Lap Joint of 6061 Aluminum Alloy Sheet and DP590 Steel Sheet by Magnetic Pulse Welding and Characterization of Its Interfacial Microstructure
2019 年 60 巻 1 号 p. 121-129
詳細
2019 年 60 巻 1 号 p. 121-129
Lap joint sheets of 6061-T6/SPCC and 6061-T6/DP590 (Dual phase) steel were fabricated by magnetic pulse welding (MPW). Strong lap joints were achieved at discharge energy W of >2.0 kJ and gap length d of 1.0 mm for the 6061-T6/SPCC, and W of 3.0 kJ and d of 1.4 mm for the 6061-T6/DP590 steel, respectively. This result suggested that the high-collision speed is required for lap joint of the 6061-T6/DP590 steel compared with that of the 6061-T6/SPCC. Weld interface showed wavy joint interface and weld width of the lap joint sheets tend to increase with increasing of discharge energy for MPW. An intermediate layer consisted of FeAl, Fe2Al5 and FeAl3 was recognized at the weld interface discontinuously, due to localized melting and a subsequent high rate cooling of molten Fe and Al confined to the weld interface. Furthermore, work hardening by accumulated plastic strain and grain refinement of Al and Fe at the welded interface were recognized by SEM-EBSD.
From microstructure observation, strong lap joint of the 6061-T6/DP590 steel by MPW was thought to be due to an increase in weld width, an anchor effect, and strengthening of the weld interface by work hardening and grain refinement.
This Paper was Originally Published in Japanese in J. JILM 68 (2018) 141–147.
Global warming countermeasures have been implemented recently in transportation equipment, such as automobiles, railway vehicles, and aircraft. In the automotive field, weight reduction of vehicles has been established to reduce particularly CO2 emissions.1) In vehicle weight reduction, high-tensile strength steel sheets (590 to 1490 MPa) have been proactively adapted to automotive parts.2–4) Weight reduction by decreasing plate thickness is assumed to continuously progress to improve collision safety. However, plate thickness reduction has a limitation due to the rigidity of vehicle parts. If reducing the weight by 30% or higher than the existing ones is necessary, then partially considering multimaterialization by using lightweight materials is important.5) With regard to practical application of this lightweight material, the considered substitute material is aluminum alloy (hereafter Al alloy). When an Al alloy is partially used, different metal welding technologies for steel and Al alloy must be utilized.
The following methods used in joining steel and Al alloy have been investigated. Melt welding methods include spot6) and laser7) welding. Meanwhile, solid-state welding methods comprise friction welding,8,9) explosive welding,10) diffusion bonding,11) roll bonding,12) stud junction,13) mechanical joining method (self-piercing rivet14) and mechanical clinch15)), and adhesion method. When using melt welding method, brittle intermetallic compound is formed at the weld interface, strong bonding strength is difficult to obtain, and galvanic corrosion is generated because of the contact of dissimilar metals.16) Therefore, the joining of steel and Al alloy through a combination of mechanical and adhesion methods was promptly applied. The joining of hatchback doors via friction stir spot welding has also been practically considered as one of solid-state welding methods.9,17) Meanwhile, magnetic pulse welding (MPW) is also a type of solid-state welding method.18) MPW is a high-speed joining method that uses electromagnetic force. Magnetic flux is rapidly applied to a flyer sheet, and it collides with a fixture sheet at high speed to join these sheets. Its joining mechanism is similar to explosive welding.19) The joining of similar metals (e.g., pure Al, Cu,20,21) and 2000 series Al alloy22) sheets) and dissimilar metals (e.g., pure Al and Cu23) sheets and 6000 series Al alloy and steel24) sheets) were reported in previous works. It was known that a strong bonding that fractures on the weaker base material side rather than the weld interface is possible depending on the joining conditions, and that of an intermediate layer is partially formed at the weld interface in joining dissimilar metals.23,24) A wavy pattern similar to that in the weld interface in explosive welding is observed from the weld interface of the lap joint sheet wherein a good bonding state is obtained. However, details of the joining properties and joint interface structures are not identified yet. In addition, no report existed yet on the joining of Al alloy and high-tensile steel sheets via MPW.
In this study, the joining conditions of cold-rolled steel (i.e., SPCC) and 6061-T6 sheets via MPW were determined. Furthermore, the joining of dual phase (DP)590 steel and 6061-T6 sheets was established. The joining properties of the lap joint sheet were evaluated through the tensile shear test, and the weld interface of the lap joint sheet was examined through electron microscopy. The formation of the weld interface was also discussed.
In MPW, two metal (i.e., fixture and flyer) sheets were installed with a gap and then joined on the coil. The electric energy charged in the capacitor is one of the joining conditions and is expressed as discharge energy W. Given that the coil has a small cross-sectional area at the center, the current density at the center increases. Thus, high-density magnetic flux was generated around the central area when large pulse current flows. When the magnetic flux crosses the flyer sheet, an eddy current is induced into the sheet to prevent the penetration of the magnetic flux. The metal with high conductivity has a large eddy current value. This induced eddy current and high-density magnetic flux intersect to generate an upward electromagnetic force inside the flyer sheet. This sheet subjected to the electromagnetic force deforms and collides at a high speed and is lap joined to the fixture sheet. The coil used was a flat, E-shaped, one-turn coil made of chromium copper. The capacity of the capacitor was 400 µF, and the frequency was 33 kHz. A detailed joining principle of MPW was reported by Okagawa et al.25) A 6061-T6 sheet was used as a flyer sheet, and an SPCC sheet or a DP590 steel (JSC 590Y) sheet was employed as a fixture sheet. A JSC 590Y is a DP steel with a tensile strength of 590 MPa class composed of ferrite and martensite phases. The dimensions of 6061-T6 and steel sheets were both 80 × 100 × 1.0 mm. These sheets were joined through MPW perpendicular to the rolling direction. Tables 1 and 2 show the chemical compositions of 6061-T6, SPCC, and DP590 sheets used in this experiment. Table 3 shows the tensile properties of these sheets. The W was set to 1.1–3.0 kJ, and the experiment was conducted with the d adjusted to 1.0 or 1.4 mm. Figure 1(a) shows the 6061-T6 and SPCC lap joint sheets (hereafter abbreviated as 6061-T6/SPCC). Figure 1(b) shows the cross section of the lap joint sheet. As shown in the figure, the flyer sheet during MPW receives electromagnetic force and is deformed.
(a) Macroscopic appearance of a 6061-T6/SPCC lap joint sheet fabricated by MPW. (b) Cross-sectional view of the 6061-T6/SPCC lap joint sheet. (c) Dimension of specimen for tensile shear test. (d) Macroscopic appearances of specimens before and after tensile shear test.
The bonding strength of the lap joint sheet was evaluated through a tensile shear test via an Instron-type universal testing machine. A test piece of the JIS 13 B (1/2 reduction) was sampled from the prepared lap joint sheet by using a wire electric discharge machine. The dimensions of the specimen are shown in Fig. 1(c). In the tensile shear test, the crosshead speed was set to a constant of 1.0 mm/min. The test was performed in parallel with the rolling direction of the sheet at room temperature. In addition, the auxiliary plate was sandwiched and fixed together with the tensile shear specimen in order to apply a shearing force to the joint during adjustments. Figure 1(d) shows the tensile shear test piece and its appearance before and after the test. The tensile shear test for the lap joint sheets prepared for each W was performed three times at room temperature. When the bonding strength is low, the test piece is peeled off at the weld area, whereas the test piece is fractured at the base metal (6061-T6 sheet) when the bonding strength was high. In addition, a hardness test in the vicinity of the weld interface was performed using a micro-Vickers hardness tester with an indentation load of 3 gf.
2.2 Interfacial microstructure observation of the lap joint sheetA microstructure observation of the weld interface was established on a cross section perpendicular to the seam direction. This observation was realized using a scanning electron microscope (SEM: SU 6600). The chemical composition was also evaluated using an energy dispersive X-ray spectroscopy (EDS) attached to the SEM. A crystal orientation analysis via an electron backscatter diffraction (EBSD) was performed using the AZtec-HKL EBSD analysis software manufactured by OXFORD in combination with SEM (JSM7800). A detailed structure observation of the weld interface was achieved through a high-angle annular dark-field scattering method by using a transmission electron microscope (TEM: JEM 2100 F). The TEM specimens were prepared using an ion slicer.
Figure 2(a) shows the dimensions of the flat, E-shaped, one-turn coil, and Fig. 2(b) shows the schematic view of the weld area of the lap joint sheet produced via MPW. When the flyer sheet receives an electromagnetic force and collides with the fixture sheet, the flyer sheet deforms from the center of the coil and then collides with the fixture sheet. Similar to explosive welding, the joining via MPW was performed in a region where the moving speed of the collision point and collision angle between the fixture and flyer sheets satisfies the wave-like pattern condition of the weld interface. Therefore, although the collision speed of the flyer sheet on the coil center line was fast because of the small collision angle, the joining at the initial collision point (at the center line) was not successful. As the collision point moves to the left and right from the center line, the flyer sheet is continuously deformed, the gradient of the flyer sheet to the left and right becomes steep, the increase of the collision angle becomes conspicuous, and the joinable angle is 5° or more, the joining starts. Therefore, the weld areas were formed symmetrically from the center line and joined in a seam shape. The deformation mode of the flyer sheet was closely related to the joining condition. Hence, the change in the d affected the joining condition, such as the weld area and its width.
(a) Dimension of E-shaped one turn coil. (b) Schematic illustration of welding positions for lap joint sheet prepared by MPW.
Figure 3(a) shows a backscattered electron (BE) image of a cross section (W = 3.0 kJ). As presented in the figure, the upper side is the fixture sheet (i.e., SPCC), whereas the lower side is the flyer sheet (i.e., 6061-T6). After the flyer sheet collided with the fixture sheet during MPW, the joining progressed from the center line of the coil (as shown in the figure) toward the joint end side. Therefore, the two places on the right and left of the area (denoted by white line in the figure) were linearly and symmetrically joined with respect to the center line. Figures 3(b), 3(c), and 3(d) show the BE images of the weld areas of the lap joint sheets fabricated at W = 1.2, W = 2.0, and W = 3.0 kJ, respectively. The left and right sides of the figure are the coil and joint end side. A wavy pattern is observed at the weld interface. Periodical wavy patterns are observed from the weld interface of the lap joint sheet that obtained a good joining state similar to explosive welding.19) Generally, the process of wave pattern formation due to an impact force, such as explosive welding, is the same as the wave pattern observed at the weld interface, and it is attributed to hydrodynamic instability phenomenon of the material at high pressure and high speed. However, its importance was not fully elucidated yet, and only discussions of Karman vortex and Helmholtz instability were conducted.26,27) In order to evaluate the impact joining mechanism in explosive welding, Kumai et al. performed impact simulation by using the smoothed particle hydrodynamics method. This method is a type of particle method by using meshless impact analysis technique.28) The researchers simulated the collision phenomenon and impact joining process of similar and dissimilar metal sheets by using Cu/Ni and Al/Fe joint sheets as models. They succeeded in reproducing the metal jet release behavior of high-speed inclined collision and the wavy pattern formation behavior at the metal surface. In the case of explosive welding, adjusting the collision speed according to the amount of explosive is possible. Moreover, performing a joining for various metal sheets at impact speeds of several hundred meter per second or 1000 m/s or higher (with the collision pressure of 10 GPa or higher) has high probability.29) Although the MPW is a joining technique that uses impact force, it is a line joining method with weak impact force that uses electromagnetic force, which can be generated using a minor device compared with explosive welding. The white line in the figure shows the weld width. As the W increases, the weld width widens. The enlarged images of the weld interface are enclosed by broken lines at the right side of each figure. Moreover, an intermediate layer with a thickness of about 5–10 µm was partially formed in the sheet thickness direction, as indicated by arrows at the weld interface. A detailed observation result of an intermediate layer will be discussed in the following section.
(a) Cross-sectional BE image of the 6061-T6/SPCC lap joint sheet. BE images of right side of the lap joint sheet prepared by (b) W = 1.2 kJ, (c) W = 2.0 kJ, and (d) W = 3.0 kJ, respectively. Enlarged BE images enclosed by white frames in BE images of (b)–(d) are shown in right side, respectively.
Figure 4(a) shows the weld width measured from the SEM image for the lap joint sheet prepared for each W. ◇ indicates the weld width on the left side from the center, △ denotes the weld width on the right side, and ● is the total of the weld widths on the right and left sides, which are indicated by numerals in the figure. As shown in the figure, the weld width of the left and right sides is substantially the same in each W, and the weld width increases from 1.9 to 2.6 mm as W increases. Figure 4(b) shows the relationship between the fracture load and the W produced in the lap joint sheet. The fracture load of the 6061-T6 sheet is also indicated by a solid line in the figure. Although the lap joint sheet produced at W = 1.1 kJ was able to be welded, but the lap joint sheet was peeled off when cutting into the shape of the test piece with the electric discharge machine. The lap joint sheets produced with W = 1.2–3.0 kJ were tested as well. The fracture load is indicated by a white bar for the sample with interfacial failure, and the sample broken with the base metal of the 6061-T6 sheet is denoted by a black bar. When W = 1.3 kJ, an interface fracture occurs at 1.7 kN. However, the base material fractures with the 6061-T6 sheet when W are 2.0 and 3.0 kJ. These results showed that a strong lap joint sheet, which fractures at the base material of the 6061-T6/SPCC, can be produced if the W is 2.0 kJ or higher. As shown in Fig. 4(a), the bonding strength becomes high as the weld width widens, thereby indicating a highly stable joining state.
(a) Relationship between weld width and discharge energy for the 6061-T6/SPCC lap joint. (b) Fracture load of tensile shear test for the 6061-T6/SPCC lap joint sheets welded at various discharge energies.
On the basis of these results, obtaining a strong bonding was possible when the joining condition of the W is 2.0 kJ or higher. Hence, the production of the 6061-T6/DP590 steel was achieved when W = 3.0 kJ. Although the joining was possible, the joint was peeled off consequently at 1.3 kN based on the tensile shear test. Thus, a strong bond that fractured the base metal was not produced even at a relatively high W. Focusing on the tensile properties (rolling direction) in Table 3, the yield strength of the DP590 steel sheet is higher than that of the SPCC sheet. On the basis of the previous studies, we produced a lap joint by using the 2024-T3 sheet as the flyer and 2024-T3 and 2024-T6 sheets as the fixture. Hence, a fast collision velocity was required for the joining of the 2024-T6 sheet with high yield strength.30) Therefore, in the preparation of the lap joint sheet, the joining of materials with the combination of high strength and poor ductility through MPW is difficult. The increase in W promotes collision speed, thereby increasing the collision pressure when the flyer sheet collides with the fixture sheet. Therefore, a high-collision pressure is required for the joining of a steel sheet with high strength as described previously. In such a case, there is a method to increase the collision speed by widening the d as another joining condition. The reason was that when the d widens, the flyer sheet can sufficiently expand until it is deformed toward the fixture sheet side by electromagnetic force. When the 2024-T3 sheet is used as the flyer sheet, the collision speed becomes fast as d increases, reaches a maximum of 380 m/s at d = 2.0 mm, and then decreases again.30) Therefore, the d was further expanded by 0.4 mm to obtain d = 1.4 mm, and the experiment was performed under the condition of W = 3.0 kJ similar to the preparation of the 2024-T3/2024-T6. Figures 5(a) and 5(b) show the BE images of the weld interface of the 6061-T6/DP590 steel produced with d = 1.0 and d = 1.4 mm, respectively. These figures show the right side of the joints. The weld width indicated by white line in the figure increased by increasing of d. The sum of the weld widths of the left and right two points at d = 1.0 and d = 1.4 mm, which were 2.0 and 2.9 mm, respectively. The right side of each figure shows an enlarged view of the weld interface surrounded by broken lines. Evidently, an intermediate layer with a thickness of about 5–10 µm is formed discontinuously in the sheet thickness direction similar to the weld interface of the SPCC sheet at any joint interface, as indicated by the arrows.
(a) Cross-sectional BE image of right side in the 6061-T6/DP590 steel lap joint sheet prepared by (a) d = 1.0 mm (W = 3.0 kJ), and (b) d = 1.4 mm (W = 3.0 kJ), respectively. Enlarged BE images enclosed by white frames in BE images of (a) and (b) are shown in right side, respectively.
Figure 6(a) shows the fracture load of the tensile shear test for the 6061-T6/DP590 steel joined at various gap lengths. A solid line in the figure indicates the fracture load of the 6061-T6 sheet. According to the tensile shear test, the lap joint sheet produced at d = 1.0 mm peeled at the joint area with a load of 65% of the 6061-T6 sheet. However, when d = 1.4 mm, fabricating a strong bonding lap joint sheet is possible because of the fracture at the base metal of the 6061-T6 sheet. Figure 6(b) shows the macroscopic appearances of specimens obtained via tensile shear test. From this image, after the tensile test, it is found that it breaks at the 6061-T6 sheet. Similarly, in the case of the SPCC/6061-T6, it fractures at the 6061-T6 sheet. The comparison of the tensile strength of each sheet in Table 3 shows that the SPCC sheet is 290 MPa and the 6061-T6 sheet is 305 MPa. The tensile strength of the SPCC sheet is low, and it may be fractured at the SPCC sheet. The 6061-T6 sheet is a flyer plate, and it deforms through electromagnetic force. At the initial stage of the deformation during MPW, the central portion of the flyer sheet collided toward the fixture sheet while maintaining the horizontal position. Meanwhile, the boundary between the flyer sheet and inclined surface through which the bending wave passes becomes the shape of a plastic hinge. The fracture point was the part where the sheet is deformed similar to a hinge, as indicated by the arrow at the top of Fig. 6(b). Although this part was work hardened, the sheet thickness was slightly thin. Hence, a fracture in this part is possible.
(a) Fracture load of tensile shear test for the 6061-T6/DP590 steel lap joint sheets welded at various gap lengths (W = 3.0 kJ). (b) Macroscopic appearances of specimens after tensile shear test.
The results showed that widening the weld width and then improving the bonding strength are possible through the MPW at a high-collision speed with widening of the d. Therefore, a strong bonding between 6061-T6 and DP590 steel sheets is possible.
3.3 Microstructure observation of the weld interface of the 6061-T6/DP590 steelA microstructure observation of the weld interface and a crystal orientation analysis through EBSD for the lap joint sheet were performed. Figure 7 shows (a) the SEM image, (b) Fe map, (c) Mn map, (d) Al map, (e) IQ map, (f) phase map, (g) IPF map, and (h) KAM map, investigated at the weld interface of the 6061-T6/DP590 steel (W = 3.0 kJ, d = 1.4 mm). As shown in the SEM image of Fig. 7(a), a wavy pattern caused by plastic flow at the time of collision between the fixture and flyer sheets is observed at the weld interface. The element maps of Figs. 7(b) Fe, (c) Mn, and (d) Al clearly show the weld interface between the DP590 steel and 6061-T6 sheets. In addition, a region in which all Fe, Mn, and Al elements are partially distributed inside the wavefront was observed in the white frame, and the thickness of the intermediate layer was found to be from 5 to 10 µm. As shown in the IQ map of Fig. 7(e), a region with a low IQ value (indicated by a black region) corresponds to the intermediate layer, suggesting the formation of fine crystal grains or amorphous phase. A region where Fe and Al was directly bonded without forming an intermediate layer was also observed in these EDS maps. The boundary line (grain boundary) where the orientation difference of crystal grains is 15° or higher is shown in the phase and IPF maps of Figs. 7(f) and 7(g). At the weld interface in the vicinity of the intermediate layer, fine grains with random orientations different from the parent phase were observed at the area indicated by arrows. Several studies were conducted on the grain refinement of such a weld interface.23,31–33) The reason was considered as that at the time of local dissolution via welding, the recrystallization occurs at the interface of Fe or Al by the accumulation of strain and heating. The KAM map in Fig. 7(h) shows the average value of the crystal orientation difference between the adjacent pixels at each measurement point. The distribution of misorientation values of up to 5° were also shown in the figure. On the basis of the degree of strain accumulation in the KAM map, a relatively large amount of strain was introduced to the weld interface in the DP590 steel side of about 10 µm depth and in the 6061-T 6 side of about 5 µm depth.
(a) SEM image of the weld interface of the 6061-T6/DP590 steel lap joint sheet (W = 3.0 kJ, d = 1.4 mm). EDS mappings for (b) Fe, (c) Mn, and (d) Al taken from same area of (a). (e) IQ map, (f) Phase map, (g) IPF map and (h) KAM map, investigated by same area of (a).
Figure 8 shows (a) the SEM image, (b) Fe map, (c) Mn map, (d) Al map, (e) IQ map, (f) phase map, and (g) IPF map, investigated at the weld interface between Fe and Al (without the intermediate layer) of the 6061-T6/DP590 steel (W = 3.0 kJ, d = 1.4 mm). As shown in the SEM image and EDS maps, Fe and Al have direct contact, and no intermediate layer is observed similar to that in Fig. 7. As denoted by arrows, the grains are refined in the range of 1 µm on the DP590 steel side from the weld interface and 3 µm on the 6061-T6 side. On the basis of the experiments and calculation results, the wavy pattern formation at the weld interface of impact welding, such as explosive welding or MPW, generated temperature and pressure distribution.34) Hence, the temperature and pressure generated by the collision impact were relatively small in the region where local dissolution is not made at the weld interface, as shown in Fig. 8. On the basis of these results, grain refinement was considered to occur at any weld interface regardless of whether or not the intermediate layer is formed.
(a) SEM image of the weld interface of Fe and Al (without intermediate layer) in the 6061-T6/DP590 steel lap joint sheet (W = 3.0 kJ, d = 1.4 mm). EDS mappings of (b) Fe, (c) Mn, and (d) Al taken from same area of (a). (e) IQ map, (f) Phase map, and (g) IPF map, investigated same area of (a).
Figure 9 shows the SEM image of the indentations after the micro-Vickers hardness test performed around the weld interface. The hardness was measured from the upper DP590 steel sheet to the lower 6061-T6 sheet, and a total of 30 points, which are six rows at five positions of the intermediate layer and weld interface, were measured. The hardness values are shown beside the indentation. The hardness value of the DP590 steel side varies from 233 to 448 Hv. Although the hardness value considerably varies in the DP590 steel because of the partial existence of martensite phase, these values around weld interface were 325, 396, and 448 Hv (Fig. 7(f)). The result showed that these values are harder than the inside of the sheet. The same trend was also observed on the 6061-T6 side.
SEM image of the weld interface in the 6061-T6/DP590 steel lap joint sheet (W = 3.0 kJ, d = 1.4 mm) after micro-Vickers hardness test.
As shown in the IPF and KAM maps of Figs. 7(g) and 7(h), respectively, high hardness value is exhibited at the weld interface because of the structural change, such as work hardening and grain refinement. As shown in Figs. 7(g) and 7(h), not only the anchor effect but also the hardening of the base material wound similar to a vortex due to the plastic flow at the time of joining observed at the weld interface, which is thought to contribute to the strengthening. As shown in the white frame of the figure, the hardness values of the intermediate layer are 524, 566, and or 694 Hv. Unlike the DP590 steel sheet, the 6061-T6 sheet and its hardened region were recognized to have even higher hardness values. The microstructure observation was implemented using electron microscopes in order to identify the details of this intermediate layer.
Figure 10 shows the BE image of the intermediate layer near the weld interface of the 6061-T6/DP590 steel. An EDS analysis was performed for points 1–5, as shown in the figure. The spectrum obtained at each point is shown on the right side of the figure. Analysis points 1 and 5 were the fixture and the flyer sheets, respectively, composed mainly of Fe or Al. Meanwhile, analysis points 2, 3, and 4 in the intermediate layer were composed of both Al and Fe elements. In addition, the composition ratios of Fe and Al were estimated to be approximately 4:6 (atomic ratio). Fine grains of 1 µm or less with white contrast were observed in the intermediate layer, as indicated by arrows in the figure.
BE image of weld interface around intermediate layer in the 6061-T6/DP590 steel lap joint sheet. EDS spectra obtained from points 1 to 5 are also indicated.
A microstructure observation was implemented using a high-angle annular dark-field scattering transmission electron microscopy (HAADF-STEM) method in order to investigate this intermediate layer in detail. Figure 11(a) shows the HAADF-STEM image in the vicinity of the intermediate layer. The HAADF-STEM method is started by observing a single-atom image according to Crewe.35) It is a microscopy technique that detects only electrons scattered above a certain angle among the electrons that pass through the electron beam during scanning by using an annular detector to construct an image. Elastic scattering decreases with the increasing scattering angle, but thermal diffuse scattering predominates at high angles. Given that the thermal diffuse scattering is proportional to almost a square of atomic number Z, the image shows a contrast, remarkably emphasizing the difference in atomic species. Therefore, the white and dark contrasts observed in this image indicate the Fe and Al, respectively. Although the intermediate layer is gray and considered to contain both elements, the intermediate layer has a marble contrast in which an irregularly mixed white and black contrast, was observed in the interior. Figure 11 shows (b) the Fe map, (c) Mn map, (d) Al map, and (e) O map examined in the area surrounded by white frame in Fig. 11(a). As shown in the EDS map, the intermediate layer is regarded as an alloy because it is composed of Fe, Mn, and Al. Moreover, a relatively large amount of Fe is distributed in the region, which is indicated by white contrast in the HAADF-STEM image. In addition, the Fe concentration is higher at the DP590 steel side than at the 6061-T6 one. Therefore, distribution of the composition was considered to occur within the intermediate layer. As shown in the oxygen map, oxygen concentration was not observed in the weld interface, and presence of oxides could not be confirmed also at the intermediate layer. In the intermediate layer, the portion indicated by arrows is where the compositions of Fe and Mn were larger than that of the Al. Therefore, Fe fragments were found to be 1 µm or less which existed in the intermediate layer. This result was similar to the fragments observed in the intermediate layer of Fig. 10. The electron diffraction (ED) patterns obtained from A and B, with relatively high or low Fe concentration regions in the intermediate layer, were evident. On the basis of the obtained ED pattern, FeAl was formed in A and Fe2Al5 and FeAl3 were formed in B. Therefore, the intermediate layer was mainly composed of an intermetallic compound and Fe fragments were only partially mixed. The compositional ratio (atomic ratio) of Fe and Al was 4:6 in the chemical composition of the intermediate layer examined by the EDS analysis in Fig. 10. Given that it is close to the composition range formed by Fe2Al5 and FeAl3 in the Fe–Al equilibrium phase diagram, the ED pattern analysis result was consistent with that of EDS analysis.
(a) HAADF-STEM image of weld interface around intermediate layer in the 6061-T6/DP590 steel lap joint sheet. ED patterns taken from area A and B are also shown. EDS mappings of (b) Fe, (c) Mn, (d) Al, and (e) O taken from intermediate layer enclosed by white frame in HAADF-STEM image of (a).
In previous studies, we reported that when joining an Al sheet as a flyer sheet to a Ni-plated Cu sheet via MPW, an intermediate layer composed of Ni and Al is partially formed at the weld interface.23) An amorphous phase composed of Ni and Al is formed in the intermediate layer, and dissolution/solidification is assumed to occur at the weld interface. From the alloy composition of the produced amorphous phase, the estimated cooling rate was 106 K/s or higher. Moreover, Ni fragments, which are regarded as undissolved plating, were observed in the intermediate layer. Meanwhile, Fe fragments on the fixture sheet side were present in the intermediate layer. The marble contrast was caused by composition difference between Fe and Al (Fig. 11). The weld interface was considered to partially dissolve, and then it was cooled at an extremely high speed. Therefore, an intermediate layer that exhibited composition distribution, such as marble type, was formed by quenching and solidifying without sufficient diffusion for alloying by Fe and Al and solidifying with a uniform composition. In addition, the intermetallic compound formed in the intermediate layer was refined because of rapid cooling after solidification. This result was consistent with the low IQ value in the intermediate layer region through EBSD.
Figure 12 shows the SEM observation on the DP590 steel sheet of the lap joint sheet (W = 3.0 kJ, d = 1.0 mm) peeled at the weld interface after performing a tensile shear test. As shown in the SEM image of Fig. 12(a), roughness is observed in the region corresponding to the weld width. Evidently, fracture occurred at the weld interface. Figures 12(b) and 12(c) show the EDS maps of Al and O, respectively, from the same area of Fig. 12(a). As shown in the Al map of Fig. 12(b), the Al of the 6061-T6 sheet adheres to the DP590 steel sheet because the Al concentration in the fractured area is relatively high. As shown in Fig. 12(c), the oxygen concentration in the fractured area is relatively small compared with that on the surface of the DP590 steel sheet. Hence, the sheet was fractured inside the Al grain instead of the weld interface. Figure 12(d) shows an enlarged SEM image of the fractured area. Dimples were also observed in the SEM image, and the Al is ductility deformed to the shear direction. On the basis of these results, the sheet did not fracture at the weld interface reinforced through work hardening when the lap joint sheet fractures at the interface, and it fractured mainly near the interface inside the Al grain of the 6061-T6 sheet.
(a) SEM image of joint interface fracture area on the DP590 steel sheet. EDS mappings of (b) Al and (c) O taken from same area of (a). (d) Enlarged SEM image of joint interface fracture area.
The results showed that strongly bonding (fractures at the base material) not only the 6061-T6/SPCC but also the 6061-T6/DP590 steel is possible can be prepared by the MPW. On the basis of the microstructure observation from the electron microscope, the weld interface formed a wavy pattern, and oxides were not noticed. An interface where Fe and Al are in direct contact or a weld interface via an intermediate layer composed of an intermetallic compound produced by local dissolution was observed. Furthermore, microstructural factors, such as work hardening near the weld interface and grain refinement, that are assumed to strengthen the weld interface were confirmed. Generally, the formation of the interface structure due to the impact force via the MPW contributes to high bonding strength.
Hence, joining sheets with high strength and low ductility through MPW is difficult. However, increasing the collision speed was possible by adjusting the W and d. Consequently, as in the joining of 2024-T3 and 2024-T6 sheets and 2024-T3 and 7075-T6 sheets, a strong bonding between the 6061-T6 and DP590 steel sheets was possible. In the future, in the automotive field, the use of high-strength high-tensile steel sheets is expected to develop. By clarifying the joining factors and the joining mechanism in these practical sheets, the joining guidelines by the MPW of high-tensile strength steel and Al alloy sheets can be developed.
The authors gratefully acknowledge the financial support of Grant-aided Project (2015–2016) from the Light Metal Educational Foundation of Japan.
