed through the interface. Let us consider why such un-welded area formed between two seam welded areas and why the wavy morphology gradually changed through the welding 1.5 Fig. 8 Wavelength and amplitude of the interfacial wave formed in the Al/ Al and s welded at discharge energy of 4.0 kj. Relationship between wavelength and distance from vertical centerline of bulged region. Relationship between amplitude and distance from vertical centerline of bulged region Maximum amplitude, A max / µm Discharge energy, E / kj Discharge energy, E / kj Fig. 9 Relationship between discharge energy and maximum wavelength and maximum amplitude. Table 2 Relationship among discharge energy, collision time, traveling velocity of flyer plate and collision pressure. Discharge energy, E/kJ interface. Similar interfacial morphology was reported for the explosive welded joint, in which a flat flyer plate and a semicylindrical parent plate were welded ) Bahrani examined the influence of the collision on the explosive welded interface morphology using this so-called semi-cylinder method. 20) It was demonstrated that no welding occurred where the collision was in the range from 0 to 6, but welding with wavy morphology was achieved at s in the range from 6 to 33. The wavelength of the interfacial wave increased with increasing collision, while the amplitude increased at s up to 15, but then decreased thereafter. 20) Onzawa explosively welded flat plates of Fe, Ag, Al, Cu, Mo, Ti and Ni to semi-cylindrical blocks of Fe and Cu. They also investigated the dependency of wave pressure, P/GPa time, t/ms Traveling velocity, v/ms

6 Interfacial Morphology of Magnetic Pulse Welded Aluminum/Aluminum and Copper/Copper Lap Joints 291 (c) length and amplitude of the interfacial wave on the collision and confirmed the results reported by Bahrani. 21) In the explosive welding, it was reported that interfacial morphology is determined by the combination of impact velocity of the flyer plate and a collision of the plates. This is known as a welding window, indicating weldability condition. 17,18,22) According to the welding window, welding is achieved in a certain range of collision and no welding is achieved at low around 0 and high condition. Therefore, if the collision of the plate continuously increases from 0 to certain s, interfacial condition is considered to shift from non-weldable condition to weldable condition. In the present magnetic pulse welding method, the generated electromagnetic force deforms the part of flyer plate located just above the coil and this selected part is forced to hit the parent plate at a high velocity. Figure 10 shows a schematic diagram of the collision process between two plates. It is the peak of the bulged region of the flyer plate that hits the parent plate at first (Fig. 10). At this stage, the between the plate surfaces is 0 theoretically. As progressing in deformation of the flyer plate, the collision point shifts along plate surface to the horizontal direction. In contrast, the outside regions of the flyer plate remain undeformed. Therefore, as moving the collision point from center to outside, the collision between the plates is considered to increase continuously (Figs. 10 and (c)). This is the situation that collision takes place between the semi-cylindrical flyer plate and flat parent plate. Consequently, the middle of the bulged region, which is the Fig. 10 Schematic illustrations of gradual change in collision between plate surfaces during magnetic pulse welding process. advancing front of the flyer plate, hits the parent plate with the collision of 0. This may result in the formation of un-welded area between the parallel seam welded areas. In contrast to that, in the successive stage, the oblique collision between the bulged flyer plate and the parent plate becomes dominant. This is where the wavy interface appears. It is considered that gradual change in wavelength and amplitude of the interfacial wave is due to the oblique collision with gradually changing collision. 4.2 Relationship among collision pressure, traveling velocity of flyer plate and interfacial wavy morphology The welding interface formed by oblique collision is characterized by wavy morphology similar to the explosive welded interface 22) and the water jet welded interface. 13) As shown in Table 2, in the present magnetic pulse welding, the traveling velocity of the flyer plate is estimated less than 150 m/s. This traveling velocity is slower than that of the explosive welding, which is about 1000 m/s and more. 12,19,22) As shown by eq. (1), the collision pressure is proportional to the collision velocity. With increasing discharge energy, the traveling velocity of the flyer plate increased (Table 2) and the collision pressure at the interface increased. The magnitude of the interfacial wave is considered to reflect the collision pressure at the interface. Both wavelength and amplitude of the interfacial wave increased with increasing discharge energy, as shown in Fig. 9. In the explosive welding, much higher pressure is generated and so the magnitude of the interfacial wave is also larger than that of the present welding. Nishida et al. produced Ti/Ti joints by the explosive welding and examined the interfacial morphology. 19) The collision velocity of the flyer plate was ranged from 420 to 1150 m/s. The collision pressure was estimated more than about 4.6 GPa by eq. (1). As well as the present results, the wavelength and amplitude of the interfacial wave increased with the collision pressure. Their sizes were from 50 mm to 760 mm in wavelength and from 30 mm to 330 mm in amplitude. In contrast to that, the collision pressure of the magnetic pulse welding is estimated GPa and less, as shown in Table 2. The relationship between collision pressure and wavelength and amplitude is comparatively shown in Fig. 11 for the magnetic pulse welded and the explosive welded Ti/Ti joint. The magnitude of the interfacial wave is different in Al/Al and Ti/Ti joint, however, they show comparable collision pressure dependence. We need to consider the following things at least in order to explain the quantitative discrepancy between them. The first is the difference in mechanical properties between Al and Ti. It is known that the combination of soft (low melting point) metals provides the wavy interface with larger wavelength and amplitude. 6) The second is accuracy of the estimated collision velocity for the magnetic pulse welding. In the present study, the collision velocity was obtained by l=t, where l is gap of two plates, t is time required for colliding of the plates from onset of discharge. Therefore, this is the average collision velocity and does not express the actual collision velocity when the flyer plate hits the parent plate.

7 292 M. Watanabe and S. Kumai Wavelength, λ and amplitude, A / µm Wavelength MPW(Al/Al) Amplitude MPW(Al/Al) Wavelength EW(Ti/Ti) Amplitude EW(Ti/Ti) pressure, P / GPa length and amplitude of the interfacial wave increased with increasing discharge energy. This is considered to be due to the increased traveling velocity of the flyer plate and the resultant enlarged collision pressure. Acknowledgement The authors would like to express their thanks to Emeritus Professor Tomokatsu Aizawa and Professor Keigo Okagawa of Tokyo Metropolitan College of Industrial Technology for providing experimental facilities and technical assistance. REFERENCES Fig. 11 Relationship between collision pressure and wavelength and amplitude of interfacial wave formed by explosive welding (EW) 19) and magnetic pulse welding (MPW). Direct observation of the deforming process of the flyer plate is necessary in order to obtain the actual collision velocity. Recently, the present authors succeeded in the in-situ observation of the deforming process of the flyer plate by using a high-speed video camera. 23) This will be of help to obtain the actual collision velocity. We will report this in other papers. Several formation mechanisms have been proposed for interfacial wave at the explosively welded interface. 20,21,24 27) Abrahamson 24) and Bahrani 20) suggested that high-speed oblique collision induce the formation of metal jets at the advancing collision front and the metal jets produce periodical combinations of indentation and hump at the interface. It should be mentioned that the present authors also succeeded in detecting the metal jet formation at the collision point front by using a high-speed video camera. 23) Cowan, 26) Onzawa 21) and Reid 27) proposed that the interfacial wave is produced by the vortex street generated behind the collision point. This is based on the idea that the vortex formed at the crest of the interfacial wave is similar to von Kármán vortex street formed behind an obstacle in a fluid stream. There has been no consensus on formation mechanism of the interfacial wave yet. 5. Conclusions Interfacial morphology of the magnetic pulse welded Al/ Al and Cu/Cu lap joints was examined and the following findings were obtained. (1) Two parallel seam welded areas were formed along the both side edges of the coil. No welding occurred between them. The welding interface exhibited characteristic wavy morphology. These characteristics result from gradual change in collision between the flyer plate and parent plate. (2) Both wavelength and amplitude of the interfacial wave gradually changed with the distance from the vertical central position of the bulged region. Both the wave- 1) T. Aizawa: J. JSTP 41 (2000) ) T. Aizawa, K. Okagawa, M. Yoshizawa and N. Henmi: Imp. Eng. Appl (2001) ) T. Aizawa: J. JSTP 44 (2003) ) K. Matsuzawa, K. Okagawa and T. Aizawa: Collected Abstracts of the 2007 Autumn Meeting of the Japan Inst. Light Metals (2007) pp ) K. Hanazaki, T. Aizawa and K. Okagawa: Collected Abstracts of the 2006 Autumn Meeting of Japan Inst. Metals (2006) pp ) M. Watanabe, S. Kumai and T. Aizawa: Mater. Sci. Forum (2006) ) K. J. Lee, S. Kumai, T. Arai and T. Aizawa: Mater. Sci. Eng. A 471 (2007) ) T. Aizawa, M. Kashani and K. Okagawa: Weld. J. 86 (2007) 119s 125s. 9) Y. Dor-ram, B. Z. Weiss and Y. Komem: Acta Metall. 27 (1979) ) K. Hokamoto, M. Fujita, H. Shimokawa and H. Okugawa: J. Mater. Proc. Tech. 85 (1999) ) M. Nishida, A. Chiba, Y. Honda, J. Hirazumi and K. Horikiri: ISIJ Int. 35 (1995) ) M. Nishida, A. Chiba, Y. Morizono, M. Matsumoto, T. Murakami and A. Inoue: Mater. Trans. JIM 36 (1995) ) S. A. L. Salem and S. T. S. Al-Hassani: Metallurgical Applications of Shock-Wave and High-Strain-Rate Phenomena, (Marcell Dekker Inc., New York, 1986) pp ) M. Chizari, S. T. S. Al-Hassani and L. M. Barrett: J. Mater. Proc. Tech. 198 (2008) ) V. Jaramillo, O. Inal and A. Szecket: Acta Metall. 35 (1987) ) H. Date, T. Saito and T. Suzuki: J. Soc. Mat. Sci. Japan 48 (1999) ) D. Jaramillo V., A. Szecket and O. T. Inal: Mater. Sci. Eng. 91 (1987) ) A. Szecket, D. J. Vigueras and O. T. Inal: Metallurgical Applications of Shock-Wave and High-Strain-Rate Phenomena, (Marcell Dekker Inc., New York, 1986) pp ) M. Nishida, A. Chiba, K. Imamura, H. Minato and J. Shudo: Metall. Trans. A 24 (1993) ) A. S. Bahrani, T. J. Black and B. Crossland: Proc. Roy. Soc. London A 296 (1967) ) T. Onzawa and Y. Ishii: Trans. JWS 4 (1973) ) B. Crossland: Explosive Welding of Metals and It s Application, (Clarendon, Oxford, 1982). 23) M. Watanabe and S. Kumai: submitted to Int. J. Impact Eng. 24) G. R. Abrahamson: J. Appl. Mech. 83 (1961) ) J. N. Hunt: Philos. Mag. Series 8 17 (1968) ) G. R. Cowan and A. H. Holtzman: J. Appl. Phys. 34 (1963) ) S. R. Reid: Int. J. Mech. Sci. 16 (1974)