Aizawa May 2007 layout:Layout 1 4/11/07 1:15 PM Page 119 WELDING RESEARCH Application of Magnetic Pulse Welding 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 fusion welding 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 WELDING JOURNAL 119-s Aizawa May 2007 corr:Layout 1 4/5/07 10:41 AM Page 120 WELDING RESEARCH 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). 120-s MAY 2007, VOL. 86 Aizawa May 2007 corr:Layout 1 4/5/07 10:42 AM Page 121 WELDING RESEARCH 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.