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WELDING RESEARCH Application of Magnetic Pulse for Aluminum Alloys and SPCC Steel Sheet Joints

Welding process parameters and characteristics were developed for a variety of similar and dissimilar metals magnetic pulse welds

BY T. AIZAWA, M. KASHANI, AND K. OKAGAWA

ABSTRACT. Magnetic pulse welding marized in Table 1. H-shaped plates, which we call the double (MPW) is a cold process for welding con- Magnetic pulse welding (MPW) pro- layer H-shaped coil. The overlapped sheet ductive metals to similar or dissimilar ma- vides an excellent tool for achieving alu- workpieces were inserted between these terials. Magnetic pulse welding uses mag- minum to steel sheet joints. Magnetic two H-shaped plates. When the high cur- netic pressure to drive the primary metal pulse welding is a solid-state joining rent flows through the coil, it can create against the target metal sweeping away process of conductive metals. The welding the magnetic field to both sides of the surface contaminants while forcing inti- process is heat-free, which can eliminate overlapped sheet workpieces, and as a re- mate metal-to-metal contact, thereby pro- localized annealing. This paper describes sult, the sheet metals were welded in the ducing a solid-state weld. In this paper, the MPW formation in the dissimilar joining seam state. The magnetic flux produced by MPW method and its application for sev- of aluminum alloys (A1050, A2017, this type of coil is shown in Fig. 1A. In this eral aluminum alloys (A1050, A2017, A3004, A5182, A5052, A6016, and A7075) method, the eddy currents that flow in A3004, A5182, A5052, A6016, and A7075) and steel plate cold rolled commercial both sheets are considerably different and joints in steel (SPCC) sheets were in- grade (SPCC). when dissimilar sheet metals like Al/steel vestigated, and the welding process para- A typical MPW system includes a power sheets are welded. And, also, the thickness meters and characteristics are reported. supply, which contains a bank of capacitors, of the workpieces was limited by the space a fast switching system, and a coil. The parts between two H-shaped plates. Therefore, Introduction to be joined are inserted into the coil, the ca- for more applications, some contrivance pacitor bank is charged, and the low induc- or improvement was needed. These ex- One of the most difficult problems in tance switch is triggered by a pulse trigger perimental results and welding character- welding is to weld dissimilar metals such as system and the current flows through the istics for several samples such as Al-Al aluminum and steel together. Hybrid coil. When current is applied to the coil, a (Ref. 4), Al-Cu (Ref. 5), Al-Mg, Al-Ti, and structures of aluminum alloy and steel are high-density magnetic flux is created Al-Fe (Ref. 6) were reported in previous suggested for reducing the weight of auto- around the coil, and as a result an eddy cur- papers. mobiles to improve fuel efficiency and rent is created in the workpieces. The eddy In the present experiment, a new coil control air pollution. Therefore, the join- currents oppose the magnetic field in the was designed to improve the welding char- ing of steel and aluminum alloy in differ- coil and a repulsive force is created. This acteristics of Al alloy and SPCC-steel sheet ent shapes is receiving attention. How- force can drive the workpieces together at joints. This new coil is a one-layer E-shaped ever, steel and aluminum are not an extremely high rate of speed and creates flat coil that the overlapped sheet work- compatible metals as far as an explosive or impact type of weld. For pieces were put on the one side of the coil is concerned. The reason for this is attrib- more conductive metals, such as aluminum (Fig. 1B). This type of coil can be designed uted to the large difference between their and copper, less energy is required to for applications ranging from short and melting points (660°C for Al and 1497°C achieve a weld. The conventional MPW small to large and long workpieces and also for steel), the nearly zero solid solubility of method with solenoidal coil is used for join- T-shaped joints with higher weld quality. iron in aluminum, and the formation of ing tubular parts and its features are most brittle intermetallic compounds such as well known (Refs. 1–3). However, a few pa- Experimental Procedures Fe2Al5 and FeAl3. Further, differences in pers on MPW of sheet workpieces have their thermal properties such as expansion been reported. MPW Principle coefficients, conductivities, and specific In a previous paper, we proposed a new heats lead to internal stresses after fusion one-turn flat coil instead of the solenoidal The principle of the magnetic pulse welding. Therefore, fusion welds of steel coil. This coil consisted of upper and lower welding method for one Al/Fe sheet sam- and aluminum suffer from heavy cracking ple is shown in Fig. 2. When a high current with brittle failure in service. The material is applied to the coil, a high magnetic flux properties of aluminum and steel are sum- KEYWORDS density B is suddenly generated and pene- trates into Al/Fe sheets, then the eddy cur- Magnetic Pulse Welding rents (current density i) pass through TOMOKATSU AIZAWA and MEHRDAD Seam Welding them to hinder its further penetration. As KASHANI ([email protected]) are with Depart- Dissimilar Metal a result, an electromagnetic force of i ×B ment of Electronic and Information Engineering, acts mainly on the Al sheet and the Al and KEIGO OKAGAWA is with Department of Aluminum Alloys Electrical Engineering, Tokyo Metropolitan Col- Steel sheet is accelerated away from the coil and lege of Technology, Shinagawa-Ku, Tokyo, Japan. collides rapidly with the steel sheet. At the

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A B

Fig. 1 — MPW coil structure. A — Double-layer, H-shaped flat coil; B — one-layer E-shaped flat coil.

A

B

Fig. 2 — Principle of MPW for welding of the Al/steel sheet sample (cross- section view).

moment of collision, the colliding surfaces Equation 1 shows that can be cleared by a large kinetic energy more eddy currents are getting before the collision. After the col- produced at the surface lision, the cleared surfaces are being of the metal sheet for pressed together by electromagnetic force materials that have a and a fixture. higher electrical con- The eddy current i and the magnetic ductivity κ and, as a re- C pressure p are given as follows: sult, the stronger mag- netic pressure p and a ⎛ ∂ ⎞ large amount of Joule ∇× =−κ B ii ⎜ ⎟ (1) heat are generated ⎝ ∂ t ⎠ during the weld ⎛ 2 ⎞ process. In addition, ⎛ ⎞ ⎜ B ⎟ ⎛ − δ ⎞ pBB=−22/ 2μ = o 1 − e 2x/ from Equation 2 it can ⎝ oi⎠ ⎜ μ ⎟ ⎝ ⎠ be obtained that the ⎝ 2 ⎠ Fig. 3 — General outlines of apparatus. A — Cross-section view of the coil magnetic pressure also containing lap of Al/steel (SPCC) sheets and discharge circuit; B — plan anndδωκμ= 2/ (2) increases for higher view of coil with discharge circuit; C — collision speed measurement. C conductive materials. denotes the capacitor bank and G the gap switch. κ μ τ where , , , Bo, and Bi are the electrical conductivity, magnetic permeability, Table 1 — Aluminum and Steel Properties thickness, and magnetic flux density at lower and upper surfaces of Al sheet, re- Melting Specific Density Thermal Electrical spectively. The depth of skin effect can be Point Heat kg/m3 Conductivity Resistivity obtained by δ=√2/ωκμ, where ω is the an- °C J/kg.°C J/m3.°C.s μΩ.cm gular frequency of changing field. When the eddy current flows through Aluminum 660 900 2700 220 2.65 the workpieces, Al sheet is pressed to the Steel 1497 460 7870 73 13.30 Fe sheet by magnetic pressure and is Al/Steel Ratio 0.44 1.96 0.34 0.33 0.20 heated by a Joule heating effect (Q = i2/κ).

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Fig. 4 — The block diagram of the discharge system. Fig. 5 — Divided region of the welded sample for shearing tensile test, optical micrograph, and SEM image observations.

A B

Fig. 6 — A — Typical current signal at 1.4-kJ discharge (200 μF/3.8 kV); B — bank energy vs. maximum discharge current.

A B

Fig. 7 — The speed of the Al sheet just before collision vs. the following: A — Discharge bank energy; B — maximum discharge current.

Experimental Apparatus generate a high-density magnetic flux ductance transmission line. The circuit is around the coil area. designed to keep the inductance as low as Figure 3 shows the general outlines of The capacitor bank that drives the dis- possible to carry out fast welding. The flat, the magnetic pulse welding apparatus, charge system of the MPW device consists E-shaped, one-turn coil was made of a Cr- which consists of a capacitor bank (C) and of two capacitors of 100 μF/10 kV in par- Cu alloy. The coil thickness is 2 mm and a spark gap switch (G) with a one-layer, E- allel. The inductance of the bank capaci- the inductance of the coil is 0.04 μH. The shaped flat coil. It was attempted to make a tor is 0.02 μH, and it is connected to the block diagram of the discharge system is low-inductance discharge circuit that can gap switch and one-turn coil by a low-in- shown in Fig. 4. When the gap switch is

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AB

Fig. 8 — Typical microstructure of joined interface zone for A1050/A1050 and Fig. 9 — A — Cross-section image of welded sample (the red zone is the ob- A5052/SPCC. servation area by SEM); B — SEM image of joined interface for A6016/SPCC sample.

Fig. 10 — EPMA result for Fe and Al distribution across the SPCC/A1050 in- terface layer.

Fig. 11 — SEM-SE image and EPMA result for Al, Mg, and Cr distribution for A1050/A5052 sample.

Fig. 12 — Typical microstructure of the interface layer of the A6111/SPCC sam- ple, including Berkovich hardness indentation across the interface.

Table 2 — The Aluminum Alloy and SPCC Steel Characteristics

Sample Specification A1050 A2017 A3004 A5182 A5052 A6016 A7075 SPCC Conductivity [IACS•] 61 49 41 33 35 53 45 13 Tensile Strength [MPa] 165 187 255 360 290 212 292 350

closed, an impulse discharge current from abrasives and methanol. The 0.1~0.3- pears in the base metal. The coil is clamped the capacitor bank (C) passes through the mm-thick insulating sheets were loaded with the fixture during the welding opera- coil and the MPW process begins. between the coil surface and the over- tion. After welding, the welded sample was Aluminum alloy (A1050, A2017, lapped ends of the workpiece sheet. It was divided into ten pieces for tensile shearing A3004, A5182, A5052, A6016, and A7075) ascertained that the welding characteristics strength tests and optical micrograph and and steel (SPCC) sheets were prepared to could be improved by fixing the initial root scanning electron microscope (SEM) image carry out the weld process. The character- opening (0.5~1 mm) between two metal observations — Fig. 5. istic parameters of the aluminum alloys sheets. The optimization of root opening and SPCC steel that were used in the ex- distance is described in the section titiled Experimental Results and periments are shown in Table 2. “collision speed measurement.” Discussion The size of all samples was 100 mm It should be noted here that the more long and 100 mm wide with a thickness of conductive metal works as a base metal (Al Discharge Current and Flux Density 1.0 mm. The contact surface between two sheet is the base metal for the Al/SPCC samples was polished and cleaned with workpieces), and the main eddy current ap- A typical current waveform is shown in

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A

A

B

B

Fig. 15 — Comparison of the maximum tensile shear strength for the following: A — The same aluminum alloy combinations; B — different aluminum alloy combinations: Fig. 13 — Distribution of tensile shear strength for ten divided pieces ■ The maximum tensile shearing strength of welded sample. ❑ The maximum tensile of the welded samples. A — A1050/A1050, A3004/A3004, and shearing strength of the base alloy without weld. A5182/A5182 sheets; B — A1050/SPCC, A5052/SPCC, and A6016/SPCC sheets: ❍ The rupture of base metal, ● the rupture of the welded area.

Fig. 6A. This current signal was obtained sure the time of travel of the base metal in at 1.4-kJ discharge (200 μF/3.8 kV) by the root opening that existed between the using a magnetic probe. The current signal two workpieces before welding. The cir- shows that a damping and oscillating cur- cuit consists of a coaxial cable and match- rent flows through a one-turn coil for the ing resistance — Fig. 3C (Ref. 7). duration of about 50 μs and the oscillating When the impulse discharge current period is about 22 μs. The maximum cur- passes through the coil, a voltage is induced rent was measured at about 150 kA at 1.4 on the two workpieces by magnetic coupling kJ bank energy discharge. The relation be- between the coil and these workpieces. Just tween the bank energy and discharge cur- after the collision, the voltage appears at rent in our system is shown in Fig. 6B. input terminals of the measuring circuit and If the discharge current flows uniformly that voltage signal can be detected by a dig- on the surfaces of the middle portions of the ital oscilloscope. If we assume that the sheet coil, then the depth of skin effect movement is like a uniform acceleration (δ=√2/ωκμ) was calculated at 0.38 mm for motion, the collision speed just before weld- Al sheet, and under this condition, the max- ing can be estimated by using the time of imum magnetic flux density is estimated at travel and root opening distance. The colli- about 20T, while the maximum magnetic sion speed has a relation with the bank en- pressure is calculated at about 150 MPa ergy and the discharge current and the max- from Equation 2. imum collision speed can be obtained at the first maximum in the current signal. There- Collision Speed Measurement fore, by inserting the appropriate root opening between sample sheets, the colli- Fig. 14 — Typical rupture of Al alloy in the tensile In order to measure the collision speed sion time can be nearly the same as quarter shear strength test of SPCC/Al joints. of the aluminum sheet just before weld- period of the current signal at the first max- ing, a simple circuit was prepared to mea- imum current peak. The optimum root

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WELDING RESEARCH opening has a relation with the capacitor measurement across the interface layer heat-affected zone. The capability of our bank energy and the discharge system in- clearly shows that the hardness of the in- MPW method has also been successfully ductance. However, our experimental re- termediate layer is higher than the alu- examined for several other types of metal sult shows that a 0.5~1 mm root opening minum base metal in all points. The rea- joints, such as T-joints, circular joints, and between sheets is necessary for achieving son for this higher hardness at the long sheet samples (up to 500 mm). high weld quality in the aluminum alloy and intermediate results from intense plastic steel sheet joint. Figure 7 shows the Al sheet deformation due to a high-velocity colli- speed just before collision vs. the maximum sion or to a fine-grain microstructure that Acknowledgments current and bank energy. was formed by rapid solidification of the welded interface. The authors wish to express thanks to Microstructure of Joined Interface Professor K. Ikeuchi of Osaka University Tensile Shear Test and Dr. M. Kumagai of Sumitomo Light The width of the weld zone was nearly Metal Industries Ltd. for fruitful and use- equal to the middle part of the coil (b = 5 Welded samples were investigated on a ful discussions. The authors also would mm). The welded sheets were divided into standard tensile shear testing machine at a like to thank Professor S. Kumai of Tokyo ten 10-mm-wide test pieces as shown in Fig. test rate of 10 mm/min. Tensile shear tests Institute of Technology for the observa- 5, and one longitudinal side of the division were made for each ten divided pieces to tion of the joined interface and Mr. M. of No. 5 was polished for observing the determine the maximum shear tensile Matsuda of Chuo Seisakusho Ltd. for use- joined interface. Several welded combina- strength. The test results for Al/SPCC and ful help about the EPMA test. tions of Axxxx/Axxxx and Axxxx/ SPCC aluminum alloy combination are shown in steels were tested. For the similar work- Fig. 13, where a mark (❍) indicates the rup- pieces, the joined interface was not very ture of based metal and (●) a rupture of the References clear. However, in the aluminum alloy and welded area. Based on the shear strength SPCC steel combination after etching and test results, the tensile shear of divisions No. 1. Brower, D. F. 1969. Metals Handbook 4 polishing, the interface layer were clearly 1 and No. 10 were less than the others. How- . Metals Park, Ohio: ASM Interna- tional, p. 256. seen against the base metals. Typical ever, in other divisions the failures always 2. Shribman, V., Stern, A., Livshitz, Y., and macrostructure of the joined interface zone occurred in the weaker metal and outside of Gafri, O. 2002. Magnetic pulse welding pro- for A1050/A1050 and A5052/SPCC are the welded area. Figure 14 shows the typi- duces high-strength aluminum welds. Welding shown in Fig. 8. As a result of magnetic cal rupture of based metal for SPCC/Al Journal 81(4): 33. pulse welding, a nonuniform wavy interface joints. 3. Masumoto, I., Tamaki, K., and Kojima, is visible for all welded samples. The wavy The comparison of the maximum tensile M. 1980. Journal of the Japan Welding Society interface zones were formed with ampli- shear strength for the same aluminum alloy 1(49): 29. tudes as high as 20 μm and widths of 100 μm. combinations and different aluminum alloy 4. Aizawa, T., Okagawa, K., Yoshizawa, M., Figure 9 also shows the macrostructure of combinations are shown in Fig. 15. The re- and Henmi, N. 2001. Proc. of 4th Int. Sympo- sium on Impact Engineering the joined interface zone for an sults of division No. 5 in Fig. 5 was used for , Kumamoto, Japan, p. 827. A6016/SPCC combination. this consideration. 5. Aizawa, T., and Yoshizawa, M. 2001. Proc. The SEM image of A6016/SPCC also The comparison of the maximum tensile of 7th Int. Symposium of Japan Welding Society, shows that the wavy morphology weld-in- shear for the same alloy combination (Fig. Kobe, Japan, p. 295. terface was formed in the interface layer 15A) shows that except for A5182/A5182 6. Aizawa, T., and Kashani, M. 2004. Proc. of without any significant heat-affected zone and A7075/A7075 joints, the maximum ten- IIW International Conference on Technical (HAZ). sile shearing for all other cases is nearly the Trends and Future Prospective of Welding Tech- same as the tensile shear strength of base nology for Transportation, Land, Sea, Air and Electron Probe Microanalysis (EPMA) metal without a weld and for different alloy Space, Osaka, Japan. combinations (Fig. 15B), the maximum ten- 7. Okagawa, K., and Aizawa, T. 2004. Proc. The result of an EPMA test for of International Conference on New Frontiers of sile shear strength of the welded sample is Process Science and Engineering in Advanced SPCC/A1050 combination is illustrated in also the same as a weaker base metal value. Material, Kyoto, Japan, p. 501. Fig. 10. A single step decrease was ob- It can be pointed out that the sheet metals served in the EPMA profile for all combi- retain their original properties without the nations (Al/SPCC) across the interface. heat-affected zone problems during the The EPMA result shows that the 5-μm- weld process, and the welded zone is wide transition layer is formed in the weld- stronger than the weaker base metals so ing interface. failure always occurred outside of the Figure 11 shows the secondary electron welded zone for these combinations. These Change of Address? images obtained by scanning electron mi- results would be expected for a solid-state croscopy (SEM-SE) and also EPMA of joining process. Al, Mg, and Cr in the A1050/A5052 inter- Moving? face layer. The EPMA result for Mg Conclusions Make sure delivery of your Welding clearly shows that a wavy bond interface Journal is not interrupted. Contact the was formed in the welded zone. We can determine the solid-state weld Membership Department with your Microhardness Profile quality achievable for most aluminum al- new address information — (800) 443- loys and SPCC steel combinations using 9353, ext. 217; [email protected]. To obtain the nano-hardness profile of the MPW method. Our experimental re- the interface layer, the Berkovich indenter sults show that the weld joint is always was used. Figure 12 shows the interface stronger than the weaker metal and in all microstructure along with traces of the tested combination a discontinuous or Berkovich nano-hardness indentations for continuous pocket-type, wavy transition the A6111/SPCC sample. The hardness layer was formed without any significant

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