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Wave formation mechanism in magnetic pulse welding
International Journal of Impact Engineering 37 (2010) 397–404
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International Journal of Impact Engineering journal homepage: www.elsevier.com/locate/ijimpeng
Wave formation mechanism in magnetic pulse welding A. Ben-Artzy a, *, A. Stern b, N. Frage b, V. Shribman c, O. Sadot b a
Rotem Industries Ltd., P.O. Box 9046, 84190 Beer-Sheva, Israel Ben-Gurion University, P.O. Box 653, 84105 Beer-Sheva, Israel c Pulsar Ltd., 11 Hayezira St., P.O.Box 2617, 43663 Raanana, Israel b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 May 2009 Received in revised form 29 July 2009 Accepted 30 July 2009 Available online 3 August 2009
Wavy interface morphology is observed in Magnetic Pulse Welding (MPW) similarly to that of the Explosion Welding process (EXW). It is recognized that interfacial waves are formed in a periodic manner and have well defined wavelength and amplitude. The phenomenon of wave formation in EXW has been subjected to extensive investigations in which empirical and numerical models have been published. In the present study, a wave formation mechanism for MPW is presented. This wave-creation mechanism was studied by evaluating the influence of sample geometry on wave morphology using stereoscopic optical microscopy. It was found that interfacial waves are formed in a Kelvin–Helmholtz instability mechanism. Reflected shock waves interact with the welding collision point at the weld interface, where interferences are the source for the wave’s initiation. The collision energy, impact angle, and the geometry of the joint, were found to have the most significant influence on the waves’ characteristics. An empirical relationship between interfacial wavelength and the free moving distance of the shock waves in the welded tubular parts was found. Ó 2009 Elsevier Ltd. All rights reserved.
Keywords: MPW Wave Impact Interface Shock
1. Introduction MPW is a solid state welding process similar to EXW. The MPW process uses a magnetic field, applied to an outer workpiece, in order to collapse it onto the inner part at a velocity high enough to achieve a metallurgical bond (Fig. 1). The magnetic pressure is produced through a coil, while the impulse current is created via a capacitor bank. The repulsion between the coil magnetic field and the induced field on the outer part results in a J B force (Lorentz force) that causes a collision of the outer tubular part onto the inner part to be welded, at speeds in the region of 250–500 m/s [1,2,9,10,25]. The collision of the outer workpiece is oblique, due to the amplitude distribution of the magnetic field, with a collision point velocity (Vc) of about 1500–3000 m/s. The collision of the metals creates a jet consisting of a mixture of metal, air and oxides, ejected from the surfaces of both metals. The two parts of the joint are then forced together to form a solid-state weld, while the whole process takes up to 100 ms [3–5]. Since the open end of the welded sample is located in the middle of the coil (where the magnetic flux density is maximum), this area is subjected to the maximum magnetic pressure. The distribution of pressure along the weld area and its increase in time
* Corresponding author. Tel.: þ972 50 6231910; fax: þ972 8 6510881. E-mail address: [email protected] (A. Ben-Artzy). 0734-743X/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijimpeng.2009.07.008
is illustrated schematically in (Fig. 2a). The acceleration of the outer tube through the gap is higher at the open end of the tube, due to higher magnetic pressure at this area and decreasing down to zero at the weld end where there is no movement of the outer tube. As a result the collision is oblique and the impact angle (a) (Fig. 7) is formed. The velocity profile (Fig. 2b) follows the magnetic pressure profile with a negligible time difference. The initial part of the joint collides at a high collision angle (a) and at too high a collision speed; therefore, usually no bond is formed in this area. As the weld progresses, the collision angle is decreased and stabilizes, and the collision velocity (Vc) decreases gradually to zero. Since the welding process is rapid, the magnetic pressure is maintained for several microseconds after the impact welding is over. The kinetic energy of the outer tube is converted to heat by massive plastic deformation of the metals. This deformation affects the interface and vicinity of approximately 50–100 mm, resulting in a significant temperature increase [10,17,19,22,36] and strain hardening, as was measured in this and others’ [23,24] work. The temperature rise of the interface softens the metals, and in some cases, melts local pockets or a thin continuous layer along the interface [18–20,22,36]. In some cases wavy interface morphology (Fig. 3) appears with short-term periodicity [17,25]. Several mechanisms have been suggested to explain the interface wave creation in EXW [8,11,12,26]. In Refs. [15,16,26,29] it was suggested that the source for wave formation is a shear movement of the flyer and base plates creating local vortices in the plastic
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Fig. 1. Magnetic pulse welding setup [4].
zone. A stagnation region of the jet acting creates each period vortex as a solid obstacle behind it. In another mechanism [7,15,37] it was assumed that indentation of the re-entrant jet into the parent plate surface forms a ‘‘hump’’ ahead of the stagnation point and due to the instability of this region and momentary change in velocity of the metals, the stagnation point ‘‘jumps’’ over the ‘‘hump’’, and a new ‘‘hump’’ is formed periodically. Another theory was published [26,28,31–33,37], based on the Kelvin–Helmholtz instability mechanism, in which waves are formed as a result of flow velocity discontinuities across the interface. The Kelvin– Helmholtz instability model is a model that deals with hydrodynamics. The main principle of the model is that whenever two fluids with different velocities interact, instabilities will occur at the interface as a result of the interferences. These instabilities involve mass flow, usually from the higher density material to the lower one. Whenever instability occurs, it has a direction and a certain velocity (energy) and it causes a movement of material from one side of the interface (Fig. 4a) to the other, and immediately a movement of material from the other side back (Fig. 4b), because of energy complementary to the system. The newly created interface wave gets its directionality and shape (Fig. 4c) under the influence of the mutual velocity of the metals. The welded metals can be defined as viscous solids for this model. An alternative theory attempting to explain the wave formation mechanism in EXW was mentioned in Refs. [6,11,30,34,35]. It was suggested that compressive waves are generated at the collision point and reflected at the free surface of the base plate. The
reflected stress waves interact with the compressive waves and cause periodic interferences at the base plate surface, at the softened high pressure collision area. During the attempts to develop a wave formation model for EXW, experimental effort has been made. The wave formation model that was published in Ref. [12] was found to be analogous to the formation of vortex stresses in fluid flow around an obstacle or in a collision of liquid streams. The fluid flow model, describes the observed transition from a smooth metal-to-metal bond zone to a wavy bond zone, above a critical collision velocity. In Refs. [7,15] it was found difficult to reconcile the periodical manner of wave formation with theories that are based on random obstacles such as ‘‘humps’’, that interrupt the steady flow region at the interface. As for the indentation of the reentrant jet into the parent plate, they found that it cannot be resolved because, if so, the interface wavelength should be dependent on the flyer plate thickness, and independent of its velocity. On the other hand, some experimental work [12–14] has clearly shown that interface waves’ wavelengths depend on the base plate thickness. Other researchers [13,14] used various base plate thicknesses in order to control the shock wave propagation path through the plate and an empirical model, describing the relationship between base plate thickness and wavelength of interfacial waves, was established. Some attempts to elucidate the wave formation mechanism for MPW have been done recently [38–40], but no detailed model was established since it was assumed that the process is identical to that of EXW [27]. In Ref. [16] it was assumed that waves are formed by
Fig. 2. Pressure and velocity profile in MPW.
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Fig. 3. A wavy interface in an Al–Mg weld.
the penetration of the jet into the inner part surface, at the collision point, as seen in EXW. Different waveforms are reported according to differences in plastic deformation dissipated by the inner part. It was also found in this research that interface waves might appear only when there is enough energy for plastic deformation of the inner part. The conditions for wave formation, from the metallurgical point of view, were defined in Ref. [17] regarding plastic deformation in the region of interaction of the weld metals. Several deformation mechanisms were suggested; displacement of dislocations, collective movements of crystal lattice defects or rotation of grains by micromechanisms. Hence this theory clarifies the feasibility of very high strain rate massive deformation, at a narrow interface zone, during interface wave creation. The interface temperature in MPW rises due to the jet and massive deformation of the surfaces, and in some cases, melting and solidification occur similarly to EXW [7,15–17,36]. The temperature rise at the interface encourages wave formation by softening of the interface and its vicinity.
Fig. 4. Kelvin–Helmholtz instability creation (from left to right u1 > u2).
This research attempts to elucidate wave formation mechanism in MPW. The present contribution of this work is an attempt to confirm the compliance of a wave formation mechanism theory, based on a Kelvin–Helmholtz instability model, with interferences triggered by shock wave propagation through the metals, interacting with the welding zone at its interface. 2. Experimental The objective of the present experimental work was to verify that interface wave formation in MPW is a consequence of a Kelvin– Helmholtz mechanism. An attempt was made to control the shock wave propagation free path, in order to prove the hypothesis that shock waves that travel through the inner part of the weld are the source for interface interferences that initiate wave formation. This approach is similar to experimental work that was aimed to influence wave formation, by controlling the base plate thickness in EXW [13,14]. In this work, the free path of shock wave propagation was controlled by drilling a hole in the inner part. In one case a hole was drilled concentrically, so that waves could propagate in a symmetrical manner, from the collision surface to the free surface of the drilled hole, and reflect as an expansion wave back to the interface where the weld is formed. In other cases, holes were drilled eccentrically, so that shock waves from different directions had different free paths of propagation. The use of eccentrically drilled holes made it possible to compare between waves propagating through different paths in a single sample, while all other parameters remained unchanged. The flyer tube (Fig. 5: part a) was 1 mm thick and 10 mm long in all the experiments. The inner part (Fig. 5: part b and 1–8) was concentrically fitted into the outer tube by a fastening pin. The inner part of the reference sample (Fig. 5: 1) was made out of a solid round bar, while in other experiments, inner parts with various diameter holes were used (Fig. 5: 2–6). Other inner part holes were drilled in different positions with
Fig. 5. Controlled wave propagation experimental setup. a) Outer tubular part, b) Inner part, 1–8) Inner part with different wall thicknesses, 9–11) Assembled samples.
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Fig. 6. Cross-section of welded samples: a) Reference, b–f) Concentric holes of various diameters, g–h) Eccentric holes.
respect to the centre (Fig. 5: 7,8). The holes, offset from the centre, were designed to create different path lengths (from the surface to the hole) for the incoming shock waves. An assembly, with a 15 mm concentric hole inner part sample, is presented in (Fig. 5: 9). It should be observed that this inner part is actually a tube. In (Fig. 5: 10), a 5 mm eccentric hole assembly is presented and a reference sample assembly can be seen in (Fig. 5: 11). The initial acceleration gap between the outer tube and the inner part was set by the diameter of the inner part (BA) at the welding zone, as can be seen in (Fig. 5: 2). The Al–Mg couple was chosen for this research, since there is a great difference in the physical properties of these metals; such as sound velocity and module of elasticity. Although there is a similarity in melting temperature, aluminum has an fcc structure while magnesium a hexagonal one, therefore deformation mechanisms are very different. These metals are easily traceable after welding by optical microscopy and backscattered electron microscopy means, due to their different structures. Commercial AA1050 and AZ31-B magnesium alloy were used for this research. AA1050 is a pure Al containing 0.25% Fe with some Si, while AZ31-B was preferred for this research over pure magnesium, due to the high strain rate load of the welding process. The experimental parameters were chosen empirically and were specified to be at the minimum energy for welding the selected couple, in order to achieve a wavy interface morphology and avoid melting at the interface zone. The welded workpieces were cross-sectioned and prepared for evaluation by standard metallographic procedures: mechanical polishing up to 1 mm mesh, followed by a light chemical etching. The samples were then examined by SEM, optical stereoscopic microscopy and montage pictures were taken. The interface waves’ wavelengths and amplitudes were measured on the stereoscopic images using ‘‘Image Pro 4’’ software. In most cases, 3–5 full scale interface waves were included, in order to eliminate secondary interference interruptions, as shown in examples (Figs. 3, 8 and 9). Welding experiments using this setup were performed with 8, 9 and 10 kJ.
in (Fig. 5: 2–6), can be seen in (Fig. 6: b, c, d, e and f) respectively. Welded samples with eccentric 5 mm holes located in different offset positions from the centre, with inner part as seen in (Fig. 5: 7 and 8), can be seen in (Fig. 6: g and h) respectively. In (Fig. 6: g), one wall thickness is three times that of the opposite wall thickness (on the same plane), while in (Fig. 6: h) the thick wall is four times thicker. Inner diameter deformation of the drilled holes can be observed in each of the hollow samples. This deformation is a wellknown phenomenon in industrial MPW of hollow welds and usually a backup steel mandrel is used to support the tube. In this case, the deformation of the sample didn’t influence the results, due to a different timetable of events. The shock wave travels in the inner part at approximately the speed of sound in magnesium, i.e. 4600 m/s [21], while the deformation of the tube is a consequence of metal acceleration, due to impact in the velocity range of 250– 350 m/s [36]. Also the propagation velocity of the collision point (Vc) is more then 1500 m/s, which means that the welding process is over before the tube deformation is initiated. Macro-observation of the welded zone revealed some important information about the MPW process. Measurement of both the aluminum tube flyer and the inner Magnesium part showed that they are subjected to severe deformation, as a consequence of the impact. The flyer thickness is reduced by 15–20% and the inner part is squeezed by the indentation of the aluminum flyer in approximating the same amount of deformation. Due to the high strain rate impact, the plastic deformation affects the adjacent metal surfaces, increasing the local temperature and creating the conditions for
3. Results and discussion A set of images of each welding experiment cross-section is shown in (Fig. 6). The reference sample (Fig. 6a) was found to have a sound continuous weld with no intermetallic phase present, thus confirming the selected experimental parameters. Welded samples with concentric holes 2, 5, 6, 10 and 15 mm, with inner part as seen
Fig. 7. The collision zone and angle of the welded couple (welding direction from left to right). (The original surface line of the inner part is marked by the white dashed line.)
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Fig. 8. MPW of a) solid inner bar (reference) and b) 15 mm concentric hole.
wave formation [36]. The impact deformation, the diametral reduction of the collapsing aluminum outer tube, and in addition, a magnetic pressure force vector in the weld propagation direction, creates a mutual velocity difference between the two metals. Hence, the aluminum flyer is moving faster then the inner magnesium part during the welding process. A cross-section of the Al–Mg reference sample weld is shown in (Fig. 7). The collision angle (a) which was found to be approximately 19 for a 1.5 mm gap, is 2.5 higher then the calculated angle, as a result of the indentation of the aluminum flyer into the inner Mg part surface. It is also evident (in this sample) that interface waves can only be observed in the centre of the welded area, which might be a result of sample geometry. There is a clear difference between the nature of interfacial waves in a joint with an inner solid part (reference) to that of a 15 mm concentrically drilled hole part, as can be seen in (Fig. 8: a and b). The 8 kJ weld was formed from left to right, while the upper part is the aluminum flyer tube and the lower part is the inner magnesium bar. The reference sample (a), with inner solid bar part, is not welded in the first third of its 9 mm contact zone. In the central area of the welded zone, some undeveloped waves are observed, while in the third part of the weld, two full scale waves can be observed. The first third of the 15 mm concentrically drilled inner part weld is bonded. The central zone is characterized by full scale interface waves and the third part of the weld, as can be observed, is a smooth and sound bonded zone with no waves observed. As for the other diameter drilled inner parts, they behave similarly to the 15 mm drilled sample with a variety of wavelengths, according to the geometry of the sample. Fig. 9 demonstrates the influence of wall thickness difference for a 1:3 ratio. Both montage pictures were taken from the same crosssectional plane in a single sample, that was drilled with an eccentric 5 mm hole. From only 5 full scale waves, it can be seen that the wavelength of one side (Fig. 9b) is much longer then the other. Also
it can be observed that the first full scale wave is closer to the process initiation point, as shown in Fig. 9a. The incident shock waves in Fig. 9c are represented schematically by the white arrows while the reflected waves from the hole wall are represented by the gray arrows. It is suggested that interfacial waves in MPW are the result of the Kelvin–Helmholtz hydrodynamic instability mechanism, caused by velocity difference between the outer tube and the inner part. The interferences that initiate the Kelvin–Helmholtz instability are the shock waves caused by the welding impact. The shock waves, generated at the impact point, travel in both metals with a radial front [26,35], with an angle proportional to a (Fig. 10a). The compression waves in the flyer tube are reflected from the back surface as refraction waves (gray arrows in Fig. 10b) with assorted periodic time. Since the MPW setup is axi-symmetric (Fig. 1), the compression waves in the inner part meet their corresponding waves at the centre of the bar in a rigid collision and are reflected as compression waves towards the interface (black arrows in Fig. 10b). The pressure peak is always at the collision point [8], since the speed is higher at that point and decreases retrogressively. Fig. 10c represents a superposition p–x diagram of Fig. 10a and b, as each new impact point creates new shock waves. Using the p–x diagram in Fig. 10 might give the impression that the interaction between the waves is occurring remotely from the collision point. In fact, the interaction of the compression and refraction waves, that generate interface waves, must occur at the collision point and in its vicinity. The interaction of the compression and refraction waves at the impact point would only occur in cases where their periods match. This interaction should take place in step with the propagation of the impact point. Since the collision point is under extreme pressure and heat, the interaction of the shock waves, in combination with mutual movement of the metals, is the source of interface waves. It was verified that the interaction of shock waves, with welded interface remote from the impact zone, do not cause interfacial waves. The wavelength is
Fig. 9. Wave pattern vs. shock wave propagation path. a) Short propagation path side, b) long path. (The interface weld line has been accentuated to illustrate the wave generation.)
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Fig. 10. Wave-creation model for MPW.
Fig. 11. Shock wave propagation during MPW.
proportional to the inner part thickness, since the radius of the pressure front of the reflected shock wave increases with the propagation distance. In cases where the inner welded part is a tube (Fig. 10d), the compression waves are reflected from the free surface of the hole as refraction waves and interact with refraction waves at the interface to create interface interferences. It is quite common to find spalling or separation of the weld interface [28], due to the interaction of refraction waves. The reaction timescale, in the case of the inner tubular part, is shorter, due to the short distance that waves need to travel through the thin tube wall. Therefore interface waves are initiated quite close to the beginning of the weld. For all Al–Mg welds the initial collision velocity Vi was approximately 410 m/s. Vc was approximately 1500 m/s since Vi ¼ Vc sin(a) where a ¼ 16 . The shock waves in Fig. 11 are traveling from the collision point in the inner part and back to
Fig. 12. The periodic manner of wave formation in MPW.
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Fig. 13. Wavelength vs. wall thickness in MPW.
the surface in 4.4 106 s (the total path being X1 þ X2). At this time, since sin(a) is small, assuming X ¼ inner part radii ¼ 10 mm, the collision point advances a distance of approximately 5.5 mm. The distance ‘‘Z ’’ is the distance that the impact point (the weld front) would pass in the same time and it was found to be 6.6 mm. This means that the collision point is ahead of the shock wave interferences and this might explain why the first part of the welded interface in Fig. 8a is undulating. In the final portion of the weld, where the collision point velocity (Vc) is severely reduced, two well-developed waves with wavelength of about 500 mm are visible. In the case of the tubular inner part (where X ¼ 2.5 mm), the collision point would advance only 0.55 mm (in the given time) and for this reason, the weld in Fig. 8b, using the tubular inner part, is characterized by a wavy interface approximately 1 mm from the weld start point, with wavelength of about 300 mm. In order to initiate wave formation, stress waves should meet at the collision point, where the heat and pressure are at a maximum. Fig. 12a illustrates the initiation of the first shock wave (in both metals). At this stage, Vc is high and the collision point is ahead of the interference zone (Fig. 12b). When the inner part is magnesium, the first wave would be generated, when Vc ¼ 1250 m/s and Vi ¼ 350 m/s (Fig. 12c), regardless of the inner part diameter. From this stage, a Kelvin–Helmholtz instability mechanism takes place and waves are created periodically (Fig. 12d). The instability generates waves, as long as nothing decays it. New collision points are created and continuously generate new shock waves, but a new interference cannot be created while waves are formed by massive metal movements across the interface. The next wave is initiated by the interference continuity (Fig. 12e). The propagation velocity, Vc, decreases severely as the weld progresses, so that the shock wave interferences meet the collision point further along and for this reason, the wavelength increases (Fig. 12f). This change in wavelength usually doesn’t occur in EXW, since the energy source is explosive and Vc is constant [8]. From some point, Vc is so small that the interferences are ahead of the collision point and new waves cannot be generated (Fig. 12g). Since stress waves are traveling both in the inner and outer parts, with different thicknesses and speeds of sound, there might be more then one mode of interference. In this case, the Kelvin– Helmholtz instability would have a multimode case, and this might be the reason for the asymmetry of the interface waves in Fig. 9b.
Nevertheless, eventually one mode dominates and the interface waves are generated in this mode. A summary of the experimental data is presented in Fig. 13, where each dot represents a single experiment. The linear line fit represents the relationship between interface wavelength and wall thickness (half of free propagation path of shock waves). The relations are constant for each different pulse energy level (8, 9 and 10 kJ). This behaviour is expected to be the same for other metal couples, in accordance with the speed of sound of the selected metals. 4. Summary and conclusions Wave formation in MPW was investigated in order to establish a common mechanism. A special sample was designed, in order to control the free path of shock wave propagation in the selected metals, similar to those used in EXW research experiments, with various base plate thicknesses. This shock wave theory claims that shock waves propagate through the metal parts, creating periodic interference perturbation at the weld interface. These interferences initiate a Kelvin– Helmholtz instability that creates the interface waves. It was calculated and has been proven experimentally that wavelength of interface waves are proportional to the free path of shock wave propagation in the inner part of the welded couple. The initial gap influences the impact angle that sets the relationship between the collision point velocity and the weld propagation velocity. Also the energy of the capacitor and the initial gap determine the collision velocity of the outer tube. The interface wavelength was found to be proportional to the geometry of the inner part. These are the main process parameters that influence interface wave formation. It was established that in tubular MPW joints, interface waves are formed in a Kelvin–Helmholtz instability mechanism, whereby reflected shock waves are the source for interferences at the weld interface. Interface waves are formed only at the impact zone and its vicinity, due to induced metal flow in this elevated temperature and high pressure region. Acknowledgement The authors wish to acknowledge Y. Livshitz and O. Gafri from Pulsar Welding Ltd. (Yavne, Israel), as well as Rotem Industries Ltd.,
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International Journal of Impact Engineering journal homepage: www.elsevier.com/locate/ijimpeng
Wave formation mechanism in magnetic pulse welding A. Ben-Artzy a, *, A. Stern b, N. Frage b, V. Shribman c, O. Sadot b a
Rotem Industries Ltd., P.O. Box 9046, 84190 Beer-Sheva, Israel Ben-Gurion University, P.O. Box 653, 84105 Beer-Sheva, Israel c Pulsar Ltd., 11 Hayezira St., P.O.Box 2617, 43663 Raanana, Israel b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 May 2009 Received in revised form 29 July 2009 Accepted 30 July 2009 Available online 3 August 2009
Wavy interface morphology is observed in Magnetic Pulse Welding (MPW) similarly to that of the Explosion Welding process (EXW). It is recognized that interfacial waves are formed in a periodic manner and have well defined wavelength and amplitude. The phenomenon of wave formation in EXW has been subjected to extensive investigations in which empirical and numerical models have been published. In the present study, a wave formation mechanism for MPW is presented. This wave-creation mechanism was studied by evaluating the influence of sample geometry on wave morphology using stereoscopic optical microscopy. It was found that interfacial waves are formed in a Kelvin–Helmholtz instability mechanism. Reflected shock waves interact with the welding collision point at the weld interface, where interferences are the source for the wave’s initiation. The collision energy, impact angle, and the geometry of the joint, were found to have the most significant influence on the waves’ characteristics. An empirical relationship between interfacial wavelength and the free moving distance of the shock waves in the welded tubular parts was found. Ó 2009 Elsevier Ltd. All rights reserved.
Keywords: MPW Wave Impact Interface Shock
1. Introduction MPW is a solid state welding process similar to EXW. The MPW process uses a magnetic field, applied to an outer workpiece, in order to collapse it onto the inner part at a velocity high enough to achieve a metallurgical bond (Fig. 1). The magnetic pressure is produced through a coil, while the impulse current is created via a capacitor bank. The repulsion between the coil magnetic field and the induced field on the outer part results in a J B force (Lorentz force) that causes a collision of the outer tubular part onto the inner part to be welded, at speeds in the region of 250–500 m/s [1,2,9,10,25]. The collision of the outer workpiece is oblique, due to the amplitude distribution of the magnetic field, with a collision point velocity (Vc) of about 1500–3000 m/s. The collision of the metals creates a jet consisting of a mixture of metal, air and oxides, ejected from the surfaces of both metals. The two parts of the joint are then forced together to form a solid-state weld, while the whole process takes up to 100 ms [3–5]. Since the open end of the welded sample is located in the middle of the coil (where the magnetic flux density is maximum), this area is subjected to the maximum magnetic pressure. The distribution of pressure along the weld area and its increase in time
* Corresponding author. Tel.: þ972 50 6231910; fax: þ972 8 6510881. E-mail address: [email protected] (A. Ben-Artzy). 0734-743X/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijimpeng.2009.07.008
is illustrated schematically in (Fig. 2a). The acceleration of the outer tube through the gap is higher at the open end of the tube, due to higher magnetic pressure at this area and decreasing down to zero at the weld end where there is no movement of the outer tube. As a result the collision is oblique and the impact angle (a) (Fig. 7) is formed. The velocity profile (Fig. 2b) follows the magnetic pressure profile with a negligible time difference. The initial part of the joint collides at a high collision angle (a) and at too high a collision speed; therefore, usually no bond is formed in this area. As the weld progresses, the collision angle is decreased and stabilizes, and the collision velocity (Vc) decreases gradually to zero. Since the welding process is rapid, the magnetic pressure is maintained for several microseconds after the impact welding is over. The kinetic energy of the outer tube is converted to heat by massive plastic deformation of the metals. This deformation affects the interface and vicinity of approximately 50–100 mm, resulting in a significant temperature increase [10,17,19,22,36] and strain hardening, as was measured in this and others’ [23,24] work. The temperature rise of the interface softens the metals, and in some cases, melts local pockets or a thin continuous layer along the interface [18–20,22,36]. In some cases wavy interface morphology (Fig. 3) appears with short-term periodicity [17,25]. Several mechanisms have been suggested to explain the interface wave creation in EXW [8,11,12,26]. In Refs. [15,16,26,29] it was suggested that the source for wave formation is a shear movement of the flyer and base plates creating local vortices in the plastic
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Fig. 1. Magnetic pulse welding setup [4].
zone. A stagnation region of the jet acting creates each period vortex as a solid obstacle behind it. In another mechanism [7,15,37] it was assumed that indentation of the re-entrant jet into the parent plate surface forms a ‘‘hump’’ ahead of the stagnation point and due to the instability of this region and momentary change in velocity of the metals, the stagnation point ‘‘jumps’’ over the ‘‘hump’’, and a new ‘‘hump’’ is formed periodically. Another theory was published [26,28,31–33,37], based on the Kelvin–Helmholtz instability mechanism, in which waves are formed as a result of flow velocity discontinuities across the interface. The Kelvin– Helmholtz instability model is a model that deals with hydrodynamics. The main principle of the model is that whenever two fluids with different velocities interact, instabilities will occur at the interface as a result of the interferences. These instabilities involve mass flow, usually from the higher density material to the lower one. Whenever instability occurs, it has a direction and a certain velocity (energy) and it causes a movement of material from one side of the interface (Fig. 4a) to the other, and immediately a movement of material from the other side back (Fig. 4b), because of energy complementary to the system. The newly created interface wave gets its directionality and shape (Fig. 4c) under the influence of the mutual velocity of the metals. The welded metals can be defined as viscous solids for this model. An alternative theory attempting to explain the wave formation mechanism in EXW was mentioned in Refs. [6,11,30,34,35]. It was suggested that compressive waves are generated at the collision point and reflected at the free surface of the base plate. The
reflected stress waves interact with the compressive waves and cause periodic interferences at the base plate surface, at the softened high pressure collision area. During the attempts to develop a wave formation model for EXW, experimental effort has been made. The wave formation model that was published in Ref. [12] was found to be analogous to the formation of vortex stresses in fluid flow around an obstacle or in a collision of liquid streams. The fluid flow model, describes the observed transition from a smooth metal-to-metal bond zone to a wavy bond zone, above a critical collision velocity. In Refs. [7,15] it was found difficult to reconcile the periodical manner of wave formation with theories that are based on random obstacles such as ‘‘humps’’, that interrupt the steady flow region at the interface. As for the indentation of the reentrant jet into the parent plate, they found that it cannot be resolved because, if so, the interface wavelength should be dependent on the flyer plate thickness, and independent of its velocity. On the other hand, some experimental work [12–14] has clearly shown that interface waves’ wavelengths depend on the base plate thickness. Other researchers [13,14] used various base plate thicknesses in order to control the shock wave propagation path through the plate and an empirical model, describing the relationship between base plate thickness and wavelength of interfacial waves, was established. Some attempts to elucidate the wave formation mechanism for MPW have been done recently [38–40], but no detailed model was established since it was assumed that the process is identical to that of EXW [27]. In Ref. [16] it was assumed that waves are formed by
Fig. 2. Pressure and velocity profile in MPW.
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Fig. 3. A wavy interface in an Al–Mg weld.
the penetration of the jet into the inner part surface, at the collision point, as seen in EXW. Different waveforms are reported according to differences in plastic deformation dissipated by the inner part. It was also found in this research that interface waves might appear only when there is enough energy for plastic deformation of the inner part. The conditions for wave formation, from the metallurgical point of view, were defined in Ref. [17] regarding plastic deformation in the region of interaction of the weld metals. Several deformation mechanisms were suggested; displacement of dislocations, collective movements of crystal lattice defects or rotation of grains by micromechanisms. Hence this theory clarifies the feasibility of very high strain rate massive deformation, at a narrow interface zone, during interface wave creation. The interface temperature in MPW rises due to the jet and massive deformation of the surfaces, and in some cases, melting and solidification occur similarly to EXW [7,15–17,36]. The temperature rise at the interface encourages wave formation by softening of the interface and its vicinity.
Fig. 4. Kelvin–Helmholtz instability creation (from left to right u1 > u2).
This research attempts to elucidate wave formation mechanism in MPW. The present contribution of this work is an attempt to confirm the compliance of a wave formation mechanism theory, based on a Kelvin–Helmholtz instability model, with interferences triggered by shock wave propagation through the metals, interacting with the welding zone at its interface. 2. Experimental The objective of the present experimental work was to verify that interface wave formation in MPW is a consequence of a Kelvin– Helmholtz mechanism. An attempt was made to control the shock wave propagation free path, in order to prove the hypothesis that shock waves that travel through the inner part of the weld are the source for interface interferences that initiate wave formation. This approach is similar to experimental work that was aimed to influence wave formation, by controlling the base plate thickness in EXW [13,14]. In this work, the free path of shock wave propagation was controlled by drilling a hole in the inner part. In one case a hole was drilled concentrically, so that waves could propagate in a symmetrical manner, from the collision surface to the free surface of the drilled hole, and reflect as an expansion wave back to the interface where the weld is formed. In other cases, holes were drilled eccentrically, so that shock waves from different directions had different free paths of propagation. The use of eccentrically drilled holes made it possible to compare between waves propagating through different paths in a single sample, while all other parameters remained unchanged. The flyer tube (Fig. 5: part a) was 1 mm thick and 10 mm long in all the experiments. The inner part (Fig. 5: part b and 1–8) was concentrically fitted into the outer tube by a fastening pin. The inner part of the reference sample (Fig. 5: 1) was made out of a solid round bar, while in other experiments, inner parts with various diameter holes were used (Fig. 5: 2–6). Other inner part holes were drilled in different positions with
Fig. 5. Controlled wave propagation experimental setup. a) Outer tubular part, b) Inner part, 1–8) Inner part with different wall thicknesses, 9–11) Assembled samples.
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Fig. 6. Cross-section of welded samples: a) Reference, b–f) Concentric holes of various diameters, g–h) Eccentric holes.
respect to the centre (Fig. 5: 7,8). The holes, offset from the centre, were designed to create different path lengths (from the surface to the hole) for the incoming shock waves. An assembly, with a 15 mm concentric hole inner part sample, is presented in (Fig. 5: 9). It should be observed that this inner part is actually a tube. In (Fig. 5: 10), a 5 mm eccentric hole assembly is presented and a reference sample assembly can be seen in (Fig. 5: 11). The initial acceleration gap between the outer tube and the inner part was set by the diameter of the inner part (BA) at the welding zone, as can be seen in (Fig. 5: 2). The Al–Mg couple was chosen for this research, since there is a great difference in the physical properties of these metals; such as sound velocity and module of elasticity. Although there is a similarity in melting temperature, aluminum has an fcc structure while magnesium a hexagonal one, therefore deformation mechanisms are very different. These metals are easily traceable after welding by optical microscopy and backscattered electron microscopy means, due to their different structures. Commercial AA1050 and AZ31-B magnesium alloy were used for this research. AA1050 is a pure Al containing 0.25% Fe with some Si, while AZ31-B was preferred for this research over pure magnesium, due to the high strain rate load of the welding process. The experimental parameters were chosen empirically and were specified to be at the minimum energy for welding the selected couple, in order to achieve a wavy interface morphology and avoid melting at the interface zone. The welded workpieces were cross-sectioned and prepared for evaluation by standard metallographic procedures: mechanical polishing up to 1 mm mesh, followed by a light chemical etching. The samples were then examined by SEM, optical stereoscopic microscopy and montage pictures were taken. The interface waves’ wavelengths and amplitudes were measured on the stereoscopic images using ‘‘Image Pro 4’’ software. In most cases, 3–5 full scale interface waves were included, in order to eliminate secondary interference interruptions, as shown in examples (Figs. 3, 8 and 9). Welding experiments using this setup were performed with 8, 9 and 10 kJ.
in (Fig. 5: 2–6), can be seen in (Fig. 6: b, c, d, e and f) respectively. Welded samples with eccentric 5 mm holes located in different offset positions from the centre, with inner part as seen in (Fig. 5: 7 and 8), can be seen in (Fig. 6: g and h) respectively. In (Fig. 6: g), one wall thickness is three times that of the opposite wall thickness (on the same plane), while in (Fig. 6: h) the thick wall is four times thicker. Inner diameter deformation of the drilled holes can be observed in each of the hollow samples. This deformation is a wellknown phenomenon in industrial MPW of hollow welds and usually a backup steel mandrel is used to support the tube. In this case, the deformation of the sample didn’t influence the results, due to a different timetable of events. The shock wave travels in the inner part at approximately the speed of sound in magnesium, i.e. 4600 m/s [21], while the deformation of the tube is a consequence of metal acceleration, due to impact in the velocity range of 250– 350 m/s [36]. Also the propagation velocity of the collision point (Vc) is more then 1500 m/s, which means that the welding process is over before the tube deformation is initiated. Macro-observation of the welded zone revealed some important information about the MPW process. Measurement of both the aluminum tube flyer and the inner Magnesium part showed that they are subjected to severe deformation, as a consequence of the impact. The flyer thickness is reduced by 15–20% and the inner part is squeezed by the indentation of the aluminum flyer in approximating the same amount of deformation. Due to the high strain rate impact, the plastic deformation affects the adjacent metal surfaces, increasing the local temperature and creating the conditions for
3. Results and discussion A set of images of each welding experiment cross-section is shown in (Fig. 6). The reference sample (Fig. 6a) was found to have a sound continuous weld with no intermetallic phase present, thus confirming the selected experimental parameters. Welded samples with concentric holes 2, 5, 6, 10 and 15 mm, with inner part as seen
Fig. 7. The collision zone and angle of the welded couple (welding direction from left to right). (The original surface line of the inner part is marked by the white dashed line.)
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Fig. 8. MPW of a) solid inner bar (reference) and b) 15 mm concentric hole.
wave formation [36]. The impact deformation, the diametral reduction of the collapsing aluminum outer tube, and in addition, a magnetic pressure force vector in the weld propagation direction, creates a mutual velocity difference between the two metals. Hence, the aluminum flyer is moving faster then the inner magnesium part during the welding process. A cross-section of the Al–Mg reference sample weld is shown in (Fig. 7). The collision angle (a) which was found to be approximately 19 for a 1.5 mm gap, is 2.5 higher then the calculated angle, as a result of the indentation of the aluminum flyer into the inner Mg part surface. It is also evident (in this sample) that interface waves can only be observed in the centre of the welded area, which might be a result of sample geometry. There is a clear difference between the nature of interfacial waves in a joint with an inner solid part (reference) to that of a 15 mm concentrically drilled hole part, as can be seen in (Fig. 8: a and b). The 8 kJ weld was formed from left to right, while the upper part is the aluminum flyer tube and the lower part is the inner magnesium bar. The reference sample (a), with inner solid bar part, is not welded in the first third of its 9 mm contact zone. In the central area of the welded zone, some undeveloped waves are observed, while in the third part of the weld, two full scale waves can be observed. The first third of the 15 mm concentrically drilled inner part weld is bonded. The central zone is characterized by full scale interface waves and the third part of the weld, as can be observed, is a smooth and sound bonded zone with no waves observed. As for the other diameter drilled inner parts, they behave similarly to the 15 mm drilled sample with a variety of wavelengths, according to the geometry of the sample. Fig. 9 demonstrates the influence of wall thickness difference for a 1:3 ratio. Both montage pictures were taken from the same crosssectional plane in a single sample, that was drilled with an eccentric 5 mm hole. From only 5 full scale waves, it can be seen that the wavelength of one side (Fig. 9b) is much longer then the other. Also
it can be observed that the first full scale wave is closer to the process initiation point, as shown in Fig. 9a. The incident shock waves in Fig. 9c are represented schematically by the white arrows while the reflected waves from the hole wall are represented by the gray arrows. It is suggested that interfacial waves in MPW are the result of the Kelvin–Helmholtz hydrodynamic instability mechanism, caused by velocity difference between the outer tube and the inner part. The interferences that initiate the Kelvin–Helmholtz instability are the shock waves caused by the welding impact. The shock waves, generated at the impact point, travel in both metals with a radial front [26,35], with an angle proportional to a (Fig. 10a). The compression waves in the flyer tube are reflected from the back surface as refraction waves (gray arrows in Fig. 10b) with assorted periodic time. Since the MPW setup is axi-symmetric (Fig. 1), the compression waves in the inner part meet their corresponding waves at the centre of the bar in a rigid collision and are reflected as compression waves towards the interface (black arrows in Fig. 10b). The pressure peak is always at the collision point [8], since the speed is higher at that point and decreases retrogressively. Fig. 10c represents a superposition p–x diagram of Fig. 10a and b, as each new impact point creates new shock waves. Using the p–x diagram in Fig. 10 might give the impression that the interaction between the waves is occurring remotely from the collision point. In fact, the interaction of the compression and refraction waves, that generate interface waves, must occur at the collision point and in its vicinity. The interaction of the compression and refraction waves at the impact point would only occur in cases where their periods match. This interaction should take place in step with the propagation of the impact point. Since the collision point is under extreme pressure and heat, the interaction of the shock waves, in combination with mutual movement of the metals, is the source of interface waves. It was verified that the interaction of shock waves, with welded interface remote from the impact zone, do not cause interfacial waves. The wavelength is
Fig. 9. Wave pattern vs. shock wave propagation path. a) Short propagation path side, b) long path. (The interface weld line has been accentuated to illustrate the wave generation.)
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Fig. 10. Wave-creation model for MPW.
Fig. 11. Shock wave propagation during MPW.
proportional to the inner part thickness, since the radius of the pressure front of the reflected shock wave increases with the propagation distance. In cases where the inner welded part is a tube (Fig. 10d), the compression waves are reflected from the free surface of the hole as refraction waves and interact with refraction waves at the interface to create interface interferences. It is quite common to find spalling or separation of the weld interface [28], due to the interaction of refraction waves. The reaction timescale, in the case of the inner tubular part, is shorter, due to the short distance that waves need to travel through the thin tube wall. Therefore interface waves are initiated quite close to the beginning of the weld. For all Al–Mg welds the initial collision velocity Vi was approximately 410 m/s. Vc was approximately 1500 m/s since Vi ¼ Vc sin(a) where a ¼ 16 . The shock waves in Fig. 11 are traveling from the collision point in the inner part and back to
Fig. 12. The periodic manner of wave formation in MPW.
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Fig. 13. Wavelength vs. wall thickness in MPW.
the surface in 4.4 106 s (the total path being X1 þ X2). At this time, since sin(a) is small, assuming X ¼ inner part radii ¼ 10 mm, the collision point advances a distance of approximately 5.5 mm. The distance ‘‘Z ’’ is the distance that the impact point (the weld front) would pass in the same time and it was found to be 6.6 mm. This means that the collision point is ahead of the shock wave interferences and this might explain why the first part of the welded interface in Fig. 8a is undulating. In the final portion of the weld, where the collision point velocity (Vc) is severely reduced, two well-developed waves with wavelength of about 500 mm are visible. In the case of the tubular inner part (where X ¼ 2.5 mm), the collision point would advance only 0.55 mm (in the given time) and for this reason, the weld in Fig. 8b, using the tubular inner part, is characterized by a wavy interface approximately 1 mm from the weld start point, with wavelength of about 300 mm. In order to initiate wave formation, stress waves should meet at the collision point, where the heat and pressure are at a maximum. Fig. 12a illustrates the initiation of the first shock wave (in both metals). At this stage, Vc is high and the collision point is ahead of the interference zone (Fig. 12b). When the inner part is magnesium, the first wave would be generated, when Vc ¼ 1250 m/s and Vi ¼ 350 m/s (Fig. 12c), regardless of the inner part diameter. From this stage, a Kelvin–Helmholtz instability mechanism takes place and waves are created periodically (Fig. 12d). The instability generates waves, as long as nothing decays it. New collision points are created and continuously generate new shock waves, but a new interference cannot be created while waves are formed by massive metal movements across the interface. The next wave is initiated by the interference continuity (Fig. 12e). The propagation velocity, Vc, decreases severely as the weld progresses, so that the shock wave interferences meet the collision point further along and for this reason, the wavelength increases (Fig. 12f). This change in wavelength usually doesn’t occur in EXW, since the energy source is explosive and Vc is constant [8]. From some point, Vc is so small that the interferences are ahead of the collision point and new waves cannot be generated (Fig. 12g). Since stress waves are traveling both in the inner and outer parts, with different thicknesses and speeds of sound, there might be more then one mode of interference. In this case, the Kelvin– Helmholtz instability would have a multimode case, and this might be the reason for the asymmetry of the interface waves in Fig. 9b.
Nevertheless, eventually one mode dominates and the interface waves are generated in this mode. A summary of the experimental data is presented in Fig. 13, where each dot represents a single experiment. The linear line fit represents the relationship between interface wavelength and wall thickness (half of free propagation path of shock waves). The relations are constant for each different pulse energy level (8, 9 and 10 kJ). This behaviour is expected to be the same for other metal couples, in accordance with the speed of sound of the selected metals. 4. Summary and conclusions Wave formation in MPW was investigated in order to establish a common mechanism. A special sample was designed, in order to control the free path of shock wave propagation in the selected metals, similar to those used in EXW research experiments, with various base plate thicknesses. This shock wave theory claims that shock waves propagate through the metal parts, creating periodic interference perturbation at the weld interface. These interferences initiate a Kelvin– Helmholtz instability that creates the interface waves. It was calculated and has been proven experimentally that wavelength of interface waves are proportional to the free path of shock wave propagation in the inner part of the welded couple. The initial gap influences the impact angle that sets the relationship between the collision point velocity and the weld propagation velocity. Also the energy of the capacitor and the initial gap determine the collision velocity of the outer tube. The interface wavelength was found to be proportional to the geometry of the inner part. These are the main process parameters that influence interface wave formation. It was established that in tubular MPW joints, interface waves are formed in a Kelvin–Helmholtz instability mechanism, whereby reflected shock waves are the source for interferences at the weld interface. Interface waves are formed only at the impact zone and its vicinity, due to induced metal flow in this elevated temperature and high pressure region. Acknowledgement The authors wish to acknowledge Y. Livshitz and O. Gafri from Pulsar Welding Ltd. (Yavne, Israel), as well as Rotem Industries Ltd.,
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