2021 Volume 62 Issue 8 Pages 1151-1159
Welding of aluminum and aluminum-coated steel plates was performed using magnetic pulse welding. A1050 pure aluminum and Al–Si-coated steel plates were used in this study. The aluminum and the aluminum-coated steel plates were used for a flyer plate and a parent plate, respectively. The welding interface was observed with an optical microscope, scanning electron microscope and scanning transmission electron microscope. Tensile-shear tests were conducted for evaluation of the joint strength. After the welding, thickness of the aluminum coating slightly decreased at the welded area and Si particles containing in the aluminum coating were refined. The welding interface exhibited characteristic wavy morphology. A banded structure consisting of fine aluminum grains with diameters of approximately 500 nm was formed along the wavy interface. When the welding was performed at longer plate gap condition, void formation was observed in the banded structure and fracture occurred at the welding interface by tensile-shear test. The void formation is considered to lead to a decrease in joint strength.
The goal of increasing the performance and functionality of industrial products has led to a focus on dissimilar metal welding. In the automotive industry in particular, the concept of multi-material components has been proposed, in which the steel that was conventionally used is partially replaced by aluminum in order to reduce the weight of the vehicle with the goal of improving fuel consumption. For achievement of the multi-material components, welding of dissimilar metals such as aluminum and steel is needed. When aluminum and steel are welded using fusion welding methods, thick layers of brittle intermetallic compounds (IMCs) such as Al3Fe and Al5Fe2 are formed at the welding interface and are known to become an origin of fracture.1) Therefore, solid-state welding methods have been attracting attention for suppression of the IMC formation.
Magnetic pulse welding is a kind of impact welding, as exemplified by explosion welding, and is classified as the solid-state welding method.2) Figure 1 shows a schematic diagram of the principle of magnetic pulse welding. In this method, a discharge circuit comprising a capacitor, a discharge gap switch and a one-turn coil is used (Fig. 1(a)). The two or multiple metal plates are affixed above the coil with a small gap (plate gap). The metal plate of the coil side is called the flyer plate and the other metal plate is called the parent plate. The electrical charge in the capacitor (charging energy) is discharged into the coil by closing the discharge gap switch, creating a high-density magnetic flux around the coil (Fig. 1(b)). Eddy currents are induced on the flyer plate surface by intersecting of the magnetic flux and the flyer plate. An electromagnetic force is thereby generated in the upward direction (towards the parent plate) in accordance with Fleming’s left-hand rule based on the relationship between the eddy current and the magnetic flux. The flyer plate is deformed at high speed by this electromagnetic force, and a joint is formed by high-speed collision of the flyer plate and the parent plate (Fig. 1(c)). For generation of the electromagnetic force, a metal plate with high electrical conductivity is suitable for the flyer plate. The welding is normally finished within 10 microseconds with a negligible temperature rise.3–5)
Schematic diagrams of magnetic pulse welding. (a) Discharge circuit and setup of metal plates. (b) Generation of electromagnetic force. (c) Lap joint.
Magnetic pulse welding can be used for dissimilar metals with different physical and mechanical properties, and strong joints have been obtained for metal combinations such as aluminum and steel,6–8) aluminum and copper9,10) and aluminum and nickel.11) However, reports on magnetic pulse welding of the surface-treated metals are few. Surface of steel plate is often coated with zinc or aluminum layer for improving corrosion resistance. In magnetic pulse welding, metal jets are ejected from collision point of the flyer plate and the parent plate.12) These jets are considered to cause foreign matter on the surface of the metal plates to be ejected outside the welded area, allowing a weld between clean surfaces. In case of welding of the coated metal plates, the ejection of metal jets is possible to result in removal of the coating layer. Itoi et al. and Wang et al. performed magnetic pulse welding of Ni-coated copper or Zn-coated steel and aluminum, and reported that strong joints were obtained.13,14) However, there have been no reports on magnetic pulse welding of aluminum-coated steel. The aluminum-coated steel is attracting attention as a next-generation coated steel with excellent corrosion resistance. In this study, we performed magnetic pulse welding of aluminum-coated steel and aluminum, and the interfacial microstructure and strength of the obtained joints were investigated.
A 0.5-mm-thick A1050-H24 aluminum (hereafter Al) plate and a 1.0-mm-thick aluminum-coated steel (Al-coated steel) plate were used in this study. The Al-coated steel was fabricated by hot-dipping of Fe–0.04%C–0.01%Si–0.18%Mn–0.016%P–0.011%S (mass%) alloy to Al–Si alloy melt. Chemical composition of the Al coating is Al–9%Si (mass%).15) The plate length and width were 200 mm and 70 mm, respectively. Before welding, the surface of the Al plate was polished using #1000 waterproof abrasive paper, and rinsed in acetone in an ultrasonic cleaner. Figure 2 shows optical micrographs of the cross section of the Al-coated steel plate surface before welding. An approximately 10-µm-thick Al coating layer was deposited on the steel surface (Fig. 2(a)). The grain size in the steel was 10–20 µm. Needle-like Si particles were observed in the Al coating layer (Fig. 2(b)). In addition, an approximately 3-µm-thick IMC layer was produced at the Al coating/steel interface. STEM-EDX analysis showed that the IMC layer was composed of layers of three phases such as Al2Fe, Al5Fe2 and Al3Fe. The Si of 8 at%, 10 at% and 11 at% was included in the Al2Fe, Al5Fe2 and Al3Fe phases, respectively. The Al2Fe layer and the Al3Fe layer were formed at the steel side and the Al side in the IMC layer, respectively, and the Al5Fe2 layer was sandwiched by the Al2Fe and Al3Fe layers. Thicknesses of the Al2Fe, Al5Fe2 and Al3Fe layers were approximately 200 nm, 2 µm and 1 µm, respectively. Although the Al coating/IMC interface was relatively flat, the IMC/steel interface had a saw-tooth shape. Also, vertical cracks, indicated by arrows in Fig. 2(b), were observed in the IMC layer.
Optical micrographs of cross section of Al-coated steel plate before welding. (a) Grain structure of steel. (b) Al coating. Arrows indicate vertical cracks formed in IMC layer.
Welding of the Al plate and the Al-coated steel plate was performed using a magnetic pulse system (Bmax, MP 12.5/25). The coil was made of Cu–Cr–Zr alloy and its cross-sectional shape was trapezoidal with an upper surface length of 3 mm. The capacitance of the capacitor in the discharge circuit for providing the pulse current to the coil was 40 µF. The Al plate and the Al-coated steel plate were used as the flyer and parent plates, respectively. The rolling direction of each plate was set in direction perpendicular to the longitudinal direction to the coil. The Al plate overlapped the coil for a distance of 3 mm and the Al-coated steel plate was set above the Al plate with a small gap between them. The charging energy stored in the capacitor and the plate gap was changed from 2.0 to 8.0 kJ and 0.5 to 2.5 mm, respectively.
2.2 Microstructural observation and analysisThe microstructure of the joint was observed using optical microscope (BX51M), scanning electron microscope (SEM, S-4500, acceleration voltage: 15 kV), transmission electron microscope (TEM, Tecnai Osiris, acceleration voltage: 200 kV) and scanning transmission electron microscope (STEM, Tecnai Osiris, acceleration voltage: 200 kV). The chemical composition was analyzed using energy-dispersive X-ray spectrometer (EDX) equipped with the SEM and STEM systems.
Samples for optical microscope and SEM observations were fabricated by mechanical polishing of cross sections perpendicular to the seam direction. The cross section was chemically etched in solutions for observation of the welding interface and the grains. The solutions used are HF:H2O = 1:50 in volume for Al and HNO3:C2H6O = 1:50 for steel. For the TEM and STEM samples, small regions including the welding interface were collected from the joint by micro-sampling using focused ion beam (FIB, FB-2100, acceleration voltage: 40 kV) milling with Ga ions, and the collected sample was thinned.
2.3 Evaluation of joint strengthThe joint strength was evaluated using tensile-shear test. The test specimens were cut from the joint using electric discharge machining to a shape that conformed to Japanese Industrial Standard 13B. The tensile speed was fixed at 1.0 mm/min. The tests were conducted at room temperature in air. Three or more specimens for each welding condition were tested.
Figure 3(a) shows macroscopic appearance of Al/Al-coated steel lap joint welded at the charging energy of 6.0 kJ and plate gap of 1.0 mm. Edge part of the Al plate, which was set on the coil, was deformed toward the Al-coated steel plate. Figure 3(b) shows an optical micrograph of cross section of the joint. Width of the pressed area of the Al plate was approximately 8 mm, and welded area, whose width was indicated by arrow in Fig. 3(b), was observed in a part of the pressed area. Figure 4 shows the relationship between the charging energy or plate gap and the width of the welded area. The width of the welded area was measured as the horizontal distance of the welded area when the surface of the Al-coated steel was horizontal at the welded area. The width of the welded area was approximately 2 mm regardless of the charging energy, but decreased with increasing the plate gap.
(a) Macroscopic appearance of Al/Al-coated steel lap joint welded at charging energy of 6.0 kJ and plate gap of 1.0 mm. Observation was performed from Al side. (b) Optical micrograph of cross section of the joint welded at charging energy of 6.0 kJ and plate gap of 1.0 mm.
Relationship between charging energy or plate gap and width of welded area.
In order to investigate the characteristics of the welded area, the interfacial microstructure formed at the central part of the welded area was observed. Figure 5 shows an optical micrograph of the welded area formed in the joint welded at charging energy of 6.0 kJ and plate gap of 1.0 mm. The upper side in the micrograph is the steel part of the Al-coated steel and the lower side is the Al. The Al coating layer was clearly observed at the area between the steel and Al. This indicates that the Al coating layer still remained at the welding interface after welding regardless the metal jet emission. The welding interface between the Al coating layer and the Al plate is indicated by an arrow. The welding interface exhibited a characteristic wavy morphology. The wavy morphology was formed through the entire welding interface whereas the magnitude of the wavy interface changed as well as Al/Al joint.3) The formation of wavy welding interface has been previously reported in the impact welded joints,16–21) and this is considered to be an indicator that a strong joint was obtained. When this wavy interface was observed in detail, it was found that whereas the base part on the Al side of the wavy interface was smooth, the ridge part on the steel side was crushed and exhibited a relatively flat morphology parallel to the interface between the Al coating layer and the steel. The thickness of the Al coating layer periodically increased and decreased with the wavy interfacial morphology, because Al coating/Al interface was wavy and Al coating/steel interface was flat.
Optical micrograph of welded area in joint welded at charging energy of 6.0 kJ and plate gap of 1.0 mm. Arrow indicates location of welding interface.
Figure 6 shows cross-sectional SEM-EDX Si maps of (a) the welded area in joint welded at charging energy of 6.0 kJ and plate gap of 1.0 mm and (b) the surface of the Al-coated steel plate before welding. Whereas needle-shaped Si particles with a length of approximately 10 µm and a width of approximately 1 µm were distributed in the Al coating layer before the welding, after the welding these Si particles became finer and more uniformly distributed throughout the entire Al coating layer.
Si maps of (a) welded area in joint welded at charging energy of 6.0 kJ and plate gap of 1.0 mm and (b) Al-coated steel before welding.
Figure 7(a) shows an SEM image of the welded area in joint welded at charging energy of 7.0 kJ and plate gap of 1.0 mm. The interface between the Al and Al-coated steel cannot be clearly observed. This suggests that the Al and the Al coating layer were welded well. Furthermore, the thickness of the IMC layer after welding was similar to that before welding, and the shape was also similar in that it was flat on the steel side and saw-toothed on the Al coating layer side. Figure 7(b) shows an enlarged SEM image of the IMC layer at Al coating/steel interface formed at charging energy of 7.0 kJ and plate gap of 1.0 mm. Horizontal cracks were formed with the original vertical cracks in the IMC layer. The horizontal cracks were often observed in Al5Fe2 layer or at Al5Fe2/Al2Fe interface. Formation of the horizontal cracks in the IMC layer is considered to be due to the transmission of pressure induced by the high-speed collision of the Al plate and the Al-coated steel plate.
SEM images of (a) welded area and (b) IMC layer at welded area in joint welded at charging energy of 7.0 kJ and plate gap of 1.0 mm.
The morphology of the wavy interface has been proposed to be related to the density ratio of the flyer plate to the parent plate (density of flyer plate/density of parent plate).22) A sinusoidal wavy interface is observed in similar metal joints such as Al/Al and Cu/Cu joints for a density ratio of 1,3) and in Cu/Ni joint for a density ratio close to 1.23) Furthermore, dissimilar Al alloy joints have also the sinusoidal wavy interface.4,24,25) In this study, since welding of A1050 and Al-coated steel was performed, the metals which sandwich the welding interface are A1050 and Al coating (Al–Si alloy). Therefore, formation of the sinusoidal wavy interface was expected but it appears that the ridge parts of the interface were crushed (Fig. 5). This is assumed to be because the Al coating layer is thin and the hard IMC layer and steel exist directly on the thin Al coating layer. The height of the wavy interface obtained in this study was approximately 20 µm, which is larger than the thickness of the Al coating layer (10 µm). Furthermore, the needle-shaped Si particles in the Al coating layer became finer after welding (Fig. 6(a)). Although cracks were formed in the IMC layer, no changes of shape occurred at the Al coating/IMC layer and IMC layer/steel interfaces after welding (Fig. 7). Grain structure of steel at the welded area in joint welded at charging energy of 6.0 kJ and plate gap of 1.0 mm was shown in Fig. 8. Comparing the grain size of the steel before and after welding, no change was observed. These results indicate that deformation occurred in the Al coating and IMC layers during formation of the wavy interface by high-speed collision of Al and Al-coated steel but did not occur in the steel. Therefore, it is assumed that growth toward steel side of the interfacial wave was prevented by hard IMC layer and steel, resulting in crushing of the ridges of the wavy interface. On the other hand, since interfacial wave can grow toward Al side because thickness of the Al plate is enough larger than the height of the interfacial wave, the base part of the wavy interface may be smooth.
Grain structure of steel at welded area in joint welded at charging energy of 6.0 kJ and plate gap of 1.0 mm.
Figure 9(a) shows an optical micrograph of the welding interface. The arrow indicates the welding interface of the Al and Al-coated steel. A banded structure which is different from both the Al coating and the Al base metal is observed along the wavy interface. This structure had a thickness of approximately 3 µm and contained fine Si particles. Furthermore, voids were observed within the banded structure for certain welding conditions especially large plate gap conditions (Fig. 9(b)).
Optical micrographs of welding interface. (a) Formation of banded structure along welding interface (charging energy: 6.0 kJ, plate gap: 1.0 mm). Arrow indicates location of welding interface. (b) Void formation in banded structure (charging energy: 6.0 kJ, plate gap: 2.0 mm).
Figure 10 shows (a) a TEM bright-field image, (b) a high-angle annular dark-field (HAADF)-STEM image, (c) a superimposed STEM-EDX map of Al, Fe and Si and (d) an O map of the welding interface formed at charging energy of 6.0 kJ and plate gap of 1.0 mm. The result of the STEM-EDX analysis clearly indicated the regions of the steel and IMC layer. The region below the IMC layer was split into two zones depending on the size of the Al grains and the distribution of Si particles. The first zone extended to a distance of approximately 3 µm from the IMC layer/Al coating interface, and was composed of Al grains with a diameter of approximately 1 µm and relatively coarse Si particles with a size more than 1 µm. The second zone was approximately 3 µm or more away from the IMC layer/Al coating interface, and consisted of Al grains with diameters of several hundred nanometers. In the second zone, no Si was observed at this magnification but extremely fine Si was detected by higher magnification analysis, which will be showed in detail later. SEM-EDX results shown in Fig. 6 revealed that fine Si particles were dispersed in the Al coating layer, indicating that Si in the Al coating layer after welding has a detectable size by SEM-EDX (more than 1 µm). Therefore, the region at a distance of approximately 3 µm from the IMC layer/Al coating interface was assumed to be the Al coating layer. Also, the second zone was considered to be the banded structure region formed along the wavy interface. No existence of any oxides was observed because weak O signal was detected uniformly. This is considered to mean that oxide layer which existed at the surface of the metal plates before welding was ejected to outside as the metal jet.
(a) TEM bright-field image, (b) HAADF-STEM image, (c) Al, Fe and Si map and (d) O map of welded area in joint welded at charging energy of 6.0 kJ and plate gap of 1.0 mm.
Figure 11 shows (a) a TEM bright-field image, (b) a HAADF-STEM image and (c) a superimposed STEM-EDX map of Al and Si of the banded structure formed at charging energy of 6.0 kJ and plate gap of 1.0 mm. The banded structure consisted of equiaxed Al grains with a diameter of 500 nm. In the HAADF-STEM image, these grains were surrounded by regions with brighter contrast. STEM-EDX analysis revealed that these brighter contrast is Si particle with diameters of several ten nanometers. Therefore, the banded structure was composed of fine Al grains with extremely fine Si particles distributed along the grain boundary.
(a) TEM bright-field image, (b) HAADF-STEM image and (c) STEM-EDX image of banded structure in joint welded at charging energy of 6.0 kJ and plate gap of 1.0 mm.
The banded structure consisted of fine Al grains with a diameter of 500 nm, with extremely fine Si particles at the grain boundary. Grain refinement of Al was clearly observed, because the Al base metal had grains extended in the rolling direction before the welding. Such fine grains are known to be formed by rapid solidification from the liquid phase and by severe plastic deformation of the solid phase.26,27) Since the main difference between these processes is the phase of the metal being worked, it is necessary to compare the temperature rise by high-speed collision with the melting point of the metals being worked. However, since the welding time is extremely short (several microseconds), it is difficult to measure the temperature directly. Furthermore, since we cannot feel temperature increase when the joint is touched by bare hands immediately after welding, even if a temperature rise occurs, it is limited to a localized region at the welding interface. Therefore, the temperature rise has been investigated by numerical analysis. Nishiwaki et al. performed a numerical analysis of the temperature rise at the Al/Cu wavy interface during magnetic pulse welding, and reported that the temperature of the vortex part of the wavy interface reached approximately 3000 K.28) This temperature is much higher than the melting point of Al (933 K). The melting point of metal under high pressure is known to differ from that under atmospheric pressure, and it is reported that the melting point of Al increases almost linearly from 933 K to 1050 K when the pressure is increased from atmospheric pressure to 2 GPa.29) The collision pressure during magnetic pulse welding, as calculated based on the velocity of the flyer plate, is approximately 1 GPa,3) and according to Fischer’s report,29) the melting point of Al at this pressure can be estimated to be approximately 1015 K. We previously performed a numerical analysis of the temperature distribution near the magnetic pulse welded interface between Al and Al, and found that a region with higher temperature than the melting point of Al was formed continuously along the wavy interface.30) Furthermore, voids were formed in the banded structure (Fig. 9(b)). As shown later in Fig. 13(d), in these void regions, particles with a diameter of 500 nm were observed after tensile-shear test. In addition, any traces of deformation or fracture were not observed at the particle surface. If these voids were formed by cracking, surface indicating ductile or brittle fracture will be observed. The existence of the particles with smooth surface in the void regions is considered to mean that the local molten metal dendritically solidified and grew from the interfaces with both solid-state base metals, and the tips of the dendrite never touched at the final solidified part. The non-touch of the tips of the dendrite is assumed to indicate that the void was formed by solidification shrinkage. The thickness of the region with higher temperature than the melting point of Al indicated by the numerical analysis was less than several ten micrometers,30) and since this is much smaller than the thickness of the base metal (0.5 mm). Therefore, the local molten region will solidify rapidly. According to a report by Sapanathan et al.,31) the cooling rate at the magnetic pulse welded interface can be estimated in the range of 107 to 108 K/s. Since the banded structure consisted of Al grains with a diameter of 500 nm (Fig. 11), it is considered that the grains become smaller due to rapid cooling. Therefore, the banded structure is considered to be formed by localized melting and rapid cooling of the Al coating layer and the Al near the welding interface.
3.3 Effects of charging energy and plate gap on joint strengthFigure 12(a) shows the relationship between the fracture load and charging energy obtained by tensile-shear test of joints fabricated at the plate gap of 1.0 mm. The fracture load was constant at approximately 800 N regardless of the charging energy, and these values are similar to fracture load of the Al base metal. Figure 12(b) shows the fracture part of the specimen after the tensile-shear test. For all specimens, fracture occurred in the Al base metal far away from the welded area.
(a) Relationship between charging energy and fracture load. Dotted line indicates fracture load of the Al base metal. (b) Macroscopic appearance of joint after tensile-shear test.
Figure 13(a) shows the relationship between the fracture load and plate gap obtained by tensile-shear test of joints fabricated at the charging energy of 6.0 kJ. The fracture load was approximately 800 N for the joint welded with a plate gap of 1.0 mm, and decreased with increasing the plate gap. Observations of the fracture part indicated that whereas the joint welded at the plate gap of 1.0 mm was fractured in the Al base metal as shown in Fig. 12(b), the welded area was peeled for the joints welded at the plate gap of 1.5 mm and 2.0 mm (Fig. 13(b)). When the surface of the peeled region of the Al-coated steel side was observed, it appeared brighter than the surface of the Al-coated steel base metal, as indicated by the arrow in Fig. 13(b). Figure 13(c) shows an optical micrograph of cross section of the bright region. At the surface of the bright region, the banded structure was clearly observed. When the surface of the bright region was observed by SEM, particles with smooth surface were observed in some regions as shown in Fig. 13(d). The existence of these particles indicates that these regions were void regions in the banded structure. Therefore, it is considered that the cracks generated by the tensile-shear test propagated by connecting the void regions.
(a) Relationship between plate gap and fracture load. Dotted line indicates fracture load of the Al base metal. (b) Macroscopic appearance of joint after tensile-shear test. (c) Optical micrograph of cross section of Al-coated steel. (d) SEM image of fracture surface.
Figure 14 shows the relationship between the charging energy or plate gap and the ratio of the total horizontal void width Wv (summed over all voids, see arrows in Fig. 9(b)) to the width of the welded area. The void ratio increased slightly with increasing the charging energy. In contrast, the void ratio increased significantly with increasing the plate gap. The increase of the void ratio is considered to result in the peeling of the welded area.
Relationship between charging energy or plate gap and width ratio of void to welded area.
In this study, we investigated the welding interface and strength of magnetic pulse welded Al/Al-coated steel joint, and obtained the following results.
Part of this research received assistance from a Light Metal Educational Foundation scholarship (2018 to 2020). We would like to express our gratitude.
